Electron drift velocities in fast Argon and CF4 based drift
gases (NIKHEF 98-030)
D.A. Wijngaarden
September 22, 1998
Abstract
Electron drift velocities in gas mixtures were measured in a tabletop experiment using a
nitrogen laser to create the primary electrons. The maximum drift times for electrons in a
5 mm (10 mm) honeycomb drift cell at 2200 V anode voltage were 28 ns (53 ns) and 21 ns
(61 ns) for Ar-CF4-CH4 (76/18/6) and Ar-CF4-CO2 (68/27/5), respectively. Changing the
ratio of the latter mix did not change the drift velocity very much. The gains of the gases are
104 for a single primary electron. CF4 causes electron attachment.
1
Contents
1 Introduction
5
2 Background
7
2.1 The HERA-B Experiment . . . . . . . .
2.1.1 The Detector . . . . . . . . . . .
2.1.2 LHC-B . . . . . . . . . . . . . . .
2.2 B-Physics . . . . . . . . . . . . . . . . .
2.2.1 CP Violation . . . . . . . . . . .
2.2.2 The B-meson system . . . . . . .
2.2.3 Other physics goals at Hera-B . .
2.3 Honeycomb Drift Chambers . . . . . . .
2.3.1 Fundamental processes in gases .
2.3.2 Drift chambers . . . . . . . . . .
2.3.3 The Honeycomb Drift Chambers
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
3 Experimental setup
3.1
3.2
3.3
3.4
Setup . . . . . . . . . . . . . . . . . .
Drift cell and gas system . . . . . . .
Optics . . . . . . . . . . . . . . . . .
Electronics . . . . . . . . . . . . . . .
3.4.1 Timing signals . . . . . . . . .
3.4.2 Pulse height measurement . .
3.4.3 Amplier . . . . . . . . . . .
3.4.4 ADC and preamp calibration
3.5 Signals . . . . . . . . . . . . . . . . .
7
7
7
7
7
9
10
10
10
11
12
14
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
4 Drift velocity measurements
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
14
14
16
17
17
18
19
19
20
22
4.1 Drift distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2 Anode voltage plateau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1
4.3
4.4
4.5
4.6
Drift time measurements . . .
Calculation of drift velocity .
Drift velocity versus E/p . . .
The Hera-B Hamburg premix
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
5 Gas gain and eciency measurements
24
25
27
28
33
5.1 Gas gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.2 Electron attachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6 Drift Times for 5 and 10 mm Honeycomb Drift Chambers
37
7 Comparing data with simulations
38
7.1 Comparing with GARFIELD and Magboltz . . . . . . . . . . . . . . . . . . . . . . 38
7.2 Sources of error in GARFIELD - experiment comparison . . . . . . . . . . . . . . . 39
8 Conclusions
42
9 Acknowledgements
44
A Scaling the anode voltage
45
List of Figures
1
2
3
4
5
6
7
8
9
10
The Hera-B detector. . . . . . . . . . . . . . . . . . .
CKM unitarity triangle. . . . . . . . . . . . . . . . . .
Hera-B detector layer. . . . . . . . . . . . . . . . . . .
LHC-b detector layer. . . . . . . . . . . . . . . . . . . .
Track reconstruction using honeycomb drift chambers.
The "drift cell" mounted on the support table. . . . . .
The gas pot. . . . . . . . . . . . . . . . . . . . . . . . .
The gas mixing setup. . . . . . . . . . . . . . . . . . .
Beam optics setup . . . . . . . . . . . . . . . . . . . .
Step motor calibration . . . . . . . . . . . . . . . . . .
2
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
8
9
13
13
13
14
15
16
17
18
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
Schematic drawing of electronics setup. . . . . . . . . . . . . . . . . . . . . .
Signal pulse and ADC gate window. . . . . . . . . . . . . . . . . . . . . . . .
Setup for ADC and preamp calibration. . . . . . . . . . . . . . . . . . . . . .
Calculated charge vs. ADC counts for dierent capacitors. . . . . . . . . . .
Graph of ADC counts vs. charge collected on capacitor. . . . . . . . . . . . .
Pulse generator pulse and ADC gate window . . . . . . . . . . . . . . . . . .
Signal distributions for drift time, signal pulse height, and laser pulse height.
ADC counts and drift time single and multiple electron events. . . . . . . .
x-t Graph of scan along x-direction. . . . . . . . . . . . . . . . . . . . . . . .
Dependence of spline t on number of knots. . . . . . . . . . . . . . . . . . .
Ar/CF4/CH4 (76/18/6) and Hamburg premix (7420/6). . . . . . . . . . . . .
Anode high-voltage plateau curve for dierent gas mixtures. . . . . . . . . .
r-t Prole plot for dierent gas mixtures. . . . . . . . . . . . . . . . . . . . .
Data minus spline t results. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drift velocity as a function of the electric eld. . . . . . . . . . . . . . . . . .
Inuence of lter transmission coecient on eciency. . . . . . . . . . . . . .
Signal pulse height spectrum and gaussian t. . . . . . . . . . . . . . . . . .
Gas gain for Ar/CF4/CH4 (76/18/6) and Ar/CF4/CO2 (68/27/5). . . . . . .
Detection eciency as a function of the distance to the wire. . . . . . . . . .
Detection eciency in Ar/CH4, lter transmission coecient = 3.3%. . . . .
Detection eciency in Ar/CH4, lter transmission coecient = 11.2%. . . .
Residual of garfield simulation results and experiment data. . . . . . . . .
Drift velocity obtained from garfield simulation. . . . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
19
20
20
21
22
23
24
25
26
26
28
29
30
31
32
33
34
34
35
36
36
40
41
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
15
37
38
39
List of Tables
1
2
3
4
Conversion factors for gases. . . . . . . . . . . . . . . . . . . .
Drift times for 5 mm cells as a function of r. . . . . . . . . . .
Drift times for 10 mm cells as a function of r, router = 5:7mm.
Maximum and minimum drift velocity and \slope" error. . . .
3
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
5
6
Drift times for 30 mm cells at 2200 V. . . . . . . . . . . . . . . . . . . . . . . . . . 45
Re-scaled anode voltages for drift cells. . . . . . . . . . . . . . . . . . . . . . . . . . 45
4
1 Introduction
The Hera-B experiment at DESY aims to measure CP Violation parameters in B physics events.
The detector uses an extensive tracking system to reconstruct the events. Honeycomb drift chambers constitute the outer tracker. The honeycomb chambers are made by folding a foil with either
a conducting layer or bulk conductivity into hexagonally shaped drift tubes.
The drift gas to be used for the honeycomb chambers has not yet been determined. Ar-CF4-CH4
(76/18/6) is fast, but seems to cause ageing problems. Ar-CF4-CO2, in varying constitution, is the
other main candidate. It is slower, but has nicer ageing properties.
A main consideration in the selection of the gas is the drift velocity of electrons in the gas. As
Hera-B will operate at a high rate, the detection time of a charged particle track in the detector
must be short.
Another reason why the drift velocity is important is the choice of the diameter of the honeycomb
chambers: 5 mm chambers will reduce occupancy, but increase the number of readout channels,
while 10 mm chambers require fewer readout channels but have a larger occupancy. The plan is
to use 5 mm chambers closer to the beam, where the occupancy is greatest, and 10 mm chambers
further from the beam. If the drift gas is too slow, however, the 10 mm chambers may not be used
at all.
For this graduation thesis I have measured the drift velocity of electrons in Ar-CF4-CH4 (76/18/6)
and several mixes of Ar-CF4-CO2 as a function of the electric eld. To do this, the drift time was
measured as a function of the distance of a primary ionization cluster to a high-voltage anode wire.
The primary ionizations were created with a pulsed nitrogen laser. A cylindrical aluminum tube,
3 cm in diameter, was used as a \drift cell". The drift cell was placed in a gas tight steel pot
constructed at nikhef.
Another measurement that could be done with the same setup was the gas gain (amplication) of
the gas. As the electron energy increases near the anode wire, they can knock other electrons free
from the molecules that bind them, starting an avalanche of electrons which is nally measured on
the anode wire. For a single primary electron, the height of this pulse gives the gas amplication.
A nal eect in the gas mixes used - due to CF4 in the mixes - is electron attachment. For certain
ranges of the electron energy, there is a chance that a drifting electron colliding with a gas molecule
will be captured by the molecule, forming a negative ion. In an avalanche, this results in a reduced
avalanche for a single primary electron, this reduces the probability that the electron wil start an
avalanche and be detected.
5
Note
The exact gas mixing ratios were calculated after the experiment. To make this report more
readable, the mixing ratios are given to an accuracy of 1%. The actual (calculated) ratios are given
below:
Ar-CF4-CO2 (68/27/5): 68.3/27.1/4.6
Ar-CF4-CO2 (54/41/5): 53.7/41.6/4.7
Ar-CF4-CO2 (82/13/5): 82.3/13.3/4.5
Ar-CF4-CO2 (66/27/7): 65.9/27.2/6.9
Ar-CF4-CO2 (71/27/2): 70.7/27.0/2.3
Ar-CF4-CH4 (76/18/6): 76.5/17.8/5.7
Ar-CH4 (93/7): 93.0/7.0
The calculated ratios were also used for the garfield calculations in section 7.1. While the error
on the gas ows was about 3% of the ow (section 3.2), much of this error would be systematic for
all gases (temperature dependence, mounting altitude, power supply), so the accuracy used for the
garfield calculations does make sense.
6
2 Background
2.1 The HERA-B Experiment
The Hera-B 1] experiment is set up to search for CP violation in decays of B mesons through the
\gold plated" decay mode B ! J=KS0 . The B mesons are produced in interactions of 820 GeV
protons in the Hera proton beam with an internal wire target in the beam halo. The experiment
can accumulate sucient statistics to detect the rare CP-violating decays by running at high
interaction rates - between 30 and 50 MHz - for several years of Hera operation. B decay candidates
are selected by requiring a dilepton in the J= mass range at the rst trigger level, and rening
the signature at higher trigger levels.
Apart from the study of the B ! J=KS0 mode, the detector and its trigger system can be used to
study a wide range of additional physics topics, ranging from Bs mixing and B hadron spectroscopy
to the investigation of heavy-quark, J= and production mechanisms, and to high-statistics
investigations of rare charm decay modes.
2.1.1 The Detector
Figure 1 shows the layout of the Hera-B detector. The silicon vertex detector has stand-alone
pattern recognition abilities over most of the angular range, to determine the vertex. The tracking
system consists of dierent detectors, with granularity and technology varying with distance from
the beam to limit the occupancy of each detector and yet minimize the number of channels. Most
relevant to this report are the outer tracker chambers - the honeycomb drift chambers. The RICH is
the only available technology to identify a tagging kaon with its momentum between a few GeV and
about 50 GeV. A relatively small transition radiation detector helps improve electron idencation
in the small-angle region. A Pb/Scintillator and W/Scintillator calorimeter and a conventional
muon system complete the setup.
2.1.2 LHC-B
Almost all detector systems are based on LHC development projects. Much of the technology used
in Hera-B will again be used in the LHC-B detector 2]. The Hera-B experiment is sometimes seen
as a testcase for the LHC-B experiment: both have forward detectors tuned to b quark events.
In many cases parts of the LHC R&D groups are members of the Hera-B collaboration, looking
forward to a rst real application of their products.
2.2 B-Physics
2.2.1 CP Violation
CP symmetry supposes that particles behave exactly as their antiparticles under simultaneous
inversion of charge conjugation and parity. CP violation - the breaking of this symmetry - was rst
7
Top View
250 mrad
Proton Beam
Electron Beam
Si-Strip
Vertex
Detector
Target
Wires
Inner/Outer Tracker
Muon Detector
20
Ring Imaging
Cherenkov Counter
Calorimeter TRD
15
Magnet
10
5
Photon
Detector
160 mrad
Vertex Vessel
0m
Side View
Beam
Pipe
Proton Beam
Electron Beam
C4 F10
Spherical Mirrors
Planar Mirrors
The HERA-B Experiment
at DESY
Figure 1. The Hera-B detector.
observed in neutral kaon decays in 1964, an experiment that won the Nobel prize 3].
CP violation plays as important a role in cosmology as in high energy physics, as it goes part of
the way to explain the excess of matter over antimatter in the universe.
The Standard Model with three quark families can naturally generate CP violation in both weak
and strong interactions. CP violation in strong interactions has never been detected. CP violation
in the weak interactions is generated by the unitary CKM matrix 4] describing the mixing of
quarks:
0
1
V
ud Vus Vub
VCKM = B
@ Vcd Vcs Vcb CA
Vtd Vts Vtb
The matrix is uniquely dened by four parameters. The most convenient parametrization is that
proposed by Wolfenstein 5]:
3
VCKM VCKM
+ VCKM
8
3
where the expansion up to third order in is given by VCKM
:
0
3
VCKM
=B
@
1
1 ; 2=2
A3( ; i)
;
1 ; 2=2
A2 C
A
3
2
A (1 ; ; i) ;A
1
The parameter is given by the sine of the Cabibbo mixing angle 6], measured from decays
involving s-quarks.
For a qualitative discussion of CP violation in B-meson systems the second term VCKM is usually
ignored.
Six of the nine unitarity conditions of the CKM matrix can be drawn as triangles in the complex
plane (gure 2). (If the second term VCKM is ignored, the triangles are identical). The angles
of the triangles can be measured either indirectly by measuring the length of the sides, or, within
the Standard Model, directly from CP asymmetries. If the angles extracted by the two dierent
methods disagree, this would indicate new physics.
Im
2
α
V
λ V td
cb
cb
(1− 2
λ/2
λ V )Vub*
η(1−λ/2)
γ
0
β
2
ρ(1−λ/2)
Re
1
Figure 2. CKM unitarity triangle.
2.2.2 The B-meson system
Since its discovery, CP violation has been detected only in the decay amplitude of KL mesons. In
the B -meson system many more decay modes are available, and the Standard Model makes precise
predictions for CP violation in a number of these. Thus, measurements of CP violation in the
B -system can be used as a very precise test of the Standard Model.
The CKM triangle(s) are completely determined by , , and . Conversely, these parameters can
be calculated from the angles of the triangle.
Hera-B will aim to measure the angle in the CKM triangle, through the size ACP of the
asymmetry between the B 0 ! J=KS0 and the B0 ! J=KS0 decay modes:
ACP = sin 2sinxt.
A new item on the Hera-B menu is the possibility to measure the angle of the CKM triangle,
based on the mode B 0 ! +
;.
9
There are even speculations about possibilities to measure the third angle, , by studying decay
channels such as B + ! D0 K + or Bs0 ! D K . However, the event samples, O(100) events per
year, appear marginal for a meaningful asymmetry measurement.
Another measurement constraining the CKM triangle is the Bs0 mixing frequency. From this frequency, the value of jVtsj can be extracted.
2.2.3 Other physics goals at Hera-B
Other physics possibilities at Hera-B include:
The study of hadronic B decays:
the Hera-B experiment will detect many thousands of
elusive decays of charged, neutral and strange mesons and of B baryons.
Semileptonic B decays and CKM physics: a good fraction of the fundamental CKM matrix elements are accessible through B decays, such as Vcb (semileptonic decays to charmed mesons),
Vub (semileptonic decays to light mesons), Vtd (B 0 mixing) and Vts (Bs0 mixing).
The study of rare B decays and exotic states (e.g. Bc).
An automatic byproduct of the Hera-B experiment are large samples of J= and mesons
decaying into lepton pairs. These can be studied as well.
Finally, at Hera energies, the charm production cross section is more than three orders of magnitude larger than the b cross section. Due to bandwidth and trigger rate limitations, charm studies
will largely be limited to channels which are accepted by the various triggers aimed primarily at B
decays.
2.3 Honeycomb Drift Chambers
2.3.1 Fundamental processes in gases
A particle traversing a gas can lose energy by elastic scattering, by excitation and by ionization of
the gas constituents. In the case of ionization, one or more electron-ion pairs are created in the
gas. If the energy of the electrons is high enough (specically in a strong electric eld) they too can
cause excitation and ionization. The process of ionization is the basis of avalanche multiplication.
The number of electrons can continue to multiply until an avalanch of 108 is formed. At this
point space charge repulsion may prevent further growth.
Avalanche multiplication occurs in noble gases at much lower elds than in complex molecules.
However, excited noble gases can only return to the ground state through emission of a photon. The
minimum energy of the emitted photon (11.6 eV for argon) is well above the ionization potential for
any metal constituting the cathode. Photoelectrons can therefore be extracted form the cathode,
and initiate a new avalanche. Ions of noble atoms will drift to the cathode and are neutralized
by extracting an electron. The balance of energy is either radiated as a photon, resulting in
photoelectrons, or by extraction of another electron from the cathode. All these processes result in
10
a delayed spurious avalanche. Even for moderate gains, the probability of these processes is high
enough to create a permanent discharge.
Weakly bound polyatomic molecules, hydrocarbons for example, can absorb photons over a wide
range of energies through excitations of rotational and vibrational levels. Polyatimic ions are also
less likely to cause secondary emission of electrons at the cathode. These gases are therefore known
as quenchers. Addition of a quencher will allow the absorption of the photons, which is essential
for high gain and stable operation.
Other processes aecting the drifting electron are recombination, absorption by the walls of the
volume, and electron attachment. An electron can recombine with a positive ion or be absorbed
by the wall of the drift volume, in both of which cases it is lost.
In collision with molecules with high electron anity (\electronegative" molecules) the electron
can be captured by the molecule, creating a negative ion. The molecule may also dissociate into
smaller parts, one of which captures the electron. Electron attachment reduces the gain factor,
and, in events with only one primary electron, the eciency, as the electron might not be detected
at all.
The number of electrons produced by a single electron traveling 1 cm along a uniform electric eld
is known as Townsend's rst ionization coecient . Conversely, ;1 is the electron mean free path
for ionization in the gas. In a mixture displaying electron attachment the growth of the avalanche
can be described by an eective ionization coecient = ; , where is the probability of
attachment per unit length. Both and are dependent on the value of the electric eld over the
gas pressure E/p 7], 8].
The ionization cross section peaks for electron energies around 100 eV for most gases, so the
avalanche starts very close to the wire (tens of microns from the anode wire for anode voltages in
the kilovolt range). Electron attachment typically starts at electron energies of 2-7 eV, which occur
at about 1-2 mm from the wire.
2.3.2 Drift chambers
A drift chamber uses the drift time of ionization electrons in a gas to measure the spatial position
of an ionizing particle. Typically, a reference time is generated by a fast scintillation counter. The
time of arrival of the electron avalanche at the anode wire gives a measure of the distance of the
particle track to the wire.
Important properties of drift chamber gases are the drift velocity, the diusion of electrons in the
gas, and specic energy loss. A high drift velocity is necessary in a high rate environment (such
as Hera-B and LHCb), but reduces the spatial resolution. Small diusion is desirable for good
spatial resolution. A gas with high specic energy loss would be used for ecient detection of
charged particles and photons, while a lower specic energy loss minimizes multiple scattering of
particles in the detector.
Argon is widely used as a drift chamber gas. It has a large multiplication at relatively low working
voltage, has a high specic ionization and is not as expensive as, for example, xenon or krypton.
Pure argon chambers are limited to gains of 103 . At higher gains, operation becomes instable
11
because of secondary avalanches caused by photoelectrons and emission of secondary electrons from
the cathode (section 2.3.1) .
Argon is usually mixed with quenchers: gases consisting of heavy organic molecules, such as CO2
and CH4. These gases have many degrees of freedom and can eciently absorb photons. Addition
of these polyatomic molecules allows gains in excess of 106 . Also, the drift velocity is increased,
and diusion decreased.
Dissociation products of some polyatomic gases (e.g. isobutane) can cause contamination of the
chamber electrodes. The molecules can form radicals, combining into solid or liquid residues.
Electric charge can build up on this layer in high rate environments and distort the electric eld.
Other eects of this kind of ageing are a drop-o of signal size and eciency, increased noise and
a decrease of the extent of the high voltage operating plateau.
CF4 gas mixtures are very advantageous because CF4 is very fast, oers good spatial resolution
and suppresses wire coating caused by avalanches. However, CF4 is also expected to cause electron
attachment 7]. In addition, uor, which may be produced with dissociation of CF4 molecules, is
very agressive and could damage the detector.
The spatial resolution of drift chambers is limited by the timing resolution. Ionization along a
charged particle track occurs in clusters. Particles passing very close to the sense wire might not
create a cluster at the point of closest approach, but a little further along the track. This results
in an incorrect measurement of the distance of the track to the wire. For tracks far from the wire,
diusion of the cluster limits the resolution, as some electrons drift faster than others. Because the
rise time of the signal is a constant for an amplier, the setting of the threshold voltage for the
timing signal also inuences the resolution. High signals will trigger the threshold earlier than low
signals, causing jitter in the timing signal the threshold should be set as low as possible for low
pulse heights. The resolution depends on the distance of the track to the wire. The mean resolution
(however dened) of drift chambers is typically of the order of 100 m.
2.3.3 The Honeycomb Drift Chambers
The nikhef involvement in the Hera-B and LHCb experiments is focused on the outer tracker
chambers. The outer tracker modules are made up of honeycomb chambers. A conducting foil (eg.
pokalon-C) is folded to make up one side of a layer of honeycomb tubes ve such layers make up one
Hera-B detector layer (gure 3). The LHC-b layers are made up of two single honeycomb layers
separated by a at conducting foil (gure 4).. The honeycomb chambers have several advantages,
especially in manufacturing: the anode wires can be strung across a single layer before glueing the
layers together. In straw tubes, the wires have to be drawn through the tube. Disadvantages of
the honeycomb chambers are their non-cylindrical eld in the corners and outgassing of glue. The
eld in the corners inuences the resolution for particles passing through the chamber far from the
anode wire.
The Hera-B chambers are made with both 5 and 10 mm heights - 5 mm chambers closer to the
beam to reduce the occupancy, 10 mm further away to reduce the number of channels. Drift times
in the 10 mm chambers are, of course, considerably longer depending on the drift velocity of the
nally chosen drift gas, the collaboration might decide not to use 10 mm chambers altogether.
12
Figure 3. Hera-B detector layer.
Foil
Cell layer
Figure 4. LHC-b detector layer.
The honeycomb chambers are used to reconstruct charged particle tracks. A charged particle will
create free electrons in the gas along its track (gure 5). If the r-t relation of the drifting electrons
is known, the drift time of the electrons can be used to calculate the distance of the track to the
anode wire.
r(t)
charged particle track
Figure 5. Track reconstruction using honeycomb drift chambers.
13
3 Experimental setup
3.1 Setup
The purpose of this experiment was to calculate certain properties of fast drift gases for the HeraB and LHC-b outer tracking chambers: drift velocity of electrons in an electric eld, and gas gain
(amplication) near the anode wire.
The drift velocity and electric eld can not be measured directly. Instead, the drift time can be
measured as a function of the distance of the ionization spot to the anode wire.
To create the primary electron clusters a nikhef two-stage pulsed nitrogen laser was focused into
a drift volume - a 3 cm diameter aluminum cylindrical tube placed in a gas tight pot. A sense
wire running through the center of the tube provided both the end time and the magnitude of the
signal. A reference (start) time was given by a laser diode placed in a split-o fraction of the laser
beam. The laser beam was parallel to the wire, so the drift distance could be measured to good
precision.
3.2 Drift cell and gas system
The drift volume for the experiment was an aluminum cylinder, 3 cm in diameter and 12 cm long.
An anode wire was suspended along the center of the cylinder, supported by two ceramic posts.
The wire had an outer diameter of 25 m. It was connected to a high voltage supply and to an
amplier to read out the wire signal. The cylinder was grounded. The cylinder, the ceramic posts
and the table on which the cylinder was mounted are shown in gure 6.
Figure 6. The "drift cell" mounted on the support table.
The aluminum \drift cell" was placed in a cylindrical, gas tight stainless steel pot. The pot was
constructed at the nikhef workplace and has a quartz glass entrance window for the laser light
on both ends, and two outlets for the gas system. The windows were optically at. A high-voltage
connector mounted in one of the sides connected the anode wire to the amplier and high-voltage
system. In gure 7 the pot is shown with the high voltage connector to the left and the entrance
windows the right and at the back. The gas in - and outlets are located next to the windows.
14
Figure 7. The gas pot.
Gas Measured Brooks
argon
1.39
1.40
CO2
0.74
0.77
CF4
0.44
0.48
CH4
0.78
0.81
Table 1. Mass ow meter conversion factors for argon, carbon dioxide, tetrauormethane and methane,
and the factors according to the producer.
All measurements were done on gas mixtures with up to three components. Mass ow meters
(Brooks model 5850E) were used to control the dierent ow rates. The meters are calibrated for
air at specic temperature and pressure. Conversion factors for dierent gases were accounted for.
The accuracy of the ow meters was 1% full scale other errors (temperature sensitivity, voltage
output, power supply sensitivity, mounting attitude sensitivity) contributed to a total error of 3%
(upper limit) on the ow.
The ow meters were calibrated for the experiment with a soap bubble ow meter. It turned out
that, while reasonably close to the values given by Brooks, the conversion factors were dierent
from those used to set the gas ows. The calibration of the ow meters made it possible, however,
to calculate the actual ows during the experiment and the constitution of the gas. The conversion
factors found during calibration are listed in table 1. The conversion factors for especially argon
seemed to depend on the gas pressure the value listed here was measured at 2 and 8 bar.
The air calibration for the mass ow meters was incorrect this, and the dierence between these
conversion factors and those used to mix the gases accounts for the rather \odd" gas mixtures (as
opposed to rounded gures such as 65/30/5, for example).
The volume of the pot was about 8.5 dm3. The purity of the gas mix is given by the following
formula:
N
; F t
N0 = e V
where N/N0 is the fraction of \old" gas left in the pot, and F is the ushing rate. Flushing at a
15
typical rate of 5 l/h, overnight ushing guaranteed that the gas mixture was over 99.9% pure.
flow control
unit
gas containers
drift volume
gas rack with brooks
mass flow meters
Figure 8. The gas mixing setup.
3.3 Optics
A nikhef nitrogen laser 9] was used to produce the primary electrons. The nikhef nitrogen laser
essentially consists of two identical lasers, the 'oscillator' and the 'amplier'. Each consists of two
thick metal plates. The gaps beteen the plates in each section are the cavities that generate the
laser light the oscillator cavity is smaller than the amplier cavity.
A voltage drop on one of the plates in each section leads to a fast rising electric eld in the gaps
between the plates. A gas discharge develops over the full length of the cavities, ending in the
formation of a highly conductive plasma. Collisions between electrons and gas molecules result in
a population inversion at the end of the discharge.
The light so generated is emitted over a fairly large angle, limited only by the exit window. Because
the time needed for the discharge to develop depends on the magnitude of the electric eld, the
smaller oscillator cavity res rst. The beam is reected back into the cavity, where it is amplied.
It is then directed into the amplier cavity. By adjusting the width of the amplier cavity, the
resulting time dierence can be made to just compensate the path length between the cavities.
The laser emits pulses of about 100 J - a peak power of the order of 100 kW for 1 ns pulses - at
a wavelength of 337 nm.
The beam optics are shown in gure 9.
To produce a highly localized ionization, the laser beam is strongly focussed. A diaphragm is used to
remove spurious light. After the diaphragm, a beam splitter is placed. Part of the beam is focussed
onto a fast photodiode to provide a reference time and amplitude. Filters prevent saturation of the
diode.
A couple of positive lenses is used to adapt the beam width to about 15 mm diameter. After the
beam width adapter a mirror system directs the (parallel) beam into the pot, through a focussing
lens which focuses the beam inside the drift cell. The focal length of the lens used in this experiment
was 20 cm.
16
mirror
system
focusing
lens
laser
diode
Beam Width Adapter
filter
111
000
optional
filter
drift cell
nitrogen
laser
beam
splitter
f1
f2
Figure 9. Beam optics setup
The mirror system used in this experiment consisted of two mirrors mounted on slides. The
geometry of these mirrors guarantees that an incident beam will be reected under a 90 degree
angle. The focusing lens could be moved through the plane perpendicular to the anode wire in the
drift cell, so it was possible to direct the beam at a very precise distance of the wire. Step motors
were used to move the beam through the drift cell. The accuracy of the step motors was 2 m,
but the error on the precision of the mirrors was a little larger - about 5 m. Over longer periods
(days) the reference coordinates of the wire in the gas pot changed somewhat, most notably the
vertical coordinate (the step motors were actually made for horizontal use only but even thermal
expansion and other meteorological eects on the table on which the experiment was mounted could
have caused this eect). A third slide adjusted the distance of the focussing lens to the entrance
window of the gas pot. During measurements this distance was kept constant.
The accuracy of the step motors was measured with a micrometer gauge. The motor position as
entered in the driver software has been plotted against the measured position in gure 10. A linear
t on the data gave slopes of 1.0 and -0.99 for the x- and y motors, respectively. The measured
accuracy of the slides was 5 m
3.4 Electronics
3.4.1 Timing signals
Figure 11 shows the (schematic) setup of the electronics for the experiment.
To measure drift speeds, timing signals from the laser diode and the sense wire were used. The
sense wire was connected to an amplier, providing a digital (ECL) timing signal and an analog
pulse.
The laser diode gives the start signal for the timing discriminator the ECL gas pulse is delayed
to account for other electronics and gives the end pulse. A dual timer started by the diode signal
provides a veto window for the TDC. Outside this window, no timing signals are accepted. This
prevents the TDC from registering signals outside this window (due to cosmics or noise) that would
result in nonsensical drift times. The end of the veto window also serves as an end of window signal,
17
measured position
7
x motor
6
5
4
3
2
1
0
59
60
61
62
63
64
65
66
67
measured position
motor reference coordinate
7
y motor
6
5
4
3
2
1
0
29
30
31
32
33
34
35
36
37
motor reference coordinate
Figure 10. Step motor calibration
resetting the TDC for the next event.
The digital gas pulse in coincidence with the same dual timer also provides a gate signal for an ADC
to measure the analog pulse height. This coincidence signal starts a dual timer which provides the
gate signal for the ADC.
Finally, the diode signal starts another dual timer, providing a gate signal for another ADC to
measure the diode pulse height.
The TDC had a resolution of 0.5 ns. The delays introduced in the timing are not relevant to
the nal goal: electron drift velocity. Being constant, they disappear after dierentiating the r-t
relation.
3.4.2 Pulse height measurement
To measure pulse heights the ADC gate windows must be set to include the analog signal, but a
minimum number of noise (pedestal) counts. Pedestals are counted when the ADC gate is open
with no signal on the input. The number of pedestals depends on the duration of the gate pulse.
The number is constant for a xed gate setting and can be subtracted from the pulse signal. Using
an oscilloscope to monitor the signals, the pedestals were reduced to about 20% of the total signal
(gure 12). The pedestal counts could be measured by taking a measurement without the analog
signal connected to the ADC.
18
dual gate
generator
end
out
marker out
laser diode pulse
50 ns
ADC
gate
delay
laser
diode
analog
splitter
quad
discriminator
dual gate
generator
end
out
marker out
1000 ns
coincidence
unit
dual gate
generator
end
out
marker out
amplifier analog pulse
200 ns
ADC
gate
delay
analog
Vth
TDC
preamp
veto
ECL/NIM
converter
discriminator
end
window
shielding
drift
cell
common
1
amplifier ECL pulse
Figure 11. Schematic drawing of electronics setup.
3.4.3 Amplier
The amplier used for this experiment was built at nikhef by Jan David Schipper, using a Lecroy
TRA402S chip. It consists basically of a current amplier and a pulse shaper to shorten the signal.
The amplier also contained a discriminator to provide a logical (ECL) \stop" signal for the TDC
(gure 11). The response of the amplier was measured for the experiment (section 3.4.4). The
gain according to the constructor of the amplier was 12 mV=A.
3.4.4 ADC and preamp calibration
To know the actual charge collected on the anode wire (caused by the electron avalanche) the
preamplier and ADC must be calibrated. To do this, the drift cell was replaced by a setup as
shown in gure 13. A capacitor was connected to the amplier. A known pulse from a pulse
generator and an attenuator provided a range of signals on the capacitor. The charge on the
capacitor can be calculated and compared with the number of counts on the ADC. A secondary
pulse from the pulse generator provided a trigger for the electronics system (as the "laser diode
pulse").
Two further eects had to be accounted for: the shaper in the amplier setup, and the capacity of
the system itself. A suciently long pulse from the generator accounted for the shaping eects. To
be able to safely ignore the input capacity the capacitor had to be small, compared to the input
capacity. For capacitors of the order of 1 to 2 pF the system capacity had negligible inuence on
the measurement (gure 14). The system capacity was estimated larger than 100 pF, reducing the
error due to the system capacity to the order of 1%.
19
Figure 12. Signal pulse and ADC gate window.
dB
attenuator
pulse
generator
C
preamp
"anode wire signal"
to electronics
shielding
1
"laser diode signal"
Figure 13. Setup for ADC and preamp calibration.
The curve of ADC counts versus collected charge is shown in gure 15. The collected charge for
actual signal pulse measurements could be interpolated from these data. The capacity of the used
capacitors was measured to an accuracy of 0.01 pF. The error on the pulse amplitude of the pulse
generator was 3 mV. The error on the attenuation was small enough to be ignored, so the error
on the amplitude scales down with the attenuation. The nal error on the collected charge was
about 1.5%.
3.5 Signals
For each event, three observables were measured: the drift time (as the time between the laser
diode pulse and the digital amplier pulse), the laser diode pulse height and the amplier analog
pulse height. For each coordinate (x,y) a number of events were taken the drift time, amplier
20
V x C versus ADC counts
ADC counts
300
250
1.05 pF
1.92 pF
200
2.13 pF
9.87 pF
150
100
50
0
3
10
4
5
10
10
collected charge (electrons)
Figure 14. Calculated charge vs. ADC counts for dierent capacitors.
pulse height and laser pulse heights could be binned in histograms (gure 17).
Figure 18 shows the distribution of the pulse height for a large sample run at relatively low eciency
(about 50% this was done by placing a lter in the laser beam (section 5)). The pulse height
distribution (top left) has a long tail (like a Landau distribution). The bulk of events was due
to a single ionization (one primary electron). Events with more than one primary electron (the
probability drops o exponentially with the number of primary electrons) cause bigger avalanches
and higher pulses - hence the longer tail.
The top right graph is a scatterplot of the pulse height (in ADC counts) versus the measured drift
time. Greater pulse heights clearly correspond to short drift times. It is even possible to discern
two \tails" for longer drift times, one at about 200 ADC counts and one at about 400 (which is
much less populated). These are caused by dierent numbers of primary electrons. The drift time
is determined by a threshold in the amplier a higher pulse, with a sharper rise, will trigger the
threshold faster than a lower pulse.
The peak in the drift time histogram rises sharply, but has a longer tail. In normal, high eciency
operation the bulk of the events will be due to more than one primary electron. The higher pulses
for these events result in minimal drift times. The longer tail is an eect of events with fewer (one)
primary electrons. Avalanche uctuations spread the measured drift time for these events.
At low eciency (as in gure 18), for longer drift times, the events are almost all due to a single
primary electron. The pulse height for single electron events follows a gaussian distribution (bottom
left), an eect of the statistical nature of the avalanche. For the shortest drift times, the pulse height
21
ADC counts
ADC and preamp calibration
600
500
400
300
200
100
0
4
5
10
10
6
10
collected charge (electrons)
Figure 15. Graph of ADC counts vs. charge collected on capacitor.
distribution is closer to a gaussian as well, this time mainly due to two-electron events (bottom
right).
4 Drift velocity measurements
To calculate the drift velocity function we measured the drift time as a function of the drift distance.
The derivative of this function, with the distance values translated to the electric eld over gas
pressure, gave us the v - E/p relation we needed to translate the experiment to a "real" honeycomb
drift chamber.
4.1 Drift distance
To calculate the drift distance, the wire position must be known. By "scanning" - taking measurements along a line close to the estimated wire position - both horizontally and vertically, a curve as
shown in gure 19 was obtained. The minimum drift time conforms to the closest approach to the
wire. A 2nd degree polynomial t to the curve was used to calculate the position of the minimum.
Two to four scans, both horizontally and vertically, gave us a statistical error on the wire position
of at most 100 m, but typically 10 to 30 m. The error on the vertical coordinate was usually the
larger of the two (up to 40 m), because the wire support post inhibited scans close to the wire in
22
Figure 16. Oscilloscope view of the analog amplier pulse generated by charging a capacitor. The shaper
shortens the pulse to fall within the gate window. The overshoot is suciently delayed to fall outside the
window.
that direction.
A series of ne horizontal scans (measurements taken about 200 m apart, over a total distance of
4 or 5 mm) gave us the desired number of data. All these scans were taken into account to calculate
the nal xwire position, giving a spread on the mean of about 10 m.
The nal error on the drift distance r is due to the error on the wire position, the error on the
motor position and the error on the focus and ionization point. The error on the motor position
was small enough to be ignored (gure 10).
The laser beam was expanded to a 2 cm wide parrallel beam, and focused to a point with a lateral
dimension of 3-4 m in the drift cell 9]. The error on the ionization point was estimated at about
20 m. The error on the drift distance was also increased somewhat because the slides on whch the
mirrors were mounted (gure 9) tended to \drag" a little bit, over longer periods of time (a day),
to a few tens of microns. The nal error on the drift distance was about 45 m.
4.2 Anode voltage plateau
To determine the anode voltage for the experiment, the detection eciency (number of anode wire
signals in 100 laser pulses) was plotted against the high voltage on the wire (gure 22). A voltage
of 2200 V was chosen for all gas mixtures so the eld would be identical for all measurements. The
23
Drift time spectrum
Anode pulse height spectrum
t
h
Nent = 100
Nent = 218
Mean = 100.16
Mean = 135.968
9
RMS = 1.30935
50
RMS = 16.8369
8
7
40
6
30
5
4
20
3
2
10
1
0
98
0
100
102
Laser pulse height spectrum
104
106
108
t (ns)
100
120
140
160
180
200 220
ADC counts
l
Nent = 100
5
Mean = 199.45
RMS = 31.8051
4
3
2
1
0
80 100 120 140 160 180 200 220 240 260 280
ADC counts
Figure 17. Signal distributions for drift time, signal pulse height, and laser pulse height.
drift cell operated in the plateau region for all gas mixtures at that voltage.
4.3 Drift time measurements
For each coordinate (x,y) and corresponding distance r to the anode wire, a number (usually 50)
of events were generated. The drift times were binned in a histogram with a bin size of 1 ns. The
weighted average of the mean peak was taken as the drift time corresponding to that distance.
The error on this mean was taken as the rms of the same peak. The error was typically of the
order of 0.3 ns. This statistical error comprises all the errors caused due to electronics, noise, and
uctuations in the actual drift times. Any systematic errors would not matter - the drift times
were recorded with a large oset anyway (about 50 ns). Any oset would disappear after taking
the derivative of the r-t relation - systematic errors would not show up in the nal v-E/p relation.
Drift time measurements have been performed for Ar-CF4-CH4 (76/18/6), a candidate because of
its high drift velocity, and Ar-CF4-CO2 (68/27/5), which is slower, but has nicer ageing properties.
Additional measurements have been done on dierent mixes of Ar-CF4-CO2 - twice varying the
argon/CF4 ratio (54/41/5 and 82/13/5) and twice varying the argon/CO2 ratio (66/27/7 and
24
gp
ADC counts for all events
htemp
Nent = 2277
Mean = 253.039
RMS = 81.1232
160
gp:t {t<100}
ADC counts versus recorded drift time
800
counts
700
140
120
600
100
500
80
400
60
300
40
200
20
100
0
100
200
300
400
500
600
700
800
80
85
90
95
t (ns)
ADC counts
gp {gp<300 && t > 85}
ADC counts for t > 85 ns
gp {t<83}
htemp
Nent = 713
Mean = 187.842
RMS = 19.091
ADC counts for t < 83 ns
100
htemp
Nent = 226
Mean = 370.491
RMS = 87.415
10
30
25
8
20
6
15
4
10
2
5
0
140
160
180
200
220
240
0
260
ADC counts
200
300
400
500
600
700
800
ADC counts
Figure 18. ADC counts and drift time single and multiple electron events.
71/27/2) to look at the eect on the drift velocity. Prole plots of the drift time versus the drift
distance are plotted in gure 23.
The shortest drift times are obtained for Ar-CF4-CH4 (145 ns at 13 mm from the wire). Ar-CF4CO2 (68/27/5) was about a factor 1.6 slower at 274 ns. As expected, a higher CF4 content increased
the drift velocity, but not much: 254 ns at 13 mm for Ar-CF4-CO2 (54/41/5) and 305 ns for ArCF4-CO2 (82/13/5). Changing the CO2 content of the mixture proved to have more inuence:
364 ns and 204 ns at 13 mm for Ar-CF4-CO2 (71/27/2) and Ar-CF4-CO2 (66/27/7), respectively.
(The xed electronics oset in the drift time has been subtracted to obtain these gures.)
4.4 Calculation of drift velocity
To calculate the drift velocity, the data were tted with B-splines. B-splines are polynomial ts
over a part of the data. At the connecting points - or knots - the splines are continuous. Splines of
higher degree also have continuous derivatives. So, for a 3rd degree B-spline t, the derivative of
the derivative is continuous - giving a smooth derivative.
The result of the t depends on the degree of the splines and the number of knots. The degree
of the t used here was 3 the number of knots was dierent for each of the gas mixtures. The
spline t was performed using CERN library functions 13]. The result of the spline ts is given in
25
t (ns)
Scan along x direction
220
200
180
160
140
120
100
80
45
50
55
60
65
70
x (mm)
Figure 19. x-t Graph of scan along x-direction.
gure 24, as the dierence between the data points and the t.
To get the velocity, the derivative of the t was computed numerically. Because the spline t
routines did not return errors on the parameters, it was not possible to calculate the error on the t
(apart from a 2 calculation, which seemed to depend more on the quality of the data than on the
quality of the t, especially for larger drift distances). Other methods to calculate the derivative
failed miserably - a simple dx=dy on the prole of t verus r gave no signicant results. Because the
outcome of the derivative of the spline depended very strongly on the chosen parameters, the error
on v (1/derivative) is rather large, of the order of 20%. This is due to technical problems, and not
to the quality of the data, where the error is much smaller - gure 24 shows that the error due to
a bad t is nowhere larger than 2 ns. The larger \divergence" for larger drift distances is due to
the spread in the data for large r (this is an eect of the error on the wire position, as discussed in
section 3.3).
Figure 20 shows the eect of too few, and too many knots in the t. A t with too few knots doesn't
show the characteristic \peak" in the drift velocity, a t with too many results in a wildly oscillating
v versus E/p graph. Between these extremes, the peak velocity and the minimum reached after
the peak vary strongly with the number of knots. This is the region most sensitive to the t, as
well as the measurements (as the drift times are shortest).
Too many knots
v (cm/µs)
v (cm/µs)
Too few knots
14
12
14
12
10
10
8
8
6
6
4
4
2
2
0
0
5
10
15
20
25
30
0
0
35
40
E/p (V/cm/torr)
5
10
15
20
Figure 20. Dependence of spline t on number of knots.
26
25
30
35
40
E/p (V/cm/torr)
4.5 Drift velocity versus E/p
The value of the electric eld can be calculated from r according to the following formula:
E = rVlnanrrb
a
where Van is the high voltage on the anode, ra and rb are the outer radius of the wire and the inner
radius of the drift tube, respectively, and r is the distance from the wire.
The anode voltage was known to a precision of 50 V, just over 2% of the value.
The measurements were taken at atmospheric pressure - uctuating with weather conditions. The
pressure has been recorded, but not accounted for in the nal calculations. For these measurements,
the value was close enough to the average of 1013 mbar used in the calculations to be ignored. At
worst, this neglect introduced another error of about 3% to the nal value of E/p. In this case, the
nal error is 4%.
The graphs of v versus E/p, calculated using the spline ts, are shown in gure 25. The number of
knots in the t was chosen so that the peak velocity was maximal. The velocities should be read
from the graphs with careful thought: the error is large (up to 2 cm/s).
Calculations for higher values of the electric eld hardly make sense: the error on the drift distance
limits the mimimum distance of the ionization spot to the wire to about 50 m. In reality, the
geometry of the experiment limits the distance of closest approach even more: part of the laser
beam is cut o by the wire support, decreasing the intensity and causing reections. The closest
we could get to the wire was about 100 m, and that is of course not sucient to calculate the
drift velocity at that point. The value of E/p at that point is 40V cm;1torr;1 . This is the limiting
value for sensible calculations of v versus E/p.
27
4.6 The Hera-B Hamburg premix
Drift times in an Ar-CF4-CH4 (74/20/6) premix from DESY in Hamburg were measured along
with the other gas mixes, to compare the Hamburg premix with the mix created at nikhef. This
was done specically to see if the Hamburg premix was pure enough the collaboration in Hamburg
had had a few problems. Unfortunately, one low-signicant bit of the TDC was malcunftioning
just during the premix measurements. This caused a "step" pattern in the r-t graph. As can be
seen in gure 23 (top right), the premix measurements t the data for the nikhef mix very well
considering the slight dierence in constitution. Figure 21 shows the dierence between proles
of the nikhef and Hamburg mixes. The agreement is very good, even though the nikhef mix
should have a slightly higher argon content (76/18/6). Within the errors of the experiment and
the error on the Hamburg mix purity itself, there is no reason to assume there is anything wrong
with the Hamburg mix. Moreover, this measurement shows that there is little contamination of,
for example, water or oxygen in the Hamburg mix, both of which would have a signicant eect on
the drift velocity.
Ar/CF4/CH4 76/18/6 and Hamburg premix
t (ns)
200
+ Hamburg premix
solid line: NIKHEF mix
O Hamburg premix - NIKHEF mix
150
100
50
0
0
2
4
6
8
10
12
14
r (mm)
Figure 21. Ar/CF4/CH4 (76/18/6) and Hamburg premix (7420/6).
28
ar-cf4-ch4 76/18/6
ar-ch4 93/7
%
%
100
100
80
80
60
60
40
40
20
20
0
1500
1600
1700
1800
1900
2000
2100
2200
V
2300
1800
1900
2000
2100
2200
V
2300
0
1500
1600
1700
1800
1900
2000
2100
2200
V
2300
1800
1900
2000
2100
2200
V
2300
1800
1900
2000
2100
2200
V
2300
ar-cf4-co2 68/27/5
%
100
80
60
40
20
0
1500
1600
1700
ar-cf4-co2 54/41/5
ar-cf4-co2 66/27/7
%
%
100
100
80
80
60
60
40
40
20
20
0
1500
1600
1700
1800
1900
2000
2100
2200
V
0
1500
2300
ar-cf4-co2 82/13/5
%
100
100
80
80
60
60
40
40
20
20
1600
1700
1700
ar-cf4-co2 71/27/2
%
0
1500
1600
1800
1900
2000
2100
2200
V
0
1500
2300
1600
1700
Figure 22. Anode high-voltage plateau curve for dierent gas mixtures.
29
400
ar-cf4-ch4 76/18/6
400
350
350
300
300
250
250
200
200
150
150
100
100
50
0
400
ar-cf4-ch4 74/20/6 Hamburg premix
t (ns)
t (ns)
2
4
6
8
10
12
14
6
8
10
12
14
50
0
r (mm)
2
4
6
8
10
12
14
6
8
10
12
14
6
8
10
12
14
r (mm)
ar-cf4-co2 68/27/5
t (ns)
350
300
250
200
150
100
50
0
400
2
4
r (mm)
ar-cf4-co2 54/41/5
400
t (ns)
350
350
300
300
250
250
200
200
150
150
100
100
50
0
400
ar-cf4-co2 66/27/7
t (ns)
2
4
6
8
10
12
50
0
14
r (mm)
ar-cf4-co2 82/13/05
400
t (ns)
2
4
r (mm)
ar-cf4-co2 71/27/2
t (ns)
350
350
300
300
250
250
200
200
150
150
100
100
50
0
2
4
6
8
10
12
50
0
14
r (mm)
2
4
Figure 23. r-t Prole plot for dierent gas mixtures.
30
r (mm)
ar-cf4-ch4 76/18/6
10
dt (ns)
8
6
4
2
0
-2
-4
-6
-8
-10
0
2
4
6
8
10
12
14
r (mm)
ar-cf4-co2 68/27/5
10
dt (ns)
8
6
4
2
0
-2
-4
-6
-8
-10
0
2
4
6
8
10
12
14
r (mm)
ar-cf4-co2 54/41/5
10
10
dt (ns)
dt (ns)
8
8
6
6
4
4
2
2
0
0
-2
-2
-4
-4
-6
-6
ar-cf4-co2 66/27/7
-8
-8
-10
-10
0
0
2
4
6
8
10
12
14
2
4
6
8
10
12
r (mm)
10
ar-cf4-co2 82/13/05
10
dt (ns)
dt (ns)
8
8
6
6
4
4
2
2
0
0
-2
-2
-4
-4
-6
-6
ar-cf4-co2 71/27/2
-8
-8
-10
-10
0
0
2
4
6
8
10
12
14
r (mm)
14
2
4
r (mm)
6
8
10
12
14
r (mm)
Figure 24. Data minus spline t results.
31
v (cm/µs)
ar-cf4-ch4 76/18/6
14
12
10
8
6
4
2
0
0
5
10
15
20
25
E/p (V/cm/torr)
v (cm/µs)
ar-cf4-co2 68/27/5
14
12
10
8
6
4
2
0
0
5
10
15
20
25
E/p (V/cm/torr)
v (cm/µs)
ar-cf4-co2 66/27/7
v (cm/µs)
ar-cf4-co2 54/41/5
14
12
14
12
10
10
8
8
6
6
4
4
2
2
0
0
5
10
15
20
0
0
25
5
10
15
E/p (V/cm/torr)
v (cm/µs)
ar-cf4-co2 71/27/2
v (cm/µs)
ar-cf4-co2 82/13/05
12
14
12
10
10
8
8
6
6
4
4
2
2
0
5
25
E/p (V/cm/torr)
14
0
20
10
15
20
0
0
25
5
10
E/p (V/cm/torr)
15
20
25
E/p (V/cm/torr)
Figure 25. Drift velocity as a function of the electric eld.
32
5 Gas gain and eciency measurements
To measure the gas gain of the gas mixtures the experiment must be run in single-electron mode meaning that most of the events are caused by one primary electron. This way, the charge collected
at the signal wire is due to the avalanche caused by only one electron.
Placing a lter in the beam the number of ionizations can be reduced to about one in every ten laser
pulses. For this eciency 90% of all events will be single electron events (as the chance of having
two primary electrons is the square of the ionization chance). The fraction of laser light needed to
accomplish this had to be measured for each gas mixture (gure 26). For Ar-CF4-CH4 (76/18/6)
a transmission coecient of 11.2% reduced the eciency to about 10% for Ar-CF4 (93/7) a 3.3%
transmitting lter resulted in a 9% eciency. For Ar-CF4-CO2 (68/27/5) a 58.7% lter reduced
the eciency to about 8%. The lters were placed in the Beam Width Adapter part of the optics
setup (gure 9).
100
ar-cf4-ch4 76/18/6
100
%
ar-ch4 93/7
%
90
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0
5
10
15
20
25
30
35
40
45
0
0
50
2
4
6
8
Transmission (%)
100
10
12
Transmission (%)
ar-cf4-co2 68/27/5
%
90
80
70
60
50
40
30
20
10
0
50
55
60
65
70
75
80
85
90
95
100
Transmission (%)
Figure 26. Inuence of lter transmission coecient on eciency.
5.1 Gas gain
To nally measure the gas gain, the ADC gate pulse was positioned around the gas pulse. Figure 27
shows the distribution of the pulse height as recorded by the ADC, in single electron mode. A
gaussian t is superimposed on the histogram. The mean of the histogram was taken as the value
of the pulse height, the RMS as the error. Events in single-electron mode (as the name implies)
are almost all due to a single primary electron. The pulse height distribution will therefore follow
a gaussian distribution. In fact, the distribution has a slightly longer tail after converting ADC
counts to electron charge.
33
The ADC counts were translated to collected charge using the data from the ADC calibration
measurements (section 3.4.4). For Ar-CF4-CH4 (76/18/6) and Ar-CF4-CO2 (68/27/5) the gain has
been plotted as a funtion of the distance to the wire in gure 28. The error bars represent the error
in the calculation of the collected charge from the mean of the pulse height histogram. This error
was due in part to the error in the calibration measurements (section 3.4.4), and in part to the
error introduced by interpolating the charge-ADC count relation. The spread on the pulse height
as given by the RMS of the pulse histogram was about 17 ADC counts for Ar-CF4-CH4 and about
13 for Ar-CF4-CH4. This translates to about a factor 2 in the collected charge - much larger than
the errors in the graph. Four measurements with Ar-CF4-CH4 with larger errors in r were taken
on another day a slight dierence in the anode voltage could explain why the gains seem much
lower for these measurements.
The gain did not depend on the distance from the wire, and it shouldn't: the avalanche caused by
the primary electron should start at less than 50 m from the anode wire, from Gareld calculations
of Townsend's rst coecient.
The apparent gain when running at in the plateau region was much higher - due to multiple primary
electrons. For Ar-CF4-CH4 (76/18/6) and Ar-CF4-CO2 (68/27/5) the signal typically was of the
order of 5 105 electtons.
Gas pulse
g
Nent = 130
Mean = 96446.3
RMS = 28232.3
18
16
14
12
10
gaussian fit, mean = 9.3e+4, σ = 2.3e+4
8
6
4
2
x10
0
400
600
800
1000
1200
1400
1600
2
1800
2000
electron charge
11000
Single electron amplification in Ar/CF4/CH4 (74:20:6)
Single electron amplification in Ar/CF4/CO2 (65:30:5)
electrons
electrons
Figure 27. Signal pulse height spectrum and gaussian t.
10000
26000
24000
9000
22000
8000
20000
7000
6000
18000
5000
16000
4000
0
2
4
6
8
10
12
14000
0
14
r (mm)
2
4
6
8
10
12
Figure 28. Gas gain for Ar/CF4/CH4 (76/18/6) and Ar/CF4/CO2 (68/27/5).
34
14
r (mm)
5.2 Electron attachment
Addition to the gas mixture of even small quantities of an electronegative gas modies the drift
properties due to electron capture. In colision with a molecule, the electron and the molecule form
a negative ion (e.g. CF4 + e ! CF4;), or the molecule dissociates and one of the components
forms a negative ion with the electron (e.g. CO2 + e ! CO + O; or CF4 + e ! CF3 + F ;). The
probability for an electron to be attached to a molecule depends on the energy of the electron 11].
Typically, the attachment coecient peaks for electron energies of 2-7 eV. For CO2 (which has a
much lower attachment coecient) it starts at 3.5 eV 10], 12]. According to Gareld calculations,
electron attachment in Ar-CF4-CH4 (76/18/6) should start at E/p values of a few V cm;1torr;1 , a
couple of mm from the wire.
Because the electron energy depends on the value of the electic eld - and thus the distance to the
wire - electron attachment will mostly occur close to the wire, where the electron energy is highest.
Any electron produced outside this region will travel a full path through the "attachment region",
and therefore have the same chance of being captured. Electrons produced inside the region will
not travel the full path, and have a higher probability of causing a signal on the wire. If, in single
electron mode, the electron is captured before starting an avalanche, it will not be detected. The
detection eciency is therefore expected to be constant for ionizations far from the wire, and to
increase (exponentially) in the region where attachment is more probable.
Another eect of electron attachment is a reduced electron avalanche 7]. A percentage of the
secondary electrons will be captured by gas molecules, reducing the pulse height on the anode.
To measure the eciency, the experiment was run in single electron mode. The number of signals
was presumed to follow a binomial distribution. The estimated mean and error on the estimate are
plotted as a function of the distance to the wire in gure 29. The graph for Ar-CF4-CH4 peaks for
short r this eect could be reproduced in additional measurements. Even closer to the wire, the
eciency falls o again part of the laser beam was cut o by the wire support (gure 6).
The measured pulses were true gas ionization pulses a run without the focusing lens showed only
pulses resulting from photo electrons freed from the cathode. These pulses could easily be cut o
from the data.
Efficiency in Ar/CF4/CH4 (76/18/6); filter transmission = 11.2%
25
25
%
%
20
20
15
15
10
10
5
5
0
0
2
4
6
8
10
12
Efficiency in Ar/CF4/CO2 (68/27/5); filter transmission = 58.7%
0
0
14
r (mm)
2
4
6
8
10
12
14
r (mm)
Figure 29. Detection eciency as a function of the distance to the wire.
A measurement with CF4 removed from the Ar-CF4-CH4 mixture, operating in single electron mode
(with a lter transmission coecient of 3.3%), showed no such increase in eciency, indicating that
CF4 is indeed the component responsible for the eect (gure 30).
35
100
Efficiency for Ar/CH4 (93/7); filter transmission = 11.2%
%
90
80
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
r (mm)
Figure 30. Detection eciency in Ar/CH4, lter transmission coecient = 3.3%.
100
Efficiency for Ar/CH4 (93/7); filter transmission = 11.2%
%
90
80
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
r (mm)
Figure 31. Detection eciency in Ar/CH4, lter transmission coecient = 11.2%.
An extra measurement with Ar-CH4 (93/7) operating at lower anode voltage to account for dierent HV platforms (gure 22), but with the same lter (11.2%) as the Ar-CF4-CH4 measurement
showed considerably higher eciency for this mixture. Apparently, CF4 does reduce the eciency
(gure 31).
The detection eciency for the setup also depended on the positioning of the laser beam on the
lenses. Especially as the spark gap of the laser aged and the beam became somewhat instable,
the eciency would change a little from day to day. Therefore, the dierent plots should not be
compared as exact, but rather as an indication.
36
r (mm)
1.0
1.5
2.0
2.5
2.9
1)
2)
3)
14.2 to 14.4
20.8 to 21.2
25.4 to 25.8
28.9 to 29.5
32.2
10.7 to 10.8
14.5 to 14.7
18.5 to 19.1
21.9 to 22.3
24.7
t5mm cell (ns)
4)
12.3 to 12.4 10.7 to 10.8
14.5 to 14.7 15.9 to 16.1
17.7 to 18.4 21.3 to 22.1
21.8 to 22.2 25.4 to 25.8
24.5
28.9
1) Ar-CF -CH (76/18/6)
4
4
2) Ar-CF -CO (68/27/5)
4
2
3)
Ar-CF4-CO2 (54/41/5)
4)
Ar-CF4-CO2 (82/13/5)
5)
Ar-CF4-CO2 (66/27/7)
6) Ar-CF -CO (71/27/2)
4
2
5)
6)
11.6 to 11.8
15.3 to 15.5
18.1 to 18.7
22.8 to 23.3
26.2
12.9.1 to 13.0
17.6 to 17.9
20.7 to 21.2
23.9 to 24.3
26.4
Table 2. Drift times for 5 mm cells as a function of r. The
rst time is for the inner cylinder of a 5 mm
height
honeycomb, the second for the outer cylinder (radius = p3 ), r = 2:9mm.
6 Drift Times for 5 and 10 mm Honeycomb Drift Chambers
To calculate the drift times relevant to the Hera-B honeycomb chambers, the drift times measured
in the 30 mm cell need to be rescaled for 5 mm and 10 mm honeycomb chambers. The chambers can
be approximated by cylinders with the same diameter. The variation of the honeycomb chamber
eld from a cylindrical eld manifests itself only in the corners of the hexagonal tube. The limits
of this approximation are the inner and outer cylinder of a honeycomb (the radius of the outer
cylinder is a factor p23 greater). The error in the drift times for identical drift distances due to
the non-cylindrical eld is of the order of 0.5 ns, but the outer cylinder has a signicantly longer
maximum drift time - the longest drift distance is larger.
The scaling was done by keeping the wire potential xed at 2200 V, and scaling the drift distance
for the dierent cell radiuses according to the equation for the electric eld:
E = rVlnanrrb .
a
Drift times for 5 mm and 10 mm honeycomb chambers are presented in tables 2 and 3.
The drift times in tables 2 and 3 were rescaled from the experimental data. The xed electronics
oset in the drift times (already subtracted in the tables) was known to a precision of 2 ns. The
error on the oset is mostly systematic and does have a large impact on the relative drift times for
the various gases. Note that Ar-CF4-CH4 (76/18/6) is slower than the other gases for 5 mm drift
cells, but faster for 10 mm cells.
37
r (mm)
1
2
3
4
5
5.7
t10mm cell (ns)
1)
2)
3)
15.2 to 15.5
27.4 to 27.9
34.7 to 35.1
41.8 to 42.2
48.5 to 49.1
56.5
11.3 to 11.5
20.5 to 21.0
27.1 to 27.8
37.8 to 38.7
49.7 to 51.1
65.1
4)
12.8 to 13.0 11.6 to 11.9
20.7 to 21.0 23.6 to 24.1
27.2 to 27.5 32.2 to 32.9
38.5 to 39.2 40.9 to 42.0
52.1 to 52.8 51.5 to 52.4
68.3
64.3
1) Ar-CF -CH (76/18/6)
4
4
2)
Ar-CF4-CO2 (68/27/5)
3)
Ar-CF4-CO2 (54/41/5)
4)
Ar-CF4-CO2 (82/13/5)
5)
Ar-CF4-CO2 (66/27/7)
6)
Ar-CF4-CO2 (71/27/2)
5)
6)
12.4 to 12.6
21.2 to 21.7
29.2 to 30.1
41.1 to 41.6
55.0 to 56.5
74.9
13.6 to 13.8
22.7 to 23.1
28.7 to 29.2
37.2 to 37.9
46.0 to 46.9
57.7
Table 3. Drift times for 10 mm cells as a function of r, router = 5:7mm.
7 Comparing data with simulations
7.1 Comparing with GARFIELD and Magboltz
Magboltz is a simulation program for drift gases. It can be used to calculate drift velocities for
electrons in various gas mixtures.
The behavior of drift chambers can be simulated using the garfield simulation program 14].
garfield can calculate eld maps, x(t) relations and drift times, among other things. An interface
to the Dr S.F. Biagi's Magboltz program makes the calculation of electron properties in nearly
arbitrary gas mixtures possible.
Figure 32 shows the residual of the r-t data and the calculations done with garfield. A straight
line t was done on the residual the slope of the t can be seen as a systematic error on the
drift velocity. The residual also contains a constant oset to the drift times, caused by the xed
electronics in the experiment.
This constant oset contributes a small systematic error to the drift velocities: if
te ; tg = a + b r
where te and tg are the measured and calculated drift times, respectively, and a and b are the
parameters of the t, and
te = fe (r), tg = fg (r), ( drdf );1 = v,
then the contribution of a straight line to the drift velocity is:
vg ; ve = b vg ve, or !v b v2.
The relative error is: vv b v.
The slopes are all of the order of 0.5 ns/mm. For a typical \maximum speed" for this experiment
of about 10 cm/s (gure 25), the error on the drift velocity would be 5%. The drift velocities
obtained from the simulation are shown in gure 33. Table 4 lists the approximate maximum and
38
1)
v (cm/s)
2)
3)
4)
5)
6)
peak
11.0(1) 10.5(4) 11.2(8) 9.5(2) 10.5(5) 11.0(8)
minimum
5.50(3) 6.5(2) 7.3(3) 5.0(1) 6.5(2) 6.0(3)
(slope (ns/mm)) 0.10
0.39 -0.61 0.27 -0.42 -0.70
1)
Ar-CF4-CH4 (76/18/6)
2)
Ar-CF4-CO2 (68/27/5)
3)
Ar-CF4-CO2 (54/41/5)
4)
Ar-CF4-CO2 (82/13/5)
5) Ar-CF -CO (66/27/7)
4
2
6) Ar-CF -CO (71/27/2)
4
2
Table 4. Maximum and minimum drift velocity and \slope" error. The listed precision is misleading it
only serves to illustrate the error on the garfield calculations. The largest error in this case results from
reading the graphs and is about 0.5 cm/s.
minimum drift velocity (as read from the graph) and the \slope" error. Note that the error is
systematic a positive slope means the simulated drift velocity is too large, a negative slope means
the simulated drift velocity is too small.
7.2 Sources of error in GARFIELD - experiment comparison
Several eects could have contributed to the dierence (in drift times) between simulation and
experiment:
or Magboltz simulation calculations are not perfect.
The gas mixtures were not perfect either because of the error on the gas ow measurements
garfield
(about 3% - section 3.2) or because of the error on the conversion factors. Contaminations
could also have contributed. A too-high ow of CF4 would result in a faster drift gas, while
CO2 and CH4 slow the drifting electrons. In fact, garfield calculations for slightly dierent
mixtures agreed better with the experimental data for some mixtures, but worse for others.
The position of the wire was known to a nite precision (about 30 m - section 4.1).
The position of the sense wire in the drift cylinder was known to a precision of better than
100 m - typically about 10 to 30m. The error on the drift distance due to the error on the
wire position is given by:
(!r)2 = x;rxw (!xw)2 + y;ryw (!yw )2,
where xw and yw are the coordinates of the wire in the cylinder. Thus, the error depends on
the \angle" of the drift distance with respect to the (x y) coordinate system, rather than the
drift distance. It should therefore be approximately the same for all drift distances. The error
on the drift times due to the wire position error should be proportional to the drift velocity.
The dierence between the measured and simulated drift times isn't it is proportional to the
distance.
39
400
ar-cf4-ch4 76/18/6
t (ns)
350
300
+ experimental data
250
200
✳ GARFIELD data
150
❍ experimenttal data - GARFIELD data
100
50
0
0
2
4
6
8
10
12
14
r (mm)
400
ar-cf4-co2 68/27/5
t (ns)
350
300
250
200
150
100
50
0
0
2
4
6
8
10
12
14
r (mm)
400
ar-cf4-co2 54/41/5
400
t (ns)
ar-cf4-co2 66/27/7
t (ns)
350
350
300
300
250
250
200
200
150
150
100
100
50
50
0
0
2
4
6
8
10
12
0
0
14
2
4
6
8
10
12
r (mm)
400
14
r (mm)
ar-cf4-co2 82/13/05
400
t (ns)
ar-cf4-co2 71/27/2
t (ns)
350
350
300
300
250
250
200
200
150
150
100
100
50
50
0
0
2
4
6
8
10
12
0
0
14
2
4
6
8
10
r (mm)
12
14
r (mm)
Figure 32. Residual of garfield simulation results and experiment data.
40
v (cm/µs)
ar-cf4-ch4 76/18/6
14
12
10
8
6
4
2
0
0
5
10
15
20
25
30
35
40
E/p (V/cm/torr)
v (cm/µs)
ar-cf4-co2 68/27/5
14
12
10
8
6
4
2
0
0
5
10
15
20
25
30
35
40
E/p (V/cm/torr)
ar-cf4-co2 66/27/7
v (cm/µs)
v (cm/µs)
ar-cf4-co2 54/41/5
14
12
14
12
10
10
8
8
6
6
4
4
2
2
0
0
5
10
15
20
25
30
35
0
0
40
5
10
15
20
25
30
E/p (V/cm/torr)
12
14
12
10
10
8
8
6
6
4
4
2
2
0
5
10
40
ar-cf4-co2 71/27/2
v (cm/µs)
v (cm/µs)
ar-cf4-co2 82/13/05
14
0
35
E/p (V/cm/torr)
15
20
25
30
35
0
0
40
5
10
15
20
25
E/p (V/cm/torr)
30
35
40
E/p (V/cm/torr)
Figure 33. Drift velocity obtained from garfield simulation.
41
8 Conclusions
This experiment is clearly in agreement with the simulations done with garfield. It is therefore
possible to use the simulation to calculate the properties of other mixes of Ar-CF4-CO2, specically.
Varying the ratio of the gases in Ar-CF4-CO2 has a relatively small eect on the drift velocity.
Especially an increase in CF4 content does not increase the drift speed enough to be worth the
extra cost and higher working voltage needed. Decreasing the CO2 content has a much larger eect,
but is probably detrimental to the gas gain and the resolution. Ar-CF4-CH4 is much faster over
longer drift distances, but slower for 5 mm drift cells. The ageing properties of this gas might be
another factor to decide against it.
The data agree well with the garfield simulation program - within the error of the experiment
(on the drift time and distance measurements, as well as the gas purity). It should therefore be
possible to use garfield to calculate the drift time relations for other gas mixes of argon, CF4
and CO2.
The gas gains in both Ar-CF4-CH4 and Ar-CF4-CO2 are of the order of 104 for a single primary
electron. It is also clear that CF4 causes electron attachment.
42
References
1]
2]
3]
4]
5]
6]
7]
8]
9]
10]
11]
12]
13]
14]
Design Report
DESY-PRC 95/01
January 1995
LHC-B Technical Proposal
CERN/LHCC 98-4
LHCC/P4
20 February 1998
J. Christenson et al., PRL 13 (1964) 138.
M. Kobayashi and K. Maskawa, Prog. Theor. Phys. 49 (1973) 652.
L. Wolfenstein, PRL 51 (1983) 1945
N. Cabibbo, Phys. Rev. Lett. 10 (1963) 531.
Electron attachment, eective ionization coecient, and electron drift velocity for CF4 gas
mixtures
W.S. Anderson et al.
Nuclear Instruments and Methods in Physics Research A323 (1992) 273-279
Introduction to Experimental Particle Physics
Richard Fernow
Cambridge University Press, 1986
A diraction limited nitrogen laser for detector calibration in high energy physics
Fred Hartjes
PhD thesis, Amsterdam 1990
Basic Properties of Gaseous Electronics
L.B. Loeb
University of California Press, Berkeley & Los Angeles 1955
Principles of operation of multiwire proportional and drift chambers
F. Sauli
CERN 77-09
3 May 1977
M.S. Nadia and A.N. Prasad, J. Phys. 5 (1972) 983
NORBAS - CERN function library
W. Moench, B. Schorr
http://wwwcn.cern.ch/asdoc/shortwrupsdir/e210/top.html
Gareld
http://consult.cern.ch/writeup/gareld
More information on Hera-B and the honeycomb chambers can be found on the Hera-B
homepage: http://www-hera-b.desy.de/
Hera-B
43
9 Acknowledgements
I had a lot of help doing this experiment. First of all, of course, from my supervisor, Gras van
Apeldoorn, who had also set up the experiment. Olaf Steinkamp and Wouter Hulsbergen sometimes
acted as my \co-supervisors" Olaf also helped me with the garfield simulation, and Wouter with
the spline ts. Kirsten Renner helped taking data and Freya Blekman measured the gas ows. They
still have all this work ahead of them : : :
Also, thanks to all the other CP Violators at the nikhef who endured my presence during lunches:
Maarten, Marcel, Nikolai, Rutger and Rutger, and Kirsten and Freya again, who also had to put
up with me in our oce.
Finally, there are a few people outside my actual working environment I would like to mention: my
parents, who kept me focused on my present and future, my sister, who conveniently went to Italy
during the year so we could have a bit of quiet around the house, the good people at Hippopotamus
Cricket Club who provided some needed distraction over the summer, and Martijn, whom I had
many philosophical discussions with, about graduating, writing a thesis, and what the hell was the
use of high energy physics anyway.
And in conclusion, I quote:
"Half a B
Philosophically
Must ipso facto
Half not be.
But can a B
Be said to be
Or not to be
An entire B ?
When half the B
Is not a B
Due to some ancient
Injury."
- Monty Python
44
A Scaling the anode voltage
To obtain drift times for 5 and 10 mm honeycomb cells (section 6), the eld as a function of r was
scaled by the logarithm factors in E = rVlnanrrb , for the dierent cell radii. The anode voltage was
a
kept the same (2200 V).
A more relevant scaling for actual detectors would be to scale, at the same time, the anode voltage
- thus keeping the eld a xed function of r. Drift tubes with smaller radii work at lower working
voltages, because high voltages would lead to instable operation. Scaling the anode voltage down
by a factor of ln rrab (new cell) over ln rrab (30 mm cell), the drift times for a given r in the 30 mm cell
used in the experiment are the drift times for the same r in a tube with a smaller radius.
Table 5 lists the drift times (with the xed electronics oset subtracted) measured in the experiment.
The re-scaled anode voltages for cells with 5, 8 and 10 mm radii are given in table 6. All gures
are for cylinders with radii as listed.
r (mm)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
1)
2)
7.2
18.4
25.5
30.3
34.3
38.7
42.7
46.3
50.3
54.3
7.3
12.9
18.6
22.9
26.9
33.5
39.0
45.5
53.4
60.8
t30mm cell (ns)
3)
4)
5)
6)
10.7
13.8
18.1
23.0
26.9
33.9
39.9
48.4
55.5
65.0
6.3
14.0
21.5
26.7
32.0
37.5
42.9
48.6
54.2
61.5
8.6
14.1
19.2
24.0
28.9
35.6
42.4
50.3
60.5
70.1
8.8
15.7
20.8
24.8
28.6
33.0
39.3
42.8
50.4
54.8
Ar-CF4-CH4 (76/18/6)
Ar-CF4-CO2 (68/27/5)
3) Ar-CF -CO (54/41/5)
4
2
4) Ar-CF -CO (82/13/5)
4
2
5) Ar-CF -CO (66/27/7)
4
2
6)
Ar-CF4-CO2 (71/27/2)
1)
2)
Table 5. Drift times for 30 mm cells at 2200 V.
cell diameter anode voltage
30 mm
2200 V
10 mm
1860 V
8 mm
1790 V
5 mm
1644 V
Table 6. Re-scaled anode voltages for drift cells.
45