CALIBRATION AND MONITORING METHODS (C&M)
FOR
THE LIQUID XENON CALORIMETER
AND
FOR THE WHOLE MEG DETECTOR........
Xe calorimeter, wire-chamber spectrometer, timing counters
an updated discussion on:
advantages, disadvantages, open problems, etc.
of proposed methods
C&M for the entire MEG:
• at any time
• during PSI beam-off periods, tuning....
• efficient use of beam-on periods
1
BVR, July 18th 2005, CB + T. Iwamoto
an internal note
requested by the
INFN MEG Referees
(MEG-TN027)
2
MEG internal note and then NIM collaboration paper
3
KEEP MEG UNDER CONTROL
PARTICULARLY AT HIGH (AND VARIABLE) BEAM INTENSITIES.........
BR   e  ~ 10-13
Beam Intensity ~ 5 107 /s
• frequent checks of calorimeter energy scale, linearity and stability
• checks of LXe optical properties
• energy resolution, spacial resolution, time resolution
• shower properties
• at the right  energy ( 53 MeV), but also at other energies..... 4
TWO MAIN TARGETS:
1) MAINTAIN THE MEG ENERGY, SPACE AND TIME
RESOLUTIONS OPTIMIZED OVER LONG PERIODS
OF TIME
2) HAVE RECORDED PROOFS OF MEG PERFORMANCES
(WHATEVER THE FINAL MEG RESULT ON BR   e  )
emphasize the reliability of our experiment !
GOOD C&M IS THE KEY TO MEG SUCCESS
no single calibration method has all the required characteristics
use complementary (and redundant) methods,
make the best use of their intrinsic properties
5
attempt to grade the different C&M methods
6
500 KV PROTON ACCELERATOR AND LITIUM TARGET FOR A
17.6 MEV GAMMA LINE
[P.R. 73, 666 (1948), N.P. 21 1 (1960),
Zeitschrift f. Physik A351 229 (1995)]
3
7Li
(p,)48Be
Potentialities :
• strongly exothermic nuclear reaction
unique: -emission much favoured over -emission
• obtainable: at resonance (E p = 440 keV 14 keV)
 106 /s (isotropic) for Ip 50 A
• from LiF target at COBRA center; ’s on the whole cal.
entrance face
• energy and position calibration; shower properties
• rather frequent use
• privilege simple, fast, (semi-automatic) mechanical system
for proton beam and LiF target introduction and positioning
• (give up the use for the calorimeter monitoring from the back)7
further studies:
• compatibility with normal beam and target
• COBRA field, accelerator (and focusing element) position
• project for easiness of target-tube mounting
• p-beam divergence and protons on target; p29 MeV/c
• post-acceleration to scan the resonance
• thin-target, thick-target
• H2+ ions, effects on -line, (H2+ elimination by a mag.-triplet)
8
astrophysics data
http://pntpm.ulb.ac.be/nacre.htm
E
sigma error
S-factor error (MeV) (b) (b) (MeV b) (MeV
b)
0.129 4.55E-06 2.3E-07 1.37E-03 7.00E-05
0.375 1.44E-03 8.5E-05 5.10E-02 3.00E-03
0.384 5.86E-03 1.5E-04 2.02E-01 5.00E-03
0.388 4.44E-03 1.8E-04 1.51E-01 6.00E-03
1.005 7.59E-05 4.3E-06 1.23E-03 7.00E-05
at the Tp* 384 keV resonance and compound nucleus formation
+ non resonant direct reaction elsewhere
9
E0 = 17.6 MeV
E1 = 14.6
6.1
Bpeak 0/(0+ 1)= 0.720.07
3
7Li
(p,)48Be
resonant at Ep= 440 keV =14 keV
peak = 5 mb
1
NaI 12”x12”
 spectrum
0
10
other interesting possibilities..... :
1
3H
(p,) 24He E ~ 20 MeV !!
used in SNO
 in : Hahn et al. PRC 51 1624 (1995)
but Tritium....and low rate.......
5
11B
(p,)612C
resonant at Ep= 163 keV
= 7 keV
E0 = 16.1 MeV peak = 5.5 b
E1 = 11.7 + 4.4 peak = 152 b
Cecil et al. NP A539 75 (1992)
10x10 cm NaI crystal
 750
0/s (isotropic)
20.000 1/s for Ip 50 A
lower proton energy !
lower rate at 50 A !!
11
ENERGY, TARGET THICKNESS AND -LINE QUALITY
correspondence between resonance  and range interval R
“thin target” R   “thick target” R >> 
if Tp = 445 keV and R = 
R = 0.120  N=7 x 1017 LiF/cm2
at 80 A Ip Np= 5x1014 p/s N= 1.8x106 /s (up to 1.6x105 in calorimeter)
very clean -line (more difficult calibration tuning)
if Tp = 445 keV and R = Range (445 keV) >>
R = 413  N = 2.5 x 1019 LiF/cm2
at 80 A Ip Np= 5x1014 p/s N= 6x105 /s (+ N=1.8x106 /s)
-line with appreciable left shoulder from 17.6 to 17.1 MeV
(simple calibration tuning)
of the total 5x1014 p/s, 2x106 p/s produce photons at resonance,
some of the residual 2.5x108 p/s produce direct photons of lower
energy (if Tp > resonant energy, right tail also.........)
12
H2+ ion effects........(30% of CW-beam)
N= 1.8x106 /s over 4 (up to 1.6x105 /s into the whole calorimeter)
(PMT non linearity over Ia = 4 A, therefore at about 2x105 /s in the calorimeter)
Very high -intensity
(other optional reactions have smaller cross-section)
(possibility of using low-efficiency selective triggers)
MEG aquisition rate is about 100 Hz
The accelerator current can be easily limited, but one can also test
the calorimeter and the PMT behaviour
as a function of an increasing -rate in the calorimeter......
13
CHOICE OF THE ACCELERATOR
Cockroft-Walton, Van der Graaf, Radio Frequency Quadrupole
HV Engineering, NEC, AccSys, Neue Technologien GmbH
• overall price, guarantees, delivery time, test, assistance,
spare parts, etc.
• energy interval of operation, current, stability, beam phase space,
background radiation, etc.
• simplicity of use, reliability, type of computer control
• source duration, 1-year without servicing, etc.
• fast conditioning and tuning
• beam height
• possibility of moving the accelerator system
• availability and possible use at the beginning of the experiment
The collection of information on all points is a slow, multistep process......:
• visits to experiments using similar accelerators
• visit to accelerator factories
• discussion with national lab. experts
14
STRONG PREFERENCE FOR A COCKROFT-WALTON
• reliable system, in use for several precision experiments, visits to GS
• good assistance in mounting and test; “nearby” factory
• large energy interval of machine operation
• visit to HV in Amersfoort and visit of HV to Pisa (Legnaro lab. expert
present)
• adequate current, good beam properties, stability
• fast tuning and operation if 1 MV machine in the same tank of the
0.5 MV machine. (15% increase in price)
• very low-background machine
• well interfaced, good safety system, interlocks, good software
(and program source available)
• compact machine in pressurized (and shielding) container
• one year operation without service
If one wants to use the machine for the MEG start-up
an order must be issued as soon as possible (September !)
15
model: “coaxial SINGLETRON”
16
BVR February 2005
PRECISE  CALIBRATION

FULLY TESTED......
θ
Potentialities :
• energy and position calibration
• shower properties and reconstruction at
E  55 MeV, the proper energy !
• fully tested in “large prototype” runs
Open problems:
• definition of -lines by collimators or by
-hit reconstruction (for  ~ 180º).
• NaI set-up. Several positions.
NaI behind coils.
• H2 cryogenics, negative beam, different
target, target introduction in COBRA.
• how often it can be performed ?
E (MeV)
 - at rest captured on protons:
 - p  0 n
- p  n 
0   
Photon
spectrum
54.9
82.9
selection of approx. back-to-back photons
by collimators
129
MeV
17
TWO POSSIBLE WAYS TO PERFORM THE º CALIBRATION
IN MEG
1) EXTRAPOLATION FROM PREVIOUS TESTS FOR MEG
Movable NaI system
Safe solution at the beginning of the experiment.
2) CONVERSION METHOD
No movable parts.
More comprehensive applications (wire-chambers,timing counters).
It depends on a trigger systems which is presently untested.
Both methods allow Xe calorimeter calibration in 1-2 days
18
NaI Detector Stage design
Anti
Counter
•
•
•

0
up
NaI detector (~100kg) needs to be moved 2
dimensionally at the opposite side of the xenon detector.
The movable stage and motor need to be magnetic
tolerable with reasonable positioning accuracy.
Test under COBRA field  OK
Linear slider
Screw drive
No bearing ball
Prism guide
down

target
Linear slider: http://www.tollo.com
Motor:
http:// www.animatics.com
Example
Motor
19
an interesting possibility for a  calibration in MEG
• abandon NaI detector in coincidence
• illuminate the whole calorimeter at the same time with -2
• convert the -1 in a 0.1 X0 converter close to the H2 target
• detect conversion and measure conversion point with a
“special counter”
• measure e+ branch of the pair in the chambers
• use part of the information for selecting -1 by trigger
angle between ’s defined by impact points on LXe-Cal
and “ special counter”
(angles  1800 useful for calibrating at different energies)
loss at conversion but huge increase in solid angle
MC METHOD SIMULATION RESULTS (F.Cei)
20
TRIGGER UNDER STUDY
Ingredients:
• LXe Cal. and QSUM threshold
• “special counter”
good time resolution, pixelization for conversion point reconstruction,
separation of e+ e-- pairs from single particles
• positron (from n ) or pair trajectory (from n ) by the wire-chamber trigger
• timing-counters
depending on the particular calibration........
A FULL TEST OF THE WIRE-CHAMBERS SPECTROMETER21
CAN ALSO BE PERFORMED !
WIRE CHAMBER SPECTROMETER AND TIMING COUNTERS TEST
(at full COBRA field)
by  - p  0 n and -1 conversion into an e+ e– pair
and also
by  - p  n  and  conversion into an e+ e– pair
(a pair spectrometer and a -line !!)
but also the Cockroft-Walton allows a calibration of the
LXe Cal and, wire-chamber spectrometer, timing counters
• CW use is much simpler than  calibration !
• LXe Cal illuminated by 17.6 MeV ’s at high rate
• Use of -converter for testing the wire-chambers spectrometer
• maximum COBRA field for LXe Cal test
• half COBRA field for wire-chamber spectrometer test
22
 energy release: increased statistics
0.1 X0, NDC > 4,
relative angle > 1750
 Intrinsic width for photons
emitted with relative angle
> 1750: 0.3 %.
 Leakage effects: ~ 1 %.
 Remaining contributions
from natural angular width
of e+e- pair production and
multiple scattering in the
target.
FWHM
2.60.3 %
23
-p  n  (129 MeV)

e+ e Main purpose: calibration of wire-chamber
spectrometer and timing counters.
 Use e+e- pair production from 129 MeV
gamma conversion in Tungsten.
 Both e+ and e- must be detected and their
tracks reconstructed. Pair spectrometer !
 Interesting thing: it provides a fixed (total)
energy calibration point for the wire-chamber
spectrometer
(normally not easily obtainable......).
24
Efficiency vs converter thickness
Thickness
(X0)
q
(o, FWHM)
Single particle
efficiency
(e+ or e-, %)
Double particle
efficiency
(e+ && e-, %)
0.05
5.7
0.13
0.024
0.1
7.7
0.26
0.039
0.15
8.5
0.42
0.071
0.2
9.5
0.59
0.079
 4 chambers required for detection
Generated 100000 events in the
whole solid angle (4 ).
~ 400 Hz
106 events
(> 4 chambers)
Large errors due to
small statistics, but
promising results; 0.1 X0
looks the best choice.
25
Total momentum distribution ( pe   pe - )
Thickness
0.1 X0
No reconstruction
included
FWHM ~ 0.7  0.9 %
This FWHM must
be compared with
the value quoted
in the Proposal:
FWHM ( pe   pe - ) 
2 FWHM ( pe  )  1.2%
e+ + e- momentum (MeV)
26
Am SOURCES ON WIRE AND WALLS
BVR February 2005
Sources in production.
Soon available for all LXe devices.
Potentialities :
• PMT quantum efficiencies
• Xenon optical properties
• low-energy position and energy
calibration
• use in Xe gas and liquid
• stability checks ?
• a unique method for cryogenic liquid
Wire presently mounted in “Large Prototype”
detectors !!
Open problems:
• will the method be usable under full intensity beam conditions ?
To be verified by test !
27
reconstruction of the 8 -source positions in gaseous Xe.
Recent measurement with the large-prototype.
(Po-source produced in Genoa)
28
RINGS
IN
LIQUID XENON
the ring radius
depends on
the Rayleigh scattering length
in LXe
29
Determination of the relative
QE for 4 different PMTs by
the use of 4 dot-wire-sources
in Xe gas of the large-prototype
the relative QEs are given by
the slope of the linear fits.
30
C&M by NEUTRONS AND NICKEL-LINE , AT THE BACK OF THE CALORIMETER
large-prototype
NaI
/E=2.5%
in the large-prototype
the line is worse.....
(thermal neutrons in LXe !)
the measurement must be
repeated, protecting LXe from
thermal neutrons by a borated-foil
31
CONCLUSIONS
Several C&M methods tested with satisfactory results:
• wire-sources
• 0 and  from – charge exchange
• thermal neutrons and nickel -line
Other C&M methods in preparation or being modified
for MEG:
• CW accelerator and 37Li (p,)48Be reaction
• new methods for 0 and  from – charge exchange
32
EXTRA SLIDES
33
Some distributions – a)
129 MeV
Pe+ + Pe- = E
Thickness
0.15 X0
Energy loss
and MS
34
Some distributions – b)
Thickness
0.15 X0
e+/e- momenta
At least 4 chambers
(7 hits) required
Region to be
selected (both
e+/e- seen)
Relative
angle
 energy
35
RADIO FREQUENCY QUADRUPOLE ACCELERATOR
• practically monoenergetic
• pulsed operation; frequency 100 Hz 100 s pulses
• average current 50 A , pulsed current 5 mA
• beam energy bin approx. 10 keV
• small vessel, pre-accelerator
• beam optical properties ? 1mm ; 20 mR
• RF radiation ? No
• proton source ? Plasma
• cost ? acceptable (AccSys), (Neue Tech.) !!!!!
• special design....time to produce ? One year
• not an out-of-the-shelf machine
• Companies: AccSys, Neue Technologien GMBH
36
37
MC ingredients
 Liquid hydrogen (LH2) target close to the muon
stopping target (10 cm length x 5 cm diameter);
 Thin tungsten converter adjacent to the LH2
target; thickness between 0.05 X0 and 0.3 X0;
 0 decay & n  pair generated in the LH2 target with
the correct energy and angular distributions;
 Tracking of photons from  decay;
 Tracking of electron & positron from photon conversion;
 Multiple scattering in tungsten included;
 Minimum number of chambers (4) in DC system
required to define a track;
 Energy/momentum reconstructions: work in progress
 Increase of MC statistics: under way
38
1) -p  n 0 Some distributions
Converter
thickness
0.15 X0
Before
converter
FWHM < 20
After
converter FWHM ~ 60
1st –e+ relative angle
and multiple
scattering effect
q (0)
E in
LXe
(MeV)
2nd –e+ relative angle
vs energy loss in LXe
Region to be selected
for energy calibration
Higher density of points
for E < 60 MeV
E (MeV)
39
Impact point and  energy release in LXe
Converter thickness 0.15 X0
cos (q)
Uniform coverage of
the whole calorimeter
FWHM  6.50
1-e+
Relative angle 2-e+
q > 1750
FWHM(energy)  4 - 5%
40
Efficiency vs
converter thickness
Thickness
(X0)
q (o, FWHM)
Events with
50<E<60 MeV
0.05
4.2
12
0.1
5.7
235
0.15
6.5
27
0.2
7.5
54
0.25
8.4
45
0.30
9.0
63
Generated 100000 events in the solid
angle covered by the LXe calorimeter
(10 %)
~ 23 Hz
106 events
(> 4 chambers)
 4 chambers; relative angle  1750
41
Rough estimate of the time
needed for the LXe calibration
Reconstruction and trigger efficiencies under evaluation
Solid angle factor
 <e>  (20  30)/105/10 = (20  30) x 10-6
 R = R x <e> = (R/106) x 106 x (20  30) x 10-6 =
(20  30) x (R/106) Hz (max.MEG acquisition rate 100 Hz)
 Events/day  8.64 x 104 R  2 x 106 x (R/106)
 Assuming  50 locations to be calibrated
(216 PMTs in groups of 4):
(< 1000 events/location would be sufficient)
1000 events/50 s
total for 50 locations 2500 s
<421 h
Assuming N0 = 106 129 MeV photons/s:
N(e+e- pairs detected)/s =
N0 x epair ~ 400/s.
Requiring 106 pairs in the wire-chamber
spectrometer (at a rate of 100 Hz:
Time = 106/(100/s) = 104 s
(less than three hours).
43