Gain Measurements of a GridPix
detector operated in Ar/iC4H10 at
different pressures
Analysis of data recorded at the Nikhef Detector R&D-group
Eric Drechsler
Universiteit van Amsterdam
16. January 2012
Abstract
This report summarises the results of gain measurements in Ar/iC4 H10 performed at the Nikhef R&D-group. Photons with 5.9 keV from a 55 Fe-source
were shot into a TPC with 6 cm drift gap at different pressures between
1.0 bar to 2.0 bar of the Ar/iC4 H10 (90/10) gas mixture. Primary electrons
were collected with a GridPix detector.
The maximum achieved single electron detection efficiency is 0.9 at p =
1.4 bar. The obtained gain curves decrease with increasing pressure and
have maximum values around 104 .
1
Contents
1. Introduction
3
2. GridPix Detector
3
3. Experimental Setup
3.1. General Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Trigger Setup and Signal Timing . . . . . . . . . . . . . . . . . . .
3.3. Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4. Analysis of Recorded Data
4.1. Occupancy and Time Spectra
4.2. Cloud Shape . . . . . . . . . .
4.3. Number of Detected Electrons
4.4. Gain Curves . . . . . . . . . .
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5. Discussion
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A. Plots
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B. Dark Matter and WIMP Detection
19
B.1. Dark Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
B.2. Detection of WIMPs . . . . . . . . . . . . . . . . . . . . . . . . . . 19
References
21
2
1. Introduction
Modern particle physics requires detectors at the frontier of current technologies.
The detector components have to be robust and reliable. For a high spatial resolution very precise and sensitive materials are required.
released A recent development is the GridPix detector, a micro pattern gaseous
detector. A pixelated CMOS chip, the Timepix chip, provided with a mesh on
top for charge amplification is used to detect ionisation produced in a gaseous
gap by radiation. As readout for a Time Projection Chamber (TPC) the GridPix
detector allows for a very high three dimensional resolution over a large volume.
This technology is studied for example in the framework of a large TPC tracker
for the ILC [1]. Likewise low statistics experiments, e.g. dark matter searches,
investigate the use of GridPix technology.
This report summarises the gain measurements and studies of single electron detection efficiency in Ar/iC4 H10 for different pressures and grid voltages performed
at Nikhef. The measurements were performed in the framework of the dark matter WIMP search in noble liquids (DARWIN) consortium. Darwin has ambitious
projects concerning the next generation of noble liquid detectors with increased
sensitivity for the direct measurement of properties of dark matter - like mass and
interaction strength - in Europe[2].
In the following a brief introduction to GridPix detectors is followed by a description of the experimental setup and the results of the data analysis. In the appendix
a short discussion of dark matter and WIMP detection is provided.
2. GridPix Detector
A GridPix detector is a Timepix chip with a thin metal foil on top. The Timepix is
a development based on the Medipix2 -chip, which is a read-out chip using CMOS
technology. It is divided into 256 × 256 pixels measuring 55 µm × 55 µm each
yielding to an overall sensitive area of 14 mm × 14 mm. For collecting signals in
gaseous detectors the chip is mounted under an amplification structure. In case of
the GridPix detector shown in Fig.(1) this is a thin metal grid on top of insulating
pillars with a pitch of 55 µm. Furthermore the chip includes a resistive layer to
avoid damage from discharges.
Between the grid and the chip a high voltage is applied - the grid voltage VGrid .
When a drifting electron enters the amplification region electron avalanches are
induced resulting in a signal amplification. This gain - the electron multiplication
factor - depends on gas mixture, pressure and voltage. A scheme can be seen in
Fig.(1).
The Medipix2 includes a 14 bit counter originally intended for photon counting in
X-ray imaging. The Timepix can use the 14 bit memory of each pixel not only for
counting hits but also for counting clock pulses sent to the chip. Each pixel can
be set to a specific threshold. The pixel is activated by a signal with adjustable
length. This time window is called the shutter time.
3
Figure 1: Photo of a GridPix detector (left) and scheme of the working principle (right).
When a threshold exceeding signal arrives during this time the pixel counter is
triggered. There are 3 different counting modes for each pixel:
• Medipix mode: single hit counting - gives information how often the pixel
was hit during one shutter cycle. This is the basic Medipix-Chip mode.
• Time over Threshold (ToT): after the incoming charge exceeds the threshold the clock pulses are counted as sketched in Fig.(2). Since the signal has a
certain length it will fall below the threshold again and thereby stopping the
counting. The length of the signal-pulse increases with the charge. Therefore the ToT-mode allows a measurement of the collected charge. During
one shutter cycle multiple signals are integrated.
Figure 2: Scheme ToT-mode signal. When a signal arrives the number of clock cycles
elapsed while the pulse was above threshold is stored. Multiple hits are
summed up.[3]
• Time of Arrival (ToA): as sketched in Fig.(3) the counter is started at
the very first signal which exceeds the threshold and then stopped by the
shutter closing. Therefore large number of clock counts correspond to an
early signal arrival and small numbers to a late arrival. After the counter is
activated the pixel becomes insensitive for further signals during the same
shutter cycle.
4
Figure 3: Scheme ToA-mode signal. When a signal arrives the number of clock cycles
elapsed until the shutter closing is stored. Multiple hit detections during one
shutter cycle are not possible.[3]
For precise measurements the timewalk effect has to be considered. The signal
is required to reach a certain threshold per pixel before the counter starts. The
incoming charge produced by the avalanche in the amplification region produces
a signal with an amplitude depending on the size of the gain. The time needed
for reaching the signal peak is characterised by the electrostatical configuration of
the system, i.e. independent of the signal height. Therefore a large signal has a
steep rising slope and crosses the threshold earlier compared to a low signal with
a softer slope. This introduces a fluctuation in measured time caused by differing
gain.
Another effect of GridPix detectors is crosstalk, electronic communication between nearby pixels. If the capacitance between pixels is high enough, the signal
on one pixel can induce a signal with opposite sign on the neighbouring pixels.
Furthermore if the gain is high one primary electron can lead to multiple pixel
hits. In this case the avalanche produced in the amplification region spreads far
enough to exceed the threshold on neighbouring pixels as well.
3. Experimental Setup
3.1. General Setup
The setup shown in Fig.(4) consists of a TPC with a single GridPix detector as
read-out. The TPC is inside a gas-tight vessel with gas inlet and outlet allowing
for a gas flow. With a connected vacuum pump the vessel can also be evacuated.
Voltage is supplied by signal cables connected with feedthroughs on the main
flange.
The TPC volume is defined by a cathode and a 6 cm distant guard electrode
measuring 10 cm × 10 cm each. The guard electrode has an opening with the size
of the GridPix detector. The latter is mounted on a carrier board. The read-out
of the Timepix chip is realised with the RelaxD-system. Furthermore the grid is
connected to a pre-amplifier allowing for a measurement of the induced charge on
the grid. Electrons drifting away from the grid towards the pixels and ions drifting
back to the cathode induce a signal with opposite sign wrt. the one on the pixels.
5
Figure 4: Main parts of the experimental setup - the vessel with the TPC, the Pre-Amp
and the source (left) and the trigger setup (right).
The pre-amplifier delivers two signals - a fast signal used for triggering the shutter
and a slower signal for measuring the charge produced in the avalanches.
Between the cathode and the grid a drift field of 300 V cm−1 was applied corresponding to a voltage of 1800 V. In the amplification region a voltage between
300 V–450 V (∼ 80 kV cm−1 ) multiplies the incoming primary electrons. Since the
guard plate is 1 mm above the grid an uniform drift field requires a correction of
30 V between the guard and the grid.
The gas used in this setup was a mixture of Ar/iC4 H10 (90/10) at pressures
between 1.0 bar to 2.0 bar. Electrons or photons can ionise and excite the Ar
atoms. This can result in secondary ionisation since the freed electrons can ionise
other Ar atoms. These processes can lead to an avalanche by charge multiplication
giving rise to a detectable signal.
The interaction between excited Ar-atoms leads to emission of ultraviolet photons.
The energy of these photons is different than any atomic level of Argon and hence
the Ar atoms are transparent for the UV-photons. This allows for a long mean
free path of latter. The gas always carries a small fraction of easily ionisable
impurities like polyatomic molecules. Photons from de-excitation can ionise these
impurities thereby freeing further electrons. This can lead to avalanches at distinct
locations in the the detector. To avoid such multiple signals a quencher gas like
isobutane iC4 H10 is added. This quencher gas can absorb the emitted UV-photons.
The mean free path of the UV-photon therefore stays short. Also the additional
reaction between quencher and photon frees electrons and increases the charge
signal at the initial location.
The source of the events are 5.9 keV photons from an 55 Fe source aimed at the
middle of the TPC. In Ar/iC4 H10 these photons can transfer all their energy by
freeing an outer shell electron. The energy needed for ionisation is only a few
eV hence the photons energy is transferred to the kinetic energy of the liberated
6
electron. Because of its high energy this electron frees more electrons in a narrow
area. On average 221 electrons are freed in Ar/iC4 H10 [4]. The resulting liberated
electrons form a spherical cloud and start to drift along the field lines. While
drifting the diffusion spreads the distance between the electrons. Nevertheless the
shape of the cloud projected on the anode is a circle.
The 5.9 keV photon can also hit an electron from an inner shell. More energy around 2.3 keV - is required to free the electron which has therefore 2.6 keV kinetic
energy. Hence the electron from the inner shell ionises less atoms. Under emission
of a photon with 2.3 keV the hole in the inner shell is filled by an outer shell
electron. This photon can escape the detector hence preventing a detection of its
energy.
3.2. Trigger Setup and Signal Timing
Two read-out systems are used in this setup - the RelaxD Timepix read-out and a
digital oscilloscope1 for the grid signal. In order to trigger on individual events and
to synchronise the two systems a trigger system was implemented. The connection
schematics for this setup is shown in Fig.(5). Timers were used to achieve a proper
synchronisation and veto signals. The RelaxD board is used in external trigger
Figure 5: Trigger system for synchronisation and individual event triggering.[5]
mode. The shutter is started as soon as a TTL pulse from high to low is registered
and stopped when a pulse from low to high arrives. After the acquisition is finished
the chip is read out and the board sends a busy signal to the trigger system.
The oscilloscope is triggered by the fast signal of the pre-amplifier and records the
waveforms of the fast and slow signals. During the data processing the oscilloscope
sends a busy signal to the trigger system. A trigger is accepted only when the two
1
The oscilloscope is a LeCroy Waverunner6030.
7
Trigger Setup Using Relaxd ReadOut for Gain Measurements
LOGIC SIGNAL CHART of TIMERS
Accepting Triggers
Trigger signal from Grid
Timer 1
True
Shutter close
started from
trigger
False
Timer 2
True
DAQ
Read-Out Time
Restart
100 us
Shutter open
Shutter
False
Comparator (Output ECL)
Timer 3
True
Busy
False
Timer 4
1 us
True
Wait till the noise
of the shutter is
False
gone
NIM Signals
0V
-0.8 V
Timer 5
True
Veto ON
Veto
Veto OFF
False
Timer 6
Read-Out
Busy
Oscilloscope
True
Relaxd
False
NIKHEF Amsterdam: Gijs Hemink & Matteo Alfonsi 25 February 2011
Figure 6: Logical signal chart showing the timing of the signals.[5]
system are finished with processing of the earlier event. In Fig.(6) the timing
of the different signals is shown. After both read-out systems are finished with
processing the shutter is opened. To avoid collection of electronics noise from the
shutter opening the trigger veto is released only 1 µs after. As soon as a signal
arrives on the grid the oscilloscope triggers the shutter closing and the veto. The
closing has a 100 µs delay to ensure the complete collection of the signal. Thereafter
the shutter is closed and the data acquisition of the pixels stopped. The global
busy signal is activated since the read-out starts. The RelaxD read-out takes place
after the shutter closing and is faster than the oscilloscope read-out. The whole
system is restarted after the processing is finished.
The 40 MHz pulse frequency results in an maximum shutter time of 295.25 ns
since the maximum storable count value of the pixels is 11810. For a successful
photon event the 100 µs delay should yield to a mean of ∼ 3900 counts in the time
distribution.
3.3. Data Acquisition
The RelaxD-board is connected to a computer with a gigabyte Ethernet cable and
connected to the carrier board with a 100 pin cable. The software for processing RelaxD-data is called RelaxDAQ. In this setup RelaxDAQ was used to store
the binary RelaxD-data and to apply changes in the setting of the RelaxD-board.
Another software used for threshold-equalisation and creating pixel masks was
Pixelman.
The oscilloscope is connected to the computer. During the data taking the oscilloscope stores the waveforms directly on the computer in binary format. A setting
8
on the oscilloscope allowed a fast processing of the data resulting in a maximum
event frequency of ∼ 20 Hz limited by the speed of the oscilloscope.
For each voltage 10.000 events were recorded. This took between 12 min to 25 min
for a run without problems depending on the voltage.
The binary data files were converted to text-files with conversion programmes.
9
4. Analysis of Recorded Data
The data taking was performed at Nikhef in October/November 2011. For different
pressures between p = 1.0 bar and p = 2.0 bar data in ToA mode was taken. Also
data in ToT-mode for pressure between 0.9 bar and 1.6 bar was recorded. In the
following only the analysis for the ToA data is reported.
If not stated otherwise the following figures are for run with p = 1.4 bar and
VGrid = 410 V.
4.1. Occupancy and Time Spectra
As a first approach to understand the chip performance a two dimensional figure
showing the occupancy per run for each pixel was investigated. Such a figure shows
the effective detection area as well as possible dead columns and other distortions.
The figure in Fig.(7) reveals that the shielding from the guard plate narrows the
sensitive area. Furthermore the right upper corner which was broken and later fixed
with glue is insensitive to incoming electrons [5]. Overall the effective detection
area is between 25 and 220 for pixels in x and 40 and 225 for pixels in y. The pixel
column at x = 88 is dead and was masked by means of the Pixelman software.
The time spectrum in Fig.(8) shows the number of clock pulses stored on each
Figure 7: Occupancy figure. The hits are required to be within 25 < xpx < 220 and
40 < ypx < 225.
hit pixel. As soon as the signal on the pixel exceeds the threshold the pixel starts
counting until the shutter is closed. Each pulse corresponds to 25 ns since the clock
pulse frequency was set to 40 MHz. The mean of the distribution is given by the
shutter closing which happens ∼ 100 µs after the signal is detected by the grid.
The distribution in Fig.(8) is not symmetric but has a tail towards smaller times
(later arrival). This is caused by the timewalk effect explained in Sec.(2).
Longitudinal diffusion also influences the time spectrum. The signal on the grid
has to reach a certain threshold to trigger the shutter closing. If the incoming
charge gets spread in z-direction the first arriving electrons do not exceed the
10
threshold. Only when enough electrons arrive the shutter closing is triggered.
Hence the pixels activated by the first electrons have a higher pulse count and vice
versa the pixels hit by late electrons - after the shutter closing is triggered - will
have a lower pulse count. In that way the longitudinal diffusion spreads the time
spectrum.
As a simplification a Gaussian was fitted to the distribution. The mean λ and the
width σ were used to determine cuts on the time distributions by requiring the
time count of each pixel to be within λ ± 5σ. With this cut all hits which do not
belong to a certain event with well defined time information are discarded.
Figure 8: Time spectrum with Gaussian fit. The width is determined by the longitudinal diffusion and statistical fluctuations. The tail towards lower pulse counts
is caused by the timewalk effect.
4.2. Cloud Shape
In Fig.(9) an example event in the effective detection area is shown. The expected
circular cloud shape is visible. A first cut is obtained by requiring a minimum
dimension of the cloud. Events generated very close to the grid get rejected by
such a cut, since the drift distance is too small to obtain a sufficient spread. For
such a cut the distribution containing the distance ∆x between the maximum
and the minimum hit pixel is shown in Fig.(10) for x and Fig.(App.18) for y. A
minimum size of 5 per dimension is required for each event.
The right figure in Fig.(10) reveals a decrease in the cloud size for increasing
pressure. While the mean of ∆x for p = 1.4 bar is around 40 pixels it decreases to
∼ 20 pixels for p = 1.6 bar. The same holds for the y-dimension.
11
Figure 9: An example event. The pixels are required to be in time and within the
effective detection area.
This behaviour can be explained by
• a decrease in the initial cloud size. Since a higher pressure corresponds to
more particles in the volume the ionisation takes place in a smaller area.
• the decreasing mean free path of the electrons. The transversal diffusion
coefficient gets smaller.
Figure 10: The relative distance ∆x for p = 1.4 bar (left) and p = 1.6 bar (right) with
Vgrid = 410 V.
The shape of the projection of the cloud on the anode can be used to classify the
event. An estimator for the circularity c of the cloud is constructed by dividing
the spread in x by the spread in y resulting in c ≡ ∆x/∆y. Ideally c ≈ 1 but
because of the statistical nature of the transversal diffusion small deviations have
to be expected. This can be seen in Fig.(11). Large deviations could originate
from events close to the border of the effective detection area since parts of the
expected circle are cut away. The estimator is required to be 50/77 < c < 77/50.
12
Figure 11: The ratio c between the cloud dimension in y and x. The chosen range is
50/77 < c < 77/50.
4.3. Number of Detected Electrons
The GridPix detector has a very high single electron detection efficiency. Since
the transversal diffusion spreads the initial cloud an estimation of the number of
primary electrons is possible by counting the number of hit pixels. The spread of
the cloud should be high enough to resolve each primary electron with the GridPix
(with a hole pitch of 55 µm).
Fig.(12) shows the distribution of number of hit pixels per event. Two distinct
Figure 12: The number of hit pixels. The first peak is the escape peak, the second the
photo peak. The latter is expected to be around 221 but is shifted towards
lower values.
13
peaks are visible. When the 5.9 keV photon from the 55 Fe source hits an outer shell
electron of the Ar the photo peak is formed at high hit counts. The peak at lower
hit counts is the escape peak from events with a liberated inner shell electron. The
peaks in Fig.(12) are shifted towards lower numbers than expected, indicating a
loss of primary electrons. As a cut the number of hit pixels is required to be within
5 to 350.
Fig.(13) shows the integral values of the slow signal on the grid recorded by the
Figure 13: The integral of the charge measured by the oscilloscope. The photo and the
escape peak are visible.
oscilloscope. The signal is caused by the electrons and the back drifting ions. It is
proportional to the number of electrons after the amplification. Since both signals
- the charge on the grid and the number of hit pixels - should be proportional a
scatter figure between both signals is shown in Fig.(14). Two dense areas which
are the photo and the escape peak are visible and connected by a straight line of
entries. This shows that both signals are indeed proportional.
The entries in between the dense areas are events near the border since neither
pixels are hit nor charge is induced on the grid. By applying the cuts on the
position and the shape of the cloud the straight connection between the two dense
areas gets removed as well as some noisy hits. This can be seen in Fig.(15).
Nevertheless an unexpected contribution of entries with oscilloscope signal in the
photo/escape peak region but lower number of hit pixels is visible. These entries
are events in which the charge seen by the grid does not correspond to the number
of detected electrons in terms of hit pixels. A possible explanation is that two or
more primary electrons hit the same pixel by entering the same hole on the grid.
The charge induced on the grid is then the same as if two or more pixels got hit.
The reason for this is the small diffusion coefficient. The electrons do not drift a
sufficient transversal distance to be separated by the hole pitch of the GridPix.
14
Figure 14: A scatterfigure of the hit pixels and the charge integral for each event. The
two dense areas are events contributing to the photo respectively the escape
peak.
Figure 15: The implemented requirements cut away the linear transition between the
two dense areas. However a clear contribution of events with less hit pixels
but high charge integral stays.
4.4. Gain Curves
The previous discussion was repeated for every measured pressure. The results
are used to create the efficiency curves in Fig.(16). These figures show the number
of detected electrons in terms of hit pixels divided by the number of expected
electrons Neexp = 221 in dependence of the applied grid voltage.
The left and the right figures show the curves at lower and at higher pressures
respectively. The highest detection efficiency around 0.9 was achieved at 1.4 bar
and VGrid = 410 V. Between 1.2 bar–1.6 bar the single electron detection efficiency
is between 0.6–0.9 for the maximum grid voltages for each setting. The curves
of 1.8 bar and 2.0 bar show a very small efficiency smaller than 0.1. For 1.0 bar
- below atmospheric pressure - a maximum efficiency of 0.58 was achieved. The
15
overall maximum grid voltage with which a whole data set could be taken was at
1.6 bar VGrid = 450 V. Finally the achieved gain for each field configuration and
Figure 16: Number of detected electrons divided by expected number of Ne = 221.
pressure can be calculated. Therefore the oscilloscope signal has to be calibrated
to correlate the number of produced electrons in the amplification region to the
recorded signal height. The number of electrons can be calculated from the integral
int
by
of the oscilloscope signal Vosc
ne = (−1.23 ± 1.0) · 105 + (3.696 ± 0.057) · 1013
int
Vosc
exp
Ne
which is a result of the calibration of the detector realised in [5]. The gain curves
are shown in Fig.(17). The highest gain was achieved at 1.2 bar with VGrid = 420 V
Figure 17: Achieved gain curves for different pressures.
followed by 1.6 bar with 450 V. In general the gain decreases with higher pressure
at the same grid voltage. Also higher pressures allow for a higher maximum
grid voltage the detector can be operated with. Furthermore the curves show an
exponential like behaviour - disregarding 1.8 and 2.0 bar.
16
5. Discussion
In this report a short introduction to GridPix detectors was given, followed by
a description of the experimental setup used to perform gain measurements of a
GridPix detector in Ar/iC4 H10 at different pressures. The analysis of the collected
data was summarised in Sec.(4) giving insights into detector performance and the
physics processes in the detector for different pressures.
The occupancy figure in Fig.(7) shows the sensitive area of the detector which is
narrower than the chip itself. This loss is expected since the shielding of the guard
plate yields to field distortions which are inevitable in this setup. For a future
setup this could be optimised by using a field shaper.
The time spectra of the recorded data was used to determine the window in which
hits were in time. Therefore gaussians fits were performed to generate automatic
upper and lower limits. The distributions showed an asymmetry caused by the
timewalk effect and longitudinal diffusion. A more careful fitting procedure including the fitting of the asymmetric tail could give a measure for longitudinal
diffusion and timewalk effect.
The electrons freed by the 5.9 keV photon from the source form initially a spherical cloud. Hence the shape of the recorded hits was investigated. With changing
pressure the minimum and maximum cloud spread in x and y changes. For increasing pressure and constant grid voltages the cloud sizes decreases due to a
lower transversal diffusion coefficient and narrower initial clouds. Furthermore the
circularity of the hits was used to cut away events on the edge of the sensitive
area.
By counting the number of hit pixels the number of freed electrons was determined.
The photo and the escape peak are at lower counts than expected. This can be
explained by multiple hits per pixel. Some of the primary electrons do not drift a
sufficient transversal distance to be separated by the holes of the GridPix.
The integrated grid signal is proportional to the number of electrons. A scatterfigure between the number of hit pixels and the integrated oscilloscope signal
validates the multiple hit explanation. A cut on the tail would shift the position
of the peaks towards higher values but also bias the results.
The single electron efficiency curves in Fig.(16) vary with pressure. A maximum
efficiency of 0.9 was achieved.
Finally the gain curves in Fig.(17) show the achieved gain for each pressure. The
operation of the system at 1.8 and 2.0 bar was difficult since voltage drops occured
frequently and collection of noise was inevitable with the chosen threshold. Furthermore the gain is very small hence no clear signal can be detected. Nethertheless
the GridPix detector showed an impressive capability of working at high pressures.
This capability could be used to reduce background and noise in low statistics experiments like dark matter searches briefly discussed in the appendix.
17
A. Plots
Cloud Size in y
min
Figure 18: The relative distance ∆ypx = xmax
px − xpx for p = 1.4 bar (left) and p =
1.6 bar (right) with Vgrid = 410 V.
18
B. Dark Matter and WIMP Detection
One of the biggest secrets in modern astronomy, cosmology and particle physics
is the nature of dark matter. Many research groups designed and constructed
experiments with different technologies for direct dark matter searches. Noble
liquid detectors are amongst the most promising designs in terms of sensitivity of
direct WIMP searches.
B.1. Dark Matter
In 1933 the Swiss physicist Fritz Zwicky discovered a missmatch between the rotation velocity of galaxies and the prediction from the visible mass. This was the first
indirect observation of dark matter (DM). An analysis of the energy distribution
of our universe revealed distributions as shown in Fig.19.
Figure 19: Measurements of the cosmic microwave background by the WMAP experiment unveil the composition of our universe2 .
Different natures of DM were assumed, cold dark matter being a promising candidate. Latter requires new, yet unseen particles which undergo the gravitational
and weak interaction but not the strong or electromagnetic force. Such a particle
is called WIMP - weakly interacting massive particle.
Candidates for WIMP particles require an extension of the Standard Model of
elementary particle physics. In R-parity conserving supersymmetric models the
lightest supersymmetric particle (LSP) is stable and hence gives a candidate for
cold DM. The mass of the LSP is believed to be > 100 MeV and it could be detected
by the LHC.
B.2. Detection of WIMPs
A WIMP candidate like the LSP could be directly observed by the LHC. Decay
chains involving gluinos and other supersymmetric particles can lead to a final
state with a certain amount of missing energy which could be an escaping WIMP.
Furthermore indirect detection by observing cosmic decay products from WIMP
annihilation could be possible. Since WIMPs interact gravitationally a locally
higher density is expected in space areas with a high matter density like the centre
2
http://map.gsfc.nasa.gov/universe/uni_matter.html, retrieved on 06.02.2012
19
of galaxies or the sun. Hence the higher annihilation rate would yield to escaping
neutrinos with a high energy spectrum, i.e. above 100 MeV. These neutrinos can
be detected on earth and by discriminating from the background a indirect observation of WIMPs could be achieved.
A third option is the observation of an elastic scatter event between a WIMP and
a nucleus. The scattering releases recoil energy which can be detected in different
ways. By measuring the heat of lattice vibrations phonons from scatter events can
be observed. This measurement requires the target to be cooled down to a few
mK. Also ionisation is a possible result from the produced recoil energy. Latter
can be measured by using semiconductors as target material. A third option is the
production of photons by scintillation. For noble liquids, e.g. xenon or argon at
low temperatures, the recoil produces ionisation and scintillation yielding to two
measurable signals.
Using noble liquids as detection material requires a high purity ∼ 1 ppb and a
temperature between 87 K to 163 K in case of argon or xenon respectively [6].
Furthermore the energy transfer - hence the recoil energy - between nucleus and
WIMP can be maximised by choosing the nucleus mass around the expected WIMP
mass. Xenon is a good candidate for many WIMP models since its nucleus mass
is mXe
N ≈ 122 MeV. Another important requirement of dark matter search experiments is a proper shielding. The rate of background events originating from
e.g. natural radiation, electronics noise and cosmic radiation has to be minimised
in order to measure rare WIMP scatter events. For this purpose, only very pure
materials are used and the detectors are deep underground.
The European consortium dark matter WIMP search in noble liquids (DARWIN )
investigates possible concepts for the measurement of properties of dark matter.
Next generation dual phase noble liquid detectors are designed and studied to increase the sensitivity by three orders of magnitude [2].
Within this framework direct charge readout for WIMP scatter events is studied.
A candidate is the GridPix-detector operated in a dual-phase noble liquid TPC.
The detector has to be operated in the cryogenic environment of liquid Xe/Ar,
which requires intense performance studies.
A dual-phase Xenon dark matter detector like the Xenon100 detector at the LNGS
is a TPC filled with liquid xenon. On top and bottom of this vessel several PMTs
are placed. Furthermore a thin layer of gaseous xenon is maintained on top of
the liquid xenon. If a WIMP scatter event occurs, the recoiling nucleus can excite
and ionise the surrounding atoms. The excitation of the atoms leads to scintillation photons at 177.6 nm which is detected by PMTs almost immediately after the
event. The electrons from the ionisation start to drift towards the anode and hence
the gaseous layer. Once the electrons reach this layer, they excite gaseous atoms
and thereby produce more UV-photons which are detected by PMTs. This results
in two distinguishable signals S1 and S2 with different amplitudes. The distance
of the event to the anode is determined by the time difference in arrival of the signals. The ratio of the signal amplitudes S2/S1 is furthermore a good discriminator
of γ-background events.
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References
[1] T. Behnke, C. Damerell, J. Jaros, A. Myamoto, et al., ILC Reference Design
Report Volume 4 - Detectors (2007), arXiv:0712.2356v1
[2] DARWIN Consortium, DARWIN - dark matter WIMP searches in noble liquids
(02.03.2012), URL http://darwin.physik.uzh.ch/index.html
[3] F. Kloeckner, Teststrahlmessung einer GEM-basierten TPC mit simultaner
Datenauslese von acht Timepix-Chips, Diploma thesis, Physikalisches Institut,
Universität Bonn (2010)
[4] M. A. Chefdeville, Development of Micromegas-like gaseous detectors using a
pixel readout chip as collecting anode, Ph.D. thesis, Universiteit Twente (2009)
[5] G. Hemink, GridPix - A pixel sensor for noble liquid dark matter searches,
Master’s thesis, University of Twente (2011)
[6] J. Jochum, Direct dark matter detection. International school in Astroparticle
physics. (July 2011)
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