Micromegas detector developments for Dark Matter directional detection with MIMAC

Abstract

The aim of the MIMAC project is to detect non-baryonic Dark Matter with a
directional TPC using a high precision Micromegas readout plane. We will
describe in detail the recent developments done with bulk Micromegas detectors
as well as the characterisation measurements performed in an
Argon(95%)-Isobutane(5%) mixture. Track measurements with alpha particles will
be shown.

Full text

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Preprint typeset in JINST style - HYPER VERSION
Micromegas detector developments for Dark Matter
directional detection with MIMAC
F.J. Iguaza, D. Attiéa, D. Calveta, P. Colasa, F. Druillolea, E. Ferrer-Ribasa∗,
I. Giomatarisa, J.P. Molsa, J. Pancinb, T. Papaevangeloua, J. Billardc, G. Bossonc,
J.L. Boulyc, O. Bourrionc, Ch. Fourelc, C. Grignonc, O. Guillaudinc, F. Mayetc,
J.P. Richerc, D. Santosc, C. Golabekdand L. Lebretond
aCEA/DSM/IRFU,
CEA, 91191 Gif sur Yvette, France
bGANIL,
Bvd H. Becquerel, Caen, France
cLPSC,
Universite Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut Polytechnique de Grenoble, France
dLMDN,
LMDN, IRSN Cadarache, 13115 Saint-Paul-Lez-Durance, France
E-mail: esther.ferrer.ribas@cea.fr
ABSTRACT: The aim of the MIMAC project is to detect non-baryonic Dark Matter with a direc-
tional TPC using a high precision Micromegas readout plane. We will describe in detail the recent
developments done with bulk Micromegas detectors as well as the characterisation measurements
performed in an Argon(95%)-Isobutane(5%) mixture. Track measurements with alpha particles
will be shown.
KEYWORDS: Micromegas, Time Projection Chambers.
∗Corresponding author.
arXiv:1105.2056v1 [physics.ins-det] 10 May 2011

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Contents
1. Introduction1
2.The MIMAC concept2
3.Design of the Bulk Micromegas detector: 10 × 10 cm2
Experimental set up
2
4.3
5.Characterisation measurements and results5
6.Track measurements with alpha particles6
7.Conclusions and Perspectives9
1. Introduction
The MIMAC (MIcro TPC MAtrix of Chambers) collaboration[1] aims at building a directional
Dark Matter detector composed of a matrix of Micromegas[2] detectors. The MIMAC project
is designed to measure both 3D track and ionization energy of recoiling nuclei, thus leading to
the possibility to achieve directional dark Matter detection[3]. It is indeed a promising search
strategy of galactic Weakly Interacting Massive Particles (WIMPs) and several projects of detector
arebeingdevelopedforthisgoal[4]. RecentstudieshaveshownthatalowexposureCF4directional
detector could lead either to a competitive exclusion[5], a high significance discovery[6], or even an
identification of Dark Matter[7], depending on the value of the WIMP-nucleon axial cross section.
Gaseous detectors present the advantage of being able to reconstruct the track of the nuclear
recoil and to access both the energy and the track properties. Micropattern gaseous detectors are
particularly suited to reconstruct low energy (few keV) recoil tracks of a few mm length due to their
very good granularity, good spatial and energy resolution and low threshold. Micromegas detectors
have shown these qualities in different environments [8, 9, 10, 11]. In particular thanks to the new
manufacturing techniques, namely bulk[12] and microbulk[13], where the amplification region is
produced as a single entity. In bulk Micromegas a woven mesh is laminated on a printed circuit
boardcoveredbyaphotoimageablefilmandthepillarsaremadebyaphotochemicaltechniquewith
insulation through a grid. This technique can be transfered to industry allowing the production of
large, robust inexpensive detector modules.
This paper describes the developments done with bulk Micromegas detectors in order to show
the feasability of a large TPC (Time Projection Chamber) for directional detection. Section 2
describes briefly the strategy of the MIMAC project. In section 3 we discuss in detail the design of
the first prototype detector of 10×10cm2. The experimental set-up used for the characterisation in
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the laboratory is presented in section 4 and the results are given in section 5. Section 6 is devoted to
the the results obtained for the reconstruction of tracks with alpha particles. Finally, the conclusions
and the perspectives are discussed in section 7.
2. The MIMAC concept
The nuclear recoil produced by a WIMP in the TPC produces electron-ion pairs in the conversion
gap of the Micromegas detector that drift towards the amplification gap (128µm or 256µm in
this case) producing an avalanche that will induce signals in the x-y anode and in the mesh. The
third dimension z of the recoil is reconstructed by a dedicated self-triggered electronics specifically
designed for this project [14, 15] that is able to perform anode sampling at a frequency of 40 MHz.
The concept had already been tested with a prototype detector of small size 3× 3 cm2and with the
first version of the electronics[1, 16].
The aim of building a detector of 10 × 10 cm2was to validate the feasability of a large TPC
for directional detection with a realistic size prototype. The design of the bulk Micromegas was
guided by the requirements on the granularity of the anode as well as by the operation conditions.
Simulation studies showed that the granularity of the readout plane needed strips of 200µm size.
The MIMAC project is expected to work with two different regimes: high pressure (up to 3 bar) in
order to improve the probability to have WIMP interactions and low pressure (down to about 100
of mbar) to get the directionality information of the WIMPs. The design of the bulk Micromegas
end cap should take into account these requirements. From the beginning of the design, it was
known that the detector would be first validated in the laboratory with the T2K electronics [17, 18]
before the final conclusive test with the specifically designed MIMAC electronics [14, 15]. Special
care was taken in the design to have a portable system.
3. Design of the Bulk Micromegas detector: 10 × 10 cm2
In order to have a detector that can stand operation at low and high pressure the design relies on
the idea of assembling a leak-tight read-out plane on a 2cm aluminium cap. A general sketch of
the mechanical assembly is given in figure 1.
The bulk Micromegas is on a Printed Circuit Board (PCB), called Readout PCB, of 1.6mm
thickness (a in figure 1). The active surface is of 10.8× 10.8cm2with 256 strips per direction.
The charge collection strips make-up an X–Y structure out of electrically connected pads in the
diagonal direction through metallized holes as can be seen in figures 2 and 3. This readout strategy
reduces the number of channels with a fine granularity covering a large anode surface. The pads
are 200µm large with an isolation of 100 µm resulting into a strip pitch of 424 µm. The quality of
the surface of the readout plane can be observed in figure 2. The 100µm diameter metallized holes
have been fully filled yielding a completely uniform surface. This fact is a prerequisite to obtain a
uniform performance of a bulk Micromegas detector.
The strips signals are rooted into 4 connectors prints at the sides of the Readout PCB. The
Readout PCB is screwed on a thick 0.5cm PCB, called Leak Tight PCB (d in figure 1), that will
ensure the leak tightness of the system. The Leak Tight PCB is constituted of various layers of FR4
with blind metallized vias in the inner layer. This piece is then screwed on a 2cm thick aluminium
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(a)
(d)
(c)
(b)
(b)
(f)
(e)
(a)
(d)
(e)
Figure 1. Left: Sketch of the general assembly constituting the readout plane and the interface to electronics.
Left: Zoom of the Readout PCB (a in the image), Leak tight PCB (d) and the Interface PCB (e).
cap that constitutes the bottom of the TPC (c in figure 1). The signal connections from one board to
another are done by means of SAMTEC connectors (GFZ 200 points) that are placed and screwed
between the two boards (b in figure 1 left). On the outside of the vessel an Interface card distributes
the signals to the desired electronics (e in figure 1). Two versions of this card exists: one dedicated
to the laboratory set-up and a second one for the final MIMAC electronics.
This design offers several advantages: a simple, compact and leak-tight way for the signal
connections and a versatility for two different types of electronics. Bulk Micromegas with two
different amplification gaps were produced in order to choose the best gap for different running
pressure conditions. Characterisations tests described in section 5 concern three readout planes
with a gap of 128µm and two with a gap of 256µm.
x2
x1
y1
y2
200 µm
x1
y1
200 µm
424 µm
100 µm
Figure 2. Left: Sketch of the 2D readout used. Right: Microscope photograph of the 2D readout.
4. Experimental set up
A dedicated vessel was built as shown in figure 4. It consists of two aluminium caps screwed
to create the TPC. In the top cap, an iso-KF25 valve is used for pumping in order to reduce the
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Figure 3. In this picture the pixel and the readout strips can be seen. The pixels are at 45owith respect to
the readout strips (observed by transparency).
outgassing from the inner walls. This cap is also equipped with two outlets for gas circulation and
four SHV electrical connections. The bottom cap with the bulk Micromegas has been described in
section 3.
6 cm
Mylar window
Drift electrode
Field degradator
rings
Bulk Micromegas
PCB
Leak Tight PCB
Argon+5% Isobutane
Americium
holder
18 cm
Figure 4. Schematic drawing of the set-up.
To uniformise the drift field in the conversion volume of 6cm of height, the vessel is equipped
with a field shape degrador. It consists of 5 copper squared rings and a copper plate, separated by
a distance of 1cm by four peek columns. The plate and the rings are electrically connected via
resistors of 33MΩ. The voltage of the last ring is fixed with a variable resistor located outside the
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vessel. The drift electrode has been designed to accommodate an241Am source. The source in
its holder is screwed to the cathode plate. For illumination with X-ray and gamma sources a thin
mylar window has been foreseen at the top of the vessel.
5. Characterisation measurements and results
After the installation of the detector in the dedicated vessel, the TPC was closed and pumped to
reduce the outgassing of the walls. A flow of 10 l/h of an Argon(95%)-Isobutane(5%) mixture was
circulated for some hours before starting the measurements. This gas was used for the characteri-
sation measurements eventhough the chosen gas for dark matter search will be CF4. Indeed most
of Micromegas detectors are first characterised in Argon mixtures where their performance can be
compared easily.
The strips were connected to ground and the readout was illuminated by an iron55Fe source
(X-rays of 5.9 keV) located on the TPC window. The mesh voltage was typically varied from 300
to 450V for detectors with a gap of 128 µm and between 470 to 600V for those with a gap of
256µm. The drift voltage was changed from 300 to 3000V. Mesh and drift voltages were powered
independently by CAEN N126 and CAEN N471A modules respectively. The signal induced in the
mesh was read out by an ORTEC 142C preamplifier, whose output was fed into an ORTEC 472A
Spectroscopy amplifier and subsequently into a multi-channel analyzer AMPTEK MCA-8000A
for spectra building. Each spectrum contained at least 5×104events and was fitted to get the peak
position and the energy resolution.
In order to obtain the electron transmission curve, the drift voltage was varied for a fixed mesh
voltage. Figure 5 shows the typical plateau for Micromegas readout planes where the maximum
electron transmission is obtained for a ratio of drift and amplification fields lower than 0.01. For
ratios over this value, the mesh stops being transparent for the primary electrons generated in the
conversion volume and both the gain and the energy resolution deteriorate for high drift fields. As
shown in figure 5, readouts with a 256 µm-thick gap have a slightly larger plateau than those with
a 128µm-thick gap. No other significant difference was observed among the tested readouts.
After this first test, the ratio of drift and amplification fields was fixed in the region where the
mesh showed the maximum electron transmission and the mesh voltage was varied to obtain the
gain curves, shown in figure 6. The tested readouts reach gains greater than 2×104before the
spark limit for both amplification gaps.
As shown in figure 7 for the best cases, the energy resolution stays constant for a wide range of
amplification fields. At low fields, the resolution worsens due to the noise level that is comparable
to the signal height. At high fields, it deteriorates due to the gain fluctuations. The values measured
for each detector are shown in table 1. We note that the readouts with a gap of 256µm show a better
energy resolution than those with a gap of 128µm. This effect is possibly due to the thickness of
the bulk mesh. Indeed for both amplification gaps the same mesh thickness (30µm) was used.
Therefore the non-uniformity of the gap will have a bigger effect on the electric field of smaller
gaps.
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Ratio of drift and amplification fields
0 0.0050.010.0150.02
Electron transmission
0.8
0.85
0.9
0.95
1
1.05
1.1
Figure 5. Dependence of the electron mesh relative maximum transmission with the ratio of the drift and
amplification fields for the readouts with a 128µm (red squared line) and a 256µm-thick amplification gap
(blue circled line).
Amplification field (kV/cm)
1015 20253035
Absolute gain
1
10
2
10
3
10
4
10
5
10
Figure 6. The absolute gain as a function of the amplification field is shown for the readouts with a 128µm
(red squared line) and 256µm-thick amplification gap (blue circled line) obtained with a iron55Fe source
using an Argon(95%)-Isobutane(5%) mixture at 1bar. The extension at low gain (green star line) for the
256µm readout was obtained with an alpha source. The maximum gain was obtained before the spark limit.
6. Track measurements with alpha particles
The T2K electronics[17, 18] has been used to read the signals induced in the strips to fully validate
the concept of MIMAC readouts for the reconstruction of tracks. Eight flat cables connect the strips
signals to two Front End Cards. Each card is equipped with four ASIC chips, called AFTER, which
digitize in 511 samples the signals of 72 channels, which are previously amplified and shaped.
Finally, the data of each AFTER is sent by a Front End Mezzanine (FEM) card to a DAQ card
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Amplification field (kV/cm)
1618 2022 242628 30323436
Energy resolution (% FWHM)
14
16
18
20
22
24
26
28
30
Figure 7. Dependence of the energy resolution at 5.9 keV with the amplification field for the readouts of
128µm (red squared line) and 256 µm-thick amplification gaps (blue circle line).
Detector Num Energy Resolution
(% FWHM)
21.0±0.3
23.4±0.4
23.2±0.4
16.0±0.1
17.8±0.3
128µm1
2
3
1
2
256µm
Table 1. Energy resolutions measured at 5.9 keV for the 3 different readouts of 128µm and for 2 at 256µm-
thick amplification gap.
and subsequently to the computer for recording. As the external trigger mode of the T2K DAQ
has been used, a trigger signal has been created feeding the bipolar output of the ORTEC VT120
amplifier/shaper into a FAN IN/OUT Lecroy 428F and subsequently into a NIM-TTL converter.
Strips pulses have been sampled every 20 ns and the peaking time has been fixed to 100ns. The
dynamic range is of 120fC.
In order to reconstruct the two 2D projection of each event, an offline analysis software has
been developped. It extracts the strips pulses by using the amplitude of each pulse sample and the
readout decoding. An example of the strips pulses and the XZ reconstruction of one event is shown
in figure 9.
During three weeks of data-taking, a constant flow of 5 l/h of Ar+5%iC4H10was circulated in
the dedicated vessel. The detector (with 256µm of amplification gap) was maintained in voltage
(Eamp= 21.9kV/cm, Edrift= 88V/cm) acquiring events in a continous way. The detector was
calibrated with an iron55Fe source twice per day to monitor the evolution of the gain, energy
resolution and the parameters calculated. The gain fluctuations observed were below 10% due to
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Figure 8. A view of the dedicated vessel used to test MIMAC readouts when reading the strips with the T2K
electronics. A detailed description is made in text.
Time bin
0 2040 6080100 120 140 160 180 200
Number of ADC channels
0
0.5
1
1.5
2
2.5
3
3.5
4
3
10
×
Number of channel
0 50100150 200250
Time bin
0
20
40
60
80
100
120
140
160
180
200
0
500
1000
1500
2000
2500
3000
Figure 9. Left: Example of pulses induced in the strips acquired with the T2K electronics. Right: The re-
construction of the XZ projection of the same event. The physical event is an electron candidate of 44.4keV
with a final charge accumulation (or blob).
the variations of pressure and temperature inside the vessel. It must be stressed that the pressure and
the temperature were not controlled. The energy resolution varied between 18 and 20% FWHM
during the same period as can be seen in figure 10.
In the two spatial projections (x and y) of the event, different parameters characterising the
charge were calculated like mean position, width and number of activated strips. The analysis was
then extended to the perpendicular direction using the amplitudes of strips pulse in each temporal
bin. Finally, the total charge of each event was obtained summing the charge of both projections.
After having tested the readout with low energy events, we evaluated its performance at low gain
with high energy events. For this purpose, an241Am alpha source was installed in the source keeper
screwed at the center of the drift plate. The source consists of a metallic circular substrate of 25mm
diameterwheretheradioactivematerialhasbeendepositedonitscenterinacircularregionofabout
8mm of diameter. The alpha particles are emitted isotropically. The 5.5 MeValphas were used to
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Number of run
6070 80 90100110120
Strips charge (ADC channels)
0
100
200
300
400
500
600
3
10
×
Energy resolution (% FWHM)
10
12
14
16
18
20
22
24
26
28
30
Figure 10. Evolution of the strips charge (red squared line) and the energy resolution at 5.9 keV (blue
circled line) during the successive calibration runs with a source of55Fe. The temporal distance between
each calibration run is half day.
characterize the readout as it was done with the iron source in section 5, generating the electron
transmission and gain curves. The electron transmission curve, taken at an absolute gain of 85,
matched with the one showed in figure 5. Meanwhile, the gain curve was produced varying the
mesh voltage between 270 and 420V, as shown in figure 6, and follows the tendency of the one
generated by photons.
The spectra generated by the241Am source showed an energy resolution of 5.5% FWHM, as
the one shown in figure 11. This value was independent of the drift voltage and the readout gain.
To check the possible presence of attachment effects in the gas, mesh pulses were acquired by a
LeCroy WR6050 oscilloscope. In an offline analysis, the amplitude and risetime of the pulses were
calculated and the 2D distribution of these parameters was generated to look for correlations. Alpha
events showed the same amplitude, independently of their risetime, i.e., their spatial direction.
Therefore attachement effects are not observed. An example of these 2D distribution is shown in
figure 11 (right).
Several alpha tracks were also acquired with the T2K electronics, as shown in figure 12 (left).
The mesh and drift voltages were respectively set to 400 and 820V, which correspond to a gain of
85 and a drift field of 70 V/cm. For each event, the length of the track projection on the XY plane
was calculated and the distribution of this variable was generated. The maximum value obtained
(54mm) matches the theoretical length expected for a 5.5 MeValpha particles in an argon-based
mixture[20].
7. Conclusions and Perspectives
The readout plane of the Bulk 10 × 10cm2Micromegas designed and built for the MIMAC project
has been described. The first characterisation tests in the laboratory show good performance in
terms of gain, uniformity, energy resolution and track measurements. The following steps are
to test this detector in a neutron beam facility with neutrons of few keV using the specifically
designed MIMAC electronics to reach the ultimate performance of the detector for the detection of
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Amplitude (V)
0 0.1 0.20.30.40.50.6 0.70.80.91
Number of events
0
500
1000
1500
2000
2500
3000
5.5% FWHM
Amplitude (V)
00.10.2 0.30.40.50.60.70.80.91
Risetime (ns)
400
600
800
1000
1200
1400
1
10
2
10
Figure 11. Left: Spectrum generated by the mesh pulses induced by the241Am alphas, showing an energy
resolution of 5.5% FWHM. Right: Distribution of the risetime versus the amplitude of the mesh pulses
induced by the241Am alphas.
Channel number in YZ direction
0 50100150200250
Time bin
0
20
40
60
80
100
120
140
160
180
200
0
100
200
300
400
500
600
700
800
900
Projection length (mm)
0102030 4050607080
Number of events
0
200
400
600
800
1000
1200
Figure 12. Left: An alpha event acquired by the T2K electronics. The track shows a final charge accumu-
lation. Right: Distribution of the length of the track projection on the XY plane, calculated by the number
of strips activated in each direction. The maximum length of the distribution matches the Geant4 simulation
value (54mm).
Dark Matter. Plans for a test in the Frejus underground laboratory with a module containing two
10 × 10cm2detectors are envisaged in the next coming months. This measurement, in a realistic
environment for a Dark Matter experiment, will give crucial information (background rejection as
well as intrinsic contamination of the used materials) before the construction of a 1m3experiment.
In any case the results obtained up to now validate the MIMAC concept for the construction of a
large TPC for directional detection of Dark Matter.
Acknowledgments
The MIMAC Collaboration acknowledges the ANR-07-BLANC-0255-03 funding. The authors
would like to thank D. Desforges for his availability in the use of the Mitutuyo Microscope as well
as D. Jourde for his help with the degrador.
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