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Use of silicon pixel detectors in double electron capture experiments

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P UBLISHED BY IOP P UBLISHING FOR SISSA R ECEIVED: December 3, 2010 ACCEPTED: December 8, 2010 P UBLISHED: January 11, 2011

12th I NTERNATIONAL W ORKSHOP ON R ADIATION I MAGING D ETECTORS , J ULY 11th –15th 2010, R OBINSON C OLLEGE , C AMBRIDGE U.K.

P. Cermak,a,1 I. Stekl,a Yu.A. Shitov,b,c F. Mamedov,a E.N. Rukhadze,a J.M. Jose,a J. Cermak,a N.I. Rukhadze,b V.B. Brudaninb and P. Loaizad a Institute

of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, 12800 Prague 2, Czech Republic b Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Moscow region, Russia c Imperial College London, Prince Consort Road, London SW7 2AZ, U.K. d Laboratoire Souterrain de Modane, 73500 Modane, France

E-mail: [email protected] A BSTRACT: A novel experimental approach to search for double electron capture (EC/EC) is discussed in this article. R&D for a new generation EC/EC spectrometer based on silicon pixel detectors (SPDs) has been conducted since 2009 for an upgrade of the TGV experiment. SPDs built on Timepix technology with a spectroscopic readout from each individual pixel are an effective tool to detect the 2νEC/EC signature of the two low energy X-rays hitting two separate pixels. The ability of SPDs to indentify α/β /γ particles and localize them precisely leads to effective background discrimination and thus considerable improvement of the signal-to-background ratio (S/B). A multi-SPD system, called a Silicon Pixel Telescope (SPT), is planned based on the experimental approach of the TGV calorimeter which measures thin foils of enriched EC/EC-isotope sandwiched between HPGe detectors working in coincidence mode. The sources of SPD internal background have been identified by measuring SPD radiopurity with a low-background HPGe detector as well as by long-term SPD background runs in the Modane underground laboratory (LSM, France), and results of these studies are presented. K EYWORDS : Particle identification methods; Particle tracking detectors; Spectrometers 1 Corresponding

author.

c 2011 IOP Publishing Ltd and SISSA

doi:10.1088/1748-0221/6/01/C01057

2011 JINST 6 C01057

Use of silicon pixel detectors in double electron capture experiments

Contents Double electron capture

1

2

TGV experimental approach

2

3

Silicon Pixel Telescope (SPT) 3.1 Instrumentation 3.2 MC simulations 3.3 Background measurements 3.4 Design optimization and discussion

2 2 3 5 6

4

Conclusions

8

1

Double electron capture

Double beta decay (β β ) has been studied both theoretically and experimentally for many years. Double beta decay with the emission of two neutrinos (2νβ β ) is allowed within the Standard Model (SM). Neutrinoless double beta decay (0νβ β ) is only allowed in models beyond the SM, such as Grand Unified Theories (GUTs). The existence of a 0νβ β -signal would indicate that the neutrino is a Majorana particle. The 0νβ β process is sensitive to the absolute neutrino mass in the meV region [1]. The experimental and theoretical efforts have mainly concentrated on β − β − modes due their greater rate and more easily measured experimental signatures, while attention to EC/EC, EC/β + and β + β + has increased only during the last few years (e.g. [2]). The EC/EC process occurs when two electrons from atomic shells are captured by the nucleus: 2e− + (Z, A) → (Z − 2, A)∗ + 2νe , and the daughter nucleus de-excites by the emission of low energy X-rays or Auger electrons. The experimental study of this process is very challenging due to the necessity to detect these very low energy particles (up to a few tens of keV). Compared to the β − β − decay mode, where measured half-lives are already at the level of 1024 -1025 years, the present experimental limits for the EC/EC mode are in the 1017 -1020 year range. e.g. the 2νEC/EC half-life limit for 180 W was determined very recently to be T1/2 > 6.6 × 1017 years [2]. The EC/EC signal has not been directly observed yet. The only experiment claiming the existence of EC/EC used an indirect geochemical method [3], which has yet to be confirmed by direct measurements. The direct detection of EC/EC decay would also be important for theoreticians calculating nuclear matrix elements for double beta decay.

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1

2

TGV experimental approach

3

Silicon Pixel Telescope (SPT)

We believe that a Silicon Pixel Telescope based on multiple SPDs can be a new forward step in 2νEC/EC measurements. Using an analogous concept to the detection cell of the TGV calorimeter, the SPT has additional advantages due to visualization of particle tracks. This will provide several improvements compared to the TGV: i) better geometrical acceptance (part of the total efficiency) by a factor of 2 due to the ability to detect and resolve both X-rays hitting the same SPD; ii) pattern recognition and α/β /γ particle discrimination, iii) localization of registered particles allowing the use of a geometrical cut, as will be explained later in section 3.2), and iv) operability at room temperature. Items ii) and iii) are important background rejection criteria. 3.1

Instrumentation

We used hybrid SPD Timepix technology [6] developed in CERN by the Medipix collaboration [7]. The detector consists of the silicon sensor (256 × 256 pixel matrix, 55 µm pitch, size 1.4 × 1.4 cm2 , thickness 300 µm) bump-bonded to the Timepix readout chip (see figure 1 left). It provides a good spatial resolution due to the small pixel size and a spectroscopic response in each pixel when operated in Time Over Threshold (TOT) mode. The detector capabilities and a broad spectrum of applications have been described e.g. in [8]. For data acquisition we use the Timepix detector connected via a USB readout interface (see figure 1 right). As one can see from the figures, the main potential drawback of SPDs for low background measurements is the presence of an amount of material with possibly higher content of radioactive impurities (board, chips, etc.) in close proximity to the silicon sensor. That is why tests of SPD internal backgrounds are subjects of crucial interest, and that is why they are the main focus of the present article.

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The experimental method proposed here builds on an approach, which was developed and successfully realized by the TGV (Telescope Germanium Vertical) collaboration, which started in the early 1990s. The second generation TGV II project started in 2004 and focused on double beta decay measurements of 106 Cd with the 2νEC/EC (g.s. to g.s.) search as a primary goal. The TGV II spectrometer is composed of 32 HPGe planar type detectors mounted in a common cryostat. The detectors are grouped in 16 detection cells, each of them consisting of a detector pair sandwiched tightly either side of a thin (40 µm) foil with detector-foil distance ∼ 1 mm. The tower assembly is enclosed in a common cryostat surrounded by several layers of passive shielding (20cm copper, 10cm lead, 16cm borated polyethylene). The experimental setup is located in the Modane underground laboratory (LSM, France) (see [4] for further details). 2νEC/EC 106 A sensitivity level T1/2 ( Cd, g.s. to g.s.) > 4.1 × 1020 y has been reached at the present moment by the TGV collaboration [5]; this is among the best world results in this field. Data analysis is still in progress. The usual ways to enlarge the exposure, increasing either mass or measuring time, have their limitations. So, the new experimental approaches, especially those with better efficiency and background rejection, are subjects of vital interest.

Figure 2. Examples of expected SSE (a) and DSE (b) signatures. 1 - SPD, 2 - sample foil, expected EC/EC X-rays are denoted as K1 and K2 .

3.2

MC simulations

The two X-rays emitted following 2νEC/EC (g.s. to g.s.) decay can be detected either by one SPD, called a Single Side Event pattern (SSE), or by two SPDs, called a Double Side Event pattern (DSE), as shown in figure 3. Only the DSE pattern is tagged by the TGV detection cell, and that is the reason for the improvement of geometrical acceptance by a factor of 2, claimed above, in the SPT relative to the TGV. Simulations were made using the GEANT 4 [9] code and the DECAY0 [10] generator. The SPT detection cell simulation included two thick SPDs (1.4 cm × 1.4 cm × 2 mm) with a 106 Cd foil (1.2 cm × 1.2 cm × 50 µm) inserted between them. The optimal thickness of the 106 Cd foil was calculated as 30-50 µm (see figure 4). The total 2νEC/EC detection efficiency in the SPT cell was studied for two cases of sourceSPD distance: i) 1 mm, called Far Geometry (FG), ii) 1 µm, called Close Geometry (CG). The

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Figure 1. Left — The hybrid SPD Timepix with silicon pixelated sensor 300 µm thick (1) and the Timepix readout ASIC (2). Right — The detector module connected to the USB readout interface (size scale is in cm).

Figure 4. Simulated distances between pixels hit by 2νEC/EC X-rays (left) and the pixels hit distances measured in a long-term background run (right) for SSE. The RMS values are 3.2 and 1.2 mm for FG and CG, respectively.

total efficiencies 12.7% (7.1% SSE + 5.6% DSE) and 16.7% (9.6% SSE + 7.1% DSE) have been obtained for FG and CG respectively compared with 5.5% efficiency in TGV II. One can further improve the signal-to-background (S/B) ratio using a localization pattern of X-ray hit clusters and applying a geometrical cut. The simulated patterns of 2νEC/EC hits were compared with the patterns accumulated in the background measurement (see figure 4). The mean distances between detected X-rays from 2νEC/EC simulated decays are 3.2 mm and 1.2 mm for FG and CG, respectively. Selection of two pixel hits at a distance 0.7-4 mm from each other rejects 75% of background events. The same selection criterion rejects 31.3% and 33.5% of signal 2νEC/EC events for FG and CG, respectively. As a result, the S/B ratio can be improved by factor of ∼ 8/3 for both geometries using a geometrical cut. Although SSE events are discussed here the geometrical cut could also be applied to the analysis of DSE as analogous localization patterns are expected for both types of events. The origin and nature of SSE contributing to the [0–0.7] mm cluster in figure 4 (right) is further studied in detail.

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Figure 3. Factors of merit (efficiencies) of 2νEC/EC detection in the SPT cell as a function of source foil thickness.

Figure 6. The distribution of cluster sizes (numbers of fired pixels) for [19–23] keV X-rays detected by a 300 µm thick Timepix detector with pixel size of 55 µm.

3.3

Background measurements

The SPD detector was operated in the LSM in order to determine the level of internal background. The detection module together with the readout interface was enclosed in an aluminium box and further shielded by 5 cm of lead. The experimental setup is shown in figure 5. A background measurement with one pixel detector was performed as a first step focusing on evaluation of SSE signals. The background spectra have been studied in the [19–23] keV energy range where two cadmium ∼21 keV KX-rays following the 106 Cd 2νEC/EC signal are expected. From test and calibration data, the distribution of the number of pixels (cluster size) in the SPD hit by [19–23] keV X-rays is shown in Figure 6. As one can see on the plot, most of the X-rays fired 6 3 pixels; 3 pixel clusters add only 3% and the contribution from clusters with high pixel multiplicity is negligible.

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Figure 5. Experimental setup in the LSM — the detection module and the read-out interface enclosed in an aluminium box (left) and the box installed inside the lead shielding (right).

The long-term background data are recorded continuously frame by frame with one second frame exposure. Frames containing two fired 1-3 pixel clusters were selected from the data. The spectrum of these events, which can potentially mimic 2νEC/EC SSE, is shown in figure 7. After 20 days of exposure and application of a geometrical cut we have only 15 SSE candidates, formed by random coincidences below 50 keV, and still no event in the [19–23] keV region of interest (see figure 7, right). 3.4

Design optimization and discussion

Maximal reduction of background is the main goal for all low level counting applications. While external background (cosmic rays, gammas, neutrons) can be efficiently suppressed by active and passive shielding, as well as by an underground setup location, internal background reduction becomes the crucial factor for experiments. This is why the setup geometry (detectors and their accessories, construction materials) should be optimized in order to minimize the level of intrinsic radioactivity. This goal can be reached by two methods: i) minimization of the amount of material close to the detection part of the setup, ii) careful selection of radiopure construction materials using screening by dedicated low-background HPGe detectors. As was already emphasized above, the intrinsic background is quite an important issue for SPDs, which contain a considerable amount of material close to the silicon sensor (see figure 1 and related discussion). Individual components of the SPD unit were screened using a low-background HPGe detector located in LSM. Table 1 summarizes the selected natural radioactive contaminations (U, Th, and K) in materials used for bump-bonding (In+Sn), the naked readout chip, empty printed circuit board (PCB) covered by soldering mask, and the whole detection module (as seen in figure 1, right). The main source of radioactive impurities is the PCB. We have now collected a set of samples of materials used for PCB production including some PCB components (bare PCB materials, copper sheets, insulating layers). To reduce the intrinsic background we plan to choose the optimal PCB material and re-design the chipboard to minimize

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Figure 7. Total energy spectrum of SSE candidates collected by the SPD during 20 days of background measurements in the LSM (left) and corresponding scatter plot of the low-energy 0-50 keV region with the region of interest highlighted in red (right). E1 and E2 are the energies deposited in the two separate clusters within one frame.

Table 1. Radioactive impurities in various components of the SPD detection module. Numbers are normalized to mBq per detection module.

228 Th 234 Th 40 K ∗

Bonding (In+Sn) < 10−8 < 10−6 < 10−7

Readout chip < 0.2 < 0.9 < 6.2

Empty PCB 263 ± 8 168 ± 11 < 25

Detection module∗ 187 ± 11 123 ± 10 117 ± 28

Without the components measured separately, thus, not the sum of the other columns

its size and number of electronic devices which have necessarily to be located close to the readout chip. Based on the successful experience with the SPD stacking achieved in [11] we would like to test an SPD sandwich with a source foil used as a common electrode (see figure 8, left) realized as a CG. Another possible SPT unit is back-to-back SPD sensors coupled to the same PCB. The important advantages of this “2 in 1” technology are: i) factor ∼2 reduction of intrinsic background due to half the amount of material near the sensors, ii) freedom of source foil preparation (can be non-metal, packed, etc.). To stack SPDs without any PCB supports is also an option to consider. However, the ability to fabricate such a scheme should be verified. Currently we use a USB1.0 readout interface providing a maximum frame reading rate of 5 frames per second. This doesn’t allow us to further decrease our actual frame exposure (one second) without a sizeable increase of dead time. The new generation of readout, USB2.0, is faster by a factor of 10, and is now available. It will allow us to reduce the frame duration down to 100 ms and thus reduce background from random coincidences. Further improvement could be reached using SPD with the future generation ASIC called Timepix 2, which will provide both the spectroscopic information and a time stamp of each activated pixel in the frame. This would eliminate the necessity to define the time coincidence by the frame exposure time. As a first stage of the SPT R&D we propose a SPT prototype composed of 7 SPD pairs of Timepix quad detectors with 7 sandwiched source foils (2.6 × 2.6 cm2 x 50 µm) of 106 Cd (98.4% enrichment) with a total mass of 2 g containing 1.63 × 1021 atoms of 106 Cd. Assuming zero background and 4 year exposure, this small SPT prototype would reach a sensitivity to the 2νEC/EC decay of 106 Cd at the level of 1021 years.

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Figure 8. SPT detection cell concepts with source foil used as common electrode (left) realized in [11] and “2 in 1” technology with back-to-back SPD sensors coupled to the common PCB (right).

4

Conclusions

Acknowledgments This project is supported by the grants No. MSM6840770029, LC305, and LA07050. We would also like to acknowledge the staff of the LSM for their support during all low-background measurements and the Prague Medipix group for sharing their knowledge and experience with the pixel detectors.

References [1] A. Faessler, Theoretical status of the double beta decay, J. Phys. Conf. Ser. 203 (2010) 012058. [2] R. Bernabei et al.: Search for double beta decay of zinc and tungsten with low background ZnWO4 crystal scintillators, J. Phys. Conf. Ser. 202 (2010) 012038. [3] A.P.Meshik et al., Weak decay of 130 Ba and 132 Ba: geochemical measurements, Phys. Rev. C 64 (2001) 035205. [4] P. Beneˇs et al., The low background spectrometer TGV II for double beta decay measurements, Nucl. Instrum. Meth. A 569 (2006) 737. [5] N.I. Rukhadze et al., Search for double beta decay of 106 Cd in the TGV-2 experiment, J. Phys. Conf. Ser. 203 (2010) 012072. [6] X. Llopart et al., Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements, Nucl. Instrum. Meth. A 581 (2007) 485. [7] MEDIPIX, http://medipix.web.cern.ch/MEDIPIX. [8] J. Jak`ubek, Semiconductor Pixel detectors and their applications in life sciences, 2009 JINST 4 P03013.

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The SPD based on Timepix technology has been proposed for use for low-background double beta decay experiments. Besides the energy of the detected particle, the localization and particle identification is also provided by the SPD. The tracking capabilities offer promising background discrimination and an increase in efficiency. This can significantly improve the S/B ratio and also help in understanding the composition of the background spectrum in the whole energy range. According to MC simulation, application of a geometrical cut restricting particle localization can provide an additional gain factor ∼8/3 for the S/B ratio. The SPD intrinsic background has been measured in an underground environment both with a long-term SPD background run and by screening the SPD components. The main sources of background have been identified and material selection for SPD construction is in progress. Further improvements in SPD technology have been discussed, such as an increase of the frame reading rate, which will provide further discrimination of random coincidence background. Finally, the design of the SPT prototype has been discussed. Despite the small size and mass of the isotope studied (2g of 106 Cd), this prototype has the potential to reach a world best sensitivity to 2νEC/EC thanks to the novel experimental approach that has been proposed and successfully verified in the current work.

[9] J. Allison et al., Geant4 developments and applications, IEEE Trans. Nucl. Sci. 53 (2006) 270. [10] O.A. Ponkratenko, V.I. Tretyak and Yu.G. Zdesenko, Event generator DECAY 4 for simulating double-beta processes and decays of radioactive nuclei, Phys. Atom. Nucl. 63 (2000) 1282 [nucl-ex/0104018]. [11] E.H.M. Heijne et al., Vectors and submicron precision: redundancy and 3D stacking in silicon pixel detectors, 2010 JINST 5 C06004.

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