1. 1 Two-phase Ar avalanche detectors based on GEMs A. Bondar, A. Buzulutskov, A. Grebenuk, D. Pavlyuchenko, Y. Tikhonov Budker Institute of Nuclear Physics, Novosibirsk Outline - Motivation: coherent neutrino-nucleus scattering, dark matter search, solar neutrino detection - Two-phase Ar avalanche detector without CsI PC - Two-phase Ar avalanche detector with CsI PC - Summary

2. 2 Motivation: cryogenic detectors for coherent neutrino scattering, dark matter and solar neutrino detection Two-phase He or Ne detectors for solar neutrino detection using charge readout Columbia Univ (Nevis Lab) & BNL, www.nevis.columbia.edu/~ebubble Two-phase Ar detector for coherent neutrino-nucleus scattering Hagmann & Bernstein, IEEE Trans. Nucl. Sci. 51(2004)2151; Two-phase Ar detector for dark matter search WARP Collaboration [P. Benetti et al., Astro- particle physics, 28(6)(2008) 495] Two-phase Ar detectors for dark matter search using thick GEM readout A. Rubbia et al., J.Phys.Conf.Ser.39(2006)129

3. 3 Principles of two-phase avalanche detectors based on GEMs - Primary ionization (and scintillation) signal is weak: of the order of 1, 10, 100 and 500 keV for coherent neutrino, dark matter, solar neutrino and PET respectively  Signal amplification, namely electron avalanching in pure noble gases at cryogenic temperatures is needed - Detection of both ionization and scintillation signals in liquid might be desirable, the latter to provide fast signal coincidences in PET and to reject background in neutrino and dark matter detection The concept of two-phase (liquid-gas) or high pressure cryogenic avalanche detector using multi-GEM multiplier, with CsI photocathode on top of first GEM 1. Buzulutskov et al., First results from cryogenic avalanche detectors based on GEMs, IEEE Trans. Nucl. Sci. 50(2003)2491 2. Bondar et al., Cryogenic avalanche detectors based on GEMs, NIM A 524(2004)130. 3. Bondar et al., Further studies of two-phase Kr detectors based on GEMs, NIM A 548(2005)439. 4. Buzulutskov et al., GEM operation in He and Ne at low T, NIM A 548(2005)487. 5. Bondar et al., Two-phase Ar and Xe avalanche detectors based on GEMs, NIM A 556(2006)237 6. Bondar et al., A two-phase Ar avalanche detector operated in a single electron counting mode, NIM A 574(2007)493 7. Bondar et al., First result of the two-phase argon avalanche detector performance with CsI photocathode, NIM A 581(2007) 241

4. 4 Two-phase avalanche detectors based on GEMs: previous results Unique advantage of GEMs and other hole-type structures: high gain operation in noble gases -3GEM operation in noble gases at high pressures at room T Budker Inst: NIM A 493(2002)8; 494(2002)148 Coimbra & Weizmann Inst: NIM A 535(2004)341 Stable 3GEM operation in two-phase mode -In Ar: rather high gains are reached, of the order of 10 4, -In Kr and Xe: moderate gains are reached, about 10 3 and 200 respectively Bondar et al., Two-phase Ar and Xe avalanche detectors based on GEMs, NIM A 556(2006)237 Successful operation of the two-phase Ar avalanche detector in single electron counting mode -Pulse-height spectra for single and 1.4 electron at gain 4·10 4, in 3GEM. -Single and two electron events would be well distinguished by spectra slopes Bondar et al, NIM A 574(2007) 493

5. 5 Two-phase Ar avalanche detector: experimental setup - Developed at Budker Institute - 2.5 l cryogenic chamber - Operated in Ar with liquid thickness 10 mm - Liquid purity: electron lifetime larger than 3  s ( drift length 1cm) - 3GEM ( active area 3  3cm 2 ) assembly inside - Irradiated with pulsed X-rays,  -particles,  - rays and neutrons Cathode gap capacitance as a function of pressure in Ar during cooling- heating procedures Two-phase mode Gaseous mode

6. 6 Two-phase Ar avalanche detector: experimental setup 3GEM 2.5 liter cryogenic chamber

7. 7 Two-phase Ar avalanche detector: emission and gain characteristics Electron emission through liquid/gas interface Gain characteristics Ionization source: pulsed X-ray tube - Maximum reached gain 14·10 3 - Gain characteristic is well reproducible – Anode pulse-height as a function of electric field in the liquid induced by pulsed X-ray -Extraction is saturated at lower fields compare to Kr and Xe

8. 8 Two-phase Ar avalanche detector: energy spectra for different radioactive sources X-rays from 241 Am 511keV  -rays from 22 Na  -particles from 90 Sr - 60 keV X-ray peak from 241 Am was used to calibrate energy scale - Only a fraction of  -particle energy was deposited in cathode gap due to 5mm dead zone between chamber bottom and cathode - (for > 190keV) = 600keV 511keV  -ray photoelectric peak Compton edge

9. 9 Two-phase Ar avalanche detector: purity effect and energy resolution for 241 Am 60 keV  X-ray peak - Two-phase Ar, 3 GEM, 60 keV  X-rays from 241 Am, gain ~ 4000 - Effect of extraction field is well pronounced - Energy resolution is 17% LAr purity: experiment LAr purity: Monte Carlo Energy resolution 60 keV X-rays - Several purification cycles are enough to achieve electron lifetime in liquid Ar larger than 3  s ( 1cm ) - Shape and position of 60 keV  X-ray peak depends on liquid purity

10. 10 Two-phase Ar avalanche detector: detection events with small energy deposition Pulse-height spectra at gain ~ 4500 for: - Single electrons - 252 Cf neutrons and  -rays - 241 Am 60 keV X-rays Nuclear recoils due to neutron-nucleus elastic scattering Energy spectra at gain ~ 4500 - Detector is irradiated with neutrons and  -rays from 252 Cf, 22 Na through the 2.4cm Pb shield -60 keV X-ray peak from 241 Am was used to calibrate energy scale

11. 11 Two-phase Ar avalanche detector: avalanching stability - Relatively stable operation 3GEM during 20 hours in two-phase Ar at gain ~1500-4500 - Correlation between pressure and peak position (gain) is clearly seen Operation of two-phase Ar avalanche detector is rather stable

12. 12 Two-phase Ar avalanche detector with CsI PC: experimental setup - GEM1 with CsI photocathode (PC) - QE of CsI PC = 5% at 185nm -The scintillation-induced photoelectrons released at the CsI photocathode are collected into the GEM holes and then multiplied, producing a so-called “S1” signal. The ionization-induced electrons are detected after some time, needed for drifting in the liquid and gas gaps and for emission through the liquid-gas interface; they produce a “S2” signal, delayed with respect to S1. S2 S1 Gain ~ 5400 E(LAr)=0.25kV/cm Shaping time 0.5  s - Anode signals induced by  -particles from 90 Sr in two-phase Ar avalanche detector with CsI photocathode

13. 13 Two-phase Ar avalanche detector with CsI PC:  -particles from 90 Sr Peak delay spectra of S1 signal with respect to S2 signal for different drift fields in LAr - The signals are induced by 90 Sr  -particles in LAr, at gain ~ 2500, shaping time 0.5  s - Shaded spectrum corresponds to low drift field in LAr - Time delay between S1 and S2 depends on the drift field and is larger for lower fields -This confirms that S1 is induced by primary scintillation signal Anode signal, averaged over ~ 100 events of a S1+S2 type, at different drift fields in LAr - Observation both S1 and S2 signals at lower drift field 0.25kV/cm and small shaping time 0.5  s -Such conditions were necessary to have enough time delay between S1 and S2; otherwise they would overlap

14. 14 Two-phase Ar avalanche detector with CsI PC:  -particles from 90 Sr Distribution of events in the plane S2 vs. S1 amplitudes - At gain ~ 2500, drift field E(LAr) = 0.25kV/cm, shaping time 0.5  s - Most events are of the “S1+S2” type where S1 & S2 are observed and correlated to each other

15. 15 Two-phase Ar avalanche detector with CsI PC:  -particles from 90 Sr Amplitude spectra of S1 and S2 - Top scale is expressed in initial charge prior to multiplication, i.e. p.e. for S1 and e. for S2 -S1 & S2 spectrums have a single peak corresponding to high energy component of the  -particle spectrum - N pe in S1 peak is about 30. This corresponds to the detection of scintillation light due to a deposited energy of about 600keV. - Photon detection efficiency = N PE /N PH ~ 10 -3 accounting for the scintillation light yield in LAr, of 40 photons/keV

16. 16 Two-phase Ar avalanche detector with CsI PC: X-rays from 241 Am Anode signals induced by 241 Am X-rays in two-phase Ar avalanche detector - Shaping time 0.5  s - Gain ~ 6600, E(LAr) = 1.71kV/cm - S1 does not seen Single event Averaged over 100 event - Gain ~ 14000, E(LAr) =0.37kV/cm - S1 is seen - Amplitude ~ 2 p.e. S1 S2

17. 17 Two-phase Ar avalanche detector with CsI PC: 511keV  -rays from 22 Na Anode signals induced by 22 Na 511 keV  -rays - Scintillation BGO counter was used to provide coincidence between the two  -quanta -Averaged over 100 events, shaping time 0.5  s - E(LAr)=0.25kV/cm - S1 is seen S1 S2 Peak delay spectrum of S1 signal with respect to trigger signal from BGO counter - Gain ~ 6600, E(LAr) = 0.25kV/cm Trigger signal from BGO counter

18. 18 Two-phase Ar avalanche detector: thick GEM versus thin GEM RETHGEM THGEM (G10) THGEM(KEVLAR) Thick Gas Electron Multiplier (THGEM) produced by the Weizmann Institute, see talk of A. Breskin Resistive Electrode Thick GEM (RETHGEM) produced by the CERN work- shop - Thickness 0.4mm 0.4mm 0.4mm - Hole diameter 0.5mm 0.3mm 0.3mm - Pitch 0.9mm 0.7mm 0.75mm - Both THGEMs have a rim of 0.1 mm etched around the mechanically drilled holes. In this part of work we collaborate with Weizmann Institute of Science, Israel Thanks to Amos Breskin

19. 19 Two-phase Ar avalanche detector: thick GEM versus thin GEM Gain characteristics Ionization source: pulsed X-ray tube - 2THGEM in two-phase Ar have maximum reached gain 6000 for KEVLAR and 3000 for G10 at voltages four times higher than typical operation voltage of thin GEM. - There is no gain in double RETHGEM at standard condition of two-phase operation. But RETHGEM have small gain of the order 10 in non-equilibrium two-phase operation of cryogenic chamber. GEM vs. THGEM RETHGEM

20. 20 Two-phase Ar avalanche detector: thick GEM versus thin GEM Pulse-height spectra of 241 Am X-rays THGEM(KEVLAR) THGEM(G10) - Energy resolution for 60keV X-rays is the same as for thin GEM 18% - Spectrum is deteriorated due to charging up THGEM(G10) could replace thin GEM

21. 21 Summary Two-phase Ar avalanche detector without CsI PC: - Wide dynamical range of operation (detecting single electrons, gamma-rays and neutrons), with good energy resolution - Stable operation for at least one day - Efficient detection of events with small energy deposition - 2ThGEM(G10) operate in two-phase Ar with maximum gain 3000 Two-phase Ar avalanche detector with CsI PC: - Stable operation of CsI photocathode for one month in the two-phase Ar avalanche detector - Detection of both primary scintillation and ionization signals, produced by  - particles, X,  -rays in liquid Ar in the two-phase avalanche mode. The results obtained are relevant in the field of lowbackground detectors sensitive to nuclear recoils, such as those for coherent neutrino-nucleus scattering and dark matter search experiments.