1 A two-phase Ar avalanche detector with CsI photocathode: first results A. Bondar, A. Buzulutskov, A. Grebenuk, D. Pavlyuchenko, R. Snopkov, 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 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, Two-phase or high-pressure Ar or Xe detectors for coherent neutrino-nucleus scattering Hagmann & Bernstein, IEEE Trans. Nucl. Sci. 51(2004)2151; Barbeau et al., IEEE Trans. Nucl. Sci. 50(2003)1285 Two-phase Ar detector for dark matter search WARP Collaboration [P. Benetti et al. Eprint astro- ph/070286] Two-phase Ar detectors for dark matter search using thick GEM readout Rubbia et al., Eprint hep- ph/

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) Bondar et al., Cryogenic avalanche detectors based on GEMs, NIM A 524(2004) Bondar et al., Further studies of two-phase Kr detectors based on GEMs, NIM A 548(2005) Buzulutskov et al., GEM operation in He and Ne at low T, NIM A 548(2005) Bondar et al., Two-phase Ar and Xe avalanche detectors based on GEMs, NIM A 556(2006) Bondar et al., A two-phase Ar avalanche detector operated in a single electron counting mode, Eprint , NIM A, in press

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 A. Bondar et al, Eprint , NIM A, in press

5 Two-phase Ar avalanche detector: experimental setup 2.5 liter cryogenic chamber General view - Operated in Ar with liquid thickness 10 mm - Liquid purity: electron lifetime larger than 3  s (1cm) - 3GEM ( active area 3  3cm 2 ) assembly inside

6 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· 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

7 Two-phase Ar avalanche detector: pulse-height spectra Pulse-height spectra for: - At gain ~ Single electrons Cf neutrons and  -rays Am 60 keV  -rays - Energy resolution for 60keV  -ray peak is 17%

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

9 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.

10 Two-phase Ar avalanche detector with CsI PC: energy spectra for different radioactive sources  -rays from 241 Am 511keV  -rays from 22 Na  -particles from 90 Sr - 60 keV  -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

11 Two-phase Ar avalanche detector with CsI PC:  -particles from 90 Sr -Trigger: threshold on S2. -The signal waveform analysis was carried out using TDS5032B digital oscilloscope: up to 5000 waveforms per measurement run could be stored in oscilloscope memory for offline processing. - In offline data analysis was used simple algorithm for S1 recognition: finding maximum (peak) at certain time interval prior to S2. - S1 and S2 amplitude: calculating area under the curve at appropriate time intervals. Scintillation signal (S1) Ionization signal (S2) - Anode signals induced by 90 Sr  -particles in two-phase Ar avalanche detector with CsI photocathode at a gain ~ 5400, drift field E(LAr)=0.25kV/cm and shaping time 0.5  s. The scintillation signal (S1), prior to ionization signal (S2), is seen.

12 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

13 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

14 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 ~ accounting for the scintillation light yield in LAr, of 40 photons/keV

15 Two-phase Ar avalanche detector with CsI PC:  -rays from 241 Am Anode signals induced by 241 Am  -rays in two-phase Ar avalanche detector - Shaping time 0.5  s - Gain ~ 14000, E(LAr) =0.37kV/cm - S1 is seen - Amplitude ~ 2 p.e. Single event Averaged over 100 event S1 S2

16 Two-phase Ar avalanche detector with CsI PC:  -rays from 241 Am Amplitude spectra of S1 and S2 signals - Gain ~ 14000, E(LAr) = 0.37kV/cm - N E in S2 peak is about 120 Pulse-height spectrum for S1 -The spectrum is exponential that is typical for gas avalanche detector when counting a few electrons - ~ 2.1 p.e.

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 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 Two-phase Ar avalanche detector with CsI PC: - Stable operation of CsI photocathode for one month in the two-phase Ar avalanche detector was shown. - Successful detection of both primary scintillation and ionization signals, produced by  -particles,  -rays in liquid Ar, has for the first time been demonstrated in the two-phase avalanche mode. The amplitude of the scintillation signal was estimated to be about 30 photoelectrons per 600 keV of deposited energy. 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.

19 Two-phase Ar avalanche detector: experimental setup - Developed at Budker Institute l cryogenic chamber - Operated in Ar with liquid thickness 10 mm - Liquid purity: electron lifetime larger than 3  s (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

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

21 Two-phase Ar avalanche detector with CsI PC:  -rays from 241 Am Anode signals induced by 241 Am  -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

22 Two-phase Ar avalanche detector with CsI PC: pulsed X-rays Gain characteristics 3GEM, Pulsed X-ray - Gain could exceed Scintillation signal from LAr was measured at reversed drift field - Slopes of the gain curves for the ionization and scintillation signal are the same

23 Two-phase Ar avalanche detector with CsI PC:  -rays from 241 Am - Peak delay spectra of S1 with respect to S2 for E(LAr) = 0.37kV/cm, at gain ~ Distribution of events in the plane S2/S1 vs. S1 amplitudes

24 WARP TPAr prototype

25 Two-phase Ar avalanche detector with CsI PC:  -particles from 90 Sr Eff(CsI/GEM1) = Npe(S1peak)/(Nph/keV * ) = E(LAr) = 0.25 kV/cm

26 Two-phase Ar avalanche detector with CsI PC:  -rays from 241 Am How many photoelectrons are there in S1 signal ?

27 Two-phase Ar avalanche detector with CsI PC:  -particles from 90 Sr Energy spectrum of 90 Sr  -particles - Only a fraction of  -particle energy was deposited in cathode gap due to 5mm dead zone between chamber bottom and cathode - (for > 190keV) = 620keV Amplitude spectrum of scintillation signals from 90 Sr  -particles at reversed drift field - (A > 0.06V) correspond to 25 p.e.

28 Two-phase Ar avalanche detector with CsI PC:  -particles from 90 Sr Distribution of events in the plane S2/S1 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 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