1. DEVELOPMENT OF PHOTODETECTION SYSTEM BASED ON MULTIPIXEL AVALANCHE GEIGER PHOTODIODES WITH WLS FOR LXE LOW-BACKGROUND DETECTORS D.Yu. Akimov, A.V. Akindinov, I.S. Alexandrov, A.A. Burenkov, M.V. Danilov, A.G. Kovalenko, V.N. Stekhanov University Department, State Scientific Centre of Russian Federation Institute for Theoretical and Experimental Physics (ITEP), 25 Bolshaya Cheremushkinskaya str., Moscow, Russia

2. D ARK M ATTER EXPERIMENTS Xenon10,100 at Gran Sasso ZEPLIN III at Boulby mine XMASS at Kamioka LUX at SUSEL All detectors utilizes PMTs for detection of the LXe VUV light. At present, PMTs (even low-background) are the most radioactive elements of detector. Search for a replacement of PMTs for the future detectors is actual.

3. T HE AIM OF THIS STUDY The possibility of the use of MPGP (Multi Pixel Geiger Photodiode ) for detection of the LXe luminescence light together with a wavelength shifter To estimate the global PDE of such a system The aim of the current work is to demonstrate experimentally:

4. M ULTIPIXEL AVALANCHE G EIGER PHOTODIODE -V oper Readout Structure of 1 cellScheme of MRS APD View of matrix cells

5. M ULTIPIXEL AVALANCHE G EIGER PHOTODIODE CPTA “green” – blue, green, IRCPTA “blue” – blue, green, IR Typical PDE for CPTA 2x2 mm 2 Charge: Q = e*N cell *C*(U - U br ) Multiplication: M = C*(U - U br ) Photon Detection Efficiency: PDE= Q.E.*R G *ε geom

6. LXe emission, p-terphenyl absorption and emission spectra and PDE of “blue” CPTA photodiode W AVELENGTH SHIFTER

7. Scheme of measurements. a) The p-terphenyl deposited layer between two optical windows, b) p-terphenyl is coated by a poly-para-xylylene film. 1 – PMT Hamamatsu R7200, 2 – α- source 241 Am, 3 - optical window (sapphire), 4 – p-terphenyl, 5 – MRS APD, 6 – optical window, 7 – Ar gas between the window, 8 - poly-para-xylylene film. LXe 3 5 8 1 2 3 4 6 5 7 1 2 4 a) b) E XPERIMENTAL SETUP Photo of assembled constructions (Ar gas between the windows) 2 x 2 mm, 1584 pixels

8. E XPERIMENTAL SETUP Scheme of measurements Photo multiplayer MRS APD k

9. Pulse area, V·ns CPTA “blue” PMT E XPERIMENTAL SPECTRUMS noise α peak 1 cell 2 cells pedestal

10. R ESULTS & PDE CALCULATION 84 cells

11. R ESULTS & PDE CALCULATION WLS Sapphire Ω1Ω1 Ω2Ω2 N 0 =E α /w, were E α = 5.486 MeV is the energy of alpha particle w = 16.3 ± 0.3 eV f - total attenuation coefficient of the light in the windows η - efficiency of transformation of the VUV light to the visible region ξ - photo detection efficiency of the photodetector in the wave range of WLS emission MRS APD

12. R ESULTS & PDE CALCULATION Series of measuremts N cells µfΩPDE 0,% Figure 1a)24±0.50.740.681.35*10 -3 9.7 ±1.2 Figure 1b)72±1.50.170.751.99*10 -2 8.4 ±1.1 P. Benetti, et al., Nucl. Instr. Meth. A505, 89 (2003). For a blue sensitive PMT (QE ≈20%) with WLS.

13. MGPD matrix WLS GEM/THGEM LXe surface Grid LXe e-e- Electroluminescence, detected by MGPD Electroluminescence, detected by PMTs Gaseous Xe Fig. 2. Possible design of high precision detection system for two-phase noble gas detectors with THGEM + WLS +MGPD. F UTURE PLANS

14. C ONCLUSION The capability of operation of WLS in LXe with protection has been shown. Poly-para-xylylene film is good protection items in LXe. The results of experiment allow one to build a detection system based on WLS. Type of construction WLS PDE, % p-terphenyl is sealed between two optical windows 9.7±1.2 p-terphenyl is coated by a poly-para- xylylene film. 8.4±1.1 One can build a system with low radioactive background for a two-phase detector sensitive to very low ionization.