A cryogenic detector for 222 Rn

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Noreen Nelson
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1 A cryogenic detector for Rn M. Wojcik a, G. Zuzel b a) Institute of Physics, Jagellonian University, Cracow, Poland b) Max Planck Institute for Nuclear Physics, Heidelberg, Germany

2 1. Selected detection techniques of Rn Pre-concentration and counting using GALLEX/GNO low-level proportional counters - highly sensitive measurements of Rn in nitrogen and argon (liquid nitrogen/liquid argon) - detection limit: ~0.5 µbq/m 3 (Appl. Rad. Isot. 52 (2000) 691) Electrostating chambers - high sensitive online Rn monitoring (clean rooms, clean benches etc.) - detection limit mbq/m 3 (NIM A460 (2001) 272) Scintillator Lucas cells - online Rn monitoring (laboratories, air etc.) - insensitive to gas contaminations and easy to use detectors - detection limit: ~ 0.5 Bq/m 3 (NIM A345 (1994) 351)

3 2. The design of the cryogenic detector GAS OUTLET TO THE PREAMPFILER GAS INELT VACCUM 12 mm φ 50 STAINLESS STEEL LN 2 /LAr φ 40 LN 2 /LAr VACCUM DETECTOR CHAMBER COLD PLATE Detector : ORTEC ULTRA diode, 50 mm diameter Cold plate : 40 mm diameter, 12 mm distance from the diode Cooling : Liquid nitrogen Volume : 65 L Material : Electropolished stainless steel SEMICONDUCTOR DETECTOR; φ 50 mm

4 3.1 Background ORTEC diode (impurities + cosmic rays) A D = (0.93 ± 0.31) cpd Emanation of Rn (detector components, welds etc.) A E = (23.6 ± 3.5) cpd Total A B = (24.5 ± 3.5) cpd

5 3.1 Background Rn daughters deposition GAS OUTLET TO THE PREAMPFILER GAS INELT VACCUM 12 mm φ 50 STAINLESS STEEL LN 2 /LAr φ 40 LN 2 /LAr VACCUM DETECTOR CHAMBER COLD PLATE SEMICONDUCTOR DETECTOR; φ 50 mm

6 3.1 Background after many test with high Rn activities p ~ 0 mbar p = 1300 mbar Vacuum in the detector chamber bar N 2 in the detector chamber Counts Po on the detector and on cold plate Counts Po on the detector 210 Po on the cold plate Energy [MeV] Energy [MeV] A D = (174 ± 6) cpd A D = (57.6 ± 2.6) cpd ~1/3 of the 210 Po is deposited on the detector: sputtering + low temperature collection

7 3.2 Absolute detection efficiency at low pressure (~ 2 mbar) LN 2 He/N 2 Rn source Pump

8 3.2 Absolute detection efficiency at low pressure (~ 2 mbar) Nitrogen as a carrier gas Helium as a carrier gas ε N = (31.2 ± 0.9) % Average value ε He = (31.7 ± 0.9) % ε = (31.5 ± 0.6) %

9 3.3 Minimum Detectable Activity (MDA) A 0 0 min = λe λt s Δt 2 σ 2 AB Δ ta B δ 2 δ ε2 2ε 1 e λδt δ 2 δ ε2 A B background (total) σ AB standard deviation of A B ε total detection efficiency δ ε standard deviation of ε δ assumed measurement accuracy t s time between Rn filling and measurement start Δt measurement time λ Rn decay constant

10 3.3 Minimum Detectable Activity (MDA) - continued A B = (24.5 ± 3.5) cpd ε = (31.5 ± 0.9) % t s = 1.5 h δ = 30 % A min = 0.8 mbq (12 mbq/m 3 ) A 1000 = 1.3 mbq (21 mbq/m 3 )

11 3.4 Detection efficiency at higher pressures Nitrogen as a carrier gas 31,3 Detection efficiency (%) 25,0 18,8 12,5 6,3 0 mbar 15 m bar 50 m bar 100 mbar 210 mbar 0, Collection time (min)

12 3.4 Detection efficiency at higher pressures - continued Helium as a carrier gas 31,3 Detectioneficiency(%) 25,0 18,8 12,5 6,3 0mbar 130mbar 400mbar t =189min, pumpstarted 0, Collectiontime(min)

13 3.5 Energy spectrum Rn E α = 5.5 MeV 218 Po E α = 6.0 MeV 214 Po E α = 7.7 MeV Counts ,0 5,5 6,0 6,5 7,0 7,5 8,0 Energy [MeV] Energy resolution for Rn: 105 kev (FWHM)

14 4. The prototype of the cryogenic detector works as expected Reached sensitivity is acceptable (12 mbq/m 3 ) however the goal for a target detector is 1 mbq/m 3 Improvement possibilities: - background reduction careful construction and selection of materials use of an ultra-low background alpha detector - increase of the detection efficiency use of an alpha detector able to work at LN 2 temperature (smaller distances between the diode and the cold plate possible) use of liquid argon for cooling (higher Rn collection efficiency for N 2 ) - increase of the active volume of the detector up to 1 m 3 Cryogenic detector has a possibility to measure others Rn isotopes ( 219 Rn/ 220 Rn) Rn emanation tests from solids can also be performed

arxiv: v1 [physics.ins-det] 20 Dec 2017

arxiv: v1 [physics.ins-det] 20 Dec 2017 Prepared for submission to JINST LIDINE 2017: LIght Detection In Noble Elements 22-24 September 2017 SLAC National Accelerator Laboratory arxiv:1712.07471v1 [physics.ins-det] 20 Dec 2017 Radon background