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
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 0.1-1 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)
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
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
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
3.1 Background after many test with high Rn activities p ~ 0 mbar p = 1300 mbar 1000 800 Vacuum in the detector chamber 500 400 1.3 bar N 2 in the detector chamber Counts 600 400 210 Po on the detector and on cold plate Counts 300 200 210 Po on the detector 210 Po on the cold plate 200 100 0 2 3 4 5 6 7 8 0 2 3 4 5 6 7 8 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
3.2 Absolute detection efficiency at low pressure (~ 2 mbar) LN 2 He/N 2 Rn source Pump
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) %
3.3 Minimum Detectable Activity (MDA) A 0 0 min = λe λt s 1 1 4 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
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 )
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 0,0 0 20 40 60 80 100 120 140 160 Collection time (min)
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 0,0 0 50 100 150 200 250 Collectiontime(min)
3.5 Energy spectrum 600 500 Rn E α = 5.5 MeV 218 Po E α = 6.0 MeV 214 Po E α = 7.7 MeV Counts 400 300 200 100 5,0 5,5 6,0 6,5 7,0 7,5 8,0 Energy [MeV] Energy resolution for Rn: 105 kev (FWHM)
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