Optical Time Projection Chamber for radon and thoron detection Wojciech Dominik Zenon Janas Krzysztof Miernik Marek Pfützner Institute of Experimental Physics Warsaw University

Optical Time Projection Chamber PMT CCD visible light 1 ms/cm Gate 1 atm. gas: 49 % He 49 % Ar 1 % N2 1 % CH4 M. Ćwiok et al., IEEE TNS, 52 (2005) 2895

OTPC at Warsaw Materials used: Chamber active volume: 20 x 20 x 15 cm3 Stesalit fibreglass PCB plates Pyrex optical window

Registration of a particle CCD PMT

Principle of 3D track reconstruction z a vdt2 vdt1 Lxy Total track length: Inclination angle: vd– electron drift velocity  10 mm/ms

What one can measure with OTPC ? length and position on XY plane (from camera picture) length of projection on Z axis (from the length of the PMT signal) no Z coordinate energy (from the total track length) charge of the particle (from the energy loss) time and position correlation between succesive a-decays - no sensitivity for electrons

Example: 214Po a-decay CCD Dt= 5 ms PMT Lxy=115 mm

Example: energy loss along the particle track projection of CCD picture fit of Bethe-Bloch formula

Example: correlation between succesive a-decays T= 0 min a 222Rn T= 3 min a 218Po - tracks originate from the same XY position - track lengths compatible with the 222Rn and 218Po a energies

14 tracks starting from the center OTPC background measurement - 5 hours measurement - circles mark the beginning of the tracks Total of 260 tracks - most of them start from the walls 14 tracks starting from the center Y - position X - position

Tracks starting from the central region (16 x 16 x 15 cm3 gas volume ?) Background activity estimate

Search for 220Rn - a - 216Po - a decay - two triggers within 300 ms gate 2161 155 ms 216Po 9 cm 220Rn 5 ms

Range of a particles in Ar(50%) + He (50%) gas 1 atm 216Po 220Rn

Decays in the center (thoron gate)

Decays from the walls (thoron gate)

Time correlation between two a particles

Background thoron activity estimate

Super-Kamiokande radon detector 16x16 mm2 S = 2 (counts/day)/(1 mBq/m3) Background – 2.4 ± 1.3 counts/day Y. Takeuchi et al.. NIM A 421 (1999) 334

OTPC for radon detection 100 cm 50 cm 20 cm 15 cm 20 cm V = 6·10-3 m3 S = 6·10-3 m3 1 mBq/m3  24 h = 0.5 (counts/day) / (1 mBq/m3) V = 0.25 m3 S = 0.25 m3 1 mBq/m3  24 h = 21.6 (counts/day) / (1 mBq/m3) Background: 200 mBq/m3 radon 20 mBq/m3 thoron

220Rn decay products 220Rn 56 s 216Po 145 ms 212Po 300 ns 212Bi 61 m 212Pb 10.6 h 208Pb stable 212Bi 61 m 212Po 300 ns 208Tl 3 m 6.29 6.78 8.78 6.1

222Rn decay products 222Rn 3.8 d 218Po 3.1 m 214Po 164 ms 210Po 138 d 214Pb 27 m 210Pb 22.3 y 214Bi 20 m 214Po 164 ms 210Tl 1.3 m 5.49 7.69 5.45 6.00 206Pb stable 210Po 138 d 210Bi 5 d 206Tl 4.2 m 5.30 4.65

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