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

2. 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

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

4. Registration of a particle
CCD
PMT

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

6. 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

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

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

9. 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

10. 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

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

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

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

14. Decays in the center (thoron gate)

15. Decays from the walls (thoron gate)

16. Time correlation between two a particles

17. Background thoron activity estimate

18. 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

19. 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

20. 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

21. 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

24. 819