1. Very preliminary study of the random background for the BiPo detector (PhoSwich configuration)
Work done by Jonathan Ferracci

2. Principle of the study
suppose that the counting rate in a scintillator (thin or thick) is due to the interaction of external photons with them (neglect the possible contribution of surface beta (210Bi) or alpha emitters)
flux of external photons a priori very difficult to modelize
Which isotopes?
(214Pb, 214Bi,
(228Ac, 212Pb, 212Bi, 208Tl
(40K,
(60Co
(152Eu
- Where? (lead shielding, iron, PMT glass, in volume, in surface)

3. Adopt a much simpler technique :
Simulate photon with a fixed energy (100 keV,
200 keV … 2 MeV) and obtain results for this fixed
Photon energy
Quenching factor :
use of values measured
by Mathieu
For all scintillators
FWHM = 15% for e- at 1MeV
Thin scintillator
thickness = 300 microns
LightGuide
Thickness = 8 mm
Photon emission
point
Thick scintillator
Thickness = 3 mm
Selenium source
thickness = 80 microns
Transversal sizes =
2 m x 3 m

4. For the same simulation apply 2 selections
Electron selection
At least 150 keV deposited in one of the thick scintillator
At least 50 keV deposited in the thin scintillator close to the thick scintillator
which is hit
Alpha selection
At least 1 MeV/quenching factor(1MeV)= deposited in one of the thin scintillator
Less than 500 keV/quenching factor(500 keV) deposited in the thick scintillator
close to the thin scintillator which is hit

5. Number of events selected (gamma emitted with a fixed energy), 500 000 events simulated
Gamma energy
Electron selection
Alpha selection
100 keV 59
200 keV 1
447
300 keV 27
1073
400 keV
199
1353
500 keV
502
1438
600 keV
929
1390
700 keV
1493
1241
800 keV
1870
1053
900 keV
2373
981

6. Gamma energy
Electron selection
Alpha selection
1000 keV
2582
856
1100 keV
2973
816
1200 keV
3198
690
1300 keV
3406
651
1400 keV
3460
585
1500 keV
3629
535
1600 keV
3776
488
1700 keV
3642
410
1800 keV
3839
433
1900 keV
3777
366
2000 keV
3884
343

7. For normalisation : use the trigger rate (0.17Hz)
Obtained by Mathieu at Canfranc
Simulate photon with a fixed energy (100 keV,
200 keV … 2 MeV) and obtain results for this fixed
photon energy
Quenching factor :
use of values measured
by Mathieu
Scintillator
Thickness = 2 x 1 cm
For all scintillators
FWHM = 15% for e- at 1MeV
LightGuide
Thickness = 8 mm
Photon emission
point
Transversal sizes =
20 cm x 20 cm

8. Selection applied for single rate
Single rate selection
At least 100 keV deposited in one of the scintillator

9. Number of events selected (gamma emitted with a fixed energy), 500 000 events simulated
Gamma energy
Single rate selection
100 keV
478
200 keV
7886
300 keV
38235
400 keV
50482
500 keV
55343
600 keV
56828
700 keV
58444
800 keV
57771
900 keV
57769

10. Gamma energy
Single rate selection
1000 keV
57093
1100 keV
56136
1200 keV
55711
1300 keV
54700
1400 keV
54475
1500 keV
53481
1600 keV
52791
1700 keV
51482
1800 keV
51181
1900 keV
50519
2000 keV
49588

11. Principle of the calculation
Example : if single rate is due to photons at 1000 keV
events selected by single rate selection :
single rate ~ 0.17 Hz
2 582 events selected by electron selection :
electron selection rate ~
2582/57093 x 0.17 Hz x 3m x 4m/(0.2 m x 0,2 m)
~ 2.3 Hz
856 events selected by electron selection :
alpha selection rate ~
856/57093 x 0.17 Hz ~ Hz x 3m x 4m/(0.2 m x 0,2 m)
~ 0.76 Hz
212Bi random rate ~ 2.3 Hz x 0.76 Hz x (3 x s)
~ Hz
212Bi random events expected in one month
~ Hz x 3600 x 24 x 30 ~ 4.1 events expected in one month

12. 214Bi random rate ~ 2.3 Hz x 0.76 Hz x (3 x 164 10-6 s)
214Bi random events expected in one month
~ Hz x 3600 x 24 x 30 ~ events expected in one month

13. Results obtained for 212Bi
worst case : Nbackground ~ 4.7 events in a month
Gamma energy (keV)
Gamma energy (keV)
Alimit obtained on 208Tl ~ 2.6 mBq/kg in one month

14. Results obtained for 214Bi
worst case : Nbackground ~ 2550 events in a month
Gamma energy (keV)
Gamma energy (keV)
Alimit obtained on 214Bi ~ 45 mBq/kg in one month

15. Conclusion like always, more than very preliminary
suppose background is only due to interaction of external photons, and not due to surface alpha or beta emitters (the contribution of this background to single rate could be significative for thin scintillators)
-> to see it : measure single rate in a scintillator as a function of thickness (linear if contribution from external photons dominate, constant if contribution from surface alpha/beta emitters dominate)
very simple model of external photons (but difficult to put the correct amount of everything at the correct place)
To be done : try to reproduce the single rate spectrum obtained
by Mathieu in Canfranc with sum of spectrums due to external
photons at fixed energies