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

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)

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

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

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

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

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

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

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

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

Principle of the calculation Example : if single rate is due to photons at 1000 keV 57 093 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 ~ 2.5 10-3 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 300 10-9 s) ~ 1.6 10-6 Hz 212Bi random events expected in one month ~ 1.6 10-6 Hz x 3600 x 24 x 30 ~ 4.1 events expected in one month

214Bi random rate ~ 2.3 Hz x 0.76 Hz x (3 x 164 10-6 s) 214Bi random events expected in one month ~ 8.610-4 Hz x 3600 x 24 x 30 ~ 2 229 events expected in one month

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

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

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