Background index of the CaMoO4 detectors assembly from -rays of Bi-214 and Tl-208
isotopes as impurities at PMT’s material.
Simulation done by Valery Gurentsov in the framework of ISTC project #3293
(01.03.2006 - 01.06.2007)
Simulation of the background index of the experimental set up for the ROI (the Range of
Interest) which corresponds to the interval of energy of the Mo-100 double beta-decay (Е = 3,0
MeV). The background is caused by present of radionuclides Ra-226 (Bi-214) and Tl-208. The
set up consists of 16 СаМоО4 single crystals with dimensions of 3,75 * 3,75 * 20 cm3. Total
mass of the crystals is 20 kg. Each crystal is viewed by 2 PMTs placed at the end surfaces of the
crystals. Light guides made of CaMoO4 (CdWO4) crystals are placed between PMTs and the end
surfaces of the CaMoO4 crystals. Length of the light guides is from 0 cm till 20 cm.
СаМоО4 assembly is surrounded by CsI(Tl) crystals with dimensions of 7,5 * 7,5 х*30 cm3
each, total quantity of the crystals is 32 and total mass is 250 kg. Each CsI(Tl) detector is viewed
by 2 PMTs placed at the end surfaces of the crystals.
Simulation has been done by the method of direct simulation of Ra-226 (Bi-214) and Tl-208
decay into the PMT photocathode and following transport of -quanta through the CaMoO4 and
CsI(Tl) assemblies. We define the background event in a case of energy release at the ROI of
2880-3120 keV into one of the СаМоО4 crystals and absence of any energy release into each of
the other СаМоО4 and CsI(Tl) detectors.
Initial data for simulation:
- Quantum efficiency of the photocathode of the PMT is 25%;
- Light collection: full internal reflection;
- Threshold of the PMT is 1 photoelectron, created at the photocathode;
- Light guides: CaMoO4 and PbWO4.
We take into account emitting of several -quanta after the decay of Tl-208 isotope. Specific
activity of Bi-214 and Tl-208 into the PMT glass (PMT produced by ETL Company and made of
low radioactivity glass):
0.3 Bq/kg (Th-208) and 2.0 Bq/kg (Bi-214).
Mass of the PMT is 300 gr.
The results of the simulation for are presented at the Table 1. Length of light guides made of
CaMoO4 is: 0 cm, 10 cm и 20 cm.
Table 1. The background index of the experimental set up (events/keVkgyr) caused by decay of
Tl-208 and Bi-214 isotopes, present at PMT (glass). Light guides are made of CaMoO4 with
length of 0, 10 and 20 cm.
Light guide, made of
CaMoO4, cm
0
10
20
Tl-208
(3.75 х 3.75 х 20) cm3
15.0
7.9*10-2
8.1*10-3
Bi-214
(3.75 х 3.75 х 20) cm3
6.4*10-3
-
Conclusion: a present of Tl-208 at PMT glass could be a main source the background of the
set up.
Possible solution is to use the light guides made of PbWO4 (ρ=8.28 г/cм3) with the length of
20 cm. We considered the PbWO4 light guides with the lengths of 5 cm, 10 cm, 15 and 20 cm
and with the cross-section equal to the cross-section of СаМоО4 crystal. The results are in the
Table 2.
Table 2. The background index of the experimental set up (events/keVkgyr) caused by decay of
Tl-208 and Bi-214 isotopes, present at PMT (glass).
Light guides are made of PbWO4 with length of 0 cm, 5 cm, 10 cm, 15 cm and 20 cm.
Light guide, made of
PbWO4, cm
0
5
10
15
20
Tl-208
(3.75 х 3.75 х 20) cm3
15.0
1.4*10-1
1.4*10-2
1.8*10-3
< 1.5*10-4
Bi-214
(3.75 х 3.75 х 20) cm3
6.4*10-3
< 3.3*10-4
-