Transcript Заголовок слайда отсутствует

Gerda background calculation
Status report
INR, ITEP, DUBNA
Background from inner tank envelope
All dimensions are taken from the Letter of Intend (fig.17)
Method of calculation.
Calculation has been done by direct simulation of gamma
transportation
The calculation has been done for set up of the 9 detectors (2
kg each), taking into account the anti coincidences.
The background is registered in the energy region 1800-2300
keV. The background is approximately flat in this region.
Input data
Cu activity 25E-6 Bq/kg of Th-232
Fe activity 20E-3 Bq/kg of Th-232
Internal tank diameter 1885 mm
Liquid Nitrogen
Background index for nine detectors with 2 kg mass
(cpy/kg.Kev)
Cu
Center
Total
2. 5x10-4
Fe (Neck)
6x10-5
3.1x10-4
Conclusion:
1. The background index 1E-3 is achievable
1,0
0,9
0,8
Probability
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
1
2
3
4
5
Distribution of numbers of hitted crystals
6
7
Requirement to superisolation
If the weight of superisolation is 1% from cryostat
copper (~120 kg), its Th-232 activity must be less
than 1 mBq/kg. The superisolation background is
1.0x10E-4 in this case
Background of set up of 9 crystals from external gamma .
1. All construction elements of the detector are taken into
account.
3. Intensity of the 2.615 MeV gamma line is 0.0625 /cm2.c with
isotropic distribution. This intensity corresponds the index
background of unshielded detector (2 KG) – 13000 (The
experimental number 12000) according to the program
calculation.
Background of 9 crystals for the completely filled water tank.
The thickness of the water in cylindrical part is 300 cm, upper
and bottom parts are 200 cm.
The number for cylindrical part with 300 cm was obtained by
extrapolation data for 200 cm
Cylindrical part
Upper part
Bottom flat part
Open Neck
Neck with 10 cm Pb
Neck with 15 cm Pb
4.5Е-5
2.7Е-4
4.3Е-5
2.4Е-2
1.1Е-4
1.1Е-5
Background index for nine detectors with 2 kG mass from
copper crystat (cpy/kg.Kev)
Center
Cu
Fe (Neck)
2.5Е-4
6.0Е-5
Testing of the program.
Comparing with GEANT 4
Sphere with water shielding 0-2 m.
Single detector (2 kg) in the center
The coefficient of absorption (cm2/g)
1.8-2.3 MeV
Peak (>2.6 Mev)
GEANT 4
0.036
0.042
Program
0.037
0.042
Table
0.0425
Conclusions:
1. The background index ~1E-3 is achievable without any lead
shoulders in the upper part.
2. The Neck must be shielded by 15 cm of Pb.
3. The background of the set up from 252 detector with
anticoincidence (7 layers with 36 detectors in each layer) is
2.5 times less.
4. To have background index ~1E-5 from Cu tank the liquid
Ar is required.
The program is ready now for other configuration calculations.
The time of calculations is 30-40 times less in comparing with
GEANT-4.