Transcript Заголовок слайда отсутствует

Background for GERDA detector from
external gamma activity
I. Barabanov, L. Bezrukov, V. Gurentsov,
S. Kianovskiy, A. Klimenko
Institute for Nuclear Research of Russian Academy of Sciences
E. Demidova, I. Kirpichnikov,
V. Kornoukhov, A. Vasenko
Institute of Theoretical and Experimental Physics
Report on Tubingen meeting of GERDA collaboration 2005
The background index from external gamma
activity for GERDA installation has been
calculated by using Valery Gurentsov Gamma
Code (2G Code).
The germanium detectors are situated in the
central position of the copper cryostat with
liquid nitrogen or argon. The cryostat is placed
in the steel tank filled with water. The gamma
activity can arise either in the cryostat material
or in the surrounding rock. We consider these
two cases separately.
The background from the cryostat material.
Initial data.
The all dimensions of the cryostat were taken
from the last design (received from Karl-Tasso
Knoepfle). It is planned to be built from the
copper of high purity except of the neck part.
The calculations were done for the Tl-208 line
that gives the main contribution to the
background.
The initial gamma activity data:
Copper activity is 25x10-3 mkBq/kg of Th-232.
Steel activity is 20 mBq/kg of Th-232.
(dimensions of steel nack:
length = 0,98 m, diameter = 0,78 m,
wall thickness = 2x0,8 cm)
Method of calculation.
The calculations have been done by direct
simulation of gamma rays transport to the
detector through the cryostat material and liquid
nitrogen or argon. Anticoincidence between
different diodes were taken into account. The
background is registered in the 1800-2300 keV
energy region. All results below will be given
for the background index in the units count/(keV.kg.year).
Results
Table 1
Background index for nine detectors with 2 kg
mass located in the center of the cryostat.
Nitrogen
Argon
Units - counts/(kg.keV.year)
Cu (tank) Steel
Total
(Neck)
2.010-4
6.010-5
2.610-4
1.110-5
1.110-5
Conclusion:
1) The background index 10-3 c/(kg.keV.year) that is
planned for the first stages of experiment is possible.
2) Anticoincidence between the different diodes decrease
the background in ~2 times.
Requirement to the super isolation.
If the weight of super isolation is 1%
from copper cryostat (~120 kg), its Th-232
activity must be less than 1 mBq/kg.
The background index from super
isolation is less than 0.8x10E-4 in this
case.
Background from external gammas.
The second step of calculation is the
background from rock of underground room.
All construction elements of the set up are taken
into account:
1. The thickness of the steel tank – 10 mm.
2. The nine detectors are in the cryostat center.
3. The intensity of the 2.615 MeV gamma line
is 0.0625 gammas/(cm2.c) with isotropic
distribution.
4. The dimensions of the steel tank were taken
from the report of Carla Cattadori on the Dubna
meeting.
Testing of the 2G Code.
Comparing with GEANT 4
For testing of the 2G Code the additional
calculations have been done in comparing with
GEANT 4 for water sphere with radius of 0.5-2 m and
single detector (2 kg) in the center. The results for
coefficient of absorption are presented below.
The coefficient of absorption (cm2/g) for two energy
regions:
1.8-2.3 MeV
GEANT 4
Program
Handbook (MEPhI)
0.036
0.037
Peak(>2.6Mev)
0.042
0.042
0.0425
The coincidence of the results is an evidence of the
2G Code reliability.
Results.
Background index for nine detectors from the
activity of the rock
Table 2. Background index for nine detectors for the
completely filled water tank. The thickness of the
water in cylindrical part is 300 cm, upper and bottom
parts are 200 cm. Anticoincidence between the
different diodes are taken into account.
Units - counts/(kg.keV.year)
Cylindrical part
1.110-4
Upper part
1.210-4
Bottom flat part
1.910-4
Open Neck
2.410-2
Neck with 15 cm Pb
2.410-5
Total 4.410-4
For comparison we give here from Table 1
the background index for copper cryostat –
2.610-4 c/(kg.keV.year)
Conclusions:
1. The background index ~110-3, which is
planned for the first and second stages of the
experiment is possible, without any additional
shielding (without lead shoulders on the upper
part of the water tank).
2. The cryostat neck must be shielded by 10-15
cm of Pb.
3. For the case of Ar the background index
~210-5 can be achieved.
4. The 2G Code is ready now for calculations of
another configuration. The time of
calcullations is 50 times less in comparing
with GEANT-4. We are ready to give this
Code to Collaboration.
Remarks on construction of third copper
wall
Third copper wall
and self shielding effect
I, 1/(keV,kg,year)
0,0005
0,0004
0,0003
0,0002
0.0001
0,0
2
3
4
5
6
7
8
9
10
11
12
R, cm
Fig.1. The dependence of the background index I from
the copper activity versus the copper thickness R .
The 232 Th activity is 25 Bq/kg.
The starting point on the plot (3cm) corresponds to the
current cryostat project. Introducing the third copper wall
with the thickness 2 cm gives
the index 0.33x10-4 c/(keV.kg.y)
Conclusion:
In case of introducing copper third wall into
set up with thickness of a few cm the background
index from the copper becomes equal to
0.4x10-4 /(keV.kg.year) and does not depends on
copper thickness.
Simultaneously a few cm of additional copper
will decrease the contribution of rock activity in
several times.
As a result the total background index will be
less after introducing the third copper wall.