GERDA meeting8-11 November, 2005 Tuebingen Alternative option. Stainless steel cryostat: LN and LAr background

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GERDA meeting 8-11 November, 2005
TuebingenAlternative option.Stainless steel
cryostat LN and LAr background

• I.R. Barabanov1, L.B. Bezrukov1, V.I. Gurentsov1,
• V.N. Kornoukhov1,2
• - INR RAS
• - ITEP
• Moscow

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Cryostat made of stainless steel from CryogenMash
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Input Data

• The parameters of the cryostat design are
  presented on the figure (Dinner 3.5 m, H 5
  m).
• 9 germanium detectors are situated in the central
  position of the cryostat with LN (LAr).
• The thickness of the double cryostat wall 2.2
  cm
• The total mass of the stainless steel 14.4 t.
• Th-232 activity 7.4 mBq/kg (LENS), and
• 20 mBq/kg - 2.4mBq/kg (these activities
  correspond to set of investigated commercial
  samples)
• and 0.2 mBq/kg - from Germany, Gerd Heusser (!)

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Method of calculation

• The calculations have been done by Valery
  Gurentsov Gamma Code (2G code), which directly
  simulate gamma rays transport to the detector
  through the cryostat material and LN (LAr).
• 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 counts/keV/kg/year
  .

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LN Results

• Background index I, (c/kg/keV/year) for nine
  detectors with
• 2 kg mass placed into the center of the
  stainless steel cryostat.
• results in count/keV/kg/year
• 232?h activity 2.4mBq/kg 7.4 mBq/kg 20
  mBq/kg
• I 0.016 0.048 0.13
• () The anticoincidence decreases the background
  2 times
• The distribution of the background from
  different parts of the cryostat for nine diodes,
  A(232Th) 7.4 mBq/kg (LENS)
• Cylindrical part Upper part
  Lower part
• I 0.041 0.0037 0.0037
• Sum 0.048 count/keV/kg/year
• If A(Th232) 0.2 mBq/kg 0.00129
  count/keV/kg/yr (!)
• If 8 cm of cold Cu ? 10-4 c/keV/kg/yr

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SS cryostatLN conclusion

• The main contribution to the background comes
  from cylindrical part of the cryostat where the
  distance of shielding by liquid nitrogen is
  minimal.
• In all cases the background index is essentially
  higher than 0.001 c/keV/kg/yr, which is planned
  for the first stages of Gerda experiment. The
  cold copper shielding inside the cryostat is
  required to absorb the gamma activity from 232Th
  in the stainless steel material. The dependence
  of the background index on the thickness of the
  copper shielding is presented on the Fig. 1.
• The 232Th activity in copper is 25 mBq/kg.

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Fig. 1. The dependence of the background index on
the thickness of the copper shielding, A(232Th)
7.4 mBq/kg in the stainless steel and 25 mBq/kg
in copper.     1-     background from copper
2-     background from stainless steel 3-    
total background.
0,1
3
0,01
1
I, 1/(keV,kg,year)
1E-3
2
1E-4
1E-5
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
R, cm
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Cold Cu shieldingLN Conclusion

• the background index 0.001 can be achieved but
  it requires additional inner copper shielding
  with thickness in the most dangerous direction
• 13 cm for for 232Th activity 2.4 mBq/kg
• 16 cm for for 232Th activity 7.4 mBq/kg
• 19 cm for for 232Th activity 20 mBq/kg
• The profile of the copper shielding can be done
  by variable thickness for copper economy as it is
  shown in the fig. 2.

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Fig.2. Profile of cold Cu shielding
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LAr Results

• in counts/keV/kg/yr
• A(Th232) 2.4 mB/kg 7.4 mB/kg 20 mB/kg
• I 0.0009 0.0027 0.0074
• (D350 cm ?D370 cm) ? 0.0016 count/keV/kg/year
• If A(Th232) 0.2 mBq/kg 7.310-5
  count/keV/kg/year (!)
• K(LN/LAr) 18

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SS cryostatLAr conclusion

• If A(Th232) 7,4 mB/kg ? 0.0027 events/kg/keV/yr
• If we add 3 cm of cold Cu passive shielding ?
• 0.001 events/kg/keV/yr
• If A(Th232) 0.2 mBq/kg ? 7.310-5 c/keV/kg/yr
  (!)
• !We should select ss steel with minimum Th
  activity!
• Contribution from 3d wall made of SS (1 cm/10 t)
• lt 50

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SS cryostatLAr conclusion (continued)

• CryogenMash can enlarge D350 cm ?D370 cm
• 0.0016 count/keV/kg/yr
• If add 3 cm of cold Cu
• 0.0006 counts/keV/kg/yr
• If add 8 cm of cold Cu
• 0.0001 counts/keV/kg/yr
• CryogenMash (Balashikha, Moscow region)
• cost of cryostat made of ss steel (montage
  including)
• lt 400 k euros

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Cryostat (ss steel) built directly in LNGS by
CryogenMashD8.6 m(outer)/D7 m (inner)
limitation from Iris
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Cryostat (ss steel) built directly in LNGS

• D 7 m
• d (ss) 2 cm
• V 180 m3
• Heat loss
• W (SI, 40 layers) 170 W
• W (8 legs) 80 W
• W (neck) 12 W
• A (Th228) 7.4 mBq/kg (0.2 mBq/kg)
• Background from material of the cryostat
• I (9 detectorsLN) 0.0007 events/kg/keV/yr
  (210-5)
• I (9 detectorsLAr) 410-5 events/kg/keV/yr
• Cost 2 M euros