Cryostat made of stainless steel from CryogenMash
Input Data: The parameters of the cryostat design are presented on the figure (D inner = 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 (!)
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 keV energy region. All results below will be given for the background index in the units [counts/keV/kg/year].
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] 232Тh activity 2.4mBq/kg 7.4 mBq/kg 20 mBq/kg I* (*) 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* Sum count/keV/kg/year If A(Th232) = 0.2 mBq/kg – count/keV/kg/yr (!) If + 8 cm of cold Cu c/keV/kg/yr
SS cryostat&LN: 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 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 Bq/kg.
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 Bq/kg in copper. 1- background from copper; 2- background from stainless steel; 3- total background E-5 1E-4 1E-3 0,01 0, I, 1/(keV,kg,year) R, cm
Cold Cu shielding&LN: Conclusion the background index ~ 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.
Fig.2. Profile of cold Cu shielding
LAr: Results in [counts/keV/kg/yr] A(Th232)2.4 mB/kg7.4 mB/kg20 mB/kg I (D350 cm D370 cm) count/keV/kg/year If A(Th232) = 0.2 mBq/kg – 7.3*10 -5 count/keV/kg/year (!) K(LN/LAr) ~ 18
SS cryostat&LAr: conclusion If A(Th232) = 7,4 mB/kg events/kg/keV/yr If we add ~3 cm of cold Cu passive shielding events/kg/keV/yr If A(Th232) = 0.2 mBq/kg 7.3*10 -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) < 50% +
SS cryostat&LAr: conclusion (continued) CryogenMash can enlarge D350 cm D370 cm: count/keV/kg/yr If add + 3 cm of cold Cu: counts/keV/kg/yr If add + 8 cm of cold Cu: counts/keV/kg/yr CryogenMash (Balashikha, Moscow region): cost of cryostat made of ss steel (montage including): < 400 k euros
Cryostat (ss steel) built directly in LNGS by CryogenMash D8.6 m(outer)/D7 m (inner) – limitation from Iris
Cryostat (ss steel) built directly in LNGS D 7 m d (ss) = 2 cm V = 180 m 3 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 detectors&LN) = events/kg/keV/yr (2*10 -5 ) I (9 detectors&LAr) = 4*10 -5 events/kg/keV/yr Cost – 2 M euros