GERDA meeting 8-11 November, 2005 Tuebingen

Alternative option.

Stainless steel cryostat: LN and LAr background

I.R. Barabanov1, L.B. Bezrukov1, V.I. Gurentsov1,

V.N. Kornoukhov1,2

(1) - INR RAS

(2) - ITEP

Moscow

Cryostat made of stainless steel from CryogenMash

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 (!)

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].

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* 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 partI* 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

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 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 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.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 301E-5

1E-4

1E-3

0,01

0,1

3

2

1

I, 1

/(ke

V,k

g,ye

ar)

R, cm

“Cold” Cu shielding&LN: 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.

Fig.2. Profile of “cold” Cu shielding

0 25 50 75 100 125 150 175 200 225 2500

3

5

8

10

13

15

18

20

23

ProfileThickness of Cu shieldingss-cryostat, D350 x 500, cm

Thi

ckne

ss o

f C

u sh

ield

ing

Distance from central plane of the cryostat, cm

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 – 7.3*10-5 count/keV/kg/year (!)

K(LN/LAr) ~ 18

SS cryostat&LAr: 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 7.3*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: 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):

< 400 k euros

Cryostat (ss steel) built directly in LNGS by CryogenMash

D8.6 m(outer)/D7 m (inner) – limitation from Iris

С ф е р и ч е с к и й р е з е р в у а р д л я ж и д к о г о а з о т а / а р г о н а

В н у т р е н н и й с о с у д

К о ж у х

Д е т е к т о р

К о м п е н с а т о р

О п о р а - 8 ш т .

7 2 0 0

46

00

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 detectors&LN) = 0.0007 events/kg/keV/yr (2*10-5)

I (9 detectors&LAr) = 4*10-5 events/kg/keV/yr

• Cost – 2 M euros