pe vs. water and requirements on wall materials

10.11.2005 TübingenBéla Majorovits 1

PE vs. Water and requirements on wall

materialsBéla Majorovits for the

Max-Planck-Institut für Physik, München


OUTLINE:

Alternative to a third wall: PE instead of water

Radiopurity of PE Estimated background due to PE

and how could we avoid it? What can we learn? MaGe

simulations for: copper, water, superisolation


Third wall required by LNGS for safety reasons

Several disadvantages:

• Less water to shield against external gammas and neutrons

• More (potentially dirty) material in the vicinity of the detectors

• More complicated structure


Alternative design: Use PE and LAr instead of water and LN2:

Advantages of PE:

• PE can not mix with LN2 No catastrophic evaporation possible

• Self supporting: can be stacked around cryo tank easy handling

But what is the influence of PE material to expected background rate?


Radiopurity of PE:Values taken from:

Recent GERDA measurement and http://radiopurity.in2p3.fr

Provider 238U[mBq/kg]

232Th[mBq/kg]

210Pb[mBq/kg]

60Co[mBq/kg]

40K[mBq/kg]

EdelweissPE

Plastique du Rhone

214Bi: 16±10

70±50 5±3 70±50

EdelweissPE

KOPOS Kolin

214Bi: 40±30

40±20 15±10 150±130

UKDM PE barrel

ICI Tracerco < 3.7 2.8 62

UKDM PE granules

Harwell Scientifics

750±500 1100 ±670

LArGe PE plate

Simona AG 228Ac: 11±3

11±7

Assume 10 mBq/kg 208Tl to estimate overall contribution of PE


Analytical estimate of background contribution I:

Calculate the number of emitted 2.6 MeV gammas from unit volume per unit time that are emitted towards the detector volume

Take into account self absorption Integrate over thickness and sphere


Analytical estimate of background contribution II:

Scale this number with reduction factor due to nitrogen and copper in the way

Scale this number with the peak to background ratio (from simulation)

Take into account anticoincidence and detection efficiency


Achievable sensitivity of the experiment degrades rapidly with Btot≥10-3

Counts/kg/keV/y

GERDA sensitivity (see K. Kroeninger)

We need to obey severe constraint:Bmax,contr≤ 10-4Counts/kg/keV/y


2.6MeVAPE = 10 mBq/kg 2.6MeVBPE= 1.9 * 10-2 Counts/kg/keV/y

Expected background contribution of PE with 2 m

LAr tank

we need to have additional copper shield of dCu,ana=133 mm

Reduction of factor 190 required in order to meet the requirement of 10-4 Counts/kg/keV/y

Independent cross check with MC simulation: dCu,sim=138 mm

r = e -μCu L Cu = 1/190


LAr thickness Additional Cu shield needed for LAr[mm] [mm] [t]

2000 133 792100 119 762200 105 732500 63 543000 - -

PE seems feasible, but makes sense only with tank radius >>

2000 mm

PE contribution is less than 10-4 Counts/kg/keV/y for liquid Argon

as shield with vessel of more than 3000 mm radius


We have to be aware:

Results calculated for PE with liquid Argon shield will be even stricter for any surrounding materials with liquid nitrogen shield!

Water

Copper

Superisolation (~30 layers of MYLAR)

Check for radiopurity requirements of shielding materials:


Simulations made with MaGe2.6 MeV gammas randomly distributed in each

volume:

Inner Copper wall

Water around the outer Copper vessel

Outer Copper wall

Vacuum (30 layers super-isolation)


Constraints for different materials:232THAbulk,exp.

[μBq/kg]

232THAbulk,max

[μBq/kg]

2.6 MeVΦmax

[10-7 cm-2s-1]

232THAsurf,max

[mBq/m2]

Inner Copper 19 68 1.3 3.8 Superisolation

5 000 15 800 1.8 0.26

Outer Copper 19 123 2.3 6.7 Water 10 37 3.8 21.2

Copper has to be pure, but OF01 and NOSV copper meet requirements

Water needs to be of very high purity! doable: achieved for BOREXINO and SNO

!

Same requirements hold for PE: 10mBq/kg could be compensated by 200 mm of Cu shield

0.36 km2 of very-clean surface

We need to be extremely carefull with (surface) contamination of superisolation!


CONCLUSIONS

PE design with liquid Argon seems reasonable, but only with increased vessel radius r >> 2000mm

Restrictions for all materials are severe for LN2

Beware of the superisolation: 0.36km2 of (electrostatically easily chargable) very-clean surface: 260 μBq/m2

Internal note with details will be published soon

pe vs. water and requirements on wall materials

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