PE vs. Water and requirements on wall materials

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PE vs. Water and requirements on wall materials
Béla Majorovits for the Max-Planck-Institut für
Physik, München
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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

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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

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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?
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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 1610 7050 53 7050
EdelweissPE KOPOS Kolin 214Bi 4030 4020 1510 150130
UKDM PE barrel ICI Tracerco lt 3.7 2.8 62
UKDM PE granules Harwell Scientifics 750500 1100 670
LArGe PE plate Simona AG 228Ac 113 117
? Assume 10 mBq/kg 208Tl to estimate overall
contribution of PE
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• 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

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• 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

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GERDA sensitivity (see K. Kroeninger)

• ? We need to obey severe constraint

Achievable sensitivity of the experiment degrades
rapidly with Btot10-3 Counts/kg/keV/y
Bmax,contr 10-4Counts/kg/keV/y
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Expected background contribution of PE with 2 m
LAr tank

• 2.6MeVAPE 10 mBq/kg ?

2.6MeVBPE 1.9 10-2 Counts/kg/keV/y
Reduction of factor 190 required in order to meet
the requirement of 10-4 Counts/kg/keV/y
r e -µCu L Cu 1/190
? we need to have additional copper shield of
dCu,ana133 mm
Independent cross check with MC simulation
dCu,sim138 mm
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PE contribution is less than
10-4 Counts/kg/keV/y for liquid
Argon as shield with vessel of more than 3000 mm
radius
LAr thickness Additional Cu shield needed for LAr Additional Cu shield needed for LAr
mm mm t
2000 133 79
2100 119 76
2200 105 73
2500 63 54
3000 - -
? PE seems feasible, but makes sense only with
tank radius gtgt 2000 mm
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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!

? Check for radiopurity requirements of shielding
materials

• Water
• Copper
• Superisolation (30 layers of MYLAR)

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Simulations made with MaGe
2.6 MeV gammas randomly distributed in each
volume
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Constraints for different materials
232THAbulk,exp. µBq/kg 232THAbulk,max µBq/kg 2.6 MeVFmax 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
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CONCLUSIONS

• PE design with liquid Argon seems reasonable, but
  only with increased vessel radius r
  gtgt 2000mm
• Restrictions for all materials are severe for LN2
• Beware of the superisolation
  3600m2 of (electrostatically easily chargable)
  very-clean surface 260 µBq/m2
• Internal note with details will be published soon