pe vs. water and requirements on wall materials
DESCRIPTION
PE vs. Water and requirements on wall materials. Béla 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? - PowerPoint PPT PresentationTRANSCRIPT
10.11.2005 TübingenBéla Majorovits 1
PE vs. Water and requirements on wall
materialsBéla Majorovits for the
Max-Planck-Institut für Physik, München
10.11.2005 TübingenBéla Majorovits 2
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
10.11.2005 TübingenBéla Majorovits 3
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
10.11.2005 TübingenBéla Majorovits 4
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?
10.11.2005 TübingenBéla Majorovits 5
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
10.11.2005 TübingenBéla Majorovits 6
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
10.11.2005 TübingenBéla Majorovits 7
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
10.11.2005 TübingenBéla Majorovits 8
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
10.11.2005 TübingenBéla Majorovits 9
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
10.11.2005 TübingenBéla Majorovits 10
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
10.11.2005 TübingenBéla Majorovits 11
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:
10.11.2005 TübingenBéla Majorovits 12
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)
10.11.2005 TübingenBéla Majorovits 13
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!
10.11.2005 TübingenBéla Majorovits 14
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