Transcript Dark Matter Search with the CRESST experiment Cryogenic

Béla Majorovits
10.11.2005 Tübingen
PE vs. Water and requirements
on wall materials
Béla Majorovits for the
Max-Planck-Institut für Physik, München
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Béla Majorovits
10.11.2005 Tübingen
OUTLINE:
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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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Béla Majorovits
10.11.2005 Tübingen
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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
Béla Majorovits
10.11.2005 Tübingen
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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?
Béla Majorovits
10.11.2005 Tübingen
Radiopurity of PE:
Values taken from:
Recent GERDA measurement and
http://radiopurity.in2p3.fr
Provider
Edelweiss Plastique
PE
du Rhone
Edelweiss KOPOS
PE
Kolin
UKDM PE ICI
barrel
Tracerco
238U
232Th
210Pb
60Co
40K
[mBq/kg] [mBq/kg] [mBq/kg] [mBq/kg] [mBq/kg]
214Bi:
16±10
214Bi:
40±30
< 3.7
UKDM PE Harwell
750±500
granules Scientifics
LArGe PE Simona AG
plate
70±50
5±3
70±50
40±20
15±10
150±130
2.8
1100
±670
228Ac:
11±3
 Assume 10 mBq/kg 208Tl to estimate
overall contribution of PE
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11±7
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Béla Majorovits
10.11.2005 Tübingen
Analytical estimate of
background contribution I:
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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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Béla Majorovits
10.11.2005 Tübingen
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Analytical estimate of
background contribution II:
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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
Béla Majorovits
10.11.2005 Tübingen
GERDA sensitivity (see K. Kroeninger)

We need
to obey
severe
constraint:
Achievable
sensitivity
of the
experiment
degrades
-3 Counts/kg/keV/y
-4Counts/kg/keV/y
rapidly
with≤B10
≥10
Bmax,contr
tot
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Béla Majorovits
10.11.2005 Tübingen
Expected background contribution
of PE with 2 m LAr tank
APE = 10 mBq/kg
2.6MeV

BPE= 1.9 * 10-2 Counts/kg/keV/y
2.6MeV
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,ana=133 mm
Independent cross check with MC simulation:
dCu,sim=138 mm
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Béla Majorovits
10.11.2005 Tübingen
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
[mm]
[mm]
[t]
2000
2100
2200
2500
3000
133
119
105
63
-
79
76
73
54
-
 PE seems feasible, but makes sense
only with tank radius >> 2000 mm
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Béla Majorovits
10.11.2005 Tübingen
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:
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Water
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Copper
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Superisolation (~30 layers of MYLAR)
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Béla Majorovits
10.11.2005 Tübingen
Simulations made with MaGe
2.6 MeV gammas randomly distributed in each volume:
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Inner Copper wall
Vacuum (30 layers superisolation)
Outer Copper wall
Water around the outer
Copper vessel
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Béla Majorovits
10.11.2005 Tübingen
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Constraints for different materials:
Abulk,exp.
232TH
Inner Copper 
Superisolation
Outer Copper

Water
Abulk,max
232TH
[μBq/kg]
[μBq/kg]
19
5 000
19
10
68
15 800
123
37
2.6 MeV
Φmax
Asurf,max
232TH
[10-7 cm-2s-1] [mBq/m2]
1.3
1.8
2.3
3.8
3.8
0.26
6.7
21.2

 Same
Water
Copper
requirements
needs
has to
tobe
bepure,
ofhold
very
but
for
high
OF01
PE:purity!
and NOSV
doable:
achieved
10
copper
mBq/kg
meet
for
could
BOREXINO
requirements
be compensated
and SNO by 200 mm of Cu shield
 We need to be extremely carefull with (surface)
contamination of superisolation!
3600 m2 of very-clean surface
!
Béla Majorovits
10.11.2005 Tübingen
CONCLUSIONS
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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:
3600m2 of (electrostatically easily chargable)
very-clean surface: 260 μBq/m2
Internal note with details will be published
soon
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