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STATUS OF NEUTRON BACKGROUND STUDY IN CRESST
Hesti Wulandari
TU Munich
J. Jochum (Tübingen), W. Rau, F. von Feilitzsch
IDM2004 Edinburgh, September 9, 2004
OUTLINE
Neutrons from local radioactivity
Muon-induced neutrons
Summary & outlook
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Neutrons from local activity1). From the rock/concrete: (fission and (,n) reactions)
Results our simulations:
• Fission and (,n) contribute almost equally
• Flux in the lab consistent with measurement
• Flux in the lab originates mainly from the
concrete can be expected also for other
other underground labs
1-2.5 MeV 2.5-5 MeV 5-10 MeV
Hall A, dry concrete (8%water) 0.35 0.12 0.18 0.05 0.05 0.02
Hall A, wet concrete (16% water) 0.18 0.06 0.12 0.04 0.03 0.02
Hall C, dry concrete (8%water) 0.27 0.12 0.15 0.05 0.03 0.01
Belli et al. (measurement, Hall A)
0.38 0.01 0.27 0.14 0.05 0.01
IDM2004 Edinburgh, September 9, 2004
Flux in the Lab
(10-6 n/cm2/s):
0
0,5
1
1,5
2
2,5
3
3,5
4
Belli et al.
Arneodo et al.
Aleksan et al.
Belloti et al.
Rindi et al.
Cribier et al.
2.5 - 25 MeV n Energy
Flu
x (1
0-6 n
/cm
2 /s)
99
89
88
85
88
95
l
Measurement @LNGS
Neutrons from local radioactivity (1)
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• Count rate & Recoil Spectrum in the Detector
50cm PE reduces the count rate by a factor of almost 104
Neutrons from local radioactivity (2)
IDM2004 Edinburgh, September 9, 2004
Without neutron moderator:• measurement (12 – 40 keV): (0.870.22)
cts/kgd• Simulation (12 – 40 keV): 0.6 cts/kgd
PEPb
Cu
Detector
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2). From the shield: U fission reaction
PbCu
Typical impurity in Pb:
• U-238 : < 2ppb (Roman Pb), < 12 ppb (low activity Pb) (Alessandrello et al. , 1991)
• EDELWEISS: < 0.7ppb (Gerbier, private comm.)
Count rate(15-25 keV): 2 cts/kgy for 1ppb U-238 in Pb
Without PE, a few ppb is acceptable With 50cm PE a few ppt U-238 already a limiting n source
IDM2004 Edinburgh, September 9, 2004
Neutrons from local radioactivity (3)
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Muon-induced neutrons Muon-induced neutrons (1)
IDM2004 Edinburgh, September 9, 2004
intensity at the depth of the LNGS calculated with SIAM routine (V. Kudryavtsev):
c**
*
R
GeV
cosE.
.
GeV
cosE.x
GeVsrscm
E.A
dE
cos,EdI
850
111
0540
115
111
1140
002
0
0
0
2
0
0
0
1
0
dE
dE
cos,EdIE,X,EPcos,X,EI
*
1: the probability for a w/ energy E0 at the surface to have the energy E at depth X
obtained by propagating with various energies at the Earth‘s surface using MUSIC (V. Kudryavtsev)2: the intensity at sea level at zenith angle *
*: calculated from the zenith angle underground , taking into account the curvature of the Earth•Rc : ratio of promt ‘s to ‘s•Choices of A and : Gaisser‘s parametrization (A=1, =2.70) or LVD best fit for depth-vertical relation (A=1.840.31, =2.770.02, Rc ≤ 2x10-3 )•Absolute intensity depends on the surface relief. A flat surface as an approximation
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Flux of high energy n’s entering the experimental hall
Neutron Flux (E>1MeV) at the boundary between Rock and hall (with back scattering): 2.5x10-9 n/cm2/s
• spectrum: LVD best fit• Geometry optimized to save computing time but still get reliable results• „MUSUN“ (V. Kudryavtsev) was connected to FLUKA to sample energy and angular dist.
20 m
20 m
6 m
20 m 5 m
Rock
PEPb
Cu
Cryst.
IDM2004 Edinburgh, September 9, 2004
Muon-induced neutrons (2)
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Effect of shielding materials
Pb is the mostdangerous
All shields(E>1MeV):Contribution of-induced neutronsIn the rock 5%
IDM2004 Edinburgh, September 9, 2004
Muon-induced neutrons (3)
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Count Rates at the Detector
MCNP used because FLUKA doesn‘t treat individual recoils
• Contribution of -induced n‘s in the shields is not negligible for the setup without neutron moderator
• 10cm internal PE shield would reduce the contribution of -induced n‘s in the shield by a factor of 30 and -induced n‘s in the rock by a factor of 60
IDM2004 Edinburgh, September 9, 2004
Muon-induced neutrons (4)
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Summary & Outlook
• The number of events measured by CRESST is compatible with the rate expected from neutron background
• CRESST II needs a neutron moderator & muon veto being installed
• to handle muon-induced neutrons in the rock: - study of multiple scattering - internal PE
• Simulations take the surface relief of Gran Sasso into account
• Measurement of shield activity
Summary & Outlook
IDM2004 Edinburgh, September 9, 2004
WIMP mass (GeV/c2)
WIM
P,p
roto
n (p
b)
CRESST 100kg years
50cm PE
30cm PE
LE n, rock no PE
Veto 99%
Veto 90%
in shield
in rock in shield,10cm int. PE
in rock,10cm int. PE
shield activity