Download - STATUS OF NEUTRON BACKGROUND STUDY IN CRESST
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
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)
• 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
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)
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
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)
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)
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)
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