1. BACKGROUND STUDY IN CRESST
STATUS OF NEUTRON
BACKGROUND STUDY IN CRESST
Hesti Wulandari
TU Munich
J. Jochum (Tübingen), W. Rau, F. von Feilitzsch
OUTLINE
Neutrons from local radioactivity
Muon-induced neutrons
Summary & outlook
IDM Edinburgh, September 9, 2004

2. Neutrons from local activity
Neutrons from local radioactivity (1)
Neutrons from local activity
1). From the rock/concrete: (fission and (,n) reactions)
MeV
n Energy
Flux (10-6 n/cm2/s) 99 89 88 85 95 l
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
Flux in the Lab
(10-6 n/cm2/s):
IDM Edinburgh, September 9, 2004

3. Count rate & Recoil Spectrum in the Detector
Neutrons from local radioactivity (2)
Count rate & Recoil Spectrum in the Detector PE Pb Cu
Detector
Without neutron moderator:
measurement (12 – 40 keV):
(0.870.22) cts/kgd
Simulation (12 – 40 keV):
0.6 cts/kgd
50cm PE reduces the count rate by
a factor of almost 104
IDM Edinburgh, September 9, 2004

4. 2). From the shield: U fission reaction
Neutrons from local radioactivity (3)
2). From the shield: U fission reaction Pb Cu
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
Typical impurity in Pb:
U-238 : < 2ppb (Roman Pb), < 12 ppb (low activity Pb)
(Alessandrello et al. , 1991)
EDELWEISS: < 0.7ppb (Gerbier, private comm.)
IDM Edinburgh, September 9, 2004

5. Muon-induced neutrons
 intensity at the depth of the LNGS calculated with SIAM routine (V. Kudryavtsev):
1: the probability for a  w/ energy E0 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=1.840.31,  =2.770.02, Rc ≤ 2x10-3 )
Absolute  intensity depends on the surface relief. A flat surface as an approximation
IDM Edinburgh, September 9, 2004

6. Flux of high energy n’s entering the experimental hall
Muon-induced neutrons (2)
Flux of high energy n’s entering the experimental hall
 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
6 m
5 m
Rock PE Pb Cu
Cryst.
Neutron Flux (E>1MeV) at the boundary between Rock and hall
(with back scattering): 2.5x10-9 n/cm2/s
IDM Edinburgh, September 9, 2004

7. Effect of shielding materials
Muon-induced neutrons (3)
Effect of shielding materials
Pb is the most
dangerous
All shields(E>1MeV):
Contribution of
-induced neutrons
In the rock  5%
IDM Edinburgh, September 9, 2004

8. Count Rates at the Detector
Muon-induced neutrons (4)
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
IDM Edinburgh, September 9, 2004

9. Summary & Outlook WIMP,proton (pb) The number of events measured by
WIMP mass (GeV/c2)
WIMP,proton (pb)
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,
shield
activity
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
IDM Edinburgh, September 9, 2004