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


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



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



Muon-induced neutrons Muon-induced neutrons (1)


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.


Muon-induced neutrons (2)

Effect of shielding materials

Pb is the mostdangerous

All shields(E>1MeV):Contribution of-induced neutronsIn the rock 5%



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



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


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

status of neutron background study in cresst

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