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 IDM2004 Edinburgh, September 9, 2004

Neutrons from local activity Neutrons from local radioactivity (1) Neutrons from local activity 1). From the rock/concrete: (fission and (,n) reactions) 2.5 - 25 MeV n Energy Flux (10-6 n/cm2/s) 99 89 88 85 95 l Measurement @LNGS 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): IDM2004 Edinburgh, September 9, 2004

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 IDM2004 Edinburgh, September 9, 2004

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.) IDM2004 Edinburgh, September 9, 2004

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 IDM2004 Edinburgh, September 9, 2004

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 IDM2004 Edinburgh, September 9, 2004

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% IDM2004 Edinburgh, September 9, 2004

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 IDM2004 Edinburgh, September 9, 2004

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 IDM2004 Edinburgh, September 9, 2004