STATUS OF NEUTRON BACKGROUND STUDY IN CRESST

1. IDM2004 Edinburgh, September 9, 2004 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

2. Measurement @LNGS 89 Flux (10-6 n/cm2/s) 99 l 88 85 88 95 2.5 - 25 MeV n Energy IDM2004 Edinburgh, September 9, 2004 Neutrons from local radioactivity (1) Neutrons from local activity 1). 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 Flux in the Lab (10-6 n/cm2/s):

3. PE Pb Cu Detector IDM2004 Edinburgh, September 9, 2004 Neutrons from local radioactivity (2) • Count rate & Recoil Spectrum in the 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

4. Pb Cu IDM2004 Edinburgh, September 9, 2004 Neutrons from local radioactivity (3) 2). From the shield: U fission reaction 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.)

5.  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 energyE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 Muon-induced neutrons (1) Muon-induced neutrons

6. 20 m 6 m PE Pb Cu 20 m 5 m Cryst. Rock 20 m IDM2004 Edinburgh, September 9, 2004 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. Neutron Flux (E>1MeV) at the boundary between Rock and hall (with back scattering): 2.5x10-9 n/cm2/s

7. IDM2004 Edinburgh, September 9, 2004 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%

8. IDM2004 Edinburgh, September 9, 2004 Muon-induced neutrons (4) Count Rates at the Detector MCNP used because FLUKA doesn‘t treat individual recoils • Contribution of -induced n‘sin 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

9. LE n, rock no PE  in shield Veto 90% WIMP,proton(pb) 30cm PE  in rock  in shield, 10cm int. PE shield activity CRESST 100kg years Veto 99% • in rock, 10cm int. PE 50cm PE WIMP mass (GeV/c2) IDM2004 Edinburgh, September 9, 2004 Summary & Outlook 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