Neutron background studies for

Transcription

1 Neutron background studies for Edelweiss programs Introducing ETNomSiQ working group - Past measurements - Monte Carlo studies - Program of measurements and protections for EDELW II G. Gerbier CEA/Saclay, DAPNIA Fréjus Laboratory, CEA/CNRS G. Gerbier TAUP2003 5/09/2003 1

2 Edelweiss in Modane Laboratory CEA-Saclay DAPNIA and DRECAM CRTBT Grenoble CSNSM Orsay IAP Paris IPN Lyon Modane Underground Laboratory (Fréjus) FZ-Karlsruhe and Univ. Karlsruhe 4800 mwe - m flux = 4/m 2 /d - neutron flux = /s/cm 2 G. Gerbier TAUP2003 5/09/2003 2

3 ETNomSiQ working group European Team for Neutron and m Shield Qualification Lyon, Saclay, Munchen, Karlsrhue, Sheffield Low energy neutrons induced by U/Th activities, calculations : I - in surrounding rock/concrete (fission and (a,n) reactions) II - in Pb/Cu Shield (fission reactions) High energy neutrons induced by muons with GEANT / FLUKA / MCNP codes III - in the rock IV - in Pb/Cu shield m c d m n n n a b n G. Gerbier TAUP2003 5/09/2003 3

4 I) Neutron flux measurement at LSM IPN Lyon+ISN Grenoble Ast Phys 9 (1998) Proc IDM2002 Detector = Scintillator NE32O + 6Li cell Clear signature : coincidence prompt proton recoil signal + delayed capture on 6Li Counting time : d 1,15 count/day Pb +Cu 0,38 count/day Pb +Cu +paraffin => net signal = 0,77 c/day Signal/noise = 2/1 e det= to (fission only) (a n + fission) fi F = 1, /cm_/s E > 2 MeV (revised 2001 value : initial value in 1998 paper = /cm2/s but wrong simulation of propagation of neutrons through Pb/Cu shield) G. Gerbier TAUP2003 5/09/2003 4

5 Predicted neutron E spectra from rock Initial spectrum from rock : inputs =U/Th contaminants, composition of rock => mix of a n (4/5) + fission (1/5) => come from 30 first cm of rock wall Propagation through Pb/cu shields of set up with MCNP code => strong softening of spectrum Energy spectrum of neutron from alpha n reactions in LSM rock and GS rock Spectre lsm Spectre Gs 1,00E-06 1,00E-07 Initial spectrum from RA from rock (4/5 a n + 1/5 fission) ,00E-08 Spectrum after Pb/cU 150 1,00E ,00E-10 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 Spectrum after Pb/CU+ 30 cm paraffin E neutron 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 G. Gerbier TAUP2003 5/09/2003 5

6 III) Neutron production by DIS of muons in shields and rock : Main difficulty : predict right neutron production rate down to few MeV : GEANT 3 4, FLUKA Strong effect of showering on spectrum => existing exp results very sensitive to exact shield set up Strategy Define «bench marks» = standard problems => compare MC s Well defined input : target Well defined output : spectrum of neutrons (outgoing neutrons) Modified GEANT 3 used by KARMEN to fit their neutron data Check X section vs litterature => Results? In progress G. Gerbier TAUP2003 5/09/2003 6

7 Neutron production as a function of m energy experiments in scintillator/lead LSD ASD LVD /m/g/cm 2 Palo Verde /m/g/cm 2 Parametrisation in scintillator Nn = 4.14 E m /m/g/cm 2 Wang et al. Phys Rev D 64 (2001) Bergamasco et al, 73 Nn = 3.2 E m /m/g/cm 2 VK (IDM 2002) G. Gerbier TAUP2003 5/09/2003 7

8 Crosss section Neutron differential E rate G. Gerbier TAUP2003 5/09/2003 8

9 IV) U fission in lead/internal materials The ultimate source of background (Not shielded, not vetoed)? NB : with 0.1 ppb U in Pb : # neutrons created in 6.5 T = 380 /year Fraction reaching detector volume (40 kg single crystal, equivalent surface = 2100 cm 2 ) = 8 %, out of which 20 % > 1 MeV=> /s/cm 2 25 % go out and get back inside Mean # of inter = % n captured : 78 % on H, 16 % on 207 Pb, 3% on 63 Cu Albedo effect = factor 4 Int of blue curve = 4 int purple 1.00E E E E E E-09 Neutrons from U fission in lead E neutron MeV Fission U in lead at det Fission U in lead initial G. Gerbier TAUP2003 5/09/2003 9

10 Résumé : neutron spectra at EDWII detectors (MCNP / GEANT3) 1,00E-04 1,00E-05 1,00E-06 Neutron energy spectra at detector for the 4 sources arbitrary normalisation RA rock Fission U in Pb Mu rock Mu Pb Exponentielle Mu pb Geant 1,00E-07 1,00E-08 1,00E-09 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5 9,0 9,5 E neutron MeV G. Gerbier TAUP2003 5/09/

11 Measured and expected neutron recoil rates in Ge detectors of Edelweiss-I at LSM Measured upper limits Data EDW1-05/01 Pb + Cu only evts/kg/d Data EDW Pb/Cu + 30cm paraf. evts/kg/dbest Er >30 kev < 2.7 Er >30 kev < 0.17 Calculated Expected rate in EDW 1 Pb + Cu only evts/kg/d Expected rate in EDW 1 Pb + Cu + 30cm paraf. evts/kg/d Er >10 kev 3.7 Er >10 kev 0.05 Er >30 kev 1.0 Er >30 kev G. Gerbier TAUP2003 5/09/

12 Summary of neutron background flux estimations at EDELWEISS II detectors Neutron source Radioactivity from Rock U fission in Pb of shield assuming < 0.1 ppb Neutrons from muons in Pb shield Neutrons from muons in rock Predicted flux at detector after shield E > 1 MeV n/cm_/sec 1 < n /cm_/sec < n /cm_/sec ~ n /cm_/sec ~ 4 Predicted flux at detector after shield E > 0.5 MeV n /cm_/sec 1 < n /cm_/sec < n /cm_/sec ~ n /cm_/sec ~ 3 firesidual single recoil rate ~ evt/kg/d m VETO e > 95 % will reduce this too G. Gerbier TAUP2003 5/09/

13 Edelweiss II Set up: Sensitivity reached by Edw I : 0.2 evt/kg/d Expected sensitivity for Edw II : evt/kg/d => Passive shieldings 20 cm Lead + 50 cm PE => Active muon veto G. Gerbier TAUP2003 5/09/

14 EDELWEISS II m Veto Study of m, m veto underway (L. Chabert, PhD thesis) Scintillators around PE, Pb, cryostat Collaboration with FZ Karlsruhe => Recycled KARMEN veto modules : G. Gerbier TAUP2003 5/09/

15 Summary Understanding of neutron background is crucial in DM search expts Reliable simulation tools developed within EDW and ETNomSiQ Large e neutron detector in coinc with EDW veto should help understanding neutrons from m ETNomSiQ working group open Share experience on measurements/simulation codes Produce common/complementary tools Meet twice a year G. Gerbier TAUP2003 5/09/