Gaitskell “High Energy Neutrons” Background in Dark Matter Search Experiments Rick Gaitskell Brown University, Department of Physics see information at

HE Neutrons - XENON Aug 2003 Rick Gaitskell Direct Detection: History & Future Oroville (88) [m = ?? GeV - if significantly better limit obtained at different mass] 90% CL Limit on Cross section for 60 GeV WIMP (scalar coupling) ~1 event kg -1 day -1 ~1 event kg -1 yr -1 ~1 event 100 kg -1 yr -1 LHC Not meant to be a complete list - see Different Colours Indicate Different Technologies NOW rjg [m=20 GeV] Homestake (87) H-M (94) H’berg-Moscow (98), IGEX (00) DAMA (96) UKDMC (96) [m=100 GeV] DAMA (98) DAMA (00) Gaitskell (astroph ) CDMS SUF (99) CDMS SUF (02) Edelweiss (98) Edelweiss (01) ZEPLIN I Xe (02) Edelweiss (02) DRIFT II 1 kg CS 2 (T) ZEPLIN II+III 10 kg Xe (T) XENON / ZEPLIN 1t Xe (T) CDMS Soudan (T) 7 kg Ge+Si Cryodet Majorana Phase 1 (T) GENINO (T) 100 kg Ge Diode GENIUS (T) 100 kg Ge Diode CryoArray (T) tonne Cryodet DRIFT III 100 kg CS 2 (T) Ge NaI Cryodet (T) Target Signal Liq Xe Gas CS 2

HE Neutrons - XENON Aug 2003 Rick Gaitskell Current and Projected CDMS Sensitivity Limit from CDMS 1999 Ge BLIP run at Stanford Edelweiss Exclusion Limit SUSY g  -2 Baltz&Gondolo, PRL 86 (2001) 5004 No SUSY g  -2 Baltz&Gondolo, PRL 86 (2001) 5004 CMSSM Ellis et al. (2001) PRD 63, Projected sensitivity for a 1-ton CryoArray (~ 1 event / (100 kg yr) For more limit curves, see Gaitskell &Mandichttp://dmtools.berkeley.edu Large region allowed by SUSY theories shrinks to light-blue region if SUSY causes excess value of muon anomalous magnetic moment (g  -2) Projected sensitivity for CDMS at Soudan, with 5 towers 4 kg Ge, 1.5 kg Si: 0.1 events/kg/keV/year (100x better than present limit at Stanford). ~1 event 100 kg -1 yr -1 ~1 event kg -1 day -1

HE Neutrons - XENON Aug 2003 Rick Gaitskell Some of Current (2001-) and Projected Experiments (2005-) **** Not a complete list **** 500 kg 1000 kg

HE Neutrons - XENON Aug 2003 Rick Gaitskell CryoArray (Sensitivity <1 per 100 kg-yr,  ~ cm -2 ) Scale up to 1 tonne detector with target (90%CL) <1 evt per 100 kg-yr Reduce  /  backgrounds by factor 20 vs CDMSII   > cts/keVee -1 kg -1 day -1 (This compares to keVee -1 kg -1 day keV for HMDS)   > cts/keVee -1 kg -1 day -1 (Challenge to survey surfaces to this sensitivity) Improve  /  rejection by factor 1-few!   99.5% -> 99.95% (1 in 2000) CDMS I 1999 in-situ calibrations already showed 99.96% (17k event calibs)   95% -> 99.5% (1 in 200) Ge BLIP with aSi contact, (E>25 keVr ) >95% (E>40 keVr) >99.5%, Si ZIP using phonon rise times (E=10-20 keVr ) >98%, (E>20 keVr) >99.5% Without Discrimination:  needs ~10 4 reduction in background from present (HM) levels rjg Revise rej

HE Neutrons - XENON Aug 2003 Rick Gaitskell Background Projections - CryoArray (1 tonne) dru = 1 event keV -1 kg -1 day -1 Energy Range keV x20 -1

HE Neutrons - XENON Aug 2003 Rick Gaitskell Background Projections - CryoArray (1 tonne) See Schnee, Akerib & Gaitskell, DM2002 (UCLA) x20 -1 dru = 1 event keV -1 kg -1 day -1 Energy Range keV

HE Neutrons - XENON Aug 2003 Rick Gaitskell Dark Matter Depth Requirements Site Depth Requirement  Dominated by need to reduce high energy neutrons ( MeV), generated by muons, that cannot be moderated directly using poly  Shallow ~1700 mwe (1 muons/m 2 /minute) Just satisfactory for 10 kg scale experiments (  ~10 -8 pb) 1 tonne experiments would require large additional active shield (>1 m thick) — >99% veto Risk associated with systematic misidentification  Intermediate ~3800 mwe Factor ~50x reduction in muons/HE neutrons compared to shallow Additional comfort factor, general consensus that 1 tonne experiments can function comfortably wrt to HE neutrons from muons (  ~ pb) Depth may be necessary for gas target given much large surface area to shield Satisfactory for cosmogenic activation Muons passing through detector array can be vetoed by simple muon veto (>99% being achieved)  Deep ~6000 mwe (Further factor ~50x reduction in muon/HE neutrons) Does not appear to be necessary for 1 tonne (  ~ pb), but eliminates any risk, and will allow next-next generation

HE Neutrons - XENON Aug 2003 Rick Gaitskell CDMS HE Neutron Flux Simulation (Soudan/2000 mwe) Inputs into Neutron Flux Calculation  Muon Flux & Spectrum - well understood  Neutrons produced / muon track (cm^2/g) CDMS Simulations (Perera CWRU/Yellin UCSB) based on papers below — # of neutrons per cm^2/g = 1.124*(depth)^(.47) [conservative UL, 3x uncertainty] — 5.2E-3 Soudan — Production spectrum of neutrons taken from Khalchukov 1983 – ~50% prod from direct µ-nuclear (virtual photon) Khalchukov 1983 ~E -1 – Rest EM cascade ( ,n) - Khalchukov softer component ~E -2 Aglietta 1989 (inc measurements from Mont Blanc) — Imply ~+/-50% error Aglietta 1999 LVD — Factor 3 below 1989 Wang 2001 — FLUKA Calculation (µ -> n) based on µ spec, and prod  (reasonably well known) — Predictions between two refs above CODE  GEANT3 µ-nuclear  ‘s are known to be too low by 10x (Battistoni 1998) Gets energy loss right by producing too many neutrons GEANT3 hardwired to older GHEISHA libraries So Fluka is necessary  GEANT4 Starting to cross-check with above R. J. Gaitskell astro-ph (IDM2000 Conference) Y.-F. Wang et.al. Phys. Rev. D, 64, , M. Aglietta et.al. hep-ex , 1999

HE Neutrons - XENON Aug 2003 Rick Gaitskell Photon background is very manageable  Current lower limit on discrimination already good enough if raw rate reduced to 13 mdru, ~2-3 times better than best levels reached so far (by IGEX, H-M) achieve via materials selection and simplification of structures (little mounting material)  Screening to g/g can reveal contamination source of such a background Electron background more worrisome, but certainly tractable  Need to screen & clean surfaces to 2.5 x counts/ (keV m 2 day) at current rejection efficiency of 99.5% Background Projections - CryoArray (1 ton) dru = 1 event keV -1 kg -1 day -1 Energy Range keV

HE Neutrons - XENON Aug 2003 Rick Gaitskell Neutron Background in 1-Ton Detector Expect dominant component from muon interactions in rock  Use veto and water/polyethylene shield for low- energy or locally produced neutrons  Neutron shield transparent above 50 MeV Need x20 improvement vs CDMS II  Increase depth to ~ 4000 mwe (or more)  Shield/moderator sandwiching inside veto  Thick (expensive) active-scintillator veto  Instrument the rock with veto  Preliminary simulations indicate >75% of emergent HE neutrons from hadron cascades are >50-cm transverse size. Soudan

HE Neutrons - XENON Aug 2003 Rick Gaitskell Aidé Memoire (Muon Rates)

HE Neutrons - XENON Aug 2003 Rick Gaitskell Nuclear Recoil Discrimination - Event by Event Nuclear recoils arise from  WIMPs  Neutrons Electron Recoils arise from  photons  electrons  alphas (Typical Background) Ionization yield  ionization/recoil energy strongly dependent on type of recoil Recoil energy  Phonons give full recoil energy Neutrons (external source) Gammas (external source) Phonon Trigger Threshold 1334 gamma events, 616 neutron events

HE Neutrons - XENON Aug 2003 Rick Gaitskell High Energy (E>10 MeV) Neutrons from Muons Neutron production ~ Muon Flux  With slight modification for hardening of muon spectrum † mean(E  )~ Depth 0.47 †Aglietta et.al. Nuove Cimento 12, N4, page 467 Soudan

HE Neutrons - XENON Aug 2003 Rick Gaitskell Neutron Background in CryoArray Expect dominant component from muon interactions in rock  Veto in cavity difficult – neutrons from 2 – 3 meters in  Polyethylene shield transparent above 50 MeV Need factor x20 improvement vs CDMS II  Increase depth to => 4000 mwe  Instrument the rock with 2.5 m streamer tubes.  Preliminary simulations indicate >75% of emergent HE neutrons from hadron cascades are >50-cm transverse size.  Augment with ‘umbrella’ veto  Increase shield density inside veto Soudan

HE Neutrons - XENON Aug 2003 Rick Gaitskell Anatomy of Penetrating Neutron Event (ii) 330 MeV neutron from rock (iii)Pb nucleus shattered 9 n (T MeV) 9 g (E MeV) (vi)Following ~12 scatters in Cu/poly neutron (now T~100 keV) (vi) scatters in two Ge detectors (Er~5 keV), and then (vii) ultimately captures on H in poly. (v)Higher energy (30 MeV) neutron traverses poly m.f.p ~ 100 cm (i) ~100 GeV µ interacts in rock of tunnel generating neutron (iv)Lower energy neutrons moderate in polyethylene m.f.p ~ 3 MeV

HE Neutrons - XENON Aug 2003 Rick Gaitskell Neutron Penetration (Water) Attenuation of High Energy Neutron Flux in water shield 300 MeV 10 MeV 1 MeV Water 300 MeV neutron injected in +z direction Monte Carlo Simulations Performed by Thushara Perera, CWRU using GEANT/MICAP/FLUKA (10 events) Typical multiplicity of 1-few

HE Neutrons - XENON Aug 2003 Rick Gaitskell Neutron Penetration (Fe vs Water) 300 MeV 10 MeV 1 MeV Water (only 1 event shown for clarity) Multiplicity of neutrons generated per event is higher (~20) 300 MeV 100 keV 10 keV Summary Fe (KE MeV) 100 cm x10 atten (>1 MeV) 200 cm x10 atten (>10 keV) Fe (10 events) Typical multiplicity of 1-few

HE Neutrons - XENON Aug 2003 Rick Gaitskell Neutron Penetration (Fe vs Water) 300 MeV 10 MeV 1 MeV Water Fe 300 MeV 100 keV 10 keV Fe (KE MeV) 100 cm x10 atten (>1 MeV) 200 cm x10 atten (>10 keV)

HE Neutrons - XENON Aug 2003 Rick Gaitskell Shielding Eff x Flux - Balanced in range MeV Neutron production (fluxes for Soudan)

HE Neutrons - XENON Aug 2003 Rick Gaitskell CDMS II Soudan - outer Pb and poly shielding Base sections of lead and polyethylene shields test assembly at UCSB who built shield. Note Poly-Pb-Poly layers The interleaved approach reduced neutron hits by 8-10x

HE Neutrons - XENON Aug 2003 Rick Gaitskell Neutron Subtraction - CDMS/CryoArray WIMP /Neutron Kinematics/Cross-section  e.g. Mixed Ge & Si targets Multiple Scattering  Position / timing (ns) resolution Neutron Capture (Tag) in bulk

HE Neutrons - XENON Aug 2003 Rick Gaitskell Dark Matter Depth Requirements Site Depth Requirement  Dominated by need to reduce high energy neutrons ( MeV), generated by muons, that cannot be moderated directly using poly  Shallow ~1700 mwe (1 muons/m 2 /minute) Just satisfactory for 10 kg scale experiments (  ~10 -8 pb) 1 tonne experiments would require large additional active shield (>1 m thick) — >99% veto Risk associated with systematic misidentification  Intermediate ~3800 mwe Factor ~50x reduction in muons/HE neutrons compared to shallow Additional comfort factor, general consensus that 1 tonne experiments can function comfortably wrt to HE neutrons from muons (  ~ pb) Depth may be necessary for gas target given much large surface area to shield Satisfactory for cosmogenic activation Muons passing through detector array can be vetoed by simple muon veto (>99% being achieved)  Deep ~6000 mwe (Further factor ~50x reduction in muon/HE neutrons) Does not appear to be necessary for 1 tonne (  ~ pb), but eliminates any risk, and will allow next-next generation