The X Experiment - Background Model and Physics Goals D. Malling April APS, Denver, CO 2013-04-13
X Background Goals Background goal: <1 WIMP-like event in 100 kg fiducial in 300 days Dominated by PMTs (material selection difficult, proximity to active region) Counting goals: ensure contributions from other materials subdominant to PMTs D. Malling April APS Denver, CO April 13, 2013 2
X Backgrounds Program Monte Carlo Material Screening Generic ER Modeling for Small Internals Precision ER Modeling for Major Internals Precision NR Modeling for Major Internals In Situ Measurement S1/S2 Yields and Collection Efficiencies Activation Isotopes Predicted Low-Energy Background D. Malling April APS Denver, CO April 13, 2013 3
non Backgrounds Require low-energy single-scatter nuclear recoil in central (100 kg) fiducial volume X PMT ER backgrounds (0-25 kevee) with unoptimized 100 kg fiducial (dashed) WIMP search energy window: 5-25 kevnr (1-5 kevee) Gammas required to traverse length of detector, deposit <5 kevee energy, exit again without scattering; suppression seen at right Another >2 orders of magnitude rejection of ER events based on S2/S1 ratio Neutron generation 1 n/day from radioisotopes; NR events rejected by multiple scatter signatures DRUee = cts/kevee/kg/day D. Malling April APS Denver, CO April 13, 2013 4
Background Sources Internal components (PMTs, cryostats, panels, etc.) Managed primarily through material screening External radioactivity and muoninduced backgrounds External rates reduced to <x1/10 internal rates by 3.5 m thick water shield (300 tonnes) D. Malling April APS Denver, CO April 13, 2013 5
Background Modeling All Monte Carlo modeling performed using Geant4-based XSim package [NIM A675 63 (2012)] Highly faithful X detector geometry -- allows for detailed studies of backgrounds from individual internals / radioisotopes Integrates NEST code for S1/S2 yields [JINST 6 p10002 (2011)] D. Malling April APS Denver, CO April 13, 2013 6
Background Modeling Detailed ER / NR modeling performed for all large internals -- yields energy spectrum in detector as function of position, radioisotope concentration Generic activity library created to accommodate >100 small components -- whole regions of detector modeled with different radioisotopes Generic library sets required screening ULs, in addition to adding to total expected ER D. Malling April APS Denver, CO April 13, 2013 7
Counting / Modeling Results Counting results [Bq total in detector] Component Facility Amount in X 226 Ra ( 238 U) 232 Th 40 K Other PMTs + bases SOLO 122 1.3 0.35 8.2 0.3 ( 60 Co) Ti Oroville 231 <0.085 <0.185 <0.37 0.2 ( 46 Sc)* HDPE SOLO 34 kg <0.21 <0.1 PTFE SOLO 25 kg <0.12 <0.03 Stainless steel SOLO 10 kg 0.39 0.1 0.8 ( 60 Co) Cu SOLO 1100 kg 1.4 ( 60 Co)** Insulation SOLO 8 kg 0.9 <0.36 1.4 85 Kr -- <0.038 kg -1 Projected activity in 100 kg unoptimized fiducial [cts/kev/kg/day] Component ER (x10-3 ) NR (x10-6 ) * Measured from activation at Surface Lab; extrapolated for time underground ** Estimated from exposure at 1 km altitude PMTs + bases 0.5 0.1 Other 0.1 (<0.4) 0.2 (<0.2) Total 0.6 (<0.9) 0.3 (<0.3) 10-3 cts/kevee/kg/day = 0.4 evts / day BEFORE ER rejection (1-5 kevee, 100 kg fiducial) D. Malling April APS Denver, CO April 13, 2013 8
Projected ER Spectra XSim predicted ER background activity (internals only) in 100 kg unoptimized fiducial 10 2 Simulation Only cts / kev ee / kg / day 10 3 10 4 10 5 10 6 500 1000 1500 2000 2500 Energy deposited [kev ee ] Solid lines are positive measurements from SOLO counting; Dashed lines are ULs from SOLO counting D. Malling April APS Denver, CO April 13, 2013 9
Projected ER Spectra 10 0 131m Sum cts / kev ee / kg / day 10 1 10 2 10 3 10 4 (11.8 d) 129m (8.9 d) 133 127 (36 d) 10 5 (5.4 d) 100 200 300 400 500 Energy deposited [kev ee ] D. Malling April APS Denver, CO April 13, 2013 10
Projected ER Spectra 10 0 Sum cts / kev ee / kg / day 10 1 10 2 10 3 10 4 131m (11.8 d) 129m (8.9 d) 127 (36 d) 10 5 100 200 300 400 500 Energy deposited [kev ee ] D. Malling April APS Denver, CO April 13, 2013 11
Physics Reach Cross section [cm 2 ] (normalised to nucleon) 10 42 10 43 10 44 10 45 10 46 130410033001 http://dmtools.brown.edu/ Gaitskell,Mandic,Filippini 10 1 10 2 10 3 WIMP Mass [GeV/c 2 ] ZEPLIN III (2011) 319 day x 4.2 kg XENON100 (2012) 225 day x 34 kg X 0 BG 300 day x 100 kg D. Malling April APS Denver, CO April 13, 2013 12