A screening facility for next generation low-background experiments Tom Shutt Case Western Reserve University.

A screening facility for next generation low-background experiments Tom Shutt Case Western Reserve University 7/2/05 T. Shutt - Synergies2 Screening - a physics users perspective History of applying yesterdays detectors for screening today Ge counters from double beta decay Gaseous concentration and counting - Ar, 71 Ge, Rn Ge detectors, using Cu and Pb shielding Spectroscopy -> identify isotopes Easy to achieve 1 ppb U, Th or ~10 mBq/kg Best case to date: U, Th 50 ppt; K 50 ppb Limitations: Cosmogenic activation of Cu, Ge Contamination of surfaces - limited detector size. Majorana, Gerda pushing this technology. 7/2/05 T. Shutt - Synergies3 Screening Mass Spectrometry, NAA for U, Th, K With major effort: U, Th at ppt; K ppb Using specialized concentration -> , But in general not preferable to whole body counting Tiny sample size (g) May miss certain types of contaminaiton - e.g., dust. Limited sensitivtity for short-lived isotopes Other techniques Gas concentration - e.g., Rn Atom trapping -e.g., Ar, 85 Kr 7/2/05 T. Shutt - Synergies4 Next generation screening for physics Next generation experiments require large advances in lower backgrounds. Current screening few (U, Th) at best Need: g/g Previous direct screening based on small-size Ge counters from double beta decay What about screening based on a solar neutrino experiment? 7/2/05 T. Shutt - Synergies5 7/2/05 T. Shutt - Synergies6 Water shield SS Sphere 6-8 m (less with low background PMTs) scintillator PMTs 100 Sample 20 cm , 40 cm long Plastic - 13 Kg plastic Cu Kg Mini-me version of Borexino Whole-body counting of sample 14 C sets threshold near 250 KeV A new facility Laura Cadonati (MIT) 7/2/05 T. Shutt - Synergies7 Purification of scintillator Non-polar solvent Extremely low solubility for ionic impurities Purification methods developed Distillation Water extraction N2 stripping Solid-column adsorption Expect at least: g/g U,Th g/g K. 7/2/05 T. Shutt - Synergies8 Sensitive to: Photons emerging Betas, alphas on surface If sample is attacked by scintillator: Seal in 50 m film of nylon Not sensitive to alphas Alphas distinguished by pulse-shape Betas and photons distinguished by event shape 7/2/05 T. Shutt - Synergies9 Backgrounds Estimates based on Borexino work PMTs - dominant Nylon vessel ( ppt U, Th; 20 ppb K) Nylon plumbing ( 50 ppb K) Scintillator (Borexino goal: g/g U,Th) Dominant radioactivity is external, so use position reconstruction. 7/2/05 T. Shutt - Synergies10 Fiducial Volume PMT background Signal Fiducial cut Vessel radius Radius (cm) x 10 cm at 1 MeV 7/2/05 T. Shutt - Synergies11 Background Same as 95 % CL with no counts. At 30 days counting, have 3 counts. Background free detector 7/2/05 T. Shutt - Synergies12 Photons detected outside sample Inside sample Outside sample Threshold sample Detected energy Energy Absorbed in sample scintillator This simulation: Ge sphere 20 cm M = 22 Kg 7/2/05 T. Shutt - Synergies13 Detection efficiency vs. Energy Reasonably good for E > 500 KeV 7/2/05 T. Shutt - Synergies14 Consider equilibrium U chain Total Counts/day: 0.15 total 0.10 fiducial Rate outside 22 Kg Ge sphere with g/g U 7/2/05 T. Shutt - Synergies15 U, as detected E 8% at 1 MeV 7/2/05 T. Shutt - Synergies16 Sensitivity Total background (E>250 keV): 0.1 counts/day 50 cnts/day/ton U,Th, K Contamination limits, g/g: Continuum background of Compton photons: Surface emitters, E > 250 keV: 0.8 cnts/day/m 2 (not sensitive to s if need to seal sample in film) 1 day counting30 days counting U 3 E-13 1 E-14 Th 8 E-13 4 E-14 K 2 E-9 8 E-11 1 day counting2 E-4 counts/Kg/keV/day 30 days counting6 E-6 counts/Kg/keV/day 7/2/05 T. Shutt - Synergies17 What this wont do Internal beta, alpha contamination High resolution measurement of lines Modest ability to distinguish contamination, especially if several contaminants Low energy photons: Reduced efficiency < 500 keV Zero efficiency < 250 keV 7/2/05 T. Shutt - Synergies18 Conclusion Can be built with existing technology fold increase in sensitivity Old: U,Th g/g New: U, Th g/g Gets within range of needed purity for next generation experiments: low E solar DM, cnts/day/ton 7/2/05 T. Shutt - Synergies19 Conclusion How to build it? Too big for single group Might get physics also? decay using doping (e.g., Xe) p-p just above 14 C Blank: Background levels are now Blank 7/2/05 T. Shutt - Synergies20 U and background 7/2/05 T. Shutt - Synergies21 Th and background 7/2/05 T. Shutt - Synergies22 Photon sensitivity (cnts/kg/keV/day) g/g U At MeV, good sensitivity to all photons. Below 500 keV, reduced sensitivity. Emergent continuum rate internal continuum rate Inside sample Outside sample