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

7/2/05 T. Shutt - Synergies2 Screening - a physics user’s perspective History of applying “yesterday’s” 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?

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7/2/05 T. Shutt - Synergies6 Water shield SS Sphere 6-8 m Ø (less with low background PMTs) scintillator PMTs ≈ 100 Sample 20 cm Ø, 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 µm 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 Ø 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 won’t 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