Beta Screening Options

Transcription

1 Beta Screening Options in an Underground Low-Background Counting Facility Case Western Reserve University Based on work with or by Dan Akerib, Eric Dahl, Raul Hennings-Yeoman, Tom Shutt (Case), Luis DeViveiros & Rick Gaitskell (Brown), Sunil Golwala (Caltech), Steve Elliot & Tom Bowles (LANL), Harry Nelson (UCSB), Prisca Cushman (UMinn), & others?

2 Outline Large gas chambers for beta screening BetaCage - a multiwire proportional counter chamber Cloud Chamber Sensitivity and applications Betas in (thin surface layers of) samples Carbon and tritium dating (Alphas)

3 Need for beta screening facility Better sensitivity to some beta-emitting isotopes than Ge γ detectors or other techniques (ICP-MS, α screeners, etc.) Some beta-emitting isotopes can be probed only by their beta emission

4 Commercial Beta Screeners Standard is Liquid Scintillation Counters with backgrounds as small as a few counts per hour. Samples prepared in aqueous cocktail Also gas flow proportional counters semiconductors for alpha spectrometry (background ~3 counts per day)

5 Achieving Lower Backgrounds Backgrounds are proportional to mass of detector Ultraclean materials to minimize internal contamination Underground, shielded apparatus to minimize external backgrounds Deploy minimum material needed to stop βs. Gas is best method to achieve this low mass 100 kev e cm Ne (1 atm) Can identify betas with 400 kev endpoint with 50 cm height Use Xe (1 atm) for higher-energy betas and alphas Maximize counting statistics Large surface area (horizontal dimension) ~1 m 2 Guard region to reject events emitted from outside chamber

6 Option 1: BetaCage Multiwire proportional counter Wires provide minimum surface area for emissions 20 µm Ø, 1/2 cm spacing - 0.4% coverage Crossed grids could yield ~mm xy position information Is this a significant benefit for any applications? drift grids 30 cm trigger grids sample 5 mm bottom guard grids

7 Option 1: BetaCage Reject background interactions in bulk of gas by creating narrow (5 mm) trigger region near samples Most gamma interactions in gas don t cause trigger Reduces backgrounds in gas to 15% of unrejectable total due to gamma interactions in sample that eject electrons into trigger region (these look exactly like beta emission) drift grids 30 cm trigger grids sample 5 mm bottom guard grids

8 Results of Monte Carlo Simulation Gamma Background equivalent to 1 event/(kg kev day) L. DeViveiros & R. Gaitskell, Brown U. 30 cm Survey Substrates 5 mm

9 Operating MWPC Test Chamber E. Dahl & T. Shutt

10 Option 2: Cloud Chamber Cameras Processors Sample Mirrors or fiber optics Supersaturated gas condenses into clouds around ionization track Capture via digital cameras or fibers H. Nelson, UCSB Shield

11 Option 2: Cloud Chamber Excellent position information on track Reject events not from sample Sub-mm xy information on origin of radiation Stills captured from video from first prototype beta-counting cloud chamber H. Nelson, UCSB

12 Sensitivity (with background subtraction) (counts kev -1 cm -2 day -1 ) easy straightforward manageable

13 Translating This Sensitivity Sample with large surface area (~1000 cm 2 ) screened for a long time (~a month) can be probed to <10-5 counts kev -1 cm -2 day -1 Translating to Bq/m 2 is easy. Bq/kg (etc.) requires knowing thickness of beta-emitting part of sample. Can be done Isotope E max (kev) Bq/m 2 Bq/kg 14 C x Pb x H x K x 10-3 Table to be expanded as desired let me know

14 Original Motivation for These Beta Screeners CDMS experiment limited by beta contaminants on surface or in thin films of detectors Gas beta chambers ideal since potential contamination already on surface of solid to be placed in chamber W trap Bias rail Trilayer for Phonon sensors: W TES 35 nm of W 300 nm of Al 40 nm of amorphous Si Si or Ge substrate (1 cm thick NOT TO SCALE) Trilayer for Ionization grid - the B deposition: 40 nm of amorphous Si 20 nm of Al 20 nm of W

15 Questions on efficacy of chemistry, processing of sample Introduction of contamination? Losses when converting sample to assayable form? Method 1: Thin sample (solid or deposited onto planchet) Method 2: Fill counter with sample made into CO 2 or CH 4 (methane) to be used as active gas Carbon or Tritium Dating Removes advantage of background discrimination using trigger region Assume assay in 1 m 3 chamber with conservative background of 1 event kev -1 kg -1 day -1 (may expect 10x better) Background 3σ limit (1 day) 14 C/ 12 C in CO events/day 6 x C/ 12 C in CH events/day 4 x H/H in CH events/day 2 x σ limit (month) x x 10-21

16 Conclusions Large, clean, shielded, underground gas chamber could be world s most sensitive screener for all non-penetrating radiation Needed for screening thin films deposited on surface of CDMS detectors Use for other surface-screening? Excellent sensitivity for carbon or tritium dating Expect virtually no background for alphas Presumably dominated by U/Th in wires, radon/dust on sample surface Not yet examined What other applications could benefit from the ultralow backgrounds for non-penetrating radiation? Anything that should influence the screener design criteria?