The BetaCage, an ultra-sensitive screener for surface contamination
R. Bunker, Z. Ahmed, M. A. Bowles, S. R. Golwala, D. R. Grant, M. Kos, R. H. Nelson, R. W. Schnee, A. Rider, B. Wang, A. Zahn
TThe BetaCage, an ultra-sensitive screener for surfacecontamination
R. Bunker ∗ , Z. Ahmed † , M.A. Bowles ∗ , S.R. Golwala † , D.R. Grant ∗∗ , M. Kos ∗ ,‡ ,R.H. Nelson † , R.W. Schnee ∗ , A. Rider † , B. Wang ∗ and A. Zahn † ∗ Department of Physics, Syracuse University, Syracuse, NY 13244, USA † California Institute of Technology, Pasadena, CA 91125, USA ∗∗ University of Alberta, Edmonton, AB, T6G 2R3, Canada ‡ Pacific Northwest National Laboratory, Richland, WA 99352, USA
Abstract.
Material screening for identifying low-energy electron emitters and alpha-decaying isotopes is now a prerequisitefor rare-event searches ( e.g. , dark-matter direct detection and neutrinoless double-beta decay) for which surface radiocon-tamination has become an increasingly important background. The BetaCage, a gaseous neon time-projection chamber, is aproposed ultra-sensitive (and nondestructive) screener for alpha- and beta-emitting surface contaminants to which existingscreening facilities are insufficiently sensitive. Sensitivity goals are 0.1 betas keV − m − day − and 0.1 alphas m − day − ,with the former limited by Compton scattering of photons in the screening samples and (thanks to tracking) the latter expectedto be signal-limited; radioassays and simulations indicate backgrounds from detector materials and radon daughters should besubdominant. We report on details of the background simulations and detector design that provide the discrimination, shield-ing, and radiopurity necessary to reach our sensitivity goals for a chamber with a 95 ×
95 cm sample area positioned belowa 40 cm drift region and monitored by crisscrossed anode and cathode planes consisting of 151 wires each. Keywords: radiopurity screening, beta decay, alpha decay, dark matter, proportional counter, radon mitigation
PACS:
INTRODUCTION AND DETECTOR DESIGN
Nonpenetrating radiation on detector surfaces, particularly from radon daughters, provides a dominant backgroundfor many underground physics experiments. Radon daughters from the atmosphere deposit onto surfaces and decay tothe long-lived
Pb, a low-energy beta emitter, and then to the alpha-emitting
Po. Unfortunately, high-sensitivitydetection of the
Pb 46 keV gamma ray from material surfaces is generally not feasible with HPGe detectors, in partdue to its low branching fraction. Furthermore, since these surface contaminants are chemically separated and henceout of equilibrium with the photon-emitting isotopes in the parent
U decay chain, direct detection of the
Poalphas or betas from
Pb or
Bi is necessary to establish the surface contamination level. More sensitive detectionof alphas and betas on surfaces would also serve archeology, biology, climatology, environmental science, geology,integrated-circuit quality control, and planetary science [1].The BetaCage [1, 2, 3], an ultra-low-background drift chamber, provides the largest efficiency possible for detectionof alphas and <
200 keV electrons from a sample’s surface by eliminating backscattering and dead-layer effects whileproviding a large sensitive area. The detector uses the minimum amount of gas needed to stop particles of interest inorder to minimize background from ambient penetrating gammas. It has the minimum possible surface area that itselfcan be a source of background particles. Finally, it provides sufficient spatial information to distinguish events comingfrom the sample surface from those due to scattering of background particles in the gas.Figure 1 shows sketches of the proposed BetaCage. Samples are placed in the gas directly under an open 95 ×
95 cm multi-wire proportional counter (MWPC) that provides a trigger for particles emanating from the sample. Above this“trigger” MWPC is a 40-cm-high region, large enough to contain the full tracks of alphas and 200 keV betas. Anelectric field in this region drifts the ionization to the top of the chamber, where a second open (“bulk”) MWPCcollects it. Proportional avalanching and crossed grids in both MWPCs provide gain and xy -position determination,respectively. The time profile of charge collection in the bulk MWPC determines the spatial profile of the track in the z dimension, while the trigger MWPC provides a start time to establish absolute z location. The right panel of Fig. 1demonstrates how the BetaCage achieves excellent rejection of background events, especially when used for alphascreening (since alpha tracks are straight enough not to suffer reconstruction failures). a r X i v : . [ phy s i c s . i n s - d e t ] A p r IGURE 1.
Schematic side views of the BetaCage. The sample lies between the 95 ×
95 cm veto and trigger MWPCs, 40 cmbelow the bulk MWPC. Left : The shielding consists of 15 cm lead outside 5 cm low-activity lead and a 1-cm thick OFE Culiner, surrounding a PMMA vessel. Not shown is a 2-mm thick stainless-steel dogleg to feedthrough signals.
Right : Examplesof background tracks rejectable (dark lines) due to their (A) insufficient energy in the trigger MWPC, (B) lack of containment inthe fiducial drift region, (C) too much energy in the veto MWPC, (D) insufficient energy in the bulk MWPC, or (E) uniform dE / dx ,even at the apparent track end. Rejection is less effective (light lines) for betas or alphas emitted from the drift gas and cathode wiresdirectly above the sample, or from the surface of the sample itself, following Rn-daughter implantation or Compton scattering. FIGURE 2.
Left : Expected rate of photon-induced backgrounds after cumulative data-selection cuts requiring, from top to bottom,(1, black) < > > Right : Division by source of expected photon-induced backgrounds after all cuts. From topto bottom, spectra are from contamination in the Noryl MWPC frames, lead shielding, acrylic chamber, copper liner, field-shapingresistors, and stainless-steel feedthrough. The Noryl spectrum is based on screening upper limits and is likely conservative; the leadis expected to dominate. Other assumed material-contamination levels are based on upper limits or measurements in [7, 8, 9].
EXPECTED PHOTON-INDUCED BACKGROUNDS
The planned 20-cm thick lead shield should reduce backgrounds due to external photons to a sub-dominant level,based on past efforts and simulations [4, 5]. The background from
Bi in the inner 5 cm of lead with 3 Bq/kg
Pb,simulated by the technique of [6], is likely to be dominant (but is expensive to reduce). Additional photons are emittedfrom internal acrylic and copper components, and the Noryl MWPC frames. The most challenging requirement ofthe detector was to identify a plastic that is sufficiently radiopure yet strong enough to handle the wire tension (40 kgin each direction) and can be precision machined when properly annealed. Screening with the UMN Gopher HPGedetector and the UC Davis neutron-activation technique, and ICP-MS measurements at Caltech demonstrated thatNoryl has sufficiently low contamination levels ( ≤ U, 3 mBq/kg
Th, and 5 mBq/kg K). Figure 2shows the expected background spectra from photons, broken down by cuts, and by source material after all cuts.
IGURE 3.
Left : Expected rate of radon-induced backgrounds before and after cumulative data-selection cuts requiring, fromtop to bottom, (1) < > > ∼ × (bottomspectrum). Right : Fraction of emanated Rn that decays in the detection chamber rather than in the radon trap, as functions of thetrap temperature, for the flow rates listed in the legend (lpm). The × indicates the parameters for the planned carbon trap, reducingradon-induced backgrounds by ∼ × (even if the trap’s efficiency is ∼ × worse than ideal). Below a certain temperature,enough Rn atoms entering the trap decay (in the trap) that its effectiveness is limited by the fraction of Rn atoms that decay beforethey are purged from the detection chamber (via circulation), and is constant with temperature. EXPECTED RADON-INDUCED BACKGROUNDS
Radon daughter plate-out onto BetaCage wires before or during construction of the MWPCs can be a dominantbackground. If the initial
Pb surface concentration on purchased wires (currently undergoing screening) proves tobe larger than the background goal, the wire will be electropolished [10] with a partially automated setup prior to wire-frame construction. We have already demonstrated the ability to reduce contamination while removing a consistentthickness of stainless-steel wire [11]. To minimize deposition during construction, the MWPCs will be strung withinSyracuse’s low-radon cleanroom [12]. The Jacobi model predicts the achieved radon level < is sufficientlylow to keep this background sub-dominant; tests are in progress to confirm the predicted deposition rate.Radon emanation from detector materials after construction similarly can be a dominant background. The leftpanel of Fig. 3 shows this expected background, starting from an assumed total emanation rate supporting 1300 Rn decays/day, which should be readily achievable with careful materials selection. Due to the long (22.3 year)half-life of
Pb, the rate of the late beta decays in the radon chain (
Pb and
Bi) will be lower than the rate ofthe early betas, by about an order of magnitude after the detector is operated for a few years. Here we assume 130
Pb and
Bi decays/day. Since most Rn daughters are positively charged, most will plate out onto the most negativenearby surface, typically the top cathode wire of the trigger MWPC. We assume 70% of the decays occur on thesewires, 25% on the veto MWPC wires, and 5% in the gas volume. Simulations indicate that data-selection cuts reducethe resulting background rate by about an order of magnitude, as shown in the left panel of Fig. 3.In order to reduce this (otherwise dominant) rate further, the gas-handling system will include a cooled-carbon radontrap, 25 cm long with a 2.5 cm diameter, including 70 g of high-quality synthetic carbon (Blucher GmbH 102688) with1342 m /g surface area. For an 8-lpm circulation flow rate, operation of the trap at 170 K should result in a sufficientlysmall survival probability for Rn atoms entering the trap that the radon-mitigation factor will be dominated by theflush time of the 0.6 m detector volume, yielding a reduction of > ∼ . A low-radon purgearound the PMMA vessel will mitigate this background. The radon level requires that the gas handling system beleak-tight to ∼ − mbar L/s downstream of the carbon trap. Upstream of the carbon trap, a much higher leak rate of ∼ .
01 mbar L/s is allowed, potentially permitting the use of a relatively leaky mini-diaphragm circulation pump. Thecarbon trap also can be used, if needed, to reduce the radon level in the neon when first filling the chamber.
XPECTED SENSITIVITY TO BETAS AND ALPHAS
The expected total beta background, dominated by the photon-induced background ( >
80% of total), is 0.25 be-tas keV − m − day − from 6 to 200 keV. One day of background measurement would establish a 49 m − day − back-ground rate over 6 to 200 keV to a precision of 7.0 m − day − , allowing detection at > σ of a 0.1 keV − m − day − rate contaminant in a total of 3 days of running. Fifteen days counting for sample and background (each) would allowsensitivity to a 0.04 keV − m − day − rate. Even for a background rate 10 × higher than expected, fifteen days countingfor sample and background (each) would yield sensitivity to a 0.13 keV − m − day − rate.The dominant background for alpha counting is expected to be from radon daughters. Gamma rays from naturalradioactivity have too little energy to be mistaken for surface alphas, and the detector’s underground, shielded envi-ronment should make unvetoed backgrounds from cosmic rays negligible. Head/tail track discrimination is expectedto be essentially perfect. With the detector’s clean gas, low surface area of material in the fiducial region, and tracking,the expected background is > × lower than the best currently achieved, 20 m − day − with the XIA UltraLo-1800 [13]. In practice, alpha sensitivity limits are likely to be signal limited; for two weeks counting, the expectedsensitivity ∼ . − day − . ACKNOWLEDGMENTS
This work was supported in part by the National Science Foundation (Grants No. PHY-0855525 and PHY-0919278)and the Department of Energy HEP division.
REFERENCES
1. R. W. Schnee, Z. Ahmed, S. R. Golwala, D. R. Grant, and K. Poinar, “Screening Surface Contamination with BetaCage,”in
Topical Workshop on Low Radioactivity Techniques: LRT 2006 , American Institute of Physics Conference Proceedings,Melville, NY, 2007, vol. 897, pp. 20–25.2. T. Shutt, C. E. Dahl, L. de Viveiros, R. J. Gaitskell, and R. W. Schnee, “Beta Cage: A New, Large-Area Multi-Wire ScreeningDetector For Surface Beta Contamination,” in
Topical Workshop on Low Radioactivity Techniques: LRT 2004 , AmericanInstitute of Physics Conference Proceedings, Melville, NY, 2005, vol. 785, pp. 79–83.3. Z. Ahmed, S. R. Golwala, D. R. Grant, M. Kos, R. H. Nelson, R. W. Schnee, and B. Wang, “Status of BetaCage: anUltra-sensitive Screener for Surface Contamination,” in
American Institute of Physics Conference Series , edited by R. Ford,2011, vol. 1338 of
American Institute of Physics Conference Series , pp. 88–92.4. A. J. Da Silva,
Development of a Low Background Environment for the Cryogenic Dark Matter Search , Ph.D. thesis,University of British Columbia (1996).5. D. Abrams, D. S. Akerib, M. S. Armel-Funkhouser, L. Baudis, D. A. Bauer, A. Bolozdynya, P. L. Brink, R. Bunker,B. Cabrera, D. O. Caldwell, J. P. Castle, C. L. Chang, R. M. Clarke, M. B. Crisler, R. Dixon, D. Driscoll, S. Eichblatt, R. J.Gaitskell, S. R. Golwala, E. E. Haller, J. Hellmig, D. Holmgren, M. E. Huber, S. Kamat, A. Lu, V. Mandic, J. M. Martinis,P. Meunier, S. W. Nam, H. Nelson, T. A. Perera, M. C. Perillo Isaac, W. Rau, R. R. Ross, T. Saab, B. Sadoulet, J. Sander,R. W. Schnee, T. Shutt, A. Smith, A. H. Sonnenschein, A. L. Spadafora, G. Wang, S. Yellin, and B. A. Young,
Phys. Rev. D , 122003/1–35 (2002).6. P. Vojtyla, Nuclear Instruments and Methods in Physics Research B , 189–198 (1996).7. J. Loach, J. Cooley, A. Cox, A. Poon, K. Adler, M. Bruemmer, K. Nguyen, and B. Wise, “A Community Material AssayDatabase,” in
Topical Workshop on Low Radioactivity Techniques: LRT 2013 , edited by L. Miramonti, and L. Pandola, 2013,vol. 1549 of
American Institute of Physics Conference Series , pp. 8–11, radiopurity.org.8. E. Aprile, K. Arisaka, F. Arneodo, A. Askin, L. Baudis, A. Behrens, K. Bokeloh, E. Brown, J. M. R. Cardoso, B. Choi,D. Cline, S. Fattori, A. D. Ferella, K.-L. Giboni, A. Kish, C. W. Lam, J. Lamblin, R. F. Lang, K. E. Lim, Q. Lin, S. Lindemann,M. Lindner, J. A. M. Lopes, K. Lung, T. Marrodán Undagoitia, Y. Mei, A. J. Melgarejo Fernandez, K. Ni, U. Oberlack,S. E. A. Orrigo, E. Pantic, G. Plante, A. C. C. Ribeiro, R. Santorelli, J. M. F. Dos Santos, M. Schumann, P. Shagin, H. Simgen,A. Teymourian, D. Thers, E. Tziaferi, H. Wang, M. Weber, and C. Weinheimer,
Physical Review D , 082001 (2011).9. S. Cebrian, “Radiopurity control in the NEXT-100 double beta decay experiment,” in Topical Workshop on Low RadioactivityTechniques: LRT 2013 , edited by L. Miramonti, and L. Pandola, 2013, vol. 1549 of
American Institute of Physics ConferenceSeries , pp. 46–49.10. G. Zuzel, and M. Wójcik,
Nuclear Instruments and Methods in Physics Research A , 140–148 (2012), URL .11. R. W. Schnee, M. A. Bowles, R. Bunker, K. McCabe, J. White, P. Cushman, M. Pepin, and V. Guiseppe, “Removal oflong-lived 222Rn daughters by electropolishing thin layers of stainless steel,” in
Topical Workshop on Low Radioactivityechniques: LRT 2013 , edited by L. Miramonti, and L. Pandola, 2013, vol. 1549 of
American Institute of Physics ConferenceSeries , pp. 128–131.12. R. W. Schnee, R. Bunker, G. Ghulam, D. Jardin, M. Kos, and A. Tenney, “Construction and measurements of a vacuum-swing-adsorption radon-mitigation system,” in
Topical Workshop on Low Radioactivity Techniques: LRT 2013 , edited byL. Miramonti, and L. Pandola, 2013, vol. 1549 of
American Institute of Physics Conference Series , pp. 116–119.13. W. Warburton, and B. Dwyer-McNally,
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactionswith Materials and Atoms263