Surface Screening with the BetaCage Time Projection Chamber Michael A. Bowles!! South Dakota! School of Mines & Technology!! Low-Background Workshop!! May 16, 2017 This work was supported in part by the National Science Foundation (Grant No. PHY-1506033) and by the South Dakota Board of Regents
BetaCage Collaborators R. W. Schnee E. H. Miller M. A. Bowles S. Golwala R. H. Nelson Z. Ahmed R. A. Bunker D. Grant P. Davis BetaCage, Michael Bowles
BetaCage Detector Design Goal: perform high sensitivity isotopic assay of samples Time projection chamber (TPC) made from radio-pure material with a large sample area bulk grid Drift Region! has homogeneous! E-Field = 40 V/cm β track Gas stops all α s + low-energy β s Shield external radiation Deploy deep underground XY grids cm spatial resolution Trigger grid provides signal start time trigger grid Sample w/ Contaminants veto grid veto grid 3 BetaCage, Michael Bowles
BetaCage Track Production Bulk wire grid diagram legend secondary e- ionizing e- path wire grid electron avalanche Electric Field electron loses all its kinetic energy ionizing atoms and producing secondary electrons Drift Region! has homogeneous! E-Field = 40 V/cm Fast deposition of! electron s energy t ~ 1 ns Trigger wire grid Sample Area Sample Material with Surface Contaminants 4 BetaCage, Michael Bowles
BetaCage Signal Production Bulk wire grid diagram legend secondary e- ionizing e- path wire grid electron avalanche Drift Region! has homogeneous! E-Field = 40 V/cm Electric Field proportional avalanche! of secondaries entering! E-field near the wire grid t ~ 10-100 us Secondary electrons ( ) along ionized track are! drifted by electric field Trigger wire grid Sample Area Sample Material with Surface Contaminants 5 BetaCage, Michael Bowles
Superb Background Rejection Veto Principles:! A. Doesn t cross trigger grid! B. Track not 100% in target gas! Bulk Grid ` C. Crosses veto grid! ` D. No Energy in bulk grid! E. Wrong track direction: de/dx! Straightforward for α particles! F. Track doesn t start low enough! Straightforward for α particles Trigger Grid Veto Grid signal arxiv:1404.5803 6 BetaCage, Michael Bowles
Cross-Section View 7 BetaCage, Michael Bowles
Expected Photon Backgrounds 99.99% external gammas are blocked by lead shielding Simulations indicate gammas from the lead surface will provide most significant photon backgrounds Total background for betas: 0.25 kev -1 m -2 day -1 Photon Flux from Pb Surface figures: B. Wang (Syr Univ) 8 BetaCage, Michael Bowles
Radon Daughters Backgrounds Radon daughters (Po, Pb) on trigger grid wires + in gas are a dangerous alpha & beta background for the BetaCage Still can veto most events from wires: using trigger signal Beta-emitters elsewhere can be vetoed almost perfectly! Main Sources of Background 1. (Un)clean cathode/anode wire 2. During the assembly of the detector in the cleanroom Fraction of mis-identified events figure: B. Wang (Syr Univ) Will improve rejection by! applying timing, de/dx cuts 3. Plate-out from radon-222 during detector lifetime Total Depositied Energy (kev) 9 BetaCage, Michael Bowles
Radon Emanation Backgrounds Can reject >80% of events due to radon progeny decay following emanation during assay: improve w/ timing cut Expected background [Pb] still dominates without mitigation for conservatively estimated emanation Trigger &Energy! in Bulk all Rn events &pass selection cuts 100x lower &mitigation 100x improvement, background becomes subdominant, at flow rate of 8 Liters/min through 70 gm cryogenic carbon trap 1. Trigger & no veto signal 2. Bulk signal collected 3. Apply selection cuts 4. Employ radon mitigation 10 BetaCage, Michael Bowles
BetaCage Backgrounds & Expected Sensitivity BetaCage backgrounds: 0.3 /kev/m 2 /day Expected from simulation including all rejection from cuts & using radon mitigated gas! BetaCage alpha/beta particle sensitivity: α: 0.1 /m 2 /day, signal limited :) β: 0.1 /kev/m 2 /day, after background subtraction 11 BetaCage, Michael Bowles
Si-32 Contamination Experiments w/ silicon targets (SuperCDMS, DAMIC) face a major background from 32 Si (& 32 P) β decays..cosmic Ray spallation in upper atmosphere Identify correlated Si-P beta pairs via event multiplicity in XY binskb +110! - 65 32 Si β: Q = 225 kev t1/2 ~150 y 32 P β: Q = 1.7 MeV t1/2 ~14.3 d 32 S DAMIC measured 32 Si: 80 events/kg/day (95% C.L.) Hypothesize: contamination throughout CCD bulk Can the BetaCage screen silicon for rare-event searches? 12 BetaCage, Michael Bowles
Simulating Si-32 Contamination Have simulated 32 Si & 32 P contamination in 1/2 mm thick sample mimics ~3 gm DAMIC CCDs 32 Si Escape Rate (/ kev / cm 2 / day) β s escaping Surface: 50% (up) + 17.5%(down) $ BetaCage can ID 1% DAMIC rate in 35 days β s escaping from Bulk: Si = 3.7%, P = 70%, >95% of parent-daughter decays in same bin! BetaCage is sensitive enough detect β s coming from bulk @ 1/4 x RDAMIC using Si-P e-pair position correlation to 3 sigma in just 2 months 10 5 10 6 10 7 10 8 analytic estimation! includes no energy loss! in Bulk of sample DAMIC 80/kg/day GEANT4 Sim. BetaCage Bkgd Q = 224.5 kev 0 50 100 150 200 250 Final Kinetic Energy (kev) 13 Freq. for 60 day Assay Electron Pair Y (mm) 10 5 10 4 10 3 10 2 10 1 10 0 0.5 0 0.5 Isotopic Fraction! 28 Si = 92.2%! 29 Si = 4.7%! 30 Si = 3.1% Si P Pair Separation: Log Scale 1 R = 156.61 ± 1.71 µm 1 1 0.5 0 0.5 1 Electron Pair X (mm) 8 7 # events / X Y Bulk Cont. @ 1/4 DAMIC Rate + 90% C.L. Backgrounds Only Bulk 32 Si & 32 P Only All Events Si- & P-32 β thin slab of silicon 0 1 2 Event Multiplicity (Events/Bin) 6 5 4 3 2 1 0 4! 3! 2! 1! 0
BetaCage Protoype Wire grid 50 ee g g a a C C d d l el e i i FF ld e fi t f i dr m c / V 79 wire grids Wi re gri d 40 x 40 x 20 c m 33 JINST 9 P01009, 2014 14 BetaCage, Michael Bowles
15 BetaCage, Michael Bowles
Live-monitoring software with low-level trigger/pulse data & operating conditions of vessel High V oltage Supply Have turned on High Voltage to see sparks during ramp up Calibration w/ alpha sources Implement track reconstruction! n o i t i s i u q Data Ac m e t s y S ) (DAQ m oom nroo eanr Clea donn Cl Rado w-ra LowLo inside de tside outsi ou s l e s n l n e a n h n c a a ch t a a d t a e l d p i e t l l p muulti m s! nics! ctronic electro ele Digitize r Board ru dthru feedth fee 16 BetaCage, Michael Bowles
BetaCage Outlook BetaCage will provide incredible sensitivity to alphas and low-energy betas on surfaces & within the bulk of some materials Expected sensitivity: 0.1 β / kev-m 2 -day & 0.1 α / m 2 -day Design has matured: Prototype commissioning in progress Continue estimating assay sensitivity: simulating internal U/Th material contamination levels & external backgrounds Short term: Demonstrate the Prototype s sensitivity ~ 0.1 α / m 2- day 17 BetaCage, Michael Bowles