Surface Screening with the BetaCage

Transcription

1 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 ) and by the South Dakota Board of Regents

2 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

3 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

4 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

5 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 ~ 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

6 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: BetaCage, Michael Bowles

7 Cross-Section View 7 BetaCage, Michael Bowles

8 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

9 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

10 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

11 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

12 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! 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

13 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) %(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 1/4 x RDAMIC using Si-P e-pair position correlation to 3 sigma in just 2 months analytic estimation! includes no energy loss! in Bulk of sample DAMIC 80/kg/day GEANT4 Sim. BetaCage Bkgd Q = kev Final Kinetic Energy (kev) 13 Freq. for 60 day Assay Electron Pair Y (mm) Isotopic Fraction! 28 Si = 92.2%! 29 Si = 4.7%! 30 Si = 3.1% Si P Pair Separation: Log Scale 1 R = ± 1.71 µm Electron Pair X (mm) 8 7 # events / X Y Bulk 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 Event Multiplicity (Events/Bin) ! 3! 2! 1! 0

14 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, BetaCage, Michael Bowles

15 15 BetaCage, Michael Bowles

16 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

17 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