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The Development of a Segmented Detector using ZnS:Ag/6Li

David Reyna

Sandia National Laboratories, CA

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2014-20440 PE

Lawrence Livermore National Laboratory

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Previous Prototype Deployment

David Reyna

  • As part of a joint SNL/LLNL project to develop aboveground detector technology, we deployed a 4-cell prototype segmented scintillator system
    • Aboveground shielded and unshielded runs in 2011
    • Belowground deployment in 2012-2013
  • Very encouraged by performance of Segmented Scintillator prototype
    • This technology is focused on reducing the overall footprint and enabling a transportable detector that can be deployed in high-background or unshielded locations
    • Demonstrated rejection of backgrounds of 5 orders of magnitude even without an external shield

Lawrence Livermore National Laboratory

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Segmented Scintillator Detector

David Reyna

  • Individual Segments contain organic scintillator with ZnS:Ag/6LiF screens on outer surface
    • 4 cells with plastic scintillator
  • Use of ZnS:Ag with 6LiF allows identification of neutron capture
    • ZnS:Ag is sensitive to alpha from n-capture on Li
    • Very slow scintillator time constant (~100ns) allows pulse shape discrimination to separate n-capture from γ events
  • This 4-cell prototype was intended for first testing background rejection only

Lawrence Livermore National Laboratory

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Particle Identification (PID)

Positron Identification through Topology

  • Positrons are rare in nature
    • Deposit most of their kinetic energy very quickly through standard ionization losses
  • Positrons will annihilate into two back-to-back 511 keV gammas
    • Very distinctive signature
    • Gammas will travel ~2-5” through most scintillators

David Reyna

e+

n

Liquid or Plastic scintillator

Neutron identification through Pulse Shape Discrimination (PSD)

Lawrence Livermore National Laboratory

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David Reyna

Aboveground Data

No PID cuts

225,177 ev/day

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Cut 1 = neutron PID only

2095 ev/day

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Cut 2 = neutron PID +

Loose positron topology

202 ev/day

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Cut 3 = neutron PID +

Strict positron topology

6 ev/day

Unshielded

Shielded

Expectation ~ 0.5 ev/day (cut 3)

D. Reyna et al. INMM proceedings (2012)

Lawrence Livermore National Laboratory

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This Work: Solve Optical Issues

David Reyna

Detected Neutron Events Along a Segment

Optical grease coupled ZnS does not transmit light

Lawrence Livermore National Laboratory

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Addition of WLS Should Help Preserve Total Internal Reflection

  • Standard Plastic Scintillator
    • Light from ZnS is reflected at boundary
    • Optical Grease coupling increased overall transmission into the plastic scintillator, but
    • Light that is transmitted, tends to be absorbed in the opposing ZnS sheet.

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  • Plastic + WLS
    • Light that is transmitted is now absorbed and re-emitted isotropically
    • Preserves internal TIR at the cost of inefficiency in the absorption/conversion process
    • WLS wavelength also has reduced quantum efficiency in PMT photocathodes

David Reyna

Lawrence Livermore National Laboratory

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Eljen 280 WLS Scintillator

David Reyna

Lawrence Livermore National Laboratory

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Montecarlo Comparison

David Reyna

Grease Coupled Normal Plastic Scintillator

Air Gap with Wavelength Shifting Plastic Scintillator

Lawrence Livermore National Laboratory

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Direct Measurement of Optical Transmission

Old Cell: Grease-coupled Bar

New Cell: Air-coupled Bar

λ ~33 cm

λ ~1.5 m

Lawrence Livermore National Laboratory

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Neutron Data Confirms Improvement

David Reyna

Lawrence Livermore National Laboratory

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Even PSD Shows More Uniform Energy Collection

Old Cell: Grease-coupled Bar

New Cell: Air-coupled Bar

Neutron cuts:

  • pulse-shape discrimination parameter greater than 0.6
  • Individual pmt energy greater than 250 keV and
  • less than 4000 keV

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Lawrence Livermore National Laboratory

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WLS Allows Extended Length

David Reyna

Test Bar

Attenuation length

Normalized neutron efficiency §

Original grease coupled 60cm

35.6 +- 1.2 cm

10.1 +- 0.9 %

WLS, air gap, 60 cm

118 +- 7 cm

10.8 +- 0.9 %

WLS, air gap, 120 cm

154 +- 11 cm

12.6 +- 1.2 %

WLS, air gap, 180 cm

200 +- 20 cm

10.9 +- 1.1 %

§ Efficiency calculated relative to a calibrated 3He detector and normalized to detector area

Lawrence Livermore National Laboratory

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Conclusions

  • WLS based segments show good uniform detector response even at 1.8m length
    • Overall efficiency is improved compared to previously deployed prototype
    • Probably could go to 3m
  • Antineutrino efficiency will improve as more segments are added
    • Single electronics rack could house ½ ton active volume (16 cells)
    • Future work will investigate how backgrounds scale with additional segments
  • The technology is easily deployed
    • Robust solid state materials
    • Full PID allows simple analysis that can be easily automated

David Reyna

More details can be found in Sweany et al, NIM A Volume 769, 1 January 2015, 37–43

Lawrence Livermore National Laboratory