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Jason Surbrook UNC-CH TUNL-ORNL REU 2014

Understanding neutron backgrounds at Oak Ridge National Laboratory's Spallation Neutron Source by assessing neutron activation in a p-type point-contact Germanium detector. Jason Surbrook UNC-CH TUNL-ORNL REU 2014. What is the SNS. Neutron production by Hg spallation

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Jason Surbrook UNC-CH TUNL-ORNL REU 2014

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  1. Understanding neutron backgrounds at Oak Ridge NationalLaboratory's Spallation Neutron Source by assessing neutron activation in a p-type point-contact Germanium detector Jason Surbrook UNC-CH TUNL-ORNL REU 2014

  2. What is the SNS Neutron production by Hg spallation Proton acceleration to 1 GeV, about 1.4 MWatts Each P “spalls” 20-30 N Oak Ridge National Lab’s Spallation Neutron Source. (courtesy of ORNL.gov) • SNS is a source of intense PULSED neutrino flux!

  3. Motivation • COHERENT collaboration’s CEνNS [sĕns] • Coherent Elastic Neutrino-Nucleus Scattering • Assumed to be important in Supernovae • Well calculable cross-section • Strong test of the Standard Model n n Z0 A A Courtesy Wikimedia Commons

  4. Coherent Elastic Neutrino Scattering • Requires ν’s slower than MeV • Faster, and it sees individual nucleons • Deviations from cross section suggest at physics beyond SM • SNS is a great location for neutrino research • High intensity, pulsed neutrinos • Free! Already being produced at SNS via pion decay • SNS MeV; Ideal for scattering!!! M: Nuclear mass F: Form factor Qw: Weak charge E: n energy T: nuclear recoil energy GF: Fermi constant

  5. Free Pulsed Neutrinos SNS beam operates at 60 Hz flux at 20 meters from target Pulsing allows for 2000x reduction of background

  6. The Detector • 0.825 kg High Purity Germanium (HPGe) • Excellent energy resolution • Established technology • Needs to be LN cooled • Has spent time unshielded in SNS target building • Crystal Dislocations? • Activation? Broad Energy Germanium (BEGe) detector cutaway from Canberra online catalogue

  7. Detector Motivation • HPGeand/or scintillators • Is this particular detector viable? • What exposure limits can we impose on HPGe? 2-Phase LXe CsI Crystal PPC HPGe Courtesy COHERENT collaboration

  8. Crystal Damage • Ge crystal dislocation due to fast neutrons • Dislocation sites = charge traps • Poor resolution • Geometric irregularities • Often skews observed energy peaks down

  9. νMeasurement Noise • Captured neutrons may cause detector radioactivity • 68Ge e-captures to 68Ga, Ga emits x-ray at 10.4 keV • Half Life is 271 days • Coherent scattering is expected to be in the several keVee range • Neutrons also impose prompt broad energy background • Not measurable here, but requires attention!!

  10. Good News, Everyone!

  11. Looking more closely

  12. Shielded Spectra

  13. Shielded Spectra (<20 keV)

  14. Well Defined Peak at 10.22 keV

  15. Wrapping Up Results • Detector energy resolution is adequate for CEνNS • While at SNS, neutrons produced of 68Ge in this detector • About 175x rate due to cosmic rays at sea-level Future Work • Low energy calibration, to correct the 10.4 keV peak • Explore T β-decay in low energy region of spectra • Shielding effects on activation rates

  16. Questions?

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