jason surbrook unc-ch tunl-ornl reu 2014 understanding neutron backgrounds at oak ridge national...
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JASON SURBROOKUNC-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
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!
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
A A
n n
Z0
Courtesy Wikimedia Commons
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!!!
E: n energyT: nuclear recoil energyGF: Fermi constant
M: Nuclear massF: Form factorQw: Weak charge
Free Pulsed Neutrinos
SNS beam operates at 60 Hz flux at 20 meters from targetPulsing allows for 2000x reduction of background
The Detector
0.825 kg High Purity Germanium (HPGe) Excellent energy resolution Established technology
Needs to be LN cooledHas spent time unshielded
in SNS target building Crystal Dislocations? Activation?
Broad Energy Germanium (BEGe)
detector cutaway from Canberra online
catalogue
Detector Motivation
HPGe and/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
Crystal Damage
Ge crystal dislocation due to fast neutrons Dislocation sites = charge traps Poor resolution Geometric irregularities
Often skews observed energy peaks down
ν Measurement 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 rangeNeutrons also impose prompt broad energy
background Not measurable here, but requires attention!!
Wrapping Up
ResultsDetector energy resolution is adequate for
CEνNSWhile at SNS, neutrons produced of 68Ge in this
detector About 175x rate due to cosmic rays at sea-level
Future WorkLow energy calibration, to correct the 10.4 keV
peakExplore T β-decay in low energy region of spectraShielding effects on activation rates