Understanding neutron backgrounds at Oak Ridge National
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Transcript Understanding neutron backgrounds at Oak Ridge National
Understanding neutron backgrounds at Oak
Ridge National
Laboratory's Spallation Neutron Source by
assessing neutron activation in a p-type pointcontact Germanium detector
JASON SURBROOK
UNC-CH
TUNL-ORNL REU 2014
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
n
collaboration’s CEνNS
[sĕns]
Coherent Elastic NeutrinoNucleus Scattering
Assumed to be important
in Supernovae
Well calculable crosssection
n
Z0
A
A
Strong test of the Standard
Model
Courtesy Wikimedia Commons
Coherent Elastic Neutrino Scattering
Requires ν’s slower than 𝐸ν = 50MeV
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 𝐸ν = 30~50MeV; Ideal for scattering!!!
M: Nuclear mass
F: Form factor
Qw: Weak charge
E: n energy
T: nuclear recoil
energy
GF: Fermi constant
Free Pulsed Neutrinos
SNS beam operates at 60 Hz
2×
107
ν
𝑐𝑚2 𝑠
flux at 20 meters from target
Pulsing allows for 2000x reduction of background
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
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
range
Neutrons also impose prompt broad energy
background
Not measurable here, but requires attention!!
Good News, Everyone!
Looking more closely
Shielded Spectra
Shielded Spectra (<20 keV)
Well Defined Peak at 10.22 keV
Counts per day
Cosmogenic
Measured
34.1 ± 5.9
1547 ± 78
Wrapping Up
Results
Detector energy resolution is adequate for CEνNS
While at SNS, neutrons produced
68Ge
𝑎𝑡𝑜𝑚𝑠
5967 ± 288
𝑑𝑎𝑦
of
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
Questions?