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?