1. 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

2. Oak Ridge National Lab’s Spallation Neutron Source.
What is the SNS
Neutron production by Hg spallation
Proton acceleration to 1 GeV, about 1.4 MWatts
Each P “spalls” 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]Z0
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
Courtesy Wikimedia Commons

4. Coherent Elastic Neutrino Scattering
Requires ν’s slower than 𝐸 ν =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 𝐸 ν =30~50MeV; 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
2× ν 𝑐 𝑚 2 𝑠 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

8. 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

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

10. 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!!

11. Good News, Everyone!

12. Looking more closely

13. Shielded Spectra

14. Shielded Spectra (<20 keV)

15. Well Defined Peak at 10.22 keV
Cosmogenic
Measured
Counts per day
34.1±5.9
1547±78

16. Wrapping Up Results Detector energy resolution is adequate for CEνNS
While at SNS, neutrons produced 5967±288 𝑎𝑡𝑜𝑚𝑠 𝑑𝑎𝑦 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

17. Questions?