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Photoneutron Source Carachterization and Neutron Simulations
Alan Robinson
Sept 24, 2015Low Energy Calibration Workshop
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LowECalSept 24, 2015
Photoneutron Sources and Simulations Photoneutron Sources● Portable
● Monochromatic
● Low-energy
● 105 – 106 photons/neutron
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Slide 3/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Photoneutron Sources
Source Half-life Photons Neutron Energy226Ra 1600 y 1764 keV (15.3 %)
2204 keV (4.92 %)others (9.69 %)
89 keV479 keV
124Sb 60.2 d 1691 keV (47.6 %)2091 keV (5.49 %)
others (0.52 %)
23.5 keV379 keV
88Y 106.6 d 1836 keV (99.9 %)2734 keV (0.71 %)
152 keV950 keV
207Bi 31.6 y 1770 keV (6.87 %) 94 keV
Radioisotope / beryllium sources
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Slide 4/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Photoneutron Calibrations● Need
►Well defined energy spectrum
►Absolute recoil rate
● Measure source yield
● Minimize moderator
● Understand neutron recoil response.
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LowECalSept 24, 2015
Photoneutron Sources and Simulations Photoneutron History● The NIST calibrated neutron source is a 1 Ci
226Ra/Be photoneutron source.
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Slide 6/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Photoneutron History● Measurements of 9Be(γ,n) cross-section
►1940-60's & 1982: Radioisotopes.
►1960-70's: Bremstrallung photons, charged particle scattering.
►2000 & 2012: Inverse compton photons.
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Slide 7/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations
9Be(,n) Cross-sectionA selection of existing measurements
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Slide 8/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations
9Be(,n) Cross-section● Reanalyzed radioisotope measurements and
corrected for
►Branching ratios
►Detector efficiencies (using modern MCNP simulations)
● Fitted to Breit-Wigner resonance parameters.
En = 1738.8 keV, Γn = 268.7 keV, Γγ = 0.7715 eV
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Slide 9/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations
9Be(,n) Cross-section
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LowECalSept 24, 2015
Photoneutron Sources and Simulations Source Yield● Calculated from known
cross-sections
►0.659 mb at 1836 keV
►1.397 mb at 1691 keV
►955 mb at 1770 keV
● Directly measured using He-3
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Slide 11/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Source Design● Photoneutron emission from 9Be
radioisotope sources is isotropic in the rest frame.
►Energy-angle dependence in the lab frame.▻±6% spread in Sb/Be neutron energy.
● Photoneutron yield and energy spectrum depend on the source geometry.
►Diminishing returns on yield of unmoderated neutrons for Be or BeO thickness >1cm.
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Slide 12/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Source Design
Low yield uncertaintyLarger spread of
neutron energies
Large yield sensitivity to BeO – 88Y distance.
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LowECalSept 24, 2015
Photoneutron Sources and Simulations Moderation● Energy degradation in BeO may be
subdominant to other moderators.
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Slide 14/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Source Design● If using a lead shield
►Surround the source with lead on all sides so that lead is a diffuser instead of a reflector.
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Slide 15/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Neutron Moderation● Libraries for both MCNP and Geant4
incorrectly describe the angular distribution from neutron scattering for most isotopes.
►Problem not applicable to oxygen and lighter.
►New libraries for F, Si, Ar, Cr, Fe, and Pb are available. PRC 89 032801 (2014).
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Slide 16/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Neutron Moderation
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Slide 17/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Neutron Recoils
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LowECalSept 24, 2015
Photoneutron Sources and Simulations Neutron Recoils● Recoil spectrum uncertainties
►Cross-section uncertainty
►Resonance parameter uncertainty
►Unresolved resonances
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Extra slides
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LowECalSept 24, 2015
Photoneutron Sources and Simulations Neutron Elastic Scattering● Resonances from compound states
● Continuum from potential scattering
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Slide 21/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Breit-Wigner Resonances
● Parameterized by
►Resonance widths
►Neutron Energy
►Spins
►Parity
● Interference handled by R-matrix formalism
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Slide 22/18
LowECalSept 24, 2015
Photoneutron Sources and Simulations Recalculating d/d● Dipole term of scattering angle distribution
vs. energy