Prompt Fission Neutron Studies at LANSCE
Transcript of Prompt Fission Neutron Studies at LANSCE
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Prompt Fission Neutron Studies at LANSCE
Hye Young Lee for ChiNu collaboration Los Alamos National Laboratory
LANL FIESTA Fission School & Workshop, Sep. 8-12, 2014
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Outline
o PFNS of 239Pu(n,f) : previous measurements tell us how to improve systematic uncertainties
o Experimental Efforts at ChiNu including MCNP calculations
o How to deduce PFNS using the ChiNu data o Summary
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PFNS of 239Pu: High energy measurements Current uncertainty : 20~50%
Knitter (J. Atomkernenergie, 1975)���Staples et al. (Nucl. Scien. Eng., 1998)
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Knitter Staples ENDF/B-VII
E inc = 0.215 MeV E inc = 0.5 MeV
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LEAD
Measurement details on Staples vs. Knitter
1. Neutron source : 7Li(p,n) with variable-energy and pulsed protons 2. Fissile samples 3. Neutron detector : liquid scintillators (BC501 vs. NE224) 4. Shadow bar to block direct neutrons
o TOF measurements o No fission events detected o Significant multiple scattering at thick targets and shielding materials o Corrections & efficiency estimation using Monte Carlo calculations
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Detector Efficiency ���(Uncertainty : 5-7 % and 2-5 %)
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Knitter data Staples MC cal Knitter MC cal.
Relative to T(d,n)4He angular distributions
Normalized to overlapping different –
calibration reaction sets
Det. volume Measurements Calculation
Staples 117 cm3 235U fission counter (E<3.5 MeV) SCINFUL for the rest energy
Knitter 75 cm3 Multiple reactions (E<20 MeV) Maggie for angular corrections
Limitations : o Knitter made a straight line
connection (not complete) between the low- and high-energy data
o Staples relied on the SCINFUL calculation after calibrating the low energy measurement
curve
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o correction for neutron inelastic scatterings o constant background subtraction at ~15 MeV o γ-peak correction influences the deduced shape of neutron spectrum at
5-15 MeV
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239Pu(n,f)
239Pu(n,n)
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2. random coincidence
TDC (channel)
Knitter et al. Atomkernenergie (1973)
Systematic uncertainty in Knitter data
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PFNS of 239Pu: Low energy measurements Current uncertainty ~10% (compilation)
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Lajtai Nefedov Werle Bojkov Belov Starostov
Bojkov-Nefedov (re-analysis) - Starostov, Laitai, ���Werle (proton-recoil proportional counter), Belov (insufficient doc)
(E inc = thermal)
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o Time of flight measurements
o Detector : 5 different neutron detectors + 2 different fission counters - 0.1 < En < 2 : Anthracene scintillator (φ =18mm, 4mm thick) at 51 cm���
-The absolute normalization for the efficiency is calculated using Monte Carlo calculation ���
0.01<En < 5 : Gas scintillation ionization det. & IC at 10~40 cm��� -The efficiency was measured with a 252Cf source���
- For the rest of detectors, used the complied 252Cf shape (weighted average over Starostov, Blinov, Lajtai) to calculate the detector efficiencies
o After background subtraction, time spectra were corrected further due to multiple scatterings in the target room
Starostov : notes on 239Pu measurement
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Lajtai : notes on 239Pu measurement Lajtai et al. NIM A (1990)
o 6Li-glass detector was used o 7Li-glass detector to measure the delayed g-ray background o Cu shadow cone to estimate neutron background o Yield = Yield (6Li detector w/o cone) – Yield (6Li detector /w cone)���
-Yield (7Li detector w/o cone) + Yield(7Li detector /w cone)���
Limitations : ���1. Overestimation of shadow bar measurements for correcting neutron-induced background ���2. Simplified detector response simulation especially near the resonance
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Chi-Nu project : Reduce unceratinty o Dedicated Flight Path at 4FP-15L ���
The 18’ X 18’ X 7’ basement was built for reducing room-returned background at low energy
o Fission Counter���Parallel Plate Avalanche Counter : 10 foils with ~ 400 µg/cm2 thickness ���Timing resolution is ~ 1ns and light mass for low background ���
o High Energy Measurement (En > 0.7 MeV) : n-γ separation 54 Liquid scintillators at 100 cm : EJ309, 17.8 cm dia., 5.08 cm thick ��� o Low Energy Measurement (En < 1 MeV) : well-understood detector
response function ��� 22 6Li-glass detectors at 40 cm: Scionix10 cm diameter x 18 mm thick
R.C. Haight et al. (J. of Instr., 2012)
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Chi-Nu project : Identify background
o Time independent background ���a. accidental coincidences with thermal neutrons – 235U(n,f) measurements ���b. accidental coincidences with alpha decays – 239Pu(n,f) measurements ���
o Time dependent background ���a. gamma flash from the neutron beam production – beam energy gate���b. incident fast neutron scattering on PPAC – Li detector angle dependence and beam energy gate���c. gamma background from various reactions – 7Li detector measurements ���d. neutron multiple scattering – corrections obtained by MCNP calculation
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MCNP calculates detector response for monoenergetic neutrons
6Li glass detector at different energies liquid scintillator with different timing resolutions
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PFN yields of 252Cf using a 6Li-glass detector
PPAC-ver.1 in the FIGARO room
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H.Y. Lee, T.N. Taddeucci, et al. (NIM A, 2013)
Data using digitizer
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Fission chamber in the Calibration room
Low-energy tail is contributed by o Any hydrogenated material near source and detector o Multiple scattering on surrounding materials o Distance between source and detector
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MCNP shows that much of the difference between PFNS forms is preserved despite significant neutron scattering
252Cf PPAC-ver.1 at the ChiNu target room (PPAC+ 22 Li-glass detectors + array frame + target room components)
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Unfolding vs. Integral approach to deduce PFNS from ChiNu data
o Unfolding : ���Using MCNP detector responses, the PFN yield can be deconvoluted to the PFNS
o Integral – double ratio :���Using the spectrum shape-correction factor, the PFN yield can be corrected in bin-by-bin for deducing the PFNS
Double ratio = MCNP(PFNS)/MCNP(Maxw)/[PFNS/Maxw] [PFNS/Maxw] = 1/double-ratio X [Measured ChiNu/ MCNP(Maxw)]
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Summary o For low energy measurements, any hydrogenated materials near the
sample should be avoided o Full MCNP Detector response needs to be studied at each setup o Time-dependent background should be well understood and
corrected o Even with large multiple-scattering effects, ChiNu measurements
still retain sensitivity to the PFNS o Double-ratio method gives an answer with limited uncertainty, while
the full unfolding will provide the PFNS with a target precision
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Collaborators and Funding Agencies
LANL: R. C. Haight, H. Y. Lee, T. N. Taddeucci, J. M. O’Donnell, T. Bredeweg, M. Devlin, N. Fotiades, S. Mosby, R. O. Nelson, T. Seagren, S. A. Wender, J. L. Ullmann, D. Neudecker, M. White
LLNL : C.-Y. Wu, E. B. Bucher, R. Henderson Nuclear Energy University Program (NEUP): Michigan U.���
(S. Pozzi, A. Enqvist, M. Flaska, students) Kentucky U.���
(M. Kovash, postdoc, student) Brigham Young U.���
(L. Rees, J. B. Czirr, students) Texas A&M U.���
(P. Tsvetkov, postdoc) Commissariat à l'énergie atomique et aux
énergies alternatives (CEA): ���T. Ethvignot, T. Granier, A. Chatillon, J. Taieb, B. Laurent
DOE – NNSA Nuclear Energy��� Nuclear Physics ��� NEUP from DOE-NE