Prompt Fission Neutron Studies at LANSCE

| Los Alamos National Laboratory |

| 1 Operated by Los Alamos National Security, LLC for the U.S. Department of Energy's NNSA

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



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



PFNS of 239Pu: High energy measurements Current uncertainty : 20~50%

Knitter (J. Atomkernenergie, 1975)��Staples et al. (Nucl. Scien. Eng., 1998)

0

0.5

1

1.5

2

0 2 4 6 8 10 12 14

Rat

io to

Max

wel

lian

(T=1

.3 M

eV)

Neutron energy (MeV)

Knitter Staples ENDF/B-VII

E inc = 0.215 MeV E inc = 0.5 MeV



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


| 5 Operated by Los Alamos National Security, LLC for the U.S. Department of Energy's NNSA Slide 5

Detector Efficiency ��(Uncertainty : 5-7 % and 2-5 %)

0 2 4 6 8

10 12 14 16

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

rela

tive

effic

ienc

y (%

)

neutron energy (MeV)

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



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

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 200 400 600 800 1000 0.175 0.25 1 2 5 15 energy (MeV)

239Pu(n,f)

239Pu(n,n)

1. γ-peak

2. random coincidence

TDC (channel)

Knitter et al. Atomkernenergie (1973)

Systematic uncertainty in Knitter data



PFNS of 239Pu: Low energy measurements Current uncertainty ~10% (compilation)

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

0.01 0.1 1 10

Rat

io to

Max

wel

lian

(T=1

.42

MeV

)

Neutron Energy (MeV)

Lajtai Nefedov Werle Bojkov Belov Starostov

Bojkov-Nefedov (re-analysis) - Starostov, Laitai, ��Werle (proton-recoil proportional counter), Belov (insufficient doc)

(E inc = thermal)



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



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



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)



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



MCNP calculates detector response for monoenergetic neutrons

6Li glass detector at different energies liquid scintillator with different timing resolutions



PFN yields of 252Cf using a 6Li-glass detector

PPAC-ver.1 in the FIGARO room

10

H.Y. Lee, T.N. Taddeucci, et al. (NIM A, 2013)

Data using digitizer

0

1

2

3

4

5

6

0.01 0.10 1.00 dN

/dE

(cou

nts/

MeV

) (ar

b)

the convolution of 6Li(n,t)a cross section and 252Cf Watt distribution

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



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)



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)]



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



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

Documents

Prompt Fission Neutron Studies at LANSCE