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Request ID99 Type of the request High Priority request
TargetReaction and processIncident EnergySecondary energy or angleTarget uncertaintyCovariance
 94-PU-239 (n,f) nubar  Thermal-5 eV  1 Y
FieldSubfieldCreated dateAccepted dateOngoing actionArchived Date
 Fission  23-MAR-18 12-APR-18 Y

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Requester: Dr Roberto CAPOTE NOY at IAEA, AUT

Project (context):


Cf. Ref. [1,2]. A similar, but stronger than in U-235, resonance nubar effect is expected for Pu-239 due to the 1/2 GS spin.


Accuracy below 1% is required on the evaluated data. New measurements must strive to achieve a relative uncertainty below about 1% on the ratio to Cf-252(sf) nubar, as done in the best past experiments [3].

Statistical precision below about 1% at the resonances is required in order to unambiguously identify resonant fluctuations.

Justification document:

A new evaluation of the PFNS [4] in the thermal energy range has determined a lower value of the average neutron energy than that reported in the existing evaluated nuclear data libraries. This value is in agreement with Rising et al and Neudecker independent evaluations. However, a number of thermal-solution benchmarks has shown that the combined use of the new Thermal Neutron Constants and a softer prompt fission neutron spectrum at thermal energy yields k-eff values that are larger than measurements by a margin that increases as the above-thermal-leakage fraction (ATLF) increases (see Ref. [5]). Therefore a reduced criticality is needed for high-leakages solutions. Such reduced criticality may arises due to the (n,gf) process in Pu-239 resonance nubar.

Unfortunately, only measurements from the 70s and 80s are available, a critical region below 5 eV needs to be remeasured with higher incident-energy resolution and higher accuracy and precision to improve existing evaluated data files.


  1. M.T. Pigni, et al., n+235U resonance parameters and neutron multiplicities in the energy region below 100 eV, EPJ Web of Conferences 146, 02011 (2017)
  2. E. Fort et al., Evaluation of prompt nubar for 239Pu: Impact for applications of the fluctuations at low energy, Nuclear Science and Engineering 99, 375 (1988)
  3. Gwin et al., Measurements of the energy dependence of prompt neutron emission from 233U, 235U, 239Pu, and 241Pu for En = 0.005 to 10 eV relative to emission from spontaneous fission of 252Cf, Nuclear Science and Engineering 87, 381 (1984)
  4. R. Capote, et al., Prompt Fission Neutron Spectra of Actinides, Nuclear Data Sheets 131, 1-106 (2016)
  5. C. De Saint Jean (coordinator), Co-ordinated Evaluation of Plutonium-239 in the Resonance Region, Nuclear Energy Agency, International Evaluation Cooperation, NEA/WPEC-34, Report NEA/NSC/WPEC/DOC(2014)447 (2014)

Comment from requester:

Additionally to changes in nubar changes in resonance parameters may be required. We cannot split those effects on studied criticality benchmarks.

Additional file attached:

Review comment:

Entry Status:
Work in progress (as of SG-C review of May 2018)

Main recent references:
Please report any missing information to


  • F.J. Hambsch, et al., Prompt fission neutron emission in resonance fission of 239Pu, ND2004, Santa Fe (NM), USA, September 2004, AIP 769 (2005) 644


  • J.E. Lynn, P. Talou and O. Bouland, Reexamining the role of the (n,gf) process in the low-energy fission of 235U and 239Pu, PRC 97 (2018) 064601