Requester: Dr Allan D. CARLSON at NIST, USA
Email: allan.carlson@nist.gov

Project (context): NIST, Neutron Cross-Section Standards (www-nds.iaea.org/standards), CSEWG, WPEC

Impact:
This neutron cross section standard is perhaps the most important of the standards [1]. All of the other standards have been measured relative to it. Any improvement in this standard improves all standards and other cross sections that have been directly or indirectly measured relative to this standard. It helps form the basis of the neutron cross section libraries. There is very large leverage associated with improvements in this cross section. A problem at about 14 MeV is shown in reference [2].

[1] A.D. Carlson, et al., International Evaluation of Neutron Cross Section Standards, Nuclear Data Sheets 110 (2009) 3215-3324.
[2] N. Boukharouba, et al., Measurement of the n-p elastic scattering angular distribution at 14.9 MeV, Phys. Rev. C 82, 014001 (2010).
[3] N. Boukharouba, et al., Measurement of the n-p elastic scattering angular distribution at 10 MeV, Phys. Rev. C 65, 014004 (2001).

Accuracy:
1%-2% over most of the angular range. Very little data are available at small center-of-mass angles so the emphasis should be placed there.

Justification document:
The nature of the standards makes a simple answer to the question difficult. Since essentially all cross section data depend on the standards, any improvement in this standard will improve data for any neutronics calculation.
This neutron cross section standard is perhaps the most important of the standards. All of the other standards have been measured relative to it. Any improvement in this standard improves all standards and other cross sections that have been directly or indirectly measured relative to this standard. It helps form the basis of the neutron cross section libraries. There is very large leverage associated with improvements in this cross section.

Comment from requester:

Review comment:

Arjan Plompen: New measurement efforts should seriously consider measuring the angular distribution for the outgoing neutron at the incident energies of 10 and 14.9 MeV tackled by the Ohio University collaboration while covering the forward angles and with good overlap with those data at backward angle [2,3]. Ideally they should answer the question concerning the degree of the Legendre polynomial required for the targeted uncertainty and allow the coefficients to be established accurately.

Don Smith: [...] improvements in the standards can almost always be justified. The H(n,n)H standard is certainly an important one. [...] this impacts on all technical areas but certainly more so at higher energies, e.g., in the HE tail of the fission neutron spectrum (or for fusion applications) [...]

Mark Chadwick: The sensivity of many experiments to this cross section is essentially one.

Allan Carlson in response to questions raised by Arjan Plompen:

A) It is very difficult to make absolute measurements of the hydrogen angular distribution with high accuracy. Instead it is much easier to make relative measurements. But to normalize the relative data to the hydrogen total elastic cross section requires either a rather complete measurement of the relative angular distribution or very good models to assist in the extrapolation to the angular region that was not measured (in the reference [2,3]). [...] it must be emphasized that this normalization aspect is very important.

B) It is disturbing that there are differences of 0.5 to 1% between the Arndt et al. and ENDF/B-VII evaluations of the total elastic cross section in the 10-20 MeV energy region. Part of this is because of the databases being used in the evaluations. For example, the Abfalterer et al. data were not used in the Arndt PWA analysis but they were included in the Hale (ENDF/B-VII) work. They had significant weight in the Hale evaluation due to the small experimental uncertainties so the evaluation is similar to those data. [...] It would be very valuable if each group used exactly the same database and compared their results.

C) Improvements in measurements of the total cross section are also worthwhile. The most recent work has used TOF techniques with white sources. These measurements are good but I would suggest that making a measurement at a single point with a (nearly) monoenergetic source for which very good background measurements are possible could provide even smaller uncertainty. In the 50’s and 60’s, a number of total cross section measurements were made in the MeV energy region with fraction of a percent uncertainties. I expect we could probably do a little better now. Note the detailed process described in Fast Neutron Physics for accurate total cross section measurements. An important point for the total cross section work (but not for the angular distribution work) is a very accurate determination of the energy of the neutrons. An error of 20 keV in the energy scale for a 1 MeV cross section measurement causes about a 1% error in the cross section. This effect is less however at higher neutron energies.

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

Main references:
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