First measurements of the differential cross sections for the elastic n-3H and n-2H scattering at 14.1 MeV using an Inertial Confinement Fusion (ICF) facility Page: 4 of 13
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The development of an accurate description of light-ion reactions is currently of great interest
as that would provide valuable insights into low-energy nuclear reactions important to nuclear
astrophysics. Radiative capture reactions, for example, occur in red giants at temperatures low
enough that the reaction rates are too small to be directly measured in a laboratory. Extrapolation
from measurements at higher energies is also suspect without a fundamental theory for computing
these reactions. Fusion energy research also requires accurate cross sections for light-ion reactions
to constrain models of inertial confinement fusion (ICF) experiments involving deuterium-tritium
fuel. For instance, uncertainties in the differential cross section for the elastic n-3H scattering need
to be less than ~5% to reliably infer a fuel areal density (pR) from the yield ratio between scattered
neutrons and primary 14.1-MeV neutrons, called down-scatter ratio (dsr) , produced in an ICF
implosion. The determination of the pR from the dsr value is essential for understanding how the
fuel is assembled in an implosion, and for ultimately guiding the community towards the
demonstration of thermonuclear ignition and net energy gain  at the National Ignition Facility
Since the 1950's, the differential cross section for the elastic n-3H scattering at 14.1 MeV has
been subject to extensive experimental and theoretical studies. Kootsey et al.  measured the
cross section at central-mass (CM) angles ranging from 550 to 1650, resulting in data with
statistical uncertainties of ~20% and a systematic uncertainty of 11%. Shirato et al.  and
Debertin et al.  measured the cross section in the CM angular range 100-175 with an
uncertainty varying from ~10% to ~70%, and their results are in good agreement with each other,
but a factor of two smaller than the Kootsey data. Optical model calculations conducted by
DeVries et al.  and by Sherif and Podmore  reproduced the Shirato and Debertin data in this
CM angular range. Additionally, G.H Hale et al.  conducted an R-matrix analysis of all
experimental data sets, and the result from that analysis forms the basis of current ENDF/B-VII
evaluation of the differential cross section for the elastic n-3H scattering that can be found in
nuclear data bases. Although numerous efforts have been made to quantify this fundamental cross
section, significant discrepancies exist between the different measurements and between
measurements and models. However, a theoretical understanding of the n+3H scattering based on
first principles calculations is within reach . For example, ab initio variational calculations
using a hyper-spherical harmonics basis expansion performed with a modern nuclear Hamiltonian
consisting of an accurate nucleon-nucleon potential and a three-nucleon interaction provide a good
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Frenje, J A; Li, C K; Seguin, F H; Casey, D T; Petrasso, R D; McNabb, D P et al. First measurements of the differential cross sections for the elastic n-3H and n-2H scattering at 14.1 MeV using an Inertial Confinement Fusion (ICF) facility, article, June 20, 2011; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc828861/m1/4/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.