Program - Brookhaven National Laboratory

OECD Nuclear Energy Agency considers of utmost importance, and therefore includes in it High Priority
Request List (HPRL) [2], the measurement of the prompt g-ray production following fission in 235 U and
239 Pu. We will take advantage of the unique opportunity of combining the EXOGAM detector [4] with
the high intensity thermal neutron beam line PF1B at ILL to measure the prompt g-rays from fission and
capture reactions in 233 U, 235 U, 239 Pu and 241 Pu. The g-ray detection set-up will consist of 40 HPGe (10
clovers) detectors from EXOGAM combined with a set of up to ten other HPGe detectors, providing an
overall segmentation of 20. The fission tagging capabilities will be provided by a transparent MicroMegas
detector [5] capable of detecting one of the two fission fragments with efficiency better than 95%. The
combination of such a detector with an array of g-ray detectors has been attempted successfully in a recent
experiment at CERN [6]. The experiment will provide valuable data on the multiplicity and total energy
released by prompt g-ray emission in fission and also in capture, as well as detail on the energy of the
individual g-rays. In this way. the results will allow the accurate description of as much as 60% of the
contribution to prompt g-ray heat in the core of nuclear reactors. At the time of the conference, just after
the finalization of the experiment at ILL, we will present the experimental set-up and the very fresh data
from the experiment. The expected quality and scope of the results will also be discussed.
[1] A. Lüthi, R. Chawla, and G. Rimpault, Nuclear Science and Engineering 138 3 (2001). [2] OECD
Nuclear Energy Agency “Nuclear Data High Priority Request List,” see in particular http://www.oecdnea.org/dbdata/hprl/tmp/HPRLgammafission.pdf
[3] D. Blanchet, Annals of Nuclear Energy 35 731-745
(2008) [4] EXOGAM detector at GANIL http://pro.ganil-spiral2.eu/laboratory/detectors/exogam [5] S.
Andriamonje et al., Journal of the Korean Physical Society 59 (2011) 1597-1600 [6] C. Guerrero et al., Eur.
Phys. J. A (2012) 48:29
PR 42
Inelastic Thermal Neutron Scattering Cross Sections for Reactor-Grade Graphite
Ayman I. Hawari, Department of Nuclear Engineering, North Carolina State University, Raleigh, NC
27695 USA. Victor H. Gillette, Department of Nuclear Engineering, North Carolina State University,
Raleigh, NC 27695 USA.
Traditional calculations of the inelastic thermal neutron scattering cross sections of graphite are based on
representing the material using ideal single crystal models. However, reactor-grade graphite represents a
multi-phase material where graphite ideal crystals are embedded in a carbon binder matrix. Furthermore,
the density of reactor-grade graphite is usually in the range of 1.5 g/cm 3 to approximately 1.8 g/cm 3 , while
ideal graphite is characterized by a density of nearly 2.25 g/cm 3 . This difference in density is manifested
as a significant fraction of porosity in the structure of reactor-grade graphite. To account for the porosity
effect on the cross sections, classical molecular dynamics (MD) techniques were employed to simulate
graphite structures with porosity concentrations of 10% and 30% relative to ideal graphite. This type of
microstructure is taken to be representative of reactor-grade graphite. The phonon density of states for the
porous systems were generated as the power spectrum of the MD velocity autocorrelation functions. The
analysis revealed that for porous graphite the phonon density of states exhibit a rise in the lower frequency
region that is most relevant to thermal neutron scattering. Furthermore, this rise increases as the porosity
level increases. Using the generated phonon density of states, the inelastic thermal neutron scattering
cross sections were calculated using the NJOY code system and compared to experimental measurements
of total cross sections below the Bragg energy cut-off (approximately 2 meV) of graphite. In this case,
the measured total cross sections represent mainly total inelastic scattering cross sections. While marked
discrepancies exist between the calculations based on ideal graphite models and measured data, favorable
agreement is found between the calculations based on porous graphite models and measured data. This
work was supported by the DOE NEUP program.
284