aluminum,
bulk 6061 aluminum alloy, 10 PPI and 20 PPI Al foams with
and without water addition) with various thicknesses at three photon
energies were collected from the c-ray spectra. Transmission
versus thickness at the three photon energies are shown in Figs. 4
and 5. It is clear that bulk samples tested at all various photon
energies exhibit the highest attenuation as compared to foam samples,
however, water-filled foams show improved attenuation. The
low attenuation of foams is a result of their porous structure and
low density. The 10 PPI and 20 PPI foams have close attenuation
at all tested energies. The pore size of the foam has not shown clear
effect on attenuation of c-rays. It should be noted that 6061 aluminum
alloy was used as a substitution for 6101 aluminum alloy due
to the unavailability of the latter.
Mass attenuation coefficients are important parameters to illustrate
c-ray attenuation coefficients. The web version of the computer
program XCOM, developed by Berger and Hubbell [15,16]
facilitate the comparison between tabulation and measurements
and it was used in this work. It has been observed that the experimental
results for bulk samples are in good agreement with those
calculated by XCOM code. The weight fractions of ‘‘foam + water”
mixture were input into the code to obtain ‘‘equivalent” mass
attenuation coefficients from 0.1 MeV up to 10 MeV. The unique
non-uniform structure of foams determines that the
‘‘foam + water” sample is not homogenous and results in large difference
between experimental and computed data. However, as
shown in Figs. 6 and 7, theoretical and experimental data exhibit
similar tendency and ‘‘foam + water” samples show higher mass
attenuation coefficients. Since the mass attenuation coefficients already
consider the density factor, a conclusion can be made that
the ‘‘foam + water” samples have better attenuation from the point
of benefiting weight saving. At the present energies, the attenuation
of the incident beam is determined mainly by Compton scattering.
It is indicated by tabulated XCOM data that Compton
scattering contribute more than 99% of the total interaction
cross-section at the present three energies.
The thermal neutron experiment verifies the assumption that the boric acid solution
works for attenuating thermal neutron. The results have shown that by filling foam with
water or boric acid solution, the attenuation got improved compared to bulk material and
foam. Foams filled with boric acid solution have the most obvious attenuating effect. By 109
increasing the boric acid concentration and thickness of the layers, the samples even totally
stopped the beam.