After establishing this narrow beam gamma-ray transmission geometry, the
attenuation measurements were carried out with the prepared samples in the configuration
described above.
The samples, which were put in a plastic container, were stacked on top of each other
placed one by one to get various thicknesses. For sample systems with different thickness
and material, measurements were performed. The intensities of transmitted photons were
measured without (I0) and with (I) placing the samples in the container. Appropriate counting
time (fixed preset time was set as 90s in the Genie 2000 software) for each measurement was
chosen after trials so that 105
-106 counts were recorded under each photo peak. Hence the 55
statistical uncertainty was kept below ~0.3%. The first run was conducted using bulk
samples with different thicknesses. The second run of measurements was repeated in the
situation of adding water into the container to fill the foam samples. Another run was
performed with 137Cs source, by filling 2% (w/v) boric acid solution into the foam samples.
The experiments were conducted at constant temperature to avoid any shift of peak.
In order
to check the performance of the experimental set up, the linear attenuation coefficients of
pure aluminum plates were calculated using experimental results and compared with
theoretical data.