Description of the materials

There is nothing of really complicated in this problem from the point of view of material description, and few different materials are necessary. The cards MATERIAL and COMPOUND as well as the cards ASSIGNMAT are self-explanatory. Of course the reliability of results is strongly dependent on the accurate knowledge of the chemical compositions. Here some attention has to be put on the description of the specific PVC used in the experiment, since the additives and densities are variable from one PVC to another.

Another note concerns the use of air. In order to overcome the difficulties due to the low interaction probability at the operating pressure of the machine ( $13.3 \div 133.0$ nPa or $10^{-10}\div 10^{-9}$ torr), calculations are performed with gas density corresponding to the atmospheric pressure. Therefore, the obtained results will have to be linearly scaled to the operating pressure.

Last, but not least, we recommend to use standard names for the basic materials (elements) in order to have the correct low energy cross sections for neutrons (see the manual). In this particular problem neutrons are not relevant, but the use of PRECISIO default activates low energy neutron transport and interaction, unless this is inhibited by some directive which overwrites the DEFAULT itself.