Converting Magazine

Eldridge M. Mount

Beta and X-ray diffraction in cavitated films
July 10, 2008

In reply to a recent posting, DaveO asks: "Is it possible for different orientations in a 2D expanded-PET film to cause variable diffraction of beta particles of X-rays in a measurement device?"

This is an interesting question, and I think, is related to the need to measure film thickness uniformity in cavitated film, which is physically thicker than a beta gauge system will say. Why is this? 

Cavitated films are films which are produced with physical voids which fill with air and are generally made by orienting a film from the solid state (as in OPP and OPET). Also, beta rays are not x-rays but particles (electrons or positrons) released from a radioactive atom during a radioactive decay where a neutron changes to a proton (emitting an electron), or a proton changes to a neutron (emitting a positron). A beta particle has the same charge and mass as an electron (or positron) and is emitted with speeds approaching that of light, dependent on the atom from which they are released. Because they are particles, they will be absorbed and scattered by the atoms in the solid through which they pass. This attenuation in the intensity of the beta particles is proportional to the mass of the film (number of atom) through which it passes and is used to calculate the mass of the film through which it passes using something like Beer’s law. Assuming a constant density for the film then the mass is converted to the film thickness which we think we are measuring as opposed to the mass.

In a cavitated film, the distribution of the voids can impact the relative density of the film as a function of position. Generally, in cavitated films there is a difference between the optical gauge (the real thickness) and the beta gauge thickness (the mass). So in OPP a 1.6 mil (40 micron) cavitated film will have the equivalent mass of approximately a 1 mil (25.4 micron) film. This may be what DaveO is seeing, a difference in film thickness (optical gauge) and the film mass distribution.

If we assume DaveO is doing a simple particle diffraction experiment, using either particles or x-rays, then a cavitated film must give variable scattering of the x-rays or particles. This is because the x-rays or beta particles will be scattered by the atoms in the film. In x-ray diffraction, the x-rays are scattered between the rows of nuclei in the film, which is how we determine atomic spacing within a crystal. In a cavitated film, where there are air filled voids between the various portions of polymer in the cavitated film, the x-rays can be expected to undergo multiple scattering from atoms randomly oriented relative to each other (each viod wall is like a new film layer). Also, there can be expected constructive and destructive interference giving rise to variable intensities and therefore variable diffraction. The same should be true for the beta particles but they may be scattered by the electron cloud of the atom as opposed to the nuclei for the x-rays.

Posted by Eldridge M. Mount on July 10, 2008 | Comments (0)