# Homework Help: Thickness (particle physics)

1. Sep 27, 2007

### touqra

I have this weird question where the thickness of the material is given in $$\frac{grams}{cm^2}$$.
And it's not wrong. What does it mean ?

2. Sep 27, 2007

### malawi_glenn

It is given in that unit so that you can calculate the number of targets / area. If you know the density of material, thickness and molar mass etc.

Basically it is the same as mass/volume ; but you multiply with the thickness of your sample so you get that unit instead.

I hope this helps, there are a lot of "strange" units out there =)

3. Sep 27, 2007

### dynamicsolo

The units would correspond to a density times a length. Where I've seen this sort of quantity used was in dealing with scattering or absorption centers in a material. If you're working with a specimen of a pure element, for instance, the density and atomic mass would tell you how many atoms or nuclei there are per cubic unit. If you follow a line into the material and take a slice transverse to that, the depth along that line would tell you how many atoms or nuclei you would encounter per square unit, as projected onto the face of the slice. If you know the average cross-sectional area of one atom or nuclei, this gives you an idea of the fraction of the face of the slice that is "covered" by these scattering or absorption centers, and thus the fraction of incident radiation that would be blocked by or transmitted through the slice. (Actually, you set up a differential equation known as Beers' Law or by various other names.) The "thickness" (called "optical depth" in some fields of study) thus gives you an idea of how effectively radiation is blocked by or penetrates the specimen.

I'd imagine there are other uses for this concept as well...