R2_D2, do you reside at the NMMH?

In answer to the question:

The **Stopping Power Ratio** will be the ratio of SP_{water}/SP_{Si} (or SP_{Si}/SP_{water}!). So the answer may be there inside the **Stopping Power** formula (Physics is very much like this - keep drilling down!)

If you look at equation 1.61 on page 24 of Podgorsak, the equation for Stopping Power is there:

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\begin{align} (S/ \rho)_{tot} = \frac{1}{\rho}.\frac{dE_{K}}{dx} (MeV.cm^2/g) \end{align}

You will not that SP is inversely proportional to density (this is the $\frac{1}{\rho}$ term!). The question then is - what are the relative densities of water, silicon and air?

According to Wikipedia, the density of Silicon is 2.33g/cm^^3^^, the density of AIR is 0.0013 g/cm^^3^^ and the density of water is 0.998 g/cm^^3^^. Thus there is a <3 fold difference in density between the Si and H_{2}O, but a 1000x difference between AIR and either Si or H_{2}O.

To me that looks to be the essence of the dilemma -

"… the stopping power ratio water to silicon is essentially independent of electron energy …"

because, for the electron, water and silicon while not exactly the same, are pretty close, close enough to assume equivalence. However the small difference there is explains why silicon dosimeters are only used for gross approximations of dose such as for an *In Vivo* Dosimetry program - they lack sufficient accuracy without much more work, and then they start to degrade anyway!

Work though this and tell me is the problem has now gone. Actually looking at your original post, that's essentially what you said - well done!

A