# Multiple scattering formula in the physics reference manual

Hello!

I am looking for a way to better understand scattering of 1-5GeV/c electrons on thin targets (given that the Highland formula comes with some uncertainties).
For this, I noticed in the physics reference manual (page 91 or see the formula I attach below) the treatment of MCS in G4 using the Urban Model. Something does not click for me and I was hoping you can help me.

As you can see in modified Highland-Lynch-Dahl formula, the angle is calculated using the square root of y multiplied by a correction term c . The correction term is given below the formula, as a function of the atomic number. Similarly, the y term is the logarithm of the material budget expressed in units of radiation length.

Here is where my problem lies. Usually, this quantity is smaller than 1 (eg: 50um of Silicon would correspond to t/X0 = 0.0005). Taking the natural log of such a number would yield a negative y.
The square root of a native y plugged into the angle formula …

Am I getting something wrong?

Thank you very much!
Wishing you all a pleasant weekend!

NB: My goal is to find a better description (formula) of the multiple scattering than the Highland formula, which contains a dependence on the material budget. I have some scattering angle data from which I would then like to calculate the material budget of an unknown sample.
I started asking myself if the Highland formula is also appropriate for electrons (since it has no dependence on the projectile mass) or if I should look for a better description or corrections.

I have checked the computation of theta0 in the source code :
/process/electromagnetic/standard/src/G4UrbanMscModel.cc, lines 976-1023
and in an example :
/examples/extended/electromagnetic/TestEm5/src/Run.cc, lines 319-336

There is a misprint in the documentation, not in the code itself.
Namely, replace sqrt(y) by sqrt(t/X0)

Thank you very much for your comment!
Still, then the y in the expression for c remains. The y there should come with a log term then (as I see it from the code).

This topic was automatically closed 7 days after the last reply. New replies are no longer allowed.