Why are some particles more strongly ionising than others?

vertices · May 4, 2008

what makes an alpha particle more strongly ionising than an electron? It is simply because of the fact that the alpha particle has a greater (double the) magnitude of charge?

malawi_glenn · May 5, 2008

well, yes. But the most relevant thing is that it has mass 8000 times that electron mass.

http://ab-div-bdi-bl-blm.web.cern.ch/ab-div-bdi-bl-blm/Literature/short_course/hugtenburgIEEE2.pdf

vertices · May 5, 2008

yeah I understand it arises from the Bethe-Bloch equation (well specifically, the equation for "range", which does show a mass dependence).

What is I am unclear about is the physical basis for why it is so? That is, why would an electron of the same momentum as an alpha particle be less ionising?

thanks glenn.

malawi_glenn · May 5, 2008

vertices said: ↑

yeah I understand it arises from the Bethe-Bloch equation (well specifically, the equation for "range", which does show a mass dependence).

What is I am unclear about is the physical basis for why it is so? That is, why would an electron of the same momentum as an alpha particle be less ionising?

thanks glenn.

It has to do with relativistc effects, and more. But the relativistical effects and that the most promiment scatterer in a material are the atomic electrons - which has the same mass as the incoming electron. And here you can just play with your newtonian mechanics - linear momentum. Check that an object of mass 8000e , with momentum p, impinging on an obejct of mass 1e - will loose much less energy than a object of mass 1e and momentum p, impinging on an object of mass 1e will do. That was a classical analogy.

vertices · May 6, 2008

thanks glenn.

"check that an object of mass 8000e , with momentum p, impinging on an obejct of mass 1e - will loose much less energy than a object of mass 1e and momentum p, impinging on an object of mass 1e will do. That was a classical analogy."

does that not depend on how much momentum is *transferred* to the gas atoms (by the charged particle). Why would a lighter particle neccessarily *transfer* more momentum than a heavier one?

malawi_glenn · May 7, 2008

vertices said: ↑

thanks glenn.

"check that an object of mass 8000e , with momentum p, impinging on an obejct of mass 1e - will loose much less energy than a object of mass 1e and momentum p, impinging on an object of mass 1e will do. That was a classical analogy."

does that not depend on how much momentum is *transferred* to the gas atoms (by the charged particle). Why would a lighter particle neccessarily *transfer* more momentum than a heavier one?

First, are you with me on the classical analogy?

second, have you done scattering theory in QM? Scattering amplitude, momentum transfer, born approximation, and so on?

vertices · May 7, 2008

"First, are you with me on the classical analogy?"

To be honest, I'm not sure if I am. how much energy is lost depends of recoil of atom that the particle collides with does it not?

"second, have you done scattering theory in QM? Scattering amplitude, momentum transfer, born approximation, and so on?"

I'm not sure if I have. I've learnt the basics of rutherford scattering. That's why I was saying that the 'energy loss' of a particle that is scattered is equal to the momentum its transfers to the (heavier) particle it scatters off, DIVIDED by the heavier particle's mass.