Interaction of Fast Electrons With Matter

Interaction of Fast Electrons With Matter
.

The interaction of electrons with matter represent the first step in the chain of physical & chemical that lead tissue damage. This applies of primary electron beam used in therapy, so as a result of electrons collisions the following interaction may occur.

1-Excitation.
Excitation results from movement of an electron from lower to high energy shell, a small amount of energy is required & when the atom returns back to its original unexcited state , the excess energy is liberated as heat , light, or UV rays.

2-Ionization.
Ionization results from expulsion of an orbital electron as a result of collision when an ion pair is formed, a negative ion represented by the ejected electron & a positive ion represented by remaining part of the atom.

3-Bremsstrahlung Radiation.
This result from an interaction between fast high energy electrons & the nucleus, as the electron comes close the nucleus a part of its energy lost & appears as bremsstrahlung radiation.
- the probability of its generation increases with the a high atomic number of the medium & also with a high electron energy, this form a radiation emitted in all directions but as the electrons energy increases the foreword direction is predominates.
- This property is made use of in the design of linear accelerators where a tungsten target is fixed at the end of accelerator tube as the accelerated high energy electrons hit a high atomic number metal, bremsstrahlung radiation are produced & emitted in the forward directions through the target.
- In case of diagnostic X-ray tube a low energy of electrons beam is used to hit the tungsten target and bremsstrahlung radiation are emitted mainly at right angle to the target & this represent a technical advantage.
- Bremsstrahlung radiations however present a problem in case of electron beam therapy using accelerators since they can generates as a result of bombardment the sides of a heavy collimators & generation of bremsstrahlung radiation in tissues is slows down the decline in dose beyond the Bragg peak .

شكرا ع الموضوع