7.1.1 Charged Particle Interactions

Created October 19, 1995

7.1.1.1 Introduction

Gamma-rays, x-rays, neutrons, and neutrinos all have no net charge - they are electrostatically neutral. In order to detect them they must interact with matter and produce an energetic charged particle. In the case of gamma and x-rays, a photo-electron is produced. In the case of neutrons, a proton is given kinetic energy in a billiard ball collision. So we begin our discussion of charged particle interactions by demonstrating that even when detecting neutral particles we must quickly think in terms of the charged particles.

Charged particles can be divided into two families: 1) electrons, including positrons and beta particles from radioactive decay, Auger electrons, and internal conversion electrons, and; 2) heavy charged particles such as the alpha particle, fission fragments, protons, deuterons, tritons, and mu and pi mesons.

The charged particles interact with matter primarily through the Coulomb force. Differences in the energy deposition trails between the two types of charged particles are due to the mass differences, i.e., the more easily altered trajectory of the electron.

The maximum energy that can be transferred from a charged particle to an electron in a single collision is given by 4 T m(e)/m, for about 1/500 of the particle energy per nucleon for heavy particles. Due to this small fraction and the fact that at any time the particle is interacting with more than one electron, the heavy particles appear to continuously slow down, gradually losing their kinetic energy along an unaltered linear path.

7.1.1.2 Types of Interactions

When a charged particle gives sufficient energy to an orbital electron such that the freed electron itself can cause secondary ionization, we say that a delta-ray has been produced. The majority of the energy loss of a heavy charged particle such as the alpha occurs via these delta-rays.

We will concern ourselves primarily with two types of interactions: collisional and radiative.

TYPE 1, COLLISIONAL.

Inelastic collision with atomic electrons. This results in excitation or ionization. These processes ultimately end with the heating of the absorber (through atomic and molecular vibrations) unless the ions and electrons can be separated using an electric field as is done in radiation detectors.

TYPE 2, RADIATIVE.

Inelastic collision with nucleus. A quantum of electromagnetic radiation is emitted (a photon). Energy loss is experienced by the particle. Important for electrons. Probability of nuclear excitation is negligible. This process is also known as RADIATIVE energy loss. The acceleration of electrons near a nucleus is known as beam braking or bremsstrahlung. Bremsstrahlung will be discussed further in the next section.

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Douglas J. Wagenaar, Ph.D., wagenaar@nucmed.bih.harvard.edu