Answer to Question #9434 Submitted to "Ask the Experts"

Category: Radiation Basics

The following question was answered by an expert in the appropriate field:

It is commonly taught that photons and neutrons are "indirectly ionizing" (as compared to alpha and beta particles that are "directly ionizing") forms of radiation. However, this is confusing when one considers some of the common interactions of photons with matter. For instance, both the photoelectric effect and Compton scattering result in an electron being ejected from the atom. This has effectively ionized the atom. Therefore, why are photons considered to be indirectly ionizing
When charged particles pass through materials they ionize atoms by way of the coulombic forces between the moving charged particle and the electrons of the atoms/molecules. In such cases most of the individual ionization events involve relatively small amounts of energy loss (usually a few eV) by the moving charged particle, and many thousands of such ionization events often occur along the path of the charged particle. Occasionally a larger energy transfer event from the primary charged particle to an electron of the medium may result in the production of a relatively energetic secondary electron, often referred to as a delta ray, which itself will go on to produce multiple ionization events.

When an ionizing photon, such as a gamma ray, interacts with material it commonly sets free a single electron in a process such as the photoelectric event or Compton scattering, as you noted. Such an event is indeed an ionization event, but the gamma ray is not capable directly of producing the multiple ionization events that characterize the interactions of a charged particle. The energetic electron that is produced by the photon interaction, however, does go on to produce multiple ionization events just as does any other energetic electron or beta particle. Thus, the process requires two steps in order for a major part of the initial photon energy to appear as ionization; most of the ionization resulting from photon interactions results from the secondary particle, the energetic electron set free by the initial photon interaction, and this requirement for production of the secondary charged particle before significant ionization energy loss occurs seems to be the major reason for referring to the photons as indirectly ionizing.

A somewhat different description but similar rationale applies to the inclusion of neutrons as indirectly ionizing. For example, fast neutrons lose a major part of their kinetic energy in soft tissue through elastic scattering with the hydrogen nucleus. The transfer of energy to the proton is often sufficient to remove the proton from the hydrogen atom. The energetic proton then goes on to produce dense ionization along its path as it loses its kinetic energy and comes to rest. The ionization density (e.g., ion pairs per cm) from a proton expectedly will be a couple of orders of magnitude greater than that from an electron of the same energy. Here again, however, it is the production of a secondary charged particle, in this case the proton, by the initial primary radiation interaction, that is necessary before energy loss via multiple ionization events is observed.

I hope this clarifies the subject for you. Thanks for the question.

George Chabot, PhD, CH
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