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07 November 2009

Answer to Question #3549 Submitted to "Ask the Experts"

Category: Radiation Basics — Beta Particles (negative and positive) and Electrons

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

I am currently a nuclear medicine student. My question concerns the beta radiation quality factor variance and how it is used to determine my monthly absorbed dose from the data the company we use for personnel monitoring dosimeters obtains from my body badge and finger badge. If beta has a higher LET (linear energy transfer) than gamma and x rays, then why are they all weighted with a quality factor of 1.0?

A very interesting question. The short answer is that beta-particle radiation is grouped with x rays, gamma rays, and electrons (see ICRP Publication 26 Paragraph 20) and with muons (Publication 60, paragraph 25), all with the explanation that these are low linear energy transfer (LET) radiations.

An important point to keep in mind is that when gamma rays or x-rays interact in tissue they set free electrons, and it is the electrons that actually produce the dose. Since electrons are physically the same particles as beta radiation, one would expect them to have the same LET if their energies are comparable. Depending on the particular energies involved, there may exist some differences in LET distributions for beta radiations compared with gamma or x rays, but the overall lack of knowledge simply does not justify small differences in values of quality factors or radiation weighting factors (W_r).

It may be helpful to look at the material in NCRP Report 104 "The Relative Biological Effectiveness of Radiations of Different Quality" and ICRP Publication 31 "Biological Effects of Inhaled Radionuclides."

Charles B. Meinhold, CHP

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