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08 September 2008

Answer to Question #4616 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:

What is the minimum beta energy that will penetrate 8 mg/cc mylar?

The thickness units you have used, mg/cc, are not indicative of material thickness. I suspect that you meant to write 8 mg/cm², the density thickness units of mg/cm² being commonly used to express practical ranges of electrons/beta particles in materials.

Assuming this to be true, we can look up the energy of an electron that would have a CSDA (continuous slowing down approximation) range equal to 8 mg/cm²in mylar, chemically identified as polyethylene terephthalate (PET). For low-atomic-number materials, such as mylar, the CSDA range is essentially equal to the penetrating ability of the electron.

I looked up values in ICRU Report 37 (International Commission on Radiation Units and Measurements, Stopping Powers for Electrons and Positrons, Bethesda, 1984) and interpolated in the table given for PET to obtain 0.068 MeV, the minimum energy electron or beta particle that will penetrate the cited 8 mg/cm².

I hope this is sufficient.

George Chabot, PhD, CHP

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