Answer to Question #9501 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

Q
When using Luxel + with CR-39 for measuring 14 MeV neutrons, should site-specific calibration be formed to get an accurate estimate of the neutron dose?
A

I do not have an energy response curve for the CR-39 element in the Luxel badge,  but typically CR-39, used with a polyethylene radiator, as is the case for the Luxel configuration, produces a response (tracks per unit area per unit dose) that generally decreases with increasing neutron energy from a few hundred keV to about 10 MeV with a slight upturn in response for neutrons around 14 MeV. The response at a few hundred keV may be three to four times greater than that at 14 MeV. The specific response at a particular energy may vary noticeably with the thicknesses of the radiator and the CR-39 film as well as with other physical factors such as the track etching time and temperature. The typical NVLAP (National Voluntary Laboratory Accreditation Program)  accreditation testing uses 252Cf, bare or heavy water moderated, as the neutron source. Such a source produces neutrons at considerably lower energy than 14 MeV; the average energy of the bare californium source is a little above 2 MeV.

You should inquire of the dosimetry provider as to what the expected response at 14 MeV would be compared to that used by the provider to obtain its calibration factor used to interpret fast neutron doses from badges used in the field. If the dosimetry vendor cannot provide the information, and if personnel in your facility are receiving significant neutron doses, I believe it would be to your advantage to attempt to have a calibration performed at the energy of interest. Since the response at 14 MeV would expectedly be lower than that at the vendor's usual calibration energy, the vendor's interpreted doses likely would be lower than actual doses associated with the higher-energy neutrons.

I wish you well in your professional pursuits.

George Chabot, PhD, CHP
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