Answer to Question #12413 Submitted to "Ask the Experts"

Category: Radiation Basics

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

Q

How can I convert environmental equivalent dose rate H*(10) (mSv/h-1) to effective dose rate (mSv/h-1)?

In my job, we have equipment such as the microRaider ICX, which measures dose rate (mrem), but we have other equipment—the RadEye G-10, which measures environmental equivalent dose rate H*(10) (mSv/hr-1). For example, in the RadEye G-10 equipment we have a value 0.178 mSv/h-1 and in the Microraider we have a value 0.128 mSv/h-1. Why is there this large difference between these equipment readings? Is there some factor for interchange between values?

A

I should first note that the quantity H*(d) refers to the ambient equivalent dose, a specific International Commission on Radiation Units and Measures (ICRU) quantity intended to be used to calibrate field instruments for use in ionizing radiation fields. The value of d represents a particular depth below the surface of the "body" (actually a phantom for calibration purposes) at which the dose is assessed, and d = 10 mm is commonly used to provide an acceptable representation of the effective dose. The effective dose is based on the dose distribution throughout the body, and cannot practically be measured with typical instruments. It is important to recognize that, while the ambient equivalent dose is considered generally accepted as a surrogate for effective dose for routine radiation protection purposes, the two quantities are not the same, and significant differences between the two may occur at some photon energies.

For example, at about 1 MeV photon energy for a front-to-back irradiation of an acceptable phantom, the ratio of the effective dose to the ambient equivalent dose is between 0.8 and 0.9; at about 0.1 MeV for the same irradiation conditions the comparable ratio is between 0.5 and 0.6. Thus, depending on the energies of photons being measured, the ambient equivalent dose that is measured may not be an extremely good approximation of the effective dose.

Regarding the specific instruments that you mention, there are some possible reasons for differences between the two. I have not used these specific instruments, but I checked their specifications as best I could. I found the microRaider ICX instrument on the Laurus Systems website, and the RadEye G-10 I on the ThermoFisher website.

The microRaider instrument is advertised as a "spectroscopic personal radiation detector." It uses a cadmium (zinc) telluride (CZT) solid state detector. The information given does not say that the detector is energy compensated, although I would think it would be. This is an important point when measuring personal dose, because the CZT has a much higher atomic number than tissue, and the detector may produce a response quite different from tissue. I could find no information on its response as a function of energy. I also did not see anything to show that testing had been performed against any recommended standards. Also, the specifications do not state what dose quantity the instruments have been calibrated to read—e.g., ambient equivalent, equivalent dose at a point in air, or some other quantity. I got the impression from reading some reviews from users that perhaps the detector is more praised for its ability to identify radionuclides through spectroscopic analysis than it is for its dose measuring capability, although this is simply my own impression based on limited information. The RadEye G-10 uses an energy-compensated GM detector, which I believe should give reasonable measurement accuracies as long as the photon energies are not much lower than about 60 keV and the detector has been properly calibrated. This instrument is intended specifically for dose measurements. The specifications note that these instruments have been calibrated to read ambient dose equivalent and have been tested in accordance with International Electrotechnical Commission (IEC) standard 60846-1 over the energy range from 50 keV to 3 MeV, providing some confidence in their performance.

Given the differences in the two instruments, I am not surprised that they yield somewhat different readings for the same exposures. The microRaider instrument yielded a reading about 30% lower than did the RadEye instrument, according to the results given in your statement. We would all like to have conformity among different instruments used to measure a given quantity. But given the differences between these two instruments and some of the uncertainties I have mentioned, I do not find this difference to be excessive. I cannot recommend any factor for converting between the two, since I am not sure what quantity is being measured by the microRaider instrument.

You can probably get more specific information regarding some of the points I have raised by contacting the manufacturer(s). I wish you well in your program.

George Chabot, PhD

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