Answer to Question #4406 Submitted to "Ask the Experts"

Category: Industrial Radiation — Industrial Applications

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

Q

We have 1.85 GBq americium-beryllium (241AmBe) portable neutron moisture meter instruments. I need some help with the choice of a survey instrument to go with them. A highly attenuated flux of low-energy photons is emitted from the carrying case from the 241Am. I suspect there is a measurable flux of photons from neutron capture reactions in the case and instrument and there is also a signal from neutrons interacting with the survey instrument itself. What is the most suitable instrument for measuring neutrons? A neutron detector seems a bit expensive and not very user friendly. We have Geiger counters calibrated for the 60 keV 241Am photons, but I am not really sure this is the main signal. Any thoughts on this? Also can anyone recommend a good instrument and perhaps a few comments on why?

A

There are several new neutron dosimeters that are on the market now, due to the demand created by homeland security. I would suggest you search the web to find the latest in "neutron detectors" or "neutron meters." You can also read about the more classic neutron detectors in a textbook like Introduction to Health Physics, by Herman Cember, McGraw-Hill Press, 4th Edition, 2009, pages 465–476.

Depending upon if you want an instant reading, only the personnel's cumulative dose or the neutron dose rate, you could choose from bubble dosimeters, badges (that measure both cumulative gamma and neutron dose), portable instruments, electronic dosimeters, etc. You can read more about these in an International Atomic Energy Agency Publication, Compendium of Neutron Spectra and Detector Response for Radiation Protection Purposes, IAEA, Vienna, 2001, No. 403.

It is interesting that a 1.85 GBq 241AmBe source in an unshielded position has a neutron dose rate of about 2.10 µSv h-1 at 1 meter and a gamma dose rate of 157 µSv h-1 at 1 meter (a Gamma Factor of 84.9 µSv h-1 per MBq at 1 meter). But, even 2 mm of steel will stop about 82 percent of the 241Am's 60 keV gammas (a mass attenuation factor of 1.11 cm2 gm-1 for iron for 60 keV gammas). So, the neutron dose rate may be impossible to interpret from a gamma-only dose rate reading due to the unknown amount of shielding between the 241Am and the detector's inner volume. My experience shows me that gamma survey meters have no significant response to a pure source of neutrons.

John P. Hageman, MS, CHP

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