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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 50 mCi AmBe 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 neutron? A neutron detector seems a bit expensive and not very user friendly. We have Geiger counters calibrated for the 60 keV 241Am photons but 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 Internet to find the latest in neutron detectors. You can also read about the more classic neutron detectors in a textbook like Introduction to Health Physics, by Herman Cember, McGraw-Hill Press, 3rd Edition, 1996, pages 382-389.
Depending upon if you want an instant reading of the neutron dose rate or only the personnel's cumulative dose 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 50 mCi AmBe source in an unshielded position has a neutron dose rate of about 2.10 microSv/hour (0.21 mrem/hour) at 1 meter (page 395, Cember) and a gamma dose rate of 157 microSv/hour (15.7 mrem/hour) at 1 meter (Gamma Factor of 0.314 mrem/hour per mCi at 1 meter). But, even 2 mm of steel will stop about 82% of the 241Am's 60 keV gammas (mass attenuation factor of 1.11 sq. cm/gram for iron for 60 keV gammas). John P. Hageman, MS, CHP
Answer posted on 12 April 2005. The information and material posted on this website is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Website. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
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