Health Physics Corner, May 2025
Checking the Estimated Activity of Monazite Ore
Welcome back to the Health Physics (HP) Corner!
In this instalment, you are given a sample of Monazite ore, which nominally contains 10 MBq of Th-232. You want to check that the activity estimate is reasonable by measuring the gamma dose rate using a properly calibrated gamma survey meter.
Question
The ambient dose equivalent gamma constant (Г ) for Th-232 is 3.42E-5 µSv m2/h/MBq, so your estimate for the dose rate at one metre should be as follows:
- H*(10) = 3.42E-5 µSv m2/h/MBq × 10 MBq × (1 m / 1 m)2
- H*(10) = 3.42E-4 µSv/h
This is essentially zero.
You decide to try measuring at 10 cm, which should increase this by a factor of 100 to 0.034 µSv/h. This is at the very limit of what your instrument might be able to detect above background.
When you try the measurement, it is more than 1,000 times as high as you expected. Why?
Show Answer
Jeff describes himself as “a grumpy old curmudgeon” who retired from Canadian Nuclear Safety Commission’s Accelerators and Class II Facilities Division in January 2021 after 17 years with the division. Prior to that, he spent 20 years at CancerCare Manitoba, the last few years of which he was the radiation safety officer for the radiation therapy facilities. Throughout both careers, he was blessed with opportunities to explore a very broad range of radiation-safety-related topics, including non-ionizing, diagnostic X-ray, nuclear medicine, medical radiotherapy, cyclotron isotope production, and even high-energy research accelerators. For some strange reason, both institutions also allowed him to teach on topics such as radiation protection fundamentals and radiation shielding design, and to mentor (corrupt) young staff. The highlight of his professional life came in 2020, when CRPA awarded him Richard V. Osborne Founders’ Award. Now that Jeff is retired, he spends most of his time attempting to golf (very badly), making sawdust in the basement (woodworking), or collecting dirty bits of paper (a.k.a. postage stamps). He loves single malt scotch whisky, of which he has far more bottles than his liver can possibly survive. He continues to dabble in radiation protection and has maintained his membership with CRPA (usually after 18 notices to renew, as he’s growing a bit forgetful in his old age). The Bulletin is published by the Canadian Radiation Protection Association (CRPA). It’s a must-read publication for radiation protection professionals in Canada. The editorial content delivers the insights, information, advice, and valuable solutions that radiation protection professionals need to stay at the forefront of their profession. Sign up today and we’ll send you an email each time a new edition goes live. In between issues, check back often for updates and new articles. Don’t miss an issue. Subscribe now!
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