In this instalment, we ask you to describe an appropriate method for identifying an unknown source of radiation found within a sealed truck.
A radiation alarm is triggered at the entrance to a landfill site as one of the garbage trucks passes through. You receive a call asking for assistance identifying and isolating the source. Unfortunately, you do not have any type of portable spectroscopy equipment available; all you have is a calibrated survey meter.
You know the source is likely a common industrial or medical isotope, such as those identified in the Canadian Nuclear Safety Commission’s (CNSC’s) Radionuclide Information Booklet, or possibly naturally occurring radioactive materials (NORM).
When you get to the site, you measure the absorbed dose rate at the surface of the truck and find that it is fairly uniform at about 100 µGy/h on either side and through the back of the truck. You wait one hour, then measure it again and find there has been no discernable drop in the dose rate.
The sides and back of the truck are approximately 1 cm thick steel, and the truck is 2.5 m wide.
Question
What can you deduce about the source prior to opening the load for inspection? Explain your reasoning. How would you proceed with identifying, isolating, and containing the source(s)?
Answer
There is no single “correct” answer, and you may well think of many better ideas than those presented here.
Narrowing it down
Which potential sources can be easily eliminated?
Sources that do not produce significant gamma emissions by themselves or via daughter products, such as H-3, Sr-90, P-32, Ni-63, and Fe-55 (due to high measured dose rate)
Isotopes that are not in a concentrated, high activity form, such as NORM
Very short-lived isotopes, such as those used for PET imaging, C-11, N-13, O-15, and F-18 (Even the longest lived [F-18, T1/2 = 2 hours] would show a significant drop in the dose rate over a one-hour period.)
Gaseous sources
Very low energy (<50 keV) gamma and X-ray emitters, such as I-125 (At these energies, the 0.5 cm of steel in the sides of the truck would allow <5% transmission.)
Assuming a single source
If the measured dose rate comes from a single source, the source would need to be located roughly equidistant (1.25 m away) from the back and sides of the truck. The approximate source activity will depend on the gamma constant for the isotope, but can be approximated as follows:
Of the commonly used isotopes, Co-60 has the highest gamma constant (3.07E-4 mGy/h/MBq at one metre) due to its 2 high-energy and high-yield gammas (1.17 MeV and 1.33 MeV, both at 100%). This implies that the source activity must be at least:
A = (0.156 mGy/h)/(3.07E-4 mGy/h/MBq)
A ≈ 0.5 GBq
Similarly, it would require about 2 GBq of Cs-137 and activities in the 0.5 GBq to 2.0 GBq range for other common industrial isotopes, such as Ir-192 or Ra-226. These activities align well with many common sealed source activities for industrial use for these isotopes.
Note that, in terms of medical isotopes, it would require roughly 3 GBq of I-131 or 8 GBq of Tc-99m to produce this dose rate. This suggests it is extremely unlikely to be a nuclear medicine imaging isotope, but that the source could also possibly be a medical therapeutic source, such as I-131.
Other possibilities—what about a distributed source?
If it were an unsealed source (e.g., I-131) that has contaminated a “slice” of the load, might it produce a similar radiation dose profile?
Could it possibly be many smaller sealed sources somehow distributed relatively evenly across a section of the load?
Precautions for isolating the source
Check with the CNSC as to whether any lost or stolen sources have been recently reported in that catchment area.
Ideally, get your hands on some portable spectroscopy equipment and make a positive identification of the isotope from the gamma spectrum. However, assuming this is not possible:
Consider leaving the truck overnight and remeasure the next day to see if there is any appreciable decay. If so, it may be feasible to employ delay-and-decay prior to offloading.
Keep in mind there may be a contamination hazard and wear appropriate personal protective equipment (PPE). Implement measures to control and prevent the spread of contaminated materials.
Choose an appropriate, isolated area for offloading and implement measures to contain any contaminated garbage.
Ensure you have suitable containment and handling tools for retrieving and isolating the source(s).
Have the truck move forward slowly and offload the garbage in small sections. Monitor each section as it comes off, as well as the load remaining on the truck.
As soon as a source or contaminated article is identified, reassess the radiological hazard and revise your approach to address the specific needs (e.g., is it contaminated material, a single source, or multiple sources?).
So, what really happened?
Yes, this was a real situation based on my fading memory (it happened 30 years ago). No, I did not think it through as nicely as I did here. However, I did manage to do most of the right things.
There were roughly 1,500 old, commercial style, hard-wired smoke detectors in crushed cardboard boxes situated 1 to 2 metres from the back of the load. Unlike modern smoke detectors containing negligible quantities of Am-241, these older devices each contained approximately 50 µCi (1.85 MBq) of either Am-241 or Ra-226 on flat discs a few inches in diameter. The Ra-226 accounted for most of the measured dose rate; the total activity of Ra-226 was approximately 1 GBq, which is in agreement with the roughly estimated activity.
This happened in mid-winter in Manitoba, and it was freezing bloody cold. It took hours to pick out all of the source rings using a NaI detector and scoop them into canisters using a shovel. Once I had finished, various authorities subsequently tried to identify where the smoke detectors had come from, but with no success. It was evident that whatever company had removed them had decided to simply dump them in someone else’s garbage bin, rather than pay for proper disposal.
Jeff Sandeman
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).
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Thanks for the excellent article. I had a similar experience dealing with concentrated NORM. The tractor trailer set off an alarm at Turtle Island Recycling in Toronto. The truck was stopped, but the driver took off with the truck. I was notified by MOL and CNSC that the trailer was now located in a trailer storage area in Brampton. The driver had taken off with the tractor. I was employed by OPG and the bill of lading stated that the truck carried plastic from one of our coal fired plants. On further investigation, we found out that the trailer contained plastic fly ash bin liners from one of our coal fired plants. We did have access to portable spectroscopy equipment and found that it was from the heavy metal residue of fly ash. The radioactive material had become vitrified and firmly bound to the plastic liner. Since it was bound, it was determined to be okay to send to land fill.
Jeff Sandeman
Thanks for the excellent article. I had a similar experience dealing with concentrated NORM. The tractor trailer set off an alarm at Turtle Island Recycling in Toronto. The truck was stopped, but the driver took off with the truck. I was notified by MOL and CNSC that the trailer was now located in a trailer storage area in Brampton. The driver had taken off with the tractor. I was employed by OPG and the bill of lading stated that the truck carried plastic from one of our coal fired plants. On further investigation, we found out that the trailer contained plastic fly ash bin liners from one of our coal fired plants. We did have access to portable spectroscopy equipment and found that it was from the heavy metal residue of fly ash. The radioactive material had become vitrified and firmly bound to the plastic liner. Since it was bound, it was determined to be okay to send to land fill.