Selecting a Gamma Imager
A gamma-ray imager can be a powerful tool for locating radiation sources, understanding reactor operation, and reducing worker dose. But, not all imagers are created equal. With most of H3D's employees holding PhDs in radiation detection and regularly hearing feedback from our customers in nuclear power, H3D has some pretty strong opinions about what makes an imager great. Here are 7 attributes that really matter and 3 that, surprisingly, don't:
1. Energy Resolution
Energy resolution is so important we've already written about it twice (Part I, Part II). With good energy resolution, you can know which isotopes are in the spill and can see particular isotopes even in high backgrounds of other isotopes (Application #14). It enables isotopic quantification (Application #10) and allows you to distinguish between true contamination and scattered gamma rays.
The H3D H series has energy resolution better than 1% FWHM at 662 keV.
No matter how good the imager, it won't do you much good if you cannot use it inside the plant. The spec sheet can tell you the weight; make sure it is something you want to carry around all day. Be sure to inquire about any other peripherals that might be needed to operate. Are there FME hazards? Cables that can drag and get contaminated?
The H100 and H400 weigh less than 9 pounds including everything needed to operate. You can also use an included 11-ounce tablet to see the images while in the field.
3. Field of View
In many applications, for instance when locating the primary source term or optimizing shielding, you need to see sources in all directions. A system with a limited field of view will require more measurements to see the same thing. More damning than just not seeing everything in one measurement is that many systems with limited field of views (coded-aperture imagers) will give erroneous images if there is a source outside the field of view—something not usually on the spec sheet. Most imagers should report their field of view on their spec sheet, but also ask if the imaging efficiency varies greatly in that field of view. You don't want the system to be much less sensitive in some of the directions.
The H3D H series views all directions simultaneously.
4. Ease of Use
Though this one is hard to quantify, it can be a game changer. Play with the system before you buy. Is it easy to take a measurement and see results? Can you organize your past measurements and easily generate a report for your colleagues? Remember, the faster you can start the measurement, the less dose you will receive. And, the faster you can analyze the results, the sooner you can get back to reading our blog!
5. Battery Life
Battery life is pretty important if you aren't near an outlet. Determine where you most want to use the imager. Is there power nearby? Do you want to be able to use it throughout most of a shift?
The H100 and H400 have over a 6-hour battery life.
6. Maximum Dose Rate
Some of the most critical measurements you will take will be in high-dose-rate areas. Make sure the imager won't poop out when you need it most—either in its spectral or imaging performance. From an imager's perspective, a high dose of high-energy gamma rays is easier to deal with than a high dose of low-energy gamma rays because the count rate is lower. So ask what type of dose field it has been tested in (or better yet, test it yourself). Keep in mind that you can measure a higher-dose source than this maximum rating if you move the detector further away.
The H100 and H400 can produce a good spectrum and image in a 5 mSv/h (0.5 R/h) field from Cs-137.
7. Customer Service
Ok, this one's not exactly a detector attribute, but it can be critical when you are going into an important outage measurement and you accidentally break something. How quickly will the manufacturer respond to your inquiry and fix your detector? Will you have to pay an arm and a leg?
H3D gives our customers a 24/7 direct line to one of our engineers and two years of no-questions-asked free maintenance and software upgrades. H3D is always working to improve our product and go the distance for our customers.
What doesn't matter as much:
1. Angular Resolution
If you can have everything and angular resolution too, absolutely take it. But, we hear from customers that angular resolution of 30 degrees is fine for most applications, as long as angular precision is good. Resolution says you can distinguish two nearby sources, but precision says you can tell where one source is. You can know exactly which valve is hot, but not know if it is a single hot spot or two hot spots next to each other. But, remember, you can (almost) always get closer if you need to see the detailed source distribution. Don't trade off field of view for angular resolution, especially if you have good angular precision.
The H100 and H400 have 30-degree angular resolution in real time and 20-degree angular resolution in post processing. It can locate a point source to within 2 degrees (angular precision).
If you are trying to find a lost check source from down the hall, or image bananas overnight, efficiency absolutely matters. However, the count rate in most interesting nuclear-power measurements is typically high enough that efficiency isn't the most important parameter (unless the system's efficiency is abnormally low compared to other imagers). Focus more on the maximum achievable dose rate before considering efficiency, as maximum dose rate and good efficiency are often tradeoffs.
The H100 can detect a 10 μCi Cs-137 source at 1 m in 50 seconds and image it in about 5 minutes. The H400 is about three times faster.
3. Low-Energy Imaging
Everyone would love to image 60 keV from Am-241 or similar isotopes, but in practice, we have never seen this peak inside a nuclear power plant because it is very hard for this energy to escape pipes and other objects.
The H100 and H400 can image gamma rays over 240 keV. If your special application actually requires low-energy imaging, H110 and H420 are capable of imaging down to 50 keV.
Thanks for reading! Remember, imagers are not all the same. Do your own tests before buying.
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