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How Gamma Emitting
Radioisotopes Are Identified

by Frank A. Iddings*

 

Radiographers and others who use sealed radioactive sources must periodically "wipe" the sources to detect the leakage of radioactive material from them. Sometimes the origin of the radioactive contamination may not be the source that is wiped and the identification of the radioactive material is required. This article is a simplified attempt to tell how that is done. Maybe it is magic! Frank Iddings Tutorial Projects Editor

 

Introduction
When the periodic wipe tests are done on sealed radioactive sources, such as in isotope radio-graphy, a wipe may show more than 185 Bq (5 x 10^-9 Ci) of contamination. Removable contamination above that level generally means the welded seal on the source is open and radioactive material is being released from the source. But that may not be the case; the sealed source may have been contaminated from contact with radioactive material left in the guide tube from a different source or wear in the metal tube inside of the exposure device shield that allows the sealed source to rub against the U-238 (depleted uranium) radiation shield. Most radiation exposure devices for radiography use depleted uranium for radiation shielding because of its weight and durability advantages over lead.

Sometimes removable radioactive material is found on the guide tubes and collimators used with the radiographic sources. The question then becomes what radioisotope is it? The answer helps pin down what the source of the contamination is and what corrective action must be taken.

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With modern gamma spectrometry, identification usually takes only a minute or so.

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Some years ago, the question was asked in one of my radiation safety classes for radiographers, "How do you identify gamma emitting radioisotopes?" My answer began with, "It's magic!" - a factitious answer to gain attention, but couched in some truth. Before the availability of equipment for gamma spectrometry, such identification was really tough and time consuming. With modern gamma spectrometry, identification usually takes only a minute or so. The following is a brief, simplistic description of gamma spectrometry for gamma emitting radioisotope identification.

 

Gamma Spectrometry
Gamma rays (and sometimes X-rays) are emitted by the radioisotopes used in industrial radiography. These gamma rays (and the few X-ray emitting radioisotopes used in industry) have unique energies. For example: Cs-137 emits a single gamma ray at an energy of 0.662 MeV, Co-60 emits two gamma rays at 1.17 and 1.33 MeV and Ir-192 emits several gamma rays with energies from 0.14 to 1.2 (average about 0.34) MeV. Other less common radioactive materials used in radiography, as well as the very common U-238 shielding, all emit specific, identifiable gamma rays.

One of the earliest efforts at identification involved measuring the amount of gamma radiation penetrating different thicknesses of lead so that a half value layer or half thickness for the radiation was determined. This was slow, tedious work with sometimes unsatisfactory results.

The development of scintillation detectors, such as NaI(Tl) - thallium doped sodium iodide - connected to single channel analyzers was a step forward, but still slow and tedious. With the availability of multichannel analyzers, gamma spectra - and hence radioisotope identification - could be obtained relatively quickly.

Briefly, the scintillation detector and multichannel analyzer work by the conversion of the gamma ray into a flash of light in the scintillation crystal, conversion of the light flashes into proportional numbers of photoelectrons in the photocathode of a photomultiplier tube and multiplication of the photoelectrons into electrical pulses exiting from the photomultiplier tube. The multichannel analyzer sorts out the pulses into a plot of number versus size of the pulses (size or amplitude of the pulses is proportional to the original gamma energy released to the crystal.) This plot is a gamma spectrum that can be compared to tables of similar spectra or used to obtain the energy of the gamma rays detected (Heath, 1957).

Two advances have been made since the "magic" of the gamma spectrometer made identification of gamma emitters relatively quick and easy. One is the improvement of the multichannel analyzer into a small device (some are now available that fit into a shirt pocket) that can operate off batteries rather than a system that occupied most of the top of a normal desk and demanded considerable electrical power. The other improvement is the development of semiconductor detectors, such as Ge(Li) - lithium drifted germanium - intrinsic germanium and cadmium-tellurium, that provide much better resolution than NaI(Tl) for the gamma spectra. Better resolution translates into narrower peaks in the gamma spectra. See Figure 1 for a comparison of a NaI(Tl) and a germanium detector (ASNT, 2002).

Figure 1  - Comparison of thallium doped sodium iodine and a germanium detector for gamma spectroscopy.

 

Some computer driven multichannel analyzers - often just a card in a desktop or laptop computer - can give you the identity of the radioisotopes producing the spectrum accumulated. Under the proper conditions, the quantity of the radioisotopes is calculated. All of this is within a few minutes and at a fraction of the cost required before.

To give you an idea of the simplicity of identification from the gamma spectra of the radioisotopes, see Figure 2, showing the gamma spectra of Co-60, Cs-137 and Ir-192 (ASNT, 2002).

Figure 2 - Gamma spectra: (a) Co-60; (b) Cs-137; (c) Ir-192.

 

The "magic" of identification of gamma emitters is now revealed - in part. There is still plenty of "magic" in the instruments and their use. You can identify a radioisotope now if it is Co-60, Cs-137 or Ir-192. Just like any magician, with more practice, you'll be able to do more magic.

 

References
ASNT, Nondestructive Testing Handbook, third edition: Volume 4, Radiographic Testing, Columbus, Ohio, ASNT, 2002.

Heath, R.L., Scintillation Spectrometry Gamma-ray Spectrum Catalogue, AEC Research and Development Report IDO-16408, Washington DC, US Atomic Energy Commission, Office of Technical Services, US Department of Commerce, 1957.

 

* 1635 Rob Roy Lane, San Antonio, TX 78251; (210) 647-7717; email <profiddings@satx.rr.com>.

 

Copyright © 2004 by the American Society for Nondestructive Testing, Inc. All rights reserved.