Answer to Question #7387 Submitted to "Ask the Experts"

We are looking for portable personnel/contamination survey instruments that could identify a specific radionuclide (⁶⁵Zn). The problem is that we work in an environment in which ⁶⁰Co is the primary nuclide of concern and the pancake GM (Geiger Mueller) tubes (although efficient for detecting the beta from ⁶⁰Co decay) are horribly inefficient at detecting the gamma emitted from the ⁶⁵Zn decay. In fact, the minimum sensitivity of these instruments for ⁶⁵Zn is about 30 times greater than the surface contamination limit to which we must work. We have developed procedures for counting swipes/smears using a scintillation detection system but this requires a strict geometry. Personnel are currently surveyed by portal monitors. We need something portable for field use.

Because ⁶⁵Zn decays primarily by electron capture with only a minor transition by positron emission (1.46% of decays) the efficiency for detection with a GM detector is greatly reduced compared to a case, such as ⁶⁰Co, which decays by beta emission 100% of the time. As you note, the gamma detection efficiency for the GM detector is very low. Much better gamma detection efficiency can be obtained with a reasonable volume solid detector. The major gamma ray emitted by ⁶⁵Zn has an energy of 1.116 MeV and is emitted in 51% of the decays; the 0.511 MeV annihilation photons are the next most abundant photons, having a yield of 0.0292 (2.9%), two such photons being produced for each positron emitted. Other detector types, solid rather than gas, typically have much higher detection efficiencies for gamma radiation than does the GM detector.

For example, the expected intrinsic detection efficiency for 1 MeV gamma rays incident normally on the thin window of a pancake-type GM detector that has an active area of 15 cm² and a response of 6,280 cps per mGy h^-1 is about 0.7%. This compares to an intrinsic efficiency for detection of 1 MeV gamma rays incident on a 2.54 cm thick NaI(Tl) of about 40%. (Note that intrinsic gamma detection efficiency is the probability that a gamma-ray incident on the detector will produce a detectable event; intrinsic efficiency does not include the geometry factor that accounts for the fraction of gamma rays emitted by the source that intercept the detector.) Portable instruments that incorporate NaI(Tl) detectors, or other scintillation detectors, are available from various manufacturers—typical suppliers include Canberra, Ortec, SAIC, and Ludlum, among many others.

If you have a need to distinguish, in field measurement, between ⁶⁰Co and ⁶⁵Zn you may have to consider portable systems that allow determination of photon energies. Some suppliers, including some of those noted above, provide portable systems using NaI(Tl) detectors, and some use semiconductor detectors, such as germanium or cadmium telluride. High-quality germanium detection systems allow excellent resolution and often sufficient efficiency for many applications in contamination monitoring.

The germanium detectors must be cooled, and some systems require liquid nitrogen that must be supplied from a portable dewar vessel. Ortec, and possibly others, also manufactures a germanium-based system that use thermoelectric cooling.

The semiconductor detectors, especially the germanium, have the distinct advantage of providing excellent energy resolution, a feature which would be very helpful in distinguishing the 1.115 MeV gamma ray from ⁶⁵Zn from the 1.17 MeV gamma ray from ⁶⁰Co. Some manufacturers/suppliers that you might want to investigate include Canberra and Ortec noted above. XRF Corporation manufactures a cadmium telluride (Cd-Te)-based portable spectroscopy system; Cd-Te detectors have a high atomic number that favors increased gamma detection efficiency, although the detectors tend to be small in volume, which may make the efficiency lower than desired. A number of suppliers are providing various radionuclide identification systems that are advertised as appropriate for homeland security applications. Some of these may also be suitable for field assessment of surface contamination.

Some of the instruments referred to here, especially the radionuclide identification systems, are much more expensive than standard GM survey instruments, and you should discuss your application needs with knowledgeable company representatives before you decide on purchasing such a system. Good luck in your search.

George Chabot, PhD, CHP