Answer to Question #5440 Submitted to "Ask the Experts"

Q

I have a ²⁵²Cf source which is supposed to be 1 μCi but I suspect it may be somewhat larger and I need to get a reasonably close estimate. Is there any other way to estimate the activity of a ²⁵²Cf source besides by counting alphas with a silicon detector?

A

You have not stated explicitly what the physical characteristics of the source are—in particular is it a near weightless preparation with a covering over the source? I shall assume that the source is a near-weightless deposit with a thin covering that allows alpha radiation to escape. As you know, ²⁵²Cf decays most of the time by alpha emission, yielding alpha particles at slightly above 6 MeV in about 96.9 percent of the decays; the other 3.1 percent of the time it undergoes fission, producing gamma rays, neutrons, and fission products in the process.

You state that you are looking for an alternative to alpha counting with a silicon detector in order to determine the activity of the ²⁵²Cf. Because of the abundance of the alpha radiation, the ability to differentiate it from other radiations, and the relative ease of measuring the alpha particles, alpha counting is likely the preferred method to use.

However, you are not required to use a silicon detector. A good analysis can be done with other alpha detectors, such as gas flow proportional detectors or zinc sulfide (ZnS) scintillation detectors. If you use a proportional counter or a ZnS detector, use a detector with an active facial area noticeably larger than the active area of the source, and place the source against the window of the detector if possible so that a 2 π geometry is achieved.

You can take reasonable account of attenuation in the source covering by developing an attenuation plot of count rate vs. absorber density thickness. This can be done by placing increasing thicknesses of absorber directly between the source and the detector window and recording respective count rates. Thin sheets of Mylar^TM (aluminized Mylar^TM with a usual density thickness of <1 mg cm^-2) is often good for this. If the source has a covering you should be able to obtain its thickness from the manufacturer.

Other techniques to quantify the ²⁵²Cf activity are possible but likely more difficult unless you have specific facilities and additional source information available. For example, it is possible to use neutron counting to evaluate the activity. This, however, requires having a neutron detector with known detection efficiency for the fission neutrons from ²⁵²Cf. Alternatively, if you have on-site another ²⁵²Cf source of known activity, not necessarily of the same physical characteristics as the source of interest, you can perform comparative neutron measurements to determine the source activity.

There are other more complicated methods that are also possible but require techniques and/or information that you may not have. One example is to do counting, using gamma spectroscopy of prompt gamma rays produced in the fission process. This requires knowing which gamma rays are correlated with production of which fission fragments. Some work has been done in the past along this line in which coincidence counting of the prompt gamma rays associated with production of various fission product pairs has been used. There are data available, but you would probably have to do some research to find what you needed.

Another technique is to use gamma spectrometry to count specific gamma rays emitted by specific fission products that have built up in the source. (This assumes that the source has a covering of sufficient thickness to prevent the loss of fission products.) In order to quantify the ²⁵²Cf activity you need to know the age of the source since fabrication, the fission yield (direct and indirect yields) of the fission product of interest, the gamma yield for the gamma ray of interest from that fission product, and the counting efficiency for the gamma ray.

You will need to use simple first-order kinetic equations to determine the expected quantity of a particular fission product present at a specific time. Selection of fission products with reasonably high yields and relatively short half-lives is desirable from the standpoint of having sufficient activity to make the measurements in reasonable amounts of time. There is a lot of information regarding ²⁵²Cf available on the Internet. Los Alamos National Laboratory has done much work and generated considerable information on the properties of californium, and, if you decide to pursue any of the above nonalpha counting approaches, you may be able to obtain information you need from them. Good luck.

George Chabot, PhD, CHP