Answer to Question #3642 Submitted to "Ask the Experts"

I used a ²³⁵U fission chamber to measure a neutron fluence. In order to make corrections to account for the loss of fission fragments in the thickness of the ²³⁵U foil of the fission chamber I need to know the range of the main (2) fission fragments in uranium. I am looking for a good reference (journal, book, etc.) for this matter.

As you know, various high-atomic-number (Z) coatings can be used to measure neutron fluence through a fissioning of a target nucleus coating. Each coating will have a different neutron energy threshold before fission will occur. For example:

In a fission chamber one measures the gas ionization pulse of the fission products, or the current. You may want to look at a few of the older textbooks on radiation detection and measurement, since these detectors were used widely in neutron studies in the 1950s through 1980s. Both Nuclear Radiation Detection by WJ Price (1958 and 1964, McGraw-Hill, Publisher), and Radiation Dosimetry by GJ Hine and GL Brownell (1956, Academic Press, Publisher) have discussions on fission chambers. However, more direct to your question, you should look at Radiation Detection and Measurement by Glenn Knoll.

Knoll notes the range of the fission fragments (FF) are about half that of a 5 MeV alpha particle. Interestingly, because the FF have a high initial ionized state, their specific energy loss (-dE/dx) decreases as the particle loses energy in the absorber. From an alpha range curve on page 125, in the 1970 Radiological Health Handbook (published by the U.S. Department of Health, Education, and Welfare), I get a range of ~3 cm in air for a 5 MeV alpha. If you halve that range to 1.5 cm, and multiply by 1.293 mg cm^-3 (density of air), a FF range is about 1.9 mg cm^-2 in area density.

This fits with some old data sheets I have for commercially manufactured fission chambers, where the coat thickness can be specified from 100 to 1,000 µg cm^-2 (micrograms per centimeter squared). In this case the coating is thin enough where the emerging FF still has residual energy to ionize the counting gas in the chamber. Again, these textbooks and the references cited would be a good place to start. I’d also work with professional science library staff to search the journal abstracts, for example, Nuclear Instruments and Methods, or IEEE Transactions in Nuclear Science.

David J. Allard, CHP