Answer to Question #6333 Submitted to "Ask the Experts"

Q

What are the characteristics of the radiations from ²⁵²Cf; in particular, I mean the energy spectra of neutrons and gamma rays from ²⁵²Cf. Also, what are the emission rates of neutrons and gamma rays when it undergoes spontaneous fission?

A

Some fundamental data on the decay properties of ²⁵²Cf can be found in a paper by Martin and others. ²⁵²Cf has a 2.645 year half-life and decays 96.91 percent of the time by alpha emission, spontaneous fission being the alternate decay mode in the remaining 3.09 percent of decay events. The spontaneous fission process leads to the prompt emission of fission neutrons and gamma rays. The neutron emission rate from spontaneous fission of 1.0 micrograms of ²⁵²Cf is 2.314 x 10⁶ n/second. The properties of the fission neutrons and prompt gamma rays from the spontaneous fission (SF) of ²⁵²Cf are quite similar to what are seen from the thermal neutron-induced fission of ²³⁵U, although some differences exist.

In the case of ²⁵²Cf SF, the average number of neutrons produced per fission is about 3.76, which compares to about 2.5 for the thermal fission of ²³⁵U. The average neutron energy is about 2.1 MeV, and the predominant energy is about 0.7 MeV, both similar to neutrons from ²³⁵U. The neutron energy spectrum for ²⁵²Cf is also similar in shape to that from ²³⁵U fission neutrons and can be approximated by a hyperbolic sine function, one expression of which you can see in a paper by Anderson and others.

The prompt gamma rays from SF of ²⁵²Cf are, on average, somewhat less energetic than those for the thermal fission of ²³⁵U, the former being about 0.87 MeV and the latter 0.97 MeV. The total prompt gamma energy emitted per fission of ²⁵²Cf is about 6.95 MeV; considering this with the average gamma energy of 0.87 MeV implies an average number of gamma rays of almost 8 per fission, compared to 6.7 for ²³⁵U. You can review some information regarding the gamma energy and number distribution in a paper by Valentine. Given the neutron and gamma fission yields (3.757 and 8, respectively), we can estimate the prompt gamma emission rate from 1.0 micrograms of ²⁵²Cf as 4.9 x 106 gammas/second. You must also be aware that as a ²⁵²Cf source ages, the inventory of fission products increases; many of these fission products emit gamma radiation, and this contributes to the total gamma emission rate from the source.

There is considerable additional information available on the Internet that you can access through some simple searches. Hope this helps.

George Chabot, PhD, CHP