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Answer to Question #6333 Submitted to "Ask the Experts"Category: Radiation Basics — Fission, Fusion The following question was answered by an expert in the appropriate field: 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
Answer posted on 21 March 2007. The information and material posted on this website is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Website. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
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