Prompt Gamma Rays fromU235(n,f),Pu239(n,f), and Spontaneous Fission ofCf252

Abstract

The spectra of prompt $\gamma$ rays from ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}\mathrm{}(n,f)\mathrm{}$, ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}\mathrm{}(n,f)\mathrm{}$, and ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$(s.f.) emitted at 0-10 nsec after fission were measured with <4-nsec time resolution. A $\gamma$-ray spectrometer with a near-Gaussian response was used over the entire energy region, and the prompt neutrons from fission were positively rejected by time of flight. The measured $\gamma$-ray spectra show a systematic softening with increasing mass number for ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$, and ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$. The average photon energy above 0.14 MeV is 0.97 ± 0.05, 0.94 ± 0.05, and 0.88 ± 0.04 MeV/photon for thermal-neutron fission of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$, and spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$, respectively. This behavior is discussed in terms of nearness of $Z$ and $N$ of the average fission fragment to closed-shell values, of the corresponding changes in level density expected, and of corroborative evidence from x-ray measurements. The total $\gamma$-ray energy released, ${\overline{E}}_{\gamma ,\mathrm{tot}}$, is 6.51, 6.81, and 6.84 MeV/fission for ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}\mathrm{}(n,f)\mathrm{}$, ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}\mathrm{}(n,f)\mathrm{}$, and ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ (s.f.), respectively, for photons above 0.14 MeV and for 0-10 nsec after fission. The calculations of Thomas and Grover are seen to be in agreement with these data in that they predict the correct partition of fission-fragment deexcitation energy between neutron and photon emission. This partition is related to the separation energy required to emit an additional neutron, to the pairing energy, and to the spin barrier for neutron emission. Two theoretical $\gamma$-ray spectra were compared to the measured spectrum for ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}\mathrm{}(n,f)\mathrm{}$, and each was found to fit the measured spectrum well over only a limited portion of the total energy range.