Slow fission of highly excited plutonium nuclei

Background: Earlier measurements of fission lifetimes of the highly excited uraniumlike nuclei by K x-ray fluorescence and crystal blocking techniques obtained slow fission (fission time $\ensuremath{\sim}{10}^{\ensuremath{-}18}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$) for most of the fission events and were shown to be incompatible with the very short fission time ($\ensuremath{\sim}{10}^{\ensuremath{-}20}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$) obtained by the nuclear techniques and also with the very small (\ensuremath{\le}5%) percentage of such slow fission events predicted by simple statistical models. One weakness of the earlier fluorescence experiments is that the observed K x-ray peaks were very broad [full width at half maximum $(\mathrm{FWHM})\ensuremath{\approx}15\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$] and the precise energies of the relevant K x-ray lines could not be determined from such measurements.Purpose: The purpose is to look at the relevant K x-ray energy region in coincidence with the fission fragments with a high resolution (\ensuremath{\approx}1 keV) spectrometer to obtain evidence of slow fission and determine its percentage.Method: Highly excited plutonium nuclei were produced in the fusion of $^{4}\mathrm{He}+^{238}\mathrm{U}$ at $E{(^{4}\mathrm{He})}_{\mathrm{Lab}}=60\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$. The intrinsic width of plutonium K x-ray lines in coincidence with the fission fragments was determined as a direct measure (or lower limit) of the fission time of the slow ($\ensuremath{\sim}{10}^{\ensuremath{-}18}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$) fission events. The minimum percentage of slow fission events has been determined from the K x-ray multiplicity per fission event and the probability of creation of K-orbital vacancies in plutonium.Results: A narrow peak (FWHM \ensuremath{\approx} 1 keV) observed in the coincidence photon spectrum at (102.8 \ifmmode\pm\else\textpm\fi{} 0.5) keV, just below the characteristic plutonium ${K}_{\ensuremath{\alpha}1}$ line (103.7 keV) has been attributed to the plutonium ${K}_{\ensuremath{\alpha}1}$ line on the basis of supporting evidence and calculations and we deduce that most of the fission events are slow (fission time $g{10}^{\ensuremath{-}18}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$). No peak has been observed exactly at 103.7 keV.Conclusions: The shift [(0.9 \ifmmode\pm\else\textpm\fi{} 0.5) keV] of plutonium K x-ray lines is plausible, if the fissioning plutonium nucleus spends most of its long fission time in a highly deformed dumbbell shape (beyond saddle) and the corresponding results are in agreement with those obtained earlier by the atomic techniques. Alternatively, if no significant shift of plutonium K x-ray lines can be expected, the absence of a peak at 103.7 keV contradicts earlier atomic technique claims of a significant percentage of slow fission events.