Coincident Electrofission Cross Section forU238from 5 to 11.7 MeV

Abstract

There have been conflicting reports 1 " 9 about the magnitude, energy dependence, and fission probability of the isoscalar electric quadrupole giant resonance (GQR) in 238 U. The spectra of inelastically scattered protons, 1 alpha particles, 5 ' 6,8 and 6 Li ions 7 have a broad bump at energies which vary for the different experiments but are generally in the range between 9 and 13 MeV. This enhanced inelastic scattering was interpreted as a concentration of E2 strength by interpolating a "background" in the 9-to 13-MeV excitation-energy region on the basis of the inelastic scattering observed at higher and lower energies. This procedure has been used 1011 to identify concentrated £0 strength near 80A~l /s MeV and £2 strength near 65A" l/s MeV for many nuclei with A between 50 and 209. The separation of E0 and E2 strength followed the pioneering (a, a') experiments at scattering angles as low as 3° by Youngblood and his collaborators. 12 Our results, which have abetter signal-to-noise ratio than did previous experiments for the sum of £2 and £0 strengths, show a relatively energy-independent distribution of the£2/£0 strength between 7 and 11 MeV. Our results make it clear that the assumed background in the (p ,p f ), {a,a f ), and ( 6 Li, 6 Li ; ) experiments overestimated the£2/£0 strength from 9 to 11 MeV compared to the £2/ £0 strength from 7 to 9 MeV. This discrepancy raises the question of whether the£2/£0 strength is as concentrated in energy as is assumed for 17 C. . nuclei with A between 50 and 209. The background in inelastic hadron scattering comes from an unknown combination of multistep nuclear excitations and direct excitations of higher multipolarity states. Because these backgrounds involve nuclear excitation, they cannot be removed by coincident measurement of the nuclear decay. In contrast, the (e,e'f) cross sections which we measure are due to single-step nuclear excitations that include only low multipoles.The previous reports of anomalously high 2,3 and low 56 fission probability of the GQR in 238 U can be clarified by our findings. High fission probability was inferred from electrofission (e, /) experiments 2 3 which can provide reliable information about £2 strength only if accurate virtual-photon spectra are known and if accurate absolute cross sections are available both for £1 photofission and for electrofission. The large £2 component claimed 2,3 has been contradicted by experiments 9 which measured (e + ,f) as well as {e~,f). Our results cast doubt on the high fission probability partly because the reported energy concentration 2 ' 3 conflicts with our results, and partly because our data are consistent with equal £1 and £2 fission probabilities. This equality of fission probability for 1" and 2 + states is expected because the density of fission transition states at high energy is not related to the energies of the lowest fission barriers of states with different spins and parities. The argument in Ref. 3 that Coincident Electrofission Cross Section for 238 U from 5 to 11.7 MeVThe m V(...

Coincidence Electron Scattering (e,e′f) and Multipole Strength Functions in<

“…This issue has recently attracted particular attention regarding the possibility of either a strongly enhanced 1,2 or a suppressed 3 coupling of the GQR to the fission channel. The present study significantly extends earlier m U(e,e'f) data 4 in momentum transfer, excitation energy, angular correlations, and statistics, thereby facilitating a clear multipole decomposition and a rigorous comparison with recent theoretical results from the quasiparticle random-phase approximation (QRPA).…”