Symmetric and asymmetric yields in the photofission of232Th,235U,238U and239Pu

Gunther, W.H.; Huber, Klaus; Kneißl, U.; Krieger, Hanno; Maier, H. J.

doi:10.1007/bf01412945

Cited by 21 publications

(3 citation statements)

References 16 publications

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“…This seems to be reasonable, since in a simple picture the differences between the symmetric and asymmetric fission barriers [51] decrease with decreasing mass numbers. This trend has already been observed in previous photofission experiments at the lower bremsstrahlung endpoint energy of E0= 55 MeV [52]. in comparison with the 238U data reported by Ivanov et aL from Novosibirsk [26] The relative contribution of mass symmetric fission is known to be very sensitive to the excitation energy of the fissioning system.…”

Section: Fragment Velocity and Mass Distributionssupporting

confidence: 66%

Photofission of235U and238U at intermediate energies: absolute cross sections and fragment mass distributions

Frommhold

Steiper

Henkel

et al. 1994

Z. Physik A - Hadrons and Nuclei

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Abstract. The total photofission cross section c%,F for Z35U and 238U has been measured in the energy range 50< E.y < 800 MeV at the 855 MeV Mainz Microtron MAMI using energy and time tagged photons (Glasgow Tagger) and a 47r arrangement of position sensitive fragment detectors. Besides the absolute photofission cross section c%,F, which almost completely exhausts the total photon absorption cross section for these nuclei, fragment mass distributions in this energy domain were determined via time of flight techniques (TOF). The results for the total photofission cross sections c%,F normalized to the atomic number A for both isotopes coincide, and agree in the A-resonance region, within the systematic errors, with the socalled "Universal Curve" C%,T/A of the total photon absorption cross section crT,T. At higher energies the cross sections exhibit a smooth behaviour. In particular, it is shown for the first time that there is n__0o resonancelike shape near the D13 resonance (at ~ 710 MeV) as observed for the free proton. This complete suppression of the DI3 resonance in complex nuclei is not yet understood on a microscopic level. The fragment mass distributions show a predominantly mass symmetric fission. However, contributions from mass asymmetric fission at some photon energies may give a hint of an increased mass asymmetric fission after the onset of the pion and two pion channels.

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Section: Fragment Velocity and Mass Distributionssupporting

confidence: 66%

Photofission of235U and238U at intermediate energies: absolute cross sections and fragment mass distributions

Frommhold

Steiper

Henkel

et al. 1994

Z. Physik A - Hadrons and Nuclei

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“…For example, the experiment of Ref. [74] measured the mass yield for the photofission of 232 Th, reporting that the ratio of symmetric yield to asymmetric yield increases from 2% to 10% for a bremsstrahlung energy range (corresponding approximately to our excitation energy) of 15 to 55 MeV.…”

Section: Excitation Energy Dependencementioning

confidence: 99%

Third minima in thorium and uranium isotopes in a self-consistent theory

2013

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Background: Well-developed third minima, corresponding to strongly elongated and reflection-asymmetric shapes associated with dimolecular configurations, have been predicted in some non-self-consistent models to impact fission pathways of thorium and uranium isotopes. These predictions have guided the interpretation of resonances seen experimentally. On the other hand, self-consistent calculations consistently predict very shallow potential energy surfaces in the third minimum region.Purpose: We investigate the interpretation of third-minimum configurations in terms of dimolecular (cluster) states. We study the isentropic potential energy surfaces of selected even-even thorium and uranium isotopes at several excitation energies. In order to understand the driving effects behind the presence of third minima, we study the interplay between pairing and shell effects. Methods:We use the finite-temperature superfluid nuclear density functional theory. We consider two Skyrme energy density functionals: a traditional functional SkM * and a recent functional UNEDF1 optimized for fission studies.Results: We predict very shallow or no third minima in the potential energy surfaces of 232 Th and 232 U. In the lighter Th and U isotopes with N = 136 and 138, the third minima are better developed. We show that the reflection-asymmetric configurations around the third minimum can be associated with dimolecular states involving the spherical doubly-magic 132 Sn and a lighter deformed Zr or Mo fragment. The potential energy surfaces for 228,232 Th and 232 U at several excitation energies are presented. We also study isotopic chains to demonstrate the evolution of the depth of the third minimum with neutron number. Conclusions:We show that the neutron shell effect that governs the existence of the dimolecular states around the third minimum is consistent with the spherical-to-deformed shape transition in the Zr and Mo isotopes around N = 58. We demonstrate that the depth of the third minimum is sensitive to the excitation energy of the nucleus. In particular, the thermal reduction of pairing, and related enhancement of shell effects, at small excitation energies help to develop deeper third minima. At large excitation energies, shell effects are washed out and third minima disappear altogether.

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“…The preneutron mass distributions measured in Refs. [18,19,56] also show analogous fine structure around the A = 134 mass number. It is seen from Figs.…”

Section: Discussion Of the Resultsmentioning

confidence: 75%

Mass yield distributions and fission modes in photofission ofU238below 20 MeV

et al. 2015

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Photon activation technique is used to obtain photofission mass distributions of 238 U after irradiation in a bremsstrahlung beam with end-point energies 19.5, 29.1, 48.3, and 67.7 MeV. The weights of the symmetric (SL) and asymmetric (STI and STII) modes in the mass distributions of photofission of 238 U and neutron-induced fission of 235,238 U at the average excitation energies of up to 20 MeV are calculated on the basis of the obtained results and other available data. The weight of the SL mode increases by about an order of magnitude as the average excitation energy increases from 8.5 to 17 MeV, while the weight of the STI mode decreases by two times, and the weight of the STII mode slightly increases. It is shown that the peak-to-valley ratios in photofission of 238 U and neutron-induced fission of 235 U are very close across almost the whole studied energy range. A similar conclusion is made concerning the behavior of asymmetric and symmetric fission modes in photofission of 238 U and neutron-induced fission of 235 U and 238 U.

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Symmetric and asymmetric yields in the photofission of232Th,235U,238U and239Pu

Cited by 21 publications

References 16 publications

Photofission of235U and238U at intermediate energies: absolute cross sections and fragment mass distributions

Photofission of235U and238U at intermediate energies: absolute cross sections and fragment mass distributions

Third minima in thorium and uranium isotopes in a self-consistent theory

Mass yield distributions and fission modes in photofission ofU238below 20 MeV

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