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1.
《Physics letters. [Part B]》1988,215(3):469-472
Complete in- and out-of-plane fission fragment angular correlations for 238U have been measured in (e, e′f) coincidence experiments for momentum transfers qeff=0.20 and 0.28 fm− and excitation energies ω=5.5–15 MeV. A combined analysis with previously extracted E1 and E2/E0 multipole strength distributions yields strength distributions for all 6 near barrier fission channels (Jπ, K) = (0+, 0), (1−, 0), (1−, 1), (2+, 0), (2+, 1), and (2+, 2) up to ≈8 MeV. An estim ate of the respective fission barriers, deduced from the increase of the fission cross sections, is given. 相似文献
2.
S. Baba K. Hata S. Ichikawa T. Sekine Y. Nagame A. Yokoyama M. Shoji T. Saito N. Takahashi H. Baba I. Fujiwara 《Zeitschrift für Physik A Hadrons and Nuclei》1988,331(1):53-62
Evaporation-residue excitation functions for16O and12C+197Au reactions were measured by means of the activation technique. The competition between evaporation and fission of the compound nuclei was studied by comparing the observed evaporation-residue data with the published fission excitation functions. A newly devised analysis was applied in order to deduce a fission barrier height at a specified angular momentum and determine the relevant fissioning nucleus as well. We found the fission barriers to be 8.2 MeV for the211Fr nucleus at 16? and 8.2 MeV for the207At nucleus at 27?. 相似文献
3.
J. Van Der Plicht M.N. Harakeh A. Van Der Woude P. David J. Debrus H. Janszen J. Schulze 《Nuclear Physics A》1981,369(1):51-76
The fission probabilities and angular distributions of the fission fragments for the (α, α'f) reaction on 232Th and 238U at a bombarding energy of 120 MeV have been measured from about 4 to 14 MeV excitation energy. Evidence for sub-barrier resonances has been found, the negativeparity ones occurring at the same excitation energy where photofission resonances have been observed. The data are analyzed with the two-humped barrier model. For 238U the data are reasonably well fitted with barriers similar to those known from the literature. For 232Th though, the outer barrier parameters are quite different: the height EB = 6.6 MeV and the width (khω)B = 1.2 MeV. Also for 232Th, introducing an additional mass symmetric and axially asymmetric outer barrier, as was previously found necessary for 238U, does not result in a good fit to the data at higher excitation energies. 相似文献
4.
P. Möller J. R. Nix P. Armbruster S. Hofmann G. Münzenberg 《Zeitschrift für Physik A Hadrons and Nuclei》1997,359(3):251-255
Fusion barriers are calculated in a macroscopic-microscopic model for several cold-fusion heavy-ion reactions leading to heavy and superheavy elements. The results obtained in such a picture are very different from those obtained in a purely macroscopic model. For reactions on 208Pb targets, shell effects in the entrance channel result in fusion-barrier energies at the touching point that are only a few MeV higher than the ground state for compound systems near Z = 110. The entrance-channel fragment-shell effects remain far inside the touching point, almost to configurations only slightly more elongated than the ground-state configuration, where the fusion barrier has risen to about 10 MeV above the ground-state energy. Calculated single-particle level diagrams show that few level crossings occur until the peak in the fusion barrier very close to the ground-state shape is reached, which indicates that dissipation is negligible until very late in the fusion process. Whereas the fission valley in a macroscopic picture is several tens of MeV lower in energy than is the fusion valley, we find in the macroscopic-microscopic picture that the fission valley is only about 5 MeV lower than the fusion valley for cold-fusion reactions leading to compound systems near Z = 110. These results show that no significant “extra-extrapush” energy is needed to bring the system inside the fission saddle point and that the typical reaction energies for maximum cross section in heavy-element synthesis correspond to only a few MeV above the maximum in the fusion barrier. 相似文献
5.
W. Schwab M. Bernas P. Armbruster S. Czajkowski P. Dessagne C. Donzaud H. Geissel A. Heinz C. Kozhuharov C. Miehé G. Münzenberg M. Pfützner C. Stéphan K. Sümmerer L. Tassan-Got B. Voss 《The European Physical Journal A - Hadrons and Nuclei》1998,2(2):179-191
Fragments of relativistic 750 A.MeV U-projectiles were investigated by using the fragment separator FRS for magnetic selection
of reaction products including ray-tracing and ΔE-ToF techniques. For elements between Ge and Sb, measurements of isotopic
yield distributions and velocities revealed three processes: fragmentation, low-energy fission, and high-energy fission. The
last of these regimes is presently reported. First and second moments of distributions of mass numbers, atomic numbers and
velocities of the corresponding fragments allowed us to identify 101
43Tc56 as the most probable fragment of a high energy symmetric fission reaction. Moreover, we could deduce a hypothetical mean
fissioning fragmentation product 208Rn and its highly excited pre-fragmentation parent 227Ra produced in a primary abrasion reaction at an excitation energy of about 290 MeV.
Received: 26 January 1998 / Revised version: 16 March 1998 相似文献
6.
A measurement of symmetric and asymmetric photofission yields of238U in the energy region from 4.5 MeV to 6.5 MeV has been performed. As γ-source the bremsstrahlung from a microtron has been used. The fission yields are obtained from β-counting of chemically separated isotopes 111Ag, 115Cd and 117Cd for symmetric fission and 139Ba for asymmetric fission. Some structures in the yield curves at about 5.3 MeV and a pronounced maximum at about 6.0 MeV in the valley-to-peak ratio in the fission fragment yield distribution are observed. It is suggested that these effects are connected with the double-hump fission barrier concept and indicate differences between fission barriers for symmetric and asymmetric fission. 相似文献
7.
The electrofission angular distributions for 234U in the energy range 5.5 to 25 MeV were measured and are analyzed together with those obtained previously for 236U and 238U. The competition between the K = 0 and K = 1 fission channels following E2 excitation is established, showing a dominance of the K = 0 channel for near-barrier fission. The E2 fission strength functions for 234U, 236U and 238U are deduced as well, and the E2 fission probabilities (at energies below the pairing gap) are estimated. A substantial concentration of E2 strength near the fission barrier is found, in good agreement with earlier photofission angular-distribution studies. 相似文献
8.
Absolute electrofission cross sections for238U and232Th in the energy regionE e =7 ?65 MeV and fission fragment angular distributions forE e =7–30 MeV have been measured. The angular distributions show strong anisotropies for low energies. The relative dipole and quadrupole contributions as a function of excitation energy are discussed in terms of the low lying fission transition states above the fission barriers. The cross sections show significant deviations from the results of some earlier measurements, in particular in the energy region above the giant dipole resonance. From the difficulties of absolute electrofission cross section measurements and the ambiguities in their interpretation it is concluded that by this time the quantitative analysis of electrofission cross sections with respect to the contributions of the giant quadrupole resonances to the fission decay channel should be regarded as rather tentative. 相似文献
9.
D. O. Eremenko V. A. Drozdov S. Yu. Platonov O. V. Fotina M. H. Eslamizadeh O. A. Yuminov 《Moscow University Physics Bulletin》2008,63(2):118-122
A dynamical-statistical model is used to analyze the experimental angular distributions of fission fragments in the reactions α + 238U, 237Np at E α = 20–100 MeV, as well as to determine the Am isotope fission probabilities and the shape isomer yields in the reactions d + 242,240Pu at E d = 20–30 MeV. Manifestations of shell effects are found in the fission barrier structure up to the excitation energies of 50–60 MeV. 相似文献
10.
《Nuclear Physics A》1997,620(2):171-190
Independent and cumulative product yields were measured for the photofission of 232Th with bremsstrahlung with endpoint energies 6.5, 7.0, 8.0, 11.0, 12.0, and 14.0 MeV, applying γ spectrometric techniques on catcherfoils and pneumatically transported 232Th-samples. The independent heavy fragment yields for the fission of the 232Th compound nucleus at excitation energies in the vicinity of the fission barrier were deduced. Postneutron mass, isobaric charge, isotopic mass distributions, isotonic and elemental yield distributions and proton odd-even effects were obtained from these independent yields. In the mass distributions a maximum yield is observed for mass splits with heavy fragments in the region of A = 142, corresponding with a high production of Ba(Z = 56) - isotopes. A slightly increased yield is also observed for mass splits with heavy mass in the vicinity of A = 134. The latter effect increases with increasing compound nucleus excitation energy. The similarity between the mass distributions of the N = 142 fissioning systems 232Th, 234U and 236Pu is striking. For low excitation energy the proton odd-even effect in the element distributions amounts to 30%, while on the other hand no sizeable neutron odd-even effect could be deduced from the isotonic distributions. The proton odd-even effects remain constant up to compound nucleus excitation energies of about 7.85 MeV. For higher compound nucleus excitation energies the proton odd-even effect drops rapidly. A possible explanation of these observations in terms of pair breaking at the outer barrier is proposed. 相似文献
11.
J. Van Der Plicht M.N. Harakeh A. Van Der Woude P. David J. Debrus H. Janszen J. Schulze 《Nuclear Physics A》1980,346(3):349-370
The fission decay channel of 232Th and 238U has been investigated, using the (α, α'f) reaction at 120 MeV bombarding energy. The angular distributions of the fission fragments and the fission probabilities up to around 15 MeV excitation have been measured. No evidence for the fission decay of the giant quadrupole resonance has been found, although for 238U, a weakly excited structure is seen in the (α, α'f) spectrum at about 9.5 MeV excitation at backward angles with respect to the recoil axis. This effect is similar to what has been found in a (6Li, 6Li'f) experiment reported recently. The over-all feature of the fission probability for excitation energies above the fission barrier are well reproduced by statistical calculations. 相似文献
12.
A. Prévost M. G. Porquet A. Astier I. Deloncle F. Azaiez A. Buta D. Curien O. Dorvaux G. Duchêne B. J. P. Gall F. Khalfallah I. Piqueras M. Rousseau M. Meyer N. Redon O. Stézowski Ts. Venkova R. Lucas A. Bogachev 《The European Physical Journal A - Hadrons and Nuclei》2004,22(3):391-395
The 84Se nucleus has been produced as fission fragment in the fusion reaction 18O + 208Pb at 85MeV bombarding energy and studied with the Euroball IV array. Medium-spin states of this neutron-rich isotope have been identified for the first time. Its level scheme has been obtained up to 4.9MeV excitation energy and spin I 7. Its structure is interpreted by analogy with those of the stable heavier isotones. The evolution of the energy of the N = 50 neutron-core excitation is discussed as a function of the proton number. 相似文献
13.
V. A. Drozdov D. O. Eremenko S. Yu. Platonov O. V. Fotina O. A. Yuminov 《Physics of Atomic Nuclei》2001,64(2):179-185
A dynamical approach is proposed for calculating the angular distributions of fission fragments. The relaxation time for the degree of freedom associated with the projection of the total angular momentum of the nuclear system onto the symmetry axis and the coefficient of damping of the fission mode are the basic parameters of this approach. Experimental data on the anisotropy of the angular distributions of fission fragments and on the multiplicities of prescission neutrons are analyzed within the proposed model for 16O+208Pb (E lab=110–148 MeV), 16O+232Th (120–160 MeV), 16O+248Cm (110–148 MeV), and 16O+238U (96–148 MeV). The relaxation time and the damping coefficient are estimated at τK=(5–6)×10?21 s and β=4×1021 s?1, respectively. 相似文献
14.
P. David J. Bisplinghoff M. Blann T. Mayer-Kuckuk A. Mignerey 《Nuclear Physics A》1977,287(1):179-188
Evaporation residue excitation functions have been measured for 35Cl ions up to 170 MeV (lab) on targets of 112, 120Sn and 141Pr. These values are combined with previously reported fission excitation functions to give fusion excitation functions. Classical analysis of these results yield fusion barriers and fusion radii. Earlier results are confirmed: as the target atomic number increases the required density penetration at the fusion barrier increases. The percentage of the reaction cross section undergoing fusion monotonically decreases as the targets go from 27Al to 141Pr. The nuclear contribution to the fusion barrier deduced from these measurements is compared with several model predictions and is found to be in satisfactory agreement with the models due to Bass and Krappe and Nix. 相似文献
15.
A. Ya. Rusanov G. D. Adeev M. G. Itkis A. V. Karpov P. N. Nadtochy V. V. Pashkevich I. V. Pokrovsky V. S. Salamatin G. G. Chubarian 《Physics of Atomic Nuclei》2007,70(10):1679-1693
The average multiplicity of gamma rays emitted by fragments originating from the fission of 226Th nuclei formed via a complete fusion of 18O and 208Pb nuclei at laboratory energies of 18O projectile ions in the range E lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier (E c.m. ? E B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory. 相似文献
16.
M. Sowiński M. Lewitowicz R. Kupczak A. Jankowski N. K. Skobelev S. Chojnacki 《Zeitschrift für Physik A Hadrons and Nuclei》1986,324(1):87-95
The results of coincidence measurements of4He emission with fission fragments in reactions of12C(108 MeV) ions with a197Au target and of16O(144 MeV) ions with a232Th target are presented. On the basis of a Monte Carlo kinematic simulation of nuclear reactions the experimental energy spectra and velocity distributions of alpha particles have been analyzed. A conclusion has been drawn that the main source of4 He emission is evaporation from the fissioning compound nucleus. Substantial part of alpha particles was emitted from fully accelerated fission fragments. Some of4He nuclei with an average energy of about 16 MeV (in the CM system) emitted mainly perpendicular to the fission axis were identified as being similar long-range alpha particles produced in ternary fission of heavy nuclei at a low excitation energy. The emission multiplicities of these particles are considerably higher than those observed at a low excitation energy. The experimental results are compared with the statistical model predictions. 相似文献
17.
《Nuclear Physics A》1986,456(4):701-716
Fission and particle evaporation following stopped- π− absorption were measured for 209Bi. The fission probability per captured π− was obtained as Wf = (3.01 ± 0.34) × 10−3 from the number of fission tracks observed in the mica track detector and that of π− stopped in the Bi target. The latter was derived from the observed absolute intensities of pionic Bi X-rays. The isotopic yield distributions for the (π−, xn) and (π−, pxn) reactions were also determined by in-beam and activation γ-ray measurements. From the analysis of the fission probability and the product-yield distribution, it was concluded that the thermally equilibrated nuclear states have a wide energy distribution and rather low excitation energies (∼50 MeV) compared to the initially involved energy of 140 MeV. A model was proposed for the formation and decay of 209Bi nuclei excited by the capture of the stopped π−. The model well reproduces the experimental results. 相似文献
18.
E.A. Zhagrov I.M. Kuks Yu.A. Nemilov Yu.A. Selitskii V.B. Funstein 《Nuclear Physics A》1973,213(3):436-444
Kinetic energy spectra of fragments in the 5–15 MeV neutron-induced fission of 226Ra are obtained. The fraction of fragments corresponding to the central peak of the mass yield curve is determined for each neutron energy used. It is shown that at low excitation energies of the 227Ra compound nucleus precluding second-chance fission the mass yield curve retains its triple-peaked nature. At still lower excitations, near the fission threshold, radium undergoes asymmetric fission, just as the heavier nuclei. 相似文献
19.
Fission fragment angular distributions have been observed in electron-induced fission of 232Th for electron energies 8.7 MeV ≦ Ee ≦ 30 MeV. For low energies (though above the fission threshold) the angular distributions contain both a dipole and a quadrupole component. The (90°/0°) anisotropy decreases rapidly for higher electron energies but reveals smaller peaks after the onset of second-, third- and fourth-chance fission suggesting that the effective fission barriers for 231Th and 229Th in second- and fourth-chance fission, respectively, are both characterized by . 相似文献
20.
U. Goerlach D. Habs M. Just V. Metag P. Paul H. J. Specht H. J. Maier 《Zeitschrift für Physik A Hadrons and Nuclei》1978,287(2):171-181
The prompt and isomeric fission probabilities of240Pu have been studied using the239Pu(d,p) reaction. A number of resonances are observed in the subbarrier population of the 4 ns fission isomer for excitation energies between 4.0 and 6.2 MeV. Apart from a structure at 4.3 MeV, they do not coincide with transmission resonances found in prompt fission. Calculations with an extended doorway state model which simultaneously reproduce the measured delayed and prompt fission probabilities yield revised fission barrier parameters as well as spectroscopic information on fission and gamma widths of highly excited states in the second minimum. 相似文献