Investigation of the maximum accessible kinetic energy of fragments in the neutron-induced fission of 238U nuclei

The masses, total kinetic energies (TKE), and emission angles of fragments originating from the fission of ²³⁸U nuclei that was induced by 5- and 6.5-MeV neutrons were measured by using digital methods for processing signals. A detailed analysis of the shape of digital signals made it possible to reduce substantially the contribution of fragments whose TKE values were distorted because of a superimposition of signals from recoil protons and from alpha particles produced in the spontaneous decay of uranium. The total statistics exceeded two million events for either neutron energy, and this permitted performing a detailed analysis of fission-fragment yields in the region of the highest attainable TKE values. An analysis of fragment yields made it possible to draw specific conclusions on the structure of the potential surface of fissile nuclei.     

Investigation of fragment yields from <Superscript>239</Superscript>U fission at anomalously high values of the total kinetic energy

The yields of fragments originating from ²³⁸U fission induced by 5-MeV neutrons are investigated. Accumulated statistics—2.5×10⁶ events of binary fission—make it possible to study fission-fragment yields at anomalously high values of the total kinetic energy. The spectra of the cold fragmentation of ²³⁹U are obtained. Events characterized by the total kinetic energy that is equal to the total reaction energy are found for some fragment masses. Methods of digital signal processing permit a highly reliable identification of these rare events. An interpretation of this phenomenon on the basis of the liquid-drop model of the fission process is proposed.     

Possible explanation of fine structures in mass-energy distribution of fission fragments

Within an improved scission point model, experimental data on the relative yield, mean value and variance of the total kinetic-energy distribution of fission fragments are described. It is shown that for a fixed mass and charge fragmentation, the potential energy of the scission configuration has several minima as a function of the deformation parameters of the fragments. The scission at these minima leads to a relatively enhanced yield of the fragments with a certain TKE and creates fine structures in the TKE-mass distribution which are different from those produced by the odd-even effect.Received: 24 March 2004, Published online: 12 October 2004PACS: 24.75. + i General properties of fission - 21.60.Gx Cluster models     

Nuclide yields of light fission products from thermal-neutron induced fission of 233U at different kinetic energies

The yields of light fission products from thermal-neutron induced fission of ²³³U are measured as a function of their mass A, their nuclear charge Z, their kinetic energy E and their ionic charge state q at the recoil spectrometer Lohengrin of the Institut Laue-Langevin in Grenoble. The mass yields are determined by intercepting the fragments with an ionization chamber of high energy resolution positioned at the focal plane of the spectrometer. The nuclear charges and their yields are determined with the same ionization chamber by measuring the residual energy of fission products, selected monoenergetically by Lohengrin, behind a passive absorber made of parylene-C. The nuclear charge resolution enabled by this detector device is considerably improved to Z/dZ = 58. The nuclear charge and mass distributions summed over all ionic charge states are listed within the mass range 79 ⩽ A ⩽ 106 at 6 energies: E = 85.34, 90.41, 95.46, 100.50, 105.55 and 110.55 MeV. The energy-integrated nuclear charge and mass yields are also given. The isotonic and isotopic yields are shown. An odd-even effect in the yields is found for the protons as well as for the neutrons at all kinetic energies. The yield weighted total odd-even effect for the protons is found to be (22.1 ± 2.1)%, for the neutrons (5.4 ± 1.7)%. An odd-even effect for the protons in the mean kinetic energy is also observed. The displacement of the mean isobaric nuclear charges from the unchanged charge-density values and the variances of the isobaric nuclear-charge distributions reveal fine structures in their mass dependences.     

The influence of pairing and nuclear structure on the thermal-neutron-induced fission of235U

The mass separated fission product beam provided by the mass separator “Lohengrin” has been used to determine the nuclear charge distribution for the thermal-neutron-induced fission of²³⁵U for all light fission products in the region 80≦A≦107. The measurements were performed at the most probable kinetic energy of the fission products. By using the known fission product mass yields, the independent yields for a total number of 100 nuclides were obtained under the condition of the most probable kinetic energy. The proton pairing effect modulates the average nuclear charge of the fission fragments and the isobaric charge distribution widths in a regular fashion. The probabilities of breaking a pair and of forming fragments with an energetically unfavourable neutron-to-proton ratio are found to compete with each other. Both probabilities depend on the mass split and reach their maximum values in the region of the most probable masses. The odd-even-proton effect is found to vary smoothly between 16% for the most abundant mass splits and 40% for the rare mass splits. The odd-even-neutron effect exhibits maxima nearN=50 andN=60, where it reaches 16%. These maxima and the extremely low Tcyield (0.13±0.05%) are discussed with regard to fragment shell effects.     

关於热能中子所致铀235分裂时发出的中子数目的讨论

An investigation on the average number v of prompt neutrons emitted per thermal neutron induced fission of U²³⁵ has been made with the Fermi gas statistical model. Weizsacker-Fermi semi-empirical mass equation has been used in calculating the neutron binding energies of the fission fragments. Using Stern's value for the mass of U²³⁵, the total excitation energy E_e of the fission fragments has been estimated to be of the order of 10 to 20 Mev for different hypotheses regarding the primary fission products. The results of calculation (given in the third table) show that only the hypothesis of equal radioactive chain lengths together with the assumption (A) that the excitation energy E_e is shared by the two fragments in proportion to their masses yields values of v exceeding 2. The latter assumption is not in accord with the experimental finding of Fraser that the light fragment group emits on the average 30% more neutrons than does the heavy. However, a shift of mass of U²³⁵ towards larger values or of kinetic energy of fission fragments towards lower values so that 5 Mev more excitation energy is available would make v considerably larger than 2 even with the assumption (B) that the excitation energy is shared by the two fragments in inverse proportion to their masses, thus making possible conformity with Fraser's discovery. Even then, in no case has the value of v thus calculated exceeded 3 (as shown in the fourth table), which may then be taken as a theoretical upper bound for the value of ν for thermal neutron induced fission of U²³⁵. The results of this investigation are thus seen to be in harmony with the recently announced experimental value 2.5 ±0.1 for ν.     

Determination of the fission fragment excitation energy with allowance for neutron multiplicity

The study of the excitation energy distribution of fission fragments as a function of their mass and charge is important for revealing the nuclear fission mechanism and useful for many applications. To measure directly the excitation energy of primary fission fragments (before emission of neutrons) is a great problem. A method of obtaining these excitation energies from calculated neutron multiplicities and experimental values for differential yields of fragment pairs after emission of neutrons is considered. The Empire-II code was used to calculate neutron multiplicities as a function of various characteristics of the nuclear structure, fission process, and fission fragment deexcitation.     

Effect of shell structure on energy dissipation in heavy-ion collisions

The effect of shell structure on the distribution of the excitation energy between fragments of the deep inelastic collisions is analysed in the microscopic approach. It is shown that the density of the single-particle levels of the proton and neutron subsystems near the Fermi surface determines the ratio between the excitation energies of fragments at the initial stage of the collision. It is shown also that the shell structure strongly influences the correlations between the width of the charge distributions and the total kinetic energy losses. Calculations are performed for the ^40,48Ca+²⁴⁸Cm reactions. The results obtained suggest a possible interpretation for the observed concentration of the excitation energy in the light fragment in deep inelastic collisions for a wide range of the total kinetic energy losses. Received: 27 July 1999 / Accepted: 29 March 2000     

Multiparameter studies of polar emission in 236U fission

The energy distributions and relative intensities of protons, deutrons, tritons and α-particles emitted along the fission axis during thermal neutron fission of ²³⁵U were measured simultaneously with both fission fragment energies. The mass distributions of fragments, the total kinetic energy (TKE), the dependence of the mean TKE on the fragment mass, as well as the mean kinetic energy dependence of polar particles on the fragment mass and energy were subsequently deduced from these data. Although some experimental results agree remarkably well with the hypothesis that polar particles are evaporated in flight from fission fragments, the general conclusion is that these particles are emitted according to some other mechanism.     

Kinetic energies of cluster fragments in ternary fission of <Superscript>252</Superscript> Cf

The kinetic energy distribution and potential energies of fragments from the collinear cluster tripartition (CCT), the “true” ternary fission of ²⁵²Cf, have been calculated. It is assumed that the breakup of the nucleus into three fragments happens sequentially in two steps from a hyper-deformed shape. In the first step a first neck rupture occurs of the parent radioactive nucleus, forming two fragments (one of them is usually ¹³²Sn) and, in the second step, one of the two fragments breaks into two other fragments, resulting finally in three fragments (the experiment is based on a binary coincidence where a missing mass is determined). We show the result for the principal combination of the three spherical fragments (semi-magic isotopes of Sn, Ca, Ni) observed recently experimentally. These isotopes are clusters with high Q -values, which produce the highest yields in the ternary fission bump. It is shown that the kinetic energies of the middle fragments have very low values, making their experimental detection quite difficult. This fact explains why the direct detection of true ternary fission with three fragments heavier than A > 40 has escaped experimental observation.     

Recoil studies of fission products formed in the fission of U by protons of energies 25–85 MeV

The recoil properties of ten fission products with masses ranging from 72 to 136 formed in the fission of ²³⁸U with protons of energies 25–85 MeV have been determined radiochemically by the integral-range method. From the recoil properties of the products and the Monte Carlo cascade calculations the average kinetic energy, cascade deposition energy, and anisotropy parameter for each fission product has been calculated. The kinetic energy and the excitation energy of the primary fragments leading to the observed fission product, and the total kinetic energy and the total excitation energy of the primary fragment pair have also been calculated.

The results indicate that up to a bombarding energy of 40 MeV fission takes place predominantly by the compound nucleus mechanism, with an increasing contribution of the direct interaction mechanism as the bombarding energy increases. The kinetic energy deficit was found to decrease with increasing bombarding energy. The fission products formed from the symmetric mode of fission have a larger separation distance between the charge centres of their respective primary fragments than those for the asymmetric mode of fission.     

Alpha-induced fragmentation of 28Si in a statistical model

Within the context of a statistical model, that incorporates final-state interaction between a pair of fragments, we have calculated the energy spectra associated with the production of different isobaric pairs as a function of their lab kinetic energy and isobaric and elemental distributions of nuclei produced in the ⁴He$ + $²⁸Si reaction at cm incident energies of 102.7, 173.7, 300, 500, and 1000MeV. Double differential cross-section of isobars 16, 20, and 24 as a function of their lab kinetic energies at 30^° and the same for isobar 24 at 10^°, 30^°, 60^°, and 90^° have been calculated at cm incident energies of 102.7 and 173.7MeV and compared with the data of Woo et al. Calculated yields follow the trend of the data at each angle, and calculated angular distributions also reproduce the general trend of the observed ones. A key feature of the model is that it allows for fragments to be emitted in ground states as well as in excited states that are allowed by the conservation of energy. The analysis establishes that the fragments are emitted in excited state. The excitation energies for A = 24 and 16 are deduced from the data. The observed angular distributions for A = 7, 12, 16, 20, 24, and 28 are well accounted for assuming them to be emitted in excited states. The relative production probabilities for different elements and isobars are energy dependent. The yields for unstable elements, ⁵Li, ⁸Be, and ²⁶Al, are found to be significant. The relative fragmentation probabilities of all allowed isotopic pairs have been presented.     

Investigation of polar emission in 252Cf and 235U + nth fission

The energy distributions and relative intensities of protons, deuterons, tritons and α-particles emitted along the fission axis during spontaneous fission of ²⁵²Cf were measured simultaneously with both fission fragment energies. The absolute intensity of particles, the mass distribution of fragments, the total kinetic energy and total excitation energy of both fragments were subsequently deduced from the experimental data. Statistical model calculations based on a hypothesis that the polar particles are evaporated from fission fragments have been performed for ²⁵²Cf and ²³⁶U fission. Although some experimental results agree remarkably well with the evaporation hypothesis, the considered model cannot describe many features of the polar emission phenomenon.     

The interaction of 605 MeV 63Cu with 197Au

Binary coincident fragments from the ⁶³Cu + ¹⁹⁷Au reaction at a copper energy of 605 MeV have been studied. Fragment energies were measured and fragment masses determined by a kinematic method. Three types of event are defined by suitable adjacent limits in the mass versus total kinetic energy event space. The angular distributions of cross section, average total kinetic energy and average mass have been determined for each event region. Total cross sections determined in the present experiment are compared to those found at lower bombarding energies. Further information on the sequential fission process has been obtained from measurements of yields of radioactive isotopes resulting from bombardment ofthin and thick targets of Au by 605 MeV Cu ions.     

Analysis of fragment mass distribution in asymmetric area at fission of <Superscript>235</Superscript>U induced by thermal neutrons

The fragment mass yields in fission of ²³⁵U induced by thermal neutrons for A = 145–160 and E_K = 50–75 MeV were measured using a mass spectrometer. The fine structure is observed at A = 153, 154 and E_K = 50–60 MeV. The obtained results were described in the framework of a model based on the dinuclear system concept. The analyzed correlation between the total kinetic energy and mass distribution of fission fragments is connected with the shell structure of the formed fragments of fission. From this correlation and the time dependence of the calculated mass distribution of the binary reaction products, one can conclude that the descent time from a saddle point to a scission point for the more deformed fragments is longer than that for fragments of more compact shape.     

Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus ²²⁶Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in ¹⁸O + ²⁰⁸Pb interaction induced by projectile oxygen ions of energy in the range E _lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E _lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, M_γ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence M_γ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E _lab, the gamma-ray multiplicity M_γ as a function of the total kinetic energy (TKE) of fragments, M_γ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E _lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ～Q _max.     

Mass and kinetic energy distribution in cold fission of233U,235U and239Pu induced by thermal neutrons

The time-of-flight technique was used to measure the mass and kinetic energy distribution of fragments from fission of²³³U,²³⁵U and²³⁹Pu, induced by thermal neutrons at the Grenoble High Flux Reactor. The data array is presented as equal probability lines in the high kinetic energy regions. The fluctuations observed in those experimental lines are explained by a static scission configuration model, in which the most important influence comes from the Coulomb interaction energy between the two fragments. The highest values of total kinetic energy are obtained for fragmentations with heavy fragmentsZ=50–52,N=80–82 and light fragmentsZ=40–42,N=60–64.     

Comparison of the spontaneous fission of 244Cm and 252Cf: (I). Fragment masses and kinetic energies

The energy of both fission fragments of ²⁴⁴Cm and ²⁵²Cf, respectively, was measured in coincidence with the prompt neutrons (see part II). Energy calibration of the surface-barrier detectors was done after the method of Schmitt et al. with the ²⁵²Cf fragments. Mean values and rms widths of the mass and energy distributions of both isotopes are calculated and compared with the results of other authors. The total kinetic energy of ²⁴⁴Cm fragments is at least as high as that of ²⁵²Cf fragments.     

Entrance-channel dynamics in the reaction ~(40)Ca+~(208)Pb

The entrance-channel dynamics including capture, fusion, and quasifission processes for the reaction ~(40)Ca+~(208)Pb is investigated in the fully microscopic time-dependent Hartree-Fock(TDHF) theory. The calculations are performed in three-dimensional Cartesian coordinate without any symmetry restrictions, in which the full Skyrme energy functional SLy4d and SLy5 are adopted.We study the energy dependence of capture cross sections, and find that the experimental data are well reproduced by the TDHF calculations. Both fusion and quasifission events are observed in the reaction ~(40)Ca+~(208)Pb. The contact time, mass and charge of quasifission fragments show a wide distribution in SLy4d compared with SLy5, implying that more nucleons are transferred in the SLy4d calculations. We find that the total kinetic energy of quasifission fragments in the TDHF calculations is distributed around Viola systematics, indicating that most of the relative kinetic energy is dissipated in quasifission dynamics.     

Cold fragmentation in thermal-neutron-induced fission of 233U and 235U

The mass spectrometer Lohengrin of the Institut Laue-Langevin in Grenoble was used to measure fission-fragment mass yields in the mass range 80 ≤ A ≤ 107 for light-fission-fragment kinetic energies up to about 115 MeV for the reactions ^233,235U(n_th, f). The kinetic energies corresponding to a common fixed yield level for each isobar reflect the influence of the proton pairing energy, but not of the neutron pairing energy. By using calculated Q-values for the different mass splits, mass distributions at fixed total excitation energy are deduced from the data. At a fixed total excitation energy of about 7 MeV, the yield increases from very asymmetric mass splits (A_L ≈ 80) to more symmetric mass splits (A_L ≈ 105) by more than two orders of magnitude. This strong dependence on the mass split seems to be correlated with the decreasing surface-to-surface distance of the unaccelerated fission fragments in this range of mass splits, as calculated under the assumption that the total Q-value is represented by the mutual Coulomb repulsion of the two fragments. The influence of the fission-fragment ground-state deformations on the yield in cold fragmentation could not be detected unambiguously.