Charge Distribution In Fission Research Articles

Extraction of potential energy in charge asymmetry coordinate from experimental fission data

For fissioning isotopes of Ra, Ac, Th, Pa, and U, the potential energies as a function of the charge asymmetry coordinate are extracted from the experimental charge distributions of the fission fragment and compared with the calculated scission-point driving potentials. The role of the potential energy surfaces in the description of the fission charge distribution is discussed.

Delayed neutrons as a probe of nuclear charge distribution in fission of heavy nuclei by neutrons

A method of the determination of cumulative yields of delayed neutron precursors is developed. This method is based on the iterative least-square procedure applied to delayed neutron decay curves measured after irradiation of 235U sample by thermal neutrons. Obtained cumulative yields in turns were used for deriving the values of the most probable charge in low-energy fission of the above-mentioned nucleus.

Electromagnetic dissociation of 238U at 120 MeV/nucleon.

Electromagnetic fission cross sections of a 120 MeV/nucleon $^{238}\mathrm{U}$ beam incident on five targets, $^{9}\mathrm{Be}$, $^{27}\mathrm{Al}$, $^{\mathrm{nat}}\mathrm{Cu}$, $^{\mathrm{nat}}\mathrm{Ag}$, and $^{\mathrm{nat}}\mathrm{U}$, have been extracted from measurements of projectile velocity fission fragments. The nuclear interaction contributions to the experimentally observed cross sections were determined by extrapolation from the Be target data using a geometrical scaling model and by an empirical decomposition of the fission charge distributions. The results are compared to model calculations in which electric quadrupole excitations have been included.

Coupling of relative motion and charge asymmetry coordinates for the fine structure in fission charge distribution yields

A quantum-mechanical model of coupled charge asymmetry and relative coordinates is solved analytically using the parametrised forms of the potentials calculated on the asymmetric two-centre shell model. Newton's classical equation of motion is used to estimate the scission time and the coupling are included numerically in both the quantum-mechanical and classical treatments. The result of coupling the relative motion to charge asymmetry is shown to give rise to fine-structure effects in fission charge distribution yields (the odd-even proton effects), which are in addition to those already known from the shell corrections and the structure in mass parameters. Calculations are made for the fission of 236U.

Dynamical fragmentation process in nuclear fission

Abstract The proton odd-even effect in fission charge distribution yields is shown to be the quantum mechanical fragmentation process, arising due to shell effects in adiabatic potential and structure of adiabatic cranking masses. Additional contribution is shown to come from the coupling of relative motion to charge fragmentation coordinate, that is solved analytically. The dynamics occur near the top of barrier, just past saddle shape.

Fine structure in nuclear fission charge distribution yields and the odd-even charge effects as the dynamical fragmentation process

The fragmentation theory is used to study the fine structure effects in nuclear fission charge distribution yields. The odd-even proton effects as well as the resulting non-Gaussian charge distributions are shown to be given satisfactorily, without use of any free parameter. Calculations are made for light mass products 98, 100, 102, and 104 due to thermal neutron induced fission of /sup 235/U.

Nuclear charge distribution in fission: fractional cumulative yields of isotopes of krypton and xenon in 249Cf(nth,f) and 250Cf(sp.f)

The fractional cumulative yields (%) of the following rare gas isotopes were determined radiochemically in the thermal neutron induced fission of 249Cf and in the spontaneous fission of 250Cf and the following results were obtained for the two processes, respectively: 89Kr: 77.6 ± 1.5, —; [Formula: see text], [Formula: see text]; [Formula: see text];139Xe: 32.0 ± 3.5, 23.6 ± 6.0; 140Xe: 12.5 ± 2.0, 14.7 ± 7.0; 141Xe: 4.0 ± 0.3, [Formula: see text]; 142Xe: 0.9 ± 0.1, ≤ 3.61, and 143Xe: 0.22 ± 0.08, —. The results are discussed with respect to the systematies of nuclear charge distribution in fission.

Systematics of nuclear charge distribution in fission the ZP model

An extension of the conventional ZP model has been used with data for ten fission reactions and the method of least-squares for derivation of parameters that describe the nuclear charge-distributions. Data for light and heavy fission products for each reaction were treated together by consideration of the complementarity of average mass numbers of precursors (A′L+A′H=AF). This treatment gave calculated yields that are essentially equal for complementary elements (ZL+ZH=ZF). The parameters determined,\(\bar \sigma Z\), the Gaussian dispersion width parameter, a linear function of ΔZ=|ZP-A′(ZF/AF)|, ZP being the Z at maximum of the curve, and\(\overline {EOZ} \) and\(\overline {EON} \), the even-odd proton and neutron yield enhancement factors, represented most data (AH⋟130–150 and the complementary light A's) moderately well, better than did oscillating functions of\(\bar \sigma Z\) and ΔZ. The\(\bar \sigma Z\) values were found to be approximately constant (0.48 to 0.62) and, generally, to increase with the average number of neutrons emitted,\(\bar v, (\partial \bar \sigma Z/\partial \bar v \cong 0.04)\). The ΔZ values at A′=140 were in the range of 0.42 to 0.50, and the slopes, ∂ΔZ/∂A′H, ranged from 0.0 to 0.03. The\(\overline {EOZ} \) factor was found to be several times larger than the\(\overline {EON} \) factor and to decrease with increasing excitation energy and with A and/or Z of the fissioning nucleus, as has been reported previously. The adequacy of the ZP model for representation of data and for prediction of yields in the high-yield mass-number regions mentioned above are discussed, and the large uncertainties that are involved in extrapolating the model to near symmetric or to very asymmetric mass and charge divisions are pointed out.

Charge distribution in fission ? a quantum mechanical phenomenon

It is shown that the charge varianceσ z 2 observed in the low energy fission is consistent with the quantum mechanical zero point motion of an isovector giant dipole resonance. Theσ z 2 is determined at the ‘exit point’, where the neck radius ≈2.4 fm.

Delayed Neutron Yield Calculations for the Neutron-Induced Fission of Uranium-235 as a Function of the Incident Neutron Energy

The total delayed neutron yield from 235U was calculated as a function of the energy of the neutron inducing fission. The calculations (based on fission data and fission systematics) extend from thermal to 15-MeV neutron energies. The energy-dependent delayed neutron calculations are generally consistent with experimental results from thermal to 14-MeV neutron energies. Delayed neutron yields per 104 fissions of 168.7 ± 16.7, 178.2 ± 17.2, and 88.9 ± 10.0 were obtained at thermal, fission-spectrum, and 15-MeV neutron energies, respectively. The energy dependence of the odd-even effect in the fission charge distribution was found to partially account for the near constant yield observed below 5-MeV neutron energy, as well as the rapid decrease in yield observed at the second-chance fission threshold.

Statistical calculations of the charge distribution in fission

The parameters of the charge distribution of the fission fragments are calculated as functions of the mass ratio of the fragments and the excitation energy at deformations between the saddle point and scission. The scission region is parametrized in terms of connected shapes. The non-uniformity of the LDM charge density for such connected shapes is considered as a new collective coordinate in fission. Reasonable agreement with experimental data is obtained.

Isomer yield ratios of 132 mI and 134 mI

The high spin isomers 132 m I and 134 m I were produced, detected and measured in the thermal neutron induced fission of 235U. The half-lives were found to be 83.1±1.0 min and 3.50±0.02 min, respectively. A good agreement was found between the experimental ratio of yields and the predicted ratio based on the charge distribution in fission.

Nuclear charge distribution in fission: Fractional independent yield ofXe135from thermal-neutron-induced fission ofCm245andCf249

Target foils $^{245}\mathrm{Cm}$ or $^{249}\mathrm{Cf}$ prepared by electrodeposition on high-purity aluminum foils were irradiated in a Savannah River Plant production reactor operated in a high flux mode. The $^{135}\mathrm{Xe}$ activity was measured for each foil as a function of time-after-irradiation by counting the 249.6-keV $\ensuremath{\gamma}$-ray emission with a 45-${\mathrm{cm}}^{3}$ Ge(Li) detector. From these measurements, the fraction of the $A=135$ mass chain produced directly as $^{135}\mathrm{Xe}$ relative to the total production of $^{135}\mathrm{I}$ plus $^{135}\mathrm{Xe}$ was calculated. The measured fractional independent yield of $^{135}\mathrm{Xe}$ from $^{245}\mathrm{Cm}$ fission is 0.17 \ifmmode\pm\else\textpm\fi{} 0.02, and from $^{249}\mathrm{Cf}$ fission is 0.36 \ifmmode\pm\else\textpm\fi{} 0.03. These yields are compared with those of $^{135}\mathrm{Xe}$ from thermal-neutron-induced fission of $^{233}\mathrm{U}$, $^{235}\mathrm{U}$,$^{239}\mathrm{Pu}$, and $^{241}\mathrm{Pu}$.RADIOACTIVITY, FISSION $^{249}\mathrm{Cf}(n,f)$, $^{245}\mathrm{Cm}(n,f)$; measured $\ensuremath{\gamma}$ activity of fission product $^{135}\mathrm{Xe}$; deduced fractional independent fission yields. Ge(Li) detector, 2.6 keV at 1332.5 keV. No chemical separation.

Nuclear charge distribution in fission: Fractional independent yield of 140La from thermal-neutron induced fission of 249Cf

The fractional independent yield of 140La from the thermal-neutron induced fission of 249Cf has been measured by two methods. In one method, the change in activity as a function of time of the 1596·2-keV gamma ray from 140La was measured with a Ge(Li) detector in the presence of the gross fission products. In the other method, 140La was chemically separated from 140Ba at known times after fission and its gamma activity was measured. The weighted mean of the yields from the two methods was 0·035±0·001. This yield is consistent with other yields from 249Cf and 252Cf fission which show there may be less redistribution of charge during fission of 249Cf and 252Cf than during fission of 233U and 235U. The most probable charge, Z p, for A = 134, 136, 138, and 140 appears to be 0·3 to 0·4 charge units closer to unchanged charge distribution for fission of 249Cf and 252Cf than for fission of 233U and 235U.

Determination of the Charge and Mass Distribution in the Fission ofCf252

A new technique is presented for determining mass and charge distribution in fission. This method is based on obtaining the independent yields of the even-even fission products from the measured intensities of ${2}^{+}\ensuremath{\rightarrow}{0}^{+}$ ground-state band transitions deexciting the prompt-fission products. Transition intensities for members of ground-state bands in 36 even-even fission-product nuclei have been used to determine the centroids (${Z}_{p}$) and widths (${\ensuremath{\sigma}}_{Z}$) of the charge distribution for eight chains of fission products with constant mass and also to determine the centroids (${Z}_{A}$), widths (${\ensuremath{\sigma}}_{A}$), and yields (${Y}_{Z}$) of the mass distribution for 12 chains of fission products with constant charge. The method has been applied to analyze the charge and mass distributions of $^{252}\mathrm{Cf}$ spontaneous fission. The results when compared with the standard radiochemical and $K$ x-ray technique give satisfactory agreement. The discrepancies which exist are predominantly associated with regions influenced strongly by nuclear shells. Examples are given relating the observed ground-state band transition intensities with other observed fission variables such as kinetic energy release and neutron-evaporation systematics.

Nuclear Charge Distribution in Fission: Fractional Cumulative Yields ofTe132andTe134from Thermal-Neutron-Induced Fission ofCm245andCf249

Fractional cumulative yields of $^{132}\mathrm{Te}$ and $^{134}\mathrm{Te}$ have been measured from thermal-neutron-induced fission of $^{245}\mathrm{Cm}$ and $^{249}\mathrm{Cf}$. The yields, expressed as ratios of atoms of tellurium to atoms of tellurium plus iodine, are 0.94 \ifmmode\pm\else\textpm\fi{} 0.01 and 0.59 \ifmmode\pm\else\textpm\fi{} 0.07 for $^{245}\mathrm{Cm}$ fission, and 0.76 \ifmmode\pm\else\textpm\fi{} 0.02 and 0.36 \ifmmode\pm\else\textpm\fi{} 0.05 for $^{249}\mathrm{Cf}$ fission. Fractional yields based on estimated total chain yields are the same as above for $^{132}\mathrm{Te}$ but decrease to 0.56 \ifmmode\pm\else\textpm\fi{} 0.08 and 0.29 \ifmmode\pm\else\textpm\fi{} 0.06 for $^{134}\mathrm{Te}$. The yielas are not consistent with similar yields from $^{233}\mathrm{U}$ and $^{235}\mathrm{U}$ fission.

Nuclear charge distribution in fission: Fractional cumulative yields of 131Sb and 131 mTe from the spontaneous fission of 252Cf

The fractional cumulative yields of 131Sb and 131 m Te from the 131Sb fission of 252Cf have been determined from a measurement of the radioactivity of 131I found in samples of antimony and tellurium separated from other products of fission of 252Cf. The yields are 0·65±0·03 and 0·19± 0·01, respectively. A comparison of the fractional cumulative yield of 131Sb to yields of 131Te, 136Xe, and 138Xe for fission of 252Cf and 235U indicates that the effects of nuclear structure of the products on charge distribution are nearly the same for both fission processes. The results also indicate that, for this mass region, there may be less redistribution of charge during spontaneous fission of 252Cf than during thermal-neutron induced fission of 235U. An approximate value, ∼0·8, was measured for the ratio of the fractional independent yield of 131 m Te to that of 131 g Te.

Nuclear charge distribution in fission: Independent yields of 139Ba and 140Ba from thermal-neutron induced fission of 233U

Independent yields of 139Ba and 140Ba have been measured from thermal-neutron induced fission of 233U. These nuclides were quickly separated from their cesium precursors and purified radiochemically. Independent yields were calculated from a knowledge of the activities of the nuclides as a function of separation time and the previously measured xenon fractional cumulative yields. The yields, expressed as fractional independent yields, are 0·084±0·007 and 0·27·0·04 for 139Ba and 140Ba, respectively. The relation between cumulative yields of 139Cs, 140Cs, 139Xe and 140Xe is essentially the same for 233U as for 235U. An odd-even effect is noted and for the A = 139 chain this effect may be more important than the 83-neutron effect.

Calculation of Kinetic Energy and Most-Probable Charge in Fission

Kinetic energies and most-probable charges in fission were calculated for 235 U+n, 252 Cf and 226 Ra+p by applying the statistical model by Fong. The mass formula by Myers and Swiatecki was employed to evaluate fragment deformation energies. The most-probable scission configuration for a given mass division was obtained by maximizing the effective intrinsic excitation energy of the complementary fragments which was given as a function of their charges and deformation parameters. The general trends of the kinetic energy distribution and the charge distribution in fission could be reproduced fairly well.

Independent yields of 80Br, 82Br, 128I, and 130I from the thermal neutron fission of 235U, 239Pu, 233U, and 238Np

Absolute independent yields of the shielded nuclides 80Br, 82Br, 128I, and 130I produced in the thermal neutron fission of 235U, 239Pu, and 233U have been determined by careful mass spectrometric analyses of krypton and xenon fission products. In addition, a fractional independent yield of 130I from the thermal neutron fission of 238Np has been determined, and an upper limit has been set for the independent yield of 126I from 235U fission. All the yields from 235U fission except that of 128I are consistent with the Gaussian charge distribution described by Wahl. However, conventional theories of charge distribution in fission do not account for the irregular variation in iodine yields from one fissile nuclide to another. A mechanism based on differences in neutron emission probabilities from prompt fission fragments is proposed in order to explain this variation.