Mass-yield Curve Research Articles

Fission mass asymmetry as a dynamic process described by a collective coordinate

A collective coordinate for describing reflection-asymmetric shapes of fissioning nuclei is introduced. The potential energy and mass parameter determining motion in this degree of freedom are calculated in a two-centre shell model. In quantized treatment the probability for a given mass asymmetry is determined and compared to the experimental mass yield curve of 236U.

Rare earth yields from bombardment of tungsten targets with 24 GeV protons

Cumulative and independent yields of lanthanides have been measured in tungsten targets following 24 GeV proton irradiation. A chain yield curve in this mass region was obtained by reference to a charge dispersion curve which was constructed for A = 148 by an extrapolation technique. The FWHM of this curve was about 1·8 charge units and Z p ∼ 64·6. The mass yield curves for different proton energies have been compared and the implications for possible reaction mechanisms considered.

Charge distribution in neutron-induced fission of thorium

Independent-fission yields of 117In, 130I, 132I, 133I and 134I in 14·8 MeV neutron-induced fission of 232Th were determined to be (1·6 ± 0·4) × 10 −2, (2·2 ± 0·5) × 10 −2, (1·1 ± 0·2) × 10 −1, (4·4 ± 1·0) × 10 −1 and (6·4 ± 1·5) × 10 −1 per cent, respectively. The experimental results were analyzed by applying various charge postulates. The charge distribution, which is different for asymmetric and symmetric regions of the mass-yield curve, were interpreted previously in terms of two separate modes of fission. The results of this investigation and the previous measurements of the independent yields both in asymmetric and symmetric regions of the mass-yield curve showed strongly in favor of the Two-Mode-Fission-Hypothesis.

Kinetic Energy Distributions in the Spontaneous Fission ofCm250andCf250

The kinetic energy distributions of coincident fragments from the spontaneous fission of $^{250}\mathrm{Cm}$ and $^{250}\mathrm{Cf}$ have been measured with silicon surface-barrier detectors. From these data, pre-neutron-emission kinetic energy and mass distributions have been calculated. The most probable value of the pre-neutron-emission total kinetic energy is 179.8 MeV for $^{250}\mathrm{Cm}$, and 184.3 MeV for $^{250}\mathrm{Cf}$. The resultant mass-yield curves are typical of asymmetric fission. The results are compared with measurements for other curium and californium isotopes.

Mass yields from thermal-neutron induced fission of 245CM

The fission yields of 21 mass chains were measured for the thermal-neutron induced fission of 245Cm. Fission product gamma activities from 245Cm were determined relative to those from 235U with a high resolution lithium-drifted germanium detector. The measured errors ranged from 1 to 10% over a mass number range of 85–147. No significant fine structure in the mass yield curve was apparent.

Medium energy intranuclear cascade calculations: a comparative study

Comparisons are reported among the Monte Carlo simulations of proton-induced intranuclear cascades as carried out by three different sets of investigators. The three sets of results for interaction of 150 and 300 MeV protons with 27A1 and 181Ta are in excellent agreement for observable quantities such as energy spectra of emitted protons and mass-yield curves for spallation reactions.

Mass yield distribution and charge dispersion in 450 MeV proton fission of 238U

The independent and/or cumulative cross-sections of 26 isotopes of silver, cadmium, indium, tin, and antimony from the fission of 238U with 450 MeV protons have been measured. The isotopic yield distribution for these elements is presented along with the charge dispersion for six mass chains, 109, 115, 117, 125, 127 and 129. The values of the most probable charge, Zp, for 14 mass chains were calculated and the variation of A Zp with A in the mass region 100–130 was determined. For A values between 100 and 120, the value of A Zp is relatively constant at a value of 2·424±0·004 if the charge dispersion width is assumed to be constant at a value of 2·90±0·10 Z units. In the mass region, 125–129, a width of 3·21 Z units and a slightly higher value of A Zp , 2·430±0·004, are required to account for the data. A mass yield curve encompassing this region of A was also determined.

Interaction of 28-GeV Protons with Uranium: Charge and Mass Distribution of Rare-Earth Products

Independent and cumulative yields of about 50 rare-earth nuclides produced in the interaction of 28-GeV protons with uranium have been measured. Chain yields on the neutron-excess side of $\ensuremath{\beta}$ stability were found to have the same mass dependence as in low-energy fission. Based on the results obtained in the present study and with the aid of the published Cs isotopic distribution, charge-dispersion curves at $A\ensuremath{\approx}147$ and $A\ensuremath{\approx}170$ were constructed. The curve at $A\ensuremath{\approx}147$ has two maxima, one on the neutron-deficient and one on the neutron-excess side of $\ensuremath{\beta}$ stability, separated by a deep valley; the curve at $A\ensuremath{\approx}170$ has a single peak on the neutron-deficient side of $\ensuremath{\beta}$ stability. Corrections were made for feeding and depletion of various mass chains by $\ensuremath{\alpha}$ decay. Total isobaric yields were determined for 11 mass numbers between $A=143$ and $A=171$. On the basis of these results and published data at lower masses, a mass-yield curve was constructed. The apparent constancy of the neutron-deficient yields between mass 143 and mass 171 is consistent with the yield pattern of a spallation-like process. Over the mass range $50<A<160$, binary fission and spallation cannot account for all the observed yields. Other processes must be partially responsible for the production of these nuclides, especially at the lower masses.

Mass distribution of 3MeV neutron induced fission of 232Th

The 3 MeV neutron induced fission yields of 232Th in the mass regions of A = 77–93 and A = 141–156 were radiochemically determined by isolating fission products belonging to 15 mass chains. The absolute activities for the nuclides were determined by β- and γ-counting and the cumulative fission yields were calculated relative to 99Mo. These data were used to define the extreme wings of the mass yield curve for 232Th. By comparison of complementary fission fragments it was determined that 2·4 secondary neutrons are emitted per fission of 232Th at 3 MeV. A qualitative comparison is made concerning the general features of the 232Th mass yield curves from 14·8 MeV and 3 MeV neutron induced fission.

Mass distribution and the total fission cross section in the fission of 238U with protons of energies ranging between 13 and 55 MeV

The fission of 238U with protons of energy ranging from 13 to 55 MeV was studied radio-chemically. The mass-yield curve was obtained for each of the eleven incident energies, and an analysis was performed, based on the two-mode-fission mechanism. The observed total chain yields were reasonably reproduced by the combination of the three Gaussian curves with partial distortions. The deduced number of neutrons emitted in the fission was found to agree well with the postulated value in the study of the charge distribution systematics. For the three highest incident energies, the formation of the fissile nuclei through a direct process was shown to occur and the analyzing method was discussed. The peak-to-trough ratio and the total fission cross section were obtained from thus-established mass distribution curves.

Mass yields of 241Am thermal-neutron fission

Mass yields of sub-threshold fission induced by thermal neutrons were investigated, namely, mass yields of 23 mass chains were determined for 241Am thermal-neutron fission by the conventional radiochemical method and by the direct γ-ray spectrometry method. The peak-to-trough ratio in terms of 99Mo/ 117Cd was found to be as large as 800±200. Three fission systems of spontaneous fission, sub-threshold fission and above-threshold fission with the compound mass A F = 242 were compared and discussed. The average number of neutrons released per fission was evaluated to be 4·0±0·2 from the mass yield curve.

Fission ofTh232Induced by Intermediate-EnergyHe4Ions

The cross sections for 34 nuclides covering the mass range 24 to 199 formed from the ${\mathrm{He}}^{4}$ ion-induced fission of ${\mathrm{Th}}^{232}$ at 20-41 MeV have been determined radiochemically. The resulting mass-yield curve clearly indicates contributions from at least three different types of fission: the symmetric binary, asymmetric binary, and asymmetric ternary modes. The identification of very light fragments and the low limits set on ${\mathrm{Au}}^{199}$ confirm the existence of ternary fission seen previously in this laboratory in plutonium compound nuclei. Certain similarities and differences in this fissioning system to other heavier elements are discussed.

Delayed neutrons from the τ- and α-induced fission of 238U, 232Th and 209Bi

The production of delayed neutrons from fission of 238U, 232Th and 209Bi with 20–34 MeV τ- and α-particles has been studied. For 238U and 232Th, delayed-neutron decay curves were analysed into 7 components, three of which correspond to the particular precursors 87Br, 88Br and 137I. The cross sections for delayed-neutron production increase up to about 30 MeV due to the increase in fission cross section, but then decrease with 34 MeV τ-particles. The number of delayed neutrons per fission as a function of excitation energy shows structure attributed to enhanced chain yields for certain light masses. For τ-fission of 209Bi, four components were sufficient to fit the observed delayed-neutron decay curves. Half-lives of 55, 16.2±1.5, 3.2±0.7 and 1.4±0.8 s were obtained corresponding to the half-lives of known Br precursors. Heavy-mass precursors in the iodine mass region are noticeably absent as expected from the symmetric mass-yield curve for Bi.

Mass-Yield Curve of First-Chance Fission from Proton- and Deuteron-Induced Fission ofRa226

Information has been obtained on the mass distribution and kinetic energy release for fission of $^{226}\mathrm{Ra}$ induced by protons and deuterons at moderate bombarding energies (20 MeV). The mass distribution, which is triple peaked, has previously been interpreted in terms of two separate fission modes: a single symmetric peak, characteristic of symmetric fission, associated with higher excitation energies; and a double-peaked distribution, characteristic of asymmetric fission, associated with lower excitation energies. It is shown that by a proper choice of proton and deuteron bombarding energies, and by a proper weighting of their respective mass-yield curves, the mass distribution for the fission of a single species, $^{228}\mathrm{Ac}$, can be determined for a single excitation energy. At excitation energies near 25 MeV this mass distribution for $^{228}\mathrm{Ac}$ is shown to be a single symmetric peak, in agreement with the hypothesis of two fission components. It is shown further that the measured kinetic energy distributions can also be interpreted in terms of the two-component hypothesis.

Calculations of beta energy release rates following thermal neutron induced fission of 233U, 235U, 239Pu and 241Pu

Results are presented of calculations of the beta energy release rate as a function of time following a 104 sec irradiation by thermal neutrons of the isotopes 233U, 235U, 39Pu and 241Pu. Reasons for the differences in the release rates are discussed and it is concluded, on the basis of the mass yield curves, that the results would be very similar if fission were induced by fast rather than by thermal neutrons. The calculated results for 239Pu are found to agree well with the single set of measurements for that isotope.

Charged particle fission of 238U

The mass yield curves for fission of 238U by 11·2 MeV protons and 10·6 MeV deuterons have been determined using Ge(Li) spectrometry. Gross fission spectra as well as spectra of samples after preliminary radiochemical separation were used. A few spallation yielss were also measured. The results are in good agreement with previous data and confirm the value of Ge(Li) spectrometry in fission studies of unseparated samples.

Distribution of low-mass fission yields in the 14·8 MeV neutron induced fission of 238U and 232Th

The fission yields of 238U and 232Th in the mass region A=73−81 were radiochemically determined for 5 mass chains. The absolute activities for these nuclides were determined by gross beta counting and the cumulative fission yields were measured relative to 99Mo. These data were utilized to define the slope of the left wing in the light peak of the mass yield curves for 238U and 232Th. A comparison of these fission yields was made with corresponding mirror points, assuming 3·5 and 4·5 secondary neutrons emitted per fission of 238U and 232Th respectively.

Cumulative yields in the photofission of 232Th and 238U in the symmetric region

Fission mass yields for molybdenum-99, ruthenium-103 and -105, palladium-109 and -112, silver-111, cadmium-115, and tin-121 and -125 in the 25 MeV bremsstrahlung beam fission of thorium-232 and uranium-238 have been determined using radiochemical methods. The mass–yield curve for thorium-232 shows a pronounced third peak in the symmetric region, contrary to the results of Hiller and Martin with 69 MeV brehmsstrahlung. In the case of uranium-238, the curve is flat between mass numbers 111 to 121 in contrast to the 14 MeV neutron fission of the nuclide which shows a valley in this region.

Mass distribution in the fission of 233U by 14·8 MeV neutrons

Abstract Radiochemical measurements have been made of the fission yields of eight mass chains, mostly in the symmetric region, from the fission of 232Th induced by 14·8-MeV neutrons. The mass distribution in the symmetric valley region indicate the presence of a third peak in the mass-yield curve.

Relative yields of stable tin isotopes in neutron-induced fission

A tandem mass spectrometer of high sensitivity has been used to measure the relative yields of the six stable isotopes of tin (117Sn, 118Sn, 119Sn, 120Sn, 122Sn, and 124Sn) and the long-lived isotope 126Sn, produced in the thermal- and fast-neutron fission of 233U and in a reactor fuel rod of natural uranium. Samples of the order of 10−9 g of tin were extracted and analyzed to give a range of yields in the region of symmetric fission. The results show that the mass yield curves in the region of symmetric fission covered by the stable tin isotopes are almost identical for the three irradiations. Furthermore, a smooth curve can be drawn through the yield points for the seven isotopes of tin and there is no evidence of "fine structure" or sharp depression in the curves in this region as has been predicted.