Fission Distribution Research Articles

On the calculation of charge values of fission fragments

The mass dependence of the average primary charge Z p and the charge distribution in low-energy fission are calculated within the approximation of a quasi-static scission configuration. The deviation of Z p from the unchanged charge density charge values z UCD is shown to be proportional to the first derivative in the charge coordinate of the potential energy whereas the width of the charge distribution is determined mainly by the second derivative. The experimental findings of a constant deviation ( Z p- z UCD) and constant width in the mass range of high yields in asymmetric fission is described satisfactorily within the model. An expression for a universal charge distribution in asymmetric fission is given.

A Unified Interpretation of Mass and Charge Distribution and Prompt Neutron Evaporation in Low-Energy Nuclear Fission

Abstract A simple model for a transition state configuration in low-energy fission is given. In its light the latest experimental data on mass and charge distribution and on prompt neutron evaporation are discussed.

Independent yield of 135I in the thermal neutron fission of 235U The half-life of 135Te

The independent yield of 135I in the thermal neutron fission of 235U (47 ± 2% of chain yield) and the half-life of 135Te (18 ± 2 sec) have been determined using a rapid chemical isolation of fission iodine via ion exchange on a performed AgCl precipitate. Chemical separation of iodine from its precursors could be performed within 2 sec after a short irradiation (0·05 sec). The result is discussed with respect to the effects of closed nuclear shells on charge distribution in fission.

Energetics and Mass Distribution of Fission Fragments in Triton- and Proton-Accompanied Fission ofCf252

The kinetic energies of the fission fragments and of the light particle emitted in triton- and proton-accompanied fission of ${\mathrm{Cf}}^{252}$ were measured in a four-parameter correlation experiment. Assuming that the average total kinetic energy of the fragments in long-range alpha fission (LRA fission) is 172.9 MeV, this value in triton-accompanied fission was found to be 173.5\ifmmode\pm\else\textpm\fi{}0.3, while in proton-accompanied fission the value of 173.6\ifmmode\pm\else\textpm\fi{}0.5 was obtained. The mass distribution in triton-accompanied fission was found to be very similar to that in LRA fission, while this distribution for proton-accompanied fission is somewhat different from that in both binary and LRA fission. In addition, the degree of correlation between the average total fission-fragment energy and the particle energy was measured.

Nuclear charge distribution in fission

A new prescription has been formulated for calculating the most probable charge Z p associated with a given heavy fragment from the thermal neutron induced fission of 235U. This follows from the observation that a plot of the independent fractional chain yields of the heavy fission fragments against their N Z (neutron to proton ratio), yields a single parabola with the peak at N Z = 1.587±0.005 . The independent fractional chain yield data from the thermal-neutron induced fission of 233U and 239Pu and the spontaneous fission of 252Cf conform well to this curve. Available experimental data on Z p in the case of 252Cf are in excellent agreement with the present calculations and are in disagreement with those calculated by the postulates of unchanged charge distribution and maximum energy release.

Measured and Calculated Absolute Power and Fission Distribution in the TSF-SNAP Reactor

Measured and Calculated Absolute Power and Fission Distribution in the TSF-SNAP Reactor

Charge Distribution in Fission: Fractional Cumulative Fission Yields ofBa143andBa144from Thermal-Neutron-Induced Fission ofU235

Rapid chemical separations of Cs and Ba from La and Ce have been used to measure half-lives and cumulative yields of $^{143}\mathrm{Ba}$ and $^{144}\mathrm{Ba}$ from thermal-neutron-induced fission of $^{235}\mathrm{U}$. The half-lives are 13.2\ifmmode\pm\else\textpm\fi{}0.3 sec for $^{143}\mathrm{Ba}$ and 11.9\ifmmode\pm\else\textpm\fi{}0.3 sec for $^{144}\mathrm{Ba}$. The yields, each expressed as a fraction of the total chain, are 0.88\ifmmode\pm\else\textpm\fi{}0.06 and 0.78\ifmmode\pm\else\textpm\fi{}0.06 for $^{143}\mathrm{Ba}$ and $^{144}\mathrm{Ba}$, respectively. Charge distribution for $A=144$ can be represented by a Gaussian distribution with width parameter $\ensuremath{\sigma}=0.52\ifmmode\pm\else\textpm\fi{}0.02$ and ${Z}_{p}=56.09\ifmmode\pm\else\textpm\fi{}0.05$. For $A=143$, $\ensuremath{\sigma}=0.58\ifmmode\pm\else\textpm\fi{}0.05$, and ${Z}_{p}=55.87\ifmmode\pm\else\textpm\fi{}0.05$, in agreement with previously reported values. Evidence is presented that charge distribution as determined from yields of several isotopes of the even-$Z$ elements Xe, Ba, Kr, and Sr can be satisfactorily represented by a single Gaussian distribution with $\ensuremath{\sigma}=0.56$. A possible even-odd effect in nuclear charge distribution from fission of $^{235}\mathrm{U}$ is noted.

Explanation of the Mass Asymmetry in Nuclear Fission

The asymmetric mass yield distribution in spontaneous fission is simply related to the intrinsic nuclear structure and to the nuclear spin-orbit coupling.Received 19 October 1967DOI:https://doi.org/10.1103/PhysRevLett.20.867©1968 American Physical Society

Emission of Long-Range Alpha Particles in the Spontaneous Fission ofCf252

An experimental investigation of $\ensuremath{\alpha}$-particle emission in the spontaneous fission of ${\mathrm{Cf}}^{252}$ is described. The measured angular distribution and energy distribution of the $\ensuremath{\alpha}$ particles are presented, as well as the mass-ratio distribution of the fission fragments and the single-fragment energy distribution in fission accompanied by long-range $\ensuremath{\alpha}$ particles (LRA fission). Also shown is the angular distribution of the $\ensuremath{\alpha}$ particles as a function of the $\ensuremath{\alpha}$-particle energy, the total fission-fragment energy, and the mass ratio. The experimental results show the LRA-fission process to be very similar to binary fission until the moment of scission. The angular distribution of the $\ensuremath{\alpha}$ particles as a function of the mass ratio (corrected for $\ensuremath{\alpha}$-particle recoil) confirms the earlier conclusion that the scission point moves towards the light fragment as the mass ratio increases. The experimental results provide evidence that the $\ensuremath{\alpha}$ particle is emitted very close to the scission point and within ${10}^{\ensuremath{-}21}$ sec of the moment of scission. The angular distribution data support the model which explains the variation of the number of neutrons emitted in binary fission as a function of fragment mass on the basis of a variation in the nuclear deformation of the fission fragments at scission.

The charge and mass distributions in fission of uranium induced by 170 MeV protons

New fission yields have been determined radiochemically for products between A = 100 and A = 140 from the reaction U(170 MeV p,f). The charge distribution curve is firmly established in the mass range 127 ≤ A ≤ 134 and is found to be Gaussian with a full width at half maximum of 2·8 charge units. Additional data indicate that the same curve is valid from about A = 100 to about A = 140, and it has been used together with other fractional chain yields to establish Z p , the most probable charge as a function of mass number. A complete mass distribution curve has been drawn, and this is neither symmetric around a most probable mass, nor has it a broad flat maximum.

Nuclear-Charge Distribution of Fission-Product Chains of Mass Numbers 131-133

New information on the nature of the nuclear charge distribution in thermal-neutron fission of $^{235}\mathrm{U}$ has been obtained from measurements of fission yields of the principal constituents in each of three adjacent mass chains. A regular variation in the width of the charge-distribution curves with mass number has been shown, and values of the most probable nuclear charge (${Z}_{p}$) have been determined. No effects attributable to the 50-proton or 82-neutron closed shells have been observed in the charge-distribution curves examined. Independent and cumulative fission yield values were determined for the principal constituents in each of the three mass chains 131, 132, and 133. Values measured are: $^{131}\mathrm{Sn}$, (1.28\ifmmode\pm\else\textpm\fi{}0.21)% cumulative; $^{131}\mathrm{Sb}$, (1.66\ifmmode\pm\else\textpm\fi{}0.40)% independent; $^{132}\mathrm{Sn}$, (0.59\ifmmode\pm\else\textpm\fi{}0.17)% cumulative; $^{132}\mathrm{Sb}$, (2.76\ifmmode\pm\else\textpm\fi{}0.35)% independent; $^{132}\mathrm{Te}$, (0.86\ifmmode\pm\else\textpm\fi{}0.10)% independent; $^{133}\mathrm{Sn}$, 0.015% cumulative; $^{133}\mathrm{Sb}$, (3.05\ifmmode\pm\else\textpm\fi{}0.39)% independent; $^{133m}\mathrm{Te}$, (2.19\ifmmode\pm\else\textpm\fi{}0.33)% independent; $^{133g}\mathrm{Te}$, (2.07\ifmmode\pm\else\textpm\fi{}0.31)% independent; and $^{133}\mathrm{I}$, (0.17\ifmmode\pm\else\textpm\fi{}0.06)% independent. The charge distribution curve for each mass chain was established from the fission yield data. The data were fitted to the Gaussian relationship, $P(Z)=A\mathrm{exp}\ensuremath{-}\frac{{(Z\ensuremath{-}{Z}_{p})}^{2}}{c}$. The widths of the curves as given by values of $c$ for mass chains 131, 132, and 133 are, respectively: 1.10\ifmmode\pm\else\textpm\fi{}0.05, 0.74\ifmmode\pm\else\textpm\fi{}0.08, and 0.57\ifmmode\pm\else\textpm\fi{}0.05. The data show a monotonic decrease in the width of the charge distribution curves with increasing fission-product mass number. Implications of this observation are discussed in respect to previous assumptions of a single charge distribution curve for all fission-product masses in low-energy fission. Values of ${Z}_{p}$ as given by the Gaussian curves for mass chains 131, 132, and 133 are, respectively: 50.72\ifmmode\pm\else\textpm\fi{}0.07, 51.07\ifmmode\pm\else\textpm\fi{}0.06, and 51.59\ifmmode\pm\else\textpm\fi{}0.04. These values are compared with published values derived from assumptions of a single charge distribution curve for all mass chains. Predicted effects of nuclear shells on charge distribution have been examined in the light of the present data. No effect was detected in mass chains 131, 132, and 133. New half-life values determined are: $^{131}\mathrm{Sn}$, 1.32\ifmmode\pm\else\textpm\fi{}0.23 min; $^{132}\mathrm{Sn}$, 1.00\ifmmode\pm\else\textpm\fi{}0.17 min; $^{132}\mathrm{Sb}$, 3.13\ifmmode\pm\else\textpm\fi{}0.33 min; and $^{133}\mathrm{Sb}$, 2.67\ifmmode\pm\else\textpm\fi{}0.33 min. The half-life of $^{133}\mathrm{Sn}$ was estimated to be 55 sec.

General Purpose Analog Pulse Height Computer

Experimental techniques in nuclear physics often call for arithmetic operations on pulses whose amplitudes convey the experimental data. An instrument capable of pulse height multiplication, division, exponentiation, as well as pulse height addition and subtraction is described. These operations are performed in about 0.5 μsec with a precision of ±0.1% of full scale over a pulse height range of 20:1. At room temperature the computer exhibits a drift of about 0.05%/C° or ±0.3%/day. With an input rate of 10 000 counts/sec the pulse height distribution shifts 0.2%, and with 25 000 counts/sec it shifts 1%. Multiplication, division, and exponentiation are performed by using logarithms and antilogarithms. The log and antilog function generators are based on the fact that the emitter-base voltage of a silicon planar transistor is proportional to the logarithm of its collector current. Addition and subtraction are performed by using linear operational amplifiers. Results of experiments are shown in which the computer was used for identification of charged particles, pulse shape discrimination, measurement of mass yield distributions in fission, as well as for other on-line data reduction techniques.

Effect of Increasing Excitation Energy on Nuclear Charge Distribution in Fission

The independent fractional chain yields of ${\mathrm{Y}}^{90}$, ${\mathrm{Nb}}^{96}$, and ${\mathrm{La}}^{140}$ have been measured from fission of ${\mathrm{Th}}^{232}$ with alpha particles. The excitation energy was varied from about 20 to 40 MeV. Yields are also calculated for ${\mathrm{Cs}}^{136}$ from data previously published. In addition, the independent fractional chain yields of ${\mathrm{Y}}^{92}$, ${\mathrm{Nb}}^{96}$, and ${\mathrm{La}}^{140}$ were measured from fission of ${\mathrm{U}}^{235}$ with 14.9-MeV neutrons. From these data, smoothly varying plots of independent yield versus excitation energy were constructed for ${\mathrm{Y}}^{90}$, ${\mathrm{Nb}}^{96}$, ${\mathrm{Cs}}^{136}$, and ${\mathrm{La}}^{140}$. The data from 14.9-MeV neutron fission of ${\mathrm{U}}^{235}$ are in good agreement with those from alpha-particle fission of ${\mathrm{Th}}^{232}$ at the same excitation energy. It is shown that, within the limits of certain assumptions regarding the charge dispersion curves, the shift in ${Z}_{p}$ (the most probable charge for a given mass number) with increasing excitation energy varies among the nuclides studied. In particular, it appears that $\frac{d{Z}_{p}}{\mathrm{dE}}$ may be significantly greater for heavy fragments than for light fragments. The data for ${\mathrm{Y}}^{90}$, ${\mathrm{Nb}}^{96}$, and ${\mathrm{La}}^{140}$ are consistent with linear plots of ${Z}_{p}$ versus ${E}^{*}$, except that the value for ${\mathrm{Nb}}^{96}$ for thermal-neutron fission of ${\mathrm{U}}^{235}$ is off the curve by 0.1 charge unit. The value of $\frac{d{Z}_{p}}{\mathrm{dE}}$ for ${\mathrm{Cs}}^{136}$ decreases with energy in this energy range, possibly because of shell effects at low excitation energies.

Nuclear fission as a Markov process

Starting from the assumption of a random transfer of nucleons between the two sides of a fissioning nucleus, during the time from saddle point to scission, a new theory of fission is developed to show that the mass distribution in low, intermediate and high energy fission can be explained on arguments based on the ground state properties of nuclei. The theory is extended to show that the shape of the deformation energy-mass curves follows as a direct consequence of the equilibrium conditions which determine the mass distributions and the gap in the “zig-zag” curves is essentially due to the proton transfers. The time of fission is shown to be of the order of 500 nucleonic times in spontaneous and low energy fission and results from the properties of the transition matrix. The theory is also able to explain the small range of the threshold energies of fission of heavy nuclei and the formation of a symmetry axis early in the process.

Mass distribution in fission and the theory of random flights

Mass distribution in fission and the theory of random flights

Fission Distribution in Seven-Pin Cluster Fuel Elements

The distribution of fissions within one fuel pin of a cluster is asymmetrical because of self-shielding and neutron-streaming phenomena. An implicit solution of the integral form of the Boltzmann equation indicates that, for a given neutron spectrum, this distribution is primarily a function of three dimensionless parameters; pin radius/ neutron mean free path in pin material; pin circle radius/ pin radius; and pin radius/fuel element radius. The actual distribution was determined experimentally by detector foils and autoradiographic techniques for various seven-pin, cluster-type, gas-cooled fuel elements. The experimental fuel pins were fabricated by winding alternate 0.001 inch thick layers of pure aluminum and enriched uranium (93%U/sup 235/) on a solid core until the desired pin diameter was reached. Seven of these pins, assembled into a fuel ele, ment, were irradiated in the thermal column of a research reactor. The layers of uranium and the uranium detector foils (which were exposed concurrently) were subsequently autoradiographed and the resulting x-ray fiim optical density measured on a microdensitometer. The detector foils were also counted in a gamma detector, thus providing a key between relative radioactivity and optical density. It was found that the fission distribution within the center pin of the cluster was symmetricalmore » and could be represented by D = D/sub 1/ exp STAC/sub 1/ (x/sup 2/ + y/sup 2/)!. The fission distrib ution in the outer pins of the cluster was asymmetrical with respect to the pin center but couid be represented by D == D/sub 2/ exp STAC/sub 2/ (x/sup 2/g/sup 2/ + (y -- y/sub o/)/ sup 2/!. Values of th e constants in the equations are correlated by the first two dimensionless parameters given but appear to be independent of the third. (auth)« less

Kinetic Energy and Prompt Neutron Distributions in Fission

The previously established facts that the kinetic energy of a fission pair as a function of the mass ratio of the two fragments shows a drastic dip in the symmetric fission region and the saw-tooth structure of the average number of prompt neutrons per fragment as a function of the fragment mass are considered. It is shown that statistical theory can explain these phenomena provided that all effects of the nuclear are taken into account. These effects include the effect on nuclear masses and deformability. Results are shown for the neutron fission of U/sup 235/. (C.E.S.)

Interaction of Fission Fragments with Organic Scintillators

A pulse-shape discriminator circuit was used for the measurement of the pulse heights of the slow components of light pulses from organic scintillators excited by different types of particles: electrons, protons, alpha particles, and fission fragments. Three types of organic scintillators were employed: a crystal (stilbene), a liquid (7 g PPO and 0.5 g M2-POPOP in 1 liter of toluene), and a plastic (NE-150). The relationship of the pulse heights of the slow components for excitation with electrons, protons, and alpha particles was similar for all three scintillators. However, fission fragments produced a slow component pulse height similar to protons in stilbene and the plastic but more like electrons in the liquid. The pulse-height distributions of total light pulses from the scintillators excited by the alpha and spontaneous fission emissions of Cf252 were measured. The alpha-particle and fission-event spectra were resolved only with the liquid scintillator giving a pulse-height spread of the fission distribution peak of 40%. From the maximum of the fission sepctra it was calculated that the most probable energy of fission produced the same pulse height as an electron with 1/75th that energy would produce. The alpha particles and the fission events were counted with 100% efficiency and a value of 31.0±0.5 was obtained for the alpha to fission disintegration ratio of Cf252.

Nuclear-Charge Distribution in Low-Energy Fission

New information concerning half-lives and fractional independent and fractional cumulative yields of fission products has been obtained from experiments in which adjacent elements in fission-product chains were rapidly separated.Fractional cumulative yields that have been determined for the thermal-neutron fission of ${\mathrm{U}}^{235}$ are: ${\mathrm{Kr}}^{93}$, ${0.075}_{\ensuremath{-}0.002}^{+0.010}$; ${\mathrm{Kr}}^{94}$, ${0.015}_{\ensuremath{-}0.002}^{+0.005}$; ${\mathrm{Kr}}^{95}$, (${1.1}_{0.1}^{+0.3}$)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$; ${\mathrm{Kr}}^{97}$, ${10}^{\ensuremath{-}5}$; ${\mathrm{Xe}}^{137}$, 0.978\ifmmode\pm\else\textpm\fi{}0.003; ${\mathrm{Xe}}^{138}$, 0.956\ifmmode\pm\else\textpm\fi{}0.003. Fractional cumulative yields determined for the spontaneous fission of ${\mathrm{Cf}}^{252}$ are: ${\mathrm{Xe}}^{139}$, 0.67\ifmmode\pm\else\textpm\fi{}0.01; ${\mathrm{Xe}}^{140}$, 0.45\ifmmode\pm\else\textpm\fi{}0.01; ${\mathrm{Xe}}^{141}$, 0.172\ifmmode\pm\else\textpm\fi{}0.005; ${\mathrm{Xe}}^{144}$, 0.007.Fractional independent yields determined for the thermal-neutron fission of ${\mathrm{U}}^{235}$ are: ${\mathrm{Sr}}^{91}$, 0.07\ifmmode\pm\else\textpm\fi{}0.05; ${\mathrm{Nb}}^{95m}$, 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$; ${\mathrm{Nb}}^{96}$, (1.0\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$; ${\mathrm{Nb}}^{97}$, (1.7\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$; ${\mathrm{Cs}}^{136}$, (1.10\ifmmode\pm\else\textpm\fi{}0.15) \ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$; ${\mathrm{Ba}}^{139}$, ${0.012}_{\ensuremath{-}0.003}^{+0.005}$; ${\mathrm{Ba}}^{140}$, 0.066\ifmmode\pm\else\textpm\fi{}0.026; ${\mathrm{Ba}}^{141}$, 0.27 \ifmmode\pm\else\textpm\fi{}0.08; ${\mathrm{La}}^{141}$, 0.004\ifmmode\pm\else\textpm\fi{}0.002; ${\mathrm{La}}^{142}$, 0.019\ifmmode\pm\else\textpm\fi{}0.005; ${\mathrm{Ce}}^{143}$, (4.4\ifmmode\pm\else\textpm\fi{}3.0) \ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$. Fractional independent yields determined for the thermal-neutron fission of ${\mathrm{U}}^{233}$ are: ${\mathrm{Nb}}^{95m}$, 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$; ${\mathrm{Nb}}^{96}$, (1.3\ifmmode\pm\else\textpm\fi{}0.2) \ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$; ${\mathrm{Nb}}^{97}$, 0.011\ifmmode\pm\else\textpm\fi{}0.004. Fractional independent yields determined for the thermal-neutron fission of ${\mathrm{Pu}}^{239}$ are: ${\mathrm{Nb}}^{95m}$, 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$; ${\mathrm{Nb}}^{96}$, (7.7\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$; ${\mathrm{Nb}}^{97}$, 0.015\ifmmode\pm\else\textpm\fi{}0.004. The fractional independent yield of ${\mathrm{Cs}}^{136}$ from spontaneous fission of ${\mathrm{Cf}}^{252}$ is 0.01.The above data together with other published data give direct information about the distribution of nuclear charge among fission products with mass numbers 91, 139, 140, 141, 142, and 143 from thermal-neutron fission of ${\mathrm{U}}^{235}$. The variation of fractional yield with $Z$ for constant $A$ can be represented in cumulative form by the area under a Gaussian curve from $\ensuremath{-}\ensuremath{\infty}$ to $Z+\frac{1}{2}$. The standard deviation of the curve which best represents the data for the six mass numbers is $\ensuremath{\sigma}=0.62\ifmmode\pm\else\textpm\fi{}0.06$.The variation with $A$ of ${Z}_{P}$, the value of $Z$ at the maximum in a charge distribution curve, is discussed, and a new empirical ${Z}_{P}$ function is derived on the assumption that the Gaussian curve is applicable to all mass numbers. The function and curve correlate quite well the available fractional yield data for low-energy fission processes.There are indications from the analysis of charge dispersion, and from some other observed fission phenomena, that primary low-energy fission products with $Z$ less than 50 and greater than the complementary charge (>42 for $_{92}\mathrm{U}$ fission) may be formed in very low yield. It is suggested that observed fission products with atomic numbers in this range may be formed predominantly by beta decay processes starting from products complementary to $_{50}\mathrm{Sn}$ ($_{42}\mathrm{Mo}$ for $_{92}\mathrm{U}$ fission) or from products of lower $Z$.During the course of this work, fission yields of 51-min ${\mathrm{Nb}}^{98}$ from thermal-neutron fission of ${\mathrm{U}}^{235}$, ${\mathrm{U}}^{233}$, and ${\mathrm{Pu}}^{239}$ were determined to be (0.064\ifmmode\pm\else\textpm\fi{}0.012)%, (0.20\ifmmode\pm\else\textpm\fi{}0.03)%, and (0.20\ifmmode\pm\else\textpm\fi{}0.03)%, respectively.New half-life values determined are: ${\mathrm{Rb}}^{91}$, 72\ifmmode\pm\else\textpm\fi{}8 sec; ${\mathrm{Rb}}^{92}$, 11 sec; ${\mathrm{Nb}}^{98}$, 51\ifmmode\pm\else\textpm\fi{}3 min; ${\mathrm{Cs}}^{141}$, 25\ifmmode\pm\else\textpm\fi{}3 sec; ${\mathrm{Cs}}^{142}$, 8 sec; ${\mathrm{Ba}}^{143}$, 12\ifmmode\pm\else\textpm\fi{}2 sec. ${\mathrm{Kr}}^{97}$ and a reported 14-min isomer of ${\mathrm{Rb}}^{91}$ were not found among the fission products; we believe that our data refute the published evidence for their existence.

The mass distribution in fission of lead induced by 54 MeV protons

The mass distribution in fission of natural lead induced by 54 MeV protons has been studied. The yield mass curve seems nearly symmetric in character around mass number 102. It has a width of about 28 mass units at half the maximum yield. The fission cross section is 13±4 mbarns, i.e. the same as for bismuth under comparable conditions.