arXiv subjects

J. P. Lestone

Publications and source records attributed to J. P. Lestone.

12 recordsLinked to original sources

arXiv2023

The earliest DT nuclear fusion discoveries

We describe the earliest measurements of the DT fusion cross section commissioned by the Manhattan Project, first at Purdue University in 1943 and then at Los Alamos 1945-6 and later, in 1951-2. The Los Alamos measurements led to the realization that a 3/2$^+$ resonance in the DT system enhances the fusion cross section by a factor of one hundred at energies relevant to applications. This was a transformational discovery, making the quest for terrestrial fusion energy possible. The earliest measurements were reasonably accurate given the technology of the time and the scarcity of tritium, and were quickly improved to provide cross section data accurate to just a few percent. We provide a previously-unappreciated insight: that DT fusion was first reported in Ruhlig's 1938 University of Michigan experiment and likely influenced Konopinski in 1942 to suggest its usefulness for thermonuclear technologies. We report on preliminary work to repeat the 1938 measurement, and our simulations of that experiment. We also present some work by Fermi, from his 1945 Los Alamos lectures, showing that he used the S-factor concept about a decade before it was introduced by nuclear astrophysicists.

physics.hist-ph↗

arXiv2021

Comparison between historic nuclear explosion yield formulas

During the Manhattan project a simple formula was developed by Bethe and Feynman in 1943 to estimate the yield of a fission-only nuclear explosion of a uniformly-dense bare-sphere of supercritical fissile material. We have not found any evidence that Bethe and Feynman knew of the first yield formula obtained by Frisch and Peierls contained within their famous March 1940 memorandum. Similarly, we have not found any technical documents that compare the Bethe-Feynman formula to the earlier works of Frisch and Peierls, or Serber. After adjusting for differences in the labeling of critical radii, we find that earlier formulas only differ by a scaling factor.

physics.hist-ph↗

arXiv2016

Late Time Emission of Prompt Fission Gamma Rays

The emission of prompt fission $γ$ rays within a few nanoseconds to a few microseconds following the scission point is studied in the Hauser-Feshbach formalism applied to the deexcitation of primary excited fission fragments. Neutron and $γ$-ray evaporations from fully accelerated fission fragments are calculated in competition at each stage of the decay, and the role of isomers in the fission products, before $β$-decay, is analyzed. The time evolution of the average total $γ$-ray energy, average total $γ$-ray multiplicity, and fragment-specific $γ$-ray spectra, is presented in the case of neutron-induced fission reactions of $^{235}$U and $^{239}$Pu, as well as spontaneous fission of $^{252}$Cf. The production of specific isomeric states is calculated and compared to available experimental data. About 7% of all prompt fission $γ$ rays are predicted to be emitted between 10 nsec and 5 $μ$sec following fission, in the case of $^{235}$U and $^{239}$Pu $(n_{\rm th},f)$ reactions, and up to 3% in the case of $^{252}$Cf spontaneous fission. The cumulative average total $γ$-ray energy increases by 2 to 5% in the same time interval. Finally, those results are shown to be robust against significant changes in the model input parameters.

nucl-th↗

arXiv2014

Watt parameters for the Los Alamos Model : Subroutine getab

Many neutron transport Monte-Carlo codes can randomly sample fission neutron energies from a Watt spectrum. The quality of simulations depends on how well the Watt spectrum represents the true energy spectrum of the fission neutrons, and on one's choice of the Watt parameters a and b. The energy spectra of fission neutrons have been calculated and tabulated for the neutron induced fission of 235,238U and 239Pu as a function of incoming neutron energy by Madland using the Los Alamos Model. Each of these energy spectra are mapped into time-of-flight space and fitted with a Watt spectrum. A subroutine getab has been written to interpolate these results, so that Watt a and b parameters can be estimated for all incoming neutron energies up to ~16 MeV.

physics.ins-det↗

arXiv2014

A comparison between two evaluations of neutron multiplicity distributions

Within MCNP6, users can use multiplicity distributions for spontaneous and neutron-induced fission from an evaluation performed by Lestone (LA-UR-05-0288). This evaluation assumes the multiplicity distributions are Gaussian and adjusts the standard deviations to reproduce measured 2nd and 3rd factorial moments. In MCNPX, users have the option of using multiplicity distributions for spontaneous fission using an evaluation of the available experimental data by Santi and Miller (LA-UR-07-6229). The spontaneous fission multiplicity distributions and corresponding factorial moments from these two evaluations are compared in the present paper. The differences are minor or within experimental errors for all but the spontaneous fission of 238Pu. The 238Pu evaluations are based on data obtained in 1956. We recommend that the 238Pu multiplicity distribution be re-measured. Given the good agreement between the two evaluations, the choice of evaluation will make little difference for the modeling of spontaneous fission. However, we suggest that the evaluation in LA-UR-05-0288 be adopted as the MCNP default because it also includes recommended distributions for neutron induced fission.

nucl-th↗

arXiv2014

Subroutines to Simulate Fission Neutrons for Monte Carlo Transport Codes

Fortran subroutines have been written to simulate the production of fission neutrons from the spontaneous fission of 252Cf and 240Pu, and from the thermal neutron induced fission of 239Pu and 235U. The names of these four subroutines are getnv252, getnv240, getnv239, and getnv235, respectively. These subroutines reproduce measured first, second, and third moments of the neutron multiplicity distributions, measured neutron-fission correlation data for the spontaneous fission of 252Cf, and measured neutron-neutron correlation data for both the spontaneous fission of 252Cf and the thermal neutron induced fission of 235U. The codes presented here can be used to study the possible uses of neutron-neutron correlations in the area of transparency measurements and the uses of neutron-neutron correlations in coincidence neutron imaging.

nucl-th↗

arXiv2014

Energy and Isotope Dependence of Neutron Multiplicity Distributions

Fission neutron multiplicity distributions are known to be well reproduced by simple Gaussian distributions. Many previous evaluations of multiplicity distributions have adjusted the widths of Gaussian distributions to best fit the measured multiplicity distributions Pn. However, many observables do not depend on the detailed shape of Pn, but depend on the first three factorial moments of the distributions. In the present evaluation, the widths of Gaussians are adjusted to fit the measured 2nd and 3rd factorial moments. The relationships between the first three factorial moments are estimated assuming that the widths of the multiplicity distributions are independent of the initial excitation energy of the fissioning system. These simple calculations are in good agreement with experimental neutron induced fission data up to an incoming neutron energy of 10 MeV.

nucl-th↗

arXiv2009

Physics based calculation of the fine structure constant

We assume that the coupling between particles and photons is defined by a surface area and a temperature, and that the square of the temperature is the inverse of the surface area (h_bar=c=1). By making assumptions regarding stimulated emission and effects associated with the finite length of a string that forms the particle surface, the fine structure constant is calculated to be ~1/137.04. The corresponding calculated fundamental unit of charge is 1.6021E-19 C.

physics.gen-ph↗

arXiv2008

Statistical Model of Heavy-Ion Fusion-Fission Reactions

Cross-section and neutron-emission data from heavy-ion fusion-fission reactions are consistent with the fission of fully equilibrated systems with fission lifetime estimates obtained via a Kramers-modified statistical model which takes into account the collective motion of the system about the ground state, the temperature dependence of the location and height of fission transition points, and the orientation degree of freedom. If the standard techniques for calculating fission lifetimes are used, then the calculated excitation-energy dependence of fission lifetimes is incorrect. We see no evidence to suggest that the nuclear viscosity has a temperature dependence. The strong increase in the nuclear viscosity above a temperature of approximately 1.3 MeV deduced by others is an artifact generated by an inadequate fission model.

nucl-th↗

arXiv2008

Black-body photon clustering by semi-classical means

If stimulated emission could be turned off then only uncorrelated photons would be emitted from black bodies and the photon counting statistics would be Poissonian. Through the process of stimulated emission, some fraction of the photons emitted from a black body are correlated and thus emitted in clusters. This photon clustering can be calculated by semi-classical means. The corresponding results are in agreement with quantum theory.

physics.gen-ph↗

arXiv2008

Fission Decay Widths for Heavy-Ion Fusion-Fission Reactions

Cross-section and neutron-emission data from heavy-ion fusion-fission reactions are consistent with a Kramers-modified statistical model which takes into account the collective motion of the system about the ground state; the temperature dependence of the location of fission transition points; and the orientation degree of freedom. We see no evidence to suggest that the nuclear viscosity departs from the surface-plus-window dissipation model. The strong increase in the nuclear viscosity above a temperature of ~1 MeV deduced by others is an artifact generated by an inadequate fission model.

nucl-th↗

arXiv2007

An evaporation-based model of thermal neutron induced ternary fission of plutonium

Ternary fission probabilities for thermal neutron induced fission of plutonium are analyzed within the framework of an evaporation-based model where the complexity of time-varying potentials, associated with the neck collapse, are included in a simplistic fashion. If the nuclear temperature at scission and the fission-neck-collapse time are assumed to be ~1.2 MeV and ~10^-22 s, respectively, then calculated relative probabilities of ternary-fission light-charged-particle emission follow the trends seen in the experimental data. The ability of this model to reproduce ternary fission probabilities spanning seven orders of magnitude for a wide range of light-particle charges and masses implies that ternary fission is caused by the coupling of an evaporation-like process with the rapid re-arrangement of the nuclear fluid following scission.

nucl-th↗