Los Alamos Model Research Articles

SPECTRUM – A computer code for prompt fission neutron spectrum and prompt neutron multiplicity calculation

The computer code SPECTRUM generates total prompt fission neutron spectrum, total prompt multiplicity, average energy of the spectrum and equivalent Maxwellian spectrum. Theoretical background employed in the calculations is that of Los Alamos model, with multiple chance fission, upgraded to allow a neutron incident energy dependence of the input parameters. The code handles the complete set of output data with a powerful graphical interface, allowing a comparison with reference data.

Prompt fission neutron energy spectra induced by fast neutrons

Prompt fission neutron energy spectra for 235U and 239Pu have been measured for fission neutron energies greater than the energy of the incident neutrons inducing fission. The measurements were undertaken to investigate the shape dependence of the fission neutron spectra upon both the incident neutron energy and the mass of the nucleus undergoing fission. Measurements were made for both nuclides at incident neutron energies of 0.50, 1.50, 2.50 and 3.50 MeV. The data are presented either as relative yields or as ratios of measured spectra to that of 235U at 0.50 MeV. Incident neutrons were produced by the 7Li(p,n)7Be reaction using a pulsed, bunched proton beam from the 5.5 MV Vand de Graaff accelerator at the University of Massachusetts Lowell Radiation Laboratory. Fission neutrons were detected by a thin liquid scintillator with good time resolution capabilities. Neutron energies were determined by time-of-flight techniques with pulse-shape discrimination to reduce gamma-ray background levels. The measurements are compared to calculations using the Los Alamos model of Madland and Nix to test the predictive capabilities of the model. The calculations were performed prior to the measurements. The data are fit by the Watt equation to determine the mean energy of the spectra and facilitate comparison of the results to previous measurements.

Measurement of high-degree solar oscillation frequencies

We present m-averaged solar p- and f-mode oscillation frequencies over the frequency range nu greater than 1.8 and less than 5.0 mHz and the spherical harmonic degree range l greater than or equal to 100 and less than or equal to 1200 from full-disk, 1000 x 1024 pixel, Ca II intensity images collected 1993 June 22-25 with a temporal cadence of 60 s. We itemize the sources and magnitudes of statistical and systematic uncertainties and of small frequency corrections, and we show that our frequencies represent an improvement in accuracy and coverage over previous measurements. Our frequencies agree at the 2 micro Hz level with Mount Wilson frequencies determined for l less than or equal to 600 from full-disk images, and we find systematic offsets of 10-20 micro Hz with respect to frequencies measured from Big Bear and La Palma observations. We give evidence that these latter offsets are indicative of spatial scaling uncertainties associated with the analysis of partial-disk images. In comparison with theory, our p-mode frequencies agree within 10 micro Hz of frequencies predicted by the Los Alamos model but are as much as 100 micro Hz smaller than frequencies predicted by the Denmark and Yale models at degrees near 1000. We also find systematic differences between our n = 0 frequencies and the frequencies closely agreed upon by all three models.

Observations of Naphthalene Scintillations Caused by Tritium Beta-Rays

It seems desirable to report here some preliminary observations on the scintillations produced in commercial naphthalene by the beta rays from tritium and by the bremsstrahlung coming from tritium occluded in tantalum. Of immediate interest is the lower limit set on the conversion efficiency from beta ray to visible light energy. A small amount of gaseous tritium was put in direct contact with finely powdered naphthalene crystals in a 15 cu cm glass Kjeldahl flask, an identical flask but without tritium being used as a control, to find the direct action of the betas. A tantalum disk containing tritium occluded throughout its volume was placed near a solid piece of naphthalene to investigate the effect of soft x-rays since this is almost a pure source of such radiation. Both sources produced scintillations which were observed with a 1P21 photomultiplier, the pulses amplified by a Los Alamos Model 501 amplifier. The photomultiplier was not refrigerated. The rise time and total pulse length, easily observed on a 248A Dumont scope were respectively 0.05 microsecs and 0.2 microsecs. The gas concentration was not well known because the initially too high counting rates necessitated two depletions.