Spontaneous Fission Source Research Articles

Energy straggling of fission fragments in gases and solids

A 252Cf spontaneous fission source and a heavy ion surface barrier detector have been used to measure the straggling parameters for light and heavy fragments transmitted through a number of gases, gas mixtures, and solids.

Application of the helium-jet technique for fission fragment transport from spontaneous fission sources

The helium-jet method has been adapted for the transport of fission products from a 252Cf source. It has been found that the helium carrier gas must be seeded with H 2O vapor and irradiated with an ultra-violet light source to obtain high efficiencies. There is indication of the role of free radicals in producing high- molecular-weight species that are necessary for the transport of the fission products. Transport efficiencies of about 55% and mean transport times of 30 s have been achieved.

The measurement of neutron importance functions

The theory and measurement of neutron importance functions are discussed by formulating a classification scheme for the different ways of measuring importance function distributions. The scheme is based on the statement of the theoretical interpretation of the experiment; that is on the exact equations which govern the measurements. The classification scheme consists of six methods utilizing neutron sources and one method utilizing small sample perturbations. Procedures for obtaining the value of certain reactor parameters, including the geometric buckling, β/Λ, and reactivity, from these measurement methods are also outlined. The source multiplication method using a 252Cf spontaneous fission source and the perturbation method using boron and cadmium samples are illustrated by comparing experimental data and calculated results for a reflected light water reactor. Four-group diffusion-theory calculations are in good agreement with the measurements. A value of 29·00 ± 0·05 cm is obtained for the extrapolated reactor radius from the 252Cf radial traverse as compared with 2894 ± 0·08 cm and 29·90 ± 0·66 cm that had been measured respectively from Mn and Th 232 foil activation.

A routine procedure for 4π proportional counting soft beta particles from solid sources

Freeze-drying has been used routinely for preparing low energy beta sources. The sources have a better distribution of active material then those prepared by heat or air drying and result in far less self absorption. The inherent self-absorption is estimated by an extrapolation method. Confidence limits of 1%, 2% and 4% are placed on determinations of specific activity of 60Co, 35S and 63Ni respectively. Extension of the method to alpha sources and to spontaneous fission sources has also given good results.

Energy Loss by Fission Fragments in Nitrogen

Energy loss by fission fragments in nitrogen gas was studied by means of a pulse technique that measured luminescence excited by a low-intensity Cf252 spontaneous fission source. A novel kinetic analysis of competing emission and quenching reactions was developed that gives the power law dependency of energy loss by the fragments in a luminescing gas from the pressure at which maximum luminosity is observed. For nitrogen, the relationship E = E0(1−f)1.70 ± 0.07 is valid for 0.4 E0 < E < E0. The term E is used for the kinetic energy of a fission fragment of initial energy E0 that has traveled a fraction f of its total range.

On-line separation of short-lived products from 252Cf spontaneous fission

Some problems related to on-line separation of short-lived activities have been studied using a spontaneous fission source. A series of different fission product catchers have been studied and the relative yield of the heavy Xe isotopes measured. A nuclear radiation detector employing three detectors and a moving collection tape is described.

An Application of a New Method for the Solution of Group Diffusion Equations to a Cylindrical Flux Trap Assembly with Spontaneous Fission Source

In the case of an infinite reflector, the group diffusion equations are converted to a system of integral equations by a Green's function technique. This system is then Fourier transformed. The transformed Fredholm-type equations have a degenerate kernel, and the solution may be reduced to a system of linear algebraic equations with infinitely many unknowns.The theory is developed for the case of a three-group three-region cylindrical flux trap assembly. An extension of the method to a general multigroup multiregion problem will be considered in a later paper.

Fragment Kinetic Energy Release in Heavy-Ion-Induced Fission Reactions

The most probable kinetic-energy release in fission reactions induced by 125-MeV ${\mathrm{C}}^{12}$ ions and 166-MeV ${\mathrm{O}}^{16}$ ions has been measured for the target nuclei ${\mathrm{Pr}}^{141}$, ${\mathrm{Tb}}^{159}$, ${\mathrm{Ho}}^{165}$, ${\mathrm{Tm}}^{169}$, ${\mathrm{Lu}}^{175}$, ${\mathrm{Au}}^{197}$, ${\mathrm{Bi}}^{209}$, ${\mathrm{Th}}^{232}$, ${\mathrm{U}}^{238}$, and ${\mathrm{Pu}}^{240}$. Silicon diode surface-barrier detectors were used in these measurements, and a ${\mathrm{Cf}}^{252}$ spontaneous-fission source served as the absolute energy standard. A least-squares analysis of our data and those obtained by others gives ${\overline{E}}_{K}(\mathrm{MeV})=0.1065 \frac{{Z}^{2}}{{A}^{\frac{1}{3}}}+20.1$. The dependence of the most probable kinetic energy on the scission shape of the fissioning nucleus has been examined by comparison with recent calculations based on the work of Cohen and Swiatecki. The data are consistent with scission shapes corresponding to spheroids whose shapes minimize the total energy of the system.

Response of Semiconductor Detectors to Fission Fragments

Measurements with a Cf252 spontaneous fission source have shown that semiconductor detectors exhibit a pulse-height defect for fission fragments, and that this defect is greater for the low-energy, heavy-mass group than for the high-energy, light-mass group. It has also been shown that the defect decreases with increasing bias voltage or average electric field strength within the depletion region. However, for a given electric field strength, the defect was found to be approximately independent of the type and resistivity of the detector. These results are consistent with the hypothesis that the pulse-height defect arises from an inability to collect all of the hole-electron pairs in the dense plasma produced by an energetic fission fragment.

Absolute Measurement of (n, 2n) Cross Sections at 14.1 Mev

Absolute ($n, 2n$) cross sections have been obtained by counting double-pulse events in a 240-gal cadmium-loaded liquid scintillator. The detection efficiency was determined experimentally using a spontaneous fission source and known values of $\overline{\ensuremath{\nu}}$. Conventional methods were used to measure the absolute neutron flux. Cross sections were measured for C, N, F, V, Fe, Cu, Ag, Ta, Cd, Au, Pb, Bi, D, and Be. The values in barns are: C, 0.006\ifmmode\pm\else\textpm\fi{}0.006; N, 0.019\ifmmode\pm\else\textpm\fi{}0.010; F, 0.062\ifmmode\pm\else\textpm\fi{}0.009; V, 0.66\ifmmode\pm\else\textpm\fi{}0.05; Fe, 0.50\ifmmode\pm\else\textpm\fi{}0.04; Cu, 0.76\ifmmode\pm\else\textpm\fi{}0.06; Ag, 1.73\ifmmode\pm\else\textpm\fi{}0.13; Ta, 2.64\ifmmode\pm\else\textpm\fi{}0.20; Cd, 1.92\ifmmode\pm\else\textpm\fi{}0.14; Au, 2.60\ifmmode\pm\else\textpm\fi{}0.20; Pb, 2.74\ifmmode\pm\else\textpm\fi{}0.20; Bi, 2.60\ifmmode\pm\else\textpm\fi{}0.19; D, 0.20\ifmmode\pm\else\textpm\fi{}0.02; Be, 0.54\ifmmode\pm\else\textpm\fi{}0.04. The ($n, 2n$) cross sections for the last six elements are, within error, equal to the nonelastic cross sections. Nuclear temperatures were calculated.

A comparison of the average number of neutrons emitted in fission of some uranium and plutonium isotopes

Measurements of the relative prompt neutron yields per fission of Uz 233,U 235,Pu 239, and Pu 241 by thermal neutrons are described. A coincidence technique is employed. Absolute values have been determined using a natural uranium spontaneous fission source to calibrate the neutron detector.