Thermal-neutron Fission Cross Section Research Articles

Thermal-neutron fission cross section of 26.1-min 235Um.

The thermal-neutron fission cross section of $^{235}\mathrm{U}^{\mathrm{m}}$ has been measured relative to the ground-state cross section. A rapid radiochemical separation procedure was developed to provide sizeable (${10}^{10}$ to ${10}^{11}$ atom) samples that were reasonably free of the parent $^{239}\mathrm{Pu}$. From a series of eight measurements, the value of 1.42\ifmmode\pm\else\textpm\fi{}0.04 was obtained for the ratio ${\ensuremath{\sigma}}_{m}$/${\ensuremath{\sigma}}_{g}$.

Some nuclear properties of fermium-257

A total half-life of 100·5±0·2 days and an alpha-to-spontaneous-fission decay ratio of 475±10 have been measured for 257Fm by counting in ionization chambers. Thermal-neutron fission cross sections for 257Fm (2950±160 b) and its daughter, 253Cf (1300±240 b), have been measured in a thermal-neutron column using mica as a fission-fragment track recorder.

Thermal neutron fission cross sections in the range 302?A?223

A least squares method is used to fit a smooth curve to known thermal neutron fission cross section data for nuclei of evenN. By making use of fission barriers resulting from a modified liquid drop formalism coupled with the Strutinsky Normalization Procedure, the result is generalized and applied to the prediction within this framework of some 59 cross sections in the actinide and adjacent sub-actinide and trans-actinide regions, as well as to the prediction of an additional 37 cross sections in the superheavy region. In the first group is included a number of nuclides for which presently available cross section data are necessarily cited as upper limits. In the second group, interest is focussed on the predicted island of stability surrounding a neutron shell closure at 184 and a proton shell closure at 114. The results, while they are regarded as plausible estimates and not precise determinations, indicate that many cross sections in this region can be expected to undergo a pronounced reduction in magnitude relative to the limited number of estimates previously given in the literature.

Thermal-neutron fission cross section and fission resonance integral of 254Es

The resonance fission integral and the thermal neutron fission cross section of 254Es have been measured by plastic fission track recorders. The cross sections were calculated from direct experimental determinations of the fission rate of 254Es compared to that of a 249Cf standard. The measured values for 254Es are: I f = 2200 ± 90 b (0·625 eV cutoff) and σ f = 2830 ± 130 b.

The thermal-neutron fission cross section of 227Th

The thermal-neutron fission cross section of 227Th was measured using both back to back fission counters and radiochemical techniques. A value of σ f( 22 Th) = 200 ± 20 barns was obtained from 8 independent measurements.

Nuclear constants of nine transplutonium nuclides

The total half-lives of 252Cf, 250Cf, 253Cf are 2·646 ± 0·004 years, 13·2 ± 0·5 years and 17·6 ± 0·2 days. The ratios of α-emission to spontaneous fission for 252Cf, 250Cf, 253Es and 244Cm are 31·3 ± 0·2, 1260 ± 40, (1·15 ± 0·03)10 7 and (7·43 ± 0·01)10 5. The partial spontaneous fission half-life of 246Cm = (1·66 ± 0·10)10 7 years. The thermal neutron fission cross sections for 249Cf and 251Cf are 1735 ± 70 barns and 3000 ± 260 barns. A separate determination and a re-evaluation of the data from the MIKE thermonuclear event confirms the 60·5 ± 0· 2 day 254Cf half-life reported by others.

A Thermal Neutron Fission Counter for Nuclear and Analytical Measurements

A fission counter which has been used at the Argonne CP-5 reactor is described. It has been applied to the measurement of thermal neutron fission cross sections and to the determination of very small quantities of fissionable materials. The counting system consists of a dual chamber fission counter, the associated electronic apparatus, a mechanism for positioning the counter in the thermal column, and the protective neutron shielding. The design and performance of the system and the operating procedures are discussed in detail.

The Temperature Dependence of the Fission Cross Sections of Pu239, U235, and U233

The dependence on neutron temperature of the ratio of thermal neutron fission cross sections, , has been measured in heavy water moderator. Satisfactory agreement was obtained with the temperature ...

Neutron Fission Cross Sections ofPu236andPu237

The thermal-neutron fission cross sections of plutonium isotopes 236 and 237 were determined by counting samples placed in the thermal column of the Argonne heterogeneous heavy-water reactor, CP-5. Comparison of the fission counting rate of the ${\mathrm{Pu}}^{236}$ and ${\mathrm{Pu}}^{237}$ samples with that of a ${\mathrm{Pu}}^{239}$ standard gave cross sections of 170\ifmmode\pm\else\textpm\fi{}35 barns and 2500\ifmmode\pm\else\textpm\fi{}500 barns, respectively, for the two nuclides. The $K$-to-total electron capture branching ratio for ${\mathrm{Pu}}^{237}$ was found to be 0.38\ifmmode\pm\else\textpm\fi{}0.06.

THE THERMAL-NEUTRON FISSION CROSS SECTION OF Pu240

A measurement of the thermal-neutron fission cross section of Pu240 is described. The value obtained, −0.8 ± 0.7 barn, is compared with other measurements.

Thermal-Neutron Fission Cross Sections for Isotopes of Plutonium, Americium, and Curium

The following thermal-neutron fission cross sections have been measured in the thermal column of the Materials Testing Reactor at Idaho Falls, Idaho: ${\mathrm{Pu}}^{238}$, 18.4\ifmmode\pm\else\textpm\fi{}0.9 barns; ${\mathrm{Am}}^{241}$, 3.13\ifmmode\pm\else\textpm\fi{}0.15 barns; ${\mathrm{Am}}^{242}$, 6390\ifmmode\pm\else\textpm\fi{}500 barns; ${\mathrm{Am}}^{243}$, 0.072 barn; ${\mathrm{Cm}}^{243}$, 690\ifmmode\pm\else\textpm\fi{}50 barns; ${\mathrm{Cm}}^{245}$, 1880\ifmmode\pm\else\textpm\fi{}150 barns. In addition, a pile neutron capture cross section of 520\ifmmode\pm\else\textpm\fi{}40 barns has been measured for ${\mathrm{Pu}}^{238}$.

Thermal-Neutron Fission Cross Section ofPu240

A thermal-neutron fission cross section of 4.4\ifmmode\pm\else\textpm\fi{}0.5 barns for isotopically pure ${\mathrm{Pu}}^{240}$, formed by the ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay of ${\mathrm{U}}^{240}$, has been measured in the thermal column of the Materials Testing Reactor.

Photoneutron Fission Analysis for U−235 in High Purity U−238

The photoneutron method for the analysis of U−235 in high purity U−238 is described. Special samples have been found to contain as little as 5 parts per million U−235. An upper limit for the thermal neutron fission cross section of U−238 can be set at 5×10−28 cm2.