Thermal Neutron Fission Of 239Pu Research Articles

Fine structure of mass and charge distribution in low energy fission

Studies of finer details in mass and charge distribution fission leads to a better understanding of the fission process. Experimental determination of independent and cumulative yields using radiochemical techniques as well as mass spectrometers and fission product recoil separators form the basis of such studies. It has been established that closed shells as well as an even number of nucleons influence both mass and charge distributions. The magnitudes of these effects may be estimated from existing experimental yield data and various fission models. Using our measurements of several fission yields and those existing in the literature we have calculated even-odd proton and neutron effects for various low energy fissioning systems. Where enough data existed, direct calculations were made, whereas for other cases the Zp-model of WAHL has been used. It is found that the even-odd proton effect is well established and pronounced in thermal neutron fission of235U and233U. Lesser effects were found for reactor neutron induced fission of232Th, thermal neutron fission of239Pu and spontaneous fission of245Cm and249Cf. No effect seems to exist in the thermal neutron fission of241Pu and the spontaneous fission of252Cf. The even-odd neutron effect is found to be much lower than the corresponding proton effect in235U and233U fissions and is nonexistent in the rest of the fissioning systems.

Yields of Short-Lived Fission Products Produced by Thermal-Neutron Fission of Plutonium-239

The absolute yields of 28 fission products representing 23 different mass chains produced by thermal-neutron fission of 239Pu and having half-lives between 30 and 1100 s have been determined using Ge(Li) spectroscopy methods. Spectra of 30 gamma rays emitted in the decay of the fission products between 35 and 1950 s after a 5-s irradiation were obtained. Gamma rays were assigned to the responsible fission products by matching gamma-ray energies and half-lives. Fission-product yields were then obtained from the data by first determining the appropriate gamma-ray activity as of the end of the irradiation, correcting for detector efficiency and gamma-ray branching ratio, and, finally, dividing by the number of fissions created in the sample. The number of fissions was determined by direct comparison of gamma rays emanating from fission products created during a careful irradiation of a well-calibrated 239Pu-loaded fission chamber.The resulting fission-product yields are compared with previous measurements and with recommended yields given in two recent (and independent) evaluations. Uncertainties assigned to the present results range between 6 and 45%, and are smaller than or comparable to uncertainties assigned to previous experimental or evaluated yields for six mass chains.

Fission Product Yields for Thermal-Neutron Fission of Plutonium-239

Absolute cumulative yields have been determined for 49 fission products representing 36 mass chains created during thermal-neutron fission of 239Pu, including 3 mass chains for which no prior data exist. Using Ge(Li) spectroscopy, spectra were obtained of gamma rays from decay of fission products between 1550 s and 31 days after a 100-s irradiation. Data were obtained for all fission products simultaneously. Gamma rays were assigned to the responsible fission products by matching gamma-ray energies and half-lives. Gamma-ray data associated with decay of 135I and 140Ba-140La, in particular, were thoroughly studied; uncertainties were obtained for the two largest intensity gamma rays from decay of 135I that are smaller than previously evaluated uncertainties. Fission product yields were obtained from the data by first determining the appropriate gamma-ray activity as of the end of the irradiation, then correcting for detector efficiency and gamma-ray branching ratio, and, finally, dividing by the number of fissions created in the sample. The number of fissions was determined by direct comparison of gamma rays emanating from fission products created during a careful irradiation of a well-calibrated 239Pu-loaded fission chamber. The resulting fission product yields are compared with previous measurements and with recommended yields given in two recent (and independent) evaluations. The present results are significantly larger for mass chains 101 and 105, somewhat smaller for mass chains 87 and 151, and in reasonable agreement with the remaining mass chains. Uncertainties assigned to the present results range between 2.5 and 25%, and are smaller than or comparable to uncertainties assigned to previous experimental (or evaluated) yields for 14 mass chains.

A model for calculating the angular momentum distribution of fission fragments

A scheme is presented for quantitative calculations of the distribution of rotational angular momenta in primary fission fragments. The wave function of the bending mode excitation at the scission point is expanded in angular momentum components. As an example, the distribution of rotational angular momentum is estimated for the fragment of mass 108 from the thermal neutron fission of 239Pu. The experimental literature on the subject is reviewed.

Emission of light nuclei in thermal neutron fission of 239Pu

The relative intensities and energy spectra of 1H, 2H, 3H, 4He, 6He and 8He particles from thermal neutron fission of 239Pu have been measured. The comparison with analogous data on the spontaneous fission of 252Cf and the thermal neutron fission of 235U reveals a pronounced similarity.

Levels in the odd nucleus 156Eu

The decay of 9.4 h 156Sm produced in fractional μCi amounts by the thermal-neutron fission of 239Pu has been studied by electron and gamma-ray spectrometry and by e-γ, β-γ and e-β coincidence techniques. Levels characterized by the quantum numbers ( K, I π ) have been established at the following energies in keV: 0 (0, 0 +), 22.6 (0, 1 +), 47.9 (0, 2 +) (tentative), 87.6 (1, 1 −), 125.7 (1, 2 −), 291.4 (1, 1 +). These levels are interpreted as corresponding to the three intrinsic configurations { 5 2 +[413]− 5 2 +[642]} 0 + , 5 2 +[413]− 3 2 −[521]} /in1 − and { 7 2 −[523]− 5 2 −[523]} 1 + .

The yields of the isomers of 81Se and 83Se in the thermal neutron fission of 239Pu

The cumulative yields of 81Se, 81mSe, 83Se, 83Br and 84Br in thermal neutron fission of 239Pu have been measured radiochemically. From these results and previous studies of the selenium isomer yields in 235U fission, it is concluded that the ratio of the formation cross section for the high spin isomer 83Se to the total formation cross section for both isomers is >0·8. Calculations based on a statistical theory of spin distributions of the nuclear level density indicate that the fragments produced in 239Pu fission have a mean intrinsic angular momentum of the order of eight units.