Search

We have re-examined the decay of ${\mathrm{Rb}}^{83}$, which is reported to decay by electron capture, with emission of a single $\ensuremath{\gamma}$ ray of 525 keV, to the 1.86-h isomer of ${\mathrm{Kr}}^{83}$. With the aid of a lithium-drifted germanium detector and with a high-resolution beta spectrometer we find that the 525-keV photopeak observed with NaI crystals is really a superposition of three $\ensuremath{\gamma}$ rays at 521 keV (46%), 530 keV (31%), and 553 keV (16%). An additional $\ensuremath{\gamma}$ ray was found at 790 keV with intensity of 0.9%. Various coincidence experiments showed that only the 521- and 530-keV transitions lead to the 1.86-h ${\mathrm{Kr}}^{83m}$. The 553- and 790-keV transitions lead to the 9.3-keV level of ${\mathrm{Kr}}^{83}$. Direct electron capture to this level has also been obsered with an intensity of 6%. The fraction of ${\mathrm{Rb}}^{83}$ disintegrations bypassing the isomeric state of ${\mathrm{Kr}}^{83}$ is 23%. The following new levels were assigned to ${\mathrm{Kr}}^{83}$: 562, 571, and 799 keV. The total disintegration energy of ${\mathrm{Rb}}^{83}$ was shown to be \ensuremath{\ge}825 keV.

The formation cross sections of ${\mathrm{Li}}^{9}$, ${\mathrm{C}}^{16}$, and ${\mathrm{N}}^{17}$ from B, C, ${\mathrm{N}}^{14}$, ${\mathrm{N}}^{15}$, ${\mathrm{O}}^{16}$, ${\mathrm{O}}^{18}$, F, Ne, Na, Mg, and Al targets bombarded with 156-MeV protons were measured by means of their neutron emission. The results were compared with calculations based on the two-step spallation mechanism. Certain reactions were shown to provide direct comparison with prompt cascade calculation. In general, the agreement between theory and experiment was good, and it is suggested that some discrepancies are attributable to the neglect of the effect of nuclear structure in the calculations. The importance of $\ensuremath{\alpha}$ emission in determining the pattern of spallation product cross section was demonstrated.

Experimental cross sections are reported for the formation of N/sup 13/ in the bombardment of Zn, In, Pb, and U with protons of 1.0, 1.9, and 2.9 Bev energy. These values are compared with theoretical N/sup 13/ emission cross sections for protons energies of 0.84 and 1.84 Bev. The calculations are based on the evaporation model. The previously described Monte Carlo procedure was modified in order to obtain better statistical accuracy for the calculated N/sup 13/ cross sections. Previously computed emission cross sections for He/sup 8/, Li/sup 8/, and Be/sup 7/ were also recomputed using the modified Monte Carlo procedure. The cross sections were computed for three different formulations of the interaction radius. Good fit with the experimental He/sup 6/ Li/sup 8-/ and Be/sup 7/ cross sections is obtained when the smaller values for the interaction radius are used. However, the fit with the experimental N/sup 13/ values is not good enough to exclude processes other than evaporation as contributing to the experimentally observed cross sections. (auth)

Monte Carlo calculations of nuclear reactions in the low-energy (E < 50 Mev) region are described. The calculations are based on the nuclear evaporation model of Weisskopf. Continuum theory was used for the calculation of inverse reaction cross sections. In the calculation of the level densities of excited nuclei, pairing and shell energy corrections were used in terms of charactertstic level displacements. The accurate equation rather than the approximate Maxwell distribution was used for the selection of the kinetic energy of the evaporated particle. Experimentally determined Q-values for the various reactions were used. The calculations are compared with experimental measurements for about 60 excitation functions of nuclear reactions in the mass range Cr/sup 50/-Se/sup 74/. Cameron's values for pairing energies were used at the outset; but a new set of pairing and shell energy correction values, which leads to substuntially improved agreement with the experimental curves, Is presented. The procedure which was used to arrive at this set is described and several features of the set are discussed. The need for a further downward correction of the level density of symmetrical (A = 2Z) nuclei is indicated. Computed excitation functions are shown for all the reactions studied as well asmore » for several reactions for which experimental data are not yet available. Further experiments on reaction cross sections are suggested which would allow a unique deterntination of the pairing and shell energy corrections of level densities for any value of Z and N in the region under discussion. The existence of a unique set of these correction terms would provide strong evidence for the validity of evaporation theory for the reactions considered. (auth)« less

The process of nuclear de-excitation by evaporation was calculated by using a Monte Carlo method and fast electronic computers. The competition between the emission of neutrons, protons, deuterons, tritons He/sup 3/, and He/ sup 4/ was studied as a functicn, of various parameters and experimental conditions. A systematic survey is presented for a range of atomic numbers, mass numbers and excitation energy of initial nuclei. The nuclides studied extend from A =49 to 239 with Z = Z/sub A/ plus or minus 3. The range of excitation energies for which computations are presented extends from 1OO to 700 Mev. Among the parameters affecting the process the importance of the temperature correction of the Coulomb barrier and the level density parameters are considered in detail. The average properties of the process such as average numbers of the various particles emitted, the average number of nucleons, of charged particles, and of charges emitted are summarized in graphical and tabular form for various combinations of the parameters used in the computation. Energy spectra of the various emitted particles are also shown. (auth)

Previous Monte Carlo calculations of nuclear evaporation reactions have been extended to include the emission of ${\mathrm{He}}^{6}$, ${\mathrm{Li}}^{6}$, ${\mathrm{Li}}^{7}$, ${\mathrm{Li}}^{8}$, and ${\mathrm{Be}}^{7}$ from Cu, Ag, Au, and Bi targets bombarded with high-energy protons (340-2000 Mev). Comparison with available experimental results shows good agreement in most cases. A discrepancy has been observed between the calculated and observed variation of ${\mathrm{Be}}^{7}$ formation cross section with the mass of the target nucleus, but even here the agreement is within a factor of three. It is shown that, for the usually chosen parameters of the calculation, a level density parameter of $a=\frac{A}{10}$ is necessary.

Delayed neutrons with a half-life of 11.4\ifmmode\pm\else\textpm\fi{}0.5 msec have been observed in the GeV-energy proton irradiation of targets of ${\mathrm{N}}^{15}$ and heavier nuclei but not from ${\mathrm{N}}^{14}$ and lighter nuclei. This new activity has been assigned to ${\mathrm{Be}}^{12}$. Cross sections for production of this nuclide from ${\mathrm{N}}^{15}$, ${\mathrm{O}}^{16}$, ${\mathrm{O}}^{18}$, F, Na, and Al have been measured. From cross-section systematics, it is estimated that delayed neutrons are emitted in about 7% of the decays. Upper limits for the delayed-neutron branches of ${\mathrm{He}}^{8}$ and ${\mathrm{B}}^{13}$ have been set at 3 and 0.3%, respectively.