Particles Of Various Energies Research Articles

Studies of Fragment Angular Distributions in the Fission ofBi209andU238Induced by Alpha Particles of Energies up to 115 MeV

Using semiconductor detectors, the fragment angular distributions have been measured in the cases of fission of Bi 209 and U 238 induced by alpha particles of various energies ranging from 23 to 115 MeV obtained from the Berkeley 88-in. variable-energy cyclotron. The center-of-mass angular distributions were analyzed by a least-squares fitting code to obtain the value of K 0 2 corresponding to the saddle-point excitation energy E x s for each bombarding energy. The transmission coefficients T l and the mean square of the orbital angular momentum 2 > of the fissioning nucleus required for deducing K 0 2 were determined from optical-model calculations. For both the cases of compound nuclei of At 213 and Pu 242 , it is found that the values of K 0 2 increase more rapidly with E x s than expected on the basis of the Fermi-gas model, irrespective of the assumptions made about the multiple-chance fissions. The fission cross sections of U 238 for alpha-particle energies up to 110 MeV, also measured in this work, enabled us to check the accuracy of the optical-model calculations. The presence of direct interactions and their effects on the deduced values of K 0 2 were also investigated in detail in the case of the target nucleus U 238 . Using the standard expression for Γ n /Γ f , the first-chance values of K 0 2 have been obtained and further corrected for the estimated direct-interaction effects, in the case of the target nucleus U 238 . Even after allowance is made for the direct-interaction effects, the energy dependence of K 0 2 appears to be significantly different from that expected on the basis of the simple Fermi-gas model. These results can be explained within the framework of the Fermi-gas model if it is assumed that J eff /a ½ f increases significantly with the bombarding energy. It appears likely that these results suggest a rapid increase in the effective moment of inertia J eff with the angular momentum, as can be expected on the basis of the observed steep variation in the saddle shapes with Z 2 /A.

Motions of charged particles trapped in the Earth's magnetosphere

Recent measurements by earth satellites provide conclusive evidence that the magnetosphere of the earth is distorted by the solar wind, as has been predicted on theoretical grounds for some time. In the work reported here motions of charged particles trapped in the distorted magnetosphere were computed by using, as a model of the magnetosphere, the field of a weak magnetic dipole in the presence of a strong magnetic dipole. Drift paths through this field were calculated both for particles whose motion is confined to the magnetic equatorial plane and for particles mirroring at low altitudes. The paths of particles of various energies moving in the magnetic equatorial plane under the combined influences of field gradient-induced drift and field rotation were also calculated. It was found that, in the model used, the field rotation energizes trapped particles on the morning side and de-energizes them on the evening side of the earth. It is suggested that this effect must occur in any realistic model of the distorted magnetosphere and that it may play a role in the accumulation of the energetic particles that constitute the Van Allen zones.

DISINTEGRATION-RATE DETERMINATION BY 4π-COUNTING: PART I

The response probability of a 4π-counter has been examined and conditions determined for which the response probability is unity to within ± 0.1% for a range of nuclides. Variables examined have been the effect of polarization potential, discriminator bias, source location, counting rate, counter gas purity, thickness of conductor on source mount, for particles of various energies and from various modes of disintegration.