The fraction of '"Tb activity recoiling out of a gold target into the forward (F) and backward (B) directions was determined in irradiations with protons of 1, 3, 11. 5, and 300 GeV. The values of F/B decreased from 9.6 at 1 GeV to 2.4 at 300 GeV. The values of 2 W(F+ B) decreased from 1.24 mg of Au per cm at 1 GeV to 0.89 mg of Au per cm' at 300 GeV, where W is the thickness of the gold target. These results were analyzed in terms of the standard two-step model for the reaction. The momentum transfer, which occurs in the first step, decreases with the energy of the incident protons. The average kinetic energy of the " Tb nuclei in the second step is 3.9~0.1 MeV for protons in the energy range from 0.7 GeV (earlier work) to 300 GeV. The constancy of this value indicates that the reaction mechanism is essentially unchanged over this range of bombarding energies.NUCLEAR REACTIONS~~Au(P, spallation) Tb; &=1, 3, 11.5, 300 GeV; measured thick-target recoil properties.The production of "'Tb from the proton bombardment of "'Au has been studied extensively. Values of the cross section have been reported for proton energies from 0.2 to 30 QeV. ' ' The mechanism of this reaction has been studied by the recoil technique from O. V to 6.2 QeV. " The work reported here extends the recoil measurements to 300 QeV.In these experiments the thickness of the target foils was substantially larger than the range of the "'Tb recoils. This si.mplified the analysis of the results in terms of the parameters of the reaction mechanism. ' The quantities measured were the fractions I' and B of all the "'Tb nuclei that recoiled out of the gold target into the forward and backward directions, respectively. These nuclei were captured in aluminum foils on either side of the target foil (see Fig. 1). The fraction that remained in the target foil is identified by T, and the thickness of the target foil by W.The results were analyzed by the standard twostep vector representation. ' ' The velocity of the recoiling "'Tb nucleus was taken to be the vector sum of a forward velocity v imparted in the first step to an intermediate nucleus and an isotropic velocity V imparted in the second step to the '49Tb nucleus. The average range (R) of "~Tb in gold in the second step was calculated in this analysis.The corresponding average kinetic energy (E) = (~m V') was obtained from the range-energy tables of Northcliffe and Schilling. 'These values of (E) were compared with values for earlier measurements" as an indication of how the mechanism of this reaction varies with proton energy from O.V to 300 QeV.
EXPERIMENTAL PROCEDUREIrradiations at proton energies of 1, 3, and 11. 5 QeV were carried out in the circulating beam of the zero-gradient synchroton at the Argonne National Laboratory. Irradiations at 300 QeV were performed with the primary proton beam in the meson area of the Fermi National AcceleratorLaboratory. The target assembly consisted of gold and aluminum foils arranged as shown in Fig. 1. The surface density of the gold foils varied from 2.1 mg/cm' t...
The recoil properties of Na and Mg formed by the interaction of protons. of energy 3.0 -300 GeV with targets of gold and uranium have been measured by a thick-target technique. y-ray spectroscopy without chemical separation was used to measure the nuclides of interest. The forward-to-backward ratio (F/8) of both nuclides from both targets exhibits a maximum near 3 GeV, decreasing from a value of about 2.0 at that energy to about 1.3 at 300 GeV. This behavior is strikingly similar to that of several neutron-deficient medium-mass nuclides formed from uranium, such as "'Ba, and seems to indicate that both types of products are formed in similar processes. The mean ranges of the light nuclides were found to decrease by about 25/o from 3 to 300 GeV, also similar to the behavior of the neutron deficient products, although smaller in magnitude.
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