Experimental attenuation curves have been obtained for uncollirnated bremsstrahlung radiation, incident on ilmenite-loaded concretes of densities 180, 240 and 270 lb/ft3. Measurements were made at bremsstrahlung end-point energies of 5, 10, 20, 30 and 40 MeV, at thicknesses increasing in steps of 3 in. from 0 to 30 in.It was found that for all except the Iowest energy, the breinsstrahlung weighted build-up factor was relatively small. Attenuation curves, calculated for a Schiff spectrum with a simplified photon transport equation, in which all photons were assumed to be forward Soing, agree closely with the measurements for all except the 5 MeV case.
Measurements have been made of the cross sections for some of the (y,p) and (y, n) reactions in molybdenum using the technique of residual activation analysis. The gross structure of these cross sections is compared with previous measurements and the results are interpreted as evidence for the isospin splitting of the giant dipole resonance in medium weight nuclei.The results of the present work are shown in Figure 1 together with other measurements (Duffield, Hsiao, and Sloth 1950;Ferrero et at. 1957;Mutsuro et at. 1959;Costa et al. 1965). No special effort was taken in the present experiment to define the peak positions accurately and the results from other laboratories which are shown in these figures have been adjusted in energy and (with the exception of the looMo (y,p) cross section) normalized to the peak cross sections measured in the present work. Fallieros, Goulard, and Venter (1965) and Morinaga (1965) have suggested that the observed appearance of the peak in the (I', p) cross section at a higher energy than that of the (y,n) cross section in medium and heavy nuclei arises from an isospin splitting of the giant dipole resonance. This interpretation has been applied to recent measurements for zirconium (Fallieros, Goulard, and Venter 1965;Balashov and Yadrovsky 1966;Berman et al. 1967). Owing to the action of the isospin selection rules and the Coulomb barrier, it is expected in the simple picture of isospin splitting of the dipole states that the T-Iower (T <) states should decay by neutron emission and the T-upper (T» states by proton emission, thus leading to (y,n) and (y,p) cross sections with single resonances peaked at about 15 and 20 MeV respectively. Macfarlane (1966) shows that when the (y,p) reaction is identified with the photoabsorption to the higher (T» dipole states and the neutrons are emitted entirely from the decay of the lower (T <) dipole states the expected ratio of the two cross sections is (T> -T <)/T <.The molybdenum (y,n) and (y,p) cross sections as shown in Figure 1 indeed show the main resonances to lie at the energies predicted by the isospin splitting theory. To compare the ratio of the experimental (y,p) and (y,n) cross sections with the theoretical prediction, the results shown in Figures l(b) and l(c) can be used to determine the integrated cross sections for the (y, n) and (y, p) reactions in looMo. The cross section shown in Figure 1 (b) is the sum of the looMo (y, n) and (y, p) reactions; the measurement of the cross section for the (y, n) reaction by Duffield, Hsiao, and * Manuscript
Photoneutron distributions from 14N have been obtained by time-of-flight methods, for bremsstrahlung end-point energies increasing in 2 MeV steps from 15.5 to 29.5 MeV. A large part of the neutron yield is associated with the sequential decay of 14N to 12C, through well-defined intermediate states of 13C, at 7.55, 8.86, and 11.80 MeV, which are unstable against neutron emission. The (γ,n0) cross section for neutron emission to the ground state of 13N is found to agree very closely with the corresponding (γ,p0) cross section, implying a high degree of isospin purity for the giant dipole resonance of 14N. It is observed that the decay of the giant resonance proceeds freely through those odd-parity excited states of the A = 13 nuclei which are single hole states formed by the removal of a p-shell nucleon from the parent 14N.The integrated cross section for all neutron-producing interactions is found to be 88 ± 5 MeV mb.
Photoneutron energy spectra from 40Ar have been measured at bremsstrahlung end-point energies from 13 to 23 MeV in 1 MeV steps. The differential cross section at 90° for the reaction 40Ar(γ, n0)39Ar revealed some 26 previously unreported excited states of 40Ar in the region below 13 MeV. The envelope of these states suggests a single particle resonance involving the valence 1f7/2 neutrons. The measured total neutron cross section exhibits a marked change from this single particle behavior at about 14 MeV where collective effects begin to dominate and neutrons are emitted to highly excited states of 39Ar.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.