Photoexcitation of nuclear isomers by (γ,γ’) reactions

Abstract

Photoexcitation of the isomers of 19 nuclides was examined in this work. Four accelerators were used as sources of bremsstrahlung to expose the samples and end-point energies covered the range from 0.5 to 11 MeV. No evidence was found for nonresonant processes of excitation. However, more than half the cases showed enhanced channels for the resonant photoexcitation of isomers with integrated cross sections approaching 10 ' cm keV. These results are three to four orders of magnitude larger than values usually characterizing {y, y') reactions.

Properties of the5−state at 839 keV inLu176and theMohr1, Bisterzo2, Gallino3 et al. 2009Phys. Rev. C242300View full textAdd to dashboardCite

“…Photon scattering [6] and photoactivation of 176 Lu [7] have been measured using bremsstrahlung at the Stuttgart dynamitron, and earlier experiments with bremsstrahlung have been performed using medical and technical electron accelerators [8,9,10,11] which are known to provide high photon intensities [12]. Activation after Coulomb excitation was studied at the Tandem accelerator at IPN, Orsay [13], activation using positron annihilation was measured at the Kyoto research reactor [14], and photoactivation experiments with various radioactive sources were reported in [10,15,16,17].…”

“…The obtained information is compiled in the ENSDF online data base [27] which is based on [28]. Further information on IS can be deduced from a Coulomb excitation and activation experiment [13] which has detected the activity of the isomer in 176 Lu recoil nuclei after Coulomb excitation by a 32 S projectile, and from various photoactivation experiments using bremsstrahlung and radioactive sources [8,9,10,11,15,16,17]. Finally, a photoactivation experiment has been performed [7] using the high photon flux of the bremsstrahlung setup of the dynamitron accelerator at Stuttgart [29].…”

“…However, these IS at higher energies are not relevant under s-process conditions. Consequently, it is not possible to derive properties of low-lying IS from bremsstrahlung photoactivation experiments with endpoint energies of several MeV [8,9,11]. There is also general agreement that the lowest IS is located at energies above 650 keV because photoactivation of 176 Lu could not be observed after irradiation with 137 Cs sources (E γ = 662 keV) [10,16,17] and in an experiment with bremsstrahlung with an endpoint energy of 600 keV [10].…”

γ-ray fluxes in Oklo natural reactors

“…Experimental data [11,12] confirm that long lived 176 Lu in the photon bath of celestial bodies can be partially transformed into metastable 176m Lu by photons with energies around 880, 1060, 1330, and 1660 keV. Higher energy photons may also contribute [26]. These photon energies correspond to excited states of 176 Lu with specific spins and parities which allow them to act as mediators for photo excitation of the isomeric state.…”

“…[12], for a laboratory environment, as opposed to a fully ionized environment), and higher values σ int γ,γ ′ (E γ )=140 mb·keV and 350 mb·keV, for 4-and 6-MeV bremsstrahlung irradiations respectively, with an assumed IS energy of 2.125 MeV [26].…”