PHOTOEXCITATION OF 87mSr BY (γ,γ′) REACTION

Isomers in atomic nuclei are the levels with the total angular momentum Jiso significantly different from the one of the ground state Jg.s.. They appear in the spectra due to the shell structure of a mean field and their excitation energy in the the odd-mass nuclei does not exceed a few hundred keV . For these reason, decay of isomers into the ground state is strongly hindered and their lifetime vary for milliseconds to days depending on the spin difference ∆J = |Jg.s. − Jiso| [1]. Isomers are populated after decay of intermediate state(s) with the energy of (2−4) MeV and finite branching to the isomeric level. IS are excited by bremsstrahlung radiation with the end-point energy of (2− 5) MeV . The previous experiments [2 − 5] have already shown that the number of such IS which are linked to both the ground and isomeric states, is very small, i.e. one-two states per MeV in spherical nuclei. There was also a set of experiments in which the isomeric states were populated in the (γ, n) reaction via excitation and cascade decay of the giant dipole resonance [6]. In this paper we report our results on the isomer photoproduction in Sr (the isotopic abundance is 7.00%). It has the stable ground state with the spin and parity J g.s. = 9/2 . The isomeric state in this nucleus has the excitation energy Eiso = 388.532 keV and J iso = 1/2 −. Thus, the spin difference ∆J = 4 and at least the E2−E3 sequence is need to populate the isomer from the ground state. The T1/2 value is equal 2.81 hours. The decay scheme of the isomer in Sr is presented in Fig.1. The isomer decays to ground states by M4 transition with the relative γ line intensity of Iγ = 82.1. An early work [7] examined the production of Sr by bremsstrahlung with end points which could be varied up to 3MeV . The tunability of that device allowed three distinct gateways to be identified at 1.22, 1.88 and 2.66 MeV , and their integrated cross sections to be measured.