Cross sections and rates for the electron excitation of the isomeric states 111m Cd, 113m In and 115m In

According to some calculations, intense pulses of electrons or photons can exert an influence on the deexcitation rate of isomeric nuclear states. Thus, it has been shown that the lifetime of isomeric nuclear states can be influenced by X-ray electron-nuclear double transitions (XENDT's), processes in which a transition effected by an inner atomic electron takes place simultaneously with a nuclear electromagnetic transition. The lifetime of isomeric nuclear states can also be influenced with the aid of fluxes of gamma-ray photons, when the induced deexcitation of the isomeric nucleus is a two-step process. In this work we shall report the results of measurements of the cross sections and rates for the excitation of isomeric nuclear states by a beam of 7.7 MeV electrons. The electron excitation of nuclear isomers has been previously studied by Booth and Brownson for electron energies up to 3 MeV. In the process studied in this work, the nucleus is initially in the ground state and the electric field of the incident electrons induces transitions to excited nuclear states, which then populate with a certain probability the isomeric level. After irradiating the targets with electrons we have determined the isomeric yield by gamma-ray spectroscopy. We have measured the cross section σe for the electron activation of the isomeric states of 111m Cd, 113m In and 115m In. We have then used these cross sections, the mean beam current I and the cross-sectional area S of the beam to calculate the mean electron activation rates R of the isomeric states. The targets were made of natural elements. For Cd we have used a thin foil of density ρs = 94.8 mg cm-2. For In we have used a thin foil of density ρs = 91.5 mg cm-2. Each target has been irradiated separately with 7.7 MeV electrons produced by a linac. The mean incident electron current was I = 8 μA, and consisted of τe = 3.5 μs pulses at a repetition rate of fr = 100 Hz. The cross-sectional area of the incident electron beam was S = 9.6 x 10 -2 cm2. The electron irradiation took place in air. The targets were cooled in a flow of air. The isomeric yield has been determined by gamma-ray spectroscopy, using a Ge detector. The cross sections for the electron activation of the isomeric states were 1.2 μb for 111m Cd, 5.5 μb for 113m In and 7.0 μb for 115m In. The observed mean activation rates R were 6.2 x 10 -16 s-1 for 111m Cd, 2.8 x 10-15 s-1 for 113m In and 3.6 x 10-15 s-1 for 115m In. The peak activation rate Rp, extant during the time τe, when the electrons are effectively exciting the target nuclei, is Rp = R/(τefr). The peak activation rates were 1.8 x 10-12 s-1 for 111m Cd, 8.1 x 10 -12 s-1 for 113m In and 1.0 x 10-11 s-1 for 115m In, for a peak power of the electron beam of 1.8 x 106 W cm-2. (authors)