Observation of Recoilless Emission ofNi61Gamma Rays Following Coulomb Excitation

Abstract

We have observed the recoilless emission of gamma rays from 61 Ni after the 67.4-keV energy level of this nucleus was populated by Coulomb excitation. The projectiles used for the excitation were 25-MeV oxygen ions from the Oak Ridge National Laboratory tandem Van de Graaff. The total MSssbauer absorption observed by this method for 61 Ni agrees well with previous measurements 1 using the radioactive source x Co 99 min *Ni*.The metallic nickel targets used for the present experiment were irradiated for periods up to 120 hours. These targets were held at a temperature of ~78°K during the entire irradiation period. There were no changes in the MSssbauer spectra observed during these measurements which could be attributed to radiation damage in the target. The maximum absorption, which occurs at zero velocity, remained constant throughout our longest measurement time which was 120 hours. Previous studies 2 of the MSssbauer effect following Coulomb excitation were done using 57 Fe for the target and absorber, and to within experimental accuracy of the measurements there was no effect observed.The observation of the M5ssbauer effect in these experiments can be summarized in the following scheme: (1) The relevant energy level, in this case the 67.4-keV level of 61 Ni, is populated by Coulomb excitation. (2) The Coulomb-excited nucleus which recoils during the interaction must dissipate its kinetic energy and come to rest in the crystal lattice in a time short compared to the lifetime of the excited level.(3) The gamma ray must be emitted from the target nucleus without recoil and has to be absorbed recoil free in an absorber which contains 61 Ni nuclei. (4) The transmission of these gamma rays through the absorber is measured as a function of the relative velocity between target and absorber.The criteria for the selection of a nucleus in the present experiment were these: (1) TheMSssbauer effect for the nucleus should have been observed previously such that the important parameters of the spectrum are known.(2) The target material should be metallic because we expected that a metallic crystal would return to thermal equilibrium in a time short compared to the lifetime of the excited state of the nucleus after the passage of the projectile through the target. This may not be true for insulators. (3) The MQssbauer effect should be measurable at or above liquid nitrogen temperature. (4) The gamma intensity of the Coulomb-excited Mo'ssbauer level must be high compared with the background contribution 3 at the gamma energy of interest from the following processes: Coulomb excitation populates not only the level of interest but also other lowenergy levels of the target nucleus. In addition, the passage of the ions through the target produces a high intensity of characteristic x rays. Bremsstrahlung from the projectile is not negligible in some cases. Of the possible candidates we have chosen 61 Ni because this nucleus satisfied all four conditions listed. Figure 1 shows schematically the arrangement of our apparatus. The 16 ...

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