CIV. The photo-production of an isomeric state in207Pb

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Abstract
The 0·8 second activity produced in Pb by irradiation in the x-ray beam of a 23 Mev synchrotron was shown to arise from a disintegration of 208Pb by comparing the effects produced in a target of ‘natural’ Pb and a similar target of Pb enriched in the 208 isotope. The γ-rays emitted by the short-lived activity were investigated with a scintillation spectrometer. The resulting spectrum, and the half-life of the activity agree with the known spectrum and half-life associated with the i13/2 isomeric state in 207Pb. The reaction was thus determined to be 208Pb(γ, n)207*Pb. An estimate of the threshold for the production of the isomer was made by comparing, at different settings of the maximum x-ray energy from the machine, the intensity of the isomeric activity and the activity produced in Ag. The value thus obtained was 9.0±0−1 Mev. The threshold calculated from the ground state mass differences, the energy of the isomeric level and the centrifugal barrier corresponding to the orbital angular momentum of the emitted neutron is shown to be significantly greater than 9.0 Mev. It is suggested that the isomeric state is reached, not directly, but. through a level of spin ll/2 lying slightly above the isomeric level.

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