βSpectrum ofI129and Its Decay Scheme

The decay of ${\mathrm{I}}^{129}$ has been studied with several instruments including a magnetic lens spectrometer, a scintillation spectrometer utilizing a technique in which the activity is introduced into a NaI(Tl) crystal, and a proportional counter. A soft beta radiation whose end-point value is measured to be 150±5 kev and a gamma radiation of 38 kev are observed. The shape of the beta spectrum is studied and the $K$-conversion coefficient and the $\frac{K}{L}$ conversion ratio for the 38-kev gamma radiation are determined. The beta and gamma radiations are shown to be in coincidence and an upper limit is set for a possible beta branch to the ground state of ${\mathrm{Xe}}^{129}$. Spin assignments are made which agree with shell model theories of the nucleus and with data available on the decay of the 8-day isomer of ${\mathrm{Xe}}^{129}$.