The decay of Tb 166 has been studied following its production by the Tb 159 (Y,3w)Tb 156 reaction with 64-Mev bremsstrahlung. In the 5.6-day activity, 18 gamma transitions were detected with energies between 89 kev and 2060 kev. The K x-radiation of Gd was associated with this activity, while no Dy radiation was found. No annihilation radiation was detected. The gamma spectrum was observed in coincidence with the Gd x-ray and found to be closely similar to that observed alone. No evidence of /3-decay was found and it is concluded that the observed decay is by orbital electron capture. A relatively weak 5.5-hr activity, ascribable to Tb 156w , was also produced by the (y,3n) reaction.
The design and construction of a cylindrical proportional counter 36 in. long, 12 in. i.d. is described, and its performance as a spectrometer for studying soft radiations from radioactive samples of low specific activity is given. The counter tank is operated in an axial magnetic field of 2500 gauss to constrain beta rays to helical paths. A line width of 3.4% (ΔE/E) is obtained for 625-kev electrons from a Cs137 source mounted on a thin film placed within the tank. Simple theory would predict 2.2%. Beta spectra are readily obtained with sources having total activities less than 10 mμC.
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