The beta spectrum of 4.2-min ${\mathrm{Tl}}^{206}$ was investigated in a $4\pi$ scintillation spectrometer. The source was in secular equilibrium with the 2.6×${10}^6$ year ${\mathrm{Bi}}^{210}$ parent. The electron distribution was observed to have a nonstatistical form which could be fitted with a once forbidden pseudovector shape factor. No necessity for the inclusion of any pseudoscalar contribution was observed. The energy release in the ${\mathrm{Tl}}^{206}$ decay is 1.571±0.010 Mev.

• Received 9 June 1961

DOI:https://doi.org/10.1103/PhysRev.124.519