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Parent Activity Observed threshold Other work* isotope detected (Mev) (Mev) I 127 13 d. 0~ 9.45 ±0.20 Pr"i 3.5 min. /3+ 9.8 ±0.3 9.40±0.1 As" 16 d. /3~, 0+ 10.3 ±0.2 Zr»o 4.5 m. e~ 12.1 ±0.1 \ Average 78 h. 0+ 11.9 ±0.3/12.0±0.2 12.48±0.15 Ni 6 * 36 h. £+ 11.7 ±0.2 * Hanson, Duffield, Knight, Diven, and Palevsky, Phys. Rev. 76, 578 (1949).The results are shown in Table I. Zirconium 90 decays either by 78-hour positron emission, or by 4.5-minute internal conversion or iiC-capture. The apparent threshold for the photo-disintegration of Zr 90 , as determined by observation of the 78-hour period, was slightly less than that determined by observation of the 4.5minute period. If the 4.5-minute period represents the decay by isomeric transition into the 78-hour isotope, one would expect the threshold observed by means of the 78-hour period to be somewhat less than that observed by the 4.5-minute period. However, since the errors quoted for the thresholds overlap, it is probably more logical to take 12.0=1=0.2 Mev as a mean value for the gamma-w threshold of Zr 90 .The authors would like to express their appreciation to Mr. Robert D. England for his very considerable help in keeping the betatron and the associated circuits running during the course of this work.