0 20 40 60 80 100 t20 MIN. FIG. 2. Bombardment of indium with fast neutrons.its excitation curve is almost identical with that of the 66-min. In 110 , produced by the Ag 107 (a, n)ln 110 reaction, the 23-min. period must be assigned to mass number 112 as the product of the Ag 109 (a, w)In 112 reaction. Further proof of this assignment is the fact that the 23-min. isotope decays chiefly by negative electron emission to a Sn isotope; the stable Sn isotope of lowest mass number is Sn 112 . It is seen from its excitation curve ( Fig. 1) that the 2.7day period is the product of an (a, In) reaction with a threshold of 15.5±0.5 Mev and must be assigned to In 111 . Lawson and Cork 2 produced a weak 2.7-day activity with fast (Li+d) neutrons and assigned this period of In 112 as the product of an (n, In) reaction. From our result we must conclude that this activity was the product of the In 113 (w, 3n) reaction. The 2.7-day In 111 decays by electron capture to an excited state of Cd 111 , which goes into the ground state by successive emission of two gammaquanta. 2 -3The 23-min. period of In 112 is followed by negative and positive electrons, both In and Cd x-rays, and the conversion electrons of a highly converted 0.16-Mev 7-radiation. While the Cd irradiation accompanies the iC-capture of In 112 , the emission of In irradiation can only be due to internal X-conversion of the 0.16-Mev 7-ray. This converted 7-ray hence is emitted in the transition between two isomeric states of In 112 . Since both the conversion electrons and the In iC-x-rays follow the 23-min. period from the beginning, this period is the half-life of the upper, metastable level of In 112 . The decay curve for the negative and positive electrons follows the 23-min. period only after a certain time; this curve, first recognized by R. N. Smith, 4 as such, is a growth curve, showing that a 9-min. activity grows from the 23-min. activity. 5 Figure 2 shows the decay curve of the positrons from a sample produced by the ln 113 0, 2n)ln m reaction with (Li-flO Mev ZV) neutrons.The deviation of the growth curve from the asymptotic 23-min. period during the time immediately following bombardment depends on the relative probabilities of excitation of the isomeric states by the nuclear reaction involved. It is largest for samples produced by (7, n) Q and (n, 2tt) reactions, smaller for those from the Ag 109 («, n) reaction, and not noticeable for those from the Cd m (d, w)In 112 reaction.The In 112 isomeric transition belongs in the Z = 4 group. Both the j8~ and the /3 + transitions from the ground state to the even, even-nuclei Sn 112 and Cd 112 are allowed; the spin quantum number of the lower level is hence 0 or 1. . 5 Analysis of the growth curve into exponentials led Smith to assume periods of 16.5 ±2 min. and 17.5 ±2 min. for the In 112 isomers in order to obtain the best fit. The growth curve can be reproduced, over a considerable interval, in this way; the decay curve for the conversion electrons and x-rays, however, shows that the half-life of the upper lev...
