The disintegration of I 124 (4.2 days) has been studied with the help of magnetic spectrometers and scintillation spectrometers. The disintegration occurs 71% by electron capture and 29% by positron emission. Three positron groups were found having end-point energies of 2130 (46.0%), 1531 (46.4%), and 786 (7.5%) kev. The most energetic positron group has a shape characteristic of A/= ±2, yes. Positron-gamma coincidence experiments show that this group goes to the ground state. Gamma rays of energies 2700, 2300, 2100, 1700, 1520, 1350, 723, and 603 kev together with annihilation radiation and Te K x-rays have been found and the relative intensities measured. A disintegration scheme, consistent with the levels of Te 124 as determined from the decay of Sb 124 , has been established. No beta rays were found, showing that a transition to Xe 124 is highly improbable. The former work on I 123 (13.5 hours) has been substantiated and, in addition, it seems highly unlikely that any positrons are emitted from I 123 .nor the 0.723-Mev gamma-ray energy. The scheme was drawn up showing the most energetic group going to the first excited state of Te 124 .In the interim, Stevenson and Deutsch 2 looked for coincidences between the high-energy positron group and gamma rays and could find none, implying that the highest energy group goes to the ground state. Marquez and Perlman 3 found that I 124 decayed 70% by electron capture and 30% by positron emission. Recently, the spin 4 of I 124 has been found to be 2. This makes it appear very likely that the highest energy positron group goes to the ground state. Finally, some difficulties were found in standardizing I 124 for therapeutic 5 use on the basis of the scheme given in I. In view of these facts, and since techniques in nuclear spectroscopy have improved greatly in the last ten years, it was decided to reinvestigate the disintegration of I 124 .
II. Preparation of Sources; ApparatusSources were prepared by bombarding separated Sb 121 (99.4%), as the metal, with 23 Mev alpha particles in the cyclotron. A small amount of KI, of known iodide concentration, was added to the active material to act as carrier. Nitric acid was added to the mixture and the iodine was distilled over. The iodine was purified by repeated oxidations and reductions and extractions into CCU and water. The purified source was layed down as Nal on thin Zapon and covered by a thin (< 10 microgram/cm 2 ) Zapon film.The particle spectrum-positrons and internal conversion electrons-was measured with the help of a magnetic lens spectrometer and a 180° magnetic spectrometer. The gamma-ray measurements were carried out with a scintillation spectrometer using either a 1-in. by 1-in. or a 3-in. by 3-in. Nal(Tl) crystal. The scintillation spectrometer had previously been calibrated for 2 D. T. Stevenson and M. Deutsch, Phys.
