/77f aoJun'Aeii ' MDDC 774 UNITED STATES ATOMIC ENERGY COMMISSION OAK RIDGE TENNESSEE "L-CONVERTED ISOMERIC TRANSITION' by M. Goldhaber CO. Muehlhause S. J. Turkel Published for use within the Atomic Energy Commission. Inquiries for additional copies and any questions regarding reproduction by recipients of this document may be referred to the Documents Distribution Sub- section, Publication Section, Technical Information Branch, Atomic Energy Commission, P. O. Box E, Oak Ridge, Tennessee. Inasmuch as a declassified document may differ materially from the original classified document by reason of deletions necessary to accomp- lish declassification, this copy does not constitute authority for declass- ification of classified copies of a similar document which may bear the same title and authors. Document Declassified: 1/23/47. This document consists of 2 pages. i RY MDDC 774 - 1 - L-CONVERTED ISOMERIC TRANSITION M. Goldhaber, C. O. Muehlhause, S. J. Turkel The Argonne Laboratory, University of Chicago Preliminary to an investigation of the activation of Ir by resonance neutrons, it seemed of interest to study the radiations associated with the 1.5 min Ir activity produced by slow neutron capture in Ir. These radiations had been previously beUeved to be /S -rays and on that basis an atomic activation cross-section of 2 barns was ascribed to this activity by Seren and his collaborators. By studying the absorption in Be and Al of the radiations de- tected with an end window counter we foxmd, however, that they were largely x-rays of the L region of Ir. The main L emission lines of ^^Ir and its neighbors ygOs and 78Pt are shown in the accompanying chart, together with the K absorption edges of the elements from Fe - Se. There are three possible ways to account for L radiation: 1. L electron capture in Ir leading to Os (L radiation of Os). 2. Internally converted isomeric transition in Ir (L radiation of Ir). 3. /^ -emission from Ir followed by a strongly internally converted •^-ray. (L radia- tion of Pt) . Number 3 could be excluded as no sufficiently high energy /J-rays to be compatible with the short half life of 1.5 min could be found. To distinguish between number 1 and number 2, critical absorption measurements were made on the x-rays. Because of the wide spread In wave-lengths of the L radiations, it was important to isolate some of the L components for this work. This was accomplished by using a Zn filter (23.5 mg/cm2) which practically completely removes all but the LUI - MV and Lin - MIV components of Ir. These compon- ents were then shown to be strongly absorbed in Cu indicating Ir L x-rays arising from thei removal of an Lin electron. To further exclude Os L x-rays the radiation filtered by 28.2 mg/cm2 was shown to be about equally well absorbed by further layers of Cu. Ni, or Fe. A few percent of the isomeric transitions are unconverted ^-rays, and from absorp- tion in Pb and other elements an approximate energy of 60 Kev was obtained. The internal conversion electrons were detected with an ionization chamber having a window equivalent to 1.5 gm Al/cm2 and their energy compared with those from Co"" (10.7 min). In this man- ner a value of 47.0 Kev was found for the internally converted electrons of Ir. When the value of the Ir L work function, 11.3 Kev, was added to this we obtained 58.3 Kev as the energy of the isomeric transition. This is in good agreement with the results of Pb absorp- tion. Softer photons of approxinaately half the energy of this transition have also been detected. They are possibly due to the emission of two photons in some of the transitions. The atomic activation cross-section of Ir (1.5 min) can be estimated to be of the order of 100 barns. Unpublished work at Argonne and at the University of Illinois makes it prob- able that the 1.5 min Ir activity is due to a metastable state of Irl92. UNIVERSITY OF H ORIDA 3 1262 08910 5224 MDDC 774 -2- For the heavier elements where the work function for the K-shell is of the order of 100 Kev, L-converted isomeric transitions may be relatively frequent. A systematic search for such transitions would seem worthwhile. Also an investigation of the relative probabi- lities of LI, Ln, and LHI conversions might prove of value for the theory of nuclear isomei'- ism.