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ATOMIC ENERGY COMMISSION
"L-CONVERTED ISOMERIC TRANSITION"
C. O. Muehlhause
S. J. Turkel
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Document Declassified: 1/23/47.
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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 believed to be / -rays and on
that basis an atomic activation cross-section of 2 barns was ascribed to this activity by
Seen and his collaborators. By studying the absorption in Be and Al of the radiations de-
theted with an end window counter we found, however, that they were largely x-rays of the
L region of Ir. The main L emission lines of 77Ir and its neighbors 760s 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. /8-emission from Ir followed by a strongly internally converted f-ray. (L radia-
tion of Pt).
Number 3 could be excluded as no sufficiently high energy /-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 Lm MV and LIII MIV components of Ir. These compon-
ents were then shown to be strongly absorbed in Cu indicating Ir L x-rays arising from the
removal of an Lm 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 f-rays, and from absorp-
tion in Pb and other elements an approximate energy of 60 Key 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 Co60 (10.7 min). In this man-
ner a value of 47.0 Key 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 Key as the
energy of the isomeric transition. This is in good agreement with the results of Pb absorp-
tion. Softer photons of approximately 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 FLORIDA
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For the heavier elements where the work function for the K-shell is of the ordqerof
100 Kev, L-converted isorneric transitions may be relatively frequent. A system..tilsear.a
for such transitions would seem worthwhile.- Also an investigation of the relativE dhlbi-,
lities of LI, L3I, and Lm conversions might prove of value for the theory of inWi bIniMei
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