ITR-20
SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY
INTRALABORATORY TECHNICAL REPORT
INTERPRETATION OF NUCLEAR DECAY SCHEME OF TUNGSTEN-181
Leslie M. Dunn
Southwestern Radiological Health Laboratory
P. 0. Box 15027
Las Vegas, Nevada 89114
December, 1969
U. S. Department of Health, Education, and Welfare
Public Health Service
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ABSTRACT
This report was written to clarify the Interpretation
of the Nuclear Decay Scheme of Tungsten-181. The
major confusions concerned the x-ray branching ratio,
and the K shell photon yield.
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Interpretation of Nuclear Decay Scheme of Tungsten-181
181 181
Tungsten-181 ( W) decays to Tantalum-181 ( Ta) by 100 percent electron
181
capture. Sixty-five percent of the time W goes directly to the ground
181
state of Ta by electron capture, thirty-five percent of the time it
goes by electron capture to 0.0063 MeV energy level and emits a 0.0063
181
MeV gamma ray to go to the ground state level of Ta.
0.0063 MeV
140 d
0
181
The x-rays that are used to identify and quantitate the W are the
K , K , K , and K of Tantalum-181 which is the daughter of Tungsten-181.
Values for these constants appear in Table 1:
TABLE I
K K K K
Element 2 1 |_1 ^2
Ta 56.28 57.54 65.2 67.0
(54) (100) (33) (8)
The numbers without the parenthesis represent the x-ray energies in KeV,
numbers in the parentheses are the relative intensities normalized to 100
for K-, x-ray, these x-rays can be separated by instruments with high
1
resolving power.
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-2-
A value for the electron capture ratio for the (K + L + M + N + ...)
1
shell electrons is given as:
(M + N +...)+ L + K = 100
(M + N + ...) = 0.25 L
L = 0.36 K
0.25 (0.36 K) + 0.36 K + K = 100
EC (K) = 68.97/100 dis
Therefore, there are 68.97 electron captures of K shell electrons per 100
2
disintegrations. With a fluorescent yield (W ) of 0.94, it is expected
A
that 64.8 shell photons would escape 100 disintegrations from this
radioisotope.
EC(K)'W = K shell photons
K
68.9 (.94) = 64.8 photon/100 dis
If one is using a low resolution detector such as a scintillator, the
value of 69.1 photons per 100 disintegration should be used since these
detectors cannot resolve a difference in photon energies of 10 KeV. When
a high resolution detector is used, which can resolve (K + K ) from
J. &
(K_ + KQ )* a correction in the photon yield will be necessary. The
Pl P2
total relative yield for the four x-rays is 195 see Table 1 and the K
1. "Table of Isotopes" by C.M. Lederer, J.M. Hollander, and I. Perlman
p. 352 and Figure 4 on page 576.
2. Fraction of K-vacancies which give rise to K x-rays.
* The energies of the complex Kft and K lives are the approximate
Pl ^2
weighted averages of the components.
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-3-
and K relative yield is 154/195 = 0.79. Applying this factor to the
total K shell yield we get an absolute K + K yield of 51.2 photon
1 a2
per 100 disintegrations:
(0.79)(64.8) = 51.2 (K + K ) photons/ 100 dis
181
In summary use of a K shell photon yield for W of:
1. 64.8 photons/100 dis for low resolution detectors.
2. 51.2 photons/100 dis for high resolution detectors when
using (K + K ) energy lines.
*"*^ Ctrt
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