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S. Afr. J. Agric. Sci. (1964), 7, 881-884
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RESEARCH NOTE
•
HALF-LIFE OF CHROMIUM-51
•
P. G. MARAIS, F. I. HAASBROEK AND I. H. M. KARSTEN, Fruit and Food
Technology Research Institute, Stellenbosch
Chromium-51 is extremely useful in animal studies for labelling red blood cells to
measure the circulating red cell volume, as shown by Gray & Sterling (1950). A
further use in this field is discussed by Koch-Weser, MacIntyre & Shapiro (1963),
and it was also used in tracing ground water (Lacey & Laguna, 1956). Because it
has a convenient half-life and decays with the emission of gamma rays (0'32 MeV),
chromium-51 is used in this laboratory, together with several other isotopes, for the
calibration of gamma-spectrometers, the determination of the efficiency of scintillation
counters and in studies on the absorption of gamma rays by various materials. The
same source is used for long periods and to avoid errors resulting from corrections
that are applied for radioactive decay, it is essential that its half-life should be known
accurately.
The half-life of chromium-51 was apparently first determined by Walke, Thompson & Holt (1940) who obtained a value of 26'5±1'0 days. Despite its great
inaccuracy this value was still used 11 years later by Bruner, King & Smyser (1951)
in calculating decay correction factors, and even 16 years later by Lacey & Laguna
(1956). Slightly more accurate values were later obtained by Lyon (1952), Kafalas &
Irvine (1956) and Schuman, Jones & Mewherter (1956), namely 27·75±O·3 days,
27·9±0·2 days and 27·8±0·1 days, respectively. These values are still relatively
inaccurate but are used today and the half-lives generally quoted are 27·9 days
(Fasoli, Manduchi & Zannoni, 1962) or 27· 8 days (Stehn, 1960; Radiochemical
Centre, 1964). There was therefore a definite need to determine a more accurate
value, and the purpose of this note is to report on the value obtained in this laboratory,
The chromium-51 was obtained from The Radiochemical Centre at Amersham,
England. It was prepared by the reaction 50Cr(n,y)51Cr and was supplied as sodium
chromate in isotonic saline. A small aliquot of the solution was added to water which
was used to moisten a piece of filter paper measuring 5 X 2·5 cm. After drying, the
paper was wrapped round a perspex rod which fitted tightly into a perspex tube.
The latter just fitted into the well of a well-type scintillation counter used for counting
rate determinations. The initial counting rate of the sample was approximately
200,000 cpm.
The scintillation counter was used under plateau conditions with the integral
counting rate being recorded. Excellent plateaux were obtained with the background,
chromium-51 and uranium standard, with slopes of less than 2 per cent/IOO volts
change in high voltage and the same voltage could therefore be used throughout the
experiment.
Received on 30 July, 1964, for publication
881
HALF-LIFE OF CHROMIUM-51
5·5
>c
·E
-~
c
'"
0
u
c
Reproduced by Sabinet Gateway under licence granted by the Publisher ( dated 2012)
...cu
...'"
4·5
bO
C
·OJ
C
'0"
~
bO
0
-J
4·0
Regression equation:
=
y = -(0·0108,670 ±0·0000,022)X +(5-306) ± 00011)
)·5
o
so
100
Time in days: X
FIG.
I.-Decay of chromium-51 as measured with a well-type scintillation counter
882
ISO
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P. G. MARAIS, F. J. HAASBROEK & J. H. M. KARSTEN
The decay of the chromium-51 was followed for 165 days and 127 readings were
recorded. On each occasion a sufficient number of pulses were recorded to give a
standard deviation of not more than O' 5 per cent. Following each determination,
readings were also taken with the uranium standard and in this case the standard
deviation was O' 1 per cent. After correcting for resolving time losses and background,
all chromium-51 readings were normalized in terms of the standard to correct for
fluctuations in the efficiency of the counter.
There was a highly significant linear relationship between the common logarithm
of the corrected counting rate (Y) and the decay period in days (X), as may be seen
from the results which are presented graphically in Fig. 1. The regression equation,
together with the standard error of the regression coefficient, is also given in Fig. 1.
Using the regression coefficient (b) the half-life of chromium-51 was calculated from
the relationship T==:log102/b. The limits of error of the half-life were found by using
the limiting values of b in the regression eq uation. Thus the half-life of chromium-51,
as obtained in the present study, was 27· 701±0 ·006 days.
It is of interest to point out that the half-life as calculated from the results prior to
normalizing was 27· 663±0' 005 days. This is due to the fact that there was a slight
but progressive decrease in the efficiency of the counter during the experiment as
reflected by the standard readings. However, this decrease was relatively small and
considering the mean values for successive half-life periods, the standard readings
changed only from 87,491 to 87,077 cpm. The half-life obtained with the normalized data thus exceeded that obtained with the data prior to normalizing by only
0·14 per cent. In normalizing the data the assumption is of course made that changes
in the efficiency of the counter would be similar for chromium-51 and uranium.
However, since it was operated under plateau conditions there is no reason to reject
this assumption. The actual discrepancy in half-lives noted is also of no practical
significance.
These results indicate that the half-life of chromium-51 should be taken as 27· 7
days in preference to the widely used value of 27 . 8 days.
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to Professor R. I. Nel, Chief of the
Fruit and Food Technology Research Institute, for his interest in this work, as well as
to Mr. C. F. G. Heyns for technical assistance .
.REFERENCES
BRUNER, H. D., KING, E. R. & SMYSER, M. P., 1951. Tables to correct for physical decay of
some frequently used isotopes. USAEC, Tech. Inform. Service, Oak Ridge.
FASOLI, U., MANDUCHI, C. & ZANNONI, G., 1962. Measurement of the L/K- capture
ratio in 51Cr decay. Nuovo Cimento 23, 1126-1128.
GRAY, S. J. & STERLING, K., 1950. Determination of circulating red cell volume by radioactive chromium. Science 112, 179-180.
KAFALAS, P. & IRVINE, J. W. Jr., 1956. Nuclear excitation functions and thick target yields:
(Cr+d). Phys. Rev. 104, 703-705.
883
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HALF-LIFE OF CHROMIUM-51
KOCH-WESER, D., MACINTYRE, W. J. & SHAPIRO, R. L., 1963. Measurements of the removal
of Cr 5 '-labelled endotoxin from the circulation in experimental animals. Int. J. Appl. Rad.
Isotopes 14, 75-80
LACEY, W. J. & LAGUNA, W. DE., 1956. Method of preparing radioactive cations for tracing
ground water. Science 124, 402.
LYON, W. S., 1952. Disintegration of Cr5'. Phys. Rev. 87, 1126.
RADIOCHEMICAL CENTRE, 1964. Radioisotopes and labelled compounds. Cat., R.C.C.,
Amersham, England.
.
SCHUMAN, R. P., JONES, M. E. & MEWHERTER, A. C., 1956. Half-Jives of Ce"', C 0 5., Cr 51 ,
Fe 55 , Mn 5., Pm"', Ru'os, and Sc· s. J. Inorg. Nucl. Chern. 3, 160-163.
STEHN, J. F., 1960. Table of radioactive nuclides. Nucleonics 18 (11),186-195.
WALKE, H., THOMPSON, F. C. & HOLT, J., 1940. K-electron capture and internal conversion
in Cr 5 '. Phys. Rev. 57,171-176.
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