SHORT COMMUNICATION
Journal of Radioanalytical and Nuclear Chemistry, Vol. 246, No. 1 (2000) 229–231
Precision measurements of the half-lives of some
electron-capture decay nuclides: Be, Mn, Rb, and Rb
7
54
83
84
C. A. Huh, L. G. Liu
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, R.O.C.
(Received November 15, 1999)
The disintegration of four radionuclides undergoing electron-capture decay was followed for one to ten half-lives. Least-squares analyses of the
gamma-ray counting data using non-linear Taylor differential correction yielded the following half-lives: 7Be, 53.42±0.01 days; 54Mn, 312.6±0.5
days; 83Rb, 86.2±0.1 days and 84Rb, 33.1±0.1 days. These values are consistent with literature data, with comparable or much better precision
than most previous determinations.
Introduction
Recently, in an attempt to determine the dependence
of half-lives on pressure and chemical forms of nuclides
undergoing electron capture decay, we have conducted a
series of experiments.1,2 We began by focussing on the
lightest electron-capture decay nuclide, 7Be, which was
expected to be more sensitive to the physico-chemical
effects, followed by 83Rb to seek out the dependence on
nuclide size. The 7Be source (1.1 mCi as of November 9,
1998) was obtained from Brookhaven National
Laboratory (S/M No. 054911) and the 83Rb source
(1.2 mCi as of June 7, 1999), from Los Alamos National
Laboratory (S/M No. 338920). At the beginning of our
experiments, the 7Be source contained 0.28 µCi of 54Mn
and the 83Rb source, 24 µCi of 84Rb. Thus, the decay of
all four nuclides could be monitored concurrently. It is
worthwhile noting that 54Mn and 84Rb also decay by
electron capture. Although activities of these two extra
nuclides were too low to meet the original objective of
our experiments, they were adequate for the
determination of decay rates at precision comparable to
or much better than those reported previously.
While the effects of pressure and chemical forms on
decay rate have been observed for 7Be,1,2 they cannot be
discerned for other nuclides. For 7Be, only the decay rate
in the form of Be(OH)2 at normal pressure will be given
here. The purpose of this paper is to recommend that the
results summarized below be considered as most
probable decay rates or half-lives of these nuclides.
Experimental
Both 7Be and 83Rb sources were received in carrierfree form in 1 ml of 0.1N HCl. The 7Be counting source
were prepared by transferring 100 µl of the stock
0236–5731/2000/USD 17.00
© 2000 Akadémiai Kiadó, Budapest
solution to a Kimax tube, followed by 5 mg of 9Be
(initially dissolved in 1 ml of 1N HCl) as the Be carrier,
and then NH4OH to form Be(OH)2 precipitate. After
centrifugation to separate the precipitate from the
solution, the supernatant water was removed and the tube
was ready for counting. The 83Rb counting source was
prepared by adding and dissolving 1 mg of 87RbCl in a
Kimax tube containing 100 µl of the 83Rb stock solution.
After evaporating the solution to dryness under a heat
lamp, the tube was ready for counting.
Gamma-counting of the sources was performed using
a HPGe well detector (EG&G Ortec GWL-100230)
interfaced with a digital γ-ray spectrometer (EG&G
Ortec DSPecTM). The counting system provides
extended live-time correction according to the GEDCKEHALE method3 and automatically sets threshold to reject
pile-up of pulse-pair at 0.5 µs resolution. The detector
has an active volume of 100 cm3, a well diameter of
1.45 cm and a well depth of 4 cm. It offers a counting
efficiency of 100% (relative to 3"×3" NaI) and a
resolution of from 1.45 keV (FWHM) for the 7Be peak
at 477.56 keV to 1.70 keV for the 84Rb peak at
881.6 keV. With the counting tubes placed in the
detector well, the sources were very close to the bottom
of the well, providing absolute counting efficiencies of
~21%, 17%, 9% and 8% for the single or the most
dominant photon peaks of 7Be, 54Mn, 83Rb, and 84Rb,
respectively. Figures 1 and 2 are typical spectra for the
7Be/54Mn and 83Rb/84Rb sources.
Activities of all four nuclides were assayed as
frequent as reasonably achievable for up to one year.
The stability of the system was ensured by weekly
calibration of the detector using a uraninite standard
solution and a 152Eu source. Throughout the course of
this experiment there were no signs of any changes in
energy, resolution and efficiency.
Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht
C. A. HUH, L. G. LIU: PRECISION MEASUREMENTS OF THE HALF-LIVES OF SOME ELECTRON-CAPTURE DECAY NUCLIDES
Results and discussion
The exponential decreases of 7Be, 54Mn, 83Rb and
84Rb activities (At) with time in the sources are shown in
Fig. 3. To simplify the plots, At is normalized against A0
(activity at t = 0). Since the measured activity is the
Gamma-ray spectrum of the Be(OH)2 source showing the
photon peaks of 7Be (477.56 keV) and 54Mn (834.81 keV)
Fig. 1.
“mean” activity in each counting interval and
exponential decrease occurs within the interval, the
“effective time” corresponding to the measured activity
should be earlier than the mid-point of each counting
interval. The regression line should therefore fit the
effective time instead of the mid-point of each counting
interval. In fitting the At/A0 data series, a non-linear
Taylor differential correction method4 based on least
squares was used. The effective time, the decay rate, and
their error limits can be simultaneously calculated by
computer iteration. This yields decay constants (±1σ
uncertainties) of 0.0129765±0.0000014, 0.0022174±
0.0000035,
0.008040±0.000009,
and
0.02111±
0.00006 d–1 (or half-lives of 53.42±0.01, 312.6±0.5,
86.2±0.1 and 33.1±0.1 d) for 7Be, 54Mn, 83Rb, and
84Rb, respectively. As a measure of evaluating the
accuracy and precision of these values, they are plotted
on Fig. 4 along with data published in the literature.
Note that error bars of earlier values do not always
overlap while our data compare more favorably with
those having better precision. We would therefore
recommend that the results reported here be included in
the nuclear database and considered as most accurate
and precise decay rates or half-lives for these nuclides.
Gamma-ray spectrum of the RbCl source showing the photon
peaks of 83Rb (at 520.4, 529.5, 552.5 and 788.9 keV) and 84Rb
(881.6 keV)
Fig. 2.
Fig. 3.
230
Exponential decay and the derived half-lives of 7Be, 54Mn, 83Rb and 84Rb
C. A. HUH, L. G. LIU: PRECISION MEASUREMENTS OF THE HALF-LIVES OF SOME ELECTRON-CAPTURE DECAY NUCLIDES
Fig. 4.
Comparison of half-lives determined from this study (n) with previously reported data5 (l)
References
1. C. A. HUH, Earth Planet. Sci. Lett., 171 (1999) 325.
2. L. G. LIU, C. A. HUH, Earth Planet. Sci. Lett., to be published.
3. R. JENKINS, R. W. GOULD, D. GEDCKE, Quantitative X-ray
Spectrometry, Marcel Dekker, Inc., New York, 1981, p. 266.
4. T. R. MCCALLA, Introduction to Numerical Methods and
FORTRAN Programming, New York, John Wiley & Sons, 1967,
p. 349.
5. C. M. LEDERER, V. S. SHIRLEY, Table of Isotopes, 7th ed.,
Wiley-Interscience, New York, 1978, p. 1523 and references
therein.
231