Journal of Nuclear and Radiochemical Sciences, Vol. 4, No. 1, pp. 5-7, 2003

ArticlesHalf-Life of Samarium-147
Norikazu Kinoshita,*, a Akihiko Yokoyama,b and Takashi Nakanishib
aGraduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan
Department of Chemistry, Faculty of Science, Kanazawa University, Kanazawa 920-1192, Japanb

Received: April 16, 2003; In Final Form: June 2, 2003
The alpha-decay half-life of 147Sm has been reevaluated. Known amounts of natural Sm and an alpha-emitter standard (210Po, 238U, or 241Am) were mixed well to prepare thin sources for the simultaneous counting of 147Sm and thealpha-emitter standard by means of an alpha-spectrometer using a silicon surface barrier detector. The alpha-disintegration rate of known amounts of 147Sm was determined by reference to the alpha activity of the standard. The
source preparation and counting were repeated to establish the reproducibility of the present half-life determination, and supplementary alpha spectrometry was carried out by a liquid-scintillation spectrometer. The arithmetic
mean of the experimental half-life values was obtained to be (1.17 ± 0.02) × 1011 y. This value is about 10%
longer than the currently adopted value, (1.06 ± 0.02) × 1011 y, and the possible factors for this difference are discussed.

Introduction
Samarium-147 is a naturally occurring long-lived alphaemitter (natural isotopic abundance: 14.99%; currently adopted
half-life: 1.06 × 1011 y;1 alpha-ray energy: 2.235 MeV). The
nuclide has recently been proved to be useful as an internalreference in the determination of alpha activity of mBq levelby alpha-spectrometric technique named Sm-method.2−5
The present authors are searching for 244Pu and 146Sm, now
extinct in the solar system, in deep-sea sediment to obtainevidence for injection of materials from a hypothetical nearEarth supernova.6, 7 In the research project searching for theextinct nuclides in deep-sea sediment, it has been planned toreevaluate the half-life of 146Sm (an alpha emitter; currently
adopted half-life: 1.03 × 108 y;8 alpha-ray energy: 2.455 MeV)
by reference to the half-life of 147Sm, because there is large
scatter in the literature values on the half-life of 146Sm.8−10 The
currently adopted half-life of 147Sm was determined in 1970 tobe (1.06 ± 0.02) × 1011 y,1 whereas the half-life values reported
from 1954 to 1970 showed some scatter between 1.04 × 1011 y
and 1.28 × 1011 y.1, 11−15 Hence, it is worthwhile to reevaluate
the half-life of 147Sm by means of a developed alpha-spectrometer with the counting sources prepared in a manner different
from those adopted in the earlier works.
Experimental
Materials. Each of two kinds of commercially available
Sm2O3 reagents of 99.9% purity supplied by two makers wasdissolved in conc. HNO3 followed by dilution with distilledwater to prepare known concentration (in µg-Sm/g-solution)of two kinds of Sm standard solutions (hereinafter these aredescribed as Sm-A and Sm-B). In addition, two kinds ofcommercially available Sm standard solutions for atomic
absorption spectrometry supplied by two makers were also
used (hereinafter these are described as Sm-C and Sm-D).
Radioactive and non-radioactive impurities in the Sm solutions
were measured by low-level gamma spectrometry, neutron
activation method, ICP-MS spectrometry, and thermal ionization mass spectrometry, and it was confirmed that the solutions*Corresponding author. E-mail: n-kino@stu.kanazawa-u.ac.jp.
FAX: +81-76-264-5742.

did not contain any detectable impurities and that the isotopic
composition is natural.
Calibrated solutions of alpha-radioactive nuclide were used
as internal standards in preparation of counting sources foralpha spectrometry: 210Pb-210Po supplied by LMRI, France,
natural uranium supplied by NBS, U.S.A., and 241Am suppliedby CERCA, France.
Apparatus. Alpha-spectrometry was carried out using an
EG & G Ortec OctetTM PC alpha-spectrometer equipped with asilicon surface-barrier detector of 450 mm2 for active area. A
counting source was placed in the vacuum (< 4 Pa) chamber ofthe spectrometer with a solid angle of ~20% of 4π. Typicalenergy resolution of the alpha-spectrometer was ~50 keV(FWHM) for alpha-particles from 147Sm, and typical background of the spectrometer was 2~5 cpd in each peak region of147
Sm (2.235 MeV), 210Po (5.034 MeV), 238U (4.198 MeV), and241
Am (5.486 MeV).
For a supplementary alpha-spectrometry of the mixture of
Sm and 241Am, liquid-scintillation spectrometry was carriedout using an Aloka LSC-6100 liquid-scintillation spectrometer.
Procedure. Known aliquot (weight basis) of Sm-A, Sm-B,Sm-C, or Sm-D solution was mixed well with known amount(weight basis) of calibrated 210Po, natural uranium, or 241Am
solution. From the four Sm solutions and the three alphaemitter standards, 10 kinds of combinations of the Sm solutions and the alpha-emitter standards were made. The mixture
was then evaporated on a watch glass (30 mm φ) to preparethin deposit for alpha-spectrometry. The amount of Smmounted on the watch glass was adjusted to ~100 µg (i.e.,~0.012 Bq of 147Sm), and the amount of the alpha-emitter standard was adjusted to ~0.04 Bq.
Alpha spectrometry using a silicon surface-barrier detector
was carried out for 1 to 2 weeks for each of the counting
sources prepared by the method described above, and alphadisintegration rate of the known amount of 147Sm was determined by reference to the alpha-activity of internal standard(210Po, 238U, or 241Am) (Figure 1).
The measurement of alpha-disintegration rate of the known
amount of 147Sm was also carried out concurrently by means ofa liquid-scintillation (LS) spectrometer. For the LS spectrometry only 241 Am was used as the internal standard: known
amounts of the Sm standard solution and 241Am standard solution were taken in an LS vial. The solution in the vial was© 2003 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/11/2003

6Kinoshita

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003
600

Po-210500400300Sm-147200200300

400500600

Channel number

Figure 1. An example of alpha-spectra obtained with alpha-spectrometer using silicon surface barrier detector. This spectrum was obtained
after ~6.6 days counting for a counting source prepared from themixture of ~100 µg of Sm and ~0.04 Bq of 210Po.

1.1

Figure 3. Half-life values and their errors determined with alphaspectrometer using silicon surface barrier detector. The literature
values are indicated by cross symbols, and values obtained in thiswork are indicated by closed symbols. The prepared sources arereferred to as the sample names depicted in the figure with the original
solution name, the reference standard, source number, and themeasurement method.

200Half-life/1011 yAm-241

1.4

Sm-147

1.3
1.2
1.1
1.0-A
(Am)
Sm-1/-A
LS(ACm)
Sm-2/-A
LS(ACmSm)-3-B/LS(ACm)Sm
1/LS-B(ACm)
Sm-2/-B
LS(ACmSm)-3/LS-C(ACm)
Sm
1-C/LS(ACm)Sm-2/-CLS(ACm)
Sm-3/-D
L
SC
(Am)
Sm
1-D/LS(ACm)
Sm-2/-DLS(ACm)
Sm-3/-ALS(ACm)
Sm
1-A/LS(ACm)
Sm
2/L-A
SC
(Am)
Sm-3/-B
LS(ACm)Sm-1/-B
LS(ACm)
Sm
2/LS-B(ACm)Sm-3/-C
LS(ACm)
Sm-1/-C
LS(ACm)Sm
2-C/LS(ACm)Sm-3/-D
LS(ACm)
Sm-1/-DLS(ACm)
Sm
2-D/LS(ACm)
-3/LS
C

100

Sm

Counts per channel

1.2

1.0

100
0100

1.3

4π
counter('54Ion)
LSiz.
C('58Ch)amber('61)
LSC('61)
Ion
LSiz.
C
(' 6Ch4)a
Sm mber-A(Po ('70))-1Sm/SS-A
D(P o)-2Sm/SS-B
D(P o)-1Sm/SS-B
D(P o)-2Sm/SS-C
D(PSm o)-1/SS-A
D(U)-1Sm/SS-A
D(U)-2Sm/SS-B
D(U)-1Sm/SS-C
D(U)-1Sm/SS-CD(USm)
2/S-A(Am) SD
Sm
-1/-A
S
S(ADm)
Sm-2/-A
SS(ADm)Sm-3/-B
SS(ADm)Sm
1-B/SS(ADm)
Sm
2/S
-B
SD(Am)
Sm-3/-C
SS(ADm
Sm)-1-C/SS(ADm)Sm
2/S-DSD(Am)-1/SS
DCounts per channel

Half-life / 1011 y1.4002004006008001000

Figure 4. Half-life values determined with liquid-scintillation spectrometer. The prepared sources are depicted as same as Figure 3.
Amersham ACS II scintillation cocktail (closed symbol) and pterphenyl + POPOP + toluene scintillation cocktail (open symbol)
were used.

Channel number

Figure 2. An example of alpha-spectra obtained with liquid-scintillation spectrometer. This spectrum was obtained after 5 hours counting
for a mixture of ~2 mg of Sm and ~0.8 Bq of 241Am dissolved in theAmersham ACS II scintillation cocktail.

evaporated to dryness on a hot plate (~200 ˚C), and addition of~1 mL of HCl followed by evaporation to dryness wasrepeated three times. To dissolve Sm and Am, 1 mL of glacial
CH3COOH and 1 mL of 2-ethylhexanoic acid were successively added to the vial. The content in the vial was thenheated on a hot plate (~200 ˚C) to evaporate only CH3COOH.
Finally, 10 mL of the Amersham ACS II scintillation cocktail(or p-terphenyl + POPOP + toluene scintillation cocktail) wasadded. The LS counting was continued for 5 hours for each of
the counting sources (Figure 2). A blank sample for background LS measurement was also prepared in the same way as
mentioned above.
Calculation of the Half-Life Value. For the calculation of
the half-life value of 147Sm, it was assumed that Sm and thestandard alpha-emitting nuclide were not fractionatedthroughout the present procedure to prepare the counting
sources.
Based on the data obtained by alpha-spectrometry, alphaactivity of 147Sm in the counting source was determined byreference to the alpha-activity of the internal standard in thesame counting source:
A147 =C147
× ASTDCSTD(1)

where A147 is the alpha-activity of 147Sm, ASTD is the alphaactivity of the alpha-emitter standard, C147/CSTD is the ratio of
the background subtracted peak area of 147Sm to that of thealpha-emitter standard. On the other hand, the alpha-activityof 147Sm is expressed as:
A147 =

ln 2
× N147
T147(2)

where T147 is half-life of 147Sm and N147 is the number of 147Sm
atoms in the counting source. Equation 3 is drawn from eq 1
and eq 2.
T147 =

N147 × ln 2
C147
× ASTD
CSTD(3)

The value of N147 can be calculated from the concentrationof Sm in the Sm-standard solution, weight of Sm standard
solution taken for the preparation of the counting source,Avogadro constant, isotopic abundance of 147Sm in natural Sm,
and atomic weight of Sm. Since it can be assumed that Sm
and alpha-emitter standard do not fractionate throughout thepresent procedure to prepare counting source, N147/ASTD ratio isvirtually invariant. The half-life of 147Sm (T147) can be calculated by substituting the experimental data of N147/ASTD andC147/CSTD for eq 3.
Results and Discussion
A series of half-life values of 147Sm measured by means ofan alpha-spectrometer using a silicon surface barrier detector
are shown in Figure 3 together with literature values.1, 11−15 A
series of half-life values of 147Sm measured with an LS spectrometer are shown in Figure 4. The attached error for eachhalf-life includes nominal systematic errors in the calibrationof the alpha-emitter standard and in the measurement ofisotopic abundance of 147Sm in addition to the statistical errorof 1 σ in alpha counting. The values shown in Figures 3 and 4
agree very closely with each other within the error.
In alpha-spectrometry for a deposited counting source, selfabsorption of alpha-rays in the source is a serious problem;
energy loss and absorption of alpha-particles in the windowlayer of silicon surface barrier detector (~50 nm) and in
vacuum chamber (< 4 Pa) are negligible. As to the extent ofself-absorption in the present alpha-spectrometry, since theHalf-Life of Samarium-147

thickness of the residue of Sm and the alpha-emitter standard
on the watch glass is less than 15 µg/cm 2 , energy loss andabsorption of alpha-particles from 147Sm and the alpha-emitter
standard of the counting source can be expected to be negligible.
The counting efficiency of the LS spectrometer is high(~100%), however, the spectrometer has disadvantages of highbackground and poor energy-resolution (Figure 2). Hence, inspite of the agreement between the mean value measured with
the LS spectrometer, (1.15 ± 0.02) × 10 11 y, and the meanvalue with the surface barrier detector, (1.17 ± 0.02) × 1011 y,within the error, the former value was regarded as a supporting
data for the latter because of the disadvantages describedabove.
The arithmetic mean of the half-life values obtained in thiswork for the nineteen counting sources (Figure 3) is (1.17 ±0.02) × 1011 y. The error associated with the average value is
one standard deviation. The average value is about 10% longer
than the currently adopted value, (1.06 ± 0.02) × 1011 y.1
In the earlier works, the half-life of 147Sm was measured
with a 4π gas-flow counter, 11 LS spectrometers, 12, 14, 15 andionization chambers. 1, 13 According to the conventional
method for the determination of long half-lives, the half-lifevalue for 147 Sm (T 147 ) is deduced by substituting measured
values of A147 and N147 (see eq 2) in the following equation:
T147 =ln 2
× N147 .
A147

Hence, experimental errors in measuring A147 and N147 give
inaccurate results for T147. Impurities in Sm reagent bring error
into the value of N147, and uncertainty of counting efficiency,
self-absorption of counting source and radioactive impurities
in Sm reagent bring error into the value of A 147 . It is firstsuspected that the purity of Sm reagent was not sufficient in
the earlier works. It is also suspected that the corrections forcounting efficiency and self-absorption were not appropriate in
the earlier works. High background and poor energy resolution of LS spectrometers used in the earlier works might also
result in inaccurate 147Sm half-life.
In contrast to the earlier works, in the present cautious work,
all the error sources mentioned above were excluded. Hence,
we conclude that the result obtained in this work is reliable.

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 20037Acknowledgements. We would like to thank Dr. K.
Norisuye of Institute for Chemical Research, Kyoto University
for the thermal ionization mass spectrometry. We also wish toacknowledge the assistance of the members of the Research
Reactor Institute, Kyoto University in the neutron activation
analysis.
References(1) M.C. Gupta and R.D. MacFarlane, J. Inorg. Nucl. Chem.
32, 3425 (1970).
(2) T. Mitsugashira, M. Hara, Y. Suzuki, M. Watanabe, andH. Hirayama, Proc. 1996 SERNIA Symp. on Environmental
Radioactive Impact in Asia, 6-8 Sep. 1996, Taipei, ROC,p. 291 (1996).
(3) T. Mitsugashira, M. Hara, Y. Suzuki, and M. Watanabe,Res. Rept. LNS Tohoku Univ. 30, 47 (1997).
(4) T. Mitsugashira, M. Hara, Y. Suzuki, M. Watanabe, S.
Hirai, Y. Okada, and A. Mori, J. Radioanal. Nucl. Chem.
239, 345 (1999).
(5) T. Mitsugashira, M. Hara, P. Kim, K. Nakashima, and K.
Nakayama, J. Radioanal. Nucl. Chem. 255, 201 (2003).
(6) K. Sakamoto, Y. Hashimoto, and T. Nakanishi, in Origin
of Elements in the Solar system: Implications of Post-1957
Observations, edited by O. Manuel (Kluwer Academic/Plenum Publishers, New York, 2000), p. 511.
(7) M. Paul, A. Valenta, I. Ahmad, D. Berkovits, C.
Bordeanu, S. Ghelberg, Y. Hashimoto, A. Hershkowitz, S.
Jiang, T. Nakanishi, and K. Sakamoto, Astrophys. J. 558,
L133 (2001).
(8) A.M. Friedman, J. Milsted, D. Metta, D. Henderson, J.
Lerner, A. L. Harkness, and D.J. Rokop, Radiochim. Acta5, 192 (1966).
(9) D.C. Dunlavey and G.T. Seaborg, Phys. Rev. 92, 206(1953).
(10) M. Nurmia, G. Graeffe, K. Valli, and J. Aaltonen, Ann.
Acad. Sci. Fenn. A VI, 148, 3 (1964).
(11) G. Beard and M.L. Wiedenbeck, Phys. Rev. 95, 1245
(1954).
(12) G.B. Beard and W.H. Kelly, Nucl. Phys. 8, 207 (1958).
(13) R.D. MacFarlane and T.P. Kohman, Phys. Rev. 121, 1758
(1961).
(14) P.M. Wright, E.P. Steinberg, and L.E. Glendenin, Phys.
Rev. 123, 205 (1961).
(15) D. Donhoffer, Nucl. Phys. 50, 489 (1964).