Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 109-112, 2007

Technological Development for Half-life Measurement of 146Sm Nuclide
N. Kinoshita,*,†,a T. Hashimoto,a T. Nakanishi,a A. Yokoyama,a H. Amakawa,b T. Mitsugashira,cT. Ohtsuki,d N. Takahashi,e I. Ahmad,f J. P. Greene,f D. J. Henderson,f C. L. Jiang,f M. Notani,fR. C. Pardo,f N. Patel,f K. E. Rehm,f R. Scott,f R. Vondrasek,f L. Jisonna,g P. Collon,hD. Robertson,h C. Schmitt,h X. D. Tang,h Y. Kashiv,i and M. Paulia

Graduate School of Natural Science and Technology, Kanazawa University, Ishikawa 920-1192, Japan
Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japanc
Institute for Material Research, Tohoku University, Ibaraki 311-1313, Japand
Graduate School of Science, Tohoku University, Miyagi 982-0826, Japane
Graduate School of Science, Osaka University, Osaka 560-0043, JapanfPhysics Division, Argonne National Laboratory, Argonne, Illinois 60439, USAg
Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112, USAhDepartment of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USAiRacah Institute of Physics, Hebrew University, Jerusalem 91904, IsraelbReceived: December 7, 2006; In Final Form: January 3, 2007
Techniques for the measurement of the 146Sm nuclide, which is known as an extinct nuclide on the present Earth,
were developed in order to determine its half-life. The nuclide was produced in three reactions of 147Sm(γ,n)146Sm,
147
Sm(n,2n)146Sm, and 147Sm(p,2n)146Eu(→146Sm). The Sm fraction in the target was chromatographically purified
to prepare α counting samples as hydroxide. The 146Sm/147Sm α-activity ratios in the samples were measured usinga Si surface barrier detector. The 146Sm22+ /152Sm23+ and 147Sm22+ /152Sm23+ ratios were measured by accelerator mass
spectrometry using the ECR-ATLAS-GFM system. The 146Sm ions were distinguished clearly from the other ionssuch as the abundant isobar, 146Nd in the samples.

1. Introduction
The α -emitting nuclide of 146 Sm is known as a p-process
nuclide produced in supernova explosions. Recently, search forthe nuclide is planned for various purposes in nuclear astrophysics, astrochemistry, etc. although it was not supposed toexist on the present Earth since its half-life is shorter by a factor of ~50 than the age of the Earth. The nuclide 146 Sm was
first discovered by Dunlavey et al.1 in an α -activated natural
Nd sample using a nuclear emulsion. The half-life was estimated to be 5 × 10 7 y in the study according to the GeigerNuttall rule. After that, the half-life was measured to be (7.4 ±1.5) × 107 y by Nurmia et al.2 in 1964, and (1.03 ± 0.05) × 108 y
by Friedman et al.3 in 1966. The currently adopted half-life of(1.03 ± 0.05) × 10 8 y was determined by Meissner et al.4 in
1987.
In determination of long half-lives, the half-life value is calculated usually from the relationship between radioactivity andthe number of atoms. Many researchers often have difficulty
in determining the number of atoms rather than in the radioactivity measurement. Nurmia et al.2 and Meissner et al.4 estimated the number of 146Sm atoms using γ activity of the parent
nuclide of 146Sm. For a direct measurement of atoms, Friedman
et al.3 produced the 146Sm atoms in nat Nd(α ,xn) reactions, andthe number of 14 6 Sm atoms was measured by a Thermal
Ionization Mass Spectrometer (TIMS) after removal of the target material. The 146Nd ions which are present in nature interfered with the 146Sm detection in that case. In determination of
the half-life of 146Sm by mass spectrometry, the precision of the*Corresponding author. E-mail: norikazu@post.kek.jp. Fax:
+81-29-864-4051.
†P resent add ress: Rad iation Science Center, H igh Energy
Accelerator Research Organization (K EK), 1-1 Oho, Tsukuba,
Ibaraki 305-0801, Japan.

measurements depends on the contamination by 146Nd.
Recently Accelerator Mass Spectrometry (AMS) using a
Gas-Filled Magnet (GFM) spectrograph has been well developed to attain excellent separation power, and used for themeasurements of 36 Cl, 63 Ni, etc. 5,6 However, the larger theatomic number of interest, the more difficult the separation of
isobaric nuclides. In case nuclides with a large atomic number
are measured with AMS, highly accelerated ions are required
to separate isobaric nuclides. We carried out AMS measurements using the Argonne Tandem-Linac Accelerator System(ATLAS) of the Argonne National Laboratory for the reasonsdescribed above.
In the present study, we produced 146Sm atoms using three
nuclear reactions with Sm targets in order to prepare samples
with different conditions in isotopic abundance and contamination by the other elements. Alpha spectrometry using a silicon
semiconductor detector and mass spectrometry using an AMS
technique were carried out to develop a method to determine
the half-life of 146Sm.
2. Calculation of the Half-life
The half-life of the 146Sm nuclide is expressed referring to
the naturally occurring nuclide of 147Sm, which also emits an
α -particle. The activities of 146 Sm and 147Sm are described,
respectively, as
A146 = ln 2 × N146 ,
T146

(1)

A147 = ln 2 × N147 ,
T147

(2)

and

© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

110 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Kinoshita

where T146 and T147 are the relevant half-lives, and N146 and N147
are the number of relevant atoms. Dividing eq 1 by eq 2 gives
T146 = A147 × N146 T147 .
A146
N147

(3)

According to eq 3, the half-life of 146 Sm can be determinedfrom the α -activity ratio by α spectrometry and the atomic
ratio by mass spectrometry referring to the half-life of 147Sm,(1.17 ± 0.02) × 1011 y, which was determined previously.7
3. Experimental
3.1. Production of 146Sm nuclide.
3.1.1. (γ ,n) reaction. Bremsstrahlung radiation (maximum
energy, 50 MeV; flux, ~1013 eq.q. s–1) was introduced to a targetof 93% enriched 147Sm oxide and the irradiation was performedfor ~100 hours at the Laboratory of Nuclear Science, Tohoku
University to produce 146Sm atoms by the 147Sm(γ ,n)146Sm reaction. The yield of 146Sm was estimated to be ~120 mb eq.q.–1 byusing the PICA3/GEM code.8 After the irradiation, the Sm target was dissolved in hydrochloric acid. The solution adjusted
to the acidity of ~0.5 M HCl was let pass through a strong cation-exchange resin column (4.6 mm φ × 250 mm) which wasconditioned with NH4Cl solution and kept at 80 °C in a column
oven. Then distilled water was let pass through the column tosettle the Sm ions in the resin. Finally, 0.3 M 2-hydroxyisobutyric acid (α-HIBA) solution which contained 3 g of NaCl per
500 mL in concentration and was adjusted to pH 4.2 with
NaOH was let pass through the column to elute the Sm ionschromatographically prior to the Pm ions produced in the (γ ,p)reaction. The Sm fraction was acidified with HCl, and a Fe
carrier was added. After the Sm was recovered with Fe(OH)3,
the precipitate was dissolved in 12 M HCl solution and thesolution was let pass through a Dowex 1X8 anion-exchange
column to remove the Fe ions.
3.1.2. (p,2n) reaction. A 21-MeV proton beam (~1 µA) was
delivered to a 96% enriched 147Sm oxide at Research Center for
Nuclear Physics, Osaka University. The irradiation continued
for 12 hours to produce the EC/ β + -decaying nuclide of 146 Eu(T1/2 = 4.61 d) by the 147Sm(p,2n) reaction. The cross section of
the reaction was estimated to be 1.1 b according to a calculation by the ALICE code9 and Reference 10. Europium isotopes
were isolated from the target material by the same cationexchange chromatography as mentioned in the preceding paragraph, and let stand for ~2 months to grow the 146Sm daughter.
Then, the Sm isotopes in the Eu fraction were separated using
cation-exchange chromatography to remove by-products suchas a weak α emitting nuclide, 147Eu (T1/2 = 24.1 d) which could
be produced by the 148Sm(p,2n) reaction.
3.1.3. (n,2n) reaction. Fast neutrons of ~3 × 1019 cm–2 in fluence in the energy range of 6–10 MeV were bombarded to a
96% enriched 147Sm oxide in a Japan Material Testing Reactor
at the Institute for Materials Research, Tohoku University to
produce 146Sm atoms by the 147Sm(n,2n) reaction. The target
was dissolved in HCl, and then a concentration of HCl was
adjusted to 0.25 M. The solution was let pass through an Ln
resin column (0.75 cmφ × 4.5 cm, 2 mL) supplied by Eichrom
Technologies, Inc. The Sm isotopes were recovered with 11
mL of 0.4 M HCl. This chromatographic procedure wasrepeated 5 times to improve the purity of the Sm fraction.
3.2. Alpha spectrometry. Each of the Sm samples of 20–
100 µg was precipitated with ammonia water and collected ona Fuji Film Teflon membrane filter (pore size: 0.2 µm) attached
to a plastic filter holder with a 4.9 cm2 filtering area to preparea counting sample. The α activity on the filter was measured
using an EG & G Ortec OctetTM PC α-spectrometer equippedwith a silicon surface-barrier detector of 450 mm2 for the active

area. A counting sample was placed in a vacuum (< ~4 Pa)
chamber of the spectrometer with a solid angle of ~20% of 4π
geometry. The typical energy resolution of the α-spectrometerwas ~50 keV (FWHM) for α-particles from 147Sm, and the typical background of the spectrometer was ~5 cpd in the peakregion of 147Sm (2.235 MeV).
3.3. Accelerator mass spectrometry. The Sm samples onfilters that were subjected to α spectrometry were, at a laterstage, dissolved with HCl and the 147Sm contents of the samples
were determined with a SEIKO SPQ-9000 ICP-MS. A known
amount of each of the samples was diluted with an HCl solution containing a known amount of natural Sm (100 mg) toapproximately 10 –8 of 146 Sm/147Sm atomic ratio to prepare asample of volume and atomic ratio suitable for the AMS measurement. The HCl concentration of the solution was adjusted
to 0.25 M and the solution was passed through an Ln resin column (0.75 cmφ × 4.5 cm, 2 mL). Neodymium isotopes whichinterfered with the 146Sm measurement in mass spectrometry
were eluted with 20 mL of highly purified 0.25 M HCl solution
which was prepared by diluting TAMAPURE-AA-10 reagent
supplied by Tama Chemicals Co. Ltd. with Milli-Q water.
Subsequently, Sm isotopes were eluted with 10 mL of purified
0.5 M HCl solution as described above. This separation procedure was repeated twice, and the Sm sample was precipitatedwith TAMAPURE-AA-10 ammonia water. The precipitate
was heated at 450 °C for 7 hours to convert hydroxide to oxide.
The oxide was mixed well with twice its weight of Zr metal
powder, and put into a Ta boat. Electric current of approximately 70 A was applied to the boat in a vacuum chamber (<~10 –6 Torr) to reduce the oxide to metal because Sm ions areproduced from a metal form rather than an oxide form in the
ECR plasma chamber as an AMS ion source.
Measurements of 14 6 Sm with AMS using an Electron
Cyclotron Resonance Ion Source (ECR-IS) were performed at
Argonne National Laboratory (ANL). The ATLAS system11
was entirely tuned with a 80Kr12+ pilot beam, which has a massto-charge ratio close to that of 146Sm22+, prior to the AMS measurement of 146 Sm 22+. The difference in the mass-to-charge
ratio is δ (m/q)/(m/q) = 4.6 × 10 –3. The ions produced in ECRIS were extracted from the ion source with a voltage of 200 kV,
and analyzed through a bending trajectory with three analyzing
magnets to select ions of interest, and then accelerated by threekinds of superconductor linear accelerators of Positive Ion
Injector (PII), BOOSTER Linac, and ATLAS Linac. Finally,
the ions passed through a GFM entrance, which consists ofMylar of 1.4 µm in thickness and entered a GFM spectrograph.
The arrangement of the Faraday cup, GFM, ionization chamber, Position-Sensitive Parallel-Grid Avalanche Counter(PGAC) is shown in Figure 1. The 146Sm 22+ ion was selected
for acceleration because of its suitable mass-to-charge ratio
which is not interfered with strongly by other ions except for146
Nd. The 146 Sm 22+ ions were accelerated to 840 MeV, andintroduced to the GFM spectrograph12,13 filled with 10-Torr N2
gas and subjected to 14 kG of magnetic field to separate theintense stable isobar 146 Nd from 146 Sm. The 146 Sm 22 + and146
Nd22+ ions slowed down in the GFM, and trajectories of theions were deflected by the magnetic field. The position of eachion at the focal plane was monitored with the PGAC. In orderto decrease the counting rate of the ionization chamber, blocking shield was inserted at the focal plane to block other ionsaccelerated together with 146Sm22+ and 146Nd22+. After the ionspassed through the PGAC, multiple energy-losses along the ion
path, ∆E1–∆E 5 and ∆E total , were measured in an ionizationchamber. The signal of Time Of Flight (TOF) was also measured as the time difference between the PGAC signal and RF
master signal of the accelerator. The 152Sm23+ ions which havea mass-to-charge ratio close to that of the 146Sm 22+ ion (δ (m/q)/(m/q) = 4.2 × 10 –3) were counted by a Faraday cup set up in
the GFM beamline. The measurements with the cup were per-

Technological Development for Half-life Measurement of 146Sm Nuclide
PGAC
Ionization chamber∆E1–∆E5, ∆Etotal

∆E5·

··

146

∆E1

146
80

Blocking shield

Sm

Kr

Faraday cup

MagnetFigure 1. Arrangement of Faraday cup, GFM, and IonizationChamber/PGAC. Ions were analyzed in GFM, positions of the ionsat the focal plane were monitored with PGAC, and multiple energy
losses (∆E1–∆E5 and ∆Etotal) were measured with an ionization chamber. The 147Sm22+ and 152Sm23+ ions were measured with the Faradaycup. The Faraday cup was removed to count the 146Sm22+ and 146Nd22+
ions with PGAC-IC system.

Counts / channel

105
(a)
147

103

Sm

146

Sm
147

102

Eu

101
10

Counts / channel

10

(b)

103
102
101
100

Counts / channel

104

(c)

103
102
101
100

Alpha spectra of the Sm isotopes which were produced inthree reactions measured with a silicon surface barrier detector, are shown in Figure 2. The 146 Sm/147Sm α -activity ratio
was determined from α -peak counts of these nuclides. The
FWHM and FWTM of approximately 50 keV and 90 keV,
respectively, for the proton-activated sample, 60 keV and 130
keV, respectively, for the neutron-activated sample, and 100
keV and 230 keV, respectively, for the bremsstrahlung-activatedsample were observed for the 147Sm α peak. The precision of
the 146Sm/147Sm α -activity ratio was hardly influenced by thecorrection for the tailing of α peaks under the above conditionsof FWHM and FWTM.
Two-dimensional plots of position along the focal plane vs.
∆E1 and those gated by TOF are shown in Figure 3. The twodimensional plot for the blank nat Sm sample, which was not
activated, is shown in Figure 4 for comparison. Two components are clearly distinguished in Figure 3, showing each of thecomponents has its characteristic trajectory and velocity in
GFM. One component was identified to be from 146 Sm ionsfrom the extrapolation of the measured energy losses and positions for the ions of 147Sm22+, 152Sm22+, and 154Sm22+. The othercomponent is interpreted as resulting from 146Nd22+ ions, whichhave the same m/q ratio as the 146Sm22+ ion.
The separation between 146 Sm and 146 Nd ions at the focal
plane arises mainly from a different mean charge of the ions in
GFM. The mean charge of the ions in material, –q , can be calculated with the following equation:14–q = Z[1 – A(Z)exp(–0.879V )] ,R

04

formed in between the measurements of the 146Sm ions. Ions of147
Sm 22+ were also counted with the Faraday cup to derive the146
Sm/147Sm atomic ratio through the ratios of 146Sm22+/152Sm23+
and 147Sm 22+ /152Sm 23+. In the measurements with the Faraday
cup, the other ions of contaminants are negligible comparedwith the Sm ions.
4. Results and Discussion

Nd

Beam

104

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007 111

150

300
250
200
Energy / channel

350

Figure 2. Alpha spectra of the Sm samples from (a) proton-activation, (b) neutron-activation, and (c) bremsstrahlung-activation.
Thicknesses of the Sm(OH) 3 counting samples are ~10 µg/cm 2 forproton-activation (a), ~15 µg/cm2 for neutron-activation (b), and ~25µg/cm2 for bremsstrahlung-activation (c).

(4)

with A(Z) = 1.035 – 0.4exp(–0.16Z) and V R = V /(V0 Z 0.65 ),where Z is the atomic number, V is the velocity of the projectile, and V0 is 2.188 × 108 cm/s. The mean charge of the 146Sm
ions is larger by ~1 than that of 146 Nd ions. Thus, the 146 Sm
events should appear at a lower channel in the position spectrum than that of the 146Nd events. It agrees with the assignment described above.
The different energy losses between 146Sm and 146Nd seen in
Figure 3 result from the combination of the different incident
energy to the ∆E1 detector, i.e., the different energy losses
between 146Sm and 146Nd ions in the GFM before entering intothe ∆E1, and those in the ∆E1 detector. The energy loss of the146
Sm ion in the GFM is larger than that of 146Nd, which leads
to the higher incident energy of 146Nd to the ∆E1 detector thanthat of 146Sm. The higher energy 146Nd ions may lose a larger
amount of energy in the ∆E1 detector than that of 146Sm for the
relevant incident energies, which is consistent with the observed
energy losses in Figure 3. In addition, the higher-energy 146Nd
ions in the GFM than 146Sm allow us to separate those nuclides
in the TOF spectrum; the higher energy 146 Nd ions should
appear at lower channels in TOF than 146Sm, which agrees withthe experimental results.
We measured the activated Sm samples prior to the blanknat
Sm sample in order to investigate the optimum condition for146
Sm measurement. Thus, the 146Sm events observed in Figure
4 using the blank nat Sm sample are attributed to the memory
effect. These events were found to be so low that background
events could be neglected in the measurements of the activated
Sm samples. The separation between the 146Nd and 146Sm ionsusing the GFM technique was found successful, even in case112 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
SinglesSm

Gated by TOF

146

Nd

(a-2)
146

high

Sm

low

(b-2)

∆E1 / channel(b-1)∆E1 / channel∆E1 / channel

(a-1)
146

Kinoshita

Position along focal plane / channel(c-2)∆E1 / channel(c-1)

Figure 4. Two-dimensional plot of position along focal plane vs.
∆E1 for the non-activated Sm sample gated by TOF. The 146Sm eventsin a circle might be attributed to the memory effect. The number of152
Sm 23 + ions was deduced to be 3.4 × 1013 in the same way as
described in the caption of Figure 3.

irradiation. We also wish to acknowledge the assistance of themembers of the ATLAS facility, Argonne National Laboratory
for utilization of the accelerator mass spectrometry.
References
Position along focal plane / channel

Figure 3. Two-dimensional plot of position along focal plane vs.
∆E1 in AMS. The figures of (a-1) and (a-2) are for proton-activatedsample, (b-1) and (b-2) for neutron-activated sample, and (c-1) and (c2) for bremsstrahlung-activated sample. Those of (a-1), (b-1), and (c1) show raw data, while (a-2), (b-2), and (c-2) are the data gated by
TOF. The numbers of 152Sm23+ ions were deduced to be 1.1 × 1012 forproton-activated sample (a), 8.6 × 1011 for neutron-activated sample(b), and 8.6 × 1011 for bremsstrahlung-activated sample (c) according
to ion-counting with a Faraday cup at intervals between the measurements of 146Sm.

the 146Nd ions were more intense than the 146Sm ions by a factorof 10–100. In the AMS, however, further normalization isrequired to correct for differences in experimental conditionsof 146Sm22+, 147Sm22+, and 152Sm23+ measurements for the determination of the half-life of 146Sm.
5. Summary
In order to develop techniques for the measurement of thehalf-life of 146Sm, nuclides were produced in 147Sm(p,2n)146Eu(→
146Sm), 147Sm(n,2n)146Sm, and 147Sm(γ ,n)146Sm reactions. Alphacounting samples were prepared as Sm hydroxide to measure
the 146Sm/147Sm α-activity ratios with a Si semiconductor detector. The Sm samples for AMS were prepared through chemical
procedures. The 146Sm22+ /152Sm23+ and 147Sm22+ /152Sm23+ ratios
were measured using the ECR-ATLAS-GFM system of ANL
to determine the 146Sm/147Sm atomic ratio. The 146Sm 22+ ionswere successfully distinguished from the 146Nd22+ ions by AMS.
Further normalization is, however, required to correct for differences in experimental conditions of the measurements of146
Sm 22+, 147Sm 22+, and 152 Sm 23+ for the determination of thehalf-life of 146Sm.
Acknowledgements. We would like to thank the staff ofthe Laboratory of Nuclear Science, Tohoku University for thebremsstrahlung irradiation, the Institute for Materials Research,
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