Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 38 (2005)

25(LNS Experiment : #2389, #2408, #2424, #2443, #2458, #2460, #2475, #2490, #2492, #2502,#2503, #2513, #2514, #2525, #2526, #2541, #2542)

Observation of α-decay of 229mTh produced from 229Ac
H. Kikunaga1, ＊ , Y. Kasamatsu2, K. Takamiya3, T. Mitsugashira4,
M. Hara4, T. Ohtsuki5, H. Yuki5, A. Shinohara2, S. Shibata3,
N. Kinoshita1, A. Yokoyama1, and T. Nakanishi1
1

Faculty of Science and Graduate School of Natural Science and Technology,Kanazawa University, Kanazawa 920-1192

2Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043

34Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494

The Oarai-branch, Institute for Materials Research, Tohoku University, Oarai-machi, Ibaraki 311-1313
5

Laboratory of Nuclear Science, Graduate School of Science, Tohoku University, Sendai 982-0826

We produced 229Th in the nuclear reaction 232Th(γ, p2n)229 Ac, followed by disintegration to 229m,gTh.
The α-decay signals from 229mTh were searched for and the alpha-particle events of the energy region
between 4.93 MeV and 5.05 MeV were observed in the separated thorium fraction from an actinium
source highly purified from the 232Th + γ reaction products. The energy values of the α-particles
coincide with those expected for 229mTh.

§1. Introduction
Recent investigations of 229Th have reported energy difference between the ground state 229gTh andthe ﬁrst excited state 229mTh to be (3.5 ± 1.0) eV [1, 2] or (3.4 ± 1.8) eV [3]. In those studies, 229m,gTh was
produced from the α-decay of 233U. Additional evidence for the existence of 229mTh was given by Burke etal. [4] using 230Th (d,t) particle reaction spectroscopy. The internal conversion process is prohibited inTh because the energy of the isomeric transition from 229mTh to 229gTh is lower than the ﬁrst ionization229

potential of thorium. Therefore, the investigation of the 229mTh decay is expected to provide a strong
verification of the electron bridge mechanism (EBM) [5], which is a de-excitation process competingwith isomeric transition and α-decay. Since the probability of the EBM depends on the energy state of
the outer-shell electrons, the half-life of 229mTh can vary with its chemical state; hence the isomer is alsointeresting from a chemical viewpoint.
It is believed that no one has thus far succeeded in detecting a decay signal from 229mTh. In a few
studies, the observation of photons due to isomeric or EBM transitions was attempted by ultraviolet
and visible spectroscopy [6, 7], but there is still some doubt regarding the results [8, 9]. Browne et al. [10]searched for 229mTh from a sample prepared from approximately 25g of 233U. They tried to observe thegrowth of γ-rays associated with the α-decay of 229gTh due to the feeding from 229mTh, but observed noevidence of such growth. The failure to observe these events might be attributable to two reasons: thedifﬁculty in detecting extremely low energy photons and the small branching ratio from 233U to 229mTh,

26which is about 2%.
In order to address the ﬁrst difﬁculty, it seems reasonable to devise the identiﬁcation of 229mTh by
the detection of its α-decay. The most favorable decay-mode of 229mTh should be the direct γ-transitionor the EBM [5]. The observation of photon emission from the isomeric state requires detecting
ultraviolet-visible photons under considerable interference, such as α-particle induced fluorescence.
Such a measurement is very difﬁcult. On the other hand, the α-decay of 229mTh is undoubtedly nuclear
disintegration phenomenon, though as a minor decay-mode. The technique for α-particle detection iswell established and its detection is subjected to little interference from the background. Therefore, it isexpected that if α-particles from 229mTh exist, they are observable. The α-decay of 229mTh has been
discussed by Mitsugashira et al. [11] and Dykhne et al. [12]. Both expected the favored α-particle decays
from 229mTh had energies higher than those from 229gTh, i.e. 4.93-5.05 MeV. This implies that the partial
half-life of the α-decay of 229mTh is shorter than that of 229gTh.
To address the difﬁculty of the small branching ratio from 233U to 229mTh, we produced 229mTh fromβ-decay of 229Ac following a nuclear reaction process, whereas in all previous investigations on thedecay properties of 229mTh, the nuclide was produced from the α-decay of 233U. By using a suitable
nuclear reaction, it was expected that the 229Th sample would have a m/g ratio large enough to observe
the minor α-decay from 229mTh, although complicated chemical procedures for eliminating by-products
would be required.
Based on these ideas, we previously reported the detection of α-decays of 229mTh produced by the230Th(γ, n) reaction [11]. Unfortunately, it appeared that the results contained interfering effects due topile-up of the α-particles from 230Th decay and β-rays of the other nuclear reaction products. In thisreaction system, the interference was unavoidable because it is impossible to chemically separate the229m

Th from the target 230Th and other thorium isotopes produced in the reaction.
In this study, we attempted to detect the α-decay signals from 229mTh produced using anotherreaction system. We obtained 229mTh from β-decay 229Ac that was produced through the reaction 232Th(γ,p2n). A sample of 229m,gTh was prepared using several chemical separation methods and was measured
with an α-spectrometer.

§2. Experimental
2.1 Preparation of the thorium dioxide target
A 232Th reagent, allowed to stand unprocessed for more than 40 years, contains daughter nuclei inradioactive equilibrium mixture, such as 228Ra and 228Ac. The 228Ac feeds 228Th that significantlyinterferes with the observation of the α-particles of 229mTh. Therefore, 228Ra and 228Ac should be removed
from thorium in radioactive equilibrium with its decay series nuclei just before bremsstrahlung
irradiation. To perform such chemical separation, about 1 g of thorium dioxide was dissolved in amixture of 10 mL of 13M (mol/dm3) nitric acid (HNO3) and a drop of 27M hydroﬂuoric acid (HF) under
heating, and the solution was passed through a column packed with anion exchange resin (Dowex 1X8,
100-200 mesh, 30 mmφ × 70 mm) to adsorb thorium isotopes. The resin was washed with 150 mL of 8M
HNO3 to remove radium and actinium, and then thorium isotopes were eluted from the column with27
150 mL of 2M HCl. A greater than 3-fold excess of saturated oxalic acid was added to the eluate under
heating, then the resulting precipitate, Th(C2O4)2, was aged for one night. The precipitate was washedwith distilled water and ignited at 800 ºC for 8 hours to produce thorium dioxide.
2.2 Bremsstrahlung irradiation
A sample of about 1.65 g of 232 ThO 2 was enclosed in a quartz tube for irradiation withbremsstrahlung radiation. The irradiation was carried out using the Electron Linear Accelerator at
Tohoku University. The accelerator was operated at an electron energy of 30 MeV with a mean currentof around 0.1 mA. The 232ThO2 target was placed in close contact with the back of a platinum converterand cooled with running tap water during the 1-hour irradiation. After the irradiation, the target waschemically treated according to the procedure outlined in Fig.1 and the details given below.
2.3 Isolation of actinium from the irradiated target
The target material was dissolved in a mixture of 10 mL of 13M HNO3 and one drop of 27M HF
under heating, and diluted with water to 40 mL (diluted to about 4M HNO 3). In order to remove
thorium isotopes from the solution, including aimed actinium, solvent extraction with 40 mL of 1M
HDEHP-benzene was carried out three times in series. After washing the aqueous phase with 40 mL
benzene, actinium isotopes were co-precipitated with iron hydroxide by adding 1 mg of iron and 15M
ammonia water (NH3aq). The precipitate was then dissolved in 1M HNO3 and the solution passed
through a cation exchange resin column (Dowex 50X8, 200-400 mesh, 5.5 mmφ × 40 mm), which
adsorbs actinium isotopes. The resin was washed with 8 mL of 3M HNO3 to remove radium and iron.
Then actinium isotopes were eluted from the column with 12 mL of 8M HNO3. The solution was passed
through an anion exchange resin column (Dowex 1X8, 200-400 mesh, 5.5 mmφ × 40 mm) to completely
eliminate thorium from the solution. The actinium fraction containing rare earth ﬁssion products wasadjusted to a 9M HCl solution and passed twice in series through an anion- exchange resin column(Dowex 1X8, 200-400 mesh, 5.5 mmφ × 40 mm) to adsorb completely 231Pa produced from 231Th. These
purification procedures for actinium were carried out within about one hour after the end ofbremsstrahlung irradiation.
2.4 Preparation of 229m,gTh source for α-particle counting and spectrometry
To allow the decay of 229Ac (T1/2 = 62.7 min) and the growth of 229m,gTh isotopes, the puriﬁed actinium
fraction was left to stand for 3 hours after the end of the purification of actinium. The solution
containing thorium isotopes was adjusted to 8M HNO3 solution during these 3 hours and passed
through an anion- exchange resin column (Dowex 1X8, 200-400 mesh, 5.5 mmφ × 40 mm) to adsorb thethorium isotopes. The resin was washed with 20 mL of 8M HNO3 to remove rare earth ﬁssion products
and actinium isotopes. The thorium isotopes were eluted from the column with 5 mL of 2M HCl and coprecipitated with samarium hydroxide by adding 30 μg of samarium and 15M NH3aq. The precipitate
was collected on a 0.02-μm alumina filter (Whatman, ANODISC membrane) of 18 mm diameter toprepare a counting source. The ﬁlter was ﬁxed on a stainless-steel supporting ring and dried at 130 ºC.
All the chemical procedures were ﬁnished about 5 hours after the end of irradiation. The sample wasmeasured by α-ray spectrometry with a 450 mm2 silicon detector and a 2k-channel PC-PHA system.
The background count rate for this detector was about 20 counts/day in the energy range of 3 to 8 MeV

28

Fig.1. Chemical procedure for preparation of the 229m,gTh sample.
and less than 1 count/day in the energy range of 4.5 to 5.1 MeV. Collection of α-events was started
within 10 minutes of completing the counting source preparation and the accumulation of an α-ray
spectrum for 104 -sec was repeated 90 times.

§3. Results and discussion
An α-ray spectrum obtained as a sum of the first 30 spectra is shown in Fig.2. The α-peaks of147

Sm, 232Th, 230Th, and 228Th and its daughters can be observed in the spectra. Some dozen α-counts areobserved between the 230Th peaks and the 228Th peaks, at an energy consistent with that expected for229m,gTh. It was found that the ratio of the α-counts of 230Th/229Th/228Th was about 3/1/70, while it was

about 5500/1/10 for the thorium sample isolated from 230Th(γ, n) reaction products [11]. In thisexperiment, the count number of 230Th decays was relatively small and had no influence on theidentification of 229m,gTh. However, interference from 228Th on the observation of 229Th may haveoccurred, as discussed below, because the count number of 228Th decays was greater by a factor of sevenin this experiment.
A magniﬁcation of theα-spectrum for the region related to 229Th is shown in Fig.3 (a). The spectrumof the energy region differs from the spectrum expected for only 229gTh. The causes of this difference arebelieved to be the peak tail from the peaks of 228Th and its daughters, α-counts from impurities, and/or
the existence of 229mTh.
228

Th was formed simultaneously with 229m,gTh by the β-decay of 228Ac produced in the 232Th(γ, p3n)

reaction in the source. It has the largest number of counts in the spectra and the energy region is29

Fig.2.

α-ray spectrum acquired over a counting duration of 3 × 105 s, starting 10
minutes after the chemical separation.

Fig.3. Expandedα-ray spectra acquired over a counting duration of 3 × 105 s, starting
10 minutes after the chemical separation (a), and acquired 1 week after thechemical separation (b).
higher than that of 229m,gTh. It was found, however, that the high resolution spectrum is little inﬂuenced
by the peak tail of 228Th. In Fig.3 (a) the 228Th peak has a FWHM of about 29 keV measured at 5423 keV.
A growth in time of the α-count rate was observed. These counts were identified by the energy
value as the α-peaks of 231Pa. The growth of 231Pa interfered with the identification of 229mTh. Thespectrum obtained one week after the chemical separation is shown in Fig.3 (b), overlaying Fig.3 (a),demonstrating the growth of 231Pa. It is thought that 231Pa was completely eliminated by the repetitive
anion exchange at the end of the chemical purification of the actinium fraction and, therefore, theobserved 231Pa may be the decay product of 231Th, produced in the 232Th(γ, n) reaction. The counting
source included 231Th because elimination of thorium was not complete. Although the decontamination

30
factor of thorium isotopes in the purified actinium fraction was estimated to be about 5 × 10 － 5,
sufﬁcient 231Pa was produced to interfere with the identiﬁcation of 229mTh because of the large crosssection of the 232Th(γ, n) reaction.
The spectrum in Fig.3 (b) can be reproduced by sum of the 229gTh spectrum and the 231Pa spectrum.
It indicates that the half-life of 229mTh is not as long as one week. On the other hand, the early spectrumin Fig.3 (a), before the growth of 231Pa, differs from the spectrum expected for only 229gTh. In particular
the observed α-counts between 4.93 MeV and 5.05 MeV, which coincides with the expected energy of229m

Th, were larger than those expected for 229gTh. It may be considered that the new α-emitter 229mTh

was observed in this work, although the α-peaks are not clearly deﬁned, probably due to poor counting
statistics.

§4. Conclusion
We attempted to produce 229m Th by the nuclear reaction 232 Th(γ, p2n) 229 Ac, followed bydisintegration to 229m,gTh. The alpha-events in the expected energy region for 229m,gTh were observedwith less interference than has been achieved in previous experiments. The spectral shape differs fromthat expected for only 229gTh, which may be due to presence of 229mTh. A better chemical separation ofactinium from thorium would eliminate the interference of 231Pa and allow more deﬁnitive results to beobtained.

Acknowledgement
The authors are grateful to the staff in the division of accelerators of the Laboratory of Nuclear
Science, Tohoku University.

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