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

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

Search for the Decay of 229mTh by Photon Detection
Y. Kasamatsu1, H. Kikunaga2, K. Nakashima1, K. Takamiya3, T. Mitsugashira4,
T. Nakanishi2, T. Ohtsuki5, H. Yuki5, W. Sato1, and A. Shinohara1
12

Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192
3

4Research 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, Tohoku University, Sendai982-0826§1. Introduction
The first excited state of thorium-229, 229mTh, is expected to show an intriguing decay property
because of its extremely low-lying excitation energy. The excitation energy has been reported to beeither 3.5 ± 1.0 eV or 3.4 ± 1.8 eV from the results of precise γ-ray spectroscopy conducted at the  decayof 233U.
In the disintegration process from 229mTh to 229gTh, the emission of internal conversion electrons isforbidden because the disintegration energy is lower than even the ﬁrst ionization energy of thorium
atoms. Thus the de-excitation from 229mTh to the ground state is expected to take place through a direct γ-ray transition or decay via an electron bridge (EB) mechanism in which part of the transition energy
is deposited to excite the valence-shell electrons of 229mTh. This implies that the half-life of 229mTh isdirectly affected by the chemical state of 229mTh.
Several experimental results have been reported by different groups about the detection of photons
emitted from 233U samples that contain a considerable amount of 229mTh produced through the α decay
from 233U with a branching ratio of about 2 %. These attempts were not successful, however, owing tointerference of radiation-induced ﬂuorescence from the materials around the radioactive 233U sample.
In our experiments, Th atoms were chemically separated by means of an anion exchange method
for removing the influence of intense radiation from 233U. As for the photon detection, the detection
efficiency was raised as high as possible by developing a novel detection system. In this report, thelimits of the half-life of 229mTh in HNO3 solution are deduced based on the experimental observations.

§2. ExperimentThorium-229m samples were prepared by a chemical separation from about 93mg of 233U containing
4.4 ppm of 232U by the following procedure. Uranium-233 was ﬁrst sorbed in 2 cm3 of an anion exchange
column (Dowex 1X8, 200-400 mesh) in 9M HCl solution and its descendants except for Bi and Tl wereeluted. After a given growth time, accumulated 229Th and 229mTh were eluted in 5 ml of 9M HCl solution
and separated from 233U. In order to thoroughly remove 233U, the eluate was again passed through33
another anion exchange column of the same resin in 9M HCl solution. We were able to accomplish thewhole chemical separation within only about 3 minutes. Following the separation, 1 mg of Al andapproximately 10 ml of concentrated ammonia water were added into the eluate. Thorium atoms werecoprecipitated with Al in the form of hydroxide. After the decantation of the supernatant liquid, theprecipitate was dissolved in 3 to 4 droplets of 8 M HNO3 in order to minimize the sample volume toprepare a quasi point source. The solution sample was transferred into a small quartz bowl of a size of 6mm in internal diameter and 10 mm in height, and photon measurements were performed.
Taking a wide range of the expected half-life of 229mTh into account, two different experiments werecarried out varying the growth time: 2 and 17 hours for the ﬁrst and second experiments, respectively.
It took 18 minutes to accomplish the chemical separation and treatment in the ﬁrst experiment, and 30
minutes in the second.
In this work, we consider that the detection efﬁciency precedes the energy information. Although
the output signal of a photomultiplier (PM) has no information concerning the energies of incident
photons, therefore, we adopted the PM designed for single photon counting. In order to improve thedetection efficiency, an oval reflector was employed to focus as many photons as possible on thephotocathode of the PM.
The detection efficiency was estimated taking into account several parameters: the sample
geometry, the reflection efficiency of the reflector, the effect of a finite volume of the 229mTh solution
sample, the quantum efficiency of the PM, and the absorbance by the HNO3 solution. The wholedetection efficiency was accordingly estimated to be at least 4 × 10 －４ for the photons in the range
between 340 and 600 nm (3.6 - 2.1 eV).

§3. Result and Discussion
It should be noted that 229mTh accumulates, during the growth time, depending on THCl, and 229mTh
disintegrates, during the measurement, at the rate THNO3. Here, THCl and THNO3 denote the half-lives of229m

Th as a solute in HCl and HNO3 solutions, respectively. Since the chemical condition of 229mTh turns

during the preparation of the sample, the following discussion on THNO3 is based on the number of 229mTh
atoms grown in the HCl solution.
In Fig. 1 and 2 are shown the time variation of the photon counting rate for the ﬁrst experiment(2-hour growth time) and that for the second experiment (17-hour growth time), respectively. The
horizontal dashed lines represent the constant background level obtained by a blank measurement. It
is obvious from the ﬁgures that additional photons deriving from the 229mTh source as well as from theother descendants were not evidently detected.
Although the half-life cannot be determined, the limits of the half-life can be estimated from theresults. The estimation of the limit of THNO3 requires 1) the lower limit of the number of 229mTh atoms at
the end of the growth in the HCl solution and 2) the detection efficiency which has already been
estimated. The number of 229mTh atoms can be figured from the activity of 229mTh at the end of thegrowth time. If THCl is 3 minutes, which was taken for the chemical separation, or shorter, it would be
difficult to estimate the limit of THNO3 because of their decay loss during the separation. From these34

Fig.1. Time variation of the counting rate for photons emitted from 229mTh HNO3 solution sample
grown for 2 hours in the 9M HCl.

Fig.2. Time variation of the counting rate for photons emitted from 229mTh HNO3 solution sample
grown for 17 hours in the 9M HCl.

conditions, the limits were estimated as follows.
1) If THCl is shorter than 3 minutes, it is difficult by the present experimental method to observe
evident photon events for any half-life of 229mTh in HNO3 solution.
2) If THCl is longer than 3 minutes, THNO3 can be limited as ＜ 3 minutes or ＞ 60 hours as indicated bysolid line in Fig. 1. Especially referring to the case that THCl is longer than 3 hours, the number of229m

Th atoms accumulated during the growth time is almost constant in both experiments and35
THNO3 should be ＜ 2 minutes or ＞ 400 days as indicated by the solid lines in Fig. 2.
For further discussion about THNO3, more detailed information on THCl is necessary. A new device forthe preparation of a point source of 229mTh is now under development.