- Email: [email protected]
A spectrometer for lifetime determination by β–γ–γ delayed coincidence technique at KUR-ISOL
Nuclear Instruments and Methods in Physics Research A 659 (2011) 193–197
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A spectrometer for lifetime determination by b2g2g delayed coincidence technique at KUR-ISOL Y. Kojima a,,1, H. Hayashi b,2, M. Shibata b, S. Endo a, K. Shizuma a, A. Taniguchi c a
Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Radioisotope Research Center, Nagoya University, Nagoya 464-8602, Japan c Research Reactor Institute, Kyoto University, Kumatori 590-0494, Japan b
a r t i c l e i n f o
abstract
Article history: Received 25 May 2011 Received in revised form 1 August 2011 Accepted 2 August 2011 Available online 12 August 2011
A new spectrometer for measuring nuclear level lifetimes has been installed at the on-line isotope separator of the Kyoto University Reactor. The spectrometer consists of a LaBr3(Ce), Ge and a thin plastic scintillation detector, and the lifetimes are determined using a b2g2g delayed coincidence technique. In this study, the LaBr3 detector was used to obtain time spectra, whereas the Ge detector was used to select a desired g branch. The energy dependence of the time resolutions was measured down to a photon energy of 100 keV. The lifetimes measured for the excited levels in 93Sr and 148Ce agree well with their evaluated values. The lifetime of 8.5(5) ns was obtained for the first time for the 98.2 keV level in 148Pr. & 2011 Elsevier B.V. All rights reserved.
Keywords: LaBr3(Ce) Delayed coincidence Lifetime 93 Sr 148 Ce 148 Pr
1. Introduction Extensive spectroscopic studies have been conducted on the fission products of 235U. Among these products, nuclei in the rare earth region having a mass number of around 150 still attract continuing attention because they provide an opportunity to study the transition from spherical nuclei to the quadrupole– octupole deformed nuclei. For example, Hayashi et al. recently reported that the experimental two-neutron separation energies for 147-149La deviate from their theoretically evaluated values [1,2]. This systematic deviation strongly suggests that a change in the nuclear structure which is not taken into account in the theoretical mass evaluation occurs. To investigate the nuclear structure of 147-149La and their neighboring nuclei, we are now planning to perform lifetime measurements at the on-line isotope separator installed at the Kyoto University Reactor (KUR-ISOL) [3]. In these experiments, a spectrometer consisting of a thin plastic scintillation detector and a recently developed LaBr3(Ce) scintillator [4–6] will be used to determine the lifetimes by a b2g delayed coincidence method. As listed in Table 1, LaBr3, among all Corresponding author. Tel.: þ 81 52 789 2570; fax: þ81 52 789 2567.
E-mail address: [email protected] (Y. Kojima). Present address: Radioisotope Research Center, Nagoya University, Nagoya 464-8602, Japan. 2 Present address: Institute of Health Biosciences, The University of Tokushima Graduate School, Kuramoto, Tokushima 770-8509, Japan. 1
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.08.005
the scintillation materials, shows the best energy resolution and a moderate detection efficiency for g-rays. Table 1 also shows that the LaBr3 scintillator has a good decay time of 16 ns, whereas those of most of the inorganic scintillators are a few hundred nanoseconds or longer, except for BaF2. A short decay time reduces variance of emission timing signals of scintillation photons. Thus, the LaBr3 scintillator has a good time resolution and is used for measuring the lifetimes [8–12]. In this work, we study the properties of our spectrometer installed at KUR-ISOL as well as its stability during long-term measurements. Next, we apply this spectrometer to measure lifetimes of excited levels of mass-separated 93Sr and 148Ce nuclides. By this experiment, we demonstrate the feasibility of lifetime determination, in the range of nanoseconds, by means of a slope method. Finally, the lifetime of the 98.2 keV level in 148Pr was measured, for which no experimental data is yet available.
2. Details of the spectrometer 2.1. LaBr3 detector The LaBr3 detector used in this study is a commercially available detector (Canberra, model LABR). The crystal, which is 1.5-in. in diameter and 1.5-in. in thickness, was coupled with the Hamamatsu 14-pin photomultiplier tube (PMT) R6231. The LaBr3 scintillator enclosed by a 0.7-mm-thick Teflon reflector was
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Table 1 Properties of scintillation materials [4]. Detection efficiencies relative to NaI are calculated for a 1.5 in. 1.5 in. crystal using the Geant4 computer code [7]. Scintillation material
Energy resolution for 662 keV g-ray (typical) (%)
Decay Detection efficiency time (ns) for 1 MeV g-ray
LaBr3 BaF2 BGO NaI
o3 12 12 7
16 0.6 300 250
1.5 1.6 4.0 1
FWHM %
10 5
2 1
102
103 Gamma−ray energy in keV
Fig. 1. Energy resolution (FWHM) of the LaBr3 detector observed at an applied voltage of 800 V.
placed in a 0.5-mm-thick aluminum housing. The R6231 tube is optimized for measuring the pulse height and not for measuring time. Thus, its timing property would not be as good as that of a faster PMT. However, the R6231 PMT is expected to have a good energy resolution, and is therefore suitable for application to experiments using the mass separator, because fission products usually show complex g spectra. The output signal from the anode was directly connected to a timing module described later. The energy spectra were obtained from the dynode signal, through a charge sensitive preamplifier and a main amplifier. The operating bias voltage specified by the manufacturer was 600–800 V. Hence, before the timing measurements, the energy resolutions and pulse heights were examined in this voltage range using standard radioactive sources such as 241Am, 133Ba, 152Eu, 137 Cs, and 60Co. A shaping time of 2 ms of the main amplifier was used for this measurement. Fig. 1 shows the energy resolution of the LaBr3 detector obtained at an applied voltage of 800 V. An energy resolution (FWHM) of 2.4% was obtained for a 662 keV g-ray. Energy resolution was also measured at bias voltages of 600 and 700 V. No difference was observed in the shapes of the g-ray peaks, as expected. However, amplitudes of signals from the PMT varied significantly in this voltage range, that is, the signal height increased as the applied voltage increased, as expected. When we used electronic devices having an input impedance of 50 O, the anode signals obtained at the 600 V bias voltage were less than 20 mV for X-rays ( r 100 keV). These small anode signals were below the minimum discrimination level of the timing modules. Therefore, the bias voltage of 800 V was applied for subsequent experiments. The background spectrum was also measured in a 5-cm-thick lead shielding surrounding the LaBr3 detector. LaBr3 contains an abundance of the naturally occurring radioisotopes 138La, 227Ac and its daughter nuclides, so that a significant self-background was observed. The background rate of this crystal was 62 cps in the 35–3000 keV region and 37 cps in the 35–1500 keV region. The Ba K,L,M, . . . X rays sequentially emitted after the b decay of 138 La were summed in the crystal, so that the peak was observed at 37 keV, which corresponds to the K binding energy of Ba atom. The counting rate of the Ba X-ray sum-peak was 18 cps.
2.2. Timing properties of the spectrometer Our system for measuring the lifetimes is based on the b2g delayed coincidence technique. The LaBr3 detector described earlier is used for g-ray measurements. Beta-rays are detected using a thin plastic scintillation detector. A 1-mm-thick plastic scintillator (pilot-U, 35 35 mm) was coupled to a fast PMT assembly (Hamamatsu H2431-51), and it was light-shielded by a 41-mm-thick reflection film. The detection efficiency was calculated using the Geant4 computer code [7]. The efficiency obtained at a source-to-detector distance of 25 mm was approximately 0.5% for the b2g coincidence measurements, assuming that the b transitions have a maximum energy of 2 MeV and the g-rays have an energy of 100 keV. The electronic unit used in this experiment consisted of standard nuclear instrumentation modules (NIM): two constant-fraction discriminators (CFDs, Canberra 2126) and a time-to-amplitude converter (TAC, Ortec 567). A 2-m-long delay cable is used to set the constantfraction timing for the LaBr3 detector, and a 0.25-m-long cable is used for the plastic scintillator. The lengths correspond to approximately 80% of the fall time of the anode signals from each detector. A b signal started the functioning of the TAC unit. The energy and TAC signals are recorded in list mode. The measuring system was calibrated using the Ortec 462 time calibrator. From the calibration measurements, a time calibration curve which shows a relation between a channel number and a time was obtained. This curve showed a good linearity, and the deviation of each data point from the calibration line was less than 20 ps. Prompt time spectra were measured using standard radioactive sources, 60Co and 134Cs, to evaluate the timing properties of this system. The 60Co decay (Qb ¼ 2824 keV) is characterized by its simple decay scheme [13]. It decays to a level of 2506 keV in 60 Ni (lifetime t of 0.43 ps), and 1173 and 1333 keV g-rays are emitted via the 1333 keV level (t ¼ 1:0 ps). The 134Cs nuclide b-decays to 134Ba (Qb ¼ 2059 keV) and emits g-rays [13]. The energies of the intense g-rays are 563, 569, 605, 796, and 802 keV. It should be noted that the 1643 keV level in 134Ba has a lifetime of 112 ps. However, the lifetime is shorter than the expected time resolution of the LaBr3 detector (a few hundred picoseconds). In addition, the b feeding intensity to this level is less than 2.5%. Therefore, the effect of the 112 ps lifetime on the measurement of the prompt time peak is small and hence can be neglected. To estimate the timing properties in the energy region lower than 100 keV, a method using an X-ray converter was also applied [14]. In this method, an appropriate target such as tungsten is placed near a radioactive source. The g-rays from the source excite the target atoms, and the X-rays are emitted from the target after a short time (a time span range of nanoseconds). In order to obtain time spectra down to a photon energy of 44 keV, we used the following materials as converters: Tb (Tb4O7, powder, 5.1 g), Yb (Yb2O3, powder, 5.5 g), W and Pb (0.5-mm-thick metallic sheet, 20 20 mm). The prompt time peaks were also measured using short-lived 93 Rb and 148Ce to obtain the timing properties in the energy region of 100–500 keV. The 93Rb and 148Ce nuclides were obtained as mass-separated beams from the fission products of 235 U using the on-line isotope separator KUR-ISOL [3]. In this experiment, g-rays were also measured using a Ge detector to select the desired g branch. Further details of the source preparation of 93Rb and 148Ce are described in Section 3.1. The insets of Fig. 2 show the prompt time spectra obtained after careful adjustment of the time walks of the CFDs. The prompt time spectra were obtained by off-line sorting. In this sorting, gates were set on the full-energy g peaks detected with the LaBr3 detector. Here, a g-ray peak observed around 99 keV using the LaBr3 detector was a triplet: 98.0, 98.2 and 99.0 keV
Counts / channel
Y. Kojima et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 193–197
FWHM in ps
1200
800
3 2
[x10 ] 99 keV γ ray FWHM 601(7) ps
2
1
2.3. Time stability of the spectrometer
[x 10 ] 1332 keV γ ray FWHM 375(6) ps
1 0 1590
1600
1610
2060
2070
Channel number
400 2
10 Photon energy in keV
10
3
56 s Ce 51.3 Q β =2060 5.9 4.8
Iβ
390 . 291 7 (1.36 . ) 101 7 (17.4 .0 (4 ) 289 . 6 ) . 191 6 (6.3) .6 (2 196 . 0 6) . 98.00 (8.0) (31. 99.0 8) (30. 98.2 (5.3 1) )
Fig. 2. Energy dependence of time resolutions (FWHM) of the prompt peaks. The closed circles show FWHMs obtained using g-rays from standard radioactive sources, closed triangles show those obtained from mass-separated short-lived isotopes, and open circles show those obtained using X-rays. Error bars are drawn for all data points, but the uncertainties for some data points are smaller than the size of the mark ( o 8 ps). The curves are guides to the eyes. The insets show prompt time spectra obtained for the 99 keV (148Ce) and 1332 keV (60Co) g-rays.
148
148
195
390.7
Electronic drifts in the time measurement shift the position of the prompt time peak and degrade the time resolution. The measuring modules are sensitive to a change in temperature. To keep electronic drifts as small as possible, the experimental room was air-conditioned. Furthermore, fans were used to force air into the measuring units to balance the temperature with the environment. Under this condition, the prompt time spectra were measured using 60Co every two hours to evaluate the stability of the spectrometer. The room temperature was also monitored with a thermistor thermometer to determine whether the room temperature was correlated with drifts in prompt time peaks. The uncertainty of the thermometer was 0.3 1C. The stability of the spectrometer was studied for a period of 120 h, and during this time, the temperature varied from 23.3 to 25.9 1C; it was within 24.070.5 1C for 80% of this period. Under this condition, the maximum drift of the centroid position was 732 ps, and the maximum fluctuation of the FWHM was 716 ps. This fluctuation was much smaller than the FWHM of 375 ps. Here, in our proposed on-line experiments for mass-separated isotopes, the measuring time is approximately 50 h. Therefore, we conclude that the electronic drift is negligible in the determination of lifetimes using the slope method when fluctuations in the room temperature are kept within 1 1C. Here, we note that all data are recorded in list mode, so that we can check the drift and correct it to obtain minimum fluctuation if necessary.
289.7 196.0 99.0 98.2 0
Pr
Fig. 3. A partial decay scheme of 148Ce. Data are taken from Ref. [13]. A g-transition intensity including internal conversion is shown in the parentheses. Energies are given in keV.
g-rays associated with the b decay of 148Ce [13]. Among them, the 98.2 keV g-ray depopulates the 98.2 keV level in 148Pr, which has a long lifetime of 8.5 ns, as described in Section 3.2. To eliminate the delay component due to the 98.2 keV g-ray, the prompt curve for the 99.0 keV g-ray from the 99.0 keV level in 148Pr was obtained by applying an additional gate on the 291.7 keV g-ray (see Fig. 3), measured with the Ge detector. As shown in the inset of Fig. 2, no delay component was observed in the time spectrum of the 99.0 keV g-ray. Therefore, the lifetime of the 99.0 keV was assumed to be much smaller than 600 ps. The time resolutions (FWHM) were obtained through the least-squares fitting method using the Gaussian curve. The FWHM of 601(7) ps observed for the 99.0 keV g-ray is 2–4 times worse than that of the BaF2 scintillators [15,16] but much superior to the FWHM of other g-ray detectors such as planar Ge and NaI detectors [17]. The time resolutions for other g- and X-rays were also evaluated by a similar procedure. The energy dependence of the FWHM is shown in Fig. 2. The time resolutions obtained using short-lived isotopes (shown by triangles) were connected smoothly to the time resolutions obtained using standard radioactive sources (closed circles) in the eye-guide curve. It can be seen that the experimental time resolutions suddenly increase below photon energies of 100 keV. This is probably due to the atomic level lifetimes, because the prompt time peaks in this region were obtained using X-rays. The time resolutions shown by the open circles in Fig. 2 are the upper limits of the intrinsic values.
3. Lifetime measurements at KUR-ISOL 3.1. Source preparation The spectrometer was used for measuring the level lifetime of mass-separated fission products. These on-line experiments were performed separately in two stages. First, the lifetimes of the 213.4 keV level in 93Sr and the 158.5 keV level in 148Ce were measured to demonstrate that our spectrometer measures lifetimes without any systematic errors; the lifetimes of these levels were well determined as 6.6(4) ns and 1.46(9) ns, respectively [13]. Next, the lifetimes of excited levels in 148Pr were measured; Note that no lifetimes have thus far been reported for any excited levels in 148Pr. Gamma-rays from 93Sr and 148Pr nuclei were also used to obtain prompt time spectra, as described in Section 2.2. Their parent nuclides – 93Rb, 148La, and 148Ce isotopes – were prepared at KUR-ISOL [3], following the thermal neutron-induced fission of 235U. A 93% enriched 235UF4 target (50 mg) was irradiated with a thermal neutron flux of about 6 1011 n/cm2s from the experimental reactor operated at a thermal power of 1 MW, or a flux of 3 1012 n/cm2s at a thermal power of 5 MW. The fission products thermalized in the target chamber were transported by a He–N2 mixed-gas jet stream to a surface-ionization-type ion source. After ionization, the radioactive isotopes were extracted, accelerated to 30 keV, and mass-separated with a resolution of M=DM 600. The mass-separated ions were implanted into an aluminized Mylar tape in a computer-controlled tape transport system. The radioactive sources were periodically moved to a detector station at time intervals listed in Table 2. The LaBr3 and plastic scintillation detectors were installed in the lead-shielded detector station. The source-to-detector distance for both detectors was approximately 25 mm. To check contaminations and to select a desired decay branch through off-line sorting, the g spectra were also measured using a high-resolution Ge detector: n-type Ge detector (Princeton Gamma-Tech, crystal size of 54 mm in diameter and 57 mm in thickness) for measurements of 93Rb and 148La, and a short-coaxial low-energy photon detector (Ortec LO-AX, crystal size
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of 51 mm in diameter and 20 mm in thickness) for 148Ce. The source-to-detector distance was 20 mm for 93Rb and 148La, and 9 mm for 148Ce. The b2g2g triple coincidence data, that is, the TAC and energy signals from the three detectors, were recorded in list mode. 3.2. Lifetimes of excited levels in
93
Sr,
148
Ce, and
148
Pr
The decay curve shown in Fig. 4(a) was obtained through gating in a g-energy region of 205–230 keV observed in a mass fraction of 93. This peak is a doublet of the 213.4 and 219.2 keV g-rays from the b decay of 93Rb. Here, the 219 keV g-ray is a result of a transition from the 433 keV level in 93Sr, whose Table 2 Experimental conditions for preparing mass-separated isotopes at KUR-ISOL. The reactor KUR was operated at two different thermal powers, 1 or 5 MW, as indicated in the last column. Parent nuclide
Tape cycle (s)
Measuring time (h)
Power (MW)
93
11.7 4.0 97 97
5.5 12.4 39.0 1.5
1 1 1 5
Rb La Ce 148 Ce 148 148
10
half-life is reported to be o 0:3 ns (lifetime o0:4 ns) [13]. Because this lifetime is shorter than the time resolution of 0.5 ns (see Fig. 2), a Gaussian-like peak is observed around the 1600 channel. The steep slope observed in Fig. 4(a) is attributed to the 213 keV level in 93Sr. A lifetime of 6.48(10) ns was deduced via the least-squares fitting method using an exponential function. In Fig. 4(a), the fitting region used for obtaining the lifetime value is represented by closed circles. Note that the fitting results varied to some extent as the fitting region changed. However, the fluctuation ( o8 ps, in this case) was much less than the uncertainty estimated for this value of lifetime through the fitting procedure. The lifetime obtained in this study agrees well with the previous value of 6.6(4) ns [13]. Long lifetimes were not observed in 93Sr, except for the 213 keV level. This is also consistent with the published data. Fig. 4(b) shows a decay curve of the 158.5 keV level in 148Ce. It was obtained through gating on the 159 keV g-ray observed with the LaBr3 detector. This g peak is a singlet. In fact, no g-rays following the b decay of 148La were observed near the 159 keV peak in a spectrum obtained using the Ge detector. The lifetime of 1.49(7) ns, deduced from the slope, is in good agreement with the evaluated value of 1.46(9) ns [13]. These two results for 93Sr and 148 Ce show that our spectrometer is able to determine lifetimes accurately in the nanosecond range.
4
10
432.6 keV
4
158.5 keV 158.5 (LaBr3)
213.4 keV 213.4 (LaBr3) 93
Sr
10
2
Counts per channel
Counts per channel
219.2
148
Ce
10
2
lifetime 1.49(7) ns
lifetime 6.48(10) ns
( evaluated value 1.46(9) ns )
( evaluated value 6.6(4) ns )
10
0
1600 1800 Channel number (112.4 ps/ch)
196.0 keV 98.0 (Ge) 98.2keV 98.2 (LaBr3)
2
148
Pr
10
1
10
2000
Counts per channel
Counts per channel
10
10
0
1600 1700 Channel number (112.4 ps/ch)
0
160
170 180 190 Channel number (1124 ps/ch)
289.6 keV 191.6 (Ge) 98.2 keV 98.2 (LaBr3)
2
148
Pr
10
1
lifetime 8.46(74) ns
lifetime 8.56(67) ns 10
1800
10
0
160
170 180 190 Channel number (1124 ps/ch)
Fig. 4. (a) Decay curve of the 213.4 keV level in 93Sr, (b) decay curve of the 158.5 keV level in 148Ce, and (c) and (d) decay curves of the 98.2 keV level in 148Pr. Energy gates used to obtain these decay curves are presented in the inset of each figure; Figures (a) and (b) were obtained through gating only on the LaBr3 detector, whereas gates for (c) and (d) were set on the LaBr3 and Ge detectors. Lifetimes were deduced from the slope. The closed circles represent the fitting regions.
Y. Kojima et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 193–197
For the b decay of 148Ce, a new delay component was observed in the time spectrum, and it was coincident with the 99 keV g peak measured with the LaBr3 detector. As described in Section 2.2, the 99 keV peak is a triplet of the 98.0, 98.2, and 99.0 keV g-rays. To select a g branch, additional gates were set on the g-rays measured with the Ge detector. Among these time spectra, clear slopes were observed for the spectra gated on the 98.0 and 191.6 keV g-rays, as shown in Figs. 4(c) and (d). This means that the delay component is due to the 98.2 keV level in 148Pr. Here, we note that prompt peaks were also observed in Figs. 4(c) and (d). They correspond to the decay of 196.0 and 289.6 keV levels in 148 Pr, respectively. This is because the b feeding intensity to the 98.2 keV level is quite small (see Fig. 3) and the 98.2 keV level is populated by higher energy levels via g transitions. The observation of the prompt peaks also means that the lifetimes of the 196.0 and 289.6 keV levels are much shorter than 500 ps. From these two decay curves, the lifetimes were deduced to be 8.56(67) and 8.46(74) ns, respectively. Here, the fluctuation of the estimated lifetime values was about 0.2 ns when the fitting regions were changed; it is within the uncertainty value stated. The averaged value of 8.5(5) ns was adopted for the lifetime of the 98.2 keV level in 148Pr; it was obtained for the first time in this study. The 98.2 keV level (Ip ¼ ð1 ,2 ,3 Þ) is de-excited only by the 98.2 keV E2 g-ray [13]. Using the experimental lifetime value of 8.5(5) ns and the total internal conversion coefficient of 2.26 [18], we obtained the reduced transition probability B(E2) of 68(4) in Weisskopf units. This value roughly agrees with the typical probability B(E2) of 102 for collective E2 transitions. The nuclear structure can be discussed further after more detailed spectroscopic studies.
4. Conclusions We installed a spectrometer consisting of a LaBr3 scintillator and a thin plastic scintillator, for use in measuring the nuclear level lifetimes at the on-line isotope separator KUR-ISOL. The spectrometer showed a good time resolution of 375 ps for the 1333 keV g-ray of 60Co, after careful adjustments. The prompt time spectra were also measured down to a photon energy of 100 keV using full-energy g peaks emitted by mass-separated
197
93
Rb and 148Ce; the FWHM of 601 ps was observed for the 99 keV 93 Sr and 148Ce agree with the previous values, and the lifetime of 8.5(5) ns for the 98.2 keV level in 148Pr was successfully obtained, for the first time. We therefore conclude that the spectrometer is capable of measuring the level lifetimes of mass-separated fission products in the nanosecond range.
g-ray. The level lifetimes observed for the mass-separated
Acknowledgments This research was supported by a Grant-in-Aid for Scientific Research (no. 21740185) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The measurements of lifetimes were carried out under the Research Collaboration Program of the Research Reactor Institute, Kyoto University. We would like to thank Mr. Higuma and Mr. Fukushige for their help in performing the experiments at KUR-ISOL.
References [1] H. Hayashi, et al., in: Proceedings of the International Conference on Nuclear Data for Science and Technology, Nice, 2007, pp. 131–134. [2] H. Hayashi, et al., Nucl. Instr. and Meth. A 606 (2009) 484. [3] A. Taniguchi, et al., Nucl. Instr. and Meth. A 351 (1994) 378. [4] Saint-Gobain website: /http://www.detectors.saint-gobain.com/Materials GasTubes.aspxS. [5] E.V.D. van Loef, et al., Nucl. Instr. and Meth. A 486 (2002) 254. [6] F. Quarati, et al., Nucl. Instr. and Meth. A 574 (2007) 115. [7] S. Agostinelli, et al., Nucl. Instr. and Meth. A 506 (2003) 250. [8] E.R. White, et al., Phys. Rev. C 76 (2007) 057303. [9] D. Weisshaar, et al., Nucl. Instr. and Meth. A 594 (2008) 56. [10] J.-M. Re´gis, et al., Nucl. Instr. and Meth. A 606 (2009) 466. [11] J.-M. Re´gis, et al., Nucl. Instr. and Meth. A 622 (2010) 83. [12] N. M˘arginean, et al., Eur. Phys. J. A 46 (2010) 329. [13] Evaluated Nuclear Structure Data File (ENSDF), maintained by the National Nuclear Data Center, Brookhaven National Laboratory: /http://www.nndc. bnl.gov/ensdfS. [14] K. Kawade, et al., Nucl. Instr. and Meth. 200 (1982) 583. [15] H. Mach, et al., Nucl. Instr. and Meth. A 280 (1989) 49. [16] A. Taniguchi, et al., J. Phys. Soc. Jpn. 65 (1996) 3824. [17] Ortec Catalog, Modular Pulse-Processing Electronics and Semiconductor Radiation Detectors, 1997. [18] T. Kibe´di, et al., Nucl. Instr. and Meth. A 589 (2008) 202.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A spectrometer for lifetime determination by b2g2g delayed coincidence technique at KUR-ISOL Y. Kojima a,,1, H. Hayashi b,2, M. Shibata b, S. Endo a, K. Shizuma a, A. Taniguchi c a
Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Radioisotope Research Center, Nagoya University, Nagoya 464-8602, Japan c Research Reactor Institute, Kyoto University, Kumatori 590-0494, Japan b
a r t i c l e i n f o
abstract
Article history: Received 25 May 2011 Received in revised form 1 August 2011 Accepted 2 August 2011 Available online 12 August 2011
A new spectrometer for measuring nuclear level lifetimes has been installed at the on-line isotope separator of the Kyoto University Reactor. The spectrometer consists of a LaBr3(Ce), Ge and a thin plastic scintillation detector, and the lifetimes are determined using a b2g2g delayed coincidence technique. In this study, the LaBr3 detector was used to obtain time spectra, whereas the Ge detector was used to select a desired g branch. The energy dependence of the time resolutions was measured down to a photon energy of 100 keV. The lifetimes measured for the excited levels in 93Sr and 148Ce agree well with their evaluated values. The lifetime of 8.5(5) ns was obtained for the first time for the 98.2 keV level in 148Pr. & 2011 Elsevier B.V. All rights reserved.
Keywords: LaBr3(Ce) Delayed coincidence Lifetime 93 Sr 148 Ce 148 Pr
1. Introduction Extensive spectroscopic studies have been conducted on the fission products of 235U. Among these products, nuclei in the rare earth region having a mass number of around 150 still attract continuing attention because they provide an opportunity to study the transition from spherical nuclei to the quadrupole– octupole deformed nuclei. For example, Hayashi et al. recently reported that the experimental two-neutron separation energies for 147-149La deviate from their theoretically evaluated values [1,2]. This systematic deviation strongly suggests that a change in the nuclear structure which is not taken into account in the theoretical mass evaluation occurs. To investigate the nuclear structure of 147-149La and their neighboring nuclei, we are now planning to perform lifetime measurements at the on-line isotope separator installed at the Kyoto University Reactor (KUR-ISOL) [3]. In these experiments, a spectrometer consisting of a thin plastic scintillation detector and a recently developed LaBr3(Ce) scintillator [4–6] will be used to determine the lifetimes by a b2g delayed coincidence method. As listed in Table 1, LaBr3, among all Corresponding author. Tel.: þ 81 52 789 2570; fax: þ81 52 789 2567.
E-mail address: [email protected] (Y. Kojima). Present address: Radioisotope Research Center, Nagoya University, Nagoya 464-8602, Japan. 2 Present address: Institute of Health Biosciences, The University of Tokushima Graduate School, Kuramoto, Tokushima 770-8509, Japan. 1
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.08.005
the scintillation materials, shows the best energy resolution and a moderate detection efficiency for g-rays. Table 1 also shows that the LaBr3 scintillator has a good decay time of 16 ns, whereas those of most of the inorganic scintillators are a few hundred nanoseconds or longer, except for BaF2. A short decay time reduces variance of emission timing signals of scintillation photons. Thus, the LaBr3 scintillator has a good time resolution and is used for measuring the lifetimes [8–12]. In this work, we study the properties of our spectrometer installed at KUR-ISOL as well as its stability during long-term measurements. Next, we apply this spectrometer to measure lifetimes of excited levels of mass-separated 93Sr and 148Ce nuclides. By this experiment, we demonstrate the feasibility of lifetime determination, in the range of nanoseconds, by means of a slope method. Finally, the lifetime of the 98.2 keV level in 148Pr was measured, for which no experimental data is yet available.
2. Details of the spectrometer 2.1. LaBr3 detector The LaBr3 detector used in this study is a commercially available detector (Canberra, model LABR). The crystal, which is 1.5-in. in diameter and 1.5-in. in thickness, was coupled with the Hamamatsu 14-pin photomultiplier tube (PMT) R6231. The LaBr3 scintillator enclosed by a 0.7-mm-thick Teflon reflector was
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Table 1 Properties of scintillation materials [4]. Detection efficiencies relative to NaI are calculated for a 1.5 in. 1.5 in. crystal using the Geant4 computer code [7]. Scintillation material
Energy resolution for 662 keV g-ray (typical) (%)
Decay Detection efficiency time (ns) for 1 MeV g-ray
LaBr3 BaF2 BGO NaI
o3 12 12 7
16 0.6 300 250
1.5 1.6 4.0 1
FWHM %
10 5
2 1
102
103 Gamma−ray energy in keV
Fig. 1. Energy resolution (FWHM) of the LaBr3 detector observed at an applied voltage of 800 V.
placed in a 0.5-mm-thick aluminum housing. The R6231 tube is optimized for measuring the pulse height and not for measuring time. Thus, its timing property would not be as good as that of a faster PMT. However, the R6231 PMT is expected to have a good energy resolution, and is therefore suitable for application to experiments using the mass separator, because fission products usually show complex g spectra. The output signal from the anode was directly connected to a timing module described later. The energy spectra were obtained from the dynode signal, through a charge sensitive preamplifier and a main amplifier. The operating bias voltage specified by the manufacturer was 600–800 V. Hence, before the timing measurements, the energy resolutions and pulse heights were examined in this voltage range using standard radioactive sources such as 241Am, 133Ba, 152Eu, 137 Cs, and 60Co. A shaping time of 2 ms of the main amplifier was used for this measurement. Fig. 1 shows the energy resolution of the LaBr3 detector obtained at an applied voltage of 800 V. An energy resolution (FWHM) of 2.4% was obtained for a 662 keV g-ray. Energy resolution was also measured at bias voltages of 600 and 700 V. No difference was observed in the shapes of the g-ray peaks, as expected. However, amplitudes of signals from the PMT varied significantly in this voltage range, that is, the signal height increased as the applied voltage increased, as expected. When we used electronic devices having an input impedance of 50 O, the anode signals obtained at the 600 V bias voltage were less than 20 mV for X-rays ( r 100 keV). These small anode signals were below the minimum discrimination level of the timing modules. Therefore, the bias voltage of 800 V was applied for subsequent experiments. The background spectrum was also measured in a 5-cm-thick lead shielding surrounding the LaBr3 detector. LaBr3 contains an abundance of the naturally occurring radioisotopes 138La, 227Ac and its daughter nuclides, so that a significant self-background was observed. The background rate of this crystal was 62 cps in the 35–3000 keV region and 37 cps in the 35–1500 keV region. The Ba K,L,M, . . . X rays sequentially emitted after the b decay of 138 La were summed in the crystal, so that the peak was observed at 37 keV, which corresponds to the K binding energy of Ba atom. The counting rate of the Ba X-ray sum-peak was 18 cps.
2.2. Timing properties of the spectrometer Our system for measuring the lifetimes is based on the b2g delayed coincidence technique. The LaBr3 detector described earlier is used for g-ray measurements. Beta-rays are detected using a thin plastic scintillation detector. A 1-mm-thick plastic scintillator (pilot-U, 35 35 mm) was coupled to a fast PMT assembly (Hamamatsu H2431-51), and it was light-shielded by a 41-mm-thick reflection film. The detection efficiency was calculated using the Geant4 computer code [7]. The efficiency obtained at a source-to-detector distance of 25 mm was approximately 0.5% for the b2g coincidence measurements, assuming that the b transitions have a maximum energy of 2 MeV and the g-rays have an energy of 100 keV. The electronic unit used in this experiment consisted of standard nuclear instrumentation modules (NIM): two constant-fraction discriminators (CFDs, Canberra 2126) and a time-to-amplitude converter (TAC, Ortec 567). A 2-m-long delay cable is used to set the constantfraction timing for the LaBr3 detector, and a 0.25-m-long cable is used for the plastic scintillator. The lengths correspond to approximately 80% of the fall time of the anode signals from each detector. A b signal started the functioning of the TAC unit. The energy and TAC signals are recorded in list mode. The measuring system was calibrated using the Ortec 462 time calibrator. From the calibration measurements, a time calibration curve which shows a relation between a channel number and a time was obtained. This curve showed a good linearity, and the deviation of each data point from the calibration line was less than 20 ps. Prompt time spectra were measured using standard radioactive sources, 60Co and 134Cs, to evaluate the timing properties of this system. The 60Co decay (Qb ¼ 2824 keV) is characterized by its simple decay scheme [13]. It decays to a level of 2506 keV in 60 Ni (lifetime t of 0.43 ps), and 1173 and 1333 keV g-rays are emitted via the 1333 keV level (t ¼ 1:0 ps). The 134Cs nuclide b-decays to 134Ba (Qb ¼ 2059 keV) and emits g-rays [13]. The energies of the intense g-rays are 563, 569, 605, 796, and 802 keV. It should be noted that the 1643 keV level in 134Ba has a lifetime of 112 ps. However, the lifetime is shorter than the expected time resolution of the LaBr3 detector (a few hundred picoseconds). In addition, the b feeding intensity to this level is less than 2.5%. Therefore, the effect of the 112 ps lifetime on the measurement of the prompt time peak is small and hence can be neglected. To estimate the timing properties in the energy region lower than 100 keV, a method using an X-ray converter was also applied [14]. In this method, an appropriate target such as tungsten is placed near a radioactive source. The g-rays from the source excite the target atoms, and the X-rays are emitted from the target after a short time (a time span range of nanoseconds). In order to obtain time spectra down to a photon energy of 44 keV, we used the following materials as converters: Tb (Tb4O7, powder, 5.1 g), Yb (Yb2O3, powder, 5.5 g), W and Pb (0.5-mm-thick metallic sheet, 20 20 mm). The prompt time peaks were also measured using short-lived 93 Rb and 148Ce to obtain the timing properties in the energy region of 100–500 keV. The 93Rb and 148Ce nuclides were obtained as mass-separated beams from the fission products of 235 U using the on-line isotope separator KUR-ISOL [3]. In this experiment, g-rays were also measured using a Ge detector to select the desired g branch. Further details of the source preparation of 93Rb and 148Ce are described in Section 3.1. The insets of Fig. 2 show the prompt time spectra obtained after careful adjustment of the time walks of the CFDs. The prompt time spectra were obtained by off-line sorting. In this sorting, gates were set on the full-energy g peaks detected with the LaBr3 detector. Here, a g-ray peak observed around 99 keV using the LaBr3 detector was a triplet: 98.0, 98.2 and 99.0 keV
Counts / channel
Y. Kojima et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 193–197
FWHM in ps
1200
800
3 2
[x10 ] 99 keV γ ray FWHM 601(7) ps
2
1
2.3. Time stability of the spectrometer
[x 10 ] 1332 keV γ ray FWHM 375(6) ps
1 0 1590
1600
1610
2060
2070
Channel number
400 2
10 Photon energy in keV
10
3
56 s Ce 51.3 Q β =2060 5.9 4.8
Iβ
390 . 291 7 (1.36 . ) 101 7 (17.4 .0 (4 ) 289 . 6 ) . 191 6 (6.3) .6 (2 196 . 0 6) . 98.00 (8.0) (31. 99.0 8) (30. 98.2 (5.3 1) )
Fig. 2. Energy dependence of time resolutions (FWHM) of the prompt peaks. The closed circles show FWHMs obtained using g-rays from standard radioactive sources, closed triangles show those obtained from mass-separated short-lived isotopes, and open circles show those obtained using X-rays. Error bars are drawn for all data points, but the uncertainties for some data points are smaller than the size of the mark ( o 8 ps). The curves are guides to the eyes. The insets show prompt time spectra obtained for the 99 keV (148Ce) and 1332 keV (60Co) g-rays.
148
148
195
390.7
Electronic drifts in the time measurement shift the position of the prompt time peak and degrade the time resolution. The measuring modules are sensitive to a change in temperature. To keep electronic drifts as small as possible, the experimental room was air-conditioned. Furthermore, fans were used to force air into the measuring units to balance the temperature with the environment. Under this condition, the prompt time spectra were measured using 60Co every two hours to evaluate the stability of the spectrometer. The room temperature was also monitored with a thermistor thermometer to determine whether the room temperature was correlated with drifts in prompt time peaks. The uncertainty of the thermometer was 0.3 1C. The stability of the spectrometer was studied for a period of 120 h, and during this time, the temperature varied from 23.3 to 25.9 1C; it was within 24.070.5 1C for 80% of this period. Under this condition, the maximum drift of the centroid position was 732 ps, and the maximum fluctuation of the FWHM was 716 ps. This fluctuation was much smaller than the FWHM of 375 ps. Here, in our proposed on-line experiments for mass-separated isotopes, the measuring time is approximately 50 h. Therefore, we conclude that the electronic drift is negligible in the determination of lifetimes using the slope method when fluctuations in the room temperature are kept within 1 1C. Here, we note that all data are recorded in list mode, so that we can check the drift and correct it to obtain minimum fluctuation if necessary.
289.7 196.0 99.0 98.2 0
Pr
Fig. 3. A partial decay scheme of 148Ce. Data are taken from Ref. [13]. A g-transition intensity including internal conversion is shown in the parentheses. Energies are given in keV.
g-rays associated with the b decay of 148Ce [13]. Among them, the 98.2 keV g-ray depopulates the 98.2 keV level in 148Pr, which has a long lifetime of 8.5 ns, as described in Section 3.2. To eliminate the delay component due to the 98.2 keV g-ray, the prompt curve for the 99.0 keV g-ray from the 99.0 keV level in 148Pr was obtained by applying an additional gate on the 291.7 keV g-ray (see Fig. 3), measured with the Ge detector. As shown in the inset of Fig. 2, no delay component was observed in the time spectrum of the 99.0 keV g-ray. Therefore, the lifetime of the 99.0 keV was assumed to be much smaller than 600 ps. The time resolutions (FWHM) were obtained through the least-squares fitting method using the Gaussian curve. The FWHM of 601(7) ps observed for the 99.0 keV g-ray is 2–4 times worse than that of the BaF2 scintillators [15,16] but much superior to the FWHM of other g-ray detectors such as planar Ge and NaI detectors [17]. The time resolutions for other g- and X-rays were also evaluated by a similar procedure. The energy dependence of the FWHM is shown in Fig. 2. The time resolutions obtained using short-lived isotopes (shown by triangles) were connected smoothly to the time resolutions obtained using standard radioactive sources (closed circles) in the eye-guide curve. It can be seen that the experimental time resolutions suddenly increase below photon energies of 100 keV. This is probably due to the atomic level lifetimes, because the prompt time peaks in this region were obtained using X-rays. The time resolutions shown by the open circles in Fig. 2 are the upper limits of the intrinsic values.
3. Lifetime measurements at KUR-ISOL 3.1. Source preparation The spectrometer was used for measuring the level lifetime of mass-separated fission products. These on-line experiments were performed separately in two stages. First, the lifetimes of the 213.4 keV level in 93Sr and the 158.5 keV level in 148Ce were measured to demonstrate that our spectrometer measures lifetimes without any systematic errors; the lifetimes of these levels were well determined as 6.6(4) ns and 1.46(9) ns, respectively [13]. Next, the lifetimes of excited levels in 148Pr were measured; Note that no lifetimes have thus far been reported for any excited levels in 148Pr. Gamma-rays from 93Sr and 148Pr nuclei were also used to obtain prompt time spectra, as described in Section 2.2. Their parent nuclides – 93Rb, 148La, and 148Ce isotopes – were prepared at KUR-ISOL [3], following the thermal neutron-induced fission of 235U. A 93% enriched 235UF4 target (50 mg) was irradiated with a thermal neutron flux of about 6 1011 n/cm2s from the experimental reactor operated at a thermal power of 1 MW, or a flux of 3 1012 n/cm2s at a thermal power of 5 MW. The fission products thermalized in the target chamber were transported by a He–N2 mixed-gas jet stream to a surface-ionization-type ion source. After ionization, the radioactive isotopes were extracted, accelerated to 30 keV, and mass-separated with a resolution of M=DM 600. The mass-separated ions were implanted into an aluminized Mylar tape in a computer-controlled tape transport system. The radioactive sources were periodically moved to a detector station at time intervals listed in Table 2. The LaBr3 and plastic scintillation detectors were installed in the lead-shielded detector station. The source-to-detector distance for both detectors was approximately 25 mm. To check contaminations and to select a desired decay branch through off-line sorting, the g spectra were also measured using a high-resolution Ge detector: n-type Ge detector (Princeton Gamma-Tech, crystal size of 54 mm in diameter and 57 mm in thickness) for measurements of 93Rb and 148La, and a short-coaxial low-energy photon detector (Ortec LO-AX, crystal size
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of 51 mm in diameter and 20 mm in thickness) for 148Ce. The source-to-detector distance was 20 mm for 93Rb and 148La, and 9 mm for 148Ce. The b2g2g triple coincidence data, that is, the TAC and energy signals from the three detectors, were recorded in list mode. 3.2. Lifetimes of excited levels in
93
Sr,
148
Ce, and
148
Pr
The decay curve shown in Fig. 4(a) was obtained through gating in a g-energy region of 205–230 keV observed in a mass fraction of 93. This peak is a doublet of the 213.4 and 219.2 keV g-rays from the b decay of 93Rb. Here, the 219 keV g-ray is a result of a transition from the 433 keV level in 93Sr, whose Table 2 Experimental conditions for preparing mass-separated isotopes at KUR-ISOL. The reactor KUR was operated at two different thermal powers, 1 or 5 MW, as indicated in the last column. Parent nuclide
Tape cycle (s)
Measuring time (h)
Power (MW)
93
11.7 4.0 97 97
5.5 12.4 39.0 1.5
1 1 1 5
Rb La Ce 148 Ce 148 148
10
half-life is reported to be o 0:3 ns (lifetime o0:4 ns) [13]. Because this lifetime is shorter than the time resolution of 0.5 ns (see Fig. 2), a Gaussian-like peak is observed around the 1600 channel. The steep slope observed in Fig. 4(a) is attributed to the 213 keV level in 93Sr. A lifetime of 6.48(10) ns was deduced via the least-squares fitting method using an exponential function. In Fig. 4(a), the fitting region used for obtaining the lifetime value is represented by closed circles. Note that the fitting results varied to some extent as the fitting region changed. However, the fluctuation ( o8 ps, in this case) was much less than the uncertainty estimated for this value of lifetime through the fitting procedure. The lifetime obtained in this study agrees well with the previous value of 6.6(4) ns [13]. Long lifetimes were not observed in 93Sr, except for the 213 keV level. This is also consistent with the published data. Fig. 4(b) shows a decay curve of the 158.5 keV level in 148Ce. It was obtained through gating on the 159 keV g-ray observed with the LaBr3 detector. This g peak is a singlet. In fact, no g-rays following the b decay of 148La were observed near the 159 keV peak in a spectrum obtained using the Ge detector. The lifetime of 1.49(7) ns, deduced from the slope, is in good agreement with the evaluated value of 1.46(9) ns [13]. These two results for 93Sr and 148 Ce show that our spectrometer is able to determine lifetimes accurately in the nanosecond range.
4
10
432.6 keV
4
158.5 keV 158.5 (LaBr3)
213.4 keV 213.4 (LaBr3) 93
Sr
10
2
Counts per channel
Counts per channel
219.2
148
Ce
10
2
lifetime 1.49(7) ns
lifetime 6.48(10) ns
( evaluated value 1.46(9) ns )
( evaluated value 6.6(4) ns )
10
0
1600 1800 Channel number (112.4 ps/ch)
196.0 keV 98.0 (Ge) 98.2keV 98.2 (LaBr3)
2
148
Pr
10
1
10
2000
Counts per channel
Counts per channel
10
10
0
1600 1700 Channel number (112.4 ps/ch)
0
160
170 180 190 Channel number (1124 ps/ch)
289.6 keV 191.6 (Ge) 98.2 keV 98.2 (LaBr3)
2
148
Pr
10
1
lifetime 8.46(74) ns
lifetime 8.56(67) ns 10
1800
10
0
160
170 180 190 Channel number (1124 ps/ch)
Fig. 4. (a) Decay curve of the 213.4 keV level in 93Sr, (b) decay curve of the 158.5 keV level in 148Ce, and (c) and (d) decay curves of the 98.2 keV level in 148Pr. Energy gates used to obtain these decay curves are presented in the inset of each figure; Figures (a) and (b) were obtained through gating only on the LaBr3 detector, whereas gates for (c) and (d) were set on the LaBr3 and Ge detectors. Lifetimes were deduced from the slope. The closed circles represent the fitting regions.
Y. Kojima et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 193–197
For the b decay of 148Ce, a new delay component was observed in the time spectrum, and it was coincident with the 99 keV g peak measured with the LaBr3 detector. As described in Section 2.2, the 99 keV peak is a triplet of the 98.0, 98.2, and 99.0 keV g-rays. To select a g branch, additional gates were set on the g-rays measured with the Ge detector. Among these time spectra, clear slopes were observed for the spectra gated on the 98.0 and 191.6 keV g-rays, as shown in Figs. 4(c) and (d). This means that the delay component is due to the 98.2 keV level in 148Pr. Here, we note that prompt peaks were also observed in Figs. 4(c) and (d). They correspond to the decay of 196.0 and 289.6 keV levels in 148 Pr, respectively. This is because the b feeding intensity to the 98.2 keV level is quite small (see Fig. 3) and the 98.2 keV level is populated by higher energy levels via g transitions. The observation of the prompt peaks also means that the lifetimes of the 196.0 and 289.6 keV levels are much shorter than 500 ps. From these two decay curves, the lifetimes were deduced to be 8.56(67) and 8.46(74) ns, respectively. Here, the fluctuation of the estimated lifetime values was about 0.2 ns when the fitting regions were changed; it is within the uncertainty value stated. The averaged value of 8.5(5) ns was adopted for the lifetime of the 98.2 keV level in 148Pr; it was obtained for the first time in this study. The 98.2 keV level (Ip ¼ ð1 ,2 ,3 Þ) is de-excited only by the 98.2 keV E2 g-ray [13]. Using the experimental lifetime value of 8.5(5) ns and the total internal conversion coefficient of 2.26 [18], we obtained the reduced transition probability B(E2) of 68(4) in Weisskopf units. This value roughly agrees with the typical probability B(E2) of 102 for collective E2 transitions. The nuclear structure can be discussed further after more detailed spectroscopic studies.
4. Conclusions We installed a spectrometer consisting of a LaBr3 scintillator and a thin plastic scintillator, for use in measuring the nuclear level lifetimes at the on-line isotope separator KUR-ISOL. The spectrometer showed a good time resolution of 375 ps for the 1333 keV g-ray of 60Co, after careful adjustments. The prompt time spectra were also measured down to a photon energy of 100 keV using full-energy g peaks emitted by mass-separated
197
93
Rb and 148Ce; the FWHM of 601 ps was observed for the 99 keV 93 Sr and 148Ce agree with the previous values, and the lifetime of 8.5(5) ns for the 98.2 keV level in 148Pr was successfully obtained, for the first time. We therefore conclude that the spectrometer is capable of measuring the level lifetimes of mass-separated fission products in the nanosecond range.
g-ray. The level lifetimes observed for the mass-separated
Acknowledgments This research was supported by a Grant-in-Aid for Scientific Research (no. 21740185) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The measurements of lifetimes were carried out under the Research Collaboration Program of the Research Reactor Institute, Kyoto University. We would like to thank Mr. Higuma and Mr. Fukushige for their help in performing the experiments at KUR-ISOL.
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