The states of 73Se populated by the beta decay of 73Br

Nuclear Physics A474 (1987) 484-498 North-Holland, Amsterdam

T H E STATES OF 73Se P O P U L A T E D BY T H E BETA DECAY OF 73Br K. HEIGUCHI, S. MITARAI, B.J. MIN and T. KUROYANAGI

Department of Physics, Faculty of Science, Kyushu University, Fukuoka, Japan Received 1 July 1987 Abstract: Decay of mass-separated samples of the nuclide 73Br have been investigated by means of an

on-line isotope separator in the tandem accelerator laboratory of Kyushu University. Twenty eight y-rays were assigned to the decay of 73Br. The half-life was measured to be 3.4+0.2 min. The decay scheme of 73Br was constructed on the basis of y-ray energies, intensities of y-rays and conversion electrons, and relations of yy-,/37- and y(ce)-coincidences. Spins and parities of the ten levels of 73Se were assigned or limited according to logft values, conversion coefficients and branching ratios of the transitions. Systematical trends in the level schemes of N = 39 isotones 69Zn, 7XGe, 73Se, 75Kr and 775r are discussed.

1. Introduction

Investigation of nuclear structure of neutron deficient nuclei in the A = 70--80 mass zone has revealed many interesting features and established the existence of a new deformed region. Effects of sub-shell closure around N (or Z ) = 40 in the region of stable nuclei vanish entirely for nuclides around N = Z = 40, and large deformation and strong collectivity are seen. Furthermore, evidence of coexistence of deformed and near-spherical shape has generated interest in studies of the transitional region. The best example for shape coexistence in this region of nuclei is 728e. It has been suggested that the origin of strong deformation and shape coexistence in this region is attributed ~) to the gaps which exist at N = 40, 8 - 0, and N = 38, c5-0.28 in the Nilsson diagram. The number of protons delicately controls whether a deformed or near-spherical shape is energetically lowest in nuclei. The fl-decay of 73Br populates the states of N = 39 nuclide 73Se and offers the opportunity to study the systematical trends of nuclear structure in this region. The decay of 73Br has been reported by Murray et al. 2). They investigated fl-decay of the samples of bromine chemically separated from the activities prodjaced by the 59C0 + 160 reaction. In their work, the decay scheme of 73Br w a s proposed only on the basis of energy differences and relative intensities of the y-rays. The decay of 73roSe with the half-life of 40 min and the ground state of 73Se with the half-life of 7.2 h has been studied by several authors 3-H). In the in-beam y-ray spectroscopy, the states of 738e have been studied through the 72Ge(oc, 3 n y ) reaction 12), the 7°Ge(ot, n y ) reaction ~3) and the 58Ni(t9F, 3pny) reaction 34). In the present work, the activity of 73Br w a s separated from the products of the natNi + t 6 0 reaction using 0375-9474/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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an on-line isotope separator 14) and the states of 735e populated by the fl-decay of 73Br were studied in detail.

2. Experimental procedure The 6°Ni(160, p2n) reaction was used to produce 73Br. Natural nickel targets of 4 0 - - 8 0 m g / c m 2 thickness were b o m b a r d e d with 60 MeV beams from the tandem electrostatic accelerator of Kyushu University. The target was mounted on an anode of a F E B I A D 15) type ion source which was held at a temperature around 1400°C. Reaction products were readily released from the target and ionized in the plasma region of the source. After extraction and mass-separation of the ions, 73Br activity was implanted into an aluminum coated mylar tape and periodically transported in front of a detector in shielded counting position by a tape-transport system. Fluctuations of stopped position of the tape are less than 1 mm under the maximum tape speed of 1 m/s. The tape-transport system is controlled by a microcomputer. G a m m a rays with an energy between 60 and 2000 keV were detected by an intrinsic Ge detector with the relative efficiency of 16.7% and low energy y-rays between 5 and 60 keV were detected by a g a m m a - X - r a y Ge detector with the efficiency of 18.5%. The efficiency calibrations of the Ge detectors were made with the sources of 57Co, 241Am, 22Na, 137Csand 6°Co with calibrated intensity. In order to determine intensities of the y-rays per decay of 73Br, intensity ratios of the y-rays to annihilation radiation were measured. In the measurement, the mass-separated samples of 73Br were sandwiched by absorbers of 17 m m thick acrylic resin so that the positrons annihilated in the confined region of the absorbers. The intensity of the annihilation radiation originated from the decay of 73Br was determined by taking account of the influence of long components in a decay curve of the annihilation radiation and attenuation of the y-rays in the absorbers. Signals of time intervals from the end of transportation of mass-separated samples to detection of radiations were recorded in list mode together with energy signals by using a digital circuit which counts clock-pulses from the microcomputer and transmits the counts to a host computer when radiation is detected. Decay curves of the radiation were obtained from off-line sorting of the data. The intrinsic Ge detector described above and a Ge(Li) detector with the efficiency of 12.0% were used for yy-coincidence measurements. Energy signals from both detectors were recorded together with signals of a time-to-amplitude converter (TAC) in 3-dimensional list mode. A plastic scintillation detector (52 m m diameter, 30 m m thickness) and the intrinsic Ge detector were used for fly-coincidence measurement. To measure half-lives of the y-rays the time intervals from the end of transportation of the samples to detection of the fly-coincidences were recorded together with energy signals from the detectors in 3-dimensional list mode.

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By using a silicon surface barrier detector (1.2 m m thick Si(Au)) and the intrinsic Ge detector, coincidences of the conversion electrons and the y-rays were observed simultaneously with the singles spectra of both radiations. Conversion coefficients were deduced from relative intensities between the conversion electrons and the y-rays. In the y(ce)-coincidence experiment, energy signals of both detectors were recorded together with TAC signals in 3-dimensional list mode. For the purpose of measuring half-lives of the excited states, fly-coincidences were observed with a 3"x 8" N a I detector and the plastic scintillation detector described above. In analysis of the data, the centroid-shift method was applied to TAC spectrum gated with the y-rays. To obtain the prompt curves, a source of 22Na was used. To obtain additional information for the decay scheme of 7SBr, the singles and coincidence spectra of the y-rays from the 6°Ni(160, 2 p n y ) reaction were measured using the g a m m a - X - r a y Ge and the intrinsic Ge detectors described above. Furthermore, an isomeric state of 735e with half-life of 10 ~ s - - l O O m s was searched. A natural nickel target of 2 m g / c m 2 thickness was b o m b a r e d e d with 160 beams chopped by an electrostatic deflector and recoiled reaction products were deposited on an Au catcher of 2 m g / c m 2 thickness. After irradiation, conversion electrons and y-rays from an isomeric state were searched using the Si(Au) and the g a m m a - X - r a y Ge detector. Both the times of irradiation and measurement were individually controlled by a microcomputer.

3. Results

A singles y-ray spectrum in fig. 1 was obtained from mass-separated samples corresponding to mass 73. All peaks of which only energies are labeled in the figure are the ones assigned to the decay of 73Br. Peaks associated with the decay of 73Se and 73roSe are seen in the figure. Other peaks are y-rays from room background and weak contaminations of 72'74Br. Assignments of the y-rays to the decay of 73Br were made on the basis of half-lives, and relations of Y7- and Xy-coincidences. The energies and relative intensities of the y-rays are given in table 1. These intensities were corrected considering the effect of concidence summing of the y-rays. The 862 keV y-ray reported by Murray et al. 2) was not assigned to the decay of 73Br in the present work, because this y-ray had a half-life of 8 4 + 7 s. Low-energy y-rays with energy between 5.0 and 60 keV were searched with the gamma-X-ray Ge detector, but no low-energy photons following the decay of 73Br were observed except for X-rays of selenium. The most important coincidence spectra obtained from the yy-coincidence experiment are shown in fig. 2. Peaks in the high energy side of the spectrum gated with the 65 keV y-ray are sum peaks. The results are summarized in the fourth column of table 1.

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Fig. 1. Singles spectrum of y-rays following the decay of separated samples corresponding to mass 73. The sum p. indicates a sum peak and energies of summed y-rays are written in the parentheses. All peaks labeled by only energy are the ones assigned to the decay of 73Br. Half-lives o f the y - r a y s d e t e r m i n e d from the f l y - c o i n c i d e n c e e x p e r i m e n t are given in the t h i r d c o l u m n o f t a b l e 1. A half-life o f the g r o u n d state o f 73Br was d e t e r m i n e d to be 3 . 4 + 0 . 2 m i n f r o m the half-lives o f the intense y-rays. T h e F e r m i - K u r i e plots o f p o s i t r o n s g a t e d with a n n i h i l a t i o n r a d i a t i o n are s h o w n in fig. 3. T h e e n d - p o i n t e n e r g y o f p o s i t r o n s f r o m the d e c a y o f the 73Br activity was d e d u c e d f r o m the m e a s u r e m e n t in w h i c h the g a m m a - X - r a y G e d e t e c t o r was used as a d e t e c t o r for r - p a r t i c l e s . E n e r g y c a l i b r a t i o n s o f the d e t e c t o r were m a d e with a s o u r c e o f 56C0. Test e x p e r i m e n t s o f this m e t h o d were m a d e using 82y a n d 85Zr activities a n d the results were in a g r e e m e n t with the p r e v i o u s work'6), within e x p e r i m e n t a l uncertainties. T h e e n d - p o i n t e n e r g y o f the 73Br activity was d e t e r m i n e d

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TABLE 1 Gamma rays from the decay of 73Br Energy (keV)

Relative intensity

65.0+0.1 125.6±0.1 166.2+0.2 192.6±0.2 249.4 + 0.2 275.2±0.1 335.9±0.1 363.9±0.2 374.7±0.2 390.4±0.2 400.9±0.1 489.8 ± 0.1 511.0 540.0± 0.1 550.6±0.2 595.3 ± 0.3 615.3±0.1 639.0±0.1 699.8 ± 0.1 764.7±0.1 788.8±0.1 849.4 ± 0.2 870.7 ± 0.1 914.3±0.1 931.4±0.1 984.7 + 0.2 996.2-4-0.1 1460.2 ± 0.1 1528.8+0.3

100.0-4- 1.2 20.4+0.4 1.3-4-0.2 0.3+0.1 0.9 + 0.1 7.3 ±0.6 28.2± 1.0 2.1 ±0.5 6.0±0.6 2.4±2.1 16.1 ±0.7 2.8 ± 0.4 506.0± 18.3 5.1 ± 1.2 1.8± 1.0 0.5 ± 0.9 5.6±0.3 2.8±0.4 24.7 ± 0.8 2.0± 1.2 2.4± 1.8 15.8 ± 0.7 3.4 ± 0.3 13.9±0.5 15.8±0.7 1.0 ± 0.4 5.1 ±0.4 3.2 ± 0.3 1.5±0.2

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A s i n g l e s s p e c t r u m o f c o n v e r s i o n e l e c t r o n s is s h o w n i n fig. 4. T h e K - c o n v e r s i o n c o e f f i c i e n t s o f t h e 65 k e V a n d t h e 125 k e V t r a n s i t i o n s w e r e d e t e r m i n e d t o b e 0.29 + 0.01 a n d 0 . 0 4 9 + 0.006, r e s p e c t i v e l y . T h e s e c o n v e r s i o n c o e f f i c i e n t s w e r e d e t e r m i n e d r e l a t i v e t o K - c o n v e r s i o n c o e f f i c i e n t o f t h e 67 k e V t r a n s i t i o n f r o m t h e d e c a y o f s e p a r a t e d 735e a c t i v i t y a s s u m i n g t h a t t h e 67 k e V t r a n s i t i o n is a p u r e M 1 t r a n s i t i o n 6,~8). Comparison

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ENERGY(KeV) Fig. 5. Comparison between experimental and theoreucal values of K-conversioncoefficients. 4. The level scheme of 73Sc The level scheme of 73Se is proposed in fig. 6. The excited states of 73Se are determined on the basis of the energies, intensities and coincidence relations of the y-rays assigned to the decay of 73Br. The l o g f t values were calculated by the tables of Gove and Martin 2o) for allowed and unique first-forbidden transitions. All known odd-A isotopes of bromine have 3- ground states. Therefore, it is tentatively assumed that the ground state of 73Br has J~ of (3) 3 - • Spin and parity of the ground state of 73Br are discussed again at the end of this section. 4.1. ENERGY LEVELS 4.1.1. The ground state and the 2 5 . 7 k e V s t a t e . Besides the ground state with the half-life of 7.2 h there exists another long-living state in 73Se with the half-life of 4 0 m i n which is depopulated by an E3 transition of 25.71 keV[ref. 3)] and fltransitions.

From the measurement of L conversion electrons an energy of 25.7 + 0.2 keV for the E3 transition was obtained. Results of y(ce)-coincidence measurement showed no K X-rays or y-rays with the energy higher than 5.0 keV in coincidence with the

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L conversion electrons. The results show that the 25.7 keV E3 transition leads to the ground state. Thus the 25.7 keV level was confirmed. These results are consistent with the previous work 3) of Konijn et al. Observed properties of the fl-decay of the both levels allow only the two following 9+ alternative spin assignments: ~ for the ground state and ~3 - for the 25.7 keV level or 7+ for ground state and ½- for the 25.7 keV level. In ref. 2), the assignments of 7+ and ½- have been suggested on the basis of systematics and decay schemes of both levels. However, a number of previous works strongly suggest the assignments of 9+ 3 and Schwall 2~) measured the anisotropy of the angular distribution of the 361 keV y-ray emitted from 73As nuclei which were recoiled after the/3-decay of 735e. He showed that the anisotropy of the 361 keV y-ray has opposite sign for the ground7+ 9-t9+ state spin of ~ or ~ and the measured sign allows only the ~ assignment. Murray et al. 7) gave the l o g f t value of 7.7 for a 3 - b r a n c h from the ground state to the 67 keV level with J ~ of ~- in 73As. If the ground state of 735e is 9+, this transition is classified as a unique first-forbidden transition and the l o g f t value is too small for this classification. But, the newest measurement by ten Brink 4) gave 8.8 for this l o g f t value calculated as a unique first-forbidden transition. This value • 2~ does not contradict -) an assignment of 9+. Meeker 8) et al. gave the l o g f t value of 7.0 of the f - b r a n c h from the ground state to the 510 keV level with J ~ of~ ÷ in 73As which was excited in the (3He, d) reaction with l = 2 character 23-25). If J~ of the ground state of 73Se is 9+, this transition is classified as a nonunique second-forbidden transition and this l o g f t value is too small for this classification. In the decay scheme proposed by ten Brink 4) no l o g f t value for this branch is given. Therefore, in the present work the spins and parities of the ground and isomeric 3states of 735e are considered to be 9+ and ~ , respectively. 4.1.2. The 2 6 . 4 k e V a n d 1 9 2 . 6 k e V levels. Zell etal. 13) proposed the 26.4 and 192.6 keV levels on the basis of the fact that the energy difference between the 192.6 and 166.2 keV y-rays is 26.4 keV. They searched a y-transition from the 26.4 keV level to the ground state using an intrinsic Ge detector with the volume of 0.5 cm 3, but could not find the y-ray with an energy of 26.4 keV. Then they supposed that this transition is an E2 or M2 transition which has a conversion coefficient larger than 60 [ref. ~9)]. The 166.2 keV y-ray reported by Zell et al. 13) should be considered to be identical to the 166.2 keV y-ray assigned to the decay of 73Br in the present work, because these y-rays have the same energy and the/3y-coincidence measurement indicates that a level which emits the 166.2 keV y-ray is a low-lying one. The 192.6 keV y-ray from the decay of 73Br could not be observed because of very weak intensity of the 3'- ray. Because the energy difference of the y-rays is important, the singles y-rays and the coincidences of the y-rays from the 6°Ni(160, 2pny) reaction were measured

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and the same value of the energy difference was obtained. Then it was concluded that the intensity of the 26.4 keV ~/-ray relative to the 166.2 keV 3,-ray is less than ~7. Results of the measurement of the (ce)3, coincidences showed no conversion electrons in coincidence with the 166.2 keV 7-ray. This fact implies that the 26.4 keV level is depopulated to the 25.7 keV level. The l o g f t value for a /3-branch to the 192.6 keV level calculated as a unique first-forbidden transition is 8.8. Therefore, J ~ of (7+) is assigned to the 192.6 keV level. Zell et al. ~3) showed that the angular distribution of the 192.6 keV line is the one of a mixed z a J = l transition ( M I + E 2 ) , which is consistent with the (7+) assignment for the 192.6 keV level. The 26.4 keV level is populated by the AJ = 1 transition ~3) from the 192.6 keV level more strongly than the ground state. An assignment of 25or 9 to the 26.4 keV level is possible. The 26.4 keV level is depopulated weakly to the ground state with J'~ of 9+ and strongly to the 25.7 keV level with J~ of 3-. Therefore an assignment of 9 is excluded and an assignment of 25is probable. An assignment of positive parity to the 26.4 keV level is probable because the other negative levels do not populate this level. Thus J~ of (5+) is assigned to the 26.4 keV level. 4.1.3. The 90. 7 ke V a n d 151.3 ke Vlevels. The 90.7 and 151.3 keV levels are depopulated by M I transitions to the 25.7 keV level with J~ of 2-. Therefore the parities of these states are negative and an assignment of (3, 3, I ) - to each state is possible. However, in the present work, no transition between these states was observed and a spin change between the states should be considered to be AJ = 2. Therefore an assignment of 3- is excluded and the following two assignments are possible: 3- for 1t h e 151.3 keV level and ~- for the 90.7 keV level or ~- for the 90.7 keV level and for the 151.3 keV level. Because the 151.3 keV level is populated by a 3~-transition from the 400.4 keV level with J ~ of (7-) discussed later, an assignment of (½)- to this level is excluded. Therefore, assignments of (½)- for the 90.7 keV level and (3) for the 151.3 keV level are probable. Murray et al. 2) did not give a l o g f t value for a / 3 - b r a n c h from the ground state of 73Br to the 151.3 keV level. In the present work, the/3-branch has l o g f t value of 7.0. 4.1.4. The 400.4 k e V a n d 565.7 k e V levels. T h e 400.4 keV and 565.7 keV levels are proposed on the basis of energies and intensities of T-rays and 77-coincidence relations. The 374.7 keV 3~-ray is in coincidence with the 390.4 keV and 540.0 keV 3,-rays. The 540.0 keV ~/-ray is in coincidence with the 374.7 keV and 984.7 keV 3,-rays. Considering the energies and intensities of the 7-rays these facts indicate that the 540.0 keV line is an unresolved doublet. Thus the 400.4 keV and 565.7 keV levels are proposed. The 400.4 keV level is populated by ~/-transitions from the 790.5 keV and 940.1 keV levels which are populated by allowed /~-transitions from the (2)- ground state of 73Br, and is depopulated by the 374.7 keV transition to the 25.7 keV level with J ~ of 3- and by the 249.4 keV transition to the 151.3 keV level with J ~ of (~)- level,

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but does not decay to the 90.7 keV level with J~ of (½)-. The fl-transition from the (3)- ground states of 73Br to this level is very weak and a logft value for this level could not be given. These facts seem to indicate that the parity of the level is negative and spin change between this level and the ground state of 73Br is tO be zaJ = 2. Therefore J'~ of (7-) is assigned to the 400.4 keV level. The assignment does not contradict the result of measurement of excitation functions of the y-rays by Zell et al. 13). 4.1.5. The 426.6keV level. A logft value for a /3-branch to this level is 5.6. Therefore the/3-transition to this level is an allowed transition and an assignment of (½, 3, 3)- is possible. However analysis of coincidence data of the conversion electrons and the y-rays indicates that the 275.2 keV transition to the 151.3 keV level with J~ of (25)- and the 335.9 keV transition to the 90.7 keV level with J~ of (½)are not E2 transitions because of small conversion coefficients. Therefore an assignment of (½, 3)- is excluded and an assignment of (3)- is probable. 4.1.6. The 7 9 0 . 5 k e V a n d 940.1 keVlevels. These levels are populated by allowed fl-transitions from (3)- ground state of 73Br. Therefore an assignment of (½, 3, 5)to each level is possible, otherwise an assignment of (½)- is excluded because these levels are depopulated to the (7-) 400.4 keV level. Thus the assignment of (3, 5) to each level is probable. 4.1.7. The 641.1, 1022.1, 1550.3 and 1620.O k e V levels. These levels are populated by allowed /3-transitions from the (3)- ground state of 73Br. Therefore assignment of (½, 3, 5)- to each level is possible.

4.2. SUMMARY OF LEVEL SCHEME

The allowed /3-transition from 73Br to ~3 isomeric state of 735e permits an assignment of ½-, 5- or 3- for the ground state of 73Br. However, most of the known odd-proton nuclei between Z = 29 and Z = 37 have ground states with spin and parity of 3- with three exceptions all of which have J~ of 5-. Therefore the assignment of 3 or ~5- to the ground state of 73Br is probable. However, the assignment of ~5 is improbable because the ground state is depopulated by an allowed fltransition to the 90.7 keV level with J~ of (1)- in 73Se. Besides, the 3- state of 73Br has been proposed at 27 keV above the ground state in the in-beam y-ray spectroscopy 26) of 73Br. Thus, spin and parity of (3)- was assigned to the ground state of 73Br. Zell et al. 13) reported the 60 keV y-transition from the 151 keV level to the 91 keV level and the 193 keV y-transition from the 193 keV level to the ground state. In the present experiments these y-rays from the decay of 73Br could not be observed because the intensities of the y-rays are too weak to observe. The 193 keV y-ray from the 6°Ni(160, 2pny) reaction was observed, but the 60 keV y-ray was not observed. In the present work, six levels and ten y-rays were newly assigned to the decay scheme of 73Br, and spins and parities of ten levels of 735e were proposed.

K. Heiguchi et al. / 73Se

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5. Discussion

The low-energy part of the level scheme of 73Se is presented together with those of N = 39 isotones in fig. 7. The level structure of the isotones drastically changes from 69Zn to 775r, however, similarity between 73Se and isotones heavier or lighter than 73Se can be found. On the heavier side, the proposed (~)-(~ 5+ 7+) and (~) 3 --(~) --(~ 7--) level sequences of 73Se are very similar to the (5+) band in 75Kr and 775r and the (3-) band in 75Kr, respectively. The (3-) band in 75Kr has been interpreted 27,28) as a strongly coupled rotational band built on the 3- [301] Nilsson orbit which is close to the Fermi surface at deformation /3--0.3. The (5+) band in 75Kr has been described well in the framework of a model of the Nilsson quasi-particle coupled to a rigid core :7). In the model, it is assumed that the I+[422] Nilsson orbit is a predominant component for the band head and deformation/3 is 0.32. On the lighter side, the energy spacing between 9÷ and ~ states decreases toward heavier isotones and these states inverse at 73Se. These decrease and inversion follow the trend o f the 9+[404] and ~-[301] Nilsson orbits for negative deformation /3 = - 0 . 1 - 0 . Nuclei with Z (or N ) - 3 4 , 36 prefer oblate deformation. In 7°Se with Z = 34 and N = 36, a systematic decrease of B(E2) values along the yrast band was observed and the decrease was interpreted 29) as the destructive interference of two collective structures associated with a change from oblate to prolate deformation. The ~+ and 3- levels are energetically close and go down toward heavier isotones rapidly. Order of the levels inverses at 75Kr and this inversion follows the trend of the 3+[422] and 3-[301] Nilsson orbits at/3 ~0.3. As described above, the level structure of 735e has features of both sides. In argument of shape coexistence, these trends of the levels of 73Se seem to be connected with the coexistence of oblate and prolate minima in the potential energy surface at low spin. The coexistence of oblate and prolate shapes near Z - 3 4 , N - 3 8 has been postulated by the Strutinsky calculation 3o). -

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The authors acknowledge the helpful advice of Prof. T. Nakashima during the off-line experiments of the ion source. Thanks are due to Y. Koga for the excellent manufacturing of components of the ion source and T. Maeda for technical support in preparing the electric circuits. Assistance from J. Taguchi in constructing the beam course of the ISOL system is gratefully acknowledged. References

1) 2) 3) 4) 5) 6)

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