Z. Phys. A 354,231-232 (1996) ZEITSCHRIFT FOR PHYSlK A © Springer-Verlag 1996

Short note

The/l-decay of 88Tc A. Odahara z, Y. Gono ~, S. Mitarai ~, T. Shizuma l, E. Ideguchi I'~, J. Mukai ~'b, H. Tomura l'°, B.J. Min i'd, S. Suematsu ~, T. KuroyanagP, K. Heiguchi*-, T. Komatsubara 3, K. Furuno 3

' Department of Physics, Kyushu University, Fukuoka, Japan Graduate School of Science, Niigata University, Niigata, Japan

3 Institute of Physics, Tsukuba University, Tsukuba, Ibaraki, Japan

Received: 29 November 1995/Revised version: 15 January 1996 Communicated by B. Herskind

A b s t r a c t : Decay scheme, half life and QEC value of SSTc were deduced from a ~-decay study. The neu- tron deficient nucleus SSTc was produced by the 58Ni( 32S,pn)SSTc reaction using beam energies of 100, 101 and 105 MeV. The ground- and isomeric states of SSTc were found to decay to SSMo with similar half lives of 5.8 4- 0.2 and 6.4 4- 0.8 sec. The most probable spins and parities of these states are 3 + and 6 + , respectively, though the order in energy could not be determined. The QEC value was deduced to be 8.6 4- 1.3 MeV.

PACS : 23.40.-s, 27.50.+e

The neutron deficient nuclei in the A~80 mass re- gion are known to have strongly deformed shapes. The nuclei near the N=Z line in this region have large prolate deformation because of the single particle shell gap at a deformation parameter/3,-,0.4. The nucleus 76Sr with N=Z=38[1] lies at the center of this deformed region with the largest deformation of ~2>0.40. Some of these nuclei show interesting features, such as a shape co-existence and a drastic shape change depending on the nucleon number. Since these nuclei consist of relatively small number of nucleons, the individual nucleon plays an im- portant role in forming the shape of each nucleus. The nucleus 88Tc is located in the transitional region between the strongly deformed 76Sr and the spherical nuclei with the N or Z=50 closed shell.

The level schemes of 88Tc and 88Mo constructed based on the in-beam 3'-ray studies were previously re- ported[2,3]. In ref.3, the 7 rays appeared in delayed 77- coincidence spectra gated by the 511 keV annihilation ra- diation were interpreted as transitions following/3 decays. From this information, the 3213keV 8 + and 3350keV 7-

Present oddresses:

~' RIKEN, Wako-shi, Saitama, Japan b Tohwa University, Fukuoka, Japan

National Institute of Radiological Sciences, Inage, Chiba, Japan ~' Korea Atomic Energy Research Institute. Daejeon. Korea

states of SSMo were considered to be fed by the/3 decay of S8Tc. Accordingly, the spin and parity of the ground state of 8STc were assigned to be (7-, 8 +) in ref.2. However, the present results show inconsistency with this conclusion. In this work, the decay scheme was constructed based on the 77-coincidence measurement. The half life and the QEc value of 88Tc were determined for the first time by means of 37-coincidence measurements.

The/3 decay of SSTc was studied by ~7 spectroscopy. The nucleus SSTc was produced by the ~SNi(a2S,pn)SSTc reaction at the beam energies of 100, 101 and 105 MeV using tandem accelerators both of Kyushu University and Tsukuba University. The 5SNi target, which was 1.4 mg /cm 2 thick and enriched to 99.76 %, was made by electro- plating on a gold backing of 2.0 mg/cm 2 thickness. The cross section of the reaction channel to produce SSTc is small (,-,0.5 mb) according to calculation by the CAS- CADE code[4]. The p-decay half life was predicted to be 953 msec[5]. Therefore a rotation disk[6] and a tape transport system[7] were used in order to reduce the back- ground originating from long and short lived activities. In these systems, irradiation and data taking periods were selected in two ways, namely (5 and 20 sec) and (6 and 6 sec), to obtain the best signal to noise ratio for 88Tc. In order to search for the states which decay to SSMo with longer life times, cycles of (100 and 100 sec) and (60 and 60 sec) were also selected. However, no state was found in these time ranges. The time information of each event taken between the end of the transport of the activities and the detection of the radiations was recorded to deter- mine the jJ-decay half life of 8STc. In these experiments, singles 7-ray, 37- and 77-coincidence measurements were carried out. Gamma rays were detected b y t w o HPGe detectors with 18 and 20 % efficiency, respectively. Beta rays were detected by a HPGe detector with 20 % effi- ciency.

Three transitions of 741 (2 + --* 0+), 914 (4 + ---* 2 +) and 972 (6 + --* 4 +) keV were found to follow the

232

decay of SSTc with the relative intensities of 100, 44 and 19, respectively. These three ~/rays are in cascade with each other. This agrees with the level scheme of SSMo in ref.3. A weak 446 keV transition was found to be in coincidence with the 741 and 914 keV 7 rays. This -y ray has the relative intensity of 6 normalizing that of the 741 keV 7 ray to 100. Based on these results, the 446 keV ~/ ray was assigned to 8SMo. In ref.3, it was reported that the 8 + and 7- levels in 88Mo were populated by the/3 decay of 88Tc. However, the 3' rays of the 586 (8 + --* 6+), 992 (5- --* 4 +) and 703 (7- - , 5-) keV were not found in the present work with the intensities expected from those reported in ref.3.

The 741 and 914 keV -~ rays were found to decay with a half life of 5.8 + 0.2 sec. The 972 keV -y ray was found to decay with a half life of 6.4 4- 0.8 sec. The half lives are definitely different, though they overlap within the present experimental accuracy. The half life of the/3 decay which populates the 2101 keV state could not be determined, due to the poor statistics.

In order to determine the d-ray end point energy, the analysis of the x / T plot of the 13-ray spectrum was carried out. The energy calibration of the /3 ray was made by the known/3 decays of the nuclei, such as 42mSc (QEc=6.4258513 MeV)[8], 87Nb(Q~c=5.176 MeV)[9] and STMO(QEc=6.63 MeV)[10]. These nuclei were produced by the z2c(3~S,2pn)42mSc, 5SNi(32S,3p)S7Nb and 5SNi(32S, 2pn)STMo reactions, respectively, in the same experiment. The first reaction took place because of the carbon con- tamination accumulated on the Ni target. As a result, the 3-ray end point energy of SSTc was obtained to be 6.8 4- 1.3 MeV from the/3-ray spectrum gated on the 741 keV "r ray.

According to the 7-ray intensity balances, each of the 2 +, 4 + and 6 + state in SSMo was found to be pop- ulated by the 13 decay of 8STc. The log f t values were deduced by the v-ray intensity balances and the/]-decay end point energies. The existence of the isomer in 88Tc which decays to 8SMo is proposed, because it is not pos- sible to populate the 2 + , 4 + and 6 + states with deduced log f t values from the decay of only one state of 8STc. Al- though the two half lives mentioned above were obtained, we could not determine which half life is of the ground state or of the isomeric state.

The characteristics of 88Tc are summarized in Table 1. A new state is proposed at 2101 keV in 88Mo deexciting

T a b l e 1 Decay properties of SSTc.

Excitation "~-ray life time energy energy ( keV ) ( keV ) ( see )

740.53(5) 740.53(5) 5.8(2) 1654.76(19) 914.23(18) 5.7(11) 2100.66(48) 445.90(44) - 2626.83(29 ) 972.07(23) 6.4(8)

relative E~ '"~ intensity

( M e v ) 100 6.8(13) 44.2(58) -

6.3(27) - 18.6(36) -

levels have the similar log f t values of the allowed transi- tion. The spin and parity of the other state is assigned to the newly found 446 keV 7 ray. The branching ratios and log ft values were proposed in two cases. In one case, only the 6 + state in 8SMo is populated by the/3 decay of the state in 88Tc with T1/2 = 6.4 + 0.8 sec. In the other case, both the 6 + and 2101 keV states in SSMo are populated by the/3 decay of the state of 88Tc with T1/2= 6.4 4- 0.8 see .

;fhe spin and parity of the one state of SSTc is de- termined to be 3 + , because the feedings to the 2 + and 4 + be 6 +, since no population of the 4 + and 8 + levels in 88Mo was found. Based on these results, the decay scheme of SSTc is proposed in fig.1.

The Qec value of SSTc was deduced to be 8.6 4- 1.3 MeV from the end point energy of the/3 ray. It is worth while to point out that the QEC value of SSTc is significantly smaller than expected from the systematics and from theoretical predictions[8].

2627 6 +

2101 (19)~972

~655t, t' 4

(44)[ 914 741 t 2+

/ (loo)[ 741 0 ~ 0 +

88Mo

6.4sec 6+ 5"Ssec 3 +

QEc= 8.6 + 1.3 MeV

Case 1 Case 2 4.1(6) 4.3(8)

100% 75% 4.9(8)

25%

Case 1 Case2

5.4(8) 8%

5./(8) 5.o(7) 24% 26%

4.9(12) 4.9(6) 68% 74%

fig.1 Proposed decay scheme of 88Tc. Branching ratios and logft values are shown for two cases. In case (1), only the 6 + state in 88Mo is populated by the/3 decay of the 6 + state in 88Tc. In case (2), both the 6 + and 2101 keV states in 88Mo are populated by the/3 decay of the 6 + state in SSTc.

R e f e r e n c e s 1. C.J.Lister et al., Phys.Rev.C42(1990)Rl191 2. D.Rudolph et al.,J.Phys.G17(1991)Ll13 3. M.Weiszflog et al. Z.Phys.A342(1992)257 4. F.Piihlhofer, Nucl.Phys.A280(1977)267 5. T.Tachibana et al., Prog.Theor.Phys.84(1990)641 6. K.Heiguchi et al., Kyushu Univ.Tandem Acc.Lab.

Rep.(1985-1987)33 7. K.Heiguchi et al., Univ.of Tsukuba Tandem Acc.

Center Ann.Rep.(1991)34 8. G.Audi and A.H.Wapstra, Nucl.Phys.A565(1993)1 9. H.Sievers, Nucl.Data Sheets 62(1991)327

10. B.J.Min et al., Nuel.Phys.A530(1991)211