ray spectroscopy of - Springer Link

Z. Phys. A 345, 243-244 (1993)

ZEITSCHRIFT

FORPHYSIKA

Short note

9 Springer-Verlag 1993

In-beam 7-ray spectroscopy of ~ D. Seweryniak 1,4, B. Cederwall 2'~, J. Nyberg ~, C. Fahlander ~, A. Johnson 2'3, A. Kerek 2, j. Kownacki 4, L-O. Norlin ~, E. Adamides s, A. Atac ~, J. Blomqvist ~, H. Grawe 7, E. Ideguchi s, R. Julin ~, S. Juntinen ~, W. Karczmarczyk 4, S. Mitarai ~, M. Piiparinen ~, R. Schubart 7, G. Sletten ~, S. T6rm~inen ~, A. Virtanen ~ The Svedberg Laboratory and Department of Radiation Sciences, Uppsala University, Box 535, S-75121 Uppsala, Sweden Manne Siegbahn Institute of Physics, Stockholm, Sweden 3 Department of Physics, The Royal Institute of Technology, Stockholm, Sweden 4 Institute of Experimental Physics, University of Warsaw, Warsaw, Poland National Centre for Scientific Research, Ag. Paraskevi, Attiki, Greece 6 The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 7 Hahn-Meitner Institut Berlin GmbH, Glienicker Strasse 100, W-1000 Berlin 39, Germany s Department of Physics, Faculty of Science, Kyushu University, Fukuoka, Japan Department of Physics, University of JyvS.skyl~i,Jyv/iskyl/i, Finland Received: 30 November 1992 A b s t r a c t : Neutron deficient nuclei close to 1005n have been investigated in-beam by 3"-r~y spectroscopic methods using the N O R D B A L L detector array. A beam of 270 M e V SSNi was used to bombard a target of 54Fe. Reactio~ channel separation was achieved with a ~r charged particle multi-detector se~-up together with a I~: neutron detector wail placed in the forward direction. Ezcited states of ~~ were identified for the first time. The level scheme constructed from 3"-3"-particle-coincidence and 77-angular correlation analysis is presented. The structure of l~ is discussed and compared ~o neighboring nuclei in the framework of the nuclear shell-model.

P A C S : 23.20.Lv;25.70.Gh;27.60.+j Nuclei close to m~ are of great interest as study objects for testing the validity of the nuclear shell model. The nucleus l~176 is the heaviest self-conjugate (N=Z) doubly magic nucleus and is extremely difficult to produce and observe at presently available accelerator facilities and with present detector systems. Recently several nuclei have been identified in-beam in this region for the first time e.g. ~~ [1], l~ l~ [2] and l~176176 [1,3]. In an a t t e m p t to reach nuclei closer to l~176 than achieved earlier, a beam of 270 MeV SSNi was used to bombard a target of S4Fe (10 m g / c m 2, 99.8%). The experiment was performed at the Tandem Accelerator Laboratory of the Niels Bohr Institute in Denmark. The NORDBALL detector array [4] was optimized to yield high selectivity for different reaction channels. Therefore the present detector set-up consisted of 15 Ge-BGO spectrometers, one of which was a LEP-detector, a 4rr charged particle detector system comprising 21 AE-type St-detectors [5], a Dr neutron detector assembly consisting of 11 liquid scintillator detectors in the downstream hemisphere [6] and a 2~r 7-ray calorimeter composed of 30 BaF2 crystals covering the upstream hemisphere. The compound nucleus H2Xe is very neutron deficient and reactions leading to evaporation of neutrons are rare. Since such reactions are of the greatest interest, producing the most exotic nuclei, a major emphasis was put on the performance of the neutron detector system. By using both neutron time-of-flight and pulse shape discrimination

techniques it was possible to improve the neutron-7 separation by almost an order of magnitude compared to that when only pulse shape discrimination is used. A total of about 420 million 3"-3"-coincidence events containing information about the detected 7 rays, neutrons, protons and ~x particles were collected. In the data analysis, 7-7-coincidence matrices gated by different multiplicities of detected neutrons, protons and a particles were sorted. Intensities of observed 7-ray transitions in different matrices were calculated and compared. The intensity ratios depend on the multiplicities of particles accompanying 7 emission and on the particle detection efficiencies. Since the detection efficiency for a specific type of evaporate d particle depends very weakly on the reaction channel, comparison of such ratios with those for 3' rays from previously known nuclei that were

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Fig.2. Summed coincidence spectrum for 1~ obtained from the 2 a l p l n and 2 a l n gated 7-7 matrices. populated in the experiment enables unambiguous assignments of the final nuclei. Results of such a comparison are shown in Fig. 1 for the 145 keV fine, which is a candidate for a transition in 1~ Using the above method a total of 29 different exit channels were identified including 8 light In, Sb, Te and I isotopes not observed before [7]. The experimental yield for 1~ was estimated to be 0.03% of the total yield (0.004% for the weakest observed channel l~176 This shows the extremely high sensitivity of the present experimental set-up. Transitions assigned to 1~ are shown in Fig.2, and the proposed level scheme in Fig.3. Some transitions in Fig.2, which belong to 1~ could not be placed in the level scheme (272 and 459 keV), or were placed only tentatively (382, 376, 250 and 222 keV) due to low statistics in the individual gates. The multipolarities of the observed transitions were

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obtained by means of a simplified V-V correlation analysis. The V-V-coincidence projections (gated with the right combination of evaporated particles) were sorted for 3 detector angles with respect to the direction of the beam: 6= 79 ~ 101 ~ 143 ~ The intensity ratios I(0=143~176176 were deduced for the transitions in l~ and compared to ratios obtained for transitions of known multipolarity. All transitions observed in 1~ have a dipole character (most likely M1) except for the 1137 keV transition which has a quadrupole character (most likely E2). It was assumed that all observed transitions are stretched and that the spin and parity of the ground state is 6 + in accordance with 1~ The spins and parities in Fig.3 are therefore tentative. The proposed level scheme of x~ resembles the low energy part of the 1~ level scheme [8,9] except for the second (7 +) state observed in 1~ The (6 +) ground states in both 1~ and 104In are attributed to the lrg~@vds/2 configuration. The (7 +) state in 1~ is suggested to consist of a mixture of this configuration with the ~rgg~21| gv/2 configuration, while the (8 +) state is dominated by the 7rgg~| configuration. The higher lying states are suggested to be due to three quasi-neutrons (mainly vd5/2, vgv/2) coupled to a ~'g9-]~ proton hole. This qualitative interpretation agrees with the results of shell model calculations, which are shown in Fig.3. A 90Zr core and a configuration space consisting of the lrg9/2, vd5/2, t~gv/2, tssl/2, tJd3/2 orbitals and the same model parameters as in [1,3] were used. Reasonable agreement between theory and experiment is concluded for 1~ except for the second 7 + state predicted by the theory, which was not observed experimentally. The unobserved transitions from the (9 + ) and (8 + ) states to the second 7+ state should be weaker compared to the observed transitions feeding the (7 +) state. The estimated reduction in the 7-ray intensity is big enough to explain why these transitions were not detected. A c k n o w l e d g e m e n t s : This work was partially supported by the Swedish Natural Science Research Council. The ezeellent cooperation of the staff of the Tandem Accelerator Laboratory of the Niels Bohr Institute and the assistance of the group from Chalmers University of Technology with the set-up of the neutron detectors ks appreciated. Software written by D.C. Radford was used in the data analysis. [1] R. Schubart et al., Z.Phys. A340, 100(1091) [2] R. Schubart et al., Z.Phys. A343, 123(1992) [3] D. Alber et al., Z.Phys. A327, 127(1987) and Z.Phys. A344, 1(1992) [4] B. Herskind, Nucl. Phys. A447, 395(1985) [5] T. Kuroyanagi et al., Proc. 21st INS Int. Symp. on Rapidly Rotating Nuclei, Tokyo, October 1992 [6] S. E. Arnell et al., Nucl. Instr. and Meth. A300, 303(1991) [7] D. Seweryniak et al., Proc. Int. Conf. on Nuclear Structure and High Spin Angular Momentum, O t tawa, May 1992 [8] M. Ogawa et al., Proc. 6th Int. Conf. on Nuclei far from Stability and 9th Int. Conf. on Atomic Masses and Fundamental Constants, Bernkastel, July 1992 [9] D. Seweryniak et al. (to be published)