Approaching 100Sn in decay studies

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Nuclear Physics A583 (1995) 849-852

Approaching l°°Sn in decay studies Ernst Roeckl Gesellschaff fiir Schwerionenforschung, Postfach 110552, D-64220 Darmstadt, Germany Among nulcei far from the/3-stability line [1,2], very neutron-deficient isotopes near the doubly-magic l°°Sn (Z=N=50) have met considerable experimental and theoretical interest recently. Most of the experimental progress in this field has been achieved by in-beam spectroscopy or by studying (ground-state) decay properties, this report being focused on the second topic. There is a twofold motivation for investigating decays near l°°Sn. Firstly, owing to the fact that protons and neutrons occupy identical (g9/2) shellmodel orbits, one may encounter superallowed alpha and cluster emission beyond l°°Sn. Secondly,/3 decay in this region is dominated by a fast (superallowed) rcgg/2--~vgT/2 (or, for Z,N<50, also by a ~rgg/2--~vgg/2) Gamow-Teller transition. It is the occurrence of both features which makes the ~°°Sn region unique with respect to gaining new insight into nuclear-structure phenomena. Even though this section of the chart of nuclides, which represents an ideal testing ground for the spherical shell-model, is occasionally considered to be a "well-known doubly magic region" [3], only lifetime limits have been obtained [4,5] for l°°Sn so far and the few-nucleon excitation states around the core are also poorly known. The present experimental status on decay properties of Z~lg nuclei in the silver-to-barium region, displayed in Fig. 1, is based almost exclusively on measurements performed by using beam-line spectrolneters or isotope separators online (ISOL). In most of the ISOL experiments relevant here, the proton-rich isotopes were produced by fusion-evaporation reactions such as S°Cr (SSNi,xpyn) and mass-separated as singly-charged 60 keV beams for subsequent decay studies. Fig. 1 includes in particular recent and partly unpublished data on, e. g., 94Ag [6], the heaviest N=Z nucleus with known decay properties; the isotope identification obtained for 9Sin, 99In, ~°°Sn, ~°2Sn and 1°4Sb [4,5]; l°tSn [7], the closest decay-spectroscopy approach to l°°Sn achieved to date; the proton-radioactive nucleus l°SSb [8]; and the light barium isotopes [9]. The decay of nuclear ground-states by the emission charged-particles allows one to identify rare nuclei, to determine their mass by "linking" the measured Q values to known masses [11], and to deduce information on the particle-emitting (ground) state and on the daughter states [6]. The region of neutron-deficient isotopes above tin is well suited for a discussion of the interplay between alpha, proton and cluster radioactivities (see Fig. 1). The occurrence of an island of alpha emission between l°6Te and ll4Cs, is the most convincing experimental proof for the occurrence of a shell closure at Z=50 and N=50: Similar to the conclusions drawn from the alpha-decay data of the heaviest elements [2], it can be shown, with reference to predictions from macroscopic-microscopic mass formulae (see e.g. [3]), that the large measured Q~ values are almost entirely due to the shell effect 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0375-9474(94)00768-3

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E. Roeckl / Nuclear Physics A583 (1995) 849-852

be related to a resonance phenomenon (i.e. a superallowed decay) and may lead to an improved understanding of cluster radioactivity. The region near l°°Sn was discussed so far with emphasis put on charged-particle disintegration modes. However, this area of the chart of nuclides has also been subject of an intense programme of D-decay studies recently [6,7,14-19]. As has already been mentioned, D decay in this region of nuclei is dominated by a ~rgg/2--'vgz/2 (or, for Z,N<50, also by a ~rgg/2-"*vgg/2) GT transition. Correspondingly, D-decay experiments allow one to investigate, even with low source strengths, the GT strength distribution and to compare it with model predictions. Recent shell-model calculations (see [19] and references therein) indicate that 1°°Sn decays almost exclusively by (superallowed) GT transition to the l+(Trg~/~.,vgT/2} GT resonance state in 1°°In, whereas neighbouring nuclei show in constrast a complex and broad distribution of the GT strength. In the following, some recent D-decay studies will be discussed, obtained by using the ISOL systems at CERN and at GSI, respectively. The measur, ments include D-delayed ~/-rays for l°°Ag, 9SCd, 1°4In and l°SSn as well as D-delayed protons for 94,gSAg, 1°°,1°2In and l°l,:°3,1°SSn. The decay of even-even nuclei such as 98Cd [14] is characterized by the feeding of several 1 + levels in the odd-odd daughter nuclei instead of one 1+{7r#~-/~,v#7/2) state as expected from the extreme single-particle shell model. The excitation-energy of the observed 1 + states ranges from 1.7 to 2.5 MeV in rough agreement with what is expected from shell-model considerations for the two-quasiparticle state. The splitting of the GTstrength has been interpreted as being due to residual 7rgg/2-vg~/2 interactions. Part of the GT strength is missed by the experiment because of the restricted decay-energy window and of the limited experimental sensitivity. Additional reduction is caused by core polarization effects in the parent nuclei. However, even when all calculated hindrance factors are taken into account, part of the GT-strength still seems to be missing when comparing the theoretical values to the experimental data of N=50 isotones. The question is whether the nuclear structure and the experimental limitations are understood well enough to assign at least part of this still missing strength to subnucleonic effects (For sd-shell nuclei, 2/3 of the quenching of the GT operator has been ascribed to higher-order configuration mixing and 1/3 to A-adnfixtures [20]). The decay of even-odd nuclei such as t°SSn [15] is governed by a resonance-like, but broad beta-strength function whose maximum, at an excitation energy of approximately 3.3 MeV in the odd-even daughter nuclei, nfight be ascribed to the coupling of an odd ds/2 neutron to the GT pair l+{~rg~/2,vgz/2 }. Similar GT strength functions are predicted for 1°3Sn and 191Sn. The calculated half-lives agree well with the measured ones for l°s'l°3Sn, whereas the value predicted for l°lSn (1.4s) deviates somewhat from the experimental result (3.i+0.9s)[7,19]. Whereas high-resolution germanium detectors were applied to obtain the D-decay data described so far, a total-absorption 7-ray spectrometer was used to study the decay of the odd-odd nuclei, namely ~°°hg [17] and ~°4In [16]. Here one expects the dominant population of a four-quasiparticle structure at an excitation energy of the order of 5 MeV in the final even-even nuclei, consisting of a GT pair l+{~rg~/2,vg~/2 } from the respective core decay coupled to the spectator particles, i. e. the odd ds/2-neutron and the odd gg/2-proton. Indeed, the results obtained for the decay

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E. Roeckl I Nuclear Physics A583 (1995) 849-852

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Figure 1: Section of the chart of nuclides, showing the lightest isotopes with known decay properties from silver to barium. Arrows indicate charged-particle and ~+/EC decaymodes, with "a" and "p" marking alpha- and proton-radioactive nuclei, respectively. Note in particular the three consecutive a-decays starting from 114Ba and the 12C-decay of this nucleus. New isotopes whose decay properties have recently been measured by using the GSI ISOL facility are marked by heavily outlined boxes. Dashed boxes indicate cases of mere isotope identification obtained by means of beam-line spectrometers. (For N-Z=2 nuclei, for example, the Q, value decreases, when going away from the valley of beta stability, from 4.32 MeV for *°STe to 3.88 MeV for *l°Xe, whereas a macroscopic mass model [3] would predict an increase from approximately 1.1 to 1.4 MeV). Another interesting result is that the alpha widths measured in this region so far, namely those of l°6Te and n°Xe, do not support the idea of superallowed alpha-decay of these nuclei. The double shell-closure at 1°°Sn gives also rise to two other ground-state decay modes involving charged-particle emission. One of them is proton radioactivity [12], experimentally identified for l°SSb [8], 1°91, 112Cs [13], and liSCs. It is interesting to note that the spectroscopic factor determined for the proton decay of 1°91, n2Cs and U3Cs is below unity which is interpreted as reflecting different shape of mother and daughter nucleus [12]. Another exotic decay mode above l°°Sn is the 12C decay of 114Ba. Recent measurements, performed by using the GSI ISOL facility, have allowed one to identify the new isotopes 114-116Ba and llSBa, to study their decay, and to collect isotopically separated 114Ba atoms for a search for cluster radioactivity [9]. A preliminary evaluation of these data indicate that the ratio between lsC and alpha emission probability for 11~Bais strikingly larger than that observed for nuclei above lead. The explanation of this result may

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E. Roeckl / Nuclear Physics A583 (1995) 849-852

of l°°Ag and 1°4In indicate such a dominant four-quasiparticle component of the strength distribution, which is qualitatively confirmed by the X-ray and/3-delayed proton data measured for 1°°"°2In [16] and '°3"°5Sn [18]. The study of proton-, alpha-, cluster- and fl-decay near l°°Sn, which has been described in this report, is a continuing programme. Its success depends crucially upon the development of new or refined experimental methods. In the case of ISOL experiments, this includes (i) the development of rapid, efficient and chemically selective ion sources, which involves, e. g., laser resonance ionization or ionization of molecules, (ii) the application of total-absorption v-ray spectroscopy, and (iii) the search for cluster decay. In the case of intermediate-energy or relativistic heavy-ion experiments, performed by means of beam-line spectrometers, the recently obtained intensities of l°°Sn have been sufficently high to allow for an unambigious in-flight isotope identification and maybe even for a halflife measurement. To date, the l°lSn rate obtained by using fusion-evaporation reactions at the GSI ISOL is one to two orders cf magnitude higher than that reached by nuclear fragmentation at GANIL and GSI, respectively. On the other hand, there are apparent improvements possible for the high-energy facilities, such as the increase of the primary beam intensity. All in all, exciting nuclear-structure results have recently been obtained for nuclei in the l°°Sn region and there is every reason to believe that there is more to come along this line soon. 1. Proc. 6th Int. ConL on Nuclei far from Stability and 9th Int. Conf. on Atomic Masses and Fundamental Constants, 1992, eds. R. Neugart and A. Wfhr, IOP Conf. Ser. 132 (Institute of Physics, Bristol, 1993). 2. E. B.oeckl, Rep. Progr. Phys. 55 (1992) 1661. 3. P. M~ller et al., At. Data and Nucl. Data Tables, in print. 4. B.. Schneider et al., Z. Phys. A, in print. 5. M. Lewitowitz et al., Phys. Left. B, in print. 6. K. Schmidt et al., submitted for publication in Z. Phys. A. 7. Z. Janus et al., submitted for publication in Phys. Lett. B. 8. 1:(. J. Tighe et al., Evidence for the Ground-State Proton Decay of l°SSb, Preprint

(1993). 9. A. Gugliehnetti et al., contribution to this conference. 10. H. Keller et al., Z. Phys. A340 (1991) 363. 11. E. Ftoeckl, Alpha Decay, Preprint GSI-92-69 (1992), to be published. 12. S. Hofmann, Proton Radioactivity, Preprint GSI-93-04 (1993), to be published. 13. R. D. Page et al., Phys. l:tev. Lett. 72 (1994) 1798. 14. A. Plochocki et al., Z. Phys. A342 (1992) 43. 15. M. Pfiitzner et al., Nucl. Phys. A, in print. 16. J. Szerypo et al., Preprint GSI-94-38, submitted to Nucl. Phys. A. 17. L. Batist et al., submitted to Z. Phys. A. 18. R. Orzywacz et al., to be published. 19. B. A. Brown and K. Rykaczewski, to be published. 20. B. A. Brown and B. H. Wildenthal, Nucl. Phys. A474 (1987) 290.