Recent experiments with radioactive ion beams at GSI

NUCLEAR PHYSICS A Nuclear PhysicsA588 (1995) 41c-48c

EI£EVIER

Recent

Experiments

with

Radioactive Ion Beams

at GSI

Ernst Roeckl GSI Darmstadt, Postfach 110552, D-64220 Darmstadt, Germany

A review is given over recent experiments performed by means of A.keV to A-GeV radioactive ion beams at GSI Darmstadt. Nuclear structure information on exotic nuclei is discussed as obtained by using an online isotope separator, a velocity filter or a magnetic beam-line spectrometer.

1. I N T R O D U C T I O N In recent years radioactive ion beams have occured as novel tools for studying nuclear structure far from the fl stability line, and particular attention has been payed to the possibility of making high-intensity, small-emittance beams of radioactive ions with Coulomb-barrier energies available to experiment. Due to the very attractive research program in nuclear physics and astrophysics, that could be realized with such beams, a number of projects along this line have indeed been proposed recently. One should remember, however, that radioactive beams exist as a research tool, even though not necessarily at Coulomb-barrier energies, and that a good deal of what is contained in the above-mentioned research program can already be studied now: There are low-energy (60 keV) secondary beams, i.e. the online isotope separator (ISOL) method pioneered and actively pursued by the ISOLDE Collaboration at CERN, as well as intermediate or high-energy secondary beams, i.e. the magnetic beam-line spectrometer method, that was pioneered at the BEVALAC (Berkeley) and has extensively been used for experiments at MSU (East Lansing), GANIL (Caen), RIKEN (Tokyo) and GSI (Darmstadt). GSI is in the fortunate position of having radioactive ion beams in the A-keV to A-GeV range available as can be seen from Table 1. At the universal linear accelerator UNILAC, the relevant instruments are (i) an ISOL system [1] based on stopping of reaction products in an ion source, extraction of a 60 keV beam of singly charged ions and mass separation in a magnetic sector field, and (ii) the velocity filter SHIP [2] which separates projectiles and evaporation residues due to their velocity difference. Radioactive ion beams of relativistic energies are produced through nuclear fragmentation of stable-isotope beams from the Schwerionen-Synchrotron SIS and separated into monoisotopic beams by using the projectile-fragment separator (FRS). The FRS represents a two-stage magnetic beam-line spectrometer whose dispersive mid-plane can house an energy degrader or a target and which can be operated in achromatic mono~nergetic or energy-loss mode [3]. Secondary beams serve, after purification by means of the FRS, for reaction or decay 0375-9474/95/$09.50 © 1995 Elsevier ScienceB.V. All rights reserved. SSDI 0375-9474(95)00097-6

42c

E. Roeckl / Nuclear Physics A588 (1995) 41c-48c

Table 1: Radioactive beam experiments at GSI

Experimeat

Primary beam (ions/s;A-MeV)

Production reaction

Secondary beam ( A . M e V )

ISOL

<6.1011; <10

fusionevaporation

,,~10-3

SHIP

<3.101~; ,,~5

fusionevaporation

,,,1

FRS

<10t°;<2.103

fragmentation

100-2-103

Special features "ion sourcery", total 7-abs. spectrometer 50 magnetic deflection, correlation technique FRS beams used at ESR, Crystal Ball, ALADIN, LAND etc.

studies at the final focal plane of the FRS. Moreover, these beams are transfered to the experimental storage ring (ESR), the Heidelberg-GSI Crystal Ball, the magnetic spectrometer ALADIN, or the neutron detector LAND. In this report, I shall present nuclear-structure results on exotic nuclei, obtained at GSI, and begin by discussing data measured at the ISOL system (Section 2) and at SHIP (Section 3). Section 4 will deal with FRS experiments and Section 5 will contain summary and outlook.

2. E X P E R I M E N T S

AT THE

ONLINE

MASS

SEPARATOR

Very neutron-deficient isotopes near the doubly-magic l°°Sn (Z=N=50) have met considerable experimental and theoretical interest, recently (see also the "l°°Sn session" of this symposium). There is a twofold motivation for investigating decay properties near l°°Sn. Firstly, owing to the fact that protons and neutrons occupy identical (g9/2) shell-model orbits, one may encounter superallowed alpha and cluster emission beyond l°°Sn. Secondly, /3 decay in this region is dominated by a fast (superallowed) 7rg9/2---~vg~/2(or, for Z,N<50, also by a 7rgg/2---*vg9/2) Gamow-Teller (GT) transition, or in case of 0 + groundstates below l°°Sn, by superallowed (Fermi) 0+---~0+ transitions. Therefore, the influence of the proton drip-line on/3 decay (e.g. isospin mixing) can be investigated in an area where the spherical shell model is applicable. The present experimental status on decay properties of Z-~N nuclei in the silver-to-barium region [4] is based almost exclusively on measurements performed by using beam-line spectrometers or .isotope separators online (ISOL). In the ISOL experiments performed at GSI, the proton-rich isotopes were produced by fusion-evaporation reactions such as SSNi(4°Ca,p3n), S°Cr(58Ni,2p5n) or 5SNi(~SNi,2n) and mass-separated as singly-charged 60 keV beams. This method yields indeed superior experimental conditions for decay studies, at least in favorable cases such as 94Ag, l°lSn or '14Ba, than those available at beam-line spectrometers. The region of neutron-deficient isotopes above tin is well suited for a discussion of the interplay between alpha, proton and cluster radioaetivities. The occurrence of an island of alpha emission between l°6Te and 1'4Cs is the most convincing experimental proof for

E. Roeckl / Nuclear Physics A588 (1995) 41c-48c

43c

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 (see Section 2), it can be shown [4], with reference to macroscopic-microscopic mass formulae, that the large measured Q~ values are almost entirely due to the shell effect. However, the alpha widths measured in this region, namely those of l°6Te and n°Xe, do not support the idea of superallowed alpha-decay of these nuclei. Recently, the development of a barium-fluoride ion source at the GSI ISOL facility has allowed one to study decay properties of the new isotopes 114-n6Ba and llSBa, and to collect 114Ba atoms for a search for cluster radioactivity [5]. A preliminary evaluation of these data indicate that the ratio between 12C and alpha emission probability for 114Ba may be strikingly larger than that observed for nuclei above lead. The explanation of this result may be related to a resonance phenomenon (i.e. a superallowed cluster decay) and may lead to an improved understanding of cluster radioactivity. Recent/3-decay studies at the ISOL system at GSI include/3-delayed 7-rays for l°°Ag, 1°4In and l°SSn as well as/3-delayed protons for 94'gSAg, 1°°'1°2In and l°l'l°3'l°SSn [4]. A comparison between the measured 94Ag half-life of 420+50 ms and shell-model predictions suggests that the decay of the 9 + isomer has been measured whereas the Fermi decay of the 0 + groundstate, with a predicted half-life of 10 ms, remained unobserved [6]. Shell-model calculations (see [7] and references therein) indicate furthermore that l°°Sn decays almost exclusively by a (superallowed) GT transition to the l+{rg~-/~2,vgv2} GT resonance state in 1°°In, whereas neighbouring nuclei show in contrast a complex and broad distribution of the GT strength. The decay of even-even nuclei such as 9SCd is indeed characterized by the feeding of several 1+ levels of excitation energies between 1.7 to 2.5 MeV in the odd-odd daughter nuclei (see [8] for a discussion of the splitting and quenching of the GT-strength). The decay of even-odd nuclei such as ~°SSn [9] 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, might be ascribed to the coupling of an odd ds/~ neutron to the GT pair l+{Trg~-/12,t,g~/2}. Similar GT strength functions have been predicted for l°3Sn and l°lSn. The calculated half-lives and/~-delayed proton intensities agree qualitatively with the measured values for l°s'l°3Sn, whereas the half-life predicted for l°lSn (1.4 s) deviates somewhat from the experimental result (3+1 s) [7, 10]. In case the decay of odd-oddnuclei near l°°Sn one expects a 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+{Trg~-/12,t,gu2) from the respective core decay coupled to the spectator particles, i. e. the odd ds/2-neutron and the odd g0/2-proton. Indeed, total-absorption "?-ray data obtained for the decay of l°°Ag [11] and ~°4In [12] indicate such a dominant four-quasiparticle component of the GT strength distribution, which is qualitatively confirmed also by the decay properties measured for 1°°'1°2In [12].

3. E X P E R I M E N T S

AT

THE

VELOCITY

FILTER

The properties and, in particular, the stability of the heaviest elements are governed by the interplay between the competing decay modes of fission and alpha-emission. As is experimentally well established now, nuclei with atomic numbers beyond Z=106 have

44c

E. Roeckl / Nuclear Physics .4588 (1995) 41c-48c

large fission barriers and are thus shell stabilized. Such a shell stabilization has been predicted to occur for Z=l14, N=184, i.e. the "island of superheavy elements" already in the sixties. However, the measured properties of nuclei with Z=>_106, i.e. the dominance of alpha decay, clearly indicates an enhancement of the fission barriers, which means that the island of superheavy has in fact been reached [13]. The main contribution in this field, namely the discovery of the elements Z=107 - 109, has come from combining recoil separation with the implantation and time-correlation technique at GSI [13]. Details of these measurements, performed at least six years ago, will not be described here. However, it is important to note the impact of the experimental results on our understanding of the heaviest nuclei: The comparison of measured (groundstate) masses and calculated macroscopic masses yields evidence for a shell effect of the order of 6 MeV and for a fission barrier of similar magnitude, which both together represent the building blocks of superheavy elements. One should note that, in addition to the nuclear-structure information, a lot has been learned from these experiments on the limitation of fusion reaction [13]. It lies close at hand to ask whether these exciting experiments can be extended to even heavier elements. This question will hopefully soon be answered as attempts are underway to synthesize element 110 by means of 244Pu(~4S,5n)2ral10 reations at FLNR Dubna and by means of 2°Spb(6~Ni,ln)2Sgll0 reactions at GSI Darmstadt [13]. The GSI group relies on an approach similar to that used for the discovery of element 109 (Mt) through 2°gBi(SSFe,ln)26~Mt reactions. However, while the latter had a production cross-section of approximately 10 pb, a much lower value of about 1 pb is expected for the formation of 2s9110. Therefore, an overall improvement of SHIP and the related detection system has been realized. The improvement factor of 7 compared to previous SHIP experiments contains the following features: higher UNII,AC currents due to the use of the ECR source, increase of thickness and area of (rotating) target, increase of acceptance and decrease of dispersion of SHIP, magnetic deflection and time-of-flight analysis behind SHIP, increase of the solid angle of the (silicon strip) detector. By using this device, a In production cross section of 1 pb would correspond to 1 detected alpha or fission event per 7 days. For such low rates it is of course extremely important that beam energy and target thickness are matched to the maximum of the in excitation function. Therefore, the SHIP group decided to measure the 2°Spb(S°Ti,xn)2Ss-~"104 and the 2°SPb(SSFe, ln)2¢s108 excitation functions in order to be able to estimate the optimum 62Ni beam energy for the ~°SPb(62Ni,ln)~69110 case. The S°Ti measurements were completed this summer, whereas those with the SSFe beam were started three weeks ago and continued when I left three days ago: So far, 30 events were observed [14] for the 26s108 alpha decay, which should allow one to reliably predict the optinmm experimental conditions for the synthesis of element 110. 4. EXPERIMENTS SEPARATOR FRS

AT

THE

PROJECTILE

FRAGMENT

4.1. Isotope Identification and Study of Production Reactions For planning experiments with relativistic radioactive beams it is indispensable to have reliable predictions of the secondary beam intensity, i.e. on the production cross-section

E. Roeckl / Nuclear Physics A588 (1995) 41c-48c

45c

and the momentum distribution of the isotope wanted (The question the of excitation function is, of course, less critical than for reactions near the Coloumb barrier). A detailed discussion of this "reaction aspect" lies outside the scope of this report, but will at least partly be covered by other speakers at this symposium (see, e.g. [17]). I would like to mention, however, that such cross-section and momentum data for 500 A.MeV SSKr induced reactions were measured and compared to model predictions [18]. In addition to the identification of several new isotopes in the manganese-to-cobalt region, this work has yielded evidence for a dependence of the fragment momenta upon the N/Z ratio of the fragment: For a given element, neutron-deficient isotopes systematically show smaller (parallel) momenta than neutron-rich ones [18]. It will be interesting to establish this trend in improved experiments and to interpret it on the basis of a fragmentation model: There may even be c¢,nsequences for determining the optimum projectile energy for producing exotic nuclei through nuclear fragmentation reactions. A novel production path towards very neutron-rich mean-mass isotopes is fission at relativistic energies: By projectile fission of a 750 A.MeV 23sU beam on a lead target many new isotopes between germanium and cerium have been identified [19]. It appears attractive to study some of these new nuclei by using the methods to be described below. Incidentally, the so far unknown doubly-magic nucleus rSNi is expected to be produced at the FRS best by 2sSU+Be rather than 23sU+Pb reactions. This comes about as the larger U + P b "Coulomb fission" cross-section, caused by the enhanced virtual-phonon field due to the high-Z target material, is overcompensated by the higher number of target atoms to a U+Be available experiment. Besides these considerations related to nuclear-structure studies of neutron-rich mean-mass isotopes, FRS experiments with relativistic 23sU beams meet additional interest: They allow one to determine fission properties of actinide nuclei (see Section 4.3) and to obtain data on the planned application of high-energy proton beams for driving reactors or for incineration of reactor-produced actinides.

4.2. B e t a - D e c a y Studies It is known since the early BEVALAC experiments that radioactive beams from a beam-line spectrometer can be implanted into a detector for studies of the subsequent /3 decay. At the FRS, this technique was applied to investigate, e.g., proton-rich nuclei produced in 58Ni fragmentation [17]. Particularly interesting are the/3-decay studies of 36Ca(Tz=-2) and 3rCa(Wz=-5/2) [20], which yielded the following results: (i) first observation of 7-deexcitation of (bound and unbound) levels of the daughter nuclei ~SK and 3rE, respectively; (ii) accurate determination of the distribution of B(GT) and B(F); (iii) evidence for isospin mixing (in the 3rCa decay) and for a systematic deviation between experimental and calculated total B(GT) strength (for the 36Ca and the ~rCa decay); (iv) confirmation of the previously accepted solar-neutrinos capture cross-section of 37C1. This work has demonstrated that/3-decay data of high accuracy can be gained by using a beam-line spectrometer at a relativistic heavy-ion accelerator, and that this new method is especially powerful in combination with high-resolution data from ISOL measurements. 4.8. Reactions Induced by Radioactive Ion Beams The density distribution of nuclei far from stability has attracted considerable interest recently. On the one side, evidence has been obtained both for a "neutron halo" of light

46c

E. Roeckl /Nuclear Physics A588 (1995) 41c-48c

neutron-rich weakly bound nuclei such as 11Li, i.e. a long tail of a low density neutron gas extending far outside the 9Li core, and from the "neutron skin" that is particularly obvious for neutron-rich (less weakly bound) nuclei. On the other side, the question whether there are experimental indications for a "proton halo" is still under discussion. A very valuable tool for probing the nuclear density distribution is the study of reactions at relativistic energies. The "transmission method", that was used in the early BEVALAC measurements, is based on (i) determining the intensity of the isotopes of interest before and after having passed the target, (ii) calculating the total interaction cross-section from the intensity loss, and (iii) deducing the mean-square matter radii of projectile and target on the basis of a simple (geometrical) relation between these radii and the cross-section. Such measurements have recently been performed at the FRS, with the target being positioned at the intermediate focal plane, for sodium isotopes [15] as well as for A=20 isobar from nitrogen to magnesium [16]. The experimental data are under evaluation and will therefore not be discussed here. Furthermore, interaction cross-section for proton-rich isotopes produced in 5SNi fragmentation, were measured by means of the transmission method at the final FRS focus [17]. In order to perform a more refined experimental analysis of the nuclear density distribution, one may investigate the kinematical properties of reaction products, e.g. their momentum distributions. Experiments of this kind have been carried out either by using the FRS alone or by transfering FRS beams to the ALADIN-LAND array. Results from these measurements for llLi and lXBe will be reported elsewhere at this symposium [21]. I would like to discuss here an alternative mehtod, namely the study of density distribution of neutron-rich helium isotopes by proton elastic scatterin9 in inverse kinematics [22]. 4He, 6He and SHe beams from the FRS interacted with protons inside a high-pressure, hydrogen filled ionization chamber which simultaneously served as a gas target. While the recoil protons were detected by the ionization chamber, several tracking detectors and scintillation counters were used to measure the scattering angle of the incident particle. A preliminary analysis of the differental cross section for elastic proton scattering as a function of the squared four-momentum transfer shows that for 4He excellent agreement is obtained with earlier 4He(p,p)4He data and that the 4He data are reproduced by a Glauber calculation with a single Gaussian density distribution assuming a mean-square charge radius of 1.67 fm determined from electron scattering. For SHe, however, the experimental results can only be decsribed by using a nuclear matter distribution that is composed of two Gaussians with 1/2 values of 1.67 fm and 3.2 fm for the core nucleons and the skin neutrons, respectively. The corresponding average 1/2 of 2.52 fm is not far from the values of 2.50 fm and 2.70 fm deduced previously [23] from the transmission method. The new data represent indeed a convincing indication for an extended nuclear matter skin of SHe. Another interesting approach is inelastic proton scattering in inverse kinematics as has been shown in a p(SSNi,p') experiment performed at the finals FRS focus [24]. The SSNi beam impinged on a CIt2 target which was surrounded by a ring of silicon detectors for measuring the energy of recoil protons at an angle of approximately 90 degrees to the beam direction. Tracking and isotope identification of the scattered 5SNi ions was performed in a similar way as in the above-mentioned SHe experiment. The B(E2) value of 600+120 e2fm4, found for the first excited 2 + state of SSNi, is close to that of neighbouring non-doubly-magic nuclei and about a factor of six larger than

E. Roeckl / Nuclear Physics A588 (1995) 41c-48c

47c

that of the doubly-magic nucleus +sea. A recent shell-model calculation has been able to reproduce the measured B(E2) value of the 2+ level of S6Ni and to ascribe it to large coherent fT/2--*P3/2 and fr/2--~fs/2 contributions to the lp-lh state [24]. In concluding this description of reactions induced by radioactive ion beams, I would like to mention Coulomb fission of actinide isotopes [25]. At the final focus of the FRS, monoisotopic beams of actinide isotopes, produced by 23SU+Be reactions, were directed on to an "active" target. This target consisted of several lead layers forming the cathodes of an ionization chamber. Fission products were identified due to signals derived from the target, from an additional ionization chamber yielding the Z information, and from two scintillation detectors which defined binary fission events. The measured nuclear-charge distributions indicate a transition from symmetric fission for 2~SAc to assymmetric fission for 2361.1. This transition occurs only if low-energy fission dominates, which is the case for the 23sU+Pb system with its enhanced electromagnetic excitation. It is interesting to note that fission properties of most of those nuclei were inaccessible previously.

4.4 Mass

Measurements in the ESR Exciting new precision experiments and studies of rare decay modes have become possible by injecting radioactive ion beams into the ESR. The most impressive example of such studies was the observation of bound-state/3-decay of 163Dy [26], and there are other experinaents of this sort foreseen. Another interesting field are mass measurements in the ESR. The method used so far represents a frequency analysis of the coasting beams by means of the so-called Schottky analysis. In a pilot experiment [27] with FRS beams of A/Z=2 isotopes (among them 110 rain lSF) produced by 2°Ne fragmentation, the feasibility of accurate mass determination has been proven. More recently, radioactive isotopes produced by 5sNi fragmentation were cooled and stored in the ESR, and it was shown that an accuracy of approximately 10 -6 was reached in mass measurements [28]. This was demonstrated by the unambigious separation of the ground-state and the 21 min isomeric state of 52Mn in the Schottky spectrum.

5. S U M M A R Y

AND

OUTLOOK

The study of proton-, alpha-, cluster- and/3-decay near l°°Sn at the ISOL facility as well as the heavy-element program underway at SHIP show that the UNILAC is still going strong after almost twenty years of operation. The FRS experiments, that were performed since the beginning of the SIS operation in 1991, show great promise even though they have just "scratched the surface". Already this "surface" has revealed exciting physics results on the identification of new isotopes,/3-decay studies, reaction work including Coulomb fission, and mass measurements. So far, most of the work at relativistic energies has been performed directly behind the FRS, i.e. by collecting radioactive atoms for decay studies or by using a single-turn target for reaction measurements. It is clear, however, that superior experiments can be carried out by using the Crystal Ball for Coulomb-excitation studies or by investigating direct reactions like (p,p') or (d,p) in inverse kinematics by means of the ESR. For this new generation of experiments, as well as for all the other FRS experiments described in this report, it is good news that a program of increasing

48c

E. Roeckl / Nuclear Physics A588 (1995) 41c-48c

the intensity of SIS beams is underway. There is thus every reason to believe that, at GSI and elsewhere, radioactive beams will continue to be an important tool for gaining insight into nuclear-structure phenomena far from the/3-;stabilityline. With this optimistic outlook on the future of radioactive-beam experiments, I would like to close by thanking the organizers of this symposium for inviting m e and m a n y of m y GSI colleagues either for collaborating in experiments or for making recent results available for this review.

[1] [2] [3] [4]

C. Bruske et al., Nucl. Instr. and Meth. 186 (1981) 61. G. M~nzenberg et al., Nucl. Instr. and Meth. 186 (1981) 423. H. Geissel et al.,Nucl. Instr. and Meth. in Phys. Res. B70 (1992) 286. E. Roecld, in Proc. of the 5th Intern. Conf. on Nucleus-Nucleus Collisions, Taormina, 30 May - 4 June, 1994, Nucl. Phys. A, in print. [5] A. Guglielmetti et al.~ in Proc. of the 5th Intern. Conf. on Nucleus-Nucleus Collisions, Taormina, 30 May - 4 June, 1994, Nucl. Phys. A, in print. [6] K. Schmidt et al., Z. Phys. A350 (1994) 99. [7] B. A. Brown et al.,Phys. Rev. C50(1994) 2270. [8] A. Ptochocki et al., Z. Phys. A342 (1992) 43. [9] M. PfStzner et al., Nucl. Phys. A581 (1994) 205. [10] Z. Janas et al., Physica Scripta, in print. [11] L. Batist et al., Z. Phys. A, in print. [12] J. Szerypo et al., Nucl. Phys. A, in print. [13] P. Armbruster, Preprint GSI-94-40 (1994), in P~oc. of the Intern. Conf. Nuclear Shapes and Nuclear Structure, Antibes, 19-25 June, 1994, in print. [14] S. Hofmann, private communication. [15] K. Suzuki, private communication. [16] L. V. Chulkov, private communication. [17] B. Blank, contribution to this symposium. [18] M. Weber et al.,Z. Phys. A343 (1992) 67 and Nucl. Phys. A578 (1994) 659. [19] M. Bernas et al., Phys. Lett. B331 (1994) 19. [20] W. Trinder et al., in Proc. of the 2nd Tours Symposium on Nuclear Physics, 30 Aug.- 2 Sept., 1994, World Scientific, in print, and W. T~inder, Ph. D. Thesis, in preparation. [21] P. G. Hansen, contribution to this symposium. [22] S. Neumeier et al., in Proc. of the 5th Intern. Conf. on Nucleus-Nucleus Collisions, Taormina, 30 May - 4 June, 1994, Nucl. Phys. A, in print. [23] I. Tanihata et al.,Phys. Lett. BI60 (1985) 380, Phys. Rev. Lett. 55 (1985) 2677 and Phys. Lett. B206 (1988) 592. [24] G. Kraus et al.,Phys. Rev. Lett. 73 (1994) 1773. [25] K.-H. Schmidt et al.,Phys. Left. B325 (1994) 313. [26] M. Jung et al.,Phys. Rev. Left. 69 (1992) 2164. [27] H. Geissel et al.,Phys. Rev. Left. 68 (1992) 3412. [28] H. Geissel, private communication.