New μs-isomers and isomeric beams

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A630 (1998) 307c-315c

New #s-isomers and isomeric beams K. Rykaczewski a'b, R. Grzywaczb'c, M. Lewitowiczc, and M. Pffitznerb,a, for a collaboration of Univ. Warsaw, GANIL Caen, CEN Bordeaux, IPN Orsay, LPC Caen, CEN Bruy@res-le-Chatel, IPNL Lyon, IAP Bucharest, JINR Dubna, KUL Leuven, GSI Darmstadt, Univ. GSttingen, Univ. Surrey, Univ. Liverpool, Univ. Brighton, Univ. York, ORNL Oak Ridge ~Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA bInstitute of Experimental Physics, Warsaw University, Ho~a 69, P1-00681 Warsaw, Poland ~GANIL, 14021 Caen-Cedex, France dGesellschaft f/ir Schwerionenforschung, D-64220 Darmstadt, Germany A review of experimental results obtained within the last three years on #s-isomeric states produced in fragmentation reactions and studied at projectile fragment separators at GANIL Caen and GSI Darmstadt is given. Experiments based on secondary reactions with isomeric beams are discussed. 1. I N T R O D U C T I O N Fragmentation reactions with heavy-ions at intermediate and relativistic energies studied at projectile fragment separators have allowed the border lines of known nuclei to be extended by several mass units. Among recent spectacular examples of such studies are the identification of the doubly-magic nuclei l°°Sn [1-4] and rSNi [5,6], both located very far from the stable isotopes of these elements. In the case of most proton-rich N=50 isotone, l°°Sn, twelve neutrons were removed from the primary 63 AMeV 112Sn beam particles at GANIL [1,2], and as many as 24 nucleons were stripped from 1 AGeV 124Xe beam at GSI [3,4]. The most neutron-rich known N=50 isotone, 7SNi, was observed among the very neutron-rich fission products of relativistic 23su beams incident on a 9Be target, after short time-of-flight through the GSI FRS set-up [5]. In these experiments, many other new isotopes in the vicinity of lOOSn (over twenty [2]) and 7SNi (over hundred [6]) also were identified. This illustrates the broad distributions in mass, A, and atomic number, Z, extending to exotic nuclei, typical for fragmentation reactions with high energy heavy-ions. However, this feature limits the spectroscopic studies of excited states in weakly produced exotic fragments, since e.g. in-beam 7-spectroscopy at the target position will be blocked by the high background rates from the products closer to stable isotopes. Information on the excited states of nuclei produced via heavy-ion fragmenta0375-9474/98/$19 © 1998 ElsevierScienceB.V. All rights reserved. PI1 S0375-9474(97)00768-9

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tion has been basically limited to the data obtained by means of decay spectroscopy of implanted reaction products. A new area of nuclear structure studies has been opened three years ago, with the observation of forty known #s-isomers in heavy nuclei. They were populated in the fragmentation of a 58 MeV/u a12Sn beam [7] and studied at the final focus of LISE3 spectrometer [8,9] at GANIL. This pioneering run was followed by about ten experiments employing fragmentation of mSn, a°~Cd, 92Mo, SSKr, ~SKr, 4°Ar and 36S beams at GANIL, and :3sU projectiles at GSI. These studies profited from the implanted fragment~7 #s-correlation technique described in the next section. Among other results, almost thirty new isomeric states in exotic nuclei have been discovered, and their main decay properties have been established. Evidence for over twenty other new metastable states in exotic nuclei such as l°~mSn and T2mNi with halflives in the #s-range has been obtained. 2. T H E M E T H O D : GAMMA-RAYS

#s C O R R E L A T I O N S

OF R A D I O A C T I V E

ION AND

Experiments with high-energy heavy ion beams profit from the selectivity achieved with modern projectile fragment separators such as the SISS1-Alpha-LISE3 complex [8-11] at GANIL and FRS [12] at GSI. Tracking of the ions in space and time usually is provided by position-sensitive counters. This information, together with energy loss and total energy measurements, allows a separation of mass (A), atomic number (Z) and charge state (Q) of individual ions i.e. it is possible to identify the reaction product. During such experiments on #s-isomeric states, a standard stack of silicon detectors (or other type of the counters and catchers system) measuring the time and energy loss signals of implanted ion is surrounded by the photon detectors at the final focus of the spectrometer. To obtain high resolution information on the detected 3' radiation, an efficient array of "),-detectors consisting e.g. of several clover- or cluster-type detectors, is crucial for the success of such experiments. After the implantation of the reaction product, the acquisition system opens for a fixed time interval, of the order of 100 #s. The overall rate of the products with spectrometer setting corresponding to exotic products is usually below 1 kHz, i.e. below 1 ion per millisecond on average. It was found, see e.g. [7], that fragmentation reactions efficiently populate the isomeric states in nuclei. If the halflife of such state is long enough for the ion to be transmitted through the spectrometer, the 3,-decay of the isomer may be observed at the focal plane. The 3' energy and its time relative to the ion implantation is recorded with the same coincidence word as product identification signals. Background 3' radiation consists mainly of natural (laboratory) background, prompt photons from reactions at the catcher (detector) material and /~-delayed -/-ray transitions following the decay of previously implanted isotopes. Such events are greatly reduced by requiring implanted ion-',/ correlation time within a #s range. One may consider the 3` spectra obtained after isomer decay similar to the singles, however, recorded only for a very short time intervals after implantation of identified ions. The range of this time interval is variable and is selected during off-line analysis accordingly to the halflife of the analyzed isomer. Therefore, the resulting typical 3`-background rate is at level of 1% of all correlated events related to the short lived isomer decay. The intensity of recorded -/-transitions has

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K. Rykaczewski et al./Nuclear Physics A630 (1998) 307c 315c

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Figure 1. Selected products of the fragmentation reaction of a 63 MeV/u n2Sn beam on a 93.5 m g / c m 2 ~*Ni target identified by magnetic rigidity, time-of-flight, and energy loss with the Sissi-Alpha-LISE3 spectrometer complex at GANIL are shown in the left panel. The right panel presents the ions observed in the #s-correlation with 7-radiation (from ref. [13]). Isomeric nuclei implanted as fully stripped ions as well as hydrogen like 96mpd+45 are indicated.

to be unfolded with respect to the detector response function only and is not affected by 7-7 coincidence requirements. Another aspect of this technique is the unambiguous atomic and mass number assignment to the fragmentation products separated and identified by energy-loss (AE) versus time-of-flight (TOF) signals. Such AE - TOF spectra of heavy-ions obtained in ys correlation with 7-transitions following isomeric decay contains practically only the events corresponding to implanted nuclei in isomeric state, see Fig. 1. Since #s-isomers occur relatively rarely in the nuclidic chart, such spectra (called Grzywacz plot, see e.g. [7,13] and figures therein) provide a background-free response function of the detector system to the AE - TOF ion identification signals. This is particularly important for studies at the border of known nuclei and for reporting the observation (i.e. a proof of existence) of new, very weakly produced isotopes. A more complete discussion of this important feature illustrated with the identification of l°°Sn and the proton instability of ~gnr is given in refs. [2] and [14], respectively. The time-of-flight of fragmentation products between the target and the final detection system is from a few hundred nanosecond to about 1.5 #s. This suggests a halflife limit

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K. Rykaczewski et al./Nuclear Physics A630 (1998) 307c-315c

of about 100 nanosec for the isomers to be investigated with this method. However, the fragmentation products are transmitted as fully-stripped ions through the spectrometer (with various contribution of hydrogen-like and helium-like fragments). Since the conversion electron channel of the isomer decay is blocked during the flight for fully stripped ion, in specific cases the ionic halflife is much larger than for a neutral atom. This allows transmission of such isomeric ions almost without intensity losses during the flight between the target and implantation stations. The recent observation of a new level with Ta/2= 354-10 nanosecond (interpreted as I=0 +) in Z4Kr [15,16] in an experiment with a 92Mo beam at GANIL was possible due to the much longer ionic halflife of fully stripped 74mKr+36" The presented method of decay studies of #s-isomers populated in the fragmentation reactions, together with the Coulomb excitation of radioactive beams, has been labelled on-beam spectroscopy [17]. Both investigations are focussed on the excited states in the selected radioactive nuclei without changing the mass and atomic numbers of the beam particles. 3. E X A M P L E S

OF EXPERIMENTAL

RESULTS

3.1. S t u d i e s near t h e p r o t o n drip line

It is obvious that an efficient method of producing very neutron-deficient nuclei is achieved by fragmenting the most neutron-deficient isotope of a specific element. Therefore, the experiments aimed to study medium mass proton drip line nuclei performed at GANIL used the beams of rare isotopes like 112Sn [1,2,7,13], 1°6Cd [18], V2Mo [15] and 7SKr [14,19]. The basic features of the experimental method were established during the experiments with 58 MeV/u and 63 MeV/u 112Sn beams [1,7,2,13]. Among the few new isomers identified during these experiments were metastable states in T z = l isotopes near doubly-magic l°°Sn. Halflives and gamma transitions following the isomeric decays of 94mpd and 96mAg were measured [13]. The first experimental evidence for 9SmCd and l°2mSn was also obtained [13]. These experiments were followed by in-beam studies with fusion-evaporation reactions establishing more precisely the decay schemes and interpreting the decay properties of 94mpd [20], 9SmCd [21] and l°~mSn [22]. However, in the recent experiment using fragmentation of a 64 MeV/u l°6Cd beam, the decay data for 94mpd and vsmCd were further improved relative to the in beam results [18]. The isomeric transition of 94 keV, see Fig.2, between 14+ and 12+ states [20] has been observed for the first time resulting in the 20% correction to the previously calculated value of a Two-Body-Matrix-Element (TBME) for the interaction between 7rgg/2 and t/g9/2 coupled to the V=9, T=0 state [18]. Also the T1/2 values of I~=8 + isomers in the N=50 isotones, 96pd and 9SCd, have been remeasured to be 1.82(5) #s and 0.19(2) #s [18], respectively. These values are considerably shorter than previously reported and they also strongly affect TBME values derived for the nucleon-nucleon residual interactions in the vicinity of doubly-magic l°°Sn, see e.g. the recent review [23]. Particularly interesting are the observations of new isomers in the N=Z=33 nucleus 66As [19] and in 96Ag [13,18]. In both cases, the isomeric state has an excitation energy above the expected proton binding energy. These states are stabilized against proton emission

K. Rykaczewski et al./ Nuclear Physics A630 (1998) 307c-315c

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by the centrifugal and Coulomb barriers. Using fragmentation reactions for production of such isomeric states, it is possible to track the ions in flight i.e. consider them as beam particles. This creates new opportunities for obtaining new experimental observables such as radii of nuclei in excited, loosely bound states. 3.2. Towards t h e n e u t r o n drip line The short-lived states in neutron-deficient nuclei have been studied more than thirty years using fusion-evaporation reactions between heavy-ions. The fragmentation based studies recently became a complementary method. Their potential to study very exotic nuclei is similar (sometimes superior) to in-beam experiments using powerful large 7arrays. For neutron-rich nuclei, mainly the excited states of nuclei populated in fission have been studied. Recently, also multinucleon transfer reactions between heavy ions have been successfully applied to these nuclei [24,25]. However, the results obtained last year at GANIL and GSI indicate, that fragmentation of neutron-rich heavy projectiles is presently the best tool for studies of excited states of very neutron-rich isotopes. Indeed, this was not obvious till these studies, since the mechanism of isomer production is not well understood and the population of isomers in the fragmentation reaction cannot be predicted reliably before the experiment. High isomeric ratios were observed among the fragmentation products of a 60 MeV/u S6Kr beam at GANIL as well as with the 1 GeV/u and 750 MeV/u 23sU beams at GSI. These experiments opened a new way of studying the nuclear structure of very neutron-rich systems. ~2mA1represents the first neutron-rich isomer found with the #s correlation method [26, 27]. It was the only new neutron-rich isomer found among the fragmentation products of 4°Ar and 36S beams. Already this single case has led to the conclusion, that extrapolations based on existing shell model parameterizations may not be reliable [26] ! Sixteen new isomers (from Z=33 79mAs to Z=21 54mSc) were identified and studied among the fragmentation products of the 60 MeV/u S6Kr beam of 1011 particles per second (pps) [28,29]. Evidence for over twenty other new isomers also was obtained ranging from Z=34 sSmSe to Z=20 s°mCa, see Fig.3. Interpretation of the very rich nuclear structure information is in progress, however a few interesting conclusions already

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Figure 3. Summary of the results on the identification of new #s-isomers near 6SNi obtained during the experiment employing the fragmentation of a 60 MeV/u S6Kr beam at GANIL.

have been obtained [30]. They are related to the structure of magic Z=28 nickel isotopes between the neutron subshell and shell closures at N=40 and N=50. The isomer 7°toNi (Ta/2=210 ns, I~=8 +) and an isomer of a similar structure (the 8+ state coupled to the odd p3/2 proton) in 7aCu (T~/2=275 ns, U = 1 9 / 2 - ) were observed. Their measured decay properties, dominated by a cascade of four E2 transitions, have led to a revision of the "realistic interaction" used for shell model description of neutron-rich nuclei in this region [30]. The experimental and calculated 2+ energies in 7°Ni were in agreement, however differences became apparent when comparing observed and predicted states at higher excitation energies. It is important to note that the production rate of 7°toNi at the target was only about 1 pps. For Coulomb excitation studies, the rate of 7°Ni should be two orders of magnitude higher. Even with a 100 pps 7°Ni beam, one can probably obtain an information on the first excited state only. In the case of 7°Ni, the observation of only the first 2+ state might result in a misleading conclusion that the existing "realistic interactions" parameterization can be used for a description of the structure of very neutron-rich nickel isotopes. The experiment by Pfiitzner et al using 1 GeV/u and 750 MeV/u 23su beams of l07 pps fragmented by lg/cm 2 9Be target, was performed in December 1996 at the GSI. Two regions of neutron-rich nuclei were investigated, near 2°Spb and fission products near and above 6SNi. Preliminary analysis [31] indicates that six new neutron-rich isotopes and three new isomeric states were observed, see Fig.4. In particular, the data for 212"~Pb should allow the shell-model description along magic Z=82 shell to be extended further to neutron-rich nuclei. Also known short-lived isomers in the vicinity of 2°Spb were populated with the isomeric ratio of about 30% via the fragmentation of the 1 GeV/u

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Figure 4. The results of the experiment based on the fragmentation of a relativistic 1 GeV/u 2ssu beam at the GSI. The new #s-isomers and new isotopes near doubly-magic 2°Spb identified at FRS are indicated.

2asu beam [31]. This is only slightly lower than the typical isomeric ratios of about 40% observed for intermediate energy fragmentation reactions at GANIL, and high enough to foresee a successful continuation of such isomeric studies at relativistic energies. The #s-isomeric states were also observed in the fission region, when the 2asu beam energy has been lowered to 750 MeV/u. Under such experimental conditions, doubly-magic ~SNi nucleus was observed [5]. This time a few #s-isomers were observed on-line, e.g. 79mAs found during S6Kr experiment at GANIL and previously known 9stay. Also 7°toNi and z2'~Cu, both identified for the first time among S6Kr fragmentation products at GANIL, were found in the off-line analysis of GSI data [31]. This indicates that the identification of the observed fragments could be quickly made on-line based on the observation of known #s-isomeric decays also during fragmentation experiments at relativistic energies. This is very important for choosing a degrader thickness for the proper implantation profile of selected fragments followed, e.g., by/3- and 7-decay spectroscopy measurements. 4. E X P E R I M E N T S

WITH ISOMERIC

BEAMS

The idea of extending experimental studies to the reactions with unstable projectiles has fascinated nuclear physicists for many years. In particular, for studying high spin states and angular momentum transfer, a projectile with high spin state is most attractive, see e.g. [32]. A number of pioneering studies on the production and secondary reactions with isomeric beams have been performed at the NSCL [33-35], GANIL [36,37] and at RIKEN [38]. Intensities greater than 103 pps were achieved for lSmF at NSCL, and the elastic scattering on CH2 target as well as the total reaction cross section, have been studied [35]. At GANIL, the total reaction cross section has been derived for 42mSc projectiles, and it was found to be similar to the value measured for 4298Sc [36,37]. At RIKEN, a first Coulomb excitation experiment using an isomeric beam has been performed with the 174mHfnuclei [38]. It is clear, that for beam intensity reasons, these pioneering experiments were performed with well-bound isomers close to the/3 stability line.

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Recent theoretical developments, see e.g. [39-43], related to nuclear physics at the drip lines have resulted in the increased interest in the study of unbound systems. Microsecond isomers in exotic nuclei observed in the fragmentation reactions offer an opportunity to perform experimental studies of such systems. In addition to decay properties such as "7-transition rates and level energies above the proton binding energy or near the neutron binding energy, new observables should become experimentally available from secondary reactions with such isomeric beams. Total reaction cross section measurement for radioactive sodium isotopes made recently at the FRS (GSI) [44] could be taken as a reference study. An increase of the cross-section values observed for most neutron-rich (among studied) sodium isotopes was interpreted as due to an increased nuclear radius. Experiment was performed with quite low secondary beam intensities, e.g. about 1 pps only for 32Na ions. One should note that such (and indeed even higher) intensities of isomeric beams already have been achieved in fragmentation experiments. Therefore, a measurement of the total reaction cross section related to the density distribution for exotic isomeric states are technically already possible. 5. S U M M A R Y

Experiments with isomeric beams produced in fragmentation reactions have led to the new field of on-beam spectroscopy measurements. New isomeric states, namely protonrich 66ma'm2As, 74mKr, somy, S4mNb' 86,~Tc' 94mpd ' 96mAg' 9SmCd and neutron-rich 32reAl, 54mSc ' 54mv, 5SmCr' 61mFe' 64mMn' 6~mFe, 66ma,m2Co' 67,~Fe' 69rnCu' 70toNi' 71mCu ' 72rnCu ' 79mAs, 2°3mT1, 2°4m2T1 and 212mPb have been identified and studied. The properties of new isomeric states measured with radioactive ion--/ #s correlation technique have already allowed a refinement of the shell-model parameterizations near doubly-magic l°°Sn and "towards" doubly-magic 7SNi. The production rates of isomers already achieved should be sufficient for starting systematic total reaction cross section measurements for exotic metastable states. Information on the matter distribution in loosely bound nuclear systems can be obtained in such measurements, simulating e.g. the properties of the ground-states of much more exotic neutron-rich nuclei. ACKNOWLEDGEMENTS Oak Ridge National Laboratory is managed for the U.S. Department of Energy by Lockheed Martin Energy Research Corporation under Contract No. DE-AC05-96OR22464. This work was also supported in part by CNRS France under Project PICS-258 and by the Polish Committee for Scientific Research KBN. RG was also supported by Foundation for Polish Science. REFERENCES

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