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Nuclear structure studies using radioactive beams
NUCLEAR PHYSICS A EI~qEVIER
Nuclear Physics A583 (1995) 725-732
Nuclear structure studies using radioactive beams B.M. Sherrill ~ aDepartment of Physics and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA A review is made of the world wide program to study the structure of radioactive isotopes using radioactive nuclear beams. The discussion includes examples of elastic and inelastic scattering, inverse kinematic, charge exchange and nuclear breakup reactions. Experiments which use radioactive nuclear beams to study structure beyond the driplines are also presented. Finally, future directions at the NSCL are presented along with a possible upgrade to the present radioactive beam facility. 1. I N T R O D U C T I O N The world wide program to study the nuclear structure of radioactive isotopes using radioactive nuclear beams has developed into a diverse field. Radioactive nuclear beams offer the possibility to study nuclei at the extreme limits of particle stability and to study nuclei with special sylmnetries, such as doubly magic nuclei. These nuclei exhibit unique structure and provide an expanded testing ground for nuclear models. This expanded view of nuclear structure, even in something as basic as the measurement of systematics of 2+ excitation energies and B(E2) values [1], allow the exploration of the limits of ore" understanding of nuclear structure. In the extreme cases, such as for halo and skin nuclei, the nuclear structm'e is qualitatively different and new models (3-body in the case of halo nuclei) are necessary. Radioactive beam experiments have also had a large impact on nuclear astrophysics, as radioactive nuclei are produced and hence most cases drive astrophysical events. The ability to study in the laboratory the same reactions of radioactive nuclei which occur in the heavens is essential to the eventual understanding of our universe. The great promise of the radioactive beam field is illustrated in recent experiments and the radioactive beam programs in place and planned at various laboratories. Other contributions to this conference discuss in detail Coulomb excitation, nuclear astrophysics, and the nature and reactions of halo nuclei. This paper will concentrate on the general aspects of the study of nuclear structure. A full review under this limited scope is still not possible, however the interested reader can find a more complete survey in the Proceedings of the Third International Conference on Radioactive Nuclear Beams [2]. There are several facilities world wide which now routinely perform experiments with radioactive nuclear bemns. Many more are in the construction, planning, or conception stage. The two general approaches to producing radioactive nuclear beams are production and reacceleration, as is used at the ARENAS facility at Louvain-la-Nueve [3], and 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved.
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B.M. Sherrill /Nuclear Physics A583 (1995) 725-732
production and in-flight separation used at, for example, the NSCL, GSI [4], GANIL [5], RIKEN [6], and Notre Dame [7]. 2. T H E R A D I O A C T I V E B E A M P R O G R A M
AT T H E N S C L
Since the National Superconducting Cyclotron Laboratory (NSCL) Phase II project has been in operation approximately 50% of the beam time has been devoted to radioactive beam experiments. The studies have ranged from experiments to determine the structure of halo nuclei to reaction lnechanism studies which have used the unusual ratio of neutrons in the radioactive projectiles to look at the degree to which target and projectile equilibrate [8]. The heart of the radioactive beam program is the A1200 fragment separator [9] which is used to filter the radioactive nuclei produced in projectile fragmentation like reactions (the exact nature of the production mechanism is not fully characterized). A schematic drawing of the A1200 is included in figure 4. A full review of the operation of fragment separators and the projectile fragmentation mechanism can be found in various reviews [10,11]. A significant advantage of the system at the NSCL is that the A1200 fragment separator is located at the start of the beamlines; and since operation of the A1200 facility began in late 1990, experiments have been done in all the experimental areas. Typical secondary beam rates range from a few ions per day to ahnost 107 ions/second. Table 1 gives a summary of some of the radioactive beam experiments performed at the NSCL, the beams involved, and the rates. Most of these will not be discussed in detail in this paper. The first item in the table is not a radioactive bemn experiment, but illustrates other uses for the facility. In these studies up to 21 new isotopes have been identified [12-14] and half-lives for some of them have been measured [15]. Table 1 Brief smmnary of some of the radioactive beam experiments done at the NSCL over the past two years. Experiment Beam Rate [ions/s] Reference New Isotopes 7SKr,92Mo,l°6Cd :~ 0.0001 [12,13] Inelastic Scattering nLi,14Be,11Be 100-104 [16,17] Parallel Momentum 11Li,14BenBe 100-104 [18,19] Coulomb Excitation llLi 103 [20]
(13O,120) Mirror Charge Exchange Reaction Mech. Studies :~Rare isotope primary beams
130
103
13N
105 2 X 10~
4°Sc,4°C1
[21] [22] [8]
Another use for the A1200 facility is been as a zero degree spectrometer. This mode has been used to make a precise measurement of the 11Li mass via the measurement of the Q-value of the reaction 14C(11B,11Li)140 [23]. The binding energy of 11Li is a key ingredient for attempts to calculate its structure. This mode has also been used to study the decays of GT resonances excited in the (6Li,6He) reaction [24].
B.M. Sherrill / Nuclear Physics A583 (1995) 725-732
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3. S E L E C T E D E X A M P L E S OF R A D I O A C T I V E B E A M E X P E R I M E N T S 3.1. Elastic scattering of radioactive ion b e a m s Considerable work has been done on elastic scattering of radioactive ions. For example, measurements have been made at ARENAS for 13N elastic scattering on 12C and 13C targets[25]; this experiment will be discussed briefly latter. Much of the effort, however, has centered on the reactions of halo nuclei on light and heavy targets. For scattering on heavier targets, the naive expectation might be that the very weak binding of this type of nuclei may lead to elastic scattering which is dominated by absorption and hence the angular distributions would be characteristic of scattering from a black disk. Measurement have been made at the NSCL for a variety of radioactive ions scattering on carbon by Kolata and coworkers [16]. Similar results have been obtained at GANIL for ~lLi on silicon[27] and more recently for 8B elastic scattering [28]. The results do not show behavior anything like the simplest expectations and have been interpreted by Mermaz [29] and Hussein and Satchler et a1.[26] as instead indicating a strong far side dominance in the angular distributions. This dominance even extends to very forward angles, unlike elastic scattering of more typical nuclei. A drawback of the present experhnents is that the energy resolution is not sufficient to resolve elastic and inelastic scattering and hence attempts to interpret the data nmst include inelastic excitations. Recent work using the high resolution SPEG spectrometer at GANIL, and described in these proceedings (see the contribution of P. Roussel-Chomaz), have sufficient energy resolution to overcome this difficulty. Inverse kinematic scattering of halo nuclei on proton targets has also been performed. Moon et al.[30] and Korsheninnikov et al. [31] have studied scattering of light neutron rich nuclei at RIKEN. Studies have also been performed at higher energy at GSI for 6He and SHe beams using a high pressure hydrogen ion-chamber, IKAR [32]. The results of this experiment are described is detail elsewhere in these proceedings. Glauber analysis of these data indicate a clear need to the inclusion of an extended neutron distribution. 3.2. Inverse kinematic reactions As mentioned in the previous section elastic scattering studies of radioactive nuclear have been performed in inverse kinematic reactions. Other reactions including inelastic excitation, nucleon transfer, resonant scattering, and nucleon capture have been performed. A few of the experiments will be mentioned here. Other contributions to this conference discuss applications to astrophysics, so for the most part they will not be included here. One of the possibilities opened by the use of radioactive beams is the ability to study doubly magic nuclei away fi'om stability, e.g. 56Ni and l°°Aa2Sn. The special symmetry of these nuclei provide a standard testing ground for nuclear physics. An example of this is the recent measurement of the B(E2) value for the 0 + to 2 + in 56Ni by Kraus et al. [33] at GSI. They used inelastic proton scattering, in inverse kinematics, at a 56Ni beam at an energy of 101 MeV/nucleon. The measured value of 600(120) e2fm4 agrees with lf2p shell model calculations when single particle-hole excitations are allowed in the t7/2 orbit. The reactions 8Li(d,n)9Be and 8Li(d,t)7Li have been studied at Notre Dame University using a 14 MeV SLi of around 106 ions/s. The beam was produced from a d(7Li,SLi)p reaction and separated with the Notre Dame-Michigan-Ohio State radioactive beam facility[7]. The cross sections for these reactions are important to primordial nucleosynthesis
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B.M. Sherrill / Nuclear Physics A583 (1995) 725-732
in inhomogeneous big-bang models as the abundance of heavier nuclei scale with the SLi abundance. The experiments were able to measure excitation functions and angular distributions and provide input data on the destruction of 8Li. Inverse kinematic reaction have also been used at ARENAS to study resonance parameter in resonant proton scattering. Experiments have been performed for 13N [35] and 19Ne [36] resonant scattering. With the use of R and K-matrix analysis they are able to determine total resonance widths, energies, and spins and parities for states in 140 and 2°Na. An interesting and unexplained sidelight of these studies is that the resonance parameters measured in this way do not agree with values obtained with alternative methods, such as (p,n) and (3He,t) reactions.
3.3. C o u l o m b r e a c c e l e r a t i o n effects One of the speculations concerning the structure of halo nuclei, and in particular 11Li is that it should have a soft dipole resonance corresponding to the 9Li core vibrating against the neutron halo. Enhanced E1 strength has been measured by Ieke et al. [20] in a kinenmtically complete experiment. However they also found that the neutrons from the breakup were much slower than the 9Li nuclei, and corresponded to the breakup of the 11Li near the lead target. If the breakup is fast and near the Pb nucleus then the neutrons have the velocity of the 11Li slowed by approaching the Pb target and the final 9Li velocity is higher than the beam velocity since it is lighter. This fast breakup observed in the experiment of Ieki argues against a resonance excitation and decay picture, since the measured width of the resonance implied a lifetime which would mean the decay should take place far from the target nucleus. The strong low-lying E1 strength is then interpreted as a threshold effect [37]. The amount of the Coulomb reacceleration is related, in a classical picture, to the proximity of the 11Li to the target at the time of breakup. If the ions follow Coulomb trajectories then, the closeness, or impact parameter, is related to the observed scattering angle. Thus, the velocity shift of the fragments verses the measured scattering angle can be used as a clock for studying the breakup time scale of the reaction. This has been discussed by various authors, including calculations based on a semiclassical picture by Bauer and Bertulani [38], and one [39] and two dimensional[40] quantum mechanical models by Bertsch and Bertulani. Higher order quantum mechanical effects have also been calculated by Typel and Baur [41]. We have used the $320 magnetic spectrometer to search for this effect in the breakup of HLi on a Uranium target. The results are shown in Figure 1. The figure shows the measured centroid of the 9Li fragment momentum distribution as a function of angle. Also shown in the figure as the dotted line is the expected momentum assuming no reacceleration effects; the error in the location of this line is 1 MeV/c. The dashed line is from the semi-classical prediction of Baur and Bertulani. The data from the 9Be target are consistent with no effect, as expected. However, the data for the Uranium target show an effect, which does not have a strong angular dependence. One interpretation of the data is that the breakup happens beyond the impact parameter corresponding to the scattering angle of 2.5 degrees. A similar experiment for the breakup of 11Be has been performed at RII(EN[42]. The results of this experiments are described in the contribution of Prof. Ishihara.
B.M. Sherrill /Nuclear Physics A583 (1995) 725-732
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Figure 1. Centroid of the 9Li momentmn distribution as a function of spectrometer angle for ::Li breakup on 9Be and 238U targets. The Uranium target shows a Coulomb reacceleration effect, but does not have the angular dependence expected from the semiclassical calculation indicated by the dashed line. The dotted lines give the expected momentum with no reacceleration effect and assume a 1 MeV Q-value for the breakup.
3.4. Beyond the driplines One of the interesting possibilities opened with the availability of radioactive beams is the expanded possibility to produce and study nuclei beyond the driplines. The general idea is to use a radioactive beam on or near the dripline and perform nucleon stripping for pickup reactions to produce nuclei beyond the dripline. This capability has been demonstrated in a series of experiments. Korsheninnikov et al. has used this technique to produce and observe states in 1°He from a ::Li bean: [43]. Thoennessen and Kryger et al. have also used this technique to study unbound resonance states produced in 130 induced reactions [44]. The main goal of their experiment was to look for the 2-proton decay of the 120 ground state. The decay was observed and appears to be consistent with uncorrelated 2-proton emission.
3.5. Charge exchange reactions Experiments have also begun in charge exchange reactions of radioactive nuclei. At the NSCL the A1200 fragn:ent separator has been used in the energy loss mode to measure the mirror charge exchange reaction ~3C(13N,~3C)~3N at 57 MeV/nucleon [22]. As mentioned previously, the same reaction has been studied at lower energy at the ARENAS facility [25]. The intensity of the radioactive :3N beam was 5xl05 ions/second at the NSCL and near 109/second at ARENAS. In the higher energy experiments it was necessary to limit angular extent of the beam to 10 mr which reduced the overall intensity. The goal of these experiments was to use the special symmetry in the reaction of mirror nuclei to extract information on the mechanism of heavy ion charge exchange.
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B.M. Sherrill / Nuclear Physics A583 (1995) 725-732
At the NSCL the goal was to extract information on the mechanism of heavy, ion charge exchange and extract information of the Gamow-Teller (GT) and Fermi (F) transition strength between states at high energy. A simple picture of these nuclei has a Pl/2 ground state and a P3/2 hole excited state, with the 3 / 2 - state at about 3.5 MeV. The expectation from 13C(p,n)13N is that the 3 / 2 - state should be strongly excited in both nuclei as the B(GT) for the 1/2- to 1/2- is 0.20 while the 3 / 2 - to 3 / 2 - is 0.82 [45]. The 1/2to 1/2- transition can also go by the Fermi transition, which for mirror nuclei has a value of B(F) = 1. This dominates the g.s. to g.s. transition. The measured spectrum is shown in Figure 2 in the upper part. The preliminary results are that the relative populations of the g.s.-g.s., g.s.-1/2-, and 3 / 2 - - 3 / 2 - states are 19(3)%, 18(3)%, and 63(6)% respectively. The ratio of the ground to excited states should provide information on the relative strengths of the GT and F interactions in heavy-ion charge exchange. The lower part shows the simultaneously measured 13N6+ peak. The latter provides a measure of the intrinsic resolution and a very accurate normalization of the incident 13N beam. It also indicates the expected position of the ground state peak. 60
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Figure 2. Preliminary data from the mirror charge exchange reaction 13C(13N,13C)13N. The upper part of the figure shows the measured energy spectrum, and the lower part shows the simultaneously measured 13N6+ peak. The latter provides a measure of the intrinsic resolution and a very accurate normalization of the incident 13N beam.
4. F u t u r e D e v e l o p m e n t s in t h e N S C L R a d i o a c t i v e B e a m P r o g r a m The NSCL is developing a plan to couple the Kh00-K1200 cyclotrons and significantly increase the intensity of its primary beanas. The coupling would also proved a large increase in the energy of the heaviest beanas. For example it should be possible to accelerate Uranium ions to near 100 MeV/nucleon. This would be of interest to heavy-ion reaction mechanism studies. The increase in intensity for the lighter beams would be a great im-
B.M. Sherrill / Nuclear Physics A583 (1995) 725-732
731 c
provement for radioactive beams. It would provide a factor of up to 1000 increase in the secondary beam rates over what we now have. For example, the current 11Li rate is about 5000/second, while with the coupled project this would increase to 5 x 106 ions/second. At this rate is would be possible to perform single and double nucleon transfer experiments, charge exchange and various unique reaction mechanism studies such as the search for subthreshold pion production and study the effect of the low Fermi momentum of the halo neutrons [46]. Besides the coupling of the cyclotrons we also hope to upgrade the A1200 fragment separator. The current A1200 separator has a limited momentum and angular acceptance, especially for lighter beams and lower production energies. Also to match the higher energies we would like to increase the maximum rigidity of the separator. A final consideration is to provide sufficient space for shielding of the high radiation present at the secondary beam production target and primary beam dump. The A1900 will have a 10 msr solid angle, and a 6 % nmmentuna acceptance compared to the 1 msr and 3% of the current A1200. The net result of the improvements will be a significant advance in the number of nuclei which can be studied. This includes several more cases of halo nuclei. Further, there is a chance to be able to produce and study nuclei near 6 new doubly magic nuclei, e.g. 78Ni, l°°Sn, and 56Ni. Another exciting possibility is that it may be possible to reach the neutron dripline up to Z=17 whereas the current limit is around Z=9. This could greatly expand our possibilities to study halo nuclei. Acknolwdgements The work described here represents the efforts of many people, whom I have tried to adequately reference. I would like to in particular acknowledge the work of the whole A1200 group in the studies at the NSCL. I would like to thank M. Lewitowicz, H. Geissel, S. Neumaier, W. Henning, J. Vervier, and E. Lienard for providing resent results.
REFERENCES 1. R.F. Casten, N.V. Zampfir and D.S. Brenner, Proc. Third Int. Conf. on Radioactive Nuclear Beams, Ed. D.J. Morrissey, Editions Frontieres, (Gif-sur-Yvette, 1993) p. 229. 2. Proc. Third Int. Conf. on Radioactive Nuclear Beams, Ed. D.J. Morrissey, Editions Frontieres, (Gif-sur-Yvette, 1993). 3. M. Loiselet et al., Proc. Third Int. Conf. on Radioactive Nuclear Beams, Ed. D.J. Morrissey, Editions Frontieres, (Gif-sur-Yvette, 1993) p. 179. 4. H. Geissel et al., Nucl. Instr. Meth. B70 (1992) 286. 5. R. Anne and A.C. Mueller, Nucl. Instr. Meth. B70 (1992) 276. 6. T. Kubo et al., Nucl. Instr. Meth. B70 (1992) 309. 7. J.J. Kolata, A. Morsad, X.J. Kong, R.E. Warner, F.D. Becchetti, W.Z. Liu, D.A. Roberts, and J.W. Janecke, Nucl. Instrm. Methods Phys. Res. B40/41 (1989) 503. 8. S.J. Yennello et al., Phys. Lett. B321 (1994) 15. 9. B.M. Sherrill et al., Nucl. Instr. Meth. B70 {1992) 289.
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B.M. Sherrill /Nuclear Physics A583 (1995) 725-732
10. B.M. Sherrill, Radioactive Nuclear Beams, ed. Th. Delbar (Adam Hilger, London, 1992) p. i. 11. G. Miinzenberg, Nucl. Instr. Meth. B70 (1992) 265. 12. M.F. Mohar et al., Phys. Rev. Lett. 66 (1991) 1571. 13. S.J. Yennello et al., Phys. Rev. C46 (1992) 2620. 14. R.A. Boyd, submitted for publication in Phy. Rev. C. 15. J.A. Winger et al., Phys. Lett. B299 (1993) 214. 16. J.J. Kolata et al., Phys. Rev. Lett. 69 (1992) 2631. 17. M. Zahar, et al., Phys. Rev. C48 (1993) R1484. 18. N.A. Orr et al., Phys. Rev. Lett. 69 (1992) 2050. 19. J. Kelley et al., Proc. Third Int. Conf. on Radioactive Nuclear Beams, Ed. D.J. Morrissey, Editions Frontieres, (Gif-sur-Yvette, 1993) to be published. 20. K. leki et al., Phys. Rev. Lett. 70 (1993) 730. 21. M. Thoennessen, B. Kryger et al., private communication. 22. M. Steiner et al., Proc. Third Int. Conf. on Radioactive Nuclear Beams, Ed. D.J. Morrissey, Editions Frontieres, (Gif-sur-Yvette, 1993) p. 419. 23. B.M. Young et al., Phys. Rev. Lett. 71 (1993) 4124. 24. S. Gales, private communication. 25. E. Lienard, private communication; and these proceedings. 26. M.S. Hussein and G.R. Satchler, Nucl. Phys. A567 (1994) 165. 27. M. Lewitowicz et al., Nucl. Phys. A562 (1993) 301. 28. M. Lewitowicz private communication. 29. M.C. Mermaz, Phys. Rev. C47 (1993) 2213. 30. C.-B. Moon et al., Phys. Lett. B297 (1992) 39. 31. A.A. korsheninnikov et al., Phys. Lett. B316 (1993) 38. 32. S. Neumaier et al., private communication and these proceedings. 33. G. I(raus et al., submitted to Phys. Rev. 34. M.J. Balbes et al., Phys. Rev. Lett. 71 (1993) 3931. 35. Th. Delbar et al., Nucl. Phys. A542 (1992) 263. 36. R. Coszach et al., submitted to Phys. Rev. 37. H. Sagawa, N. van Giai, N. Takigawa, and M. Ishirara, private communication. 38. B. Baur, C.A. Bertulani, and D.M. Kalassa, Nucl. Phys A550 (1992) 527. 39. G.F. Bertsch and C.A. Bertulani, Nucl. Phys. A556 (1993) 136. 40. C.A. Bertulani and G.F. Bertsch, Phys. Rev. C 49 (1994) 2839. 41. S. Typel and G. Baur, KFA preprint, KFA-IKP(TH)-1993-28. 42. T. Nakamura et al., accepted in Phys. Lett. B. 43. A.A. korsheninnikov et al., Phys. Lett. B326 (1994) 31. 44. B. Kryger et al., private communication. 45. T.N. Tadeucci, et al., Nucl. Phys. A 469 (1987) 125. 46. X. Li, M. Hussein, and W. Bauer, Phys. Lett. B258 (1991) 749.
Nuclear Physics A583 (1995) 725-732
Nuclear structure studies using radioactive beams B.M. Sherrill ~ aDepartment of Physics and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA A review is made of the world wide program to study the structure of radioactive isotopes using radioactive nuclear beams. The discussion includes examples of elastic and inelastic scattering, inverse kinematic, charge exchange and nuclear breakup reactions. Experiments which use radioactive nuclear beams to study structure beyond the driplines are also presented. Finally, future directions at the NSCL are presented along with a possible upgrade to the present radioactive beam facility. 1. I N T R O D U C T I O N The world wide program to study the nuclear structure of radioactive isotopes using radioactive nuclear beams has developed into a diverse field. Radioactive nuclear beams offer the possibility to study nuclei at the extreme limits of particle stability and to study nuclei with special sylmnetries, such as doubly magic nuclei. These nuclei exhibit unique structure and provide an expanded testing ground for nuclear models. This expanded view of nuclear structure, even in something as basic as the measurement of systematics of 2+ excitation energies and B(E2) values [1], allow the exploration of the limits of ore" understanding of nuclear structure. In the extreme cases, such as for halo and skin nuclei, the nuclear structm'e is qualitatively different and new models (3-body in the case of halo nuclei) are necessary. Radioactive beam experiments have also had a large impact on nuclear astrophysics, as radioactive nuclei are produced and hence most cases drive astrophysical events. The ability to study in the laboratory the same reactions of radioactive nuclei which occur in the heavens is essential to the eventual understanding of our universe. The great promise of the radioactive beam field is illustrated in recent experiments and the radioactive beam programs in place and planned at various laboratories. Other contributions to this conference discuss in detail Coulomb excitation, nuclear astrophysics, and the nature and reactions of halo nuclei. This paper will concentrate on the general aspects of the study of nuclear structure. A full review under this limited scope is still not possible, however the interested reader can find a more complete survey in the Proceedings of the Third International Conference on Radioactive Nuclear Beams [2]. There are several facilities world wide which now routinely perform experiments with radioactive nuclear bemns. Many more are in the construction, planning, or conception stage. The two general approaches to producing radioactive nuclear beams are production and reacceleration, as is used at the ARENAS facility at Louvain-la-Nueve [3], and 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved.
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B.M. Sherrill /Nuclear Physics A583 (1995) 725-732
production and in-flight separation used at, for example, the NSCL, GSI [4], GANIL [5], RIKEN [6], and Notre Dame [7]. 2. T H E R A D I O A C T I V E B E A M P R O G R A M
AT T H E N S C L
Since the National Superconducting Cyclotron Laboratory (NSCL) Phase II project has been in operation approximately 50% of the beam time has been devoted to radioactive beam experiments. The studies have ranged from experiments to determine the structure of halo nuclei to reaction lnechanism studies which have used the unusual ratio of neutrons in the radioactive projectiles to look at the degree to which target and projectile equilibrate [8]. The heart of the radioactive beam program is the A1200 fragment separator [9] which is used to filter the radioactive nuclei produced in projectile fragmentation like reactions (the exact nature of the production mechanism is not fully characterized). A schematic drawing of the A1200 is included in figure 4. A full review of the operation of fragment separators and the projectile fragmentation mechanism can be found in various reviews [10,11]. A significant advantage of the system at the NSCL is that the A1200 fragment separator is located at the start of the beamlines; and since operation of the A1200 facility began in late 1990, experiments have been done in all the experimental areas. Typical secondary beam rates range from a few ions per day to ahnost 107 ions/second. Table 1 gives a summary of some of the radioactive beam experiments performed at the NSCL, the beams involved, and the rates. Most of these will not be discussed in detail in this paper. The first item in the table is not a radioactive bemn experiment, but illustrates other uses for the facility. In these studies up to 21 new isotopes have been identified [12-14] and half-lives for some of them have been measured [15]. Table 1 Brief smmnary of some of the radioactive beam experiments done at the NSCL over the past two years. Experiment Beam Rate [ions/s] Reference New Isotopes 7SKr,92Mo,l°6Cd :~ 0.0001 [12,13] Inelastic Scattering nLi,14Be,11Be 100-104 [16,17] Parallel Momentum 11Li,14BenBe 100-104 [18,19] Coulomb Excitation llLi 103 [20]
(13O,120) Mirror Charge Exchange Reaction Mech. Studies :~Rare isotope primary beams
130
103
13N
105 2 X 10~
4°Sc,4°C1
[21] [22] [8]
Another use for the A1200 facility is been as a zero degree spectrometer. This mode has been used to make a precise measurement of the 11Li mass via the measurement of the Q-value of the reaction 14C(11B,11Li)140 [23]. The binding energy of 11Li is a key ingredient for attempts to calculate its structure. This mode has also been used to study the decays of GT resonances excited in the (6Li,6He) reaction [24].
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3. S E L E C T E D E X A M P L E S OF R A D I O A C T I V E B E A M E X P E R I M E N T S 3.1. Elastic scattering of radioactive ion b e a m s Considerable work has been done on elastic scattering of radioactive ions. For example, measurements have been made at ARENAS for 13N elastic scattering on 12C and 13C targets[25]; this experiment will be discussed briefly latter. Much of the effort, however, has centered on the reactions of halo nuclei on light and heavy targets. For scattering on heavier targets, the naive expectation might be that the very weak binding of this type of nuclei may lead to elastic scattering which is dominated by absorption and hence the angular distributions would be characteristic of scattering from a black disk. Measurement have been made at the NSCL for a variety of radioactive ions scattering on carbon by Kolata and coworkers [16]. Similar results have been obtained at GANIL for ~lLi on silicon[27] and more recently for 8B elastic scattering [28]. The results do not show behavior anything like the simplest expectations and have been interpreted by Mermaz [29] and Hussein and Satchler et a1.[26] as instead indicating a strong far side dominance in the angular distributions. This dominance even extends to very forward angles, unlike elastic scattering of more typical nuclei. A drawback of the present experhnents is that the energy resolution is not sufficient to resolve elastic and inelastic scattering and hence attempts to interpret the data nmst include inelastic excitations. Recent work using the high resolution SPEG spectrometer at GANIL, and described in these proceedings (see the contribution of P. Roussel-Chomaz), have sufficient energy resolution to overcome this difficulty. Inverse kinematic scattering of halo nuclei on proton targets has also been performed. Moon et al.[30] and Korsheninnikov et al. [31] have studied scattering of light neutron rich nuclei at RIKEN. Studies have also been performed at higher energy at GSI for 6He and SHe beams using a high pressure hydrogen ion-chamber, IKAR [32]. The results of this experiment are described is detail elsewhere in these proceedings. Glauber analysis of these data indicate a clear need to the inclusion of an extended neutron distribution. 3.2. Inverse kinematic reactions As mentioned in the previous section elastic scattering studies of radioactive nuclear have been performed in inverse kinematic reactions. Other reactions including inelastic excitation, nucleon transfer, resonant scattering, and nucleon capture have been performed. A few of the experiments will be mentioned here. Other contributions to this conference discuss applications to astrophysics, so for the most part they will not be included here. One of the possibilities opened by the use of radioactive beams is the ability to study doubly magic nuclei away fi'om stability, e.g. 56Ni and l°°Aa2Sn. The special symmetry of these nuclei provide a standard testing ground for nuclear physics. An example of this is the recent measurement of the B(E2) value for the 0 + to 2 + in 56Ni by Kraus et al. [33] at GSI. They used inelastic proton scattering, in inverse kinematics, at a 56Ni beam at an energy of 101 MeV/nucleon. The measured value of 600(120) e2fm4 agrees with lf2p shell model calculations when single particle-hole excitations are allowed in the t7/2 orbit. The reactions 8Li(d,n)9Be and 8Li(d,t)7Li have been studied at Notre Dame University using a 14 MeV SLi of around 106 ions/s. The beam was produced from a d(7Li,SLi)p reaction and separated with the Notre Dame-Michigan-Ohio State radioactive beam facility[7]. The cross sections for these reactions are important to primordial nucleosynthesis
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in inhomogeneous big-bang models as the abundance of heavier nuclei scale with the SLi abundance. The experiments were able to measure excitation functions and angular distributions and provide input data on the destruction of 8Li. Inverse kinematic reaction have also been used at ARENAS to study resonance parameter in resonant proton scattering. Experiments have been performed for 13N [35] and 19Ne [36] resonant scattering. With the use of R and K-matrix analysis they are able to determine total resonance widths, energies, and spins and parities for states in 140 and 2°Na. An interesting and unexplained sidelight of these studies is that the resonance parameters measured in this way do not agree with values obtained with alternative methods, such as (p,n) and (3He,t) reactions.
3.3. C o u l o m b r e a c c e l e r a t i o n effects One of the speculations concerning the structure of halo nuclei, and in particular 11Li is that it should have a soft dipole resonance corresponding to the 9Li core vibrating against the neutron halo. Enhanced E1 strength has been measured by Ieke et al. [20] in a kinenmtically complete experiment. However they also found that the neutrons from the breakup were much slower than the 9Li nuclei, and corresponded to the breakup of the 11Li near the lead target. If the breakup is fast and near the Pb nucleus then the neutrons have the velocity of the 11Li slowed by approaching the Pb target and the final 9Li velocity is higher than the beam velocity since it is lighter. This fast breakup observed in the experiment of Ieki argues against a resonance excitation and decay picture, since the measured width of the resonance implied a lifetime which would mean the decay should take place far from the target nucleus. The strong low-lying E1 strength is then interpreted as a threshold effect [37]. The amount of the Coulomb reacceleration is related, in a classical picture, to the proximity of the 11Li to the target at the time of breakup. If the ions follow Coulomb trajectories then, the closeness, or impact parameter, is related to the observed scattering angle. Thus, the velocity shift of the fragments verses the measured scattering angle can be used as a clock for studying the breakup time scale of the reaction. This has been discussed by various authors, including calculations based on a semiclassical picture by Bauer and Bertulani [38], and one [39] and two dimensional[40] quantum mechanical models by Bertsch and Bertulani. Higher order quantum mechanical effects have also been calculated by Typel and Baur [41]. We have used the $320 magnetic spectrometer to search for this effect in the breakup of HLi on a Uranium target. The results are shown in Figure 1. The figure shows the measured centroid of the 9Li fragment momentum distribution as a function of angle. Also shown in the figure as the dotted line is the expected momentum assuming no reacceleration effects; the error in the location of this line is 1 MeV/c. The dashed line is from the semi-classical prediction of Baur and Bertulani. The data from the 9Be target are consistent with no effect, as expected. However, the data for the Uranium target show an effect, which does not have a strong angular dependence. One interpretation of the data is that the breakup happens beyond the impact parameter corresponding to the scattering angle of 2.5 degrees. A similar experiment for the breakup of 11Be has been performed at RII(EN[42]. The results of this experiments are described in the contribution of Prof. Ishihara.
B.M. Sherrill /Nuclear Physics A583 (1995) 725-732
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3.4. Beyond the driplines One of the interesting possibilities opened with the availability of radioactive beams is the expanded possibility to produce and study nuclei beyond the driplines. The general idea is to use a radioactive beam on or near the dripline and perform nucleon stripping for pickup reactions to produce nuclei beyond the dripline. This capability has been demonstrated in a series of experiments. Korsheninnikov et al. has used this technique to produce and observe states in 1°He from a ::Li bean: [43]. Thoennessen and Kryger et al. have also used this technique to study unbound resonance states produced in 130 induced reactions [44]. The main goal of their experiment was to look for the 2-proton decay of the 120 ground state. The decay was observed and appears to be consistent with uncorrelated 2-proton emission.
3.5. Charge exchange reactions Experiments have also begun in charge exchange reactions of radioactive nuclei. At the NSCL the A1200 fragn:ent separator has been used in the energy loss mode to measure the mirror charge exchange reaction ~3C(13N,~3C)~3N at 57 MeV/nucleon [22]. As mentioned previously, the same reaction has been studied at lower energy at the ARENAS facility [25]. The intensity of the radioactive :3N beam was 5xl05 ions/second at the NSCL and near 109/second at ARENAS. In the higher energy experiments it was necessary to limit angular extent of the beam to 10 mr which reduced the overall intensity. The goal of these experiments was to use the special symmetry in the reaction of mirror nuclei to extract information on the mechanism of heavy ion charge exchange.
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At the NSCL the goal was to extract information on the mechanism of heavy, ion charge exchange and extract information of the Gamow-Teller (GT) and Fermi (F) transition strength between states at high energy. A simple picture of these nuclei has a Pl/2 ground state and a P3/2 hole excited state, with the 3 / 2 - state at about 3.5 MeV. The expectation from 13C(p,n)13N is that the 3 / 2 - state should be strongly excited in both nuclei as the B(GT) for the 1/2- to 1/2- is 0.20 while the 3 / 2 - to 3 / 2 - is 0.82 [45]. The 1/2to 1/2- transition can also go by the Fermi transition, which for mirror nuclei has a value of B(F) = 1. This dominates the g.s. to g.s. transition. The measured spectrum is shown in Figure 2 in the upper part. The preliminary results are that the relative populations of the g.s.-g.s., g.s.-1/2-, and 3 / 2 - - 3 / 2 - states are 19(3)%, 18(3)%, and 63(6)% respectively. The ratio of the ground to excited states should provide information on the relative strengths of the GT and F interactions in heavy-ion charge exchange. The lower part shows the simultaneously measured 13N6+ peak. The latter provides a measure of the intrinsic resolution and a very accurate normalization of the incident 13N beam. It also indicates the expected position of the ground state peak. 60
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4. F u t u r e D e v e l o p m e n t s in t h e N S C L R a d i o a c t i v e B e a m P r o g r a m The NSCL is developing a plan to couple the Kh00-K1200 cyclotrons and significantly increase the intensity of its primary beanas. The coupling would also proved a large increase in the energy of the heaviest beanas. For example it should be possible to accelerate Uranium ions to near 100 MeV/nucleon. This would be of interest to heavy-ion reaction mechanism studies. The increase in intensity for the lighter beams would be a great im-
B.M. Sherrill / Nuclear Physics A583 (1995) 725-732
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provement for radioactive beams. It would provide a factor of up to 1000 increase in the secondary beam rates over what we now have. For example, the current 11Li rate is about 5000/second, while with the coupled project this would increase to 5 x 106 ions/second. At this rate is would be possible to perform single and double nucleon transfer experiments, charge exchange and various unique reaction mechanism studies such as the search for subthreshold pion production and study the effect of the low Fermi momentum of the halo neutrons [46]. Besides the coupling of the cyclotrons we also hope to upgrade the A1200 fragment separator. The current A1200 separator has a limited momentum and angular acceptance, especially for lighter beams and lower production energies. Also to match the higher energies we would like to increase the maximum rigidity of the separator. A final consideration is to provide sufficient space for shielding of the high radiation present at the secondary beam production target and primary beam dump. The A1900 will have a 10 msr solid angle, and a 6 % nmmentuna acceptance compared to the 1 msr and 3% of the current A1200. The net result of the improvements will be a significant advance in the number of nuclei which can be studied. This includes several more cases of halo nuclei. Further, there is a chance to be able to produce and study nuclei near 6 new doubly magic nuclei, e.g. 78Ni, l°°Sn, and 56Ni. Another exciting possibility is that it may be possible to reach the neutron dripline up to Z=17 whereas the current limit is around Z=9. This could greatly expand our possibilities to study halo nuclei. Acknolwdgements The work described here represents the efforts of many people, whom I have tried to adequately reference. I would like to in particular acknowledge the work of the whole A1200 group in the studies at the NSCL. I would like to thank M. Lewitowicz, H. Geissel, S. Neumaier, W. Henning, J. Vervier, and E. Lienard for providing resent results.
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