sign in sign up
<<
Home , Halo nucleus, Mirror nuclei, Neutron emission, Nuclear matter, Photodisintegration, Proton emission

0»

Nuclei at the limits of particle stability

Alex C. Mueller Bradley M. Sherrill IPNO-DRE 9,3-10 t MSU CL 883 \ Vr - J

Michigan State University National Superconducting Cyclotron Laboratory Nuclei at the limits of particle stability

Alex C. Mueller Bradley M. SherriJl IPNO-DRE 93-10 MSU CL 883

Michigan State University National Superconducting Cyclotron Laboratory NUCLEI AT THE LIMITS OF PARTICLE STABILITY

Alex C. Mueller CNRS-IN2P3, Institut de Physique Nucléaire, F-91406 Orsay, France

and

Bradley M. Sherritt National Superconducting Cyclotron Laboratoiy, Michigan State University, East Lansing, MI48824, USA

KEY WORDS: nuclear ^-stability, production of unstable nuclei, on-line separators and recoil spectrometers, reactions induced by unstable nuclei, properties of nuclei at the drip-lines, -halo, nuclei far off stability in astrophysics

Shortened Title: Exotic Nuclei

(to appear in Annual Review of Nuclear and Particle Science, Volume 43) CONTENTS

!.INTRODUCTION 3 2. THE LIMITS OF STABILITY 4 3. TECHNIQUES OF PRODUCTION 7 3.1 Overview 7 3.2 ISOL techniques 7 3.3 Recoil separators 9 4. -RICH NUCLEI 13 4.1 Searches for the location of the proton drip-line 13 4.2 Properties of proton rich nuclei 16 4.3 Beyond the proton drip-line 20 4.4 Astrophysics: breakout from the CNO-cvcle and the rp-process 22 4.5 Properties and synthesis of the heaviest elements 24 5. NEUTRON-RICH NUCLEI 26 5.1 Mapping of the neutron drip-line 26 5.2 Nuclear masses along the neutron drip-line 28 5.3 Decay studies 30 5.4 Nuclear moments 32 5.5 Nuclear radii, and charge distributions 33 5.6 Reaction studies of the halo nuclei: cross-sections, momentum distributions and neutron correlations 35 5.7 Some comments on the neutron halo. 38 5.8 Neutron-rich nuclei in astrophysics 40 6. FUTURE PROSPECTS 43 7. SUMMARY 48 8. LITERATURE CITED 51 9. FIGURE CAPTIONS 68 NUCLEI AT THE LIMITS OF PARTICLE STABILITY

Alex C. MUELLER and Bradley M. SHERRILL

1. INTRODUCTION

Nuclear physics addresses the behavior and stability of assemblies of nucléons. In nature, single nuclei are not found as clumps of or , but, instead, as a detailed balance between these two types of hadrons. A fundamental question is, of course, the number of stable combinations and the location of the limit between bound and unbound systems. A nucleus is bound when it is stable against spontaneous particle emission. This means that the lifetime of the system is long on the timescale of the which produced it. Indeed, whereas 10*22 s is a typical time fora direct nuclear reaction, the fastest radioactive decays via weak interactions are 10*3 s Jn me laboratory, nuclei at the limits of stability are difficult to synthesize because of their low production cross section. Yet, as we shall see, it is for the weakly bound systems, at the "drip-lines", that new and unusual phenomena occur. Historically, it was by nuclear spectroscopy that first experimental information became available about the most neutron- and proton-rich nuclei. During the last years, however, a completely new technique has given rise to a very rapid increase of our knowledge of the extreme cases situated at the drip-lines. Nuclear reactions have been induced and studied by means of the "exotic" nuclear beams, i.e. beams of radioactive drip-line nuclei. Presently, these beams are mostly generated in heavy-ion projectile fragmentation. However, the success of these experiments has to a broad discussion of future radioactive beam facilities. The field of nuclei far-from-stability has been reviewed recently by Detraz and Vieira (1), and by Roeckl (2). In this article we will summarize the work covered in these earlier reviews and present the results of more recent experimental and theoretical work. 2. THE LIMITS OF NUCLEAR STABILITY

The is very strong, but also of very short range. Thus, infinite systems of nuclei are not found in nature. Instead, nuclear stability is determined by the balance of the attractive nuclear force, which depends roughly on the nuclear volume and the repulsive Coulomb force between the protons in the nucleus. The details of the nuclear landscape is highly influenced by the fact that the n-p interaction is slightly stronger than the n-n, or p-p reaction. Excluding consideration of the Coulomb force, nuclei with equal numbers of neutrons and protons are more stable since the number of n- p interactions in maximized. Further, since the n-n system is slightly unbound; infinite, or even very large, collections of neutrons and a few protons are not stable (except perhaps in neutron stars, stabilized by gravity). The extent of stable nuclei found in nature illustrates the characteristics of the nuclear stability. The stable nuclei (found naturally) are represented by dark squares in Figure 1, the chart of . For nuclei greater than A=40 the Coulomb force has shifted the line of stability away from N=Z toward neutron rich nuclei. The Coulomb force also limits the extent of the heaviest elements since the short range of the nuclear force does not provide sufficient binding in large nuclei to overcome the long-range Coulomb force. Thus, heavy nuclei fission into two more stable pieces. The figure also shows the predicted proton and neutron driplines. These are defined to be where for the lightest or heaviest isotope the last nucléon is no longer bound and the nucleus decays on the time scale of the strong interactions (10*22s or faster). It is this limit of stability and the characteristics of the nuclei found there which will be discussed in this article. In the figure, the proton drip-line is taken from the mass model ofJânecke and Masson (3) up to Z= 106; from there to Z=I 10 the model of Mollerand Nix (4) is used. On the neutron rich side the mass model of Tachibana et al is displayed (5). For a compilation of mass models, and a prediction of the driplines for the various models, see the review of Haustein (6). The various models do not agree on the location of the driplines and can vary by 10 or more in their prediction. The predicted 5 extent of proton-rich nuclei is severity limited by the Coulomb force, while the neutron rich side is limited by, qualitatively, the fact that the nuclear well gets filled up with neutrons. As more and more neutrons are added the gain in binding is not sufficient to hold them in. It is interesting, however, that nevertheless doubly magic nuclei like 176Sn and 70Ca are predicted to be stable. The shaded area represents the limit of radioactive nuclei which have been synthesized in the laboratory. Sections 4-1 and 5-1 will discuss the searches for the driplines on the proton and neutron rich sides and the attempts to expand the region of known nuclei. Section 4-6 will discuss the limits to the stability of the heaviest elements, which have very low fission barriers. As discussed in that section, extra stability gained from shell effects can raise the and allow a certain region of nuclei to become stable. It is now thought that this effect is responsible for the stability of the elements observed up to Z= 109 (). Much beyond this all heavier elements might be expected to decay by . Actual prediction of the driplines is difficult since a few hundred keV in the of a nucleus can make the difference between whether a nucleus (3-decays or decays by particle emission. Many of the mass models discussed in Haustein's review have accuracies of only 1 MeV or worse at the driplines. Hence the actual location can vary by 5-10 nucléons. Nevertheless the location of the drip-line is a stringent test of mass models and provides a good way to judge if the model can be used to extrapolate away from stability. It is also possible for a nucleus to be beyond the drip-line, yet still be synthesized and studied. This is the case for nuclei studied via direct reactions, and for nuclei which due to a sufficiently large Coulomb or angular momentum barrier have a lifetime which is significantly longer than 10"22S. Such cases will be discussed in section 4-3. In the figure there are 273 dark squares corresponding the isotopes found in nature. In total about 30OG radioactive nuclei have been synthesized in the laboratory for study. The vast majority of the radioactive nuclei produced lie on the proton-rich side of stability, while the vast majority of nuclei remaining yet to be studied lie on the neutron- 6 rich side of stability. It is interesting that even the predicted r-process path does not reach many of the predicted stable neutron rich isotopes, and perhaps these only exist in some form in neutron stars. Depending on the mass model one chooses, perhaps in total 7000 isotopes may be particle stable. However, some more recent mass predictions base on, for example, the Fermion-Dynamical-Symmetry Model (FDSM) suggest a much smaller number of stable neutron rich isotopes(7). Such a result could have significant implications for example the r-process path. The FDSM model also predicts a possible stable region near Z= 114 and N= 164. 3. TECHNIQUES OF PRODUCTION AND SEPARATION

3.1 Overview

Except for the lightest nuclei, nuclei at the limits of particle stability lie far from the stable nuclei experimenters have to begin their synthesis with. Production cross sections are low and the nuclei produced must be effectively and efficiently separated from the more abundantly produced nuclei closer to stability. Thus, the problem is two-

fold, first to produce the nuclei of interest and then to separate them for study. The various production techniques to be discussed in the following sections are fission, target

, projectile fragmentation, fusion-evaporation, deep inelastic collisions, and nucléon transfer. The separation techniques themselves fall into two basic categories;

isotope separation on-line (ISOL) and in flight separation (so called recoil and fragment separation). The limits of particle stability have been mapped exploiting all of the above mentioned production and separation techniques and it appears all will continue to contribute into the foreseeable future. Various authors have reviewed the production and separation techniques in general (2) and more specifically the ISOL (8), recoil spectrometer(9), and fragmentation techniques( 10,11).

3.2 ISOL Techniques

The technique which has lead up to now to perhaps the largest amount of work on nuclei far-from-stability is Isotope Separation On Line (ISOL) (12). Because of this recently there has been significant interest in using ISOL techniques in radioactive beam facilities. A large number of facilities have used the ISOL techniques for the production of nuclei far from stability. A review of the facilities is given in reference 2. Figure 2 shows the layot t for the latest generation of such devices, the new PS-Booster ISOLDE separator at CERN (13). It uses 1 GeV protons from the CERN PS Booster on various targets to produce a vast array on isotopes via target Spallation, fission, and target g fragmentation reactions. As shown the new ISOLDE facility includes two mass separators, a high resolution separator (HRS) based on ISOLDE-3, and a general purpose separator (GPS). Also included is a large experimental area and radiation handling facilities. The ISOL technique is not limited to light ion beams, as somewhat similar reactions may be induced with light to heavy ions. Alternatively, for many cases favorable production rates can be obtained using near Coulonb barrier heavy ions and fusion evaporation reactions. ISOL systems based at heavy ion accelerators include GSI, Berkeley, Chalk River, Louvain-la-Neuve, and Oak Ridge (14, and see the articles in reference 15). In all cases, the isotopes which are produced are thermalized in the target, diffuse and desorb into an ionsource where they are ionized and extracted at high voltage (for example 60 KV in the ISOLDE case). The ions beams are then separated by mass in a mass spectrometer and delivered to the experimental areas. A more extensive overview of recent developments in the field may be found in the proceedings of the last Electromagnetic Isotope Separator (EMIS) conference( 1 S). Two alternative methods to the standard ion sources are first the use of ionization (16), and second, the Light Ion Guide ISOL, LIGISOL technique (17,18), where the radioactive ions recoil into and are stopped in a gas and extracted before they neutralize. The LASER technique offers great selectivity and possibly high efficiency, while the LIGISOL technique is very fast (ions in isomeric states with 100 microsecond half-lifes have been extracted and studied). Fast lived nuclei can also be studied with the jet technique or fast catcher wheel as is used at Berkeley (19); results of which will be discussed in latter sections. The general ISOL target and ion-source techniques permit separation of thennalized radioactive nuclei from up to 100 g/cm^ thick irradiated target material and continuously convert them into a mono-isotopic ion beam. The techniques combine nuclear reactions with high-temperature chemistry, metallurgy, solid-state diffusion, and ionization phenomena. Three factors that determine the production rates in the target, i.e. reaction cross-sections, primary beam properties, and target thicknesses. The actual rates of isotopes from the separator are determined by the critical factors that control the 9 efficiency with which the radioactive nuclei produced in the target can be transformed into a final ion beam. Generally, the target and ion-source unit, which forms the heart of ISOL facilities and have to be developed specifically for each element or group of elements. Today the development of such systems has reached a stage that allows the production of intense low-energy beams of the radioactive isotopes of many elements in the periodic system. Intensities can be as high as 1011 ions/s. Difficult elements to extract directly can be extracted by in situ synthesis of a more volatile compound(20,21).

3.3 Recoil Separators

An alternative technique for producing and separating radioactive nuclei is to take advantage of the forward focussing present in certain types of peripheral nuclear reactions or in inverse kinematic fusion-evaporation reactions. Recoil devices using this technique can be divided into roughly two groups, those which perform separation at low velocity and use transfer or fusion-evaporation reactions, and higher energy devices which use primarily projectile fragmentation. In projectile fragmentation facilities very high collection efficiencies are possible, coupled with the use of thick production targets and large production cross sections. For example at LBL as much as 1.0 % of the primary ' ^C beam was converted to 11C and separated (22) for biomédical applications. Such techniques have been pioneered using elements at LBL (23), and with the LISE spectrometer at GANIL (24,25). Several new devices are now in operation, including the FRS separator at GSI (26), the RIPS separator at RIKEN (27), and the A1200 at Michigan State University (28). As will be shown in sections 4 and 5 such devices have allow mapping of the driplines for the light nuclei. A full review of these devices is given in references 10 and 11. Unlike the lower energy recoil separators, electric fields, although desirable, are generally not useful at above 50 MeV/ because the attainable fields can not sufficiently bend high rigidity fragments. Many lower energy devices use electric fields to cancel the velocity dispersion of magnetic dipoles leaving only mass dispersion (see 10 the contributions to reference 9). As will be seen in latter sections these devices are also very efficient and provide very good separation of the desired ions. The newest generation of these devices now in operation and under construction have sufficient solid angle and momentum acceptance to collect a very large fraction (up to 80%) of the ions formed in fusion-evaporations reactions, even when a-particles are evaporated. Examples of the modern generation of devices are the Recoil Mass Separator (RMS) under construction at Oak Ridge (29) and the Fragment Mass Analyzer (FMA) at Argonne (30). A general discussion of the mass separator technique has been given by Wollnik(31). These devices still suffer from some contamination since the separation is only sensitive to the mass-to-charge ratio of the ion and not strictly the mass and proton number of the ions. An advantage of the fast-ion magnetic achromatic devices is that the purity of the secondary beams can be improved by the use of profiled energy degraders at an intermediate dispersive point (32). This can, in many cases, eliminate mass-to-charge ambiguities, and provide pure secondary beams for even the heavy elements(26). A marriage of the desirable features of both techniques has been made in the recent upgrade of the LISE separator, LISE3, which uses a high electric field Wien filter after the LISE separator (25). A schematic drawing of LISE3 is shown in Figure 3. The first half of the device is a standard achromatic fragment separator. The Wien filter provides an addition velocity selection, which can remove the remaining contaminants. The very high degree of selectivity allowed by this technique has permitted the decay studies of nuclei along the proton drip-line. It is also possible to use solenoid lenses for very high solid angle collection, but the field strength of current superconducting solenoids probably limits these to fragment energies below 50 MeV/nucleon. Such devices have been constructed by Becchetti et al at the NSCL (33) and another is under construction to increase the acceptance and transmission of the GANIL for secondary beams (34). The advantage of these devices is the very large solid angle for collecting fragments (up to 1 sr). The solenoid technique has been used extensively for radioactive beam studies at Notre Dame 11 University (NDU) (35). Sufficient separation from the beam has been obtained at NDU to allow many studies of elastic scattering and nucléon exchange at low energy. A question to be answered for the study of nuclei at the limits of stability is whether the separation will be sufficient to allow experiments to be performed. A similar device , Soleno, has been used at Orsay to study cluster radioactivity and measure decay properties of nuclei along the proton drip-line (36,37). One of the biggest problems with beams produced by projectile fragmentation is that the energy and angular spread of the beam is large. The emittance of secondary beams is determined by the combination of nuclear reaction kinematics and atomic processes such as multiple angular scattering and energy loss straggling in the target and the dégrader. Secondary beam emittance is unavoidably large, except at very high beam energies. The use of profiled degraders also increases the emittance by a factor of 4 or so for a standard dégrader (32). The beam energy spread is also relatively large, on the order of several percent, but can be compensated in many applications by a flight time measurement of the ions, or by the use of a monokinetic dégrader which leaves all ions with the same energy. A review of dégrader shapes, uses and effects has been published (32).

The rates of ions which can be produced and separated by these techniques depends on the production cross sections, the collection efficiency of the recoil spectrometer, and the interaction losses of the ions in the device. The production cross sections for projectile fragmentation reactions can be estimated from the EPAX code of Sûmmerer et al (38). Various codes are available for the calculation of collection and transmission efficiencies of fragment separators (LISCHEN (39), LISE(D. Bazin, private communication, GANIL), INTENSITY(40), and MOCADI(41) ). These codes use the standard fragmentation model and momentum conservation to predict the fragment momentum and angular distributions. Agreement between predicted and measured rates and distributions is generally good, although near the driplines large discrepancies are possible. 12 An interesting future application may be the use of storage rings coupled with recoil separators. Already the GSI Fragment Separator and Experimental Storage Ring system has been used to store and cool secondary radioactive beams (42). It may be possible to apply this technique to nuclei at the limits of stability. However the low production rates and short half-lifes may present a serious problem to the application of this technique. 13 4. PROTON-RICHNUCLEI

4.1 Searches for the location of the proton drip-line

In general proton-rich nuclei are more accessible than neutron rich nuclei since the proton force limits their extent much more than that of neutron rich nuclei. Further the building blocks which experimenters have to work with are all proton-rich relative to nuclei which are heavier. This is why the fusion-evaporation mechanism has allowed a vast area of proton-rich nuclei from Z = 50 to 109 to be studied, while a more limited range of nuclei on the neutron-rich side ha? been studied. Even the heaviest nuclei known are actually proton-rich relative to the expected more stable isotopes of the heaviest elements, which at present can not be reached because the appropriate neutron rich radioactive beams are not available. Another limitation is that when radioactive nuclei are formed in reactions they are formed at a finite excitation. Since neutrons do not have a Coulomb barrier as protons and other charged particles do, they are more preferentially emitted. Except for the very limits of the range of particle stability this results in products which have yet even fewer neutrons. The extent of proton rich nuclei which exits to the left of the N=Z line is limited to below mass 100. For such nuclei, there are corresponding analog nuclei with the opposite sign of T2, allowing tests of dependent effects.

The location of the proton drip-line has been searched for by four major techniques: multinucleon transfer reactions, projectile fragmentation, target spallation, and fusion-evaporation. The drip-line and the extend of the known nuclei are shown in Figure 1. As previously noted, nuclei have been synthesized near (and beyond) the proton drip-line for many elements. Key topics remaining in the mapping of the proton- drip-line are the search for lO^Sn, the extent of the heaviest elements, the role the drip- line plays in astrophysical processes, the variety of decays possible for very unstable nuclei, and the properties of N=Z nuclei near the drip-line from mass 60 to mass 100. 14

Multinucleon transfer reactions have been used to study nuclei at and beyond the drip-line up to around ^Ca. These reactions have the advantage of being able to measure the atomic mass of the proton rich nucleus. Typical resolutions are on the order of 20-40 keV. They can also measure energies and widths of excited states and unbound

4 8 resonances. Typical reactions which have been used extensively are: ( He1 He), (3He,6He), and (TT+ ,TI-). The nuclei beyond the drip-line which have been studied in

this way are 6Be(43), 7B(44), 8C(45,46), »N(47), 12O(48), 15F(49,50), 16F(Sl), 16Ne(52,53), 39Sc(54,55).With the introduction of anew generation of large solid angle magnetic spectrographs and intense beams these studies could be extended to heavier nuclei. A significant advance would be the availability of radioactive beams. However, since the drip-line lies so far from stability except for the lightest nuclei this technique will

not in the near future be viable for the general mapping of the proton drip-line. A technique which has shown more general application is projectile

fragmentation. This technique have been used at GANIL and the NSCL to search for the drip-line up to mass 80 or so. Figure 4 shows the nuclei observed for the first time in these studies. Nuclei observed at GANIL are marked with a circle, while nuclei found at the NSCL are marked with an X. Also marked in the figure with gray squares, for comparison, are nuclei produced via fusion evaporation reactions at Berkeley and Daresbury. These will be discussed in more detail later in this section. The studies at GANIL used beams of 55 to 85 MeV/nucleon 36Ar ,40Ca and 58Ni (56,57,58,59, and references therein). The experiments at the NSCL used rare isotope beams of 65 and 75 MeV/nucleon 78Kr (60,61) and 75 MeV/nucleon 92Mo (62). It is likely that up to Z=22 the drip-line has been determined. In the mid-mass range only ^Fe, 4$Mn, 42V, and perhaps two light Ni-isotopes are yet unobserved. Above the drip-line may be determined for odd-Z isotopes up to , while one or two even Z isotopes up to Yttrium are yet to be found in future studies. An interesting and important feature of the work at GANIL and the NSCL was the availability of the rare isotope beams produced by the ECR ions sources (63). An other feature of both studies was the observation that the use of Nickel targets significantly 15

enhances the yield of the most proton rich fragment compared to the use of light targets such as which is standard in fragmentation studies. Further in both series of experiments proton rich fragments with atomic numbers greater than the beams were observed. In the "traditional" (i.e. at relativistic energies) fragmentation reaction the target would not play a significant role, and hence the reaction involved in these studies are more complicated. The reaction products are however observed at near beam velocity and near zero degrees as in traditional fragmentation. The reaction mechanism, although, must involve multinucleon transfer, but is not up to now well studied. The target spallation method and ISOL techniques have been used at, e.g. ISOLDE, to produce a wide range of proton rich nuclei. Notable studies near the 3 driplines are ^K from protons on ScC2(64,65), studies of the light Ar isotopes from

protons on Ca(66,67), and the search for light isotopes in the l^Sn region (68,69). In some cases, as in the light Rb isotopes, evidence has been found for having reached the proton drip-line(70). In this study ^Rb was not observed, while based on systematics it should have been detected. This observation is consistent with the more recent fragmentation results (60). A similar search was made for ' ^3Cs (71) where the absence of observed decay events is taken as indication that 1 ^Cs is the lightest isotope. As noted by D'Auria in the ^3Rb and * *3Cs searches, the ISOL method suffers from poor

target release efficiency for isotopes with short half-lifes found near the driplines. Typical half-lifes are in the 10 ms range, while ion source release times are seldom below

the range of a second. By far the vast majority of proton rich nuclei have been produced in fusion-

evaporation reactions(72). The decay properties of many light proton rich nuclei were studied by Cerny et al (73) using (p,xn) and (3He,xn) reactions. For example they have 65 studied a series of Tz=-3/2 nuclei extending now up to Se(74), and the Tz=-5/2

nucleus 35Ca (75). It is likely 65Se is the lightest stable isotope, and 35Ca the lightest particle stable isotope. Heavy ion fusion evaporation studies have also been used widely at other laboratories (e.g. at the GSI on-line mass separator (2,14)). Two difficulties in isotope searches are presence on intense backgrounds of isotopes 16 from other evaporation channels and the low production rates for the channels of interest. The Daresbury recoil mass separator has been used to tag the (p,2n) evaporation channel and in this way have been able to study gamma-decays in proton rich nuclei near the drip- line in the mass 70-80 range (76,77). The actual location of the drip-line for heavier nuclei has been determined in a

similar way by the observation of direct proton radioactivity, first at GSI and later at Munich and Daresbury with fusion-evaporation reactions and similar techniques( see the review in reference 78). This will be discussed in detail in section 4-5. In a sense the limit of particle stability has been exceeded for awide range of proton-rich alpha emitters from Hafnium (Z=72) to Meitnerium (Z= 109); almost all produced in fusion-evaporation studies (79). As shown in Figure 1 these studies have come close to the proton-drip-line. In the near future, it is possible that the proton drip-line could be reached for almost all of

these heavy cases using radioactive beam facilities such as the Oak Ridge Radioactive Beam facility under development (80,81). With the radioactive proton rich beam available at such a facility, calculations indicate that it should be possible to produce nuclei beyond the proton drip-line for most elements up to around (Z=84).

4.2 The properties of proton-rich nuclei

This section will deal with the variety of studies which are carried out on the most proton rich nuclei. As will be discussed, our knowledge extends even beyond the proton drip-line for a wide range of nuclei. This has allowed fundamental issues of to be addressed, including the role of the Coulomb force in nuclei. One of the prime assumption of nuclear physics is that isospin is a good quantum number, and that the Coulomb force can be treated as a perturbation. Hence one might expect that the

Coulomb force does not play a large role. However, at the limits of particle stability on the proton-rich side of nuclei the Coulomb force is the determining feature of the behavior of these nuclei, and that which makes their decay modes very different than their neutron- rich counterparts. One of the most critical features is that fora medium-mass nucleus the 17 addition of a proton adds on the order of 10 MeV in Coulomb energy. For nuclei at the limits of particle stability this allows for a much larger energy window in p-decay. Thus many new decay modes become assessable, which are not available on the neutron rich side of nuclei. For recent reviews of proton-rich nuclei see Roeckl (2) and Detraz Vieira

(1). Some of the possible decay modes for proton-rich nuclei are illustrated by the case of 22si. Because of the 18 MeV for P-decay there are 7 different decay

modes possible for this nuclei. Moreover, a large fraction of the Gamow-Teller (GT)strength is accessible in P-decay. This allows a test of the quenching of the axial-

vector component of the weak interaction in nuclei, and provides a comparison of GT- strength measured by other less direct means such as (p,n) reactions. A further advantage is that most of the decays lead to charged particles which allows for good

detection efficiency and weak branches to be studied. Multi-particle decay modes allow for good tests of nuclear models and a search for nucléon correlations in nuclei.

The decay studies of light nuclei on or near the proton-drip-line are summarized in Table 1. For a review of nuclear decays from Z=50 to Z= 109, see the review by Roeckl and Schardt (79). As noted, and can be observed in the table, the large QEC values for

P-decay in the lighter nuclei allow for a variety of decay modes. The various charged particle decay modes observed so far are P-p (see the review by Hardy and Hagberg

(96)), P-2p (see the review by Moltz and Cemy (97)), and more recently p-3p(19) from the decay of 31 Ar and p-pot from the decay Of4^Cr (58). An illustration of the quality of the decay which can be obtained for the decays of nuclei at the proton drip-line is shown in Figure 5. These studies were possible due to the very good separation of the of 4^Cr ions from other unwanted products by the LISE3 spectrometer (25). The multi-particle decay modes allow for the possibility to study correlations of the nucléons in the nucleus and provide important information on the parent and daughter nuclei. In this regard, P-delayed 2p emission has been discussed by Détraz (98). Isospin mixing can be studied by isospin-forbidden P-delayed (96,99). 18 Laser spectroscopic methods have now reached a level of sensitivity where they

can be applied to nuclei near the proton drip line (100). This was demonstrated by a recent experiment on the neutron-deficient argon isotopes, in particular ^Ar and 33Ar (101). These measurements should give information on the nuclear ground state moments and radii. Besides the decay properties, the atomic masses are another fundamental property to be studied. As mentioned in Section 4-1, the masses of many proton rich nuclei along the drip-line have been determined via nucléon transfer reactions. Other techniques with have been used are the determination of p-endpoints (102), and the measurement of ECXjS-decay ratios (103). New techniques include the measurement of total decay energy

(104), and the use of Penning Trap mass spectrometers (105). Time-of-flight techniques, as discussed in the section on neutron rich nuclei, have not yet been applied to proton-rich nuclei. The masses of the most proton rich nuclei are of interest, for example in testing the Isobaric Mass Multiplet Equation (IMME). It has been shown (see the review in reference 106) that in the absence of multi-body forces the energy difference between isobaric pairs of states can be written as

M(A,T,tz) = a + b x tz + c x tz2 (4-1)

If there are higher order forces, then terms of order I2^ or higher will be present (the coefficient of the I2^ term would be the so called d-coefficient, and so on for higher order terms). The series of mass measurements using transfer reactions showed that except for the mass 9 quartet there is no evidence for nonzero terms above tz2 (106). If the correctness of equation 4-1 is assumed and the coefficients are determined from systematics, knowledge of two masses in a multiplet can be used to determine unknown masses as has been done by for example Borrel et al (SS). Thus by measurement of an IAS energy they are able to infer the mass of the parent. One of the fundamental studies which can be made by study of the decay of exotic nuclei is the extent to which the axial vector current is renormalized in nuclei. The accepted free nucléon value for g\ yields a ratio of Gamow-Teller to Fermi strengths of 19 SA/Sv = 1-26 (107). Based on the study of (p,n) reaction cross sections, which are thought to be proportional to gv it is believed that the GT matrix elements are quenched by a factor of about 0.6 (108). Similar quenching have been determined from p-decay studies ( 109). For some complete decays of nuclei very far from stability in the sd-shell, where in principle good shell model calculations are available, the observed quenching values are not at large as the 0.6. This has been observed for the light Ar isotopes (66,67) and more recently for ^7Ca (110). If the conclusions of these studies are correct, and the axial vector current is not quenched as much as believed, this would have significant implications for the (p,n) and (n,p) reactions and interpretations of their data. Brown (111), however, has pointed out that the shape of the GT distribution is very sensitive to the Hamilitonian assumed in the shell model calculations. Since experimentally only 20-40% of the total GT-strength can be observed in P-decay, shifts in the GT-distribution near the experimental cut off can significantly change the quenching value obtained. A more ideal case to study would be one where most of the

GT-strength can be seen in b-decay. The large Qec values for nuclei at the proton drip- line should provide the best cases for study. Recently the discovery of extended sizes for weakly bound nuclei on the neutron rich side has prompted a search for weak binding effects in proton rich nuclei. Such effects have been observed for some time and have been manifested in shifts in energy levels of isobaric multiplets are known as the Thomas-Ehrman Shift (see e.g. 112). Further, differences in |3-decay rates for analog transitions have been measured to be as large as 50 % for some decays (113,114). This is due in part to the change of the proton wave function for the weakly bound member of the decay. A recent measurement of this effect has been carried out by Riisager et al (115) using the recently completed ISOLDE facility connected to the PS-Booster at CERN (13). In their study of the decay of17Ne to the first excited state of 17F. They find a branching of 1.8% compared to 0.76(13)% for the analog transition (17N(P")17 O), one of the largest asymmetries yet measured.

The difference may be interpreted to be due to the very weakly proton bound nature of the 20 first excited state of 1?F (Sp=IO? keV), and the resulting expanded proton wave

function. New evidence for weak binding effects comes from the Quadrupole moment measurement of ^B by Minamisono et al (116). They use a new P-NMR technique to

measure the quadrupole moment ratio of polarized ^£/83 jOns implanted in BN and Mg crystals. They find that the measured quadrupole moment of 8B is about twice what would be expected for well bound proton orbits indicating an extended proton distribution. A potentially significant impact of the 8B halo is in relation to the solar problem (117). The reaction ^Be(p,y)8B is responsible for most with energy

greater than 2.0 MeV from the sun and thus is the reaction responsible for the neutrinos detected by most experiments. Due to the Coulomb barrier the peak of the

cross sections occurs at around 20 fm from the &B center, and hence is very sensitive to the tail of ^B wave function. Riisager and Jensen (118) have used a two body mode] to analyze recent experiments with information on the 8B proton wave function and find they are not consistent. In particular, total reactions cross section measurements by Tanihata et al. ( 119) indicate the proton wave function is smaller at large distances than is assumed in most proton capture calculations. If this is true, the effect could be large enough explain pan of the factor of two discrepancy in the solar neutrino problem.

Further experimental and theoretical work is warranted.

4.3 Beyond the proton drip-line

The Coulomb and angular momentum barrier in proton-rich nuclei allow even proton unbound nuclei to have finite lifetimes. The effect of ground state proton emission now been observed for a number of cases ranging from ^Com(l20) to J60Re(121). The observation of ground state proton emission allows the drip-line to be mapped unambiguously. The study of these nuclei is also of great interest for nuclear structure. The decay process is in principle simple, compared to for example a-emission 21 which involves a preformation factor, and can be used to obtain spectroscopic information. widths are relatively sensitive to influence of the centrifugal potential relative to the Coulomb potential. This in principle enables the orbital angular momentum of an unpaired proton to be determined, and the information can be used to determine the quantum numbers of the emitting state and perhaps to infer the collective nuclear deformation (121). The proton decay energy can be used to determine the atomic mass which can be compared to various models. Hoffman has made a recent review of the cases studied so far (122). The first observation of direct proton emission came from the 19/2" isomeric state in 53Co at Harwell(120). The first ground state proton emissions 147Tm and 151Lu

10 1 were observed at GSI(122). Shorter lived ground state proton emitters ^I J1n^ 1 ^Cs where then observed at Munich(123). Recently, advances in Si-strip technology have allowed the extension of these studies to (HI,p3n) fusion-evaporation studies (124). This has permitted the observation of a series of new proton enrfters from Z=69 to 75, 146Tm, 150Lu, 156Ta, and 16ORe (124,125). Page et al. have also found that 1OSi is a ground state a-emitter (126), the first time a case of a-decay beyond proton decay (for

J09f) has been observed. This effect is thought to be due to odd-even staggering of the proton caused by the np pairing interaction. Figure 6 shows a schematic drawing of the Daresbury recoil separator used in these studies, top part, and in the lower part the energy spectrum of all decays (a), and (b) decays gated using the features of the Si-strip detectors to allow the 160Re proton decay to be observed.

Direct proton radioactivity requires a special set of circumstances in order for it to be observable, i.e. the nucleus be proton unbound but yet within an energy window where the half-life in not too short. This situation may be found in many nuclei beyond the proton drip-line. Other searches for regions of proton emission have been made. Searches before 1990 have been reviewed by Hoffman (122). More recently, based on predicted masses, several other regions have been searched for ground state proton emission including the region near 65As, 6^Br and 77Y (127-129). The studies attempted to use the fusion evaporation reactions such as 40Ca(32s,p2n)69Br and 22 with various collection and separation schemes. No evidence for proton emission was seen in any of the studies. Recently, in P-decay studies of these nuclei it has been determined by Winger et al (61) that at least ^As and probably ^Br are ^-emitters. After single proton emission the most likely decay mode to be observed is direct two proton emission. As discussed in section 4-3 ^-delayed two proton emission has been observed, although no evidence has been found for ^He emission (98). Brown (130) has done a survey of possible ground-state two proton emitters in light nuclei.

Likely cases he identified were 39Ti, 42Cr> 45FCt 48]^ and 49Ni. The existence of a two proton decay mode is made possible by the odd-even staggering in the single-proton separation energies (Sp), which results in cases were Sp>0 while S2p<0. Détraz et al

(92) have performed an extensive study of the case of ™Ti decay and find no evidence for 2p decay, but instead that it ^-decays with a half-life of 26(8) ms. If cases can be found two proton decay measurements would allow measurement of nuclear correlations.

4.4 Astrophysics: breakout from the CNQ-cvclo and the rp-process

The properties of the most proton rich nuclei are thought to play an important role in the cosmos in the rapid-proton capture process, or rp-process. Explosive burning will take place in a number of sites, primarily though in cataclysmic binary systems (including novae and x-ray bursts), and in Type II (131). An understanding of the nuclear structure of the nuclei involved is critical to understanding this process, and interpreting information obtained with the newest generation of astronomical observatories. The capture reaction process occur at such high temperature and densities that nuclei can be synthesized out to the proton drip-line out to the point where the increasing Coulomb barrier terminates the process, or at the highest temperatures to where the stability of the nuclei themselves limits the process. By the nature of the process essentially all nuclei from stability out to the proton drip-line from mass 20 to mass 100 may play a role. Thus, one would like to know 23 (p>y). (cc,p), (3-decay, and rates for all of these nuclei. Needless to say, a number of the important reaction rates and nuclear properties remain to be measured to fully understand the rp and related processes( 131). The primary reactions of interest are (p,y) capture reactions which depend on the proton and gamma widths of the resonance states. For very unbound nuclei the temperature is high enough that photodisintegration, which depends on the atomic mass of the isotope in question, can limit the process and cause a waiting point. If the temperature is high enough these waiting points can be bridged by (a,p) reactions, which depend on proton and alpha

widths, until the Coulomb barrier for alpha penetration becomes too large and this reaction channel is too weak( 132). In this section three of the methods used to extract rp- process information from studies on nuclei near the drip-line will be presented. from high-temperature hydrogen burning is dominated by the hot CNO cycle (133) and the rapid proton capture process (134). Both processes are linked by the reaction sequence 15o(alpha,y)19Ne(p,v)20Na (134,135) if the temperature is sufficiently high. The temperature at which the breakout from the hot CNO cycle occurs depends on rates of these two reactions. The '^O(a,y)^Ne reaction has been studied via light and heavy-ion transfer reactions (134,136,137). Recently the 19Ne(p,v)2()Na reaction has been studied via the P-decay of 20Mg(85-87). The experiments at GANIL also measured y-rays and provide an interesting and complete study of the decay of a proton drip-line nucleus. Light ion transfer reactions had identified at state in 2OjVIg at 2645 keV, or 446 keV above the proton threshold (85,138,139), but its nature could not be determined. However because it is close to the proton threshold it could dominate the capture process. The recent beta-decay studies indicate that it is not a I+ since the lower limit for the log(ft) value is 6.0 indicating it is not an allowed transition. Brown, Fortune, and Sherr (140) have recently reanalyzed all information on this state and conclude that it is likely a 3* state and make estimates based on this assignment on its proton and y-widths. In very hot and dense rp-process events (T9>1.0 and hydrogen densities of on the order of 10^ g/cm^) if there is sufficient time nuclei up to Z=40 could be synthesized 24 (132). Such events are thought to take place on the surface of neutron stars in binary systems and be responsible for Type II X-ray bursters. Typical durations for these events are in the 10 s range. Above ^Ge the rp-process path meets the predicted proton drip-line and if nuclei like ^As are proton unstable the process will essentially be terminated because further proton capture would have to wait for the (3-decay of the 1 run

64Ge. Mohar et al (60) and Winger et al (61) have determined that 65As is stable to -^ proton decay and hence should not be the termination point. The nucleus ^Br has been observed in fragmentation, but it half-live has not yet been measured. Both the '*- fragmentation studies(60) and spallation studies(70) have failed to identify 73Rb. If 73Rb is proton unbound (as is indicated in the experiments) then the termination point may be 7^Kr (t 1/2= 17s). Further studies of half-livss, decay modes, and masses in this region are needed to fully confirm this hypothesis.

4.5 Properties and synthesis of the heaviest elements

Another limit of nuclear stability is found for the heaviest elements (141,142). In nature isotopes of elements heavier than 2=92 are not found, however nuclei up to

Z= 109 have now been synthesizeJ in the laboratory. In 1966 it wa/ predicted that îhere may be an island of near stable super-heavy elements ( 143), where the fission barrier is at near zero excitation but the nuclei are stabilized by shell effects. These speculations began an intensive search for such nuclei. The stability of these nuclei is a balance between proton-, a-, p*-decay, and spontaneous fission (SF). More recent calculation have put the region of stability more near Z=HO and A= 184 (144). The stability is gained through shell effects, without which the SF half-lifes would be too short to allow the nuclei to be produced. The heaviest element produced so far is element 109 (14S), made by the cold fusion reaction 209Bj(58Nj>n)266]09. The cross section for this reaction was only on the order of 10 pb. Because the beams which can be used in these reactions are all relatively proton rich, the synthesized heavy elements are also proton rich relative to the 25 predicted most stable isotopes of the heavier elements. To date there has been only one isotope of 109, 2 of 108, 3 of 107, 4 of 106, 8 of 105, and 10 of 104 produced( 141,142). Many of these, although not all, have been produced with cold

fusion reactions of 50Ti, ^Cr, and ^Ni on targets of Pb or Bi in cold fusion reactions. Cold fusion refers to the attempt to form the compound system with a minimum of excitation energy which minimizes the fission probability. Typical decay channels are 1 or 2 neutrons, compared to "hot" fusion reactions where the decay channels are more like 4n. A region of shell stabilized nuclei does appear to have found in these studies, although not in as high Z nuclei as originally predicted. The role of fission decay decreases compared to a decay when passing beyond elements 104. Whereas nearly all

isotopes of elements 104 and 105 decay by spontaneous fission, isotopes of elements 106 to 109 decay primarily by alpha emission. This was observed for element 106 at

Dubna (146) and at SHIP (147,148). The amount of shell stabilization has been estimated by Miinzenburg to increase from 5 MeV at element 104 to almost 8 MeV at element 109 (141). Beyond element 110 or so the shell effects are expected to decrease again. Because of the very low production cross sections for these heavy elements, high efficiency detection methods must be used. The most successful method recently has been the combined use of a velocity filter to select the ions of interest and implant them in a Si-detector and then look for correlated alpha or SF decays. This technique has been used at GSI by the SHIP group(141). Recently, groups at GSI(149), RIKEN(ISO), LBL(151), and Dubna(152) have constructed gas filled magnetic separators to continue these studies. The gas filled separator uses the fact that at a few MeV/nucleon the equilibrium charge state is linearly related to the velocity and thus ions kept at their equilibrium charge by passing through a gas will have magnetic rigidity which is proportional to their mass to ratio. These devices can be built with large solid angles and good charge state collection efficiency, and hence good overall efficiency, even approaching 50%. 26 5. NEUTRON-RICH NUCLEI

5.1 Mapping of the neutron drip-line

Mapping of the neutron drip-line remains an experimental challenge. Indeed very large neutron-excesses are possible, owing to the fact that neutron matter itself is almost bound. The major difficulty is that far from stability, a very rough, but useful rule of thumb tells that the production cross-section fall by one order of magnitude for each step further away (regardless of the production mechanism actually used). Thus it is instantly obvious, that mapping of the neutron drip-line had to start in the lightest elements where the drip-line is closest to stability. It is also clear, that, at least for the foreseeable future, nuclei like the doubly-magic 176Sn (Z=SO, N=126), predicted to be particle-stable by mass formulae, are beyond any experimental possibilities. The minute quantities in which the isotopes on the neutron drip-line are made in a nuclear reaction call for efficient, fast and selective experiments. This is because of the short lifetimes of these weakly bound species and the huge background induced by the more stable reaction products. Early successes were mainly achieved by particle identification methods (AE, E, time-of-flight) using detectors to identify the light reaction products from high-energy protons on uranium targets (153). This method is very fast, however, it is not very efficient (because of the small solid angle subtended by the detectors), nor is it selective. Nonetheless, these experiments culminated in the first synthesis of the nucleus 14Be, thus mapping the drip-line up to Z=4. As we have seen in the preceding chapters ISOL has been historically the most successful method for investigating unstable nuclei, because it is in general selective, efficient and (often) sufficiently fast. The programs at ISOL facilities have allowed important experiments, giving deeper insight in the nuclear structure of many neutron-rich nuclei. Unfortunately, however, the majority of the light elements, because of their chemical properties, are very difficult to extract from ISOL sources (see also our chapter on production 27 techniques). Thus, the progress in mapping the neutron drip-line was somewhat halted in the 1970's. The breakthrough in this situation was marked by the advent of high-energy heavy-ion beams, permitting reversal of the roles played by the projectile and the target. Instead of fragmenting heavy targets by protons, the collection of exotic projectile-like fragments from a high-energy heavy-ion beam impinging onto a light target allowed exploitation of the kinematic properties of this reaction very favorably. Symons et al

(154) and Westfall et al (155) showed that a magnetic spectrometer was perfectly matched for this task, producing 15 new neutron-rich isotopes at Berkeley with beams of 40Ar (205 MeV/u) and 48Ca (212 MeV/ii). A following experiment in 1984, by Musser & Stevenson (156), who used a 56Fe beam at 670 MeV/u gave first evidence for the synthesis of the heaviest, Z=5 isotope, 19B, although the mass-resolution was somewhat marginal. At about the same time, heavy-ion beam intensities which were 4 orders of magnitude higher than those at Berkeley's BEVALAC became available at the French coupled-cyclotron facility GANIL. Although this gain was in part was counterbalanced by GANIL's lower energy-range (see section 3), the highly efficient and selective recoil spectrometer LISE (157) was soon exploited to fora series of studies. Beams Of40Ar and 48Ca 44 MeV/u and 55 MeV/u, respsctively, confirmed the earlier results and

proved the existence of 22C, 23N, 29F, 29>30Ne and the particle-unbound character of 21C, 25O and 28F (158,159,160). The state-of-the-art of the experiments by Guillemaud-

Mueller et al (161), using intense 44 MeV/u 4gCa beams, is shown in Figure 7. Four counts 32Ne were now been observed, and the unbound character Of31Ne demonstrated. Comparing these results with the recent compilation by Haustein (16) of mass predictions (see also the discussion by Guillemaud-Mueller et al ( 160,161 ) ), the neutron drip-line seems now experimentally to be mapped up to Z=IO. Regarding the non- observation Of26O, generally predicted bound by the mass formulae, its experimental non-observation seems above statistical uncertainty (see insert in Figure 7.). An interesting speculation about this has been brought forward by Hansen (private communication, 1990) who pointed out that for a very marginally bound 26O one would 28 expect extremely large Coulomb break-up cross sections. This could give rise to a dramatic reduction of the transmitted 26O, most being "destroyed in situ" in the thick (and high Z!) production target at the entry of LISE. Therefore, it might be of interest, to continue the search for 26O (and possibly even the doubly magic 28O) by increasing the statistics Of48Ca fragmentation. One may also note that Jensen & Riisager discussed recently the possibility of "islands" beyond the line of particle stability (162).

5.2 Nuclear masses along the neutron drip-line

The characteristic pattern of presence and absence of nuclei in the identification experiments (compare, e.g., figure 7) underlines the essential role that pairing has to play for nuclear stability at the neutron drip-line: for each element, the heaviest isotope has equal (=2n). With the exception of the hydrogen and isotopes, all cases with 2n-l are unbound, and, for helium and , 2n-3 are still unbound. To go beyond these qualitative statements of the effects of pairing, systematic mass measurements are needed. Given the count rates observed in the experiments which map the neutron-drip line, mass measurements of all the nuclei observed up to now are of course as yet not feasible. There has been considerable progress, however, in the development of methods striving to go as far out as possible. An early example of a direct mass measurement of a neutron-drip line isotope has been on-line mass spectrometry by Thibault et al ( 163) on11 Li. Recently, two new methods have been used at LAMPF and GANIL which both rely on precise time-of-flight measurements in a magnetic spectrometer. The review by Détraz & Vieira (1) discusses quite extensively these experiments made at Los Alamos with TOFI (164), and at GANIL with ALPHA+SPEG (165), where numerous new masses of neutron-rich light nuclei could be determined. In particular, the mass of *] Li and ' 4Be have been remeasured (166), and also for the isotopes the masses are now known up to the drip-line (166,167), the last experimental mass 28Ne (167,168), for comparison, is however still 4 neutrons away from the drip-line. 29 Within regard to direct mass measurements, it is important to underline the outstanding efforts which are made in the developments of very precise methods. Measuring the cyclotron resonance frequency of nuclei stored in a Penning trap (105,169), their revolution frequency in a storage ring (42), or using a cyclotron (170) or a storage ring ( 171) as time-of-flight spectrometer may improve the precision of 0.1- IMeV in the experiments quoted above, even for species which are weakly produced although the transmission of the system is of course a major concern. For heavier radioactive nuclei, the Penning trap used at ISOLDE 2 in CERN reached resolving powers of up to m/Am=1.5xl06 and many masses far from stability have already been obtained (172,173). The device is presently re-installed at CERN's new PS-Booster ISOLDE (13), and one may soon hope for a wealth of data including also some light nuclei. For very short-lived species one has to be aware, however, that a measurement of a frequency (typically 5 MHz for mass 20) is trivially limited in precision by the duration of the measurement and hence by the nuclear life-times. The short life-times (see also our next chapter) also present a severe limitation to storage ring experiments using cooling since the time for this process exceeds seconds (42).

Concerning indirect methods for nuclear masses, one may note that the situation is in contrast to the proton drip-line where data from decay spectroscopy have allowed recent progress. The application of the isobaric multiplet mass equation (e.g. 58) is impossible at the neutron drip-line, simply because the corresponding , far beyond the proton drip-line do not exist. The technique of linking decay chains by combining p+-decay energies with alpha and proton decay data has been highly successful at the proton drip-line (103), but it would be very difficult to exploit an

"analogue" at the neutron drip-line: contrary to charged particles, there is so far no neutron detector with the required energy resolution and detection efficiency although some significant progress has been reported by Harkewicz et al (174). Spectroscopy of heavy-ion transfer or pion-induced reactions, are by principle limited to the possible combinations of the available stable isotopes for the beam and the target. At the neutron drip-line these methods are thus restricted to the lightest systems, but they have given 30 many precise data on the (ground-state) masses and also of excited states including

neutron-unbound systems. Oglobin and Penionzhkevich have thoroughly reviewed all measurements up to the late 80's in their review on light exotic nuclei (175). It is interesting to note that the potential of this powerful tool was not yet fully exhausted: Very recently Bohlen et al have used 13C and 14C beams from the VICKSI accelerator at HMI Berlin to examine the unbound systems 7He, 9He, N>Li, 13Be (176-179). Concerning TC induced reactions, 11B targets were used at the Gatchina facility by Aimelin et al in order to study 10Li (180) and at Los Alamos by Kobayashi for 11Li (181), the latter providing a mass value for 11Li, roughly consistent with the direct measurements.

Another interesting new technique for studying nuclei even beyond the limits of particle stability is under development at MSU-NSCL by Thoennessen et al (private communication, 1992) to search for resonances in 10Li. The method involves production of the nuclei of interest by an "arbitrary" reaction such as in the case of Thoenessen's experiments 18O fragmentation to 10Li at 80 MeV/u. The momentaneous formation of 10Li nuclei is detected as a coincidence between a forward going neutron and a 9Li fragment and the 10Li excitation energy is reconstructed from the velocity difference of the n-9Li pair. This technique has been used previously by Heilbronn et al to determine the energies and width of neutron-unbound levels 8Be and 9Li (182).

5.3 Decay Studies

The large 0-decay energy for a nucleus far off stability often to the population of particle-unbound states in its daughter. Hansen & Jonson have recently reviewed this process of ^-delayed particle emission (183), a further review by Jonson & Nyman is in press (184), we will concentrate here only on the newest experiments. The emission of delayed neutrons from the light nuclei situated at the neutron drip-line, a genera] feature, turned out to be a quite helpful for experiments, rather than a complication. Indeed, the simultaneous observation of a p-ray and a neutron allows a 31 coincidence measurement which strongly reduces the background. The problem of the coincidence efficiency, important because of the low count rates, was addressed by Bazin et al (185), who developed an apparatus with a geometry close to 4n, based on NE-213 liquid scintillators for the with a coincidence efficiency of 20%. This detector was used by Mueller et al ( 186,187) and Lewitowicz et al ( 188) for a first measurement of the half-life of several light isotopes close to or at the drip-line, in particular 20C, 2I-22N and 23>24O. The nuclei were collected in an implantation detector behind the recoil spectrometer LISE, allowing a "tagging" of the projectile-like fragments and therefore to correlate the time of implantation of a specific nucleus to the observation of its decay. By means of this technique, which further reduces the background, one obtains (energy-integrated) ^- emission probabilities, which, although far from level-by-level spectroscopy, give a first idea of the decay modes. In a similar way,

Dufour et al (189) used a 4* detector ball, filled with 500 liters of Gd-doped scintillator fora study of the isotopes 14Be, 17B, 19C. This detector (190) is well suited for measuring neutron multiplicities, a delayed two- branch of 5% and 7% was found for 14Be and 19C, respectively. In the case of 17B, the new mode of ^-delayed emission of four neutrons was observed at a level of 4 %. Reeder et al

(191) also used recoil-tagging behind the TOFI spectrometer ( 164), in connection with a polyethylene-moderated 3He neutron counter, for several light neutron-rich nuclei and report a first measurement for 25F and 28Ne. By means of a similar (paraffin-moderated) "long-counter" Tengblad et al (192) investigated the isotopes 26'29Ne at the ISOLDE facility. In contrast to all the experiments mentioned so far which used energy integrating neutron detectors, Harkewicz et al constructed a time-of-flight spectrometer, based on plastic-scintillators, connected to the A1200 fragment separator at MSU and were able to resolve several neutron unbound-states in 15C, populated in the p-decay of 15B (174). As shown in Figure 8, the device has now also been used to study the p-delayed neutron emission in 11Li and in 14Be (D. J. Morrissey et al., private communication, 1992). A time-of-flight spectrometer with liquid scintillators has been set up by Bounajma et al. at ISOLDE and was first used for the decay of 33Al ( 193). They simultaneously studied the 32 emitted y-rays with a -spectrometer, a technique which, for the most neutron- rich light nuclei, is still somewhat in its infancy. The use of a few hyperpure germanium detectors, now available with very large efficiencies, very closely grouped together (as used in the study of the proton drip-line isotope 20Mg (87) mentioned in section 4.4.) should soon move beyond early experiments at CERN PS (194), ISOLDE (195) and LISE (196-198). The most neutron-rich nuclei may also have weak branching ratios for charged-particle emission (182). Borge et al (199) have recently re-investigated this process at ISOLDE for 6He and 8He using a gas-counter silicon-detector telescope and report a probability of 7.6x10"6 for p-delayed deuterons in 6He and of 8 % for ^-delayed tritons in 8He.

5.4 Nuclear moments

The wealth of information about spins, moments and charge radii of nuclei far from stability which has been obtained by means of optical spectroscopy has been reviewed by Otten (200). Experiments are however very difficult for light nuclei (one of the reasons is the narrowness of the hyperfine structures), yet recent progress has been reported for the most neutron-rich isotopes by Arnold et al (201,202) who polarized 9^] Li beams from ISOLDE by means of laser induced optical pumping. After implantation in a cubic LiNOj crystal, the polarization was analyzed through the observation of the asymmetry of the 0-decay of the implanted nuclei, and an NMR technique was used for a determination of the spin, magnetic and electric quadrupole moment. The results strongly suggest a non-deformed character of these nuclei and that the charge distributions Of9Li and ' 1Li have similar shapes. Asahi et al. report another promising development for the boron isotopes. They observed, for projectile-like fragments, the presence of spin alignment at 0° (203) or polarization (204) at forward angles of a few degrees. They have found a correlation of polarization with fragment momentum, hence the fragment separator can also be used, by selecting a given momentum bin, to define the polarization. At Riken, Asahi et al (205) 33 achieved polarizations of about 10 % for the nuclei 14>15B, and derived their magnetic moment from an NMR measurement of the implanted nuclei where the polarization was observed though the anisotropy of the y-rays in the radioactive decay. One should note, that this technique is in principle applicable to many other cases, and has recently also been used for the neutron-deficient nucleus 43Ti (206).

5.5 Nuclear radii, matter and charge distributions

A completely new technique, pioneered at Berkeley by Tanihata et al in 1985 (207,208), has made possible the investigation of nuclear (interaction) radii, matter and charge distributions for the light neutron-rich nuclei. Tanihata and his collaborators showed, despite the weak intensity of the secondary exotic nuclear beams generated in heavy-ion projectile fragmentation, that it is indeed possible to benefit from their energy to introduce nuclear reactions. The first class of experiments using secondary radioactive beams impinging onto a target have been measurements of the (total) interaction cross section oj. The interaction cross section can be determined in a transmission experiment where the incident flux of a particle is compared to the flux after a target. Even fora large number of incident isotopes, charged particle identification of the incoming and the outcoming nuclei is easy at the energy of projectile fragment and can be made by a measure of the energy loss, magnetic rigidity and time-of-flight measurements. From the oj, it is possible to infer rms radii of the distribution using a Glauber model. The validity of this procedure was verified by using the known rms radii for the stable projectiles ( determined from electron scattering) in order to predict the GI. A remarkable agreement (within 2%) to the measured oj was found (209). It was in these experiments that a very strong and unexpected increase of the rms matter radius of ' 1Li was observed, which was associated by Hansen & Jonson (210) to the occurrence of a spatially extended "neutron halo" in a weakly bound drip-line nucleus. At GANIL, reaction cross sections OR have been measured for numerous light nuclei (211,212) through the detection of the prompt y-rays associated with a nuclear 34 reaction by means of a 4?r y-ball surrounding the interaction target. Also at GANIL,

Villari et al (213) determined (energy-integrated) OR for light nuclei up to , using a silicon detector as an active target and deduced the reduced strong absorption radius TQ. They note a isospin dependent increase (compared to the stable isotopes) of this quantity for neutron-rich nuclei. With the beams of different energies from Berkeley and RIKEN systematic determinations of 01 have been made for many combinations of light projectiles (including stable) and targets (including liquid hydrogen and ). The energy dependence of the elementary N-N cross sections was used to determine the matter density profile of the weakly bound nuclei 11Be (214) and 11Li (215): In the relativistic regime, the 01 are sensitive to the high-density core region, whereas the low density region of the outer part of the nucleus are probed by the intermediate energy beams. Figure 9 shows the energy dependence of 01 for 11Li beams impinging on a target, and the deduced density distribution, which features a long tail, reflecting the presence of the neutron halo. Blank et al have made at SATURNE a measurement of the total and charge- changing cross section for 8A11Li at 80 MeVAt in a transmission type experiment, using plastic scintillatois before and after the carbon interaction target (216,217). The results, see Figure 10, show that the charge-changing cross sections stay constant for 8^*11Li.

They exhaust only 40% of the total cross sections for11Li, whereas this value is 70% for 8Li (and reaches 90% in the (218)). These observations are consistent with a constant rms charge-radius for the lithium isotopes, and also in line with the measurements of spins, magnetic and electric moments Of9^1Li (see chapter 5.4.), and thus confirm the interpretation of the large rms matter increase of11 Li as due to the (last two weakly bound) neutrons They also hint that the presence of the halo has not significantly perturbed the core 9Li distribution.

For stable nuclei, a classical way to explore density distributions has been elastic scattering. First such experiments for radioactive nuclei, addressing in particular the case of 11Li are now reported. et al. (219) have studied, in an inverse kinematics 35 experiment, the elastic scattering of 9^1Li produced at RIKEN with 60 MeV/u. Scattering the radioactive beams on the protons of a polyethylene target, the trajectories before and after the target were determined by means of multi-wire proportional chambers. Also the recoiling proton was detected in coincidence. At MSU, Kolata et al (220) have with a similar set-up investigated the symmetric system 11Li + 12C at 60 MeV/u. At GANIL, Lewitowicz et al used an "active" silicon- detector target for a study of the elastic scattering of 7^ 1Li at 30 MeV/u (221 ). These three experiments have been analyzed by means of optical model calculations. It is found that the parameters of the real and imaginary part of the potential have to be substantially readjusted due to the neutron halo. For example, analyzing the data of the experiment at MSU (220), Mermaz calculates a very long tail in the real potential, up to 12 fm, and a short tail in the imaginary part, making the latter potential surface transparent and giving rise to strong refractive effects (222).

5.6 Reaction studies of the halo nuclei: cross-sections, momentum distributions and neutron correlations

Systematic studies of the strong reaction channels for the halo nuclei were undertaken at Berkeley, GANIL, MSU soon after the large total interaction cross sections had been established. Mostly the nuclei11Be and 11Li were addressed, prime candidates for experiments because of their low binding energies for the last neutron(s) and their comparatively satisfactorily production rates in projectile fragmentation. Hansen & Jonson had predicted that Coulomb stripping of halo neutrons (i.e. 11Li->9Li+2n, 11Be- >10Be+n, etc.) should play a very important role at intermediate energies (210).

Such large break-up cross sections were observed for high-Z targets at GANIL for 8He, 11Li, 11Be, 14Be (223-225). The process is also reported by Kobayashi et al. for 800 MeV/u beams of 8He, 11Li, 14Be made at Berkeley (226), but at the relativistic energies only 15-30% of the total reaction cross section is electrical. Not only Coulomb stripping but also nuclear stripping is strongly enhanced for the halo nuclei: Using a 36 neutron-detector hodoscope at GANIL to investigate the neutrons from the break-up reactions, Anne et al (223) found the neutron yield around zero degrees more than an order of magnitude greater for 11Li than for 9Li impinging with 30 MeV/u onto a beryllium target. Likewise, an experiment at RIKEN showed that the fragmentation cross section nBe+Al->Be+X (=0.65 b) at 33MeWu exceeds that of the corresponding reaction of 13C by a factor of seven (214). From the combined analysis of their interaction and their fragmentation cross sections, measured at Berkeley at 800 MeV/u for the isotopes 4>6>8He Tanihata et al (227) have verified a theoretical relationship deduced by Yabana et al. (228) who derived that the increase in the interaction cross sections with the increasing is essentially explained by the neutron removal cross sections. Tanihata at al interpret this as linked to the occurrence of a thick "neutron skin" in 6^8He (227). Measurements of momentum distributions of the fragments from the break-up reactions also have been studied now at several laboratories. Kobayashi et al found at Berkeley that the transverse momentum distribution of the 9Li recoils from the 11Li break-up reaction on a carbon target is narrow (229), and interpreted this via Heisenberg's principle of uncertainty as being directly linked to the occurrence of a spatially extended neutron halo. Anne et al. used a neutron hodoscope mounted behind LISE to measure inclusive and exclusive neutron angular distributions for the neutrons from the break-up reactions (223-225). For 11Li, the observed distributions are strikingly narrow and also their form is almost independent of the nature of the target, for the nuclei 8He,l1Be,11Li, 14Be, a clear correlation of the width with the binding energy was established. For 11Be on impinging on a beryllium target, a broad distribution was seen, and interpreted as due to diffraction and absorption dissociation (see also section 5.7). Similar measurements are now also reported from Berkeley at 800 MeVAi for 6»8He and 11Li (230), a scaling of (the "narrow" component of) the momentum width of the neutron transverse momentum distribution with the binding energy is also seen. There has been a lot of discussion about a consistent interpretation of the data from the transverse momentum measurements and their relation to the neutron momenta 37 deduced from the angular distributions. It was recently pointed out (231), however, that transverse momentum data are effected, for heavy nuclei, by Coulomb deflection and by multiple scattering (the break-up targets used in radioactive beam experiments are usually quite thick). Not affected by these effects, parallel momentum distributions, on the other hand, are thought to reflect reliably (the Fourier transform of) the wavefunction of the projectile, for both nuclear and Coulomb induced break-up (232). Experimentally, the difficulty of measuring parallel momenta is due to the large momentum spread of a secondary beam produced by projectile fragmentation (up to Ap/p=6% for the largest acceptance spectrometers). Orr et al have used the A1200 fragment separator at MSU- NSCL as a zero degree energy loss spectrometer, operated in a dispersion matched mode, in order to overcome this problem. They measured longitudinal momentum distributions (231), for "Li at 66MeWu impinging on beryllium, ard targets, an example is shown in Figure 11. Very recently, it has been possible to use the same experimental method with the FRS spectrometer of GSI, and data were taken at relativistic energies for several projectiles (H. Geissel, private communication, 1993). The neutron-correlation in the break-up of 11Li has now been investigated in experiments at the different fragment separators. A fir-t result from GANIL at 29 MeWu was based on a few hundred coincidence events, consistent within experimental errors with an uncorrelated distribution (224). leki et al. (233) report a much refined experiment at the same energy at MSU, using two consecutive rows of neutron hodoscopes, in which it was possible to measure simultaneously the velocity vector of all three particles in the exit channel. From the correlation function between the two neutrons, they were able to deduce the rms source size and find consistency of their value of 9.2 fm with other determinations. Figure 12 shows the spectrum of the decay energy of the system, deduced from the velocities. An interesting point is that leki et al observed a substantial Coulomb post-acceleration Of9Li after the break-up. This is a clear indication that the break-up occurs in close proximity to the target nucleus and hence the time between the excitation and the break-up has to be short, which , in other words, could mean that there is very little collectivity involved in this process. Another experiment, with a segmented 38 plastic scintillator neutron detector, has been made recently at RIKEN by Shimoura et al (234). Their data show, for 11Li at 43 MeV/u, a width of the relative n-n energy distribution of FWHM * 0.2 MeV (230). The excitation-energy spectrum of 11Li peaked at about IMeV with a width of about 1.5 MeV. In this context, the pion double charge exchange experiment by Kobayashi et al ( 181), which gave a mass determination of11Li (see 5.2), also observed an excited state near 1.2MeV. Finally, a real full kinematics experiment has been made for 300 MeV/u 11Li at GSI, using the ALADIN magnet for a momentum measurement of the 9Li and the large area neutron detector LAND for the neutron detection. The complete data analysis from this experiment will be a major effort, but single neutron spectra, consistent with the data from LISE at GANIL (223-225), were already reported recently by Schrieder (235).

5.7 Some comments on the neutron halo

It is clearly beyond the scope of this general review on nuclei at the limits of particle stability to present all experimental details and conclusions related to the phenomenon of the neutron halo: not only are we limited in space, but the field is also very rapidly progressing. In the preceding chapters we have therefore tried to concentrate on the, to our belief, important developments which are dominated by the new technique of investigating nuclear far from stability through the reactions induced by them.

Concerning the theoretical understanding of the neutron halo, the progress has been equally impressive. In total one counts over one hundred experimental and theoretical papers which have been written on this subject over the last years. A discussion of the phenomena involved, together with a quite extensive list of references has been given by Hansen at the 1992 Wiesbaden conference on Nuclear Physics (238). Recent theoretical developments have also been reported by Esbensen on the Bernkastel conference on Nuclei far from Stability (239). Therefore, let us only recall here some prominent properties of the neutron halo as they definitely seem to emerge now: 39 The probably most salient features of the neutron halo which were predicted by Hansen & Jonson (210) are by now clearly identified experimentally: i) the correlation between the weak binding energy of the valence neutron(s) and the spatial extension of the wave function and ii) the large electrical polarizability of the neutron halo giving rise to a lot of low-lying El strength and correspondingly to large Coulomb dissociation cross sections, are clearly manifest in the experiments at various energies at the different facilities. This interplay between the structure of the halo and the reaction mechanism is seen in Figure 13 which shows data from GANIL for the simple case of the one-neutron 11Be. The data are almost quantitatively described by a simple, parameter- free, calculation assuming a Yukawa wave function for the halo neutron (225). The Coulomb calculation in the case of the target followed the theory by Baur (240 and references therein), whereas in the case of a beryllium target, a black-disc model, derived from Glauber's calculation for the deuteron (241), was applied for absorption and diffraction dissociation. The satisfying reproduction of the experimental data by these quite simple ingredients reflects the fact that the halo is a phenomenon essentially accounting for effects at large distances or small momenta (225, 238), with little influence of the core. leki and others have discussed as to whether the strong Coulomb excitations of halo nuclei should be viewed more as collective processes or as essentially single particle strength to the continuum (see e.g. 230,233,238). Experimental evidence seems to indicate a fast process. Concerning diffractive phenomena, Roussel et al investigate theoretically also their influence on the recoiling charged fragment for11Li at 790 MeVAi

(242). The nucleus 11Li and similar nuclei, which have been called "Borromean" by

Vaagen et al (244), because they form a bound three-body system while none of the binary subsystems is stable, contain the same basic simple increments as a one-neutron halo system, although the three body aspects certainly give rise to important modifications. Many theoretical investigations of '1 Li can be found in the proceedings of the conference at Niigata (245). 40 One of the striking experimental features found in the study of the break-up reactions of11Li was that the neutron angular distributions showed (almost) no target dépendance (223). It has been noted, that there is a possible link to the parallel momentum distributions of the recoiling 9Li which behave also target independent (231 ). Indeed, very little (or no) correlation of the valence neutrons would match the momenta width from both different experiments (246). Theoretically, however, Esbensen & Bertsch predict an (anti-)correlation (134), but it certainly will take more refined

experiments to be sensitive to it. Concerning the extremely small target dependence, the very strong cross section at O degrees for 11Li, weaker and differently bound than l1 Be,

may certainly hide broader contributions (238), but the present understanding is certainly not satisfactorily.

In the preceding three sections, we have discussed the reaction experiments and part of their interpretation in the framework of the neutron halo. The halo possibly manifests itself also indirectly in decay spectroscopy experiments. For example, the even-n nuclei at the neutron drip-line show "superallowed" ^-transitions with a strength and energy that may be interpreted as arising from the decay of a quasi-free neutron pair (247). In that sense, the very strong (3-delayed triton branch in 8He (199,248) has been interpreted by Borge et al. (199) as the leftover from the decay of a "tetra-neutron".

5.8 Neutron-rich nuclei in astrophysics

Strictly speaking, the neutron-rich nuclei which are of interest in astrophysical

environments, are not located at the limits of particle stability. The experimental problems to produce and study these nuclei is however very similar, and the experimental progress which has been made only very recently, is closely linked to developments which we have discussed so far. We therefore give here a brief account on ongoing work. As argued by Boyd et al. (249), the necessity of an inhomogeneous Big Bang Model as compared to the (homogeneous) Standard Model can be tested on the abundances of11B (and some heavier nuclei). A critical nuclear reaction involved is 8Li 41 (ct,n) 11B. A first experiment (249) with the neutron-rich radioactive beam 8Li, produced by projectile fragmentation of 80 MeWu 14N at the spectrometer RIPS (27) has been made at RIKEN. Farrell et al. (250) studied competing reactions, such as 8Li (d,n) 9Be and 8Li (d,t) 7Li at the Notre Univ. of Mich, radioactive beam facility (35), using 17 MeV 7Li induced transfer reactions combined with a superconducting solenoid for the selection of 8Li and other light neutron rich nuclei. The location of the astrophysical r-process path strongly depends on the properties of neutron-rich nuclei far off stability at neutron shell closures. Since the rate will drastically decrease for these nuclei, the process has to wait for p-decay before synthesizing heavier species. For a comprehensive understanding of the resulting abundance peaks it is therefore important to measure decay properties like the half-life and the delayed neutron emission probability of selected key nuclei around closed shells. The same argument holds true for "isotopic anomalies" observed in elements lighter than for species also produced by rapid neutron capture. Experiments at ISOLDE on the waiting point nuclei 79Cu and 130Cd by Kratz et al. (251,252) using p-delayed neutron counting, have brought substantial informations about the temperature and neutron density required in explosive stellar scenarios (253,254). Similar measurements on the isotopes 44S, 45-47Cl by Sorlin et al. (255) with the LISE spectrometer at GANIL showed that the decay properties of these nuclei are the probable key to understand the strong over-abundance in the universe of 48Ca compared to 46Ca. The over-abundance Of48Ca is known to be correlated with those Of50Ti and 54Cr, an effect which seems to continue for 58Fe, 64Ni, ^6Zn (256). It is therefore of interest to progress also experimentally in the Sc-Fe region and to test predictions of nuclear models relevant for astrophysical applications. Concerning recent theoretical developments for the decay properties one may note the QRPA-calculations by Môller & Nix (4) and by Staudt et al. (257) and the discussion of mass-predictions for astrophysical applications by Môller et al. (258). First experimental advance in Fe, Co, Ni, Cu region has been made by in p- half-life measurements of isotopes produced by Bernas et al. in neutron-induced fission 42 at the reactor of the ILL Grenoble (259,260) and by Czajkowski et al. who used relativistic heavy-ion projectile fragmentation at GSI Darmstadt (261). 43 6. FUTURE PROSPECTS

In this charter we shall mention some open questions. We have no intention to give an exhaustive list here, but rather select a few examples. These are points which seem today important to us and which we feel have a realistic chance of experimental investigation in the, say, next decade. In particular, we shall include some topics which underline, to our belief, the promise of future facilities based on post-accelerated ISOL beams and on projectile fragmentation. Of course, the first open question is simply the exact location of the drip-lines. Regarding the proton drip-line, the progress has been remarkable, as shown in the preceding chapters. One may expect that projectile-fragmentation of heavy beams of improved intensity at the existing facilities will continue to be very useful tool, and eventually become joined by fusion-evaporation reactions induced by radioactive beams from the planned future facilities. It may be a reasonable speculation that it will be possible in future experiments to synthesize essentially "all" neutron-deficient nuclei, also those which are particle-unbound, but have appreciable lifetimes for direct emission of proton(s). The study of nuclei just before but also beyond the proton drip-line may thus "define" this line rather precisely. As we have seen, the neutron drip-line bends very quickly away from the valley of stability for increasing atomic number. Whereas heavy-ion fragmentation is very efficient for the production of neutron-deficient isotopes, because neutrons are "easily" stripped away from the projectile, the same is not valid for the making new neutron-rich isotopes: The projectile has already to contain already at least the sufficient number of neutrons. Attempting to synthesize the predicted drip-line for, say , i.e. 40Mg (see Figure 1), therefore asks for a projectile 28 neutrons or more. Thus it is virtually impossible to use beams of 48Ca, which were most successful up to the drip-line for neon (see section 5.1). Using a heavier projectile very quickly increases the necessary mass-loss between the projectile and wanted fragment and the process becomes quickly very inefficient, analogue arguments hold true for proton-induced target fragmentation. 44 Notwithstanding other difficulties, if one applies, e.g., Summerer's code (38) to possible combinations of targets or projectiles, very low cross sections are predicted, and it seems extremely difficult to reach the neutron drip-line much beyond the neon isotopes. Applying other standard estimates, it is easy to convince oneself, that fusion reactions with very neutron-rich radioactive beams are no alternative. It seems therefore that we are presently confined to push fragmentation experiments with 48Ca and 64Ni to their very limits, the two important factors being the energy and the intensity of the beam. For this reason the future development of heavy-ion accelerators is clearly of importance, to reach values as quoted in (262) for a heavy-ion synchrotron could be a decisive step. The use of cyclotrons, known for their high beam currents, could also be a route with substantial probability of success: Studies have been made at the MSU-NSCL (H.Blosser et al., private communication 1992), which indicate that it is possible, within a reasonable budget, to boost the present energy to a several hundred MeV/u by means of a second superconducting cyclotron without much sacrifice in die transmission. The questions one may ask and for which one realistically may hope for answers are thus somewhat different for the most neutron-rich isotopes than at the other side of the stability. For the former, for as little that we now of them today, it is interesting to examine the systematics of the binding energies of the last neutrons, determined by !he experiments described in section 5.2. Systematics by Audi show that the one- and two- neutron separation energies for the light nuclei close to the drip-lines flatten out for increasing mass-number and approach the line of zero-binding almost tangentially (recent Audi reference 263). On the other hand, as has been underlined by Hansen (238), we know today about the very unusual nuclear structure of the halo nuclei (see in particular sections 5.5-5.7), and one could imagine that (even larger) clusters of neutrons (close to being bound) may couple with a stable core to a syttem which could be very slightly bound, leading possibly to "stable islands" (162). These effects are not accounted for by any mass formula and it is to our opinion an important question to our opinion to investigate the binding of the most neutron-rich light species (including nn, 3n, 4n). Recoil separators with sensitive detectors are necessary to contribute to the answer. For 45 example, one may note (see figure 13) the very large cross section for neutrons emitted around O degrees by Coulomb break-up and their strong dépendance from their binding energy. The experiments made at HMI with transfer reactions (see section 5.2) point to the potential that one could expect from a radioactive beam facility providing post- acceleration for light beams with a few neutrons more than stable nuclei. The obtainable intensities are of course an important consideration. The new PS-Booster ISOLDE facility is certainly expected to contribute to the field, in particular within regard to decay spectroscopy, but one could also imagine Penning trap experiments, notwithstanding the major effort involved, for a few dedicated cases. Although we feel that we have by now a good understanding some of the main

features of the neutron halo, the large experimental effort which we have tried to summarize in chapter 5 has opened up new questions. Among these is the quantitative

comprehension of the transverse momentum distributions for light targets, comparatively "too" broad for the core recoils and "too" narrow for the neutrons from *1 Li. Complete

kinematic experiments, at different energies, like the one at 300 MeV/u under analysis at GSI (235) will address this problematic and, if fine enough, certainly shed light on the question of correlations of the neutrons in the halo. Another important issue is that of bound excited states. The presently known halo nuclei may not have any, excepted the 320 keV 1/2- state in 11Be (264). Using several very large volume germanium spectrometers for the observation of the E !-transition back to the l/2+ ground-state, Hansen and one of us (ACM) have recently been able to populate this 1/2- state by means of Coulomb excitation with a beam of only SxIO4 11Be, at an energy of 45MeWu from LISE3 at GANIL. Despite the mediocre beam quality

(high emittance, large momentum width) of secondary beams from projectile fragmentation and the large Doppler shift and broadening due to the high energy we believe this to be a very useful development for radioactive beam experiments. Naturally its application goes beyond the search for excited states in the halo nuclei and this method could become a very useful tool in connection with post-accelerated beams behind ISOL 46 systems, particularly in view of the much better beam quality and the more appropriated

(i.e. lower) beam energy. One should note in this context, although closer to stability, two recent experiments: Brown et al. were able to observe Coulomb excitation of 8Li by means of inelastic scattering at Notre Dame (265). Oshima et al. excited the 2+ state in 76Kr at JAERI and observed the £2 y-radiation to the ground-state (266). The intensity of the secondary beam in both experiments was 107 and 106 particles per second, respectively. The feasibility of these "test" experiments shows, to our opinion, that systematic experiments using Coulomb excitation for the lowest excited states in order to study effects of deformation and collectivity will become an important part of the programme at future radioactive beam facilities. Another class of experiments which would benefit from the availability of secondary beams of high optical quality is certainly elastic scattering for precision measurements of optical model parameters. Along the proton-drip-line, one of the most interesting prospects is to study the nuclei around the N=Z line, up to 100Sn. Due to the presence of the protons and neutrons in "identical" shells, which enhances collective effects, strong deformation effects are possible for small changes in the Fermi surface by adding or removing a few nucléons. The purity of the isospin quantum number could be explored by the reduced transition probabilities of isospin-forbidden AT=I Fermi P-decays. Isospin violation should in general be most pronounced for N=Z nuclei along the region of the proton drip-line between A=64-96. However, neutron halo nuclei manifestly also violate isospin symmetry and calculations suggest that these nuclei, despite their large isospin values, will produce comparable effects in Fermi-decay measurements. Further interest in |3- decay will be in studies of the Gamow-Teller quenching of the isovector transitions. Finally, key questions of nuclear structure for astrophysics wait to be addressed. The further study of nuclei at the of stability will therefore strongly linked to the development of radioactive beam facilities. At present there is a great deal of interest and activity in the consideration and study of the best ways to develop such facilities. It appears that a combination of techniques will be applied. The Oak Ridge facility under 47 construction was already mentioned. Many other facilities (see the contributions to reference 267), such as for example the Isospin Laboratory (268), TRIUMF, the JNP project (269), the CERN-ISOLDE facility, Catania, GANIL, Grenoble, and Louvain-la- Neuve (270), and technical developments such as the high intensity test bed at the RAL (270) are in the planning stages. 48 7. SUMMARY

In this paper we have reviewed the properties and synthesis of the nuclei at the limits of particle stability. Even for the light elements, where the particle driplines lie close by, the synthesis of these nuclei is difficult. Since the extent of proton-rich nuclei is limited by the repulsive Coulomb force, the proton drip-line has been more extensively studied. By the use of projectile fragmentation reactions, fusion-evaporation reactions, nucléon transfer reactions, and in a few cases target spallation, studies have reached near (and in a few cases of ground state proton emission exceeded) the proton drip-line for almost all elements. It is possible that with the exception of one or two isotopes all proton rich nuclei up to Nickle have been observed, and it is now possible to perform reasonably detailed decay studies of these nuclei. Examples of this are the decay studies of 2OjVIg and 43y. Improvements in the sensitivity of fusion evaporation studies has allow weaker reaction channels to be studied, and recently several new ground state proton emitters to be found. The introduction of proton rich radioactive beams may even further expand the ability to produce such nuclei. Besides detailed decay spectroscopy, techniques such as ^-asymmetry and LASER spectroscopy have now been applied to nuclei on the proton drip-line. One of key items of study has been the possible presence of an extended proton wave function for weakly bound systems and the possible impact this may have in nuclear and astrophysics. The properties of drip-line proton rich nuclei play an important part in the astrophysical rp-process. It has now been possible to synthesize in the laboratory some the nuclei important in the binary star systems and study their decays. The information has helped to define the beginnings and extent of the rp-process. Progress in the study of the heavies elements has been limited recently by the very low production cross section. These nuclei are proton rich, and not so far from the predicted proton drip-line due to the fact that all nuclei which can be used to synthesis heavy elements are more proton rich than the most stable higher Z nucleus. 49 The extent of neutron rich nuclei is predicted to be much greater than that for

proton rich nuclei, nevertheless perhaps only up to the Neon isotopes has the neutron

drip-line been mapped (and this is even not certain). Nuclei along the neutron drip-line

have shown some of the most interesting and unusual structure yet found. Due to there

weak binding the neutron wave function can expend out to very large radii (infinity if the

binding energy is zero). This allows the existence of nuclei like ^Li, a system which is

in some ways like an atomic system with two neutron orbiting a 9Li core. Further, the weak binding allows for a separation of the neutron and proton parts of the wavefiinction, a qualitatively new feature not found in other nuclei. Extensive studies of these "halo" and "skin" nuclei have begun. Measurement of masses and decay studies have also progressed. Along with improvements in facilities, experiments with nucléon transfer and stripping, elastic scattering, Coulomb excitation, and total reaction cross sections are now possible with beams of nuclei at the driplines.

Perhaps to a large extent further progress in the study on nuclei at the limits of stability rests on the progress in developing and using radioactive nuclei beams. Many laboratories are considering or have active radioactive beam facilities. At present, experiments with radioactive beams are performed with ions produced and separated in flight, such as via projectile fragmentation and nucléon transfer. In the near future facilities which reaccelerate short-lived radioactive ions produced at near thermal velocities will be available.

For along time it was argued, without proof, that the study of nuclei at the limits of particle stability should be interesting since their very different binding energies and slopes of binding energy curves might allow features not seen in stable nuclei. Indeed, perhaps the most interesting systems may He along the driplines. The standard example was the unexpectedly long half-lifes and strong binding observed in the heavy

Isotopes. With the discovery of nuclear "halos" and the study of the features of these nuclei, it appears the example of the Sodium isotopes is not an isolated one, and indeed there is a wealth of new phenomena to be discovered and research at the limits of particle stability. 50

ACKNOWLEDGEMENTS: The authors would like to express their thanks to a number of colleagues for helpful discussions during the preparation of this review and for supplying us with material from their work. We are particularly grateful to, Véronique Borrel, Dominique Guillemaud- Mueller, Gregers Hansen, Olivier Sorlin, Bertram Blank, Kazuo leki, Phil Woods, and Rainer Neugart. 51 9. LITERATURECITED:

1. Détraz C and Vieira DJ, in Annu. Rev. Nacl Part. Sd., 39:407-465 (1989). 2. Roeckl E, Submitted for publication in Rep. on Prog, in Phys., and GSI Report, GSI-91-58 3. Jânecke J and Masson PJ, At. Nucl Data Tables 39:265-71 (1988) 4. Môller P and Nix JR, At. Nucl. Data 7aWes39:213-23 (1988) 5. TachibanaT, UnoM, YamadaM, YamadaS, At. Nucl Data Tables'^: 251-8 (1988) 6. Haustein P (editor) The 1986-1987 Atomic Mass Predictions. At. Data NucLData Tables39:185-393 (1988) 7. Han X-L, Wu C-L, Feng DH, Guidcy MW, Phys. Rev. C:\ 127-44 (1992) 8. Ravn HL, Nucl Instntm. Afe/fiodsB70:107-17 (1992) 9. Cornier TM, in Annu. Rev. Nucl. Part. ScL, 37:537-65 (1987) 10. Munzenberg G, Nucl. lastrum. MethodsB70:265-75 (1992) 11. Sherrill BM, 1992, in Radioactive Nuclear Beams, ed. Delbar, Th., pp. 1-20, London: Adam Hilger 12. Ravn HL and Allardyce B, 1989, Treatise on Heavy-Jon Science, ed. D. Allan Bromley, (Plenum Press, New York) 8:363. 13. Kugler E, Fiander D, Jonson B, Haas H, Przewloka A, et al., Nucl. Instrum. MethodsB70:4l-49 (1992) 14. Kirchner R, Burkard K, Huiler W, Klepper O, Nucl. Instrum. Afcfftods 870:56-61 (1992). 15. PfXK. 12th Int. Conf. on Electromagnetic Separators and Techniques Related to their Applications, Sendai, Japan, 1991 (CGJ M. Fujioka, T Shinozuka and Y. Kawase) Nucl. Instrum. Methods B70 (1992) 1. 16. Alkhazov GDE.Berlovich Ye, and Panteleyev VN, Nucl Instrum. Methods A280:141(1989) 17. Arje J, et al. Nucl. Instrum. Methods 626:384-93 (1986) 52 18. Yoshij M, et al., Nucl. Instrum. Methods. B26::410-8 (1987) 19. Reiff JE, Hotchkis MAC., Moltz DM, Lang T, Robertson JD., et al., Nucl.

Instrum. Methods A276:228-32 (1989) 20. Puteaux JC, et al., Nucl. Instrum. Methods 186 (1981) 321.

21. Hoff P, et al., Nucl. Instrum Methods 186 (1981) 186. 22. Alonso J, 1984, Froc, of the Workshop on Research with Radioactive Beams, ed Niischke, J.M., LBL Report 18187 23. KrebsGF, AlonsoJR., FeinbergB, KalninsJG, 1992, in Radioactive Nuclear Beams, ed. Delbar, Th., pp. 59-64, London:Adam Hilger 24. Anne R, et al., Nucl. Instrum. Methods A257:215 (1987) 25. Anne R, Mueller AC, Nucl. Instrum. MerfKwfcB70:276-85 ( 1992), and Mueller AC, Anne R, Nucl. Instrum. MethodsB56:559 (1991) 26. Geissel H, Armbruster P, Behr KH, Briinle A, Kurkard K, et al., Nucl. Instrum.

AferfK*fcB70:286-97 (1992) 27. Kubo T, Ishihara M, Inabe N, Kumagai H, Tanihata I, Nucl. Instrum. Methods

870:309-19(1992) 28. Sherrill BM, Morrissey DJ, Nolen JA, Orr N, Winger JA, Nucl. Instrum. Methods 870:298-303 (1992) 29. Cole JD, Cormier TM, Hamilton JH, Ramayya AV, Nucl. Instrum. Methods 870:343-57(1992)

30. Davids CN, Back BB, Bindra K, Henderson DJ, Kutschera W, et al, Nucl. Instrum. MethodsB70:35&~65 (1992)

31. WollnikH, 1987, Optics of Charged Particles (Boston: Academic Press) 32. Geissel H, et al., Nucl. Instrum. Methods A2&2-.241 (1989)

33. Becchetti FD, et al., in 6th International Conference on Nuclei far from Stability, to be published 34. Joubert A, Baron E, Grunberg C, Larson JD, Mittig W, Ripouteau, F., 1991 GANIL Report A-91-01 53 35. Kolata JJ, Morsad A, Kong XJ, Warner R E, Becchetti FD, et al., Nucl Instrum. AfedK*fcB40/41:503-506 (1989) 36. Schapira JP, Azaiez F, Portier S, Gales S, Hourani E, Kumpulainen J, Maison JM, Nucl. Instrum. Methods 224:337 (1984) 37. Hourani E, Rosier L, Gerrier-Ronsin G, Elayi A, Mueller AC, et al, Phys. Rev. €44:1424-34(1991) 38. Summerer K, Brochle H, Morrissey DJ, Schâdel M, Szweryn B., Weifan Y,

Phys. ltev.C42:2546 (1990) 39. Hanelt E, PhD. Thesis, Technische Hochschule, Darmstadt, unpublished.

40. Winger JA, Sherrill BM, Morrissey DJ, Nucl Instmm. AfefAodsB70:380-92 (1992) 41. Schwab Th, 1991, GSI Report GSI-91-10 42. Geissel H, Beckert K, Bosch F, Eickhoff H, Franczak B, et al., Phys. Rev. Lett

68:3412-15(1992) 43. Rapaport J, Foster CC, Goodman CD, Goulding CA, Taddeucci TN, et al., Phys. /2ev.C41:1920(1990) 44. McGrath RL, Cerny J, NorbeckE, Phys. Rev. Lett 19:1442-1446(1967) 45. Robertson RGH, Benenson W, Kashy E, Mueller D, Phys. Rev.£13:1018 (1976

46. Tribble RE, Kenefick R.A., Spross R.L., Phys. Rev. C 13:50 (1976) 47. Benenson W, Kashy E, Kong-A-Siou DH, Moalem A., Nann H, Phys Rsv.

£9:2130(1974) 48. Watson DL, Oilman R, Morris CL, Zumbro JD, Mordechai S, et al., Phys. Rev.

£43:1318-1320(1991) 49. Benenson W, Kashy E, Ledebuhr AG, Pardo RC, Roberstson RGH, Robinson LW, Phys. Rev. C17:1939 (1978) 50. KeKelis GJ, Zisman MS, Scott DK, Jahn R, Vieira DJ, Cemy J, Ajzenberg-Selove F, Phys. Rev. C17:1929 (1978)

51. Orihara H, Nishihan» S, Furukawa K, Nakagawa T, Maeda K Miura K, Ohnuma H, Phys. Rev. Lett 49:1318 (1982) 54 52. Burleson GR, Blanpied GS, Daw G., Viescas AJ, Morris CL, et al., Phys. Rev. C22:l 180 (1980) 53. Woodward CJ, Tribble RE, Tanner DM, Phys. Rev. C27:27 (1983) 54. Mohar MF, Adamides E, Beneson W, Bloch C, Brown BA, et al., Phys Rev. C38.-737 (1988) 55. Woods CL, Catford WN, Fifield LK, Orr NA, Nucl. Phys. A484:145 (1988). 56. Détiaz C, Anne R, Bricault P, Guillemaud-Mueller D, Lewitowicz M, Nucl Phys.

A519:529(1990) 57. Pougheon F, Jacmart JC, Quiniou E. Anne R, Bazin D, et al., Z. Phys. A327:17-

24(1987) 58. Borrel V, Anne R, Bazin D, Borcea C, Chubarian GG, et al., Z. Phys. A344:135- 44 (1992), and references therein 59. Saint-Laurent MG, Dufour JP, Anne R, Bazin D, Borrel V, et al., Phys. Rev. Lett. 59:33-5(1987) 60. Mohar MF, Bazin D, Benenson W, Momssey DJ, Orr NA, et al., Phys. Rev. Lett. 66:1571-4(1991)

61. Winger JA, Bazin DP, Benenson W, Crawley GM, Morrissey DJ, et al., in press Phys. Lett B

62. Yennello SJ, Winger JA, Antaya T, Benenson W, Mohar MF, et al., Phys. Rev. C:2620-3 (1992) 63. Antaya T, NSCL Preprint. 64. Carraz LC, Ewan GT, Hagberg E, Hardy JC, Jonson B, et al., Phys. Lett. 885:212-214(1979)

65. Honkanen J, Aysto J, Eskola K, Phys. Scr. 34:608 (1986) 66. Borge MJG, Gabelmann H, Johannsen L, Jonson B, Nyman G, Riisager K, Tengblad O, et al., Nucl. Phys. A515:21 (1990) 67. Borge MJG., Hansen PG, Jonson B, Mattssoîi G, Nyman G, Richter A, Riisager K, et al., Z. Phys. A332:413 (1989) 55 68. Plochocki A, Rykaczewski K, Batsch T, Szerypo J, Zylicz J, et al., Z. Phys. A342:43(1992) 69. Elmroth T, Hagberg E, Hansen PG, Hardy JC, Jonson B, et al., Nucl. Phys.

A304:493(1978) 70. D'Auria JD, et al., Phys. Lett 866:233 (1977) 71. D'Auria, JD, et al., Nucl. Phys. A301:397 (1979) 72. Mosel U, 1989. in Treatise on Heavy Ion Science, Vol. 2, Bromley D.A. Ed, pp 3-

52. New York: Plenum 73. Âystô J and Cemy J, 1989. in Treatise on Heavy Ion Science, Vol. 8, Bromley D.A. Ed, pp 207-58. New York: Plenum 74. Batchelder J, Moltz D, Ognibene T, Rove M, Cemy J, Bull. Am. Phys. Soc.,

37:1278(1992) 75. Âystô J, Moltz DM, Xu XJ, ReiffJE, Cemy J, Phys. Rev. Lett., 55:1384 (1985) 76. Lister CJ, et al., Phys. Rev. C42:R1191-4 (1990)

77. Gelletly W, Bentley MA, Price HG, Simpson J, Gross CJ, et al., Phys. Lett. 8253:287(1991)

78. Hoffman S, 1989, in Particle Emission from Nuclei, eds. Poenaru, D.N., Ivascu, M.S., VoI II, pp. 1-24, Boca Raton: CRC-press

79. Roeckl E and Schardt D, 1989, in Particle Emission from Nuclei, eds. Poenani, D.N., Ivascu, M.S., VoI II, pp. 1-24, Boca Raton: CRC-press 80. Garrett JD and Olsen DK, A Proposal for Physics with Exotic Beams at the HoIifield Heavy Ion Research Facility, Physics Division, ORNL, February 1991. 81. Olsen DK., Alton GD, Baktash C, Carter HK, Dowling DT, et al., 1992, in

Radioactive Nuclear Beams, éd. Delbar, Th., pp. 131-136, LondonrAdam Hilger 82. Mikolas D, Brown BA, Benenson W, Harwood LH, Kashy E, Phys. Rev. C37:766-779(1988) 83. Asahi K.Matsuta K, Takeyama K, Tanaka KH, Nojiri Y., et al., Phys. Rev.

C41:358-61 (1990) 84. Hansen PG, Phys. Scr. T32:21 (1990) 56 85. Kubono S, Ikeda N, Funatsu Y, Tanaka MH, Nomura T, et al., Phys. Rev. C46:361-5(1992) 86. Goerres J, Wiescher M, Scheller K, Morrissey DJ, Sherrill BM, et al., Phys. Rev.

C46:R833-837 (1992) 87. Piechaczek A, Mohar MF, Anne R, Borrel V, Cône JM, et al., in 6th International

Conference on Nuclei far from Stability, in press 88. Cable MD, Honkanen J, Schloemer EC, Ahmed M, Reiff JE, et al., Phys. Rev. €30:1276-85(1984) 89. Borrel V, Jacmart JC, Pougheon F, Anne R, Detraz C, et al., Nucl. Phys. A531:353-69(1991) 90 Aysto J, Taskinen P, Eskola K, Vierinen K, Messelt S, Phys. 5cr.T5:193 (!983) 91. Bazin D, Del Moral R, Dufour JP, Fleury A, Hubert F, et al., Phys. Rev. C45:69-

79(1992) 92. Détraz C, Anne R, Bricault P, Guillemaud-Mueller D, Hubert F, et al., Nucl.

Phys. A519-.529.47 (1990) 93. Moltz DM, Batcheleder JC, Lang TF, Ognibene TJ, Cerny J, et al., Z. Phys.

A342:273-6(1992) 94. Hotchkis MAC, Reiff JE, Vieira DJ, Blonnigen F, Lang TF, Moltz DM, Xu X, Cerny J, Phys. Rev. C35:315 (1987) 95. Kureewicz W, Zganjar EF, Kirchner R, Klepper O, Roeckl E, et al, Z. Phys. A308:21 (1982)

96. Hardy JC, Hagberg E, 1989, in Particle Emission from Nuclei, eds. Poenaru, D.N., Ivascu, M.S., VoI IH, pp. 99-132, Boca Raton: CRC-press

97. Moltz DM and Cerny J, 1989, in Particle Emission from Nuclei, eds. Poenaru, D.N., Ivascu, M.S., VoI HI, pp. 133-156, Boca Raton: CRC-press 98. Détraz C, et al., Z. Phys. A340:227-230 (1991) 99. Brown BA, Phys. Rev. Lett. 65:227 (1990) 100. Neugart R, et al., in 6th International Conference on Nuclei far from Stability, to be published 57 101. Neugart R, private communication. 102. KellerH, et al, Nucl. Instr. and Meth. A300:67-76 (1991) 103. Keller H, Kirchner R, Klepper, Roeckl E, Schardt D, et al., Z. Phys. A340:363 (1991) 104. Brohm T, Clerc H-G, Gollerthan U, Schwab W, Schmidt K-H, Simon RS, Z. /%>s.A339:495-S01(1991) 105. Bollen G, Kluge H-J, Konig M, Otto T, Savard G, et al., Phys. Rev. C46:R2140- 2143(1992) 106. Benenson Wand Kashy E, Rev. Mod. Phys. 51:527-40 (1979) 107. Rho M, Nucl Phys. A231:493 (1974) 108. Goodman CD, et al., Phys. Rev. Lett. 54:877 (1985) 109. Brown BA, Wildenthal BH, Ann. Rev. Nucl. Part. Sc/.38:29 (1988) 110. Adelberger EG, Garcia A, Magnus PV, Wells DP, Phys. Rev. Lett 67:3658 (1991) 111. Brown BA, Phys. Rev. Lett 69:1034-7 (1992) 112. Comay E, Kelson I, Zidon A, Phys. Lett. 6210:31-4 (1988) 113. Borge MJG, et al., Nucl. Phys. A490:287 (1988) 114. Nyman G, et al., Nucl. Phys. A510:189 (1990) 115. RiisagerK,BorgeMJG,HansenPG,JonsonB,MôllerP,étal., in6th International Conference on Nuclei far from Stability, to be published 116. Minamisono T, Ohtsubo T, Minami I, Fukuda S, Kitagawa A, et al., Phys. Rev. Lett. 69:2058-61(1992) 117. Bahcall JN, Huebner WF, Lubow SH, Parker PD, Ulrich RK, Rev. Mod. Phys. 54:767(1982) 118. Riisager, K., Jensen, A.S., submitted to Phys. Lett. B 119. Tanihata I, et al., Phys. Lett. 8206:592 (1988) 120. Jackson KP, Cardinal CU, Evans HC, Jelley NA, Cerny J, Phys. Lett. B:33:281 (1970) 58 121. Page RD, Woods PJ, Cunningham RA, Davinson T, Davis NJ, et al., Phys. Rev. Lett. 68:1287 (1992) 122. Hoffman S, 1989, in Particle Emission from Nuclei, eds. Poenam, D.N., Ivascu,

M.S., VoI II, pp. 25-72, Boca Raton: CRC-press 123. GillitzerA,FaestermannT,HartelK,KienleP,NolteE, ZJ>/iys.A326:107 (1987) 124. SeIHn PF, et al., Nucl Instrum. Methods. A311:217 (1992) 125. Woods PJ, private communication (1993), and Woods PJ, Davinson T, Davis NJ, Hofmann S, James A, Livingston K, Page RD, SeIHn PJ, Shotter AC, in 6th International Conference on Nuclei far from Stability, in press 126. Page RD, Woods PJ, Bennett SJ, Freer M, Fulton BR, et al, Z. Phys. A338:295- 301 (1991) fa 127. Robertson JD, Reiff JE, Lang TF, Moltz DM, Cerny J, Phys. Rev. C42:1922-28 (1990)

128. Hotchkis MAC, et al., Manchester Nuclear Physics report, August 1987-1988, unpublished. 129. Hourani E, Azaiez F, Dessagne Ph, Elayi A, Fortier S, et al., Z Phys. A334:277- 83(1989) 130. Brown BA, Phys. Rev.C43:R\5l3 (1991) 131. Champagne AE and Wiescher M, Annu. Rev. Nucl. Part. ScL 42:35-76 ( 1992) 132. Taam RE, Annu. Rev. Nucl. Part. ScL 35:1-23 (1985)

133. AudouzeJ,TruranJW,ZimmermanBA1 Astmphys.J. 184:493(1987) 134. Wallace RK, Woosley SE, Astrophys. J. Suppl. 45:389-420 (1981)

135. Wiescher M, Goerres J, Thielemann F-K, Ritter H, Astron. Astrophys. 160:56-72 (1986)

136. Langanke KH, Wiescher M, Fowler WA, Goerres J, Astrophys. J. 301:629-33 (1986) 137. Magnus PV, Smith MS, Howard AJ, Parker PD, Champagne AE, Nucl. Phys. A506:332-45(1990) 59 138. Lamm LO, étal., Nucl Phys. A510:503-17 (1990) 139. Smith MS, et al., Nucl. Phys. A536:333 (1992) 140. Brown BA, Fortune HT, Sherr R, NSCL preprint and private communication 141. Munzenberg G, Nucl PhysA502:S7 lc-590c ( 1989) 142. Munzenberg G, Rep. Prog, in Phys. 51:57 (1988) 143. Myers WD, Swiatecki WJ, Nucl Phys. 81:1 (1966) 144. MôllerPand Nix JR, At. Nucl. Data Tables39:213-23 (1988) 145. Armbruster, P., Anna. Rev. Nucl. Part. Sd. 35:135 (1985) 146. Demin, A.G., Tretyakova, S.P., Utyonkov, V.K., Shirokovski, I.V., Z. Phys. A315:197(1984) 147. Muzenberg, G., et al., Z. Phys. A322:227 (1985) 148. Muzenberg, G., et al., Z. Phys. A328:49 (1987) 149. Muzenberg, G., Armbruster, P., Berthes, G., Hessberger F.P., Hofmann, S., et al., Nucl. Instrum. MethodsB26:294 (1987) 150. Miyatake, H., Nomura, T., Kawakami, H., Tanaka, J., Oyaizu, M., et al., Nucl. Instnun. Methods B26:309-13 (1987) 151. Ghiorso, A., Yashita, S., Leino, M.E., Frank, L., Kalnins, J., et al., Nucl. Instrnm. MediodsA269:192-201 (1988) 152. Oganessian, Yu.Ts., et al., private communication, DUBNA 153. Bowmann, J. D., Poskanzer, A.M., Korteling, R. G., Butler, G. W., Phys. Rev.C9: 836-51 (1974) and references therein 154. Symons, T. J. M. Viyogi, Y. P., Westfall, G. D., Doll, P., Greiner, D. E., etal., Phys. Rev. Lett. 42:40-43 (1979) 155. Westfall, G. D., Symons, T. J. M., Greiner, G. D., Heckman, H. H., Lindstrom, P. J., et al., Phys. Rev. Lett.43: 1859-62 (1979) 156. Musser, J. A. and Stevenson, J. D., Phys. Rev. Lett. 53: 2544-46 (1984) 157. Anne, R., Bazin, D., Mueller, A. C., Langevin, M., Jacmart, J. C., Nucl. Instrum. Methods \251:215-32 (1987) 60 158. Lange vin, M., Quiniou, £., Bernas, M., Galin, J., Jacmart, J.C., et al., Phys.

Le». 15OB: 71-74 (1985) 159. Pougheon, F., Guillemaud-Mueller, D., Quiniou, E., Saint-Laurent, M. G., Anne, R., et al., Europhys. Lett 2: 505-9 (1986) 160. Guillemaud-Mueller, D., Penionzhkevich, Yu. E., Anne, R., Artukh, A.G., Bazin, D., et al., Z. Phys. A332: 189-93 (1989)

161. Guillemaud-Mueller, D., Jacmart, J. C., Kashy, E., Latimier, A. Mueller, A. C., étal., Phys. Rev.CAl: 937-41 (1990) 162. Jensen, A. S., Riisager, K., Nucl. Phys. A537: 45-63 (1992) 163. Thibault, C., Klapisch, R., Rigaud, C., Poskanzer, A. M., Prieels, R., et al., Phys. Rev.CU: 644-57 (1975) 164. Wouters, J. M., Vieira, D. J., Wollnik, H., Enge, H. A., Kowalski, S., et al.,

Auc/. Instnim. Methods A240: 77-90 (1985) 165. Blanchi, L., Fernandez, B., Gastebois, J., Gillibert, A., Mittig, W. et al., Nucl. Instrum. Methods A276:509-20 (1989) 166. Wouters, J. M., Kraus, R. H., Vieira, D.J., Butler, G. W., Lôbner, K.E.G., Z. Phys.A33l: 229-33 (1988)

167. Gillibert, A., Mittig, W., Bianchi, L., Cunsolo.A., Fernandez, B., et al., Phys. Left B192: 39-43 (1987)

168. Vieira, D. J., Wouters, J. M., Vaziri, K., Kraus Jr., R. H., Wollnik, H., et al., Phys. Rev. Lett. 57: 3253-56 (1986)

169. Bollen, G., Hartmann, H., Kluge, H. J., Kônig, M., Otto, T., et al., Physica Scripta 46: 581-86 (1992) 170. Auger, G., Bajard, M., Baron, E., Bibet, D., Bricault, P., et al., /. PAj*. G17: 8463-5468(1991) 171. Fujita, Y., Trôtscher, J., Schwab, Th., Lindemann, K., Schnell, J., et al. 1991. PTOC. Int. Symposium on Structure and Reactions of unstable Nuclei, Niigata, pp. 40-46, Singapore: World Scientific 61 172. Stolzenberg, H., Becker, St., Bollen, G., Kern, F., Kluge, H. J., et al., Phys. Rev. Lett. 65:3104-7 (1990) 173. Bollen, G., Kluge, H. J., Otto, Th., Savard, G., Schweikhard, L. et al., Journ. Mod. OpticsW. 257-62(1992) 174. Harkewicz, R., Morrissey, D. J., Brown, B. A., Nolen Jr., J. A., Orr, N. A., et al., Phys. Rev. C44 2365-71 (1991) 175. Oglobin, A. A., Penionzhkevich, Yu. E., 1989. In Treatise on Heavy-Ion Science, éd. D. A. Bromley, pp. 261-360. New York: Plénum 176. Bohlen, H. G., 1991. PTOC. Int. Symposium on Structure and Reactions of unstable nuclei, Niigata pp. 83-90, Singapore: World Scientific 177. Ostrowski, A. N., Bohlen, H. G., Demyanova, A. S., Gebauer, B., Kalpakchieva, R. et al., Z. Phys. A343: 489-90 (1992) 178. Bohlen, H. G., Alexandrov, D. V., Demyanova, A. S., Gebauer, B., Kalpakchieva, R et al. 1992. Proc. Int. Conf. on Nuclei far from Stability, Bemkastel, in press, Bristol: IOP Publishing 179. Bohlen, H. G., Gebauer, B., von Lucke-Petsch, M., von Oertzen, W., Ostrowski, A. N., et al., Z. Phys. A: in press (1993) 180. Aimelin, A. L, Gornov, M. G., Gurov, Yu., B., Ilyin A. L., Morokhov, P. V., et al., Yad.Fizika52:1231-33(1990) 181. Kobayashi, T., NucJ. Phys. A538: 343c-352c (1992) 182. Heilbronn, L., Galonsky, A., Yang, X., Deak, F., Kiss, S., and Seres, Z., Phys. Rev. C40: 2576-2580 (1989) 183. Hansen, P. G., Jonson, B., 1989, In Particle Emission from Nuclei, eds. Poenaru, D. N., Ivascu, M. S., Vol. 3, pp. 157-201, Boca Raton: CRC-Press 184. Jonson, B., Nyman, G., 1993, In CRC Handbook of Nuclear Decay Modes, éd. Poenaru, D., Vol. 2, in press, Boca Raton: CRC-Press 185. Bazin, D., Mueller, A. C., Schmidt-Ott, W. D., Nucl. Instntm. MethodsA2&l: 117-27 (1989) 62 186. Mueller, A. C, Bazin, D., Schmidt-Ott W. D., Anne, R., Guerreau, D., et al., Z Phys. A330: 63-68 (1988) 187. Mueller, A. C, Guillemaud-Mueller, D., Jacmart, J. C., Kashy, E., Pougheon, F., et al., JViic/. PAjs. A513: 1-10 (1990) 188. Lewitowicz, M., Penionzhkevich, Yu. E., Artukh, A. G., Kalinin, A. M., Kamanin, V. V., et al., NucL Phys.M96:477-84 (1989) 189. Dufour, J. P., Del Moral, R., Hubert, F., Jean, D., Pravikoff, M. S., et al., Phys. Lett.B206:195-198 (1988) 190. Fréhaut, J., NucL lastrum. Methods 135: 511-18 (1976) 191. Reeder, P. L., Warner, R. A., Hensley, W. K., Vieira, D. J., Wouters, J. M., Phys. Rev. CM: 1435-53(1991) 192. Tengblad, O., Borge, M. J. G., Johannsen, L., Jonson, B., Lindroos, M. et al., Z Phys. A342:303-307 (1992) 193. Bounajma, M., Baumann, P., Buck, A., Klotz, G., Knipper, A. et al., 1991. Proc. 1st Eump. Biennial Workshop on Nuclear Physics, Megève, pp. 227-35, Singapore: World Scientific 194. Guillemaud-Mueller, D., Détraz, C., Lange vin, M., Naulin, F., de Saint- Simon, M. et al., Nucl. Phys. A426: 37-76 (1984) 195. Walter, G. 1989. Proc. Workshop Nuclear Structure of light Nuclei far from stability, Experiment and Theory, Obernai, pp. 85-97, Strasbourg: Centre de Recherches Nucléaires 196. Dufour, J. P., Del Moral, R., Fleury, A., Hubert, F., Jean, D. et al., Z Phys. A324:487-88 (1986) 197. Hubert, F., Dufour, J. P., Del Moral, R., Fleury, A., Jean, D., et al., Z Phys. A333: 237-46 (1989) 198. Pravikoff, M. S., Hubert, F., Del Moral, R., Dufour, F., Fleury, A. et al., Nucl. -PA.js.A528: 225-41 (1991)

199. Borge, M. J. GM Hansen, P. G., Johannsen, L., Jonson, B., Nilsson, T. et al., Preprint CERN-PPE/93-05,Nucl. Phys. A in press 63 200. Otten, E. W., 1989. In Treatise on Heavy-Ion Science Vol. 8, ed. D. A. Bromley, pp. 517-638. New York: Plenum Press 201. Arnold, E., Bonn, J., Gegenwart, R., Neu, W., Neugart, R. et al., Phys. Lett 6197:311-14(1987) 202. Arnold, E., Bonn, J., Klein, A., Neugart, R., Neuroth, M., et al., Phys. Lett. 8281:16-19(1992) 203. Asahi, K., Ishihara, M., Ichihara, T., Fukuda, M., Kubo, T. et al., Phys. Rev. C43:456-60 (1991) 204. Asahi, K., Ishihara, M., Inabe.N., Ichihara, T., Kubo, T., Phys. Lett. B251: 488-92 (1990) 205. Asahi, K. 1992. Proc. Int. Conf. on Nuclei far from Stability, Bernkastel, hi press, Bristol: IOP Publishing 206. Matsuda, K. Ozawa, A., Nojiri, Y., Minamisono, To., Fukuda, M., et al. 1992. Proc. Int. Conf. on Nuclei far from Stability, Bernkastel, in press, Bristol: IOP Publishing 207. Tanihata, L, Hamagaki, H., Hasimoto, O., Nagamiya, S., Shida, Y., et al., Phys. Le». B160: 380-84 (1985) 208. Tanihata, L, Hamagaki, H., Hasimoto, O., Shida, Y., Yoshikawa, N., et al., Phys. Rev. Lett. 55: 2676-79 (1985) 209. Tanihata, L, Nucl. Phys. A522: 275c-292c (1991) 210. Hansen, P. G., Jonson, B., Europhys. Lett. 4: 409-14 (1987) 211. Mittig, W., Chouvel, J. M., Zhan Wen Long, Bianchi, L., Cunsolo, A., et al., Phys. Rev. Lett. 59: 1889-91 (1987) 212. Saint-Laurent, M.G., Anne, R., Bazin, D., Guillemaud-Mueller, D., Jahnke, U., et al., Z. Phys.A332:457-65 (1989) 213. Villari, A. C. C., Mittig, W., Plagnol, E., Schutz, Y., Lewitowicz, M., et al., Phys. Ie«.B268: 345-50 (1991) 214. Fukuda, M., Ichihara, T., Inabe, N., Kubo, T., Kumagai, H., Phys. Lett. B268: 339-44(1991) 64 215. Shimoura, S. 1991. Proc. Int. Symposium on Structure and Reactions of unstable nuclei, Niigatapp. 132-40, Singapore: World Scientific 216. Blank, B., Gaimard, J. J., Schmidt, K. H., Stelzer, H., et al., Z. Phys. A340: 41-50(1991) 217. Blank, B., Gaimard, J. J., Geissel, H., Schmidt, K. H., Stelzer H., et al., Z. Phys. A343: 375-9 (1992) 218. Webber, W. R., Kish, J. C, Schrier, D. A., Phys. Rev. C41: 520-32 (1990) 219. Moon, C. B., Fujimaki, M., Inabe, N., Katori, K., Kim, J.C., et al. 1992. Proc. Int. Conf. on Nuclei far from Stability, Bernkastel, in press, Bristol: IOP Publishing 220. Kolata, J. J., Zahar, M., Smith, R., Lamkin, K., Belbot, M., et al., Phys. Rev.

Lett. 69: 2631-34 (1992) 221. Lewitowicz, M., Borcea, C., Anne, R., Artukh, A. G., Bimbot, R. et al., Nucl.

Phys. A in press 222. Mermaz, M. C., Phys. Rev. C (1993) in press

223. Anne, R., Arnell, S. E., Bimbot, R., Emling, H., Guillemaud-Mueller, D., et al., Phys. Lett.E25Q: 19-23(1990)

224. Riisager, K., Anne, R., Arnell, S. E., Bimbot, R., Emling, H., et al., Nucl. Phys. A540: 365-82 (1992) 225. Anne, R., Arnell, S. E., Bimbot, R., Dogny, S., Emling, H., et al., Phys. Lett. B, in press 226. Kobayashi, T., Shimoura, S., Tanihata, I., Katori, K., Matsuta, K., et al., Phys.

Lett. 8232:51 -55 (1989) 227. Tanihata, L, Hirata, D., Kobayashi, T., Shimoura, S., Sugimoto, K., Toki, H.

1992. Proc. Int. Conf. on Nuclei far from Stability, Bernkastel, in press, Bristol: IOP Publishing

228. Yabana, K., Ogawa, Y., Suzuki, Y., Nucl. Phys. A539: 295-315 (1992) 229. Kobayashi, T., Yamakawa, O., Omata, K., Sugimoto, K., Shimoda, T., Phys. Rev. Lett. 60: 2599-602 (1989) 65 230. Kobayashi, T., Nucl. Phys. A553: 465c-72c (1993) 231. Orr, N. A., Antaramanan, N., Austin, Sam M., Bertulani, C. A., Hanold, K., et al., Phys. Rev. Lett. 69: 2050-3 (1992) 232. Bertulani, C. A., McVoy, K, W., Phys. Rev. C46: 2638-41 (1992) 233. Ieki, K., Sackett, D., Galonsky, A., Bertulani, C. A., Kruse, et al., Phys. Rev. Lett. 70:730-3 (1993) 234. Shimoura, S., Nakamura, T., Ishihara, M., Inabe, N., Kobayashi, T. et al. 1992. PTOC. Int. Coaf. on Nuclei far from Stability, Bemkastel, in press, Bristol: IOP Publishing 235. Schrieder, G. 1992., Proc. Int. Conf. on Nuclei far from Stability, Bemkastel, in press, Bristol: IOP Publishing 236. Esbensen, H., Phys. Rev. C44: 440-45 (1991) 237. Esbensen, H., Bertsch, G. F., Nucl. Phys.A.542: 310-40 (1992) 238. Hansen, P. G., Nucl. Phys. A553: 89c-106c (1993) 239. Esbensen, H. 1992. Proc. Int. Conf. on Nuclei far from Stability, Bemkastel, in press, Bristol: IOP Publishing 240. Baur, G. 1991. Proc. Int. Symposium on Structure and Reactions of unstable nuclei, Niigata pp. 211-17, Singapore: World Scientific 241. Glauber, R. J., Phys. Rev. 99: 1515-17 (1955) 242. Roussel, P., Bacri, CO., Clapier, F., Nucl. Phys. A, in press 243. Wapstra, A. H., Audi, G., Nucl. Phys. A432: 1-362 (1985) 244. Vaagen, J. S., Bang, J. M., Zhukov, M. V., Fedorov, D. V., Danilin, B. V., Thompson, I. L., 1992. Proc. Int. Workshop on the Physics and Techniques of Secondary Nuclear Beams, Dourdan pp. 101-111, Gi f-sur-Yvette: Editions Frontières 245. Proc. Int. Symposium on Structure and Reactions of unstable nuclei, Niigata, June 1991, Ikeda, K. and Suzuki, Y. eds, Singapore: World Scientific 246. Mueller, A. C. 1992. Proc. Int. Conf. on Nuclei far from Stability, Bemkastel, in press, Bristol: IOP Publishing 66 247. Borge, M. J. G., Hansen, P. G., Johannsen, L., Jonson, B., Nilsson, T., Z. Phys. A340:255-61 (1991) 248. Borge, M. J. G., Epherre-Rey-Campagnolle, M., Guillemaud-Mueller, D., Jonson, B., Langevin, M. et al., Nucl. PhysA46Q: 373-80 (1986) 249. Boyd, R. N., Hirata, D., Inabe, N., Kubo, T., Nakagawa, T., et al., Nucl. Phys. A538:523c-530c(1992) 250. Farrell, M. M., Boyd, R. N., Kalen, J. D., Gu, X., Kolata, J. J., et al., 1992. Proc. 2nd Int. Conf. on Radioactive Nuclear Beams, Louvain-la-Ncuve, pp. 287- 92, Bristol: Adam Hilger 251. Kratz, K. L., Gabelmann, H., Hillebrandt, W., Pfeiffer, B., Schlôsser, K., Thielemann, F. K., Z. Phys. A325:489-90 (1986) 252. Kratz, K. L., Gabelmann, H., Môller, P., Pfeiffer, B., Ravn, H. L., Wôhr, A., Z. Phys. A340:419-20 (1991) 253. Kratz, K. L., Revs. Mod. Astron. 1: 184-209 (1988) 254. Kratz, K. L., Ap. J; (1993), in press 255. Sorlin, O., Guillemaud-Mueller, D., Mueller, A. C., Borrel, V., Dogny, S., Phys. Rev. C, in press 256. Harper, C. L., Nyquist, L. E., Shih, C. Y., Wiesmann, H., 1990. Proc. Int. Symposium on "Nuclei in the Cosmos", Baden, pp. 138-44, Garching: MPI fur Astrophysik 257. Staudt, A., Bender, E., Muto, K., Klador, H. V., At. and Nucl. Data Tables44: 79-132(1990) 258. Môller, P, Nix, J. R., Kratz, K. L., Wohr, A., Thielemann, F. K., 1992. Proc. 1st. Int. Symposium on Nuclear Physics in the Universe, Oak Ridge 259. Bernas, M., Armbruster, P., Bocquet, J. P., Brissot, R., Faust, H., et al., Z. PAj*. A336:41-51 (1990) 260. Bemas, M., Armbruster, P., Czajkowski S., Faust, H., Bocquet, J. P., Brissot, R., Phys. Rev. Lett. 67: 3661-3 (1991) 67 261. Czajkowski, S., Bernas, M, Armbruster, P., Faust, H., Bocquet, J. P.et al. 1992. Proc. Int. Symposium on Nuclear Astrophysics "Nuclei in the Cosmos", Karlsruhe, 357-62, Bristol: IOP Publishing 262. Kienle P, 1984. Proc. 7th Int. Conf. on Atomic Masses and Fundamental Constants, Seeheim pp. 712-24, Darmstadt: Lehrdruckerei Technische Hochschule 263. Audi, G. 1989. Proc. Workshop Nuclear Structure of light Nuclei far from stability, Experiment and Theory, Obemai, pp. 113-28, Strasbourg: Centre de Recherches Nucléaires 264. Ajzenberg-Selove, F., NucL Phys. A506:l-158 (1990) 265. Brown, J. A., Becchetti, F. D., Janecke, J. W., Ashktorab, K., Roberts, D. A., et al., Phys. Rev. Lett. 66: 2452-5 (1991) 266. Oshima, H., Gono, Y., Murakami, T., Kusakari, H., Sugawara, M., et al., MJC/. Instntm. MethodsA3\2:425-30 (1992) 267. Proc. Int. Workshop on the Physics and Techniques of Secondary Nuclear Beams, Dourdan, March 1992, Bruandet, J. F., Fernandez, B. Bex, M. eds., Gif-sur- Yvette: Editions Frontières 268. Carsten, R. F., D'Auria, J. M., Davids, C. N., Garrett, J. D., Nitschke, J. M., et al., Los Alamos Publication LALP 91-51, unpublished 269. Nomura T, NucL Instrum. andMethodsB70:4Ql-l3 (1992) 270. Report on the European Radioactive Beam Facilities from the NuPECC Study Group, Kôrner, E. G. éd., in press, Munchen: NuPECC 68 FIGURECAPTIONS Figure 1 : Chart of the nuciides illustrating the limits of particle stability. The predicted proton drip-line is taken from the mass model of Jànecke and Masson up to Z= 106 and from Moller and Nix above Z= 106. The predicted neutron drip line is from the model of Tachibana, Uno, Yamada, and Yamada. The dark squares represent the stable nuclei, or very long lived, nuclei found in nature, and the shaded area is the region of nuclei synthesized in the laboratory up to now. Figure 2: Schematic drawing of the ISOLDE-PS experimental facility. Figure 3: Schematic drawing of the LISE-3 separator which uses a combination of fragment separation techniques and electric fields for isotope separation. The excellent selectivity of this device has allowed, for example, decay studies of

very weakly produced nuclei along the proton drip-line. Figure 4: Section of the chart of the nuciides illustrating the nuclei produced along the proton drip-line in recent years using fragmentation and fusion-evaporation techniques. Solid squares represent stable nuclei. The solid line is the predicted proton drip-line from the mass model of Janecke and Masson. Figure 5: Measured data on the decay of 43cr by Borrel et al (58) using the LISE-3 spectrometer. The left side of the figure shows the proposed decay scheme, while the right side illustrates the charged particle energy spectrum (top) and measured half-life (bottom). Figure 6: A schematic drawing of the Daresbury recoil separator (RMS) used for the most recent groundstate proton decay measurements (top). On the bottom is

(a) the energy spectrum for all decays observed in the A= 160 region of the RMS focal plane for the reaction of 300 MeV 58Ni on a l°6Cd target, the broad bump extending from around 1 MeV up to the sharp a-decay peak is due to unstopped a-particles. Part (b) shows the same decays, but now

only for decays within 3 ms of an implantation in the same pixel of the double sided strip detector. 69 Figure 7. Particle identification plot, at the exit of the spectrometer LISE (157), of the projectile-like fragments from a 44MeWu 48Ca beam, impinging on a tantalum target (161). An energy-loss measurement and a time-of-flight measurement, event-by-event, ensure the identification in atomic number Z and mass A. The magnetic rigidity of the spectrometer was set for an optimum production of the unknown isotope 26O, simultaneously mapping of the all predicted neutron drip-line isotopes up to the Z= 10 (i.e. 32Ne), with the exception of 26O. Note the cleanliness of this identification, underlines by the absence of any count on the particle unstable isotopes. The insert in the figure shows a momentous drop in the observed counts for the isotopes with 2Z+2, which estimates that about 30 events of 26O should have been observed.

Figure 8. Energy spectrum of the p-delayed neutrons emitted in the decay of the isotope 1 !Li ( 192). The spectrum is obtained by recording the time-of-flight between the implantation detector and one of the 16 large-area (157cm x 7.6 cm) curved neutron-detectors which surround the implantation detector in 1 m distance. The clearly resolved peaks from neutron unbound states in 11Be are labelled and the energies and widths are given in the insert.

Figure 9. Energy dependence of the interaction cross sections of * * Li with carbon. The 2 intermediate-energy data have been obtained at RIKEN, the 2 relativistic energy points at Berkeley. The solid and dashed lines, respectively, show a best fit based on the Glauber mode] including (or not) a Yukawa-type tail for the valence neutrons outside a radius. The fît parameters are the width of the (harmonic oscillator) core, the number of nucléons in the tail and their binding energy. A reasonable description of the experimental data is obtained by the solid curve, based on two neutrons in the tail bound by 0.34 MeV. This, in turn, is consistent with the (still not very precisely) known mass of 70 11Li. The insert in Figure 8 shows the inferred density distribution which clearly has a long tail, the neutron halo. The figure is taken from (215).

Figure 10. Charge-changing (OAZ) and total reaction (otot) cross sections for the isotopes 8-]1Li interacting at 80 MeWu with a carbon target. Note the constancy of

0Az whereas the change in Otot reflects the increasing matter radius. The figure is taken from (217).

Figure 11. Angular distributions of neutrons from the reaction at 29 MeVAi on a gold target, HLi->9Li+n+X, from an experiment at GANIL (223). The full

drawn and the dashed curves are from a Coulomb excitation calculation by Esbensen (236), representing absolute and renormalized values respectively. The upper insert shows the longitudinal momentum distribution of 9Li (arbitrary scale) from the reaction 11Li->9Li+X, measured at MSU at an

energy of 66 MeV/u on a tantalum target (231). The full drawn curve is a (Gaussian) fit to this momentum distribution and the dashed is a calculation by Esbensen & Bertsch (237).

Figure 12. Decay energy spectrum of the nucleus 11Li from the reaction HLi+Pb- >9Li+2n+X, at 28 MeVAi. The experiment was made by leki et al by means of a double neutron detector hodoscope, installed at MSU (233). The full drawn curve, corresponding to a Breit-Wigner fitting of the data, gives a resonance energy of Eo=O.7MeV and a width of Fo=O-SMeV. From this leki

et al obtain a Coulomb dissociation cross section of 3.6±.4 b, consistent with the experiment at GANIL. The other curves, giving a less satisfactorily description of the data, correspond to other fits (see 233).

Figure 13. Angular distributions of the neutrons emitted in the reaction of 11Be impinging at 41 MeVAi on gold, titanium and beryllium targets (225). 71

Upper part: Comparison with schematic calculations, the full drawn curves correspond to Coulomb excitation of gold and beryllium. The calculated angular distribution arising from diffraction dissociation is shown by a dashed line. Lower part: Comparison of the gold data with Coulomb excitation calculations for different range parameters p of the Yukawa wave function, which are dependent from the binding energy Sn as p = n / V2psn, where ji

11 is the reduced mass and Sn the neutron separation energy ( Be is bound by 504 keV (243). Note the sensitivity of the cross section near O degrees to the

2 separation energy (scaling with (Sn)' ). 72

Table 1: Summary of the decay studies of light nuclei along the proton drip-line.

>-Lme Nucleus A » Measured Quantities References 9c Ep1I1721BR(Pp) 82 130 Ep,t 1/2,BR(Pp) 83 17Ne Ep^1721BR(Pp) 84 20Mg Ep,t172,BR(pp,pY),Ey 85,86,87 22Ai Ep,2p.tl/2.BR(Pp,p2p) 88 ***\ 22Si P-decay 59 26^f. p Ep,2p.tV2,BR(Pp,p2p) 88 27*\*1 ^ S t Ep,2p. l/23R(PP,P2p),IA 89 *» 1 S 3^Cl Ep,2p.tl/2.BR(PP,P2p) 90 3lAr Ep,2P,3p.tl^,BR(Pp,P2p, 89,91,19 P3pa *j ._. )\ 35 K Ep,2p»ty2,BR(Pp,P2p) 64 35ca 75 f\f\ Ep,2r»tl/23R(PP,p-2p) 39 Ti 4,2p.tl/2»BR(Pp,P2p) 92,93 A *% 43y P-decay 58 43A'y Cr t Ep,2p> l/2.BR(Pp,p2p,pa 58

>|jC. . P)MS 46Mn 58 Ai— Ep,t1/2,BR(pp,p2p) 47Fe ji- 4 *V2 58 6 f IG> a «W 61 6lGe £C . Ep.ti/2 94 65As »1/2 61 76Sr Ey 76 O/\ 80Zr Ey 76 4 8 Mo Ey 77 96Ag Ep,ti72,BR(p) 95 •0 (4

Figure 1 g \

Figure 2 W///////////,

Y/Y////////////A

Wien filter crossed ?*sV/«fds

Second achromatic focal point

Concrete shielding W/////////////A

Y//////////,

\ achromatic \ focal point

focal plane dégrader position

Primary team

Figure 3 Figure 4 Figure 5 0 Q-POLE CROSSED FIELD DEVICES Q-POLE VELOCITY SO SECTOR TRIPLET TRIPLET SLIT MAGNET

BEAM

TARGET CHAMBER

NSF RECOIL SEPARATOR STRIP DtIbCIOR OVERALL LENGTH 13m SYSTEM

IS2 (a) Er '56. 14000 £J (C >• t 03000 ^ tu S. «2000 156Hf C 3 0 1000

/Y , .-— T— M hfln^ I , O 160... (b) W 1 30 (O O 20 160Re S^L. proton decay ,5^ 15Sm I 10 60 59 U O I ' TdT W

n. JUiWkuui i Jl JU J 1234567 Decay particle energy ( MeV )

Figure 6 S 6 7 8 9 10 11

time of flight

Figure 7 1I1111I1

O O > OW N 05 O CO K Q) ^t1 C\2 CO O5 CO .^ CO —< Tj*

> rj« C\2 ^ CO CO A O) *-« 1*3 OS CO ^ VS CD CO CV2 «-« «-* O CO IO O Z

O O •""-eJI s "" - -

O IO

I I I I I I I I I I I ,. I — I II I I I O O O O O O O O O O to CO CV2

'ISMNVHO/SlNflOD

Figure 8 2000

1750

IBOO

1250

1000

750 I SO 100 500 1000 Incident Energy (UeV/nuclcon)

Figure 9 1.0

O CD 0.5 U) O

0.0 I I I I 8Li 9Li 11Li

Figure 10 100 (7 » 18.7 (0.8) MeV/c 60 1OCOc

-4 §a« 4 -a 40

v 3100 3190 3200 3290 p (MeV/c)

Figure 11 -Q

-a b "O

0.00 0.25 0.50 0.75 1.00 1.25 1.50 Ed (MeV)

Figure 12 Au 41 MeV/\i 11Be

26 ( En < 80 MeV

4f* IU

10 20 30 Neutron Lab. Angle "(deg.)

N 41 MeV/u 11Be on Au • Theoretical curves for ^^VxS.= 0.25, 0.5 and 1.0 MeV

10 10 20 Neutron Lab. Angle (deg.)

Figure 13