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Explorations Far from Stability

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Explorations Far from Stability Explorations Far From Stability Nowadays various criteria are used in evaluating the various fields of research which compete for scarce funds. Probably Alvin Weinberg, in his Minerva article in 1963, was the first person to articulate a set of criteria for scientific choice. Prior to that time, and perhaps by many even at that time, no such articulation was deemed necessary. Since Weinberg's article appeared there has been some evolution and elaboration in this area, but the criteria are still usually broken down into the so-called intrinsic and extrinsic categories; and, where possible, some idea of the impact on society is included. The intrinsic criteria deal with the importance of the area under discussion to the science itself and to such factors as its ripeness for development, whereas the extrinsic look outward to see the immediate or eventual impact the field may have on other areas of science and/or technology. It would naturally be of interest to apply these criteria to the newly de­ veloping area of nuclear explorations far from the line of beta stability. First of all we should mention the two major conferences which have been held on thinubject. The 1966 Lysekil Conference1 on Why and How Should We Investigate Nuclides Far Off the Stability Line? turned out to be more prophetic than even such a bold title at first implied. In addition to providing a forum for the pioneers high on the walls of the valley of stability, the Conference also provided some of the earliest discussion of the superheavy elements-the island of stability around element 114. V. M. Strutinski presented his now famous method for combining the liquid drop and single particle models for the calculation of the energy of the nucleus as a function of deformation, and he gave some results for various elements including 112 and 116. H. Meldner presented the first calculation indicating that 298 114 would be the center of the new island, and Torb Sikkeland gave an excellent discussion of the types of heavy ion reactions that might be used to produce superheavy elements. In 1966, then, the discussions showed the strong possibilities for expanding considerably the horizons of nuclear physics. The succeeding conference, held at Leysin, Switzerland2 in 1970, reinforced in all respects the importance given these new areas at the Lysekil Conference. From the point of view of intrinsic criteria, the interest associated with studies far from the line of beta stability is indeed great because phenomena CONAPP A 95 can be studied there that cannot be studied anywhere else. Furthermore, from an experimental as well as a theoretical point of view, the field is obviously ripe for development. The theorist, R. A. Sorenson, has pointed out the importance of studying nuclei along the N = Z line, which is only possible (except for the lightest nuclei) far from stability. Along the N = Z line the neutrons and protons fill corresponding energy levels so that their single particle wave functions have the maximum opportunity for overlap. There is correspondingly a maximum opportunity for the study of neutron-proton correlation effects. A number of phenomena including beta decay and deformation effects should be better studied along the N = Z line than near stability where the neutron excess masks the relationship between the proton and the neutron seen when they are equal in number. Sorenson has also pointed out the value of discovering more doubly magic nuclei such as 1 ~~Sn. Such nuclei far from stability would afford an oppor­ tunity to study radioactive decay that the doubly magic nuclei in the valley naturally cannot give. Since the orderings and spacings of neutron and proton levels are difficult to calculate as a function of N and Z, only approximate values of new magic numbers are now available. So the experimentalists will have to ultimately search them out. Another central question involves the validity far from stability of the various currently accepted mass formulas. In practice these mass formulas are, of necessity, semiempirical. That is to say, they are based on some theoretical model, but they contain adjustable parameters obtained by experiment. Since most of the experiments so far have been near stability, the validity of the parameters and shell corrections far from stability, where they are sometimes needed, is questionable. Indeed, as we shall see, some illuminating results have just recently been obtained in this regard. The important phenomena to be studied far from stability include neutron-proton correlation energy and the effects on shell corrections and magic numbers of nuclear deformation. The recent results of K. S. Toth and R. L. Hahn3 and their coworkers illustrate the richness of this area. These investigators have been studying alpha emitters far from the line of stability with atomic numbers ranging from 67 to 76 such as 156Yb, 159Hf, and 1690s. These nuclei are produced by bombarding suitable targets with heavy ions. Some of their product nuclei are located 19 to 21 mass units away from the line of stability. The rather extensive results obtained have enabled Toth and Hahn to see the large effects of the N = 82 magic number, neutron pairing effects in the N = 86 to 88 region, and to obtain an experimental check of various mass equations far from stability'. The Kelson-Garvey mass formula, so good near stability, was found to give poor results, for example. 96 The predictions of stability for the superheavy elements rest largely on the validity of the mass equation of Strutinsky which features his prescription for applying shell corrections to the liquid drop model. According to the various calculations, the superheavies will occur around Z = 114 and N = 184. The parameters employed naturally influence the results strongly, however. So the extent of the island of stability in terms of N and Z (as well as its very existence) awaits experimental exploration. The highlights we have hit illustrate the scientific interest in nuclei far from stability, and now we will look at how they are to be produced. Spallation reactions from high energy protons produce a broad mixture of products on the neutron deficient side. These high energy protons can also produce mixtures of products on the neutron rich side by high energy fission. Important work has already been done in this area at accelerators such as CERN in Geneva and the AGS at Brookhaven. The LAMPF at Los Alamos will have an 800 MeV one milliampere proton beam so that this type of research will soon receive a big boost. The fact that mixtures of isotopes are produced has prompted a large effort at CERN in the development of an on-line mass separation system. This highly successful system has enabled the investigators 116 there to study such isotopes far from the stability line as Xe(T112 = 55 sec), 115 118 117 Xe(T112 = 19 sec), Cs(T112 = 16 sec), and Cs(T112 = 8 sec). The Los Alamos facility will also have an on-line mass separator. Heavy ion accelerators are also most useful in exploring the walls of the valley, and they are thought to also represent the only reasonable way to reach the superheavy island of stability. Heavy ions produce fission and spallation, but they also engage in multinucleon transfer reactions and com­ pound nucleus reactions giving specific products. For example, transfer reactions have enabled A. G. Artukh and his coworkers to produce many neutron rich isotopes of C, N, 0, and F. A typical reaction would involve a 232Th target being bombarded by an 180 beam with up to four neutrons being transferred to the projectile and up to four protons being transferred to the target. Again the need for mass separation on-line is obvious and Dubna has such a system. Also the ALICE accelerator at Orsay has a highly success­ ful system. The heavy ion isochronous cyclotron at Oak Ridge is currently being fitted with an on-line mass separator as well, and such a system will also be used at the Berkeley Super HILAC. For studies on the neutron rich side, mass separators have also been installed on-line at several reactors including the ones at Ames, Iowa (U.S.A.) and Studsvik (Sweden). It is therefore clear that on-line mass separation, a "must" for serious study in this field, is well underway as a frontier of techno­ logical development. Another area of intrinsic interest to nuclear physicists involves heavy ion reactions. The study of nuclei far from stability, perhaps most particularly CONAPP A2 97 the production of the superheavy elements, will almost certainly require new understandings of the mechanisms of heavy ion reactions. At present the interplay of the various important effects is relatively unknown because high energy ions beyond argon have only become available with the krypton beam at Orsay and the xenon beam at Dubna in the last year or so. Even in the region between neon and argon few studies have been made. The develop­ ment of the tandem cyclotrons at Dubna, the Super HILAC at Berkeley, the UNILAC at Darmstadt and ALICE at Orsay will alter this picture drastically over the next few years. Indeed, the Super HILAC and the UNILAC will offer a particularly flexible approach since they will have sufficient energy to put any ion up to uranium over the Coulomb barrier of any element. Although we have only hit some of the high points, it appears clear that there is plenty of inherent interest for the nuclear physicist in the regions far from stability, and it is also clear that the means for carrying out such studies are at hand. So the field is ripe for development.
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