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Investigations of extremely neutron-rich nuclei are bringing significant information; neutron halos, neutron skin and other surprises already appear on the frontier of the nuclear binding. We have initiated the experimental study of fission of U-projectiles at relativistic energies with the FRS (GSI) in which 117 new n-rich isotopes have been separated and unambiguously identified. They will be further investigated starting with half-life (T 1/2) and mass-excess (m.e.) measurements. The first or the second of these quantities has already been measured for 42 new n-rich isotopes of elements between Cu and Ti using different production techniques at the MP-tandem (IPN-Orsay), the online mass-separator (GSI-Darmstadt), the nuclear reactor (ILL-Grenoble), and the FRS (GSI).

In the preceding joint report to this conference1, the experiment installed at CERN on-line with ISOLDE for measuring masses of exotic nuclei, with a two stage double-focusing mass-spectrometer, has been described.

A Q-value of -31.5762(30) MeV has been measured for the 12C (3He, 6He)9C reaction. The mass of 9C, when compared to its Coulomb analog levels in 9B, 9Be, and 9Li, confirms a significant deviation from the quadratic form of the isobaric multiplet mass equation, with a cubic coefficient of d = 5.7 ± 1.5 keV.

The possibility of using mass spectrometric techniques to determine the masses of short lived radioactive isotopes was investigated in 1970 after the successfull measurements of short half lives using on line mass spectrometry1. At the AMCO IV conference -1971- two projects were presented; one from the Minnesota group2 using a double focusing mass spectrometer and the other from the Orsay group3 using a single stage instrument. At the AMCO V conference -1975- the results of the direct measurements of the masses of Li and 26–32Na were presented by Thibault et al.4. They showed that an accuracy of 100 keV could be reached for light elements even with a single stage mass spectrometer and that the masses of a chain of isotopes could be determined in a single experiment, both facts would allow us to gain information on the global properties of the nuclear mass surface as well as on the local peculiarities of nuclear structure.

In the last decade a large experimental program has been in progress at MSU with the aim of testing the isobaric mass multiplet equation (IMME) first proposed by Wigner.l This relation holds that the masses of the members of an isospin multiplet of mass A and isospin T can be expressed in a simple quadratic form: $$M(A,{T_z}) = a + b{T_Z} + cT_z^2$$ , where Tz is the isospin projection. There are numerous derivations of this formula in the literature,2,3 which employ perturbation methods to expand the nuclear Hamiltonian. In the above form, the lowest isospin multiplet which can be used to test the IMME is T = 3/2.

The isobaric multiplet mass equation (IMME), first propounded by Wigner1 in 1957, states that the masses M of members of an analog multiplet should be related by an equation quadratic in Tz $$ M = a + b{T_z} + cT_z^2. $$ A non-trivial test of the equation requires a multiplet of at least 4 states (i.e. TZ=3/2), and in 1964 Cerny and his collaborators completed the first isobaric quartet.2 Since that time, some 18 quartets have been measured, a few with extreme precision, and in only one case, the lowest mass 9 quartet, is there a significant disagreement with the IMME. The present status of isobaric quartets is summarized in this conference by Benenson and Kashy. There is considerable incentive to make new tests of the equation by considering multiplets other than quartets, but only very recently has it become experimentally feasible to test the IMME even in quintets. A number of features make such tests interesting. If one represents deviations from the IMME by additional terms dT Z 3 , eT Z 4 , etc., only one such term can be determined in a quartet, but two in a quintet. Thus one can test the IMME more rigorously, and, in the event of a violation, gain better insight into the possible mechanisms causing it. Furthermore, many quintets include both particle-stable and unbound nuclei, and if changes in the spatial wave functions across a multiplet can cause deviations from the IMME, then quintets may be rather sensitive to that influence. Finally, if there exists a many-body charge-dependent force, the presence of two determined parameters (d and e) makes quintets an attractive testing ground.

In the course of spectroscopic work with (p,r) reactions in the A = 90 mass region, we had found discrepancies between the reaction Q-values, i.e., the proton binding energies, derived from our measurements and those given in the 1964 mass table /1/ and also in the 1971 mass table /2/. Therefore, we made a systematic investigation in order to obtain more accurate values. The reactions studied were 88Sr(p,r)89Y, 89Y(P,r)90Zr, 9OZr(p,r)91Nb and 92Mo(p,r)93Tc.

Recent direct mass measurements are compared to precise Qβ-values. The corresponding beta spectra were recorded with an intrinsic Ge-detector which was calibrated with conversion electrons from the BILL spectrometer. By the use of a magnetic pre-separation recently only the high energy portion of the beta spectrum was recorded. Thus count rate limitations of the beta detector were circumvented allowing the use of stronger β-sources and yielding beta spectra with essentially zero background.

The characterization of the nuclidic mass in those regions which are far removed from the line of beta stability is of fundamental importance for testing and improving mass theories. An increasing component of our knowledge on nuclear properties of light exotic nuclei is derived from the analysis of mass-separated radioactivity, complementing the much more extensive investigations by this technique of heavy nuclei off the stability line.1 Such studies provide new insights into the limits of nuclear stability, the behavior of nuclear shapes and the onset of new radioactive decay modes.

In this paper some new unpublished mass-spectrometric measurements on sodium isotopes will be presented. In AMCO V1 mass measurements for A = 26 to 32 were already presented and had shown the onset of a deformation for A = 31 and 32. This had encouraged us to pursue these experiments in order to investigate what happens for isotopes further from stability: 33Na which was already observed to be bound but for which the mass could not be measured, and 34Na, the existence of which was questionable according to predictions. In addition, it seemed also important to improve the precision on 31,32Na for which an error bar of 1 MeV had been obtained while the observed anomaly was 2–4 MeV only.

Development within the past decade of improved shell model and deformation corrections to the liquid-drop mass law has not only improved its accuracy but also allowed use in more aspects of nuclear physics, such as calculation of fission barriers. Additional experimental data are thus available as input for determination of mass-formula parameters. Preliminary results including fission barrier heights and fission Q-values as input data will be presented. Statistical tests have been performed on fits to the 1971 mass table and to the input reactions and doublets, indicating a possible discrepancy between the two forms of data.

A description of the principles of operation and essential elements of the rf spectrometer was presented in 1967(1). Further details of procedures, a discussion of precision, and measurements of sixteen doublets yielding the mass excesses of H, D, 14N, 16O, 35C1 and 37C1 were given in a recent paper (2). It has been shown: (a)that, with the technique of visual peak matching, the standard “setting” error of a single reading is about 1/2500 of the half-width of a peak; and (b) that the frequency ratio (f2/fl) may easily be determined within a few parts in 1010 so that no calibration uncertainty arises from equating this to the mass ratio (M1/M2) for the highest precision attainable. Thus, in the absence of other, greater sources of error, masses should be determinable for resolutions achieved (2–4x105) from the mean of ten readings with an accuracy between 3 and 6 parts in 1010. After elimination of a number of purely instrumental sources of error, the primary known remaining sources that limit precision are small electric fields that do not shift with the applied voltages in inverse proportion to the two masses being compared. Their presence is readily shown by the study of wide “calibration” doublets for which the differences in mass numbers are one or two units. Considerable progress in neutralizing the effects of these fields has been made by applying small adjustable d.c. voltages in series with the high voltage applied to the ion source and with the lesser voltages applied to one of each pair of external deflectors and to each internal deflector as explained in reference (2).

In many respects, the techniques used in preparing the 1971 Atomic Mass Evaluation (1–4) were taken from the several papers relating to the 1964 Atomic Mass Adjustment (5–7) and the 1961 Atomic Mass Adjustment (8,9). Indeed, the connections are very close, in that one of the authors (A. H. Wapstra) of the ‘71 version was also an author of the ‘64 and ‘61 adjustments and both authors of the ‘71 version benefited from frequent assistance of the Mainz group. Research supported in part by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation.

For several reasons, the interest of nuclear physicists is continuously shifting towards nuclei further and further removed from stability. In low mass nuclides one wants to study nuclei with high values of the isospin. For very heavy ones, the matter of the possible existence of an island of stability near at mass number 300 is a popular subject. And at intermediate mass number, the possible existence of new regions of deformation receives much attention.