Mass Measurements of Neutron Excessive Nuclei by the (11B,8B) Reaction
In recent years a number of experimental techniques have been developed to establish the existence of light nuclei which are particle stable on the neutron excessive side of the valley of beta-stability (1). Figure 1 shows the present status of observed particle stable nuclei below silicon, together with the boundaries predicted by two theoretical calculations (2, 3), Most of the new nuclei have been produced in the bombardment of heavy targets with high energy protons or heavy ions. In the work of Poskanzer’s group (4,5,6) a uranium target was bombarded with protons of several GeV, and the new isotopes of 11Li, 14,15B and 17C were detected in a telescope of solid state counters. A similar technique has been used by Thomas et al. (7) to demonstrate the stability of 19N, 21O and recently (1) they may have observed 19C. A further eleven new isotopes, 18C, 20, 21N, 22, 23, 24O, 23, 24, 25F and 25, 26Ne were reported by Artukh et al. (8,9,10), formed in the bombardment of Th by 18O. The stability of 11Li and of five neutron rich isotopes of Na has been demonstrated by Klapisch et al. (11) using an on-line mass spectrometric technique. As illustrated in figure 1, none of these methods is able, at the present time, to yield accurate information on nuclear masses, and such information is essential for the development of theoretical predictions on the limits of nuclear stability. For example, the nuclei 11Li and 19C were initially predicted to be unbound by the Garvey-Kelson mass relations (2,12) although these nuclei are now known to be particle stable. Figure 1 shows further that Garvey and Kelson predict the last stable isotope of oxygen to have mass 28, whereas Vinogradov and Nemirovsky predict (3) the limit at mass 24. Similar discrepancies exist for other isotopes. It seems likely that further progress will only be made by making accurate mass measurements of neutron rich nuclei in order to make a systematic comparison with theoretical predictions. In this paper we report on the feasibility of using the three neutron transfer reaction (11B, 8B) as a technique for direct mass measurements, and apply the method to a determination of the mass-excess of the T=5/2 nucleus 29Mg.
KeywordsMass Measurement Composite Spectrum High Energy Proton Neutron Rich Isotope Uranium Target
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