Mass Measurements of Neutron Excessive Nuclei by the (¹¹B,⁸B) Reaction

Abstract

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, 21}N, ^{22, 23, 24}O, ^{23, 24, 25}F and ^{25, 26}Ne 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.