Observation of Substantialp-Wave Capture Strength in theLi7(

Abstract

The 7Li(p, y)SBe reaction has been studied using polarized protons having an incident energy in the range E~(lab) = 80 -0 keV. The measured angular distributions of the cross section cr(8)/Ao and analyzing power AY(8) were interpreted using a model independent transition-matrix element analysis.The large (-40%) analyzing power at 90' indicates the presence of an appreciable p-wave capture strength at these energies. The direct-capture model is used to show that this could reduce the astrophysical S factor by as much as 40%. PACS numbers: 25.40.Lw, 24.70.+s, 95.30.CqThe radiative capture of low energy protons on Li to excited states of Be is of interest for several reasons. Understanding the dynamics of the capture process is in itself worthwhile. And this understanding should lead to a more accurate and reliable extraction of the astrophysical S factor and the thermonuclear reactivity for the 7Li(p, y)sBe reaction. In addition, isospin symmetries can be used to relate the 7Li(p, y)sBe reaction to the 7Be(p, y) B reac-

8Be Level Properties from the Low-energy 7Li(p, γo)8Be Cross Section

“…(1992) are not supported. This best fit gives values of aI, a2 and b2 at Ep = 70 keVin agreement with those obtained in the earlier fits to the Chasteler et al (1994) data, but there is significant disagreement for b1 , leading to X 2 = 51·55. As in the fit of Zahnow et at.…”

“…There is also good agreement with the low-energy values of aI, a2 and b1 obtained in earlier fits (Chasteler et al 1994;Barker 1995) to the data of Chasteler and Schlueter et al (1964) (triangles). The curves are as in Fig.…”

“…In our earlier work (Barker 1995), the fit to the low-energy data (Chasteler et al 1994) led to relative signs of the reduced width amplitudes of the sBe 1 + levels at 17·64 and 18 ·15 MeV that were different from those obtained in the fit (Barker 1979) to the higher-energy data (Ulbricht et al 1977), and the latter fit in particular suggested signs different from shell model values. Here we have found satisfactory fits to all the higher-energy data, including the recent measurements of Zahnow et al (1995), using two approaches in which the El component of the 7Li(p, /'o)SBe cross section is attributed to either 1-levels of sBe or direct capture (Figs 1-4).…”

“…There has recently been renewed interest in the low-energy 7Li(p, /'o)SBe cross section, following work by Chasteler et al (1994) which gave evidence for a substantial p-wave strength, of uncertain origin. They pointed out that this could significantly reduce the zero-energy astrophysical S factor obtained by assuming pure s-waves, and suggested that a similar phenomenon might be present in the 7Be(p, /'o)8B reaction, which is of importance in the solar neutrino problem.…”

“…1. The points at 70 keV are from previous fits to the data of Chasteler et al (1994); they are the best fit for all transition matrix elements independently adjustable (Chasteler et al 1994, as revised in Barker 1995) (square), and for the Ml contribution attributed to the 17·64 and 18· 15 MeV levels of 8Be, as in Fig. 1 of Barker (1995) (diamond).…”