ON THE EXISTENCE OF THRESHOLD STATES IN ALUMINUM-26 AND THEIR CONTRIBUTION TO ITS PRODUCTION AT LOW STELLAR TEMPERATURE.

摘要

Certain meteoritic inclusions have been shown to have an abundance of ('26)Mg which is in excess of the solar system abundance by as much as 40%. These observations further suggest that this excess has resulted from the in-situ decay of ('26)Al (T(, 1/2) = 7.2 x 10('5) years), an important astrophysical chronometer. It appears that a likely site for the synthesis of ('26)Al responsible for the observed abundance anomaly is in the hydrogen-burning zone of a supernova (characterized by temperatures near 1 x 10('8)(DEGREES)K). Accordingly, detailed knowledge of the behavior of the ('25)Mg(p,(gamma))('26)Al reaction at such low temperatures is clearly necessary.;The present results indicate that the 6400-keV state plays an insignificant role in the production of ('26)Al, but that production through the 6343-keV state dominates the production rate for temperatures T (LESSTHEQ) 8 x 10('7)(DEGREES)K, and provides a significant enhancement over previous determinations. In consequence, a viable mechanism for the production of ('26)Al through hydrogen burning at temperatures below 1 x 10('8)(DEGREES)K may exist.;A candidate for a low energy resonance in the ('25)Mg + p system has been discovered, through study of the ('25)Mg(('3)He,d)('26)Al reaction, at an excitation energy E(,x) = 6343 keV, which corresponds to an incident proton energy E(,c.m.) = 37.2 keV. The shape of the angular distribution for this state is characteristic of l = 0 + 2 angular momentum transfer and therefore a spin-parity J('(pi)) = (2,3)('+). Measurements on the ('25)Mg(d,p)('26)Mg reaction indicate that the 6125-keV state in ('26)Mg is the analog to this ('26)Al state; s-wave spectroscopic factors S (TURNEQ) 0.14 are determined for both cases. Studies on the ('26)Mg((alpha),(alpha)')('26)Mg reaction indicate that the 6125-KeV ('26)Mg state possesses unnatural parity, and so J('(pi)) = 3('+) is indicated for the analog pair. The gamma-ray branching ratios for the 6343-keV ('26)Al state have been measured using the ('27)Al(('3)He,(alpha)(gamma))('26)Al reaction and have established that the decay of this state ultimately populates the ground state 57% of the time. As a result, a resonance strength (omega)(gamma) = 5.7 x 10('-16)eV is determined for this state. Similarly, an upper limit (omega)(gamma) (LESSTHEQ) 1.0 x 10('-11) eV is found for the possible, but un-observed resonance at E(,x) = 6400 keV, studied via the ('24)Mg(('3)He,p(gamma))('26)Al reaction.