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Register a new userAbsence of low-temperature dependence of the decay of 7Be and 198Au in metallic hosts
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The electron-capture (EC) decay rate of 7Be in metallic Cu host and the beta-decay rate of 198Au in the host alloy Al-Au have been measured simultaneously at several temperatures, ranging from 0.350 K to 293 K. No difference of the half-life of 198Au between 12.5 K and 293 K is observed to a precision of 0.1%. By utilizing the special characteristics of our double-source assembly, possible geometrical effects that influence the individual rates could be eliminated. The ratio of 7Be to 198Au activity thus obtained also remains constant for this temperatures range to the experimental precision of 0.15(0.16)%. The resulting null temperature dependence is discussed in terms of the inadequacy of the often-used Debye-Huckel model for such measurements.
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The half-life of 7Be implanted in a C60 pellet and gold foil has been measured to be about the same within about 0.2%. Using a radiochemical technique, we also measured that the probability of formation of endohedral 7Be@C60 by nuclear implantation technique was (5.6+-0.45)%. It is known from earlier works that the half-life of endohedral 7Be@C60 is about 1.2% shorter than that of 7Be implanted in gold. An analysis of these results using linear muffin-tin orbital method calculations indicates that most of the implanted 7Be ions in fullerene C60 stay at a distance of about 5.3 Angstrom from the centers of nearest C60 molecules forming exohedral compounds and those who enter the fullerene cages go to the centers of the cages forming endohedral 7Be@C60 compounds.
We have measured the cross section of the 7Be(p,gamma)8B reaction for E_cm = 185.8 keV, 134.7 keV and 111.7 keV using a radioactive 7Be target (132 mCi). Single and coincidence spectra of beta^+ and alpha particles from 8B and 8Be^* decay, respectively, were measured using a large acceptance spectrometer. The zero energy S factor inferred from these data is 18.5 +/- 2.4 eV b and a weighted mean value of 18.8 +/- 1.7 eV b (theoretical uncertainty included) is deduced when combining this value with our previous results at higher energies.
The interpretation of the most recent solar neutrinos experiments requires a good knowledge of the cross section of the reaction 7Be(p,gamma)8B at very small energy (Ecm=18 keV). We have recently measured this cross section for Ecm=0.35-1.4 MeV and for Ecm=0.112-0.190 MeV. We report here on the description of the preparation of the radioactive targets of 7Be used in these experiments.
The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis calculations. The present work reports on a new precision experiment using the activation technique at energies directly relevant to big-bang nucleosynthesis. Previously such low energies had been reached experimentally only by the prompt-gamma technique and with inferior precision. Using a windowless gas target, high beam intensity and low background gamma-counting facilities, the 3He(alpha,gamma)7Be cross section has been determined at 127, 148 and 169 keV center-of-mass energy with a total uncertainty of 4%. The sources of systematic uncertainty are discussed in detail. The present data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3He(alpha,gamma)7Be astrophysical S-factor to solar energies.
An exclusive measurement of the Coulomb breakup of 8B into 7Be+p at 254 A MeV allowed to study the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates and that E2 multipolarity can be neglected. By using a simple single-particle model for 8B and treating the breakup in first-order perturbation theory, we extract a zero-energy S factor of S-(17)(0) = 18.6 +- 1.2 +- 1.0 eV b.
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