The 3H(alpha,gamma)7Li Reaction at Low Energies

Thesis by Carl Richard Brune
California Institute of Technology, 1994


The absolute cross section of the 3H(alpha,gamma)7Li reaction has been measured for 50 <= Ec.m. <= 1200 keV. Specially prepared Ti-3H targets were bombarded with an alpha beam, and gamma rays were detected using an 85% high-purity germanium detector. Total S-factors and the branching ratios for radiative capture to the two final bound states are reported for the entire energy range. Angular distributions of the capture gamma rays were measured for nine energies in the range 115 <= Ec.m. <= 1200 keV. Legendre coefficients extracted from fits to the angular distributions are also reported. The preparation and characterization of the radioactive Ti-3H targets are discussed.

This experiment is motivated by models of big-bang nucleosynthesis, which require the 3H(alpha,gamma)7Li reaction rate for computing the primordial 7Li abundance. Our results have considerably smaller uncertainties and reach lower energies than previous experiments. For E <= 150 keV, we find the S-factor to be considerably smaller than indicated by previous experiments. The measured branching ratio is found to be approximately energy independent, with a value of 0.45. The energy dependence of the measured S-factors is in agreement with existing theoretical calculations. The new results are used to calculate the thermonuclear reaction rate for temperatures below 10 GK.

Here you can download the final version of my thesis, complete with figures. This copy is identical to the one that I provided to the Caltech library. Also included is a text file which provides the essential results in numerical form.

Also note that most of the results in my thesis have been published in:
The 3H(alpha,gamma)7Li reaction at low energies
C. R. Brune, R. W. Kavanagh, and C. Rolfs, Phys. Rev. C 50, 2205-2218 (1994).

Carl R. Brune / brune _at_
Last updated 5 November, 2010.