A unique set of transition matrix elements for the deuterium(vector deuteron,photon) 4-helium reaction at E(beam) = keV.

摘要

The synthesis of the light elements in the first thousand seconds of the evolution of the Universe can be used to test the standard cosmology. Big-Bang Nucleosynthesis (BBN) is the only observational test of the hot big-bang model at early times, and the comparison between the calculated and observed abundances of 2H, 3He, 4He, and 7Li can be used to constrain the predicted baryon density of the Universe as well as the number of species of light particles. To reliably calculate the light element abundances it is necessary to reduce uncertainties in the BBN network reaction rates. The deuteron-deuteron radiative capture reaction is one of the deuterium-burning processes for which the cross section is not known well at very low energies (10–100 keV). Polarized beams allow accurate extraction of the complex transition matrix elements (TMEs) from the measured analyzing powers, such studies of the reaction mechanisms should lead to a reliable extrapolation of the cross section to energies relevant to BBN.; Prior to any polarized beam measurements, the 2H(d,γ) 4He reaction was believed to proceed entirely via electric quadrupole (E2) radiation at very low energies, a result of s-wave (angular momentum of the two deuterons l = 0) capture. However, low-energy measurements of 2H(,γ)4He analyzing powers revealed the presence of non-E2 radiation, appearing because of the presence of other than s-wave capture terms. Those studies showed that 50 to 85% of the total capture strength is due to p-wave (l = 1) capture. Unfortunately, the data analysis did not allow an unambiguous set of TMEs to be extracted, so that a critical test of theoretical results could not be performed.; In my work we were able to reduce uncertainties and expand the angular range in the measurements of the polarization observables. We obtained angular distributions for vector and tensor analyzing powers A_y and T₂₀ by stopping a polarized deuteron beam of E _d(lab) = 115 keV in a heavy-water ice target. The analysis of the data included l = 1 partial waves in addition to the s-wave (E2) capture term, corresponding to electric dipole (E1) and magnetic quadrupole (M2) radiation. A unique fit was obtained with an s-wave (E2) capture strength of 55 ± 8% of the total strength, and a p-wave strength of 29 ± 6% (E1) and 16 ± 3% (M2), for a total of 45 ± 7%. The measured analyzing powers and the results of the TME analysis were compared to refined resonating-group-model calculations. The observed and the calculated transition amplitudes were found to be in good agreement, although the p-wave phases disagree with the experimentally determined results, which will require further comparison of our analysis with the calculation.; Finally, the p-wave strength established by this work leads to the extrapolated value of the astrophysical S-factor for the 2H(,γ)4He reaction of 2 times lower than the previous best calculation.