Measurement of the H 2 (p,γ) He 3 S factor at 265-1094 keV

Recent astronomical data have provided the primordial deuterium abundance with percent precision. As a result, big bang nucleosynthesis may provide a constraint on the universal baryon to photon ratio that is as precise as, but independent from, analyses of the cosmic microwave background. However, such a constraint requires that the nuclear reaction rates governing the production and destruction of primordial deuterium are sufficiently well known. Here, a new measurement of the H2(p,γ)3He cross-section is reported. This nuclear reaction dominates the error on the predicted big bang deuterium abundance. A proton beam of 400-1650 keV beam energy was incident on solid titanium deuteride targets, and the emitted γ rays were detected in two high-purity germanium detectors at angles of 55° and 90°, respectively. The deuterium content of the targets has been obtained in situ by the H2(He3,p)He4 reaction and offline using the elastic recoil detection method. The astrophysical S factor has been determined at center of mass energies between 265 and 1094 keV, addressing the uppermost part of the relevant energy range for big bang nucleosynthesis and complementary to ongoing work at lower energies. The new data support a higher S factor at big bang temperatures than previously assumed, reducing the predicted deuterium abundance.