Ruhlig’s 1938 First-Ever Observation of the Fusion of A = 3 Ions with Deuterium: An Analysis of Secondary Reactions Following Deuteron-on-Deuterium Fusion in a Heavy Phosphoric Target

While studying d(d,n)³He fusion in 1938, Ruhlig observed protons with energies larger than 15 MeV. Ruhlig suggested that these high-energy protons were generated by tritium-on-deuterium fusion neutrons scattering protons out of a thin cellophane foil placed inside a cloud chamber. This led Ruhlig to hypothesize that he was observing secondary (in-flight) tritium-on-deuterium fusions and conclude that the d(t,n) reaction “must be an exceedingly probable one.” This was the first attempt to quantify the probability of d(t,n) fusion, using the ~1-MeV tritons generated by d(d,p)t fusion. This caused some Manhattan Project scientists to suggest that the d(t,n) cross sections are significantly higher than those for deuteron-on-deuterium fusion and led to the first measurement of d(³He,p) and d(t,n) cross sections in 1943. Here, we have used modern cross sections and stopping powers to estimate the expected numbers of high-energy protons associated with in-flight d(t,n) reactions in Ruhlig’s experiment. Our estimate is four orders of magnitude lower than Ruhlig’s observed rate. However, the number of high-energy protons in Ruhlig’s experiment can be obtained via simulation if the protons are assumed to have been emitted by secondary in-flight d(³He,p) reactions, with various plausible assumptions about the experimental geometry and target-backing thickness. Our calculations demonstrate that quantitative information about the fusion of A = 3 ions with deuterium could have been obtained via experiments similar to Ruhlig’s well in advance of the advent of ³He ion and triton beams in 1943. This opportunity seems to have been missed.