The measurement of the thermal neutron absorption cross sections of graphite and carbon by W. Bothe and P. Jensen – a recalculation of the experiments and the graphite problem during the early uranium work

Abstract The experiments of W. Bothe and P. Jensen from the year 1940 to determine of the thermal neutron absorption cross sections of graphite and carbon were recalculated with current neutron transport codes and latest nuclear data in order to find out the reasons for the negative results, which excluded the use of both graphite and pure carbon as moderators in a natural uranium reactor. One result of the recalculation is that the diffusion theory underlying the measurement of the graphite cross section was appropriate to the problem, that the boron content of the graphite investigated was at 6 appm, and that the thermal absorption cross section of 8 mb (or 7.58 mb in a re-evaluation of the experiment) as given by W. Bothe and P. Jensen was correct. On the other hand, the measured absorption cross section of the graphite impurities of 1.61 mb was too low by a factor of 2.64, resulting in a too high carbon absorption cross section of 6.4 mb. According to the current calculation, the absorption cross section of the carbon should only have been 3.38 mb. The recalculation of the ash disk experiment, on the basis of which W. Bothe and P. Jensen had determined the neutron absorption of the impurities, shows that there are four reasons for their underestimation, two of which can be quantified. The measurements of W. Bothe and P. Jensen take center stage of a more comprehensive investigation of the graphite work of the German Uranium Club (Uranverein) in the years 1939–1942. This shows, among others, that the negative result of the Bothe-Jensen measurement was not of decisive importance in the decision to reject graphite as a moderator in a nuclear reactor, but rather that it was erroneously assumed that it was practically impossible to remove the boron impurity from the graphite. The successful approach of L. Szilard and E. Fermi to solve the graphite problem during the early phase of the Manhattan Project is also escribed. Thereby, it reveals, that the measuring method attributed to W. Heisenberg, which was used in Germany by W. Bothe and P. Jensen to determine the thermal neutron absorption cross section of graphite, most probably originates from L. Szilard. Overall, it turns out that the members of the German Uranium Club, compared to the American scientists, made only minor or no efforts in order to develop procedures for the accurate characterization, the lossless ashing, and the sufficient purification of graphite, which are necessary for a successful application of this material in nuclear engineering.