Resonance Transmutations of Carbon by Protons

Abstract

DURING the investigation of the capture radiation of light elements bombarded by protons, we have found for carbon the following results. The intensity of capture radiation was measured for different proton energies by the number of coincidences produced in a pair of Geiger-Müller counters. The coincidences were caused by electrons released from an aluminium plate by the incident Î-radiation. The quantum energy of this radiation was determined by finding the range of the electrons in millimetres of aluminium absorber placed between the counters. The intensity of Î-ray emission at different bombarding voltages is shown by Curve 1 of Fig. 1. Two resonances are clearly observed at 400 and 500 kv. approximately. With a bombarding proton energy of 560 kv., corresponding to the second peak of this curve, we find the Î-ray energy to be 7.4 Mv., as deduced from the end-point at 11 mm. of aluminium (Curve 1, Fig. 2). This is in marked disagreement with the value of the Î-ray energy to be expected from the reaction However, if we consider the reaction it is found by calculation, using accepted mass values and taking into account the energy of the incident protons, that the Î-ray energy is 8.2 Mv. for this process. This reaction has not previously been observed, and the following experiments confirm that the resonance peak of Fig. 1 at 560 kv. must be ascribed to the rarer isotope ¹³C and not to ¹²C.