Total Cross-Section of Hydrogen and Carbon for 153-MeV. Neutrons

WE have used the 110-in. Harwell cyclotron 1 as a source of high-energy neutrons for total cross-section measurements in good geometry. An internal beryllium target 8.9 mm. thick was bombarded by about 1 µ amp. of 171-MeV. protons, and neutrons emerging near the forward direction were collimated by a series of 5-cm. diameter holes through three concrete screening walls. A polythene disk 19 mm. thick and 6.3 cm. in diameter was set up in the beam at a distance of 16 metres from the target. Protons ejected from the disk at an angle of 10° to the forward direction were detected by a triple-coincidence proportional counter telescope. Carbon absorbers placed between the counters in the telescope ensured that only protons with an energy greater than 141 MeV. were counted. This placed a lower limit of 145 MeV. on the energy of the neutrons detected, while an upper limit of 169 MeV. was set by the energy of the primary protons in the cyclotron. In a subsidiary experiment, the high-energy part of the neutron-energy spectrum was investigated by varying the amount of carbon absorber in the telescope. The results showed that the effective mean energy of the neutrons during the total cross-section work was 153 MeV.