Nuclear Physics

Abstract

THE British Association discussion at Norwich on nuclear physics on September 5 was opened by Lord Rutherford. After a review of progress resulting from the application of high voltages to nuclear transmutation, he passed on to discuss the rapid development of our knowledge of artificial radioactivity and in particular the production of such radioactivity by neutron bombardment. Neutrons, being uncharged, penetrate the heaviest nuclei without difficulty, and radioactive isotopes of the great majority of the elements have already been discovered. The effectiveness of neutrons in producing some types of transmutation is increased largely by slowing them down by passage through paraffin, water or other substances containing hydrogen, the neutrons losing energy by collision with the protons. By slowing them down in this way to thermal velocities, their adsorption by boron is increased 350 times, by cadmium 3,000 times, and by gadolinium 30,000 times, the effective cross-section for capture then being of the order of 10-20 sq. cm. A demonstration that neutrons actually obtain thermal velocities is afforded by slowing them down in paraffin wax cooled to liquid air or liquid hydrogen temperatures. This extra cooling reduces the velocity still further, and an increase in the efficiency of disintegration has been observed by P. B. Moon and others.