The accurate determination of the energy released in certain nuclear transformations

Abstract

During the last few years, very definite information has been obtained on the various modes of transformation of certain elements, particularly of lithium, when bombarded by protons and heavy hydrogen. The nature of these transformations has received strong confirmation by examining the effects produced when each of the isotopes of lithium of mass 6 and 7 is separately bombarded. Sufficient quantity of these two isotopes—of the order of a microgram—has been obtained to settle the groups of liberated particles to he ascribed to each isotope under the two types of bombardment. In the pioneer experiment on these transformations, the range in air of the emitted α -particles or protons was approximately determined by finding the thickness of mica required to stop the particles In general, no special precautions were taken to measure the ranges in air with accuracy. The counting chamber and the first valve of the linear amplifier were Usually mounted on rubber sponge to avoid disturbances due to vibration, so that the position of the counting chamber relative to the source was difficult to fix with certainty. In addition, even in the most favourable cases, the range of the particles is difficult to measure with the same accuracy as the range of a group of α -particles emitted from a clean radioactive source. Apart from the probability variations dependent on the number of particles counted, the intensity of the bombarding source is liable to vary capricious during the time required for a complete experiment. Moreover, the straggling of the particles is more pronounced than in the usual radioactive case, partly on account of the difficulty of preserving a smooth radiating surface under intense bombardment, and partly also on account of the sensible depth of penetration of the bombarding ions. The actual stopping power of mica relative to air for the fast particles is not known with the certainty desired, introducing a serious difficulty in accurate measurements. In order to test whether the conservation of energy holds in these transformations, it is necessary to know the energy of the emitted particles with considerable precision. With this information, we are able to deduce the relative mass numbers of the isotopes involved, and thus obtain a check on the values obtained by direct measurements with various types of mass spectrograph. When there is clear evidence of the mechanism of transformation, the measurement of the energy changes allows us to deduce with considerable certainty the masses of new isotopes which in some cases are produced during transformations.