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Simple Nature - Light and Matter

Simple Nature - Light and Matter

Simple Nature - Light and Matter

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h / Chadwick’s subatomic pool table. A disk of the naturally occurringmetal polonium provides a source of radiation capable of kickingneutrons out of the beryllium nuclei. The type of radiation emitted bythe polonium is easily absorbed by a few mm of air, so the air has to bepumped out of the left-h<strong>and</strong> chamber. The neutrons, Chadwick’s mysteryparticles, penetrate matter far more readily, <strong>and</strong> fly out through the wall<strong>and</strong> into the chamber on the right, which is filled with nitrogen or hydrogengas. When a neutron collides with a nitrogen or hydrogen nucleus, itkicks it out of its atom at high speed, <strong>and</strong> this recoiling nucleus then ripsapart thous<strong>and</strong>s of other atoms of the gas. The result is an electricalpulse that can be detected in the wire on the right. Physicists had alreadycalibrated this type of apparatus so that they could translate the strengthof the electrical pulse into the velocity of the recoiling nucleus. Thewhole apparatus shown in the figure would fit in the palm of your h<strong>and</strong>, indramatic contrast to today’s giant particle accelerators.just put on a scale <strong>and</strong> weigh. Chadwick came up with an ingenioussolution. The masses of the nuclei of the various chemical elementswere already known, <strong>and</strong> techniques had already been developed formeasuring the speed of a rapidly moving nucleus. He therefore setout to bombard samples of selected elements with the mysteriousnew particles. When a direct, head-on collision occurred betweena mystery particle <strong>and</strong> the nucleus of one of the target atoms, thenucleus would be knocked out of the atom, <strong>and</strong> he would measureits velocity.Suppose, for instance, that we bombard a sample of hydrogenatoms with the mystery particles. Since the participants in thecollision are fundamental particles, there is no way for kinetic energyto be converted into heat or any other form of energy, <strong>and</strong> Chadwickthus had two equations in three unknowns:equation #1: conservation of momentumequation #2: no loss of kinetic energyunknown #1: mass of the mystery particleunknown #2: initial velocity of the mystery particleSection 3.1 Momentum In One Dimension 139

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