Chadwick's actual work is somewhat obscure and rarely taught in any detail. A mention of the discovery, in 1932, is the only reference in an introductory course. The topic is rarely raised in more advanced lectures.

IB published a question in the May 2000 that is helpful. The question, (which in a one small part was somewhat unfair), reports one of Chadwick's efforts to find the mass of the neutron. Most of the question is not about Chadwick at all, but is a simple conservation of momentum and energy question, that many candidates would have found straight forward.

The interested reader is asked to read Chadwick's original paper, which can be found on the web. They will find that the year was 1932 not 1936, but they will find, that the velocities of ejected protons and nitrogen atoms were estimated as repored. The mass estimate is also reported correctly. Readers will also find that Chadwick did not do some of the work himself - he used the results of others. More importantly, his estimate of the accuracy of the mass obtained in this way was ±10%. He went on to determine the mass of the neutron in a more reliable way, finding it to be only very slightly more than that of the proton.

The reader will note his belief, current at the time, that the nucleus was made up of alpha particles with the balance of protons and electrons. The paper is interesting, easy to follow, and written for the most part in the past passive. The style is remarkably modern apart from the occasional use of archaic expressions like ... whence ... and ... grave. He has the excellent referencing of other people's work that one would expect from a Fellow of the Royal Society, but the references are in footnotes, rather than collected at the end of the paper.

Ed.

Question 4

Part 1 Chadwick and the discovery of the neutron.

(a) In 1936 Chadwick carried out an experiment in which he bombarded beryllium (⁹Be) with a-particles. He found that a very penetrating radiation was produced and also another element. He believed that this penetrating radiation consisted of energetic neutrons. Assuming that neutrons are produced in this interaction, write down the equation for the interaction of an a-particle with a nucleus of beryllium(9) and identify the other element produced.

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(b) To determine the mass of the particles he carried out two further experiments.

(i) In the first of these Chadwick bombarded a slab of paraffin wax with the radiation produced in the above reaction. The radiation ejected protons from the wax. Describe briefly the principle by which Chadwick measured the velocity of these ejected protons.

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(ii) In the second experiment Chadwick allowed the unknown radiation to enter a Wilson cloud chamber containing nitrogen(14). He observed tracks that he assumed were produced by ionised nitrogen atoms being "knocked on" by collision with the neutrons.

Describe briefly how Chadwick determined the velocity of the nitrogen atoms.

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(c) Chadwick now applied the ideas of conservation of momentum and energy to the data he obtained from the recoil experiments. The diagram below shows a neutron mass m moving with speed v about to collide head on with another particle of mass M.

 

The collision takes place along a line joining the two particles.  After the collision the neutron rebounds with a speed vÅå and the other particle moves off with a speed V as shown in the diagram below. The collision is perfectly elastic.

Write down an equation in terms of the respective masses and velocities of the two particles that expresses the ...
                 

(i)   ...  conservation of momentum.

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(ii) ...  conservation of mechanical energy.

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(d) It can be shown that in such a collision ...

In the experiments with protons and nitrogen in 1936 Chadwick obtained the following data:

Maximum speed of protons ejected from paraffin wax = 3.3x10⁷ m/s
Maximum speed of nitrogen ions in a cloud chamber = 4.7x10⁶ m/s

If the mass of a nitrogen atom is 14 mp where mp is the mass of a proton, show, using the equation above and this data, that the mass of the neutron mn is determined as....

mn =1.16 mp

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