Winner of the 1935 Nobel Prize in Physics, he discovered the neutron for the atomic bomb. ALS signed “J. Chadwick,” one page both sides, 8 x 10, personal letterhead, February 7, 1967. Responding to a student regarding recent experiments, Chadwick writes, in part: “I cannot answer properly your first question because you do not state the exact conditions under which the experiment was done. Moreover, in one paragraph you say that you noted 71 forking alpha particles and in the next paragraph you say that the number of forking alpha particles was almost zero. I can only take this to mean that you observed 71 large, or rather marked, deflexions of the alpha particles but that very few of these showed a fork or spur. This is only to be expected. The chance of a large deflexion depends on the atomic number of the atom which the alpha particle meets; in the collision energy will be transferred to the atom, according to the ordinary laws of impact, but the range of a heavy atom, heavily ionised, will be short and the spur will seldom be visible as a definite track. In the case of the B rays the deflexions are of course still mainly due to the atomic nuclei near which they pass, but the amount of energy transferred is very small. The occasions on which a B particle suffers such a close collision with an electron as to cause a fork, resulting in two electron tracks, will be very rare. The range of the alpha particle in the gas in your expansion chamber will depend only on the number of atoms it meets. The range, on average, will therefore vary inversely as the pressure of the gas and increase with the absolute temperature. This is indeed an elementary question to which any book on the subject will provide the answers. I must suggest that on all questions of this kind you should consult your teacher, who will be able to give a more detailed explanation and one which applies to the conditions of your experiment.” Central horizontal and vertical fold, a few light areas of brushing, and two small impressions at the bottom left corner, none of which affect the crisp signature, otherwise fine condition. In an impressive letter brimming with ample scientific jargon, Chadwick takes a step down from his own work to mentor a pupil in need. RRAuction COA.
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