Image from page 703 of "The Bell System technical journal" (1922)
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observed at Cambridge that themaximum range of protons projected straight forward is greater than the maximumfound among those projected almost straight backward: for boron^the two valueswere 58 and 38, for aluminium 90 and 67 (in air at 760 mm. and 15° C.) 650 BELL SYSTEM TECHNICAL JOURNAL solid angles to the least practicable values. But in practice they can-not be reduced to low values, because the ejection of a proton is soinfrequent an event that if one observed only those coming off withinsay a degree of a certain chosen direction, they would be altogethertoo few to be profitably observed during any reasonable period of time.The same thing would happen, if one used a beam of alpha-particlesof similar narrowness; the impacts would be few because the impingingcorpuscles were few. It is therefore to be feared that under the bestof possible conditions, the horizontal segments in the curves will beshorter, the descents broader and smoother, than under ideal conditionsthey would be.*
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10 15 25 30 Fig. 9- 20CM AIR -Distribution-in-range curve obtained by differentiating the curve of Fig. 8for boron (\V. Bothe and H. Franz). Another result emerges from the experiments of Franz, and those ofthe Cambridge school: the mean speeds of the groups apparentlydiminish with the speed of the a-particles. This is the effect whichPose observed with the slowest of the groups which he detected, nothowever with the faster and sharply-marked two; but from the ex-periments of the others, it seems to be the rule—not that the otherstested all of the groups by varying the speed of the a-rays, far from it!but rather, for all which they did test, they found that sort of a de-pendence. Whether the constancy of speed of the groups which Posestudied is a peculiar feature of these, or his were the better experiments,I would not venture to say. At all events it is obvious that whereverthis effect enters in, the natural sharpness of the groups is bound to beblurred by the differences in the sp
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