Open AccessThe decrease in velocity of α particles from radium C
Author(s) -
G. H. Briggs
Publication year - 1927
Publication title -
proceedings of the royal society of london. series a, containing papers of a mathematical and physical character
Language(s) - English
Resource type - Journals
eISSN - 2053-9150
pISSN - 0950-1207
DOI - 10.1098/rspa.1927.0045
Subject(s) - deflection (physics) , atomic physics , magnetic field , electron , geiger counter , drift velocity , physics , chemistry , optics , nuclear physics , quantum mechanics
The early measurements of the decrease of velocity of α particles in passing through matter were made by Rutherford and Geiger, using the magnetic deflection method and observing by means of a zinc sulphide screen. The lowest velocities recorded were 0·43 V0 by Rutherford and 0·2 V0 by Geiger. Later Marsden and Taylor, using the same method, made measurements with air, mica and metal foils as the absorbing materials. It appeared from their experiments that something abnormal happened when the velocity had been reduced to about 0·415 V0 , for as the thickness of absorbing material was increased beyond this point, the observed velocity remained unaltered. The simplest explanation of this is that these early experiments were probably carried out with a residual gas pressure high enough for the exchange He++ ⇆ He+ to be frequent. Under these conditions it can readily be shown from the data given by Rutherford for capture and loss of electrons by α particles that when the velocity lies between 0·4 V0 and 0·3 V0 the deflection in a magnetic field remains practically constant and equal to that of He++ with a velocity of 0·4 V0 . The problem of the true shape of the velocity curve towards the end of the range has been attacked in a number of ways. Kapitza measured the energy of a beam of α particles at points along the range by the heating effect. He has also examined by the Wilson cloud method the curvature of α ray tracks in very strong magnetic fields. From this data, by calculating the average charge on the α particle from Rutherford’s experiments mentioned above, he deduced the velocity curve for the region between 5 and 20 mm. from the end of the range in air. I. Curie has assumed that the rate of loss of energy for a single α particle is proportional to the number of ions produced per unit length of the path, and from her measurements of the ionisation or Bragg curve for a beam of rays and measurements of the straggling, she has deduced the form of the Bragg curve for a single average α particle. From this she has calculated on the above assumption the form of the velocity curve over the whole range. Blackett has also deduced the form of the velocity curve near the end of the range from the scattering observed by the Wilson cloud method.