Particle Velocity Measurements Made by Observing Cerenkov Ring Images with Image Intensifiers

Ring images produced by a lens from C&#x0010Cerenkov radiation have been observed by using an image-intensifier system. As a result of the passage of a single fast particle through the Cerenkov radiator, a photograph of the output of the system shows a ring image defined by some 12 to 26 spots, the precise number varying according to the conditions. The calculated number, and distribution in number, of spots is shown to be in agreement with the experimental results. A least squares method is used to fit a circle to the set of points defining a ring. The precision in fitting is such as to give a standard deviation in measurement of particle velocityß(= v/c) of 2 x 10-4 and in measurement of particle direction of ~5 milliradians. The least squares method used is of an unorthodox nature and its performance is checked by a simulation programme in which points representing a ring are generated by a computer. It is found that the accuracy in determining ring radius for the experimental rings is similar to that for the computer-simulated ones.