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The output of betatron and synchrotron accelerators consists of a regular succession of pulses of a few microseconds duration. This fact is generally of little consequence in applications involving integrating exposures of detecting apparatus. The use of nuclear emulsions to detect disintegration products from nuclei and the induction of artificial radioactivity are the most important examples of this group. With the Wilson cloud chamber, however, i t is essential to limit the irradiation of the cloud chamber to one or two output pulses which are timed to occur during the sensitive period following expansion of the chamber. The intense ionization produced by an accelerator, which is operated to give continuous output, paralyses the chamber and prevents the formation of tracks which are separately distinguishable. The following account deals with a circuit allowing control of the number and timing of the X-ray pulses from a 14 MeV. electron synchrotron when operated in conjunction with a rubber diaphragm-type cloud chamber which has been described previously (Darby and Swan 1948). The chamber automatic control mechanism causes expansions to occur approximately once per minute and a timing signal for the circuit is derived a t the completion of the movement of the expansion valve. Thus the synchronizing action is started a t the beginning of the sensitive time, regardless of variations in the time taken for the mechanical motion. This signal gives rise to a gate pulse of variable duration which allows one of the periodically occurring electron injection trigger pulses to pass on and fire the injection modulator. After the time taken for acceleration, about eight milliseconds, an X-ray pulse is produced which occurs well within the sensitive time of most cloud chambers. This method has the advantage of causing the least possible interference with the independent operation of the accelerator or cloud chamber. As it requires the continuous operation of the magnetic field of the accelerator, i t might be undesirable for large accelerators, for which the amount of power dissipated between expansions would prove uneconomical. However, a similar method of synchronization has been successfully employed by Gaerttner and Yester (1949) for a 70 MeV. synchrotron used in conjunction with a fast cycling expansion chamber.

A Circuit for the Operation of a Synchrotron in conjunction with a Wilson Cloud Chamber