The Radio-Frequency Fine Structure of the Photon Beam from the Berkeley Synchrotron

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Abstract
The full energy spread‐out bremsstrahlung beam of the Berkeley synchrotron was shown to be emitted in sharp bursts with the 47.7‐Mc/sec frequency of the electrons in the doughnut. A two‐counter telescope, located about 20 inches away from a one‐fourth inch thick lead target and about 20 degrees from the photon beam direction, was used to measure an accidental coincidence counting rate as a function of the length of delay line in one input to the coincidence circuit. When the length of delay cable was chosen equal to integral multiples of the period of the radio‐frequency oscillator, the coincidence counting rate was of the order of 100 counts per unit of integrated beam; on the other hand, when the length of delay line was chosen to be equal to one‐half of odd integral multiples of the period of the radio‐frequency oscillator, the coincidence counting rate was less than about two counts per unit of integrated beam. An estimate of the width of a single pulse of photons can be obtained from the measurement by unfolding a Gaussian resolution function of the coincidence circuit. If the time variation of the intensity of the photon beam is assumed to be a Gaussian function, then the rms value of that Gaussian is found to be less than 1.5×10−9 second. A calculation by Lloyd Smith indicates that the measured radio‐frequency fine structure should provide a rather accurate picture of the phase‐stable bunch of electrons at the end of the acceleration period.

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