A traveling-wave photomultiplier

Abstract

A new high-frequency photomultiplier has been developed for use at microwave modulation rates. This device consists basically of a circularly symmetric form of the crossed-field secondary electron multiplier together with a helical output circuit, so that the frequency response is of a band-pass form centered at approximately 2 GHz. The four-stage electron multiplier, which has a coaxial geometry, has a transit time-dispersion of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> seconds when operated in a solenoidal magnetic field of 900 gauss with an anode voltage of 4000 volts. The traveling-wave photomultiplier has been successfully operated in a modulation-demodulation experiment in the frequency range of 1 GHz to 3 GHz with typical values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M^{2}R_{eq}</tex> of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> . The high-impedance helical output coupler makes a very sensitive tube possible with only a limited number of multiplication stages so that internal sources of noise, such as field emission and dark current, are not serious problems. When compared with the nonmultiplying version of the traveling-wave phototube, this new photomultiplier provides a direct increase in signal level of 40 dB for the same optical input signal. Minimum detectable optical signals are of the order of 0.5 microwatts when the modulation index is only 10 percent. The dynamic range is quite large, both because of the relatively rugged dynode structure and because of the limited number of multiplication stages which are necessary to reach a given detected signal level. Traveling-wave gain has not yet been achieved in the helix region because the beam shape is not optimum due to various focusing problems, but improvements in this area should result in amplification in the helix region as well as current multiplication in the multiplier region in the case of strong input signals. It should be possible to extend the frequency range of devices of this type up at least 8 GHz by suitable helix design.