RFPMT

Radio Frequency Photo-Multiplier Tube

Traditionally photons are detected in photo cathodes and converted to electrons. The electrons are multiplied and produce electrical signals with nanosecond (ns) resolution and then processed by traditional shaping electronics – amplifiers, discriminators, and time to digital converters to produce a time signal measuring the arrival of the initial photon. The challenge is in the picosecond (ps) resolution; even though modern digital circuits operate at high speeds of tens of GHz, they are not fast enough to directly count individual photons or electrons with ps resolution. Also, these devices have significant deadtimes that can be as large as 80ns. Improvements have been made by using superconducting nanowire single-photon detectors with temporal resolutions below 15ps by MIT and the Jet Propulsion Lab (JPL). The best resolution they achieved is 3.0ps and a deadtime of 100ns with a maximum data rate of 10 MHz.

Measurement of time to very high precision is a prerequisite in many fields of science and technology. A new timing processor, the Radio Frequency Timer, will be capable of ps resolution for single electron detection for high-rate electrons. Consequently, a photon sensor based on the RFT, namely the Radio Frequency Photo-Multiplier Tube (RFPMT), developed at the Alikhanyan National Laboratory (ANL) will be capable of detecting single photons with ps resolution. Currently there is no optical sensor capable of matching the combination of ultra-high timing resolution for single photons and very fast readout speed promised by the RFPMT, making it ideal for applications in ultra-high resolution optical microscopy.

The RFPMT, after some development, will be able to detect with 1 ps resolution and essentially be free from dead time, so that multiple single photons speeding, for example from a laser, the induced fluorescence could be recorded and time resolved. With fast readout from a suitably pixelated anode, the RFPMT will have enormous data throughput, potentially increasing the speed of image reconstruction by large factors. It is expected that the RFPMT will offer major improvements to several imaging techniques. For example, in high precision time-correlated, Stimulated Emission Depletion (STED) microscopy precise timing offers improved coordinate resolution. Similarly in time-correlated Diffuse Optical Tomography, the ability of the RFPMT to map and de-convolute scattered photon time distributions with extremely high precision would be a huge advance compared to conventional photon sensors. Ultimately, with ps resolution or better, the RFPMT offers a window of opportunity to access dynamical processes in biological molecules as they take place. The Radio Frequency Timing Technique In a typical timing technique, the time interval is measured between the leading edges of two electronic pulses applied to the start and stop inputs of a time-interval meter. A typical circuit might measure the difference in arrival time of two photons. The detectors, e.g. vacuum or Silicon photomultipliers, produce close to nanosecond (ns) rise time pulses, with constant-fraction discriminators providing sub-ns, time-pick-off precision for the logic pulses fed to time-to-digital converters. The basic principle of the RFPMT is the conversion of information in the time domain to a spatial domain by means of a high frequency RF field. Streak Cameras, based on similar principles, routinely operate in the ps and sub-ps time domain, but have substantial dead time associated with the readout system.