P25 USB module

USB Photodetector modules include a 25 mm diameter photomultiplier tube with a 22mm active photocathode diameter (280-630nm or 300-850nm options). Other sizes of active areas are also available.

The photomultiplier is powered by a very efficient, +HV, power supply. Low power dissipation ensures negligible heat generation within the package. The high voltage is factory set, at the optimum operating voltage, to ensure stability of operation and stable photon counting performance.

The signal from the photomultiplier is connected to a high performance amplifier discriminator combination, which is factory preset to accept signal but to reject electrical noise.

Pulses from the output of the amplifier/discriminator are passed to a counter which is gated on/off by the microcontroller according to the gate period setting. The count is latched through to a parallel to serial converter before continuing to the next counting period. This ensures the dead time of the counter is kept to an absolute minimum (11.1μs gap between gate periods).

The microcontroller has a maximum count rate capability of 100 Mcps after, linearity correction.  All counts are automatically corrected for dead-time losses, caused by pulse pile up. This is particularly important at high count rates.

The module also incorporates a watchdog circuit such that in the event of internal software failure, the package will automatically reset itself to its default settings.

The module housing is mumetal providing good shielding from transverse magnetic fields.  These photodetector modules use the photomultiplier in the photon counting mode.

Photon Counting is a very sensitive method for measuring ultra low light levels. Typical spectral response characteristics for the photocathodes used in photon counting detectors are shown below:

quantum efficiency

Photoelectrons emitted by the photocathode are accelerated and focussed onto the first dynode of the electron multiplier section of the photomultiplier. On impact, each photoelectron liberates a number of secondary electrons which are, in turn, accelerated and focussed onto the second dynode. The process is repeated at each subsequent dynode and the secondary electrons from the last dynode are collected at the anode.

Individual charge pulses from the photomultiplier vary in magnitude because of the statistical nature of the gain process but, so long as they exceed the preset threshold of the discriminator, each will be counted as one pulse. Only small pulses, resulting from single electrons originating from intermediate dynodes, will fall below the threshold and, thus, rejected.

The voltage applied to the photomultiplier in the detector is factory set to provide the optimum gain for photon counting. At this operating point on the signal counts vs HV plateau characteristic, the slope gradient is very low and, hence, stable performance is achieved.

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Link to counting software: