5 - Measuring parameters

The conventional photoelectron multiplier (PEM) is a vacuum tube applying the photoelectron emission of metal surfaces for transduction. Electrons are emitted from the cathode when photons impinge on it. The electrons are ejected from the cathode surface when the energy of the radiation quanta is greater then the work function of the cathode material. In the PEM tube, additional electrodes (dynodes), at subsequently higher positive potential, are located between cathode and anode so as to amplify the electron current by means of secondary emission from the dynodes. The electrons are collected by the anode and cause a current flow, which can be used to produce an output voltage impulse on the load resistor connected in series with the anode. Despite of its large size and the necessary high voltage, the PEM has unique advantages such as low noise and great sensitivity.

The photoelectron multiplier can be replaced with large area photodiodes (PIN, APD) in the scintillations detectors.

Indirect detector

Direct detector

Photodiodes are based also on semiconductor pn-junctions. They apply the current-voltage characteristic shift of the device due to the electron-hole generation caused by the absorbed light. Practically, the reverse biased current of the diode is measured. The devices have a built-in field enabling them to operate in the photovoltaic cell mode but have much better performance in the photoconductive mode. Photodiodes apply Si or Ge pn-junctions and compound semiconductors often with heterojunctions. Devices operated within the visible range are based on the latter materials.

Ionization effect: the incident radiation results in the ionization of gases, which can be detected as a current impulse between electrodes in a closed chamber filled with a specially optimized gas mixture. The basic device is called ionization chamber. Proportional counters are operated at voltages high enough for multiplication; thus, the output pulses are proportional to the energy of the incident particle. Geiger counters are operated at higher voltages, in the so called Geiger plateau range, where the collected charge is entirely independent of the amount of initial ionization.