You need to provide a series of voltages to the dynodes of a photomultiplier to make it work.

An active voltage divider ensures constant gain up to a mean anode current of 100 µA, the maximum permitted in most photomultiplier types. Power consumption is typically 100 mW making them particularly suited to portable instrumentation.

An active divider consists of a string of high impedance resistors connected in parallel with a string of FETs. The resistors provide reference voltages for each stage of the active string to pick off and apply to the dynodes. An ideal active divider is one that can sense these voltages over the full dynamic range of anode currents without unwanted load effects. They  are suitable for pulsed applications with the addition of decoupling capacitors.

Active dividers can be constructed with FETs only in final stages and resistors for earlier stages, or with FETs throughout, as shown in figure 1. There does have to be at least one resistor, R, in the active string in order to set the standing current. Reliability and temperature stability are excellent in active dividers and the improvement in linearity of performance at reduced cost to power consumption is a rare prize.

Further information on voltage dividers can be found in our photomultiplier brochure, technical reprint Photomultiplier Voltage Dividers RP/085 which can be obtained by contacting us.

Pulse Encoding

Here information is contained in the number of photons in each pulse, the rate of pulses and their individual pulse heights.

The pulses usually range in width from 1ns to 1µs. For example the pulses from a Na(TI) detector measuring gamma emissions from a 137Cs are characterised by a decay time of 230ns.

An typical requirements is:

Pulse height resolution with Na(TI) is 137Cs

≤7%

Background gamma rays (≥0.1MeV)

12dpm

Pulse height non-linearity at 25mA

≤5%

Dynamic range

1000-1

 

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