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PMT gain

Dependence of PMT gain on supply voltage

The gain of a photomultiplier varies dramatically with the photomultipler tube (PMT) voltage. As a guide, it doubles roughly every 50 V increase.

[Ii vs UPMT]

The relationship between measured intensity and PMT voltage is

[Ii]

where Ii is the measured intensity, Ii0 is the intensity above background at zero volts, Ib is a background intensity (dark current plus any electronic offset in the amplifier), and a is a parameter that varies from one PMT to another , for most PMT's used in optical emission spectrometry it is between 7 and 8.

Taking logarithms of both sides we get

[ln(Ii-Ib)]

Hence if we plot ln(Ii - Ib) versus ln UPMT we should get a straight line. The slope then equals the parameter a.

[ln(Ii-Ib) vs ln(UPMT)]

The results to the left were measured using a stainless steel sample on a RF glow discharge source with fixed source conditions. The value for Ib was taken as the intensity measured with UPMT = 300 V immediately before turning on the plasma. Only the PMT voltage was changed between measurements. As shown, the slope is typically 7.5, ie a ~ 7.5. The exact value of the slope 'a' depends on the design of the PM tube, in particular on the number of ' multiplying' dynodes used.

The high voltage supplied to the PM tube allows to optimise the sensitifity of the detector to the observed light intensity. This optimisation process can be used dynamically. With a 'simple' controle loop, the HV of the PMT can be controled by the out-put signal of thePMT. This control loop must have a negative slope, i.e.a high signal leads to a low PMT voltage and vice versa. This feature is used in the "High Dynamic Detection" system offered by Horiba Jobin Yvon, Longjumeay France, instruments. Strictly speaking the poper term would be dynamic range compression: variations of a low light level will lead to a large change in the PMT out-put signal, whereous variations of a strong emission signal will alter the PMT out-put signal only little.

Authors: Richard Payling and Thomas Nelis

First published on the web: 6 March 2001.