光电倍增管特性介绍

Photomultipliers

Photomultiplier Tubes

In optical emission spectrometry, photomultipliers are commonly used as detectors. They are photocell detectors. The incident photons coming from the exit slit liberate electrons from the photocathode and the electron flow is then amplified by a set of dynodes. The final anode current is proportional to the incident photon signal received by the photocathode.

The measurement dynamic range is very broad, i.e. 10¹⁵, and sensitivity is high, as the dark current is low. These detectors allow the detection of low intensities emitted by trace elements, as well as strong signals from major elements. They have very fast response times, typically 1-2 ns for a 10%-90% change in signal. The main inconvenience of photomultipliers is their cost.

PMT structure

There are several types of photomultipliers, which differ in the nature of the entrance window, either crystal or fluoride, and in the nature of the sensitive layer on the photocathode. Some are only sensitive in the far ultraviolet while others are more sensitive in the visible. The type of photomultiplier to be used is selected according to the wavelength of the line to be detected.

A fatigue lamp (a small incandescent light source) is often used with photomultipliers to keep the temperature of the tube and its associated electronics constant. The fatigue lamp is switched on when the emission source is off and switched off when the emission source is on.

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.

The relationship between measured intensity and PMT voltage is

where I_i is the measured intensity, I_i⁰ is the intensity above background at zero volts, I_b 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

Hence if we plot ln(I_i - I_b) versus ln U_PMT we should get a straight line. The slope then equals the parameter a.

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 I_b was taken as the intensity measured with U_PMT = 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.

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