What is a photomultiplier tube and how does it work

What is a photomultiplier tube

Photomultiplier tubes can detect light and these tubes are used in applications such as fluorescence and spectroscopy. Photomultiplier tubes are a versatile device that provides ultra-fast response and extremely high sensitivity. The photomultiplier tube consists of a photo-emissive cathode followed by a focusing electrode, an electron multiplier, and an electron collector. It is very extremely sensitive detectors of light in the ultraviolet, visible and near-infrared ranges of the electromagnetic spectrum. It is similar to phototubes which are consisting of the photocathode and a series of dynodes in an evacuated glass enclosure.

How does a Photomultiplier tube work

When light enters the photocathode, then the photocathode emits photoelectrons into the vacuum and this emission of electrons is done by the photoelectric effect. Then the photoelectrons are directed towards the electron multiplier by the help of electrode voltages. During the impact, each electron will liberate a number of secondary electrons which are in turn, electrostatically accelerated and focused on to the next dynode. Due to the secondary emission multiplication, the photomultiplier tubes provide extremely high sensitivity and exceptionally low noise compared to the photosensitive devices. The photomultiplier tube features a fast time response and a choice of large photosensitive area.

What is photoelectric effect

If light or electromagnetic waves with high frequency falls upon a metal surface, then electrons are emitted from the metal surface and this process is called the photoelectric effect and the electrons which are emitted in this manner are called photoelectrons. The photoelectric effect requires photons with energies from a few electron-volts to over 1Mev in elements with a high atomic number. The energy of the emitted electrons does not depend upon the intensity of the light but only on the energy of individual photons.

What does a photocathode do

The photocathode of the photo-multiplier tube converts energy from incident lights to electrons. The conversion efficiency or the photocathode sensitivity varies with the wavelength of the incident light. The relation between photocathode sensitivity and wavelength is called the spectral response characteristics. In photomultiplier, tube photocathode acts as a negatively charged electrode

How do dynodes work and what does an anode do in a photo-multiplier tube

In the photomultiplier tube, the dynode acts as an electron multiplier, this multiplier consists of 8 to 19 stages of electrodes. Photoelectrons which is emitted from the photocathode are multiplied by the first dynode through the last dynode with current amplification and are finally sent to the anode. Major dynode materials will be gallium phosphide, beryllium oxide, and magnesium oxide. When the primary electrons strike at the dynode secondary electrons are emitted and the number of secondary electrons per primary electron is called the secondary emission ratio.

Anode which is used in a photomultiplier tube is an electrode it collects the electrons from the dynode and then outputs the electron current to an external circuit. An important character that is required for the anode is that an adequate potential difference must be established between the last dynode and the anode in order to prevent space charge effects and to obtain a large output current.

What are the factors that must be considered while choosing a photomultiplier tube

There are some important factors for selecting a photomultiplier tube, they are window material, photocathode spectral response, number of dynodes and type of dynode. A photomultiplier must be selected which can detect the incident light and also select the optimum circuit conditions that match the application. Photomultiplier tube must be selected according to the operating conditions and circuit configurations according to the incident light wavelength, intensity, beam size and the speed of optical phenomenon.

The photomultiplier tubes have high sensitivity, so there is a chance that it could detect extraneous light other than the light to be measured. This decreases the signal to noise ratio, so an external housing is needed for the external light shielding. Photo-multiplier tube characteristics may vary with external electromagnetic fields, ambient temperature, humidity, or mechanical stress applied to the photomultiplier tube and because of this, a magnetic or electric shield is required to protect the photomultiplier tube. So all of this must be checked while selecting a photomultiplier tube.

What are the applications of photo-multiplier

• Photomultipliers can be used for civil, military and aerospace applications including moving maps and aircraft monitoring system

• Photomultipliers can also be used for chromatography, it is the process of analyzing the chemical mixtures by separating the constituents in a column

• Spectroscopy

• It is used in medical equipment such as PET scanner

• It is used in planar imaging device