Step-By-Step Photomultiplier Selection Guide

Step 1 - What is the wavelength of your source?

The following photocathodes are used in Electron Tubes photomultipliers. The quoted wavelength ranges refer to standard borosilicate windows.

Bialkali (K - Cs - Sb), 280-630nm
Offers high blue and good green response with low dark current.

Rubidium Bialkali (Rb - Cs - Sb), 280 - 680nm
Offers high blue and enhanced green response but twice the dark current of the bialkali.

Multialkali S20 (Na - K - Cs - Sb), 280 - 850nm
Sensitivity extends ffrom the uv to the infrared but may require cooling to reduce dark curent.

High Temperature Bialkali (Na - K - Sb), 280 - 630nm
Recommended for high temperature operation at temperatures above 60 °C.

Solar Blind (KBr, Csl, RbTe, CsTe), 110 - 360nm
When sensitivity in the UV and VUV only is required.

Step 2 - Do you want to detect at wavelengths below 300nm?

Window materials used in Electron Tubes photomultipliers are:

Borosilcate Glass
This is the standard, low cost, window material and is suitable for wavelengths greater than 300 nm.

UV glass (W)
Extends sensitivity down to 185 nm.

Quartz (Q)
Made from fused silica, this material transmits down to 160 nm.

Magnesium Fluoride (MgF₂)
Transmits uv radiation down to 110 nm.

Step 3 - Is low background glass an advantage?

ET Enterprises is the leading supplier of photomultipliers with very low levels of natural radioactivity. Photomultipliers with minimal levels of naturally occurring K, Th and U, are recommended for low background scintillation counting and most photomultipliers from ET Enterprises are made with low background windows. Radionuclide activities are given in the table below:

Step 4 - What detection area and geometry do you require?

For light sources remote from the pmt, such as a star, you can choose from the full range of window geometries. When the light source is directly coupled to the pmt, as in the case of a scintillation crystal, a flat window is best.

The range of window geometries is shown below. Further information is available in our photomultiplier brochure.

Circular
Diameters from 15 to 135 mm available to suit diffuse and directly coupled light sources. Typical applications include scintillation counting and general uses.

Reduced Area
Sensitive diameters of 2.5 mm to 10 mm for applications where light can be concentrated or focused. The reduced photocathode area provides the benefit of lower dark current (see step 6).

Domed Window
The inherent strength of this construction allows us to use a very thin window, reducing its radionuclide content. The window is often supplied sand blasted to enhance quantum efficiency.

2 Pi
Sidewall sensitivity allows wide angle light detection. A typical application is in probes for scintillation counting.

Hemispherical
Wide angle light detection from diffuse light sources such as the atmosphere or large liquid scintillation counters. Applications are mainly in fundamental research.

Side Window
A geometry that offers a small footprint in analytical instruments, such as spectrophotometers.

Step 5 - Are high light levels or low temperature a consideration?

For the purpose of photomultiplier selection, high light levels are defined in terms of the magnitude of the expected photocurrent or, more directly, in relation to the sensitivity of the human eye.

If you can see the light, no matter how faint, then you are dealing with high light levels! Equivalently, if photocathode current is greater than 1 nA then light levels are high.

The current carrying capacity of a photocathode is dependent on the operating temperature, their rankings, at 20 ºC, are given in the table below. All become less conductive with decreasing temperature.

Step 6 - Have you considered signal / background?

Background is the unwanted output from the photomultiplier when operating in the absence of signal - that is in the dark.

Dark current or dark count is always a consideration in low light level applications or where the dynamic range exceeds 105 (dynamic range is simply the ratio of the highest to the lowest light level measured).

• Dark current and count rate increase with pmt diameter.
• Dark current and count rate increase with pmt temperature.
• Dark current increases approximately linearly with gain.
• Dark count rate is essentially independent of gain.

Further information is available in our photomultiplier brochure.

Step 7 - What is the optimum photomultiplier gain?

Photomultipliers are available with gain capabilities ranging from 103 to 108 - the more dynode stages in the photomultiplier, the higher the gain capability.

The figure below indicates the variation of photomultiplier gain, g, with applied voltage for the range of fast, BeCu, 52 mm photomultipliers, illustrating the effect of increasing the number of dynodes.

Step 8 - Which dynode structure best meets your performance needs?

Photomultipliers are manufactured from any of the four dynode structures:

• venetian blind (VB)
• circular focused (CF)
• box and grid (BG)
• linear focused (LF)

with many available in a choice of two dynode surface materials

• decide: the gain required
• decide: the dynode structure
• decide: the number of stages to best meet the performance demands of your application

Photomultipliers with plano-concave windows and circular focused multipliers give the best timing performance. Other considerations such as the number and type of dynodes, the overall voltage, and the diameter of the photocathode, are also relevant.

Pulsed anode currents, of peak amplitude up to 150 mA, can be drawn from the photomultiplier, depending on the type of dynode and the type of secondary emitting surface.

Step 9 - Are you operating in a harsh environment?

A harsh environment has two aspects - operating temperature and mechanical stress.

Photomultipliers with high temperature bialkali cathodes are capable of operating from -60 °C up to temperatures of +175 °C. We have developed special photocathodes that operate at liquid nitrogen temperatures (-180 °C) but you will need to speak to us about your application.

We offer a range of ruggedised, conventional photomultipliers for severe industrial and space applications and a range of ultra-rugged high temperature types, of metal ceramic construction, intended for the oil well logging industry. These types are highlighted in the special features column in the specifications section of our photomultiplier brochure. Detailed shock and vibration specifications are provided in the data sheets for these products.

Further information is available in our photomultiplier brochure.

Step 10 - Which mechanical configuration for the base is best for you?

There are three base configurations for our photomultipliers, although not all of these are available on all types. The options for any pmt are listed in the specifications section of our photomultiplier brochure. Select the base option that best suits your requirements, noting that:

In the hardpin base, the pins exit directly from the glass envelope to match a mating socket. These photomultipliers have the shortest length.

The capped base (K) has more robust pins set in a blue, opaque thermoplastic cap. The photomultipliers are longer than the hardpin version but offer the advantage of best electrical contact and mechanical support.

The flexible lead base (FL) has a set of flexible wires for soldering to a printed circuit board. These wires can also be terminated in a blue cap loosely or closely fitted to the pmt. Types supplied with loosely fitted caps are designated KFLB.

ET Enterprises can offer all hard pin pmts with welded-on flying leads to special order.

Further information is available in our photomultiplier brochure.