window material

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The lower limit of detection is determined by the window material. The optical transmission of the window influences the spectrum of light reaching the photocathode. The window material is particularly important when measuring uv light. Certain applications, such as low level scintillation counting, also require window material free from naturally occurring radioactive contaminants.

You should select the photomultiplier with a spectral response that best matches the spectrum of the light you want to detect. Most photomultipliers include a borosilicate window, suitable for the detection of visible light down to wavelengths of 290 nm.

To detect shorter wavelengths, between 106 and 290 nm, requires non-standard window material such as MgF₂, quartz or uv transmitting glass. Photomultipliers with these window materials are more costly and they should only be selected where really necessary.

Photomultipliers are manufactured with the following window materials.

borosilicate glass   This is suitable for incident light of wavelengths greater than 260 nm and is the standard glass used. For critical applications, low background borosilicate glass is also available.

uv glass (W)   this extends the sensitivity down to180 nm.

quartz (Q)   made from fused silica, this material transmits down to 160 nm and has the added
advantage of low radioactive background.

magnesium fluoride (MgF₂)   transmits ultraviolet radiation down to 110 nm and is free from
radioactive contaminants.

sapphire (S) Al₂O₃   is used in metal-ceramic photomultipliers for harsh environments. It has good
UV transmission and low background.

For more detailed information, request our brochure "Understanding photomultipliers" via this form.