Editing Geiger–Müller tube (section)

==Photon energy compensation==
[[File:GM tube compensation.gif|thumb|Comparative response curves for G-M tubes with and without gamma energy compensation]]
[[File:Zählrohr 70 031 E 01.jpg|thumb|Thin-walled glass G–M tube showing a spiral wire cathode. The tape bands are for fixing compensating rings]]
[[File:Zählrohr 70 031 E 02.jpg|thumb|Thin-walled glass G–M tube with energy compensating rings fitted. The complete assembly fits into the aluminum housing.]]
If a G–M tube is to be used for gamma or X-ray [[Dosimetry|dosimetry measurements]], the energy of incident radiation, which affects the ionizing effect, must be taken into account. However pulses from a G–M tube do not carry any energy information, and attribute equal dose to each count event. Consequently, the count rate response of a "bare" G–M tube to photons at different energy levels is non-linear with the effect of over-reading at low energies.  The variation in dose response can be a factor between 5 and 15, according to individual tube construction; the very small tubes having the highest values.

To correct this a technique known as "energy compensation" is applied, which consists of adding a shield of absorbing material around the tube. This filter preferentially absorbs the low energy photons  and the dose response is "flattened". The aim is that sensitivity/energy characteristic of the tube should be matched by the absorption/energy characteristic of the filter. This cannot be exactly achieved, but the result is a more uniform response over the stated range of detection energies for the tube.<ref name="knoll"/>

Lead and tin are commonly used materials, and a simple filter effective above {{nowrap|150 keV}} can be made using a continuous collar along the length of the tube. However, at lower energy levels this attenuation can become too great, so air gaps are left in the collar to allow low energy radiation to have a greater effect. In practice, compensation filter design is an empirical compromise to produce an acceptably uniform response, and a number of different materials and geometries are used to obtain the required correction.<ref name="centronics"/>