To measure radiation it must be absorbed. The absorption is higher as more dense the absorber material is. There are gas filled ionisation chambers or Geiger tubes available but the most sensitive is a crystal of NaI with a density of 3.7 g/cm3. This crystal produces a small light flashes when exposed to radiation which must be detected by a light amplifier called photo-multiplier. The whole detector must be built in a mechanical protection and light tight housing and also stabilized against any changes.
The two sources of radiation have to be shielded in lead containers to prevent the rays to travell in all directions. By a thin channel it is ensured that only in one direction the working beam is allowed to come out. The radiation is measured by a scintillation counter which is the most sensitive detector available for industrial applications. Both, source and detector, is called measuring path. Two measuring pathes are necessary for ash determination. It would be possible to use only one detector for both radiations but because physically in the detector each radiations influences the other it is more reliable to use separate ones.