Limitations of Geiger Tubes

 

A geiger tube  will not detect every particle or gamma photon  which enters it. The percentage of the incoming radiation that is counted is known as the efficiency of the geiger tube. The factors which affect the efficiency are :

1. Type of radiation being detected-a geiger tube detects beta particles  much more efficiently than gamma-rays  which can pass right through without interacting. As a general rule a tube will give a pulse for each beta particle entering it, but for only 1% of the gamma photons.
2. Geiger tube construction-geiger tubes made of different materials, and of different size have different efficiencies. Generally smaller tubes of the same type have lower efficiency.
3. Dead time  losses-after an ionizing event the tube takes a short period of time to recover, and will not record anything during this interval known as the dead time (about ). For high count rates, an appreciable number of counts will not be registered.

When surveying for radiation it is important to start with the instrument switched to the highest range and then to select more sensitive ranges as required. Never walk into an area in which a high field may exist with the survey instrument set on a sensitive range. It may saturate and read very low or zero even though a high field is present.

In general any Geiger counter  is designed to account for the limitations of the detector by suitable design of the scale and by minimizing dead time losses. The latter is achieved by using a tube suitable for the intensity of radiation . For this reason many survey instruments are equipped with two geiger tubes, a larger one for lower fields, and a smaller one for high fields.