• Description of Apparatus
• Apparatus Specifications
• Instructions for use of Apparatus

GEIGER-MULLER TUBE

Description

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1. The tube is to be operated in the housing supplied, in order to protect the window, and the assembly connected to a Picker Nuclear Compact Scaler. Samples can be placed in pans or other appropriate containers and inserted in the housing at set distances beneath the window. Absorbers can also be inserted between the sample and the tube.
2. Before conducting any measurements the correct operating voltage of the G-M tube should be determined. Set up a moderately weak source (use the nominal voltage on the tube to determine this) under the tube and repeatedly count it while changing the high voltage. Start at about 700 V and work up first in 50 V and later in 20 V steps until the count levels off to about a 10%/100 change. Plot a graph as you go. Keep increasing the voltage until the count begins to increase again significantly. DO NOT GO TO HIGHER VOLTAGES. The optimum operating point is about 1/3 of the way from the bottom of the plateau to the top.

Principles of Operation

The tube contains gas at a low pressure (~ 2 Torr). When a beta particle or gamma ray passes through it, a trail of ionized atoms will be left behind. (alpha particles will ionize effectively but cannot usually penetrate the window. gamma rays can enter through the window or the walls). The charged ions will be accelerated strongly to one or other electrode and will further collide and produce more ions. In this manner the originally small number of ions is greatly multiplied, and when they are finally collected at the electrodes an easily measurable pulse of current is drawn from the high voltage supply. The presence of a small quantity of a halogen element aids in stopping the multiplication process promptly after each excitation. Because the multiplication process proceeds to a saturation, (approx 10 to the 10 ions) each time it is excited (if the high voltage is in the plateau region) it is not possible to tell the energy or nature of the ionizing beta or gamma. Likewise, simultaneous passages of two or more particles will produce only a single current pulse.

Precautions

• Do not let anything touch the window. It is very fragile.
• Do not ever apply a high voltage beyond the plateau region, as the tube will be damaged.
• Count rates of more than 100000/min will likely be subject to some coincidence error (see below).

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