21 June 2002
Gieger-Mueller Tube Interface Circuits
by Joseph DiVerdi
The successful use of a Geiger-Mueller (GM) tube as an ionization detector depends critically on the interface circuitry to which it is connected. However, the fundamental operation of these tubes and their interface circuitry is sometimes imperfectly understood. This can result in unexpected or even erroneous measurements. Often circuit designers will provide a complete tube and interface circuit system intended to meet a set of technical expectations and when built exactly as prescribed, these systems will deliver as promised. Occasionally, a published design can not be followed exactly for various reasons. In such cases, suitable modifications of the interface circuitry may be required or to prevent the resulting system performance from suffering.
It is with these considerations in mind that the following notes on the operation of GM tubes are provided along with a few different, representative interface circuits for their use.
A GM tube is a sealed chamber with a pair of internal electrodes and filled to low pressure with an easily ionizable gas. The enclosing chamber is either partially or totally transparent to the type of ionizing radiation which the tube is intended to detect. One electrode is an extended piece of metal foil and the other consists of one or more extremely fine wires in common electrical contact. The most popular physical configuration is a tube with one fine wire electrode extending along the cylindrical axis and surrounded by a second electrode consisting of a metal foil cylinder. Another popular configuration is a flat plate and has been described quite nicely by Shawn Carlson in one of the last Amateur Scientist columns appearing in Scientific American. In this configuration, one electode consists of a series of parallel fine wire electrodes in common electrical contact and suspended between the second electrode, a pair of continuous metal foil parallel plates in common electrical contact.
In its quiescent state, the gas in a GM tube is electrically neutral and the electrodes are electrically isolated from each other. When a bit of ionizing radiation (matter or energy) enters the tube it encounters one of the neutral gas molecules, the collision between them results in the ionization of the atom with the ejection of an electron and the consequent creation of a positively charged and much heavier atomic ion. The electron moves under the influence of the tube's very steep electric field gradient gaining velocity and kinetic energy very rapidly. It then collides with a second neutral atom, ionizing it, creating a second electron and ion pair with the collision products sharing the incident electron's kinetic energy. This process continually and rapidly repeats generating a flood of electrons which sweeps towards the positive electrode, ionizing all atoms in its path. As many as 109 electrons impinge upon the posit! ive electrode during a time period of around 10-6 second from a single ionization event. After this avalanche has subsided, the electrical neutrality of the gas atoms is soon re-established and current in the tube ceases to flow.
From an electrical circuit perspective, the GM tube can therefore be modeled as a fast acting on-off switch. In its quiescient "off" state or non-conducting, that is, no net electrical current flows through it. When an ionizing event occurs, it immediately switches to its "on" or fully conducting state where the passage of electrial current occurs. After a short time period it switches "off" again and current ceases to flow through the tube.
It is useful to know what the magnitude of electrical current this electronic flow represents. The definition of one Ampere (A) (the unit of electrical current) is: one Columb (C) of charge passing through a circuit for one second (s). In addition, it well known that one mole (6.022 x 1023) of electrons carries 96,500 C of charge. A little algebra shows that the current flow of 109 electrons flowing during a time period of around 10-6 second constitutes a current of approximately 10-4 A or 100 uA.
For a GM tube to operate properly the interface circuit must satisfy several requirements:
A representative circuit which will accomplish the stated requirements is shown in the following figure.
The GM tube is shown schematically with its two electrodes, a fine wire and a cylinder, connected in series with a battery and a resistor. The battery represents a source of high voltage. The resistor serves two functions: it limits the current flowing through the tube and provides a resistance across which a voltage is developed which is proportional to the tube current. Typical values are for these elements are 500 volts (V) and 10 million ohms (W). The application of Ohm's Law shows that a maximum current of 50 uA can flow through this circuit which is below the threshold for continuous discharge. The output voltage (Vout) is measured with respect to the indicated ground in all the example circuits shown here.
Vout is zero when the tube is "off". When the tube switches "on", the voltage will rapidly go negative and then will return to zero albeit at a somewhat slower rate, for some practical reasons. This is shown schematically in the inset figure. The peak voltage will approach -500 V but may be observed to be somewhat smaller depending on the characteristics of the external measurement device, that is, its input resistance, input capacitance, and response time.
The next figure shows an alternate GM tube circuit.
In this circuit, when the GM tube is "off" Vout is +500 V. When the tube turns "on", Vout drops to a value approaching zero and then returns to +500 V. As most semiconductor circuitry is unable to withstand high DC voltage, this circuit typically requires a small capacitor capable of withstanding the high DC voltage in series with the output to block it from the subsequent circuitry. The addition of such a capacitor renders this circuit's Vout virtually identical to the first.
The next figure shows yet another alternate GM tube circuit.
Here the battery is replaced with one which supplies twice the voltage and the resistor is replaced with two resistors in a voltage divider configuration, each with twice the value of the original. This circuit was offered by Carlson in the Amateur Scientist article and has the advantage of being able to reduce the voltage applied to the GM tube when the voltage supply is itself unadjustable and too high for the GM tube. Its Vout is identical to the previous circuit.
The last figure shows a particularly useful GM tube circuit which has been used in some commercially available, handheld GM counters.
In this circuit, the cylindrical electrode is connected to ground through a second resistor. The Vout is taken at the junction of this second resistor and the tube electrode. In practice, the battery is set to +500 V, R1 to 10 MΩ, and R2 to 100 kΩ. When the tube is "off" the Vout is zero. When the tube switches "on", the voltage will quickly go positive and then will return to zero when the tube switches back to "off". The maximum positive voltage is governed by the voltage divider created by the two resistors which in this case is about +5 V. This value is particularly convenient for directly interfacing to popular CMOS logic. The Vout can be varied over a considerable range by varying R2 without substantially affecting other circuit characteristics. Further, no high voltage DC blocking capacitor is required.
In summary, it can be seen that
it isn't just the GM tube which makes the measurement but the interface circuit
which plays an important role in the process and that there are many alternatives
available to imaginative experimentalists. 