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Radioactivity Detector Circuit

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The term "Geiger Counter“ refers to a device which actually counts the number of radio active particles passing through a specified area and while this circuit does not actually count, it does produce the familiar "click-click" sound which most of us associate with a Geiger counter.

Simple and inexpensive though the circuit is, it is just as accurate and as sensitive as any commercial device and whilst we hope that no one ever has reason to use the device seriously it does open a whole field of experimenting. The circuit is battery operated and so the device can easily be made portable. The operational part of the circuit is the Geiger tube.

How Gieger Tube Works

They require a high voltage supply but the current consumption is minute and so a very simple type of power supply is needed. The operation of a Geiger tube is as follows.

When the correct voltage is applied between the anode and cathode of the tube in normal circumstances no current will pass. When however a radio active particle passes through the tube, a gas inside the tube is ionized and creates a momentary conduction of current between the two.

Radioactive particles are emitted by any radioactive material and are very, very small physically.

Three types of rays are emitted by any source but it is only the dangerous (in large quantities) gamma rays that are of interest.

They travel at very high speeds and have tremendous penetration powers; only thick lead screens are effective unless great thicknesses of other materials are used. These gamma rays have no difficulty in penetrating the surround of the Geiger tube.

However it is not simply a matter of applying any voltage across the tube. Below a critical voltage there is insufficient potential for the conduction to take place and above a certain voltage it will take place continuously once a particle has created a path.

What is Plateau Voltage

The voltage between these two is known as the "plateau" voltage and it varies with the individual tube though it is generally about 400V. As long as the voltage applied to the tube lies in this plateau region the tube will operate successfully.

The output is taken from across a high value load resistor, R2. The value of this resistor limits the current flow through the tube to a low level.

How the Circuit Works

The clicks produced by the radioactive rays can be heard in a crystal earpiece connected via a d.c. blocking capacitor across the load resistor.

If desired this output could be taken to an amplifier feeding a loudspeaker but the source impedance of the amplifer should not be much below 1M ohms.

The high voltage supply comprises Tr1 and the associated components. The transformer Tl is a low current 250V primary, 9-0-9V secondary but here we are using it in reverse; the secondary is connected into a Hartley oscillator circuit with Trl .

C2 provides the feedback to the base to sustain oscillation and C3 tunes the transformer winding. The frequency of operation is far from critical and it does not have to be the 501-lz normally applied to such a component. The transformer used in the prototype with the component values shown worked at several hundred hertz. The base bias is provided by R1.

In order to control the voltage applied to the geiger tube, a variable resistor, VRl, is connected in the supply to the complete stage. At maximum resistance this reduces the voltage applied across the trans- istor and this in turn affects the voltage in what has become the secondary.

The varying current in the primary induces very high voltages in the secondary but even these are not sufficiently high to drive the tube and so a voltage doubler is included, this comprises D1, D2, C4 and C5. This arrangement doubles the voltage of the transformer and rectifies and smooths it.

Note that there is high potential at this point, albeit at very high impedance. The voltage here is at so high an impedance that reliable readings are not possible with a multimeter, though an indication of several hundred volts will be given on meters with a sensitivity of 20, 000 ohms per volt or better.

Note that for this reason the components used for C4 and C5 (especially the latter) must be of high quality and have low leakage. Because the frequency of the oscillator is high and because so small a current is being taken, the value of the smoothing capacitor C5 is quite adequate although it is very low in comparison to conventional values.

There should be no problems in getting the circuit to operate once the high voltage supply is working. The value of VR1 should be set so that each radio active particle produces a nice clean click, this level on the pot can be noted. Over a certain voltage all that is heard is a continuous "mush".

How to Hear the Device

No special equipment is necessary to hear the device working. There is always a background level of radioactivity and for this type of tube there are on average 40 clicks per minute though these are about as random as you can get; there may be long intervals and then a whole series of clicks

Several common place objects are radioactive. Luminous watch dials always used to be highly radioactive (and some still are) and some oi the luminous paint used on ex-Government equipment is very high indeed, in fact it is well above the now accepted safety limits.

The radioactive background level also varies considerably with geography. Around Aberdeen the level is very considerably higher than the average and this also applies to other areas built on granite. A number of everyday objects are also slightly radioactive including gas mantles and lighter flints.

At high levels the "clicks" will become far more frequent and in areas of high level the clicks will all blend together into a single sound though this is distinguishable from the mush produced by setting the voltage too _high. The current consumption is about 10mA.

It is worthwhile avoiding touching the high voltage sections, for although it will only give a slight shock which is not dangerous, this is nevertheless unpleasant.

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