Discharge Derangement Syndrome

Some time ago, I purchased a sample of five GM (Geiger-Müller) tubes, received through the post.  They are cheap, SI3BG tubes.  These are small and, as a direct result, will have quite low sensitivity, but for applications where we are only interested in the danger of radioactive contamination or fallout warning systems then they are probably a reasonable choice of device.

Why did I buy them?  Because I have a sneaking suspicion that we will soon be needing all manner of nuclear radiation detection and protection but, firstly, it was out of curiosity.  I have already purchased potassium iodide tablets for myself and a few others so I am taking seriously the possibility of a real nuclear war over the coming years.

Note that this type of tube is not suitable for the detection of alpha particle radiation or neutrons but it will detect gamma rays (top end of the electromagnetic energy range) and beta particles (free electrons or positrons moving at high speed).  Gamma rays are strongly associated with nuclear fission, whether that is occurring on the scale of a massive weapon or on the atomic scale.

For some time I have been considering purchasing a cheap Geiger counter and so I did just that, in addition to the tubes.  At least it will enable me to measure a very rough radiation source in order to use the source as an approximate calibration reference for the SI3BG tube circuit.

The typical GM tube works by detecting the ionisation of one or more gas molecules within the tube due to the collision of an energetic particle, be it a photon, electron, positron or alpha particle (helium nucleus moving at high speed).  Fundamentally, the tube consists of an anode in the centre and a cathode which is typically part of the surrounding tube.  A gas is used to fill the tube (not air) and a high voltage is applied across the anode and cathode of the tube.  If an energetic particle collides with a gas molecule within the tube, it can ionise the gas molecule, if there is sufficient energy transfer in the collision, relieving the gas molecule of an electron.  This electron is, in turn, accelerated towards the anode by the electric field which exists within the tube between the anode and the cathode.  The additional energy of the free electron can serve to free another electron in a subsequent collision with another gas molecule and that electron, in turn, is accelerated by the electric field, and so the process continues resulting in an avalanche of electrons which eventually arrive at the anode which, when they strike it, generate a current pulse in the external circuit, the mad rush of electrons causing a limited discharge of the capacitively stored energy across the tube's terminals.  This pulse of current can be amplified, detected and, finally, recorded by the counter circuit.  Each electron that exits at the anode is balanced by an electron entering the gas at the cathode.  Charge transfer back to the ionised gas molecules ensures that the injected electrons contribute to the neutralisation of any ions in the gas.  There are issues such as the need for "quenching" the tube if certain gases are used which may result in self-triggers after an event has occurred but I will not discuss those issues here.

So, all in all, it is a very simple device and anybody who has done high school physics will understand the mechanisms therein.  However, making this simply work is not what interests me but finding an effective way of using such a cheap device to provide some meaningful measure of not just the frequency of radiation events but also of their strength, even if only approximately.  This will involve various control mechanisms to control the GM tube supply voltage so a carefully designed feedback control loop is in order.  However, a simple circuit to get the tubes up and running will be the first priority.

The circuit to drive and control them should be quite simple compared to the sort of thing I generally design but it does have something I do not frequently have to deal with except on the odd occasion... high DC voltage.  Typically, this will involve using some kind of inductor or transformer and a voltage multiplier circuit (capacitors and diodes).  With a little more complexity than the standard fare, I should be able to design a power and control circuit with minimal operating current in order that the whole device can be operated from a few series alkaline cells.  Once operating, I intend to implement an LCD display (typical 44780 interface for a character LCD, cheap and abundant) as well as an ISM-band link (probably at 434MHz for the range).  A USB interface would also be useful, just some com-port emulator type of setup, also known as a CDC in USB parlance.  It will typically be communicating with a Linux box when connected, perhaps even a Raspberry Pi used as a remote logger.

Typical of GM tubes, these have a pulse operating range from 380 to 460 volts.  The output pulse mode of operation is, perhaps, the most commonly used, the easiest to use and possibly the most useful in terms of a first line of defense emergency alert system sensor.  Another nice thing about these little tubes is that they are mounted in a glass tube with metal end-caps that provide the electrical connection to the actual sensing tube inside.  These end caps are about the size of those found on common garden-variety fuses so this allows the use of simple, common-as-muck fuse holders to hold the tube in place.  Easy!

So, once I get this all going, I will purchase a more expensive and considerably more sensitive tube, perhaps one with a mica window for alpha particle detection.  In the meantime, a search for reasonable quality radiation reference sources shall have to be made since the thorium in the unused gas lamp mantle that I have, although useful for simple go/no-go testing, provides no information on the expected emission rate (the amount of thorium in the mantle is unknown).  Guess I will have to get on with it soon enough if I get a bit of breathing space between jobs.  There is always a tree to prune, a lot of grass to cut, some repairs to do on the house, another piece of firmware to write and debug and a PCB to design or update.  At least I keep myself occupied.