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.

Locating the Wily Electron

In the previous post I mentioned the revelation of quantum physics that we cannot pin-point the location of the electron.  Of course, this is actually true of anything at all and a cursory understanding of the uncertainty principle (more accurately, the Heisenberg position-momentum uncertainty relation) reveals that what we cannot obtain is precise position and momentum information simultaneously.  The more accurately we ascertain one, the more poorly the other, with theoretical extrema being that perfect information of momentum (essentially, velocity) means completely unknown whereabouts, and perfect knowledge of location means completely unknown momentum.  Of course, the extrema are only theoretical in that they cannot actually be achieved and serve only as asymptotic limits on the theoretical model that was proposed.  Measuring exact position requires a process whose time differential is zero, which is impossible, and measuring perfect momentum information requires infinite time, again impossible, especially given that conditions change, the electron has not always existed (as far as we know) and will, perhaps, cease to exist in the future, or at the very least, its given physical state may dramatically change, through collisions for example.  As such, there is always a level of uncertainty about both.  Thus my statement that we can never really know where the wily electron is.

To those familiar with the time-frequency relationship of signals (or, in general, inverse-space relationships), this problem is akin to the problem of the accurate measurement of a signal.  We can use an extremely small sampling time window and capture the signal's time amplitude to a high degree of certainty but the spectral content of a near-impulse process is broad, and of a perfect theoretical impulse is infinite in extent, so we can approach perfect time amplitude information at the complete loss of spectral amplitude information.  Conversely, we can sample the signal over an infinite time window (as is the theoretical outcome of a traditional Fourier analysis) and grow towards perfect spectral information but completely lose any definition of signal amplitude in time because we have completely lost any definition of specific time.  The two extremes are only theoretical, of course, and perfect information on both spectral amplitude and time amplitude of a signal are impossible, which is why we always talk about approximations of these things, and we use various specifically designed "windowing" functions that give us better information on one particular aspect of a signal at the cost of the degradation of another.  Simultaneous representation both across all time and all frequency is impossible by virtue that they are essentially inverse spaces.  Pick a moment and there's no defined spectrum, or pick a frequency and there is no specific moment.  We are finite beings with finite minds and finite information processing capabilities no matter how sophisticated our tools become, so we must always accept that all of our observations of the natural realm are going to be finite with inherent uncertainty and, worse still, that our very actions in making observations of natural processes affect those processes, since an observer is always part of the system he observes or he could not, otherwise, observe it.  Generally we can constrain these effects to such a small degree that they need not bother us at all but, in this day and age of extremely low energy electronic devices, the problem of obeserver interplay in the observed system is becoming an issue right along side the effects of quantum physics, in that the energy transfers between integrated components within modern semiconductors are so small that quantum effects are almost a significant, if not dominant, source of uncertainty leading to large increases in error probabilities in modern computing devices.  The concurrent explosion in research into the areas of error correction algorithms is not at all surprising and all such algorithms involve some form of information redundancy, which leads me to ask a question:  Is there a lower, but non-zero, finite limit to the energy requirements for information transfer in any given medium such that attempting a lower energy process leads to the need to counter information uncertainty with a concurrent increase in redundant information which, by its added energy requirement, at least counteracts the original energy reduction?  In practical terms, this would place an absolute lower bound on bit energy.

Another point I would like to make to qualify my statement about knowing where something is with certainty is that we accept a given amount of uncertainty of anything and everything.  No human has the capacity for complete, perfect information for it would require the mind of God, literally.  However, the scale of the uncertainty is such that the geometric scale of the probability density function of the position of something as small as an electron is sizeable, even huge, compared to the electron itself (viewing the electron as a finite spatial realm within which "most" of its defining energy is contained at a given instant) but the same cannot be said for something the size of a grain of sand, where the "fuzziness", if you like, of its positional information is small compared to its overall perceived size.  In the first instance, we cannot say where the electron is most of the time without a large probability of error but in the second instance we can say, with some quite small probability of error, where the grain of sand is spending most of its time.  The hard-and-fast defintion of physical properties we are so used to in the macroscopic world are not strictly correct but are sufficiently so, most of the time, with only an infinitesimal probability of error, an error level we happily accept.

Noteworthy, from this brief commentary, is that the mathematical models we use to model physical processes are never perfect.  There is no mathematical model of physics that I am aware of which is not an approximation to reality.  This, of course, forces us to ask the question, "Is mathematics invented or discovered?"  If no mathematical model we use to represent a physical process is perfect and all are just approximations, on what basis do we make the claim that nature follows mathematical principles and that mathematics is inherent in nature?  Then, because the neural networks of the brain must follow natural principles, and those very principles allow the formulation of mathematical logic, we must accept that math is somewhow inherent in nature.  Perhaps we will recognise that nature does follow mathematical logic but the models required to perfectly represent natural processes are themselves infinitely complex despite the finiteness of the natural process.  Natural systems are deeply non-linear and mathematical modelling of non-linear systems, even simple ones, can lead to infinite complexity, as anybody who has looked into or studied non-linear dynamics (or "chaos theory", as some call it) would realise.  I cannot escape the conclusion of this, that the universe is the product of unlimited intelligence, the product of an infinite mind, since the information underpinning its existence, the mathematics, must pre-exist its physical manifestation or there would be no reason for it to take a logical form and it would be unlikely, in the extreme of extemes, to exist at all... and yet, here we are!  We know from thermodynamic consequences that natural systems do not increase in irreducible complexity, only decrease, so we know that Darwinian evolution and its cosmic counterparts fly directly in the face of what are the most well established physical principles known which, if such were not at all correct in order that macro-evolution be even remotely possible, none of our technology would work, no information system such as our brains could even exist, the universe itself could not exist, et cetera.  Invariably, I come back to where the ancients and the greatest natural philosophers come back to again and again... GOD IS.

Well, enough of that... now to some tinkering.

What's All This Then?

The plethora of very low cost, highly integrated digital, analogue and mixed signal semiconductor circuits has resulted in a revolution in hobbyist endeavours as well as a flood of low cost, high performance, professionally designed contraptions.  I have watched, over my thirty years in electronics engineering, enormous changes and advances in technology take place and a concurrent and dramatic rise in complexity and accessibility and fall in cost.

I have no desire to write detailed posts on the design or construction of electronic gizmos and will completely avoid the tedium of specifications and compliance requirements.  There is already far too much of that in my professional life.  The aim, here, is to tinker and talk about this from time to time.  Hopefully I can avoid letting out too much of the magic smoke that makes modern electronic gadgets work but, hey, that is sometimes just par for the course.

Although we sometimes think that we have mastered the electron, it is an illusion.  The domesticated electron is still a wily character and nobody can really be certain of its whereabouts at any time, as quantum physics has revealed.  However, we can coax it into doing a few things for us now and then but it does take a grand effort on our behalf to domesticate the electron on even the most basic level.