2014. június 28., szombat

Building a radiation detector from scratch 1 - the basics.

Before you begin

If you follow this series of articles and maybe do some research on your own, you'll be able to build a simple radiation detection device and perform some simple, interesting experiments.

I chose to build a simple, but surprisingly effective device called an ionization chamber or ion chamber, which is just a very sensitive current sensor, or resistance measuring device, the difference being only the point of view.

A great source of information on ion chambers is: http://www.techlib.com/science/ion.html

Ion chambers are not Geiger counters, however they work according to the same basic principles, as will later be explained.

You will need to be able use soldering iron, and perform current and voltage measurement. If you don't know how to do these things, it's better to ask someone to show you. Please only perform the tasks written here if you know what you are doing. Soldering requires high temperature. We will work with high voltages at some point, and of course, there will be some radioactivity involved. Any of these things alone can be dangerous, so exercise caution and use common sense. Hurt yourself, blame yourself.

To really build from scratch, one needs to understand the basic principles on which the device operates. I'll describe these basics in two articles, this one will be on nuclear radiation itself, the second will be on basic electronics.

This series of articles should be considered loose notes on these subjects, and will only contain material vital for our purposes that is, building radiation detectors. If you don't understand something, do your research, or ask in the comments.



Bill of materials in advance

I tried to keep the circuits and the overall project as simple as possible, using the most common resources available. Below are an approximate bill of materials. This might change as we go along, but will certainly be enough for the first one or two experiments.

  • Transistors. I used BC547C types, because of their high gain, and their price, which is next to nothing. You will need 3 in the basic circuit or 6 for the more advanced one, but I recommend you to buy at least 50, since we will need to match a certain parameter of them to be able to get any reasonable stability. You can probably use any NPN bipolar transistors with current gain around 500 in a TO-92 package. We will connect three of these to achieve something around 1 to 2 hundred million current gain.
  • Resistors. The actual value you might want to use will be between 1 KOhm (kilo ohm) and 100 KOhm (or 1K and 100K). 10K is a safe bet. These are also very cheap, you might want to buy 10 each from the following values: 3.3K, 4.7K, 6.8K, 10K, 15K, 22K, 33K. Ask for through-the-hole, 1/4 or 1/2 watts, metal film types. These are probably the cheapest too. Don't buy SMD (surface mounted) types, unless you really know what are you're up to :)
  • If you want to build the high precision version, you might also want to buy a trimmer potentiometer, a 10K will probably be all right.
  • A digital multimeter - Make sure it can measure millivolts too. One with a short circuit indicator will help a lot. Probably this one will be the most expensive item in this list. If you want to build a really cool looking device, buy a 100uA (microAmper) analog current meter too, and use it with a 1K-10K resistor in series, as later will be demonstrated.
  • A 78L12 integrated circuit device. It should look exactly like the transistors above (it has a TO-92 package), and needed to keep the voltage fairly constant around 12 volts.
  • Two 1uF (microFarad) capacitors, aluminium electrolytic, 50V, needed for the 78L12voltage stabilizer.
  • I'm planning to build a high voltage power supply, but I'm not completely sure about the exact design. It probably will be a Dickson charge pump driven by an astable multivibrator.
    • For that, you'll need two more from the transistors above, they don't even have to be matched.
    • You'll also need 20 diodes, 1n4148 will be OK.
    • 20 100p (picoFarad) capacitors, at least 50 volts.
    • one 10nF capacitor, 400V
    • This version might need a "guard ring", a 1-2cm long coaxial cable will suffice. The exact type of cable doesn't matter much.
  • A soldering iron. Use low power type suitable for the components above. A 30W iron is a good choice. Choose one with a small, pointy tip (need not to be needle sharp though).
  • Solder. Use thin, flux core solder.
  • Empty tins cans, two of the same shape, circular, flat. Try to get ones with 8-12cm diameter. The ones I used contained tuna. When buying the cans, use magnets to check their material. You need cans that stick to the magnet (steel cans). Aluminium cans cannot be soldered (easily), and must be drilled and equipped with screws, washers & pins to create reliable electrical contacts. As long as you can get steel cans, don't bother with aluminium ones.
  • Some fine sandpaper for sanding the tin cans, since they are likely to be coated with a thin layer of plastic foil or paint. This need to be removed.
  • A sheet of thin aluminium foil (the thinner the better)
  • Some electrical tape / duct tape
  • Some wires
  • 2 9V Batteries
  • 2 Battery clips
  • Some source of radiation for testing - you might already have some ;)

What is radiation?


The word "radiation" can mean a lot of things depending on the context. It is used in many scientific fields and pseudo-scientific settings. For our purposes the word radiation will mean nuclear radiation, the kind of energy emanation that happens as a consequence of nuclear processes, or those coming from the nuclei of atoms.

We will take a close look the three most common types of nuclear radiation. These three share an important propery: they are all a kind of ionizing radiation.

The makeup of matter

To make any sense of the stuff above, you have to know a thing or two about the structure of matter, about how the everyday stuff you see is built from basic building blocks.

If you've been through elementary school, you probably know that stuff around you and even inside you, is made out of tiny particles, called atoms. Though once thought to be the final, indivisible constitutents matter, atoms too are built from even smaller particles.
First of all, every atom has an outer cloud of electrons loosely jiggling around the small, and very dense nucleus. The nucleus itself can be divided into particles called nucleons of two types: protons and neutrons.

All the chemistry and electronics happens in the outer electron cloud of atoms. Nuclear decay and other nuclear processes such as fusion and fission happens inside the nucleus or between atomic nuclei.


Alchemy FAIL


One very important difference between the chemical and nuclear processes is the amount of energy involved. The electrons are really just very loosely around the nucleus relatively to how densely the latter is packed. This means that the amount of energy in the nucleus is much, much higher than in the electron cloud.

This means that no chemical process can ever influence nuclear processes. This is why alchemists have never succeeded in creating gold by mixing any number of materials together. The energy levels in a beaker are simply much, much too low to possibly be able to influence the nucleus. Given that gold is an element, and has a unique nucleus, it would require changing one kind of nucleus into an other one. Neither ancient alchemy nor modern chemistry had or has any chance doing so.

So it is utterly impossible to influence nuclear events with chemical ones, it is quite possible and natural the other way around. Nuclear processes do effect chemistry and electronics. This is what we will exploit in our nuclear radiation detector.

Electronvolts

OK, we've talked much about energy levels, but never mentioned numbers, just "large" and "small". So, what counts a "large" and "small" energy at the atomic and subatomic level?

First, we need a good unit for measuring energy. The SI unit is joule, which is useful for measuring energies in everyday life. But this unit is too big. A joule is the amount of energy that needed to move something with 1 newton force through a 1 meter path, or the energy you get when a coulomb charge passes through a resistor with 1 volt through it's pins. One would need to accelerate particles to relativistic speeds to achieve such energy levels.

There is a nice, small, non-SI unit of energy that can be used for our purpose. This is the electronvolt, or eV. One eV is very small compared to joule. 1 eV = 1.602177*10-19 J

One electronvolt is the amount of inertial energy an electron has once it has been accelerated with one volt. If someone would build an electron tube and connect a single volt to it's accelerator plate, the electrons arriving at the plate would have one electronvolt inertial energy.

Visible light comes in packets of energy called photons, with energies between 1.6 eV and 3.4 eV. Red photons have lower, blue photons have higher energy.

When burning hydrogen, the amount energy released is 286 KJ/mol. For one  molecule that is 286 / (6*10-23) = 4.77*10-22 KJ = 4.77*10-19 J = 4.77 eV. Such energies are typical in chemistry.

The energy of radiation particles are greater by a factor of hundreds of thousands to millions.

So what nuclear radiation really is?



When an atomic nucleus decay, it turns into an other kind of nucleus. Yes, what alchemy failed to do, happens in nature spontaneously all the time. Unfortunately gold won't form just like that from any cheap stuff, so this fact won't make us magically super rich overnight (or at all). However, the byproduct of nuclear decay is a big burst of energy - nuclear radiation. There are three types of nuclear radiation we will consider here.

Alpha radiation


is a stream of very high speed helium nuclei, consisting of two protons and two neutrons. The inertial energy of these nuclei typically in the ballpark with that of an electron accelerated with a voltage of five million volts, or to put it differently, is around five mega electronvolts or 5 MeVs. This is about two million times higher than the energy of a single photon of visible light (1.6 eV to 3.4 eV).

Alpha radiation can be very harmful if the source gets into the body, but quite harmless otherwise, since alpha particles are stopped by a single sheet of paper, the dead skin cells on your hand, or a few centimetres of air. Alpha particles can only penetrate very thin materials, such as the gold lead in the famous Geiger-Marsden experiment (also called the Rutherford gold foil experiment).

These effects are all because the alpha particles are heavy, charged, and interact quite readily with any kind of ordinary matter, so they deposit their energy into pretty much the first thing they bump into. They can easily rip the electrons from the electron clouds of atoms, and can even fuse with light nuclei (such as aluminium). The products of such fusion are often unstable, and themselves undergo radioactive decay, they can emit neutrons, protons, electrons or gamma rays (see below).

Beta radiation

Beta radiation is a stream of high speed electrons. They are produced together with a so-called anti electron-neutrino (wich is extremely hard to detect), and a proton when a neutron decays.

The typical energy of a gamma particle is around 1MeV, but it can be as low as a few KeVs, or as high as a few tens of MeVs.

Beta rays tend to penetrate deeper into matter than alpha rays, so they are harder to stop. A few millimeters of aluminum, or a centimetre or two of clear plexiglas or polystyrene can stop most of the gamma particles.

One dangerous property of beta radiation is that it can produce secondary brake radiation, or Bremsstrahlung. When a fast electron hits something, it decelerates and some of it's energy is emitted as a high energy photon. This is how X-Ray machines work: they shoot electrons at a metal plate, and the collision produces X-Rays.

The heavier atoms the obstacle the electron hits has the more Bremsstrahlung is produced, so beta radiation is better stopped with materials containing light atoms: aluminium and plastics (carbon, oxygen, nitrogen, hydrogen) are good choices.

As we already mentioned, beta radiation also a kind of ionizing radiation. These high-speed electrons can knock off other electrons from around atomic nuclei, much like alpha radiation, but with much less vigour.

Gamma radiation

Gamma radiation is electromagnetic radiation. It is the same kind of energy than light, or radio waves, only quite a bit more powerful. The typical gamma ray has a few hundred KeVs of energy, but can be much lower or higher than that. This is no way a definition, only a rule of thumb.

Gamma rays interact more reluctantly with matter than alpha or beta rays, but (or because of that) they penetrate deeper. One needs several tens of centimetres of lead to "stop" gamma rays. Even with such shielding, gamma rays cannot be stopped 100%. You can get arbitrarily close to that, but never truly reach it, there’s always a tiny chance of one or two of them slipping through. In practice, a few large lead bricks can protect even highly sensitive equipment from being affected too much by natural background radiation.

Gamma rays can interact with the electrons in atoms by giving them some of their energy probably knocking them off from their orbits. This phenomenon is called Compton scattering or the photoelectric effect.

There are materials that can change their electronic properties when hit by ordinary, visible light because of similar effects. Such materials are used in digital cameras for example.

Other kinds of radiations

There are as many type of radiation as particles - one could even accelerate mercury nuclei and call it "mercury radiation". However, in nature, and for our purposes, these three kind of radiation is all we care about, but there are some other cases that I find worth mentioning.

I already wrote about neutron and proton radiation, but these are relatively rare. Also beta radiation can consist of positrons (anti-electrons), but they will quickly annihilate with a regular electron producing (in the most common case) two photons with an energy of about 511 KeVs.

There are some other exotic particles that do happen in nature, mostly because of cosmic radiation. The most common particle radiation at and around sea levels is the muon. This particle can put out quite a show in spark chamber (look one up at YouTube).


Detecting radiation

All detectors rely on the ionizing nature of nuclear radiation. Our solution will measure the conductivity of air. When air subjected to radiation, some of the air molecules will be stripped of an electron or two. The electrons and the resulting ions will drift away from each other if there's an electric field around.

The conductivity of air is very, very low (it doesn't conduct electricity very well). Even high voltage wires hanging on poles won't cause much current to flow in the air.

However, air can become quite conductive when ionized. Very high voltages can rip electrons off from the air molecules. Such high voltages than accelerate these electrons so quickly than when they encounter an other air molecule, they themselves can knock off more electrons from it. An avalahce effect takes place. This happens during thunderstorms, or most of the time when an arc forms.

Ions can form in the air if there are energetic particles passing through it. If we measure the conductivity of air, we can guess the amount of radiation passing through it.


Ion chamber


We will build a metal chamber, and stick a wire inside it, than apply some voltage between the chamber and the wire probe. If we can measure the tiny current flowing between the probe and the chamber walls, we can see how much ions are in the chambers. Since the currents are in the femto- to picoampere range, we need to amplify the current a few million times.

How can we do that? See the next article.