The Beeb Body Building course part 40
Counting on Geiger
By Mike Cook
This is a project that I have wanted to do for some time. However I have not undertaken it as I thought there would be insufficient interest. Now, due to the unfortunate accident at the Chernobyl nuclear power station, interest in monitoring radiation levels is high, so let's see how we can turn our trusty BBC computer into a radiation monitor. If you are not familiar with the jargon of atomic and nuclear physics, please read the accompanying panels on radiation.
I first worked with radioactive isotopes when I left school. The company I worked for made level detectors for blast furnaces. Basically, a beam of radiation is sent through the furnace and detected at the other side. As the coke, limestone and iron ore that fill the furnace are not very dense, the radiation passes straight through. As the iron is smelted from the ore the level of liquid iron rises inside the furnace. When the liquid iron reaches the radiation beam it absorbs the radiation, therefore nothing is detected and you know how full the furnace is. The circuit presented here could be used for exactly the same purpose for those of you with a blast furnace in your back garden.
More useful however might be the experiments that can be performed as part of a course of study in physics. The circuit could also be used to monitor changes in background radiation for those of you who do not believe the official figures, although I should stress that the measurements I have been taking show no cause for the slightest alarm.
There are many ways to detect ionising radiation, perhaps the simplest is with a piece of photographic film. This is the basis of the blue radiation monitering badge worn by all regular worker with radioactive material. Radiation will fog photographic film, the badge has a piece of film inside it and small areas are covered by varying thicknesses of aluminium. When developed the degree of fogging under the various thicknesses of metal indicate the types of radiation and the dosage received over the time the badge has been worn. There are two types of bagde wearers, those who wear the badge on the lapel and those who wear it on the belt!
As the film badges are sent away for processing every six months or so, they do not immediately indicate that an abnormal dose has been received. Immediate readings can be take with a pocket dose meter which is basically a gold leaf electroscope. A static charge is induced on two thin leaves of gold. As they have the same charge, they repel each other and so are forced apart as shown in figure I. Radiation entering the chamber causes the air or gas to ionise and so conduct a small amount of charge off the gold leaves, hence they move closer together. The closer they are, the more radiation there is about. This type of detector was used when radioactivity was first investigated.
The radiation detector most well-known to the public is the Geiger counter. This works on the same principle as the electroscope, that of making some gas ionise and thus conduct electricity. At the heart of a Geiger counter is the Geiger-Muller tube. This has two electrodes:- one is simply a hollow conducting tube and the other a single wire running down the middle, this is shown in figure II. It is filled with a mixture of gasses and a voltage is placed across it. This voltage is normally in the range of 300 to 1500 volts and depends on the tube's geometry and the gas pressure and mixture. What happens is that some radiation comes along and knocks an electron off an atom of the gas. This electron is then accelerated by the voltage across the tube towards the anode or positive electrode. As it picks up speed it bumps into other atoms of gas knocking out more electrons, these extra electrons are also accelerated and are involved in further collisions. The nett effect is that the tube becomes a virtual short circuit and so the voltage across it collapses. When this happens there is no longer any voltage to accelerate the electrons and so the tube stops conducting and a voltage can develop across it again.
The overall effect of all this is that, if we measure the current through a Geiger-Muller tube, we will see a pulse for every radioactive ionisation event which occurs in the tube. Some of the gasses in the tube are used to mop up the positive ions left behind by the electrons and "quench" the tube, this is normally done with a halogen. During this quenching time, the tube can not detect any radiation and so we call this the dead time of the tube. The "dead time" is normally in the order of a few tens of micro seconds and, as we shall see later, we can compensate for any counts missed due to the dead time. The voltage characteristic of a Geiger-Muller  is shown in figure III. It takes a certain voltage before any radiation is detected, then if you put more voltage across the tube you will get a higher count as the tube is more sensitive. There is a plateau region where the count is essentially independent of voltage and, finally, the count increases towards continuous discharge. The ideal operating voltage is in the middle of the plateau.
What a Geiger-Muller tube can't tell you is what sort of radiation caused the initial ionisation. To do this you need to use scintillation counter. These work by making the radiation cause a small flash of light in a crystal and then amplifying the light. From the distribution of the brightness of the flashes you can get the signature of the isotope and its radiation. Unfortunately scintillation detectors cost several hundred pounds.
Therefore for our radiation monitor we will use a Geiger-Muller tube as these give adequate results at reasonable cost. Geiger-Muller tubes come in all sorts of shapes and sizes with the tube voltage being the major variant but they can all be driven by a similar circuit. However, for this month's project I have narrowed down the field to a choice of two. The first is the cheapest I could find and is called the ZP1300 and is made by Mullard. Due to its price it does not look all that impressive, being a small glass tube 15 mm long and 6 mm in diameter. It looks rather like a neon lamp. However, at one end is a cathode strap that can be soldered directly into the circuit. To prevent heat damage solder it at the very end of the strap. The anode connection is made via a clip. No attempt must be made to solder the anode on directly, otherwise damage will certainly occur.
This tube has a sensitive length of 8mm and is filled with helium, neon and halogen. It is sensitive to gamma rays and also high energy beta particles (>0.5MeV). This is perfectly adequate for measuring background radiation even though it will not detect the lower energy beta particles nor alpha particles. To do this you need a tube with a very thin window. This is provided by the second tube on offer; the ZP1401, also made by Mullard. This is about twice the price of the ZP1300 but is the cheapest tube in the range which will detect all three types of radiation.
This tube is 43mm long with a 9mm diameter end window of mica which allows weak radiation to enter the tube. Care must be taken not to touch this end window as this will damage it. To protect against this, there is a thin plastic grating over the front of the tube. The sensitive length is 39mm and it is filled with neon, argon and halogen. Just like the ZP1300 it has a cathode strap and anode clip.
Some people are worried about X-Ray emission from monitors and TV sets. This is so low that it is very difficult to detect, Geiger-Muller tubes that are sensitive to X-Rays are available but they need some 1600V and cost over £150.
So, to detect any radiation we must place the correct voltage across the Geiger-Muller tube. In the case of the ZP1300 this is 550V and for the ZP1401 it is 500V. As they have plateau lengths of 100V and 200V, this is not critical and both tubes could be run at the same voltage. There are various methods of generating such a high voltage. My main criterion was to  produce the high voltage at as low a cost as possible using standard components. The basic idea is to use a standard transformer backwards. If you take a standard low voltage transformer and feed a low voltage signal into it you will get mains voltage out of it. So basically what we are doing is to pull current through a transformer, this sets up a magnetic field that induces a voltage in the other winding. When you stop the current flowing, the field collapses like an elastic band snapping back and an opposite voltage is induced in the other winding. If you do this rapidly enough you can rectify and smooth the induced voltage and get a voltage step-up effect.
If you do this current switching too fast there will not be enough time to let the magnetic field build up and so the step-up effect will be less. Therefore, by altering the speed of switching, you can alter the voltage output from the transformer. This is shown in figure IV along with the rest of the circuitry required for the radiation detector. Transistor T1 is switched by bit 7 of the user port. We can use timer 1 in the free run mode to produce a continuous square wave at this pin. The period of the square wave is determined by the value in timer 1's latches. Therefore, by altering this value, you can change the output voltage to suit other tubes. I  managed to produce a maximum of 650V from my prototype, so it should be suitable for both types of tube.
I used a transformer with a 3V secondary, in fact it was a 3-0-3 winding so I used only one side and left the other unconnected. To get the step-up to be greater than the 240V of mains, the winding is connected to the 12V aux. power supply of the computer. The circuit draws negligable current so you can continue to use it for other purposes.
Having got the high voltage, it is rectified and smoothed by a high working voltage diode and capacitor. The values of resistors around the Geiger-Muller tube are different for each tube and follow the manafacturers recommendations. Any pulse from the tube is capacitively coupled into a transistor and clamped by the diodes D2 and D3. This clamping prevents the transistor being subject to large voltages. The transistor then feeds its signal into bit 6 of the user port.
All the parts including a printed circuit board can be purchased as Body Build Pack 33. As you will see from the coupon on page XX you can get the tube and the rest of the circuitry separately. This allows you to chose the tube you want. Note also that you will need to have a user port cable. This is the one we always use and is sold as Body Build pack No.2.
Bit 6 of the user port can be used to decrement timer 2 when it is acting as a counter. By decrementing the hardware counter in the VIA you make sure that you see all the pulses produced by the Geiger-Muller tube. The other alternative would be to make the pulse produce an interrupt. However, there is a danger of missing a pulse as the interrupts are not always serviced immediately.
The software needed to drive this circuit is basically very simple. An example is given in listing I. First of all, you need to set timer 1 going at a rate to generate the correct voltage for the tube. Then you need to set up timer 2 as a counter clocked from bit 6 of the user port. All you need to do now is sit in a loop periodically sampling the value in the counter and subtracting it from what you had last time. ( remember the timer counts down ) Using the computer's internal clock you can collect the number of counts in a minute and display the result.
You could write a more complex program which would give you the count averaged over the last hour updated every minute. Or you could plot the  count rate on a graph and store the values on disc for later processing. You can correct for the dead time of the Geiger-Muller tube by using the formula:- True Rate= Observed Rate/(1-Observed Rate*Dead Time)
On the low rates of background counting this correction will give a negligible change.
Background radiation occurs randomly, so this means that there may be sudden bursts of activity and equally sudden lulls. Try placing a luminous clock or watch next to the tube and you can see the count rate increase. This is more obvious if you use older ones as the modern ones emit low energy beta particles that are harder to detect. For a really dramatic effect try some dials from ex-army equipment.
Relating count rate to dose rate is not easy as, to be really accurate, you must know what sort of radiation you are measuring. Each tube will be supplied with a copy of the manafacturers calibration curve, but to give you some idea, a count of 10 per second will indicate a dose of 2 mrads per hour with the ZP1401 and 25 mrads per hour with the ZP1300 tube. These figures are based on a Cerium 137 isotope which is a gamma emitter. This relationship between count and dose rate is logarithmic, so beware of extrapolating the figures.
So there you have it, radiation detection by your own fireside and, in the event of a nuclear war, the whole thing wont work because the mains will be out, but I'm working on that one. Hopefully I'll see you next month.


UNDERSTANDING RADIOACTIVITY
By Mike Cook

In order to understand what exactly is meant by radioactivity we must first understand something about atoms. Everyone knows that all matter is made up of atoms but what are atoms made of? Basically there are three different particles that make up an atom:- electrons, protons and neutrons. These are arranged with the protons and neutrons clustered in the centre or nucleus of an atom with the electrons surrounding them at some distance. A common model to help us visualise this is where the electrons orbit round the nucleus like planets around the sun.
The feature which differentiates atoms into different elements is the number of electrons surrounding the nucleus. All the properties of an element are determined by the number of electrons. For each electron surrounding the nucleus there is a proton in the nucleus, because electrons and protons carry equal and opposite charges and so balance each other. Neutrons carry no charge but act to bind the protons together. There is an optimum number of neutrons needed to bind together any given number of protons, but in practise there can be more or less neutrons than the optimum. When this happens we say we have an isotope. Isotopes have exactly the same properties as a  normal atom except that, having a different number of neutrons, they weigh a little bit more or less. Most people have heard of heavy water; this is simply water (two atoms of hydrogen and one of oxygen) with the hydrogen being an isotope. Heavy water occurs naturally and  in fact most elements have naturally occurring isotopes. For example silicon comes in three types:- Silicon 28 (92.3%) Silicon 29 (4.7%) and Silicon 30 (3.0%). The number following the element name is the number of protons and neutrons in the nucleus, we say this is the atomic number of the element. The percentage figures are the naturally occurring rates.
Some naturally occuring isotopes are radioactive but it is also possible to make isotopes artificially.
The alchemists' dream of turning base metal into gold was achieved in the thirties in a Cambridge laboratory. Simply take a little mercury,  bash it about a bit until you disloge a proton and an electron and you are left with an atom of GOLD. Unfortunately it is much cheaper to dig out of the ground than to make it an atom at a time. Still, just how do we bash an atom about? Basically we shoot bits of atoms (electrons, protons and neutrons) into the substance and hope to chip some bits off the atom. When we do chip a bit off, the resulting atom is usually unstable, as is it contains too much energy for its own good. In time it releases this energy  and it is this energy release that we call radioactivity. We can't tell how long an atom will hold onto this extra energy but we can say for a large number of atoms half of them will have released their energy in a certain time. This time is known as the half life  and is different for each isotope. Half lives are the most widely varying natural phenomena in the universe. They range from 10-28 of a second right up to half the age of the universe.
There are three major types of radioactive emmission:- Alpha particles are made from two neutrons and two protons. In atomic terms this is a very heavy particle. Beta particles are simply electrons which have been ejected from their atoms. Finally, Gamma rays are bursts of electromagnatic radiation. Electromagnetic radiation is a wave and the length of the wave determines what use we make of it. If the wave is very long (about 200 metres) we use it to carry radio programs and call it a  radio wave. As the waves get shorter we use them to carry TV signals or cook our food (Microwaves). Getting even shorter we have infra red or heat rays. Then, at a certain wavelength, we can see them so we call them light. When they get shorter still, the Ultra Violet is used for giving us (and EPROMs) a suntan. Then, shorter still, we have X-Rays and finally, Gamma rays. So you see, there is an awful lot of electromagnetic radiation about under different names. As the wavelength gets shorter, the wave carries an increasing amount of energy, so gamma rays have quite a bit of energy.
In this context we measure energy in units of "Electron Volts- eV " that is the amount of energy of an electron receives when acclerated to a potential of one volt. In practice this is a very small amount and so we talk about thousands or millions of electron volts (KeV or MeV). 
The energy of a particle or a wave can be thought of as being rather like its speed; the more energy it has, the further it can travel. This is important when we consider the effects of radiation. What happens is that the alpha, beta or gamma radiation travels along knocking into atoms and breaking bits off them. If it dislodges an electron we say that the atom has been ionised so we speak of this sort of radiation being ionising radiation. Each type has its own characteristics. The alpha particle is very heavily ionising, blundering along like a cannon ball. Fortunately it can't travel very far in air before all its energy is used up. It has a range of only a few centimetres and won't even pass through a piece of paper. On the other hand gamma rays can penetrate the earth to a depth of several miles but fortunately they do not do nearly so much damage. In the middle come beta particles. Depending on their energy you can stop them with a few inches to a foot of lead. They are more ionising than gammas but not nearly so much as alphas. By the way, lead is used as a radiation shield because it is dense and relatively cheap.
So, how harmful is radiation? Well everything is harmful if you get enough of it, for example daylight (electromagnetic radiation) can cause skin cancer, water will drown you and electric shocks can kill. It is all a matter of degree. The same is true of radioactivity. The safest amount of radiation would be nil but this is impossible to achieve as it is all around us. This residual or background radiation comes mainly from naturally occring radioactive isotopes in the soil and rocks and from cosmic rays from the sun. There are even some naturally occuring radioactive isotopes in our own bodies so we are constantally getting dosed from within.
What ionising radiation does is to ionise the fluid that makes up cells and upset their chemical balance. The result is that the cell dies. Normally this is of little consequence as the body continually replaces cells. The difficulty comes when a lot of cells are killed off at once with a large dose of radiation. As the body can't replace them quickly enough you will die. You are also in trouble if non-replacable cells are destroyed such as the germ cells in the ovary or brain-cells. At lower dosage levels a cell is not killed but can become damaged leading to changes in the characteristics of the cell. This can reduce growth, produce cancerous cells or produce genetic mutations. It must be remembered that genes may mutate spontaneously but radiation can act as an agent to increase the frequency of mutation. The sorts of mutations produced by radiation are no different from the naturally occuring ones. If a mutated cell gives rise to a new individual then the damage can range from harmless abnormalitites such as the inability to taste the chemical phenylthiocarbamide to gross abnormalities of the brain.
What level can we consider safe? Well there is a wealth of information concerning recommended safe levels. Basically if you are working in an industry concerned with radioactivity you are "allowed" to get a higher dose than a normal member of the public. Dosage is measured in Rads which stands for Radiation Absorbed Dose and is a measure of the amount of energy absorbed from radiation, 1 Rad is equal to the energy absorbtion of 0.1 Jules per Kilogram of tissue. At these low levels dosage is accumulated rather like green stamps, it matters not that the dose was received slowly over several years or quickly in shorter bursts. In calculating the amount of radiation that can be received the Rem is used. A Rem is simply the dose in  Rads multiplied by an ionising factor dependent on the type of radiation. This would be 10 for alpha particles and 1 for nearly everything else. The maximum dose can be calculated from the age of a worker and is given by:-
D=5*(Age-18) Rems
This is simply an empirical formula but it shows that a 30 year old could have accumulated a maximum dosage of 60 Rems if he started work at the age of 18. At retirement he could have received 235 Rems and providing the rate was not greater than 3 Rems per 13 weeks it is considered that there is a negligible probability of any damage being caused.
For non workers in the nuclear industry the "safe" dosage has been set at 50 mrems (milli rems) per year compare this with the normal background dose averaged over the whole UK of about 100 mrems per year. This will be higher in some areas of the country than others. In Aberdeen for example, due to the natural uranium in the granite, it can be 120 mrems per year and in limestone areas such as the Mendip hills it can be down to 40 mrems per year. So a "safe" dosage is deemed to be about half the natural level found in this country. In other countries natural radiation can be very much higher. For example in Rio de Janiero the background dose is 1,000 mrem per year. However the record goes to the state of Kerala in India with a rate of 2,600 mrems per year. The 100,000 people living there will get a larger dosage of radiation in their lifetimes than we allow our nuclear workers to accumulate. In the light of this, the cloud of radiation from Chernobyl that passed over my house amounted to an increase of one fortieth of the normal background dose for the year.
For further information on the subject of radiation hazards see:- The Hazards to Man of Nuclear and Allied Radiations. H.M.S.O. 1960.