Building a radioactivity meter (a Geiger counter)
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Year 2008 is the centenary of the
invention of Geiger tube by Hans Geiger (right) and Ernst
Rutherford in 1908. The original tube design, able to detect only alpha
particles, was later modified by his PhD student Walther Müller in
1928, becoming the Geiger-Müller tube (GM tube) and used since then for
many years to detect and measure ionizing radiation (alpha, beta and
high energy gamma radiation). |
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Radioactivity: get your fears away
Radioactivity is among us
and as natural as a sunset. There is a general fear levelled to a
general ignorance about it. There are radioisotopes like uranium, radium
and thorium present in soil, in ocean water, in drinking water, in fruits, vegetables and
edible meat from both wild and farm animals. Even our bodies
have radioactive isotopes, like carbon-14 or potassium-40. Most building
materials for our homes like granite, wood or gypsum also contain
radioisotopes...
Does this mean we live in a dangerous and polluted world
and should blame the nuclear industry and scientists? Not at all. Most
of these radioactive materials are present since this planet was born.
Life appeared here with radioactivity and evolved with it. Life on
Earth is totally compatible with the level of radioactivity found in our
environment.
Why building a Geiger counter?
A Geiger counter can be build easily at home or as an university project and is a very rewarding gadget. The GM tube is itself a small living lesson on physics. Just connecting it to a DC source and plugging an oscilloscope to catch the voltage peaks on the resistor every time it's ionized it's a revelation of the presence of the radioactivity around us.
Another interesting side of building a Geiger counter is the electrical/ electronic side. The high DC voltages necessary to polarize the tube (500 - 900 V) can be nowadays obtained from batteries with some few and not expensive electronic components. The addition of a small microcontroller and an LCD, affordable components to hobbyists and students, plus a little software can result in a nice and almost professional equipment.
Yet another reason to build a Geiger counter is simply to have it at home. Smiling? Well, many people buy small nice weather stations even if the best forecasts can be seen for free on TV with the data of many stations and satellites. Why not having a Geiger counter at home? Personally, I find them lovely amazing when they sing their random song: beep.... beep... beep. Some people find this macabre, but it's Nature as well, we must accept it. It's there.
The heart of a Geiger counter is the tube. Even if it's a century old invention and has not changed very much since then, it is still a good and reliable sensor. We can find basically three types of GM tubes, depending on the radiation we want to detect.
| GM tube type | Detected radiation | Characteristics |
| no window | gamma | General purpose. Cheapest. Good enough for ambient radioactivity measurements |
| glass window | gamma, beta, X-rays |
More flexible and professional. Medium cost. Allows a qualitative analysis of radioactive elements present. |
| mica window | gamma, beta, X-rays, alpha |
Very professional. Not very high cost. Provides a good analysis of radioactive elements present. |
How does a GM tube works?
Most GM tubes look
like metal covered glass cylinders with just two connections, see
pictures below. Inside
they are filled with a noble gas (Neon is usual, but Helium or Argon can
also be used) plus a small amount of a halogen (Penning's mixture). Electrically, a GM tube is a cylindrical capacitor
with the gas as the dielectric. A
wire placed along the axis acts as one electrode and the cylindrical
metal shield as the other. A large DC voltage (between 500 and 1200 V) is set up between the electrodes with no current
normally flowing through the gas.
If any ionizing
radiation enters the tube and breaks some gas atoms into ions (that is,
if it has enough energy) the ions
are accelerated by the electrical field and collide with other atoms
thus multiplying hugely the number of ions inside the tube. This is
known as 'avalanche effect'. The result is the dielectric break and an
electrical charge flowing through the gas and the electrical
circuit the tube is connected to.
Close-up of a GM tube. In the
picture below, the mica window is shown. The right picture shows
the contact for the central electrode and the outer shell. |
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Below is the basic circuit to operate a GM tube. Very hard to imagine any simplification. When the tube is ionized, the tube's electrical charge is discharged through the resistor R1, therefore with a time constant of R1·CT (CT is the GM tube capacitance, usually around 5 pF for medium size ones). Values of R1 in the order of mega ohms produce pulses of 2 - 3 V peak and lasting for some milliseconds, enough to drive directly an MCU pin triggering an interrupt.
How to create GM tube voltages?
One could think that obtaining voltages above 500 V is not easy for the home builder and even dangerous. Surprisingly, with today's technology is not only easy and affordable, but also cheap. And circuits producing these high voltages have such low power that are absolutely harmless. A simple circuit with a 555 IC and some few more components can generate up to 1 kV for your GM tube from batteries!.
Circuit below shows a generic step-up circuit able to deliver up to 900V to a Geiger-Mueller tube from battery voltages. The key elements are inductor L1, MOSFET M1 and diode D1.
M1 has to be a high voltage (600, 800 or 1000V)
N-channel power mosfet. Not necessarily a high current type, but low Ron
types dissipate less power which is important with batteries. It must
have a low VGS(ON) voltage.
D1 has to be an ultrafast, high Vrr diode. Inductor L1
is usually about 10 mH and rated for currents in the range 100-200 mA.
Low DC resistance are preferred as they have lower losses and higher
performances.
VCC can be 6V from 4 AA-size batteries. If using lower voltages, for example from NiMH (1,2 V each), an intermediate step-up circuit to 10 - 15 V is recommended to avoid losses in L1 and M1. Pulses frequency depends on the design but usually aren't troublesome high frequencies, in most cases below 50 kHz and usually around 10 kHz.
How to choose the right GM tube?
It's simple. There are just two factors to take into account. One is the desired sensitivity (size and price are proportional to it). It has to be said that a high sensitivity tube is just faster, a lower sensitivity tube can obtain the same measure if a longer time is allowed. The other is the type of radiation to be measured. Gamma tubes are the cheapest and good for measuring natural radioactivity. A beta + gamma tube can detect any 'unnatural' radiation by doing a second measure blinding the window with a lead lid. An alpha + beta + gamma tube will provide a lot of information about the type of radiation present as three measures can be done using a filter for alpha and alpha + beta.
An example of a home built Geiger counter
This is a Geiger counter built by the author. Even if assembled as a prototype, it's quite handy (160 x 80 mm) and fits into a portable box. It has an LND7121 beta + gamma tube operated at 550 V. It lives on three AA NiMH batteries (lower right corner) and has circuitry to recharge them with a DC supply. The MCU (longest IC) manages step-up voltage conversion (dual from 3V6 to 12V and then to 550V), measurement conversion (from cps to other units), and communication with an interface panel (mounted above) with alphanumeric LCD, four pushbuttons, three LED and a buzzer. |
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Geiger tubes details: sensitivity chart, dead time and saturation
Detection of radioactivity by a GM tube has some
statistics involved. First of all, any radioactive source emits a
spectrum of energies. Not all quantum crossing the tube have the same
energy. The weakest ones maybe will never be detected by a given tube.
Those energetic enough will have a chance
(depending on their energy and tube characteristics) to ionize the gas
and be detected.
Every time a GM tube gets ionized by the radiation,
there is a little spark inside it that discharges it and has to be
charged back to the operating voltage through the RC circuit. This takes
a short time, usually some hundreds of microseconds. That time is called
the tube's dead time, as for that time the tube will
not detect anything.
Looking at the example sensitivity chart below, we see a good
straight line at low radiation levels. As the radiation level goes up,
the line becomes flat and constant at about 10k cps (counts per second). This is directly
related to the dead time: the tube gets ionised and when it recovers
back, gets ionised again immediately and the cps is the same
independently of the radiation.
A GM tube should be used in the linear zone and a good Geiger counter software should detect and warn about tube saturation. The home builder measuring at home and outside just for fun should not worry, as normal radiation levels are well below 1cps (for tubes like this one) and 10 cps is just enough to go straight away of that place and inform the authorities!