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How an ionization chamber works

GM tubes and ionization chambers work on broadly similar principles

A GM tube and an ionization chamber work on similar principles but there are some important differences.

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With a GM tube the source is outside the tube so most of the radiation goes undetected.  With an ionization chamber the source is inside the chamber so it detects all the radiation that makes it out of the source.

A GM tube is a fairly complex piece of apparatus and requires an air-tight seal to house the low-pressure gas.  An ionization chamber is simply a metal can with a lid.  It doesn't need to be air-tight.

A GM tube operates at a high enough voltage for each ionization to cause an avalanche of further ionizations.  An ionization chamber operates at a lower voltage.  The ionizations cause the air in the can to conduct and the current is measured.

GM tubes can detect single particles but take time to recover after each count so can’t measure very high activities.  Ionization chambers aren't good at measuring very low levels of radioactivity but will cope with very high levels.

Ionized air molecules if left alone quickly become neutral again

If we put an alpha source in our ionization chamber the alpha particles ionize the air molecules in the can.  When it loses an electron the air molecule is left with a positive charge.  The electron and the positive ion quickly recombine and the ion turns back into an uncharged molecule.

With zero voltage all the ions recombine.

Increasing the voltage keeps the ions and electrons separate

As the voltage is increased to say 100 V the can becomes negative so the positive ions move outwards towards the sides of the can.  The source becomes positive and the electrons are attracted to it.

Since charges are moving (the electrons and positive ions) this constitutes a very small electric current.  It might be a few microamps (millionths of an amp) but it can still be measured.

Above a given voltage, say around 300 V none of the ions recombine with their electrons.  Even if you continue to increase the voltage the current doesn't change.  It reaches a maximum value.

Why current reaches a maximum value

Ionization chambers always operate at a voltage high enough for the current to saturate like this but not so high that there’s an avalanche.

Let’s look at why the current reaches a maximum value.

An electric current is a flow of charged particles.  In a simple electric circuit the charged particles are electrons.  These free electrons are already there in the wires.

The electrons just go round and round the circuit.  Like a wheel, there’s no real limit to the speed they can go.  When electrons leave one part of a wire they are replaced by the bit next to it and so on all the way round.

However in an ionization chamber, the ions and free electrons aren’t already there.  They are created by the radiation.  As they come into contact with the walls of the chamber they are lost to the circuit.

A very radioactive source replaces ‘lost’ ions very quickly so the maximum current is big.  A less radioactive source replaces ‘lost’ ions more slowly so the maximum current is smaller.

Ionization current is proportional to radioactivity

The important point is that the ionization current is proportional to the radioactivity.  So if the radioactivity changes with time then this can be monitored by simply monitoring the current.

Conventional current: two halves make a whole

Each alpha produces lots of ion/electron pairs.  But because we’re dealing with pairs do we have twice the current?

Let’s think about conventional current.  This is the flow of positive charges from positive to negative.  If negative charges are flowing (like electrons) then we have to replace them with imaginary positives flowing in the other direction.

Imagine a single ion/electron pair created in the middle of the chamber.  The positive ion travels to the wall of the chamber, which is negative.  This is fine because a positive charge is moving.  The electron travels to the source, which is wired to the positive.  This is a problem because it’s the flow of a negative charge.

We have to imagine replacing the electron with a positive charge flowing from the source to the creation point.

So we have one complete journey.  The first half is made by an imaginary positive charge, the second by a real positive ion.  So one ion/electron pair means one complete journey from positive to negative, not two.

So when we think about the current flowing we can think about the positive ions OR the electrons but not both.

Some common calculations

We measure the current.  It’s the same as the charge carried by all the ions (or electrons) passing a point each second.

If we know the current then we can calculate the number of ion/electron pairs produced per second.  Typically of the order of several billion.

And if we know the number of alphas produced per second we can calculate the number of ionizations per alpha, which is something of the order of a hundred thousand.

back to Lesson 11: Ionization and Detection