[Catalist] Physics phun - detecting ionising radiation on the cheap

[Leon Harris]

The crypt opens and the ghosts of past sciences emerge.

Once when I was poor and cheap I wanted a geiger counter. A month or so 
was wasted before I realised that the neon globe I sought to make a 
geiger tube actually worked due to the tiny bit of thorium inside it, 
and that I was stuffed if I wanted to use outside radiation sources to 
make it go, because it already had its own. This was the time when I was 
working on "Bunnings Nuclear Physics" and always on the scrounge to make 
a cheap lab.


This was a successful spinoff from those times. It is an ionisation 
chamber, with an extremely sensitive semiconductor called a field effect 
transistor (fet) at its heart. They are called mpf105, and are cost 
about a buck.

It works like this: two 9 volt batteries supply a potential between the 
inner copper electrode and the tin can. When ionising radiation collides 
with the air particles inside, they, well , ionise it. Now we have a 
fluid (that which can flow) with free moving charges (the ions) in it. 
This allows a tiny current to flow and charge to build up on the gate of 
the field effect transistor.


FETS are cool - in theory they need no current, only a charge on their 
gate (in practice that means enough current to make some electrons 
accumulate on the tiny gate). The forces those charges exert on the gate 
push electrons away from the gate into the part of the FET that can 
conduct. Now that region has extra mobile charges, and a much larger 
current can flow. You can see this as the increase in voltage on the 
volt meter (or and extra extra sensitive current meter, if you have one).


What is cool about this is for a couple of bucks you have a radiation 
detector. It also detects ionised air. Placing a burning flame, such as 
from a candle or a match in it makes it go nuts ! These kind of 
experiments in lower school are very good for reinforcing the idea of 
what current and electricity is - it also shows a lot how modern 
electronics uses that electrostatics stuff that seems so fun but 
irrelevant in year 8 !


Into the skip you go ! <toss>

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