Geiger counters (and Geiger counting) for beginners. Especially suitable for you if you thought Geiger counters were beyond your understanding!
By Dorian Stonehouse
With equipment borrowed from a radiation test facility
TO get a better idea about Geiger counter tubes and Geiger counters, before you go on, please visit:
Handy radioactive check sources for when your are ready to explore with your Geiger counter
But don’t forget: you do not need any check sources to make your Geiger counter click.
It will always be clicking, as it registers natural background radiation events from our environment.
I used to go hunting for luminous watch and clocks in Bric-a-brac shops (stores)!
Geiger counters are not perfect by any means. Even if no radiation existed on Earth, spurious clicks – fake events born inside the Geiger detector tube, will still occur.
I bought old mantles to get an idea of what radiation is all about
Even the Russian Geiger detector tube SI-8B described previously on electrosparkles.com is quoted as having “Own Background < 2 Pulses/s” (explanation later).
The old Tilley lamp – a fantastic piece of kit
So, even if you placed your Geiger counter instrument in a thick lead box, it will still be clicking away because of its “own background,” – fake events born inside the Geiger detector tube itself.
I bought this old lamp with mantle at a bargain basement price in Ammanford
So how do I know when radiation readings are real or fake?
There is only one sure way of overcoming “own background” fake readings (clicks) born inside the detector tube, and that is:
get to know your Geiger counter; AND get to know your radiological area – the place you are testing for radioactivity.
Glass coloured by radioactive mineral (Depression glass)
Let me explain some more: Suppose that under normal conditions (no nuclear incident has occurred) you are using an SI-8B Geiger tube and our homemade Geiger counter in Cwmllynfell and its clicking away nicely.
Check source from Ludlum Geiger counter – discussed later
You know that your SI-8B produces:
“Own Background < 2 Pulses/s” So, (fake events – clicks) are less than 2 counts per second.
Okay, lets even things out and say fake events take up: 1 count per second.
But after one minute your counter shows 80 counts.
So, check sources are for testing if your Geiger counter is working properly and to make sure it reads accurately
You take away 60 from your total of 80 straight away, because 60 are “Own Background” (fake event clicks). You are left with 20, which cannot be “Own Background” (fake event clicks). So, 20 counts is the normal background for Cwmllynfell!
Don’t worry, we’ll soon get to the Professional 1202
One last example
Looking at it another way: you’re a conductor on a bus in Cardiff. You know that 1000 passengers use the bus per day.
Each passenger is a real person (an actual event) and every time the conductor clicks his machine a ticket is dispensed. However, every day the conductor gets bored and fiddles with his ticket machine, making it go click! click! click…
These clicks – 500 of them do not represent real people. His daily total of real people are 1,500 total clicks, minus 500 (fake event clicks) = 1000 real people – events – yes?
A neat first line Geiger counter and effective dosimeter
The “Rad Alert Professional 1202” is an excellent Geiger counter (effective dosimeter) for the beginner and professional alike.
It has been used by the fire brigade (fire service) as a “first line” instrument when called to industrial fires.
But alas! I do not think they make them anymore.
The “counter? (events)” display
First, I choose “counter? (events),” and the model 1202, starting at 1, begins counting upwards.
After 1 minute (whoops!) the display looks like this:
So in the Ammanford area, you can expect about 12 counts “tot” per minute (12cpm), minus x for the fake event (clicks), say minus 2 cpm = about 10cpm
Let’s get a bit more accurate
Letting the model 1202 click for one minute is all well and good for starters, but radiation figures vary up and down over the day (like my blood pressure!); so to be more accurate, we need to see what happens over a longer period and take the average for that period.
So this time we’ll let the model 1202 run for just over 20 minutes (1,207 seconds).
Looking at the total number of counts for that period = “316 tot.”
So, for one minute (60 seconds), its 60 divided by 1,207 (60 sec over the 1,207 sec) = 0.050 (rounded off)
X 316 (the tot counts) = 15. 7 (average cpm)…
Another useful Geiger counter: the Mini-monitor reads about 0.5 – 1 counts per second in Ammanford, so reads >30 counts per minute
The false event readings (clicks) total approximately 2 counts per minute x 20 minutes = 40 counts per 20 minutes total.
So, the average is 40 divided by the number of minutes = 20.
This gives an average false event (clicks) of 2.
Take 2 from the 15.7 (average cpm) and we get 15.7 – 2 = 13.7 real events per minute (cpm), based on a run time of 20 minutes.
Compare this to the 60 second run time, which gave 10 cpm⇑⇑.
What a dose!
Imagine a constant water mist that covers everything in sight
Just imagine being under a fine water mist for several hours every day. The dosage of water hitting me could be measured in litres per hour.
Say the atomiser bottle in the picture can deliver 5 litres of mist per hour. Spread over my whole body, that’s not too bad, as I could go and dry off, so getting rid of the water (scrub down)?
But what if I got a dose of 50 litres of mist an hour? Now, I’d be soaked and likely to get a nasty cold!
If it makes you feel better, you could call them Johns per hour, or Bettys per hour; but radiation dose rate is understood as being:
Sieverts per hour
Up to now we have been looking at counts per minute; per 20 minute. But we could have let the Geiger counter run for an hour. This would have given us counts (clicks) per hour.
But as we know, Geiger counters can vary in the number of counts they show per same period. For example, the Model 1202 will register around 13 AVERAGE counts (clicks) per minute and a slightly higher AVERAGE count per hour…
These clocks represent our Geiger counters – all can have different number of clicks (counts) per minute
But they must all be calibrated to show the right time!
The mini monitor shows more than 0.5 counts per second, so > 30 counts per minute, again with an AVERAGE hourly reading a bit higher.
It does not matter how many Geiger counters in one spot that show differing counts per same period, as long they all show the same dose rate per hour – that is sacrosanct!
Remember: dose rate is in Sieverts per hour (just think of that mist!)
So, Sieverts per hour is another way of looking at your radiological environment in DOSE PER HOUR, as opposed to counts per minute or counts per hour.
When measuring radiation dose rate per hour, the Model 1202 can read the environment for a maximum of 15 minutes, before giving us an average dose rate per hour figure.
So, we start the 1202 at 900 seconds, press the green button and she starts to count down to zero. Have a look:
The Rad Alert Professional 1202 Geiger counter and effective
radiation hourly dose meter
Still counting down
After 880 seconds (just under 15 minutes) the dose rate per hour in Ammanford is
The Inspector ALERT Nuclear Radiation Monitor – a good comparison
Comparing against the pro 1202, we get the following reading on the Inspector Alert:
Neat little window on the Inspector Alert nuclear radiation detector showing 0.05 microsieverts (µSv) per hour
So, in Ammanford the 1202 gives about 13 counts (clicks per minute) and the Inspector gives 38 counts per minute.
But both meters give around 0.05 to 0.06 microsieverts per hour, meaning they both tell the right time – so to speak! (Look at the clocks above).
So, I no longer have to know how many clicks (counts) occur per minute?
It’s always handy to know about counts per second/minute – sure.
But if I shoved a strange Geiger counter in your hand (THAT YOU DID NOT KNOW MEASURED 200 COUNTS/MINUTE NORMALLY) and asked you to measure the radiation level in area X, you wouldn’t have clue what to do – unless it was calibrated to read in microsieverts!
So, I arrive at area X
but what is a dangerous level of radiation?
In the Amman Valley, a figure of 0.1 microsieverts (recurring) per hour might indicate a nuclear incident somewhere!
Readings in the 1 microsievert range (and slightly upwards of this figure) would indicate a serious radiological incident, but would not present personal danger.
Radiation workers might be expected to carry on working in levels greater than 5.7 microsieverts an hour (uSv/hour) for one year = > 50 millisieverts (mSv) per year.
It is extremely unlikely that we will ever be exposed to a radiation dose of 1 Sievert per hour⇑⇑ (1000,000 microsieverts).
Such a dose would probably not kill us instantly. It would, however, give us radiation sickness: nausea, vomiting, bleeds and weakness; and possibly give us cancer later on.
So, three minutes exposure at 16.6millisieverts for every one minute = 49 millisieverts would be our exposure limit for safety with 1 Sievert an hour present.
Again, normal radiation should be in fractions of microsieverts ie: 0.06 microsieverts ⇑⇑per hour (compare that to 1000, 000!).
What you are now able to do
You should now be able to take a Geiger counter out in the field and measure GAMMA and BETA radiation.
To measure Alpha radiation, you will need to study just a tiny teensy-weensy, bit more.
I hope to be able to write up on Alpha radiation testing, when I get to borrow a Ludlum instrument in the near future.
It’s here at last!
Excellent links that you may also wish to check out:
I do hope that you have enjoyed this introduction to Geiger counters, and please don’t forget – if you would like to contact me to discuss any aspect of the article, I would be humbled and pleased to help you to the best of my ability.
In the meantime, please may I ask all visitors to post links on their social media accounts directing visitors to electrosparkles.com so that more people can enjoy the website, and join in to present their technical ideas for featuring.
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