Now we are going to look at my oscilloscope to see the waveforms generated by ionising radiation.
I am assuming that you are students at school, or novice readers. If you have not done much with an oscilloscope, please email me and I will do a tutorial on this subject.
The setup: Check source Caesium-137,137Cs screwed on to body of Geiger counter), lead-covered scintillation probe, gammaspectacular.com driver unit, oscilloscope:
Side view of 137 Cs and Ludlum Model 3 instrument:
Let’s have a closer look at the 137 Cs radioactive check source . Notice that the actual amount of the radioactive material is about one drop, deposited in the recesses either side of the disk↓
TAKE A CLOSER LOOKAT THE LEFT HANDRECESS. NOTICE THAT SOME 137 Cs HASSHIFTED TO ONE SIDE,REVEALING ASHALLOW PLASTIC BOTTOM:
Why was 137Cs attached to these Geiger counters inthe first place?
This radioactive material is used for checking whether the Geiger counter is working, and is used for calibrating theinstrument. Also, it has a fairly low lifespan (half life about 30 years), so compared to some radioactive material, it is less of an environmental hazard!
137 Cs is also great for making classic gamma spectroscopy pictures (see later), in that it is a fairly straightforward sourceto identify, with its idiosyncratic peaks.
Inside the leadtube in the above pictureis the Canberrascintillation detector:
Knowing a bit about the gamma scintillation detector is worth its weight in gold, and if you would like me to make a tutorialon the subject, please email me.
Why use a big lead tube to wrap around the detector?
In the workshop all sorts of interference can affect the scintillation detector: stray gamma waves, background radiation and even strong electric or magnetic fields. The lead tube, therefore, acts as a shield, preventing extraneous readings.
Absence of lead sealing cap at the end of the tube!
I do have such a cap and, yes, the 137 Cs should be placed well insidethe lead tube, then sealed with said cap. However, I just didn’t want to unscrew the check source from the Ludlum monitor, as the instrument is on loan!
And now for something completely different:
Sometimes it is useful to explain technical things by way of ananalogy
Take Spikey’s hair for example: it consists of large spikes, made with loads ofBrylcreem. These spikes are of the sameheight, but they may appearanywhere on spikey’s scalp andare, therefore, a bit random.
Beneath the spikes,we have (occurring in greater numbers)much shorter hair,which cover his entire scalp.
Spikey’s energy is clearly indicated by his long spiky hair (he loves to party ’till dawn).
Compare MrSpikey man’shairstyle with thewaveform obtained on my scopefrom above (live) circuit:
The scope (which had been stopped),shows higher energy spikes (like Mr Spikey’s hair), standing out on top.
They are all about the same spike size, but there are less of them (lower repetition). These are gamma pulses.
Just before some of these tall spikes are lower energy gamma pulses, which appears to be present due to “Comptonscattering.” (look it up!).
Beneath the tall spikesare lower-height spikes (tiny fine hairs); but there aremore of them than the bigger spikes (lower energy pulses of higher repetition).These appear to be X rays, mixed up with “noise.”
When we talk about a spectrum of things, we think about rearrangingthings: staking things up along a line – a line which starts from the left and travels to the right (left to right).
Time for another (closer) look at Mr Spikey- man’s hair and half an oscilloscope picture⇓⇓
Mr Spikey has basically three distinct types of hair styles that make up his whole head of hair:short hair beneath, mediumspiked hair in the middle, accompanied by long spiked hair(liberty spikes) on top.
Now check out the half scope picture. Like Spikey, there arethree distinct types of waveforms that give us the whole picture:
Small low energy, rapidly-occurring (higher frequency- counts)waveform at the bottom
Medium energy waveform, occurring less often (lowfrequency-counts) in the middle,
Higher energy waveform, occurring even less often (verylow frequency-counts) and peaking on top
Gamma-ray Spectrum graph
It’s time to get rid of the rummage sale and place everything into neat clothes rails – our new spectro graph.
The graph reads from the left to the right, and the bottom horizontal line (x axis) shows energy increasing the further right we go.
The graph also readsvertically (y axis), and the higher it goes,the higher the count-rate(frquency) of the pulses.
X across:for energy
Y up: forcounts
And now for the real thing
Remember what I said about placing everything into neat clothes rails. Well,there are two brilliant ways of doing thisby installing free gamma spectroscopy software on your computers (which you must do first):
Just go back to our layout for a second⇑⇑ (the one showing the lead tube, check source and oscilloscope).
Now, instead of connecting thegammaspectacular box to the oscilloscope, we connect it to our computer soundcard, and then power it up by connecting the usb cable that comes with it to the computer usb port:
All radioactive material will have a unique signature, and here are two gamma spectroscopy graphs that I recently producedusing the more affordable spectroscopy driver GS 1100A by Gammaspectacular.com:
PRA Spectroscopy result after following the short tutorial and getting used to the software (this might take you an hour or three, so don’t panic!)
Above the PRA free spectroscopy software andbelow the Theremino version
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