Spectroscopy for beginners


By Dorian Stonehouse.

IN PREVIOUS features on electrosparkles.com we looked at ways of detecting ionising radiation using the Geiger counter and scintillation detector. Check out:


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


Why was 137 Cs attached to these Geiger counters in the first place?

This radioactive material is used for checking whether the Geiger counter is working, and is used for calibrating the instrument.  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 source to identify, with its idiosyncratic peaks.

Inside the lead tube in the above picture is the Canberra scintillation detector: 

Knowing a bit about the gamma scintillation detector is worth its weight in gold, and if you would like me to make a tutorial on the subject, please email me

Check out:   https://electrosparkles.com/alpha-radiation-and-popping-popcorn-a-minds-eye-view

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 inside the 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!

To recap:

And now for something completely different:

 Sometimes it is useful to explain technical things by way of an analogy

Take Spikey’s hair for example: it consists of large spikes, made with loads of BrylcreemThese spikes are of the same height, but they may appear anywhere on spikey’s scalp and are, 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).

Hold that thought…

Compare Mr Spikey man’s hairstyle with the waveform obtained on my scope from 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 “Compton scattering.” (look it up!).

Beneath the tall spikes are lower-height spikes (tiny fine hairs); but there are more of them than the bigger spikes (lower energy pulses of higher repetition). These appear to be X rays, mixed up with “noise.”  

A  spectrum

When we talk about a spectrum of things, we think about rearranging things: 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, medium spiked hair in the middle, accompanied by long spiked hair (liberty spikes) on top.

Now check out the half scope picture.  Like Spikey, there are three distinct types of waveforms that give us the whole picture: 

  1.  Small low energy, rapidly-occurring (higher frequency- counts) waveform at the bottom
  2.  Medium energy waveform, occurring less often (low frequency-counts) in the middle,
  3. Higher energy waveform, occurring even less often (very low 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 reads vertically (y axis), and the higher it goes, the higher the count-rate (frquency) of the pulses.

X across: for energy

Y up: for counts

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 this by 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 the gammaspectacular 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 produced using 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 and below the Theremino version

I do hope that you have enjoyed this feature on spectroscopy for beginners.  If you would like to support the website by contributing to its upkeep, please press the Patron button at the start of the feature; or the one below.

Thank you


[whohit]Spectroscopy for beginners [/whohit]