Alpha Radiation and Popping popcorn – a mind’s eye view
By Dorian Stonehouse
THIS feature follows from two earlier features on radioactivity and Geiger counters. They may be found here:
A little bit about Alpha radiation
When monitoring radiation contamination, it is essential to be aware that there are three types of radiation to be concerned about :
Gamma radiation (waves) has been covered above, and is very penetrating and is not easily stopped
Beta radiation, is a stream of particles (electrons) which are stopped by something like a thin aluminium sheet (already shown in pictures – follow above links)
Alpha Radiation jumps only a couple of centimetres
Please have a look at the following YouTube clip, which makes clearer the nature of the things compared: popcorn in the pan; and alpha radiation. Most important: look what happens when the popcorn gets hot:
Eberline radiation survey meter Model ASP-1 and Ludlum Alpha survey (scintillation) probe Model 43-5: a big window probe to cover a large area – fast!
In the video, the hot and heavy popcorn shoots up into the air just a few centimetres from the surface of the wok, hits the mesh cover before falling back to the surface.
This is a great way to consider Alpha radiation, which consists of parts of the atom: two protons and two neutrons. These are heavy particles and only rise a few centimetres from the surface, before falling back to the surface.
To measure the presence of Alpha radiation, just a little more knowledge is required:
Because Alpha radiation is so heavy, the only sure way of detecting it is to place the detector probe very close to the contaminated surface to be surveyed. This is why Alpha radiation is the most difficult to detect – you have to be close up and personal!
Often, but not always, a certain type of detector probe will be used to detect the presence of Alpha radiation – the scintillation detector probe.
The scintillation detector probe consists of just two contacts: the inner contact is for application of high voltage, which will rise and fall, and produce a “click;” and the outer is just the negative (chassis) contact
Basic operation of scintillation detector probe in a few words!
“Scintillation” simply means: the production of bursts of light.
So here goes: first item to encounter the Alpha radiation is a crystal, which is sandwiched between the radiation source and the vacuum detector tube bottom (photomultiplier tube).
The crystal takes in the (Alpha) radiation bursts, and gives out bursts of light (the crystal scintillates).
The more intense the radiation, the more light bursts the crystal produces.
The Eberline survey meter produces a high voltage for the two contact Alpha scintillation probe
As already stated, the crystal is sealed against the bottom surface of a detector (vacuum) tube.
This photographic surface will absorb the bursts of light from the scintillation crystal, and convert these light bursts in to bursts of electrons inside the vacuum detector tube. The detector tube is called a “photomultiplier tube”
These electrons will be multiplied and produce a cascade of current to flow across the two contacts of the detector tube (the scintillation tube only has two external contacts in total); creating a “click” sound every time a radioactive event occurs.
Alpha radiation = bursts of light from the crystal = converted to bursts of electrons inside the tube = electron bursts multiplied = electrons cascade (Avalanche) = high bursts of current = rising and falling voltage (“clicks”) at the output of the vacuum photomultiplier tube = pulses are fed through a simple capacitor = amplified and feed to a simple pulse counter – and that’s it!
An expensive problem when in the field: the scintillation crystal is covered by a super light Mylar foil, which can easily tear, exposing the crystal to sunlight, which makes the detector “photosensitive”
One of the drawbacks when using this type of probe has been outlined above, namely the aluminized Mylar covering the detector probe crystal is typically about 0.25 mm thick and tears easily.
The Mylar allows Alpha radiation penetration, but blocks both sunlight and moisture absorption by the crystal (crystal is hygroscopic).
The owners of the survey equipment featured above tell me that they recently took both units down to Oxford for calibration.
The probe came back in a photosensitive state and was less able to reject gamma and beta radiation. As a result of this, they decided to mask off the sides of the probe with silicon window sealant: job done – no more photosensitivity!
Comparing the above with a gamma scintillation probe
Remember, gamma penetrates, while Alpha does not. So, with a gamma scintillation probe, ionising gamma radiation will penetrate the thin aluminium end of the case and reach the scintillation crystal, which is now safely housed inside the can – out of reach of light and moisture.
The crystal is safely protected inside the solid alloy can containing the whole gamma scintillation probe assembly
Gamma scintillation probes of this calibre can focus with precision on the radioactive sample under test
The big window of the Ludlum Model 43-5 Alpha probe, and its ability to reject strong gamma and beta activity, has to make it the quintessential detector probe for Alpha radiation detection; and barring a new semiconductor probe turning up that can be fine-tuned to detect just one type of radioactivity (Alpha), the future of the 43-5 seems assured – for a while!
I hope that you have enjoyed the short feature on Alpha detection, and that you will spread the word about electrosparkles.com amongst your colleagues, fellow students and friends.
I thank you
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