1 Rand (Pieter W. Botha) - South Africa – Numista
Today is World Radiography day and I am a Radiographer, so this is timely. I previously X-rayed a 1 Rand coin and this is one of the radiographs:
When it comes to X-raying coins, there are some technical variables which you have to consider:
1. The penetrating power of the X-ray beam, which is a setting I can adjust on the X-ray unit. It is given in kV and the range of options on a general X-ray machine are usually between 40kV and 150kV. Metallic structures like a coin represent a significant barrier to the X-ray beam and you therefore have to raise the kV quite high. In the image above it was 135kV!
2. The number of X-ray photos in the beam, which is a setting I can adjust on the X-ray unit. It is given as a combined variable known as mAs which is milliAmps x seconds. When X-raying a patient you want the “s” component of the exposure as low as possible because the patient could move, but with a coin you can increase the “s” component if required.
If you X-ray a large patient, X-ray photons can get diverted or absorbed in the tissues and they might not make it to the detector so you have to increase the mA to compensate. With this coin you do not need a high mA, so a value between 3 and 4 is adequate.
3. Grid. All medical X-ray units have some sort of anti-scatter grid. The physics of it is complex (for this forum) so I won't dwell on it. But it needs to be in place when X-raying a coin. Not only that, the grid must be reciprocating (not a stationary grid) otherwise you will get fine grid lines on the image.
4. Focal spot size. This is a setting which I can adjust on the X-ray unit. There are two options: broad and fine. On a coin no thicker than this 1 Rand coin where you do not have a high mAs, the focus can be put on fine, which gives slightly better sharpness. As soon as you have a high mAs, you have to use broad focus in order to protect the life of the X-ray tube. The physics behind that is interesting but not for this audience.
All of this assumes that the coin can be penetrated in the first place. I am betting that a really thick coin will be a problem and certain metals will attenuate the beam to a greater extent. However as you can see from the above image, it works fine on a common nickel coin.
Note that both sides of the coin are superimposed on the image. It does not make a difference on a thin coin, which side is up at the time of the X-ray. On a very thick coin, the side that is down (towards the detector) will be marginally sharper.
The superimposition explains why the springbok is merged with P.W. Botha.
This is what happens if you select a kV that cannot penetrate. This is the same coin with kV of 41:
