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Radiation meters and crystals

Dear Reader,

I have noticed that so many people are so willing and so fast to comment on the hotspots which have been found at Fukuashima, a range of comments have been written on one blog have suggested that the used nuclear fuel has relocated up into the stack.

I think that such a upwards relocation is very unlikely, I suspect that Gunderson’s suggestion that during the worst part of the accident that the inner surfaces of the pipe which goes up the stack was coated with cesium and that now the condensation from the humid air from the containments has washed the cesium down the stack back to ground level is reasonable.

One way to tell if the fuel has gone up the pipe or if the cesium salts have come back down the stack is to use gamma spectroscopy. If I was in charge of the Fukuashima site I would want to place a gamma spectrometer close to the bottom of the stack to find out what radioisotopes are inside the pipes.

It is important to understand that radiation detectors are not magic, they work in quite simple ways. One gamma ray detector which is energy dispersive is the sodium iodide detector. Typically a sodium iodide crystal (doped with a little thallium) is placed in a light proof box with a photomultiplier tube attached.

The idea is that when a gamma photon is absorbed by the sodium iodide crystal that its energy is converted to light, the emitted light is then measured by the photomultiplier tube. The greater the energy of the photon then the more photons of light which are generated by the crystal. The photomultiplier tube is then used not only to detect that an event occurred but also how much light was emitted during the event.

The sodium iodide crystal is a cubic crystal which has the same type of arrangement of atoms as found in common salt (sodium chloride). As we have done before we are going to have a go at making a unit cell.

The cell is a cube whose sides are 6.5 Å long

The fractional coordinates of the sodium atoms are

0.0 0.0 0.0

0.0 0.5 0.5

0.5 0.0 0.5

0.5 0.5 0.0


The iodine atoms are at the following fractional coordinates

0.0 0.0 0.5

0.0 0.5 0.0

0.5 0.0 0.0

0.5 0.5 0.5


Now the clever ones out there should be able to work out that the sodium to iodide ratio is  1:1.

The cell contains 8 sodiums which are shared between eight unit cells


8 x 0.125 = 1 sodiums

The cell contains six sodiums which are each shared between two cells

6 x 0.5 = 3 sodiums

Total of four sodiums

The cell contains twelve iodines which are each shared between four unit cells


12 x 0.25 = 3 iodines

The cell contains one iodine which is totally inside the box


1 x 1 = 1 iodine.

So as the box contains four sodiums and four iodines then the empirical formula is NaI which confirmed what we were expecting.

Back to radiation detectors, what I would want is to have a lead box containing a NaI or better still a germanium detector, the lead box would have a hole facing the hot spot. This device could then be used to collect a gamma spectrum of the hot spot. The germanium (not geranium) detector is a very expensive and large diode which is used as a detector. The idea is that when a gamma photon is absorbed in the germanium it causes free electrons and holes to be formed. The germanium diode is biased with about 2000 or 3000 volts, the charge carriers move through the crystal and charge is passed. The detector measures how much charge is passed during each event, the more energy in the photon the more charge is transported through the crystal. I will tell you more another day about such detectors.

But back to scintillation counters.

Two of the classic solids which have been used in scintillation counters are anthracene and zinc sulphide, these solids can be made to emit light using UV light or X-rays as well as alpha, beta and gamma rays. I have made a short film in which I have exposed the two solids to UV light. The anthracene is on the left and the zinc sulphide on the right. It is important to note that the lifetime of the blue glow for the zinc sulphide is longer than it is for the organic compound.

Enjoy the film, I hope to explain to you soon more about sctintillation counters and other radiation detectors.


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