• Blog Stats

    • 76,905 hits
  • Archives

  • Enter your email address to subscribe to this blog and receive notifications of new posts by email.

    Join 157 other followers

  • Copyright notice

    This blog entry and all other text on this blog is copyrighted, you are free to read it, discuss it with friends, co-workers and anyone else who will pay attention.

    If you want to cite this blog article or quote from it in a not for profit website or blog then please feel free to do so as long as you provide a link back to this blog article.

    If as a school teacher or university teacher you wish to use content from my blog for the education of students then you may do so as long as the teaching materials produced from my blogged writings are not distributed for profit to others. Also at University level I ask that you provide a link to my blog to the students.

    If you want to quote from this blog in an academic paper published in an academic journal then please contact me before you submit your paper to enable us to discuss the matter.

    If you wish to reuse my text in a way where you will be making a profit (however small) please contact me before you do so, and we can discuss the licensing of the content.

    If you want to contact me then please do so by e-mailing me at Chalmers University of Technology, I am quite easy to find there as I am the only person with the surname “foreman” working at Chalmers. An alternative method of contacting me is to leave a comment on a blog article. If you do not know which one to comment on then just pick one at random, please include your email in the comment so I can contact you.

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.

Advertisements

Go on, Have your say !

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: