Super bright spinthariscope
By Leslie Wright
I once built one of the most basic nuclear radiation detectors, when I was 15, the Spinthariscope.
This wonderfully simple device, allows you to `see` nuclear disintigrations in realtime, and consists of nothing more than a lens, a phosphor screen (coated with zinc sulphide), and a radioactive source.
The screen is simply mounted at the end of a short tube, with a lens at the other end, to magnify the image.
The assembly is then taken into a darkened room, and a radioactive souce is offered upto the screen.
Once the eyes haver become dark adapted (some 10 minutes!) faint flashes of light are visible on the screen, each corresponding to the impact of a single charged particle!
For the screen in my original Spinthariscope, I broke open a cathode ray tube from a television, and scraped the phosphor off the inside of the screen. I then scattered the powder on the sticky side of a short length of sellotape, and shook off the excess.
For the radioactive source, I bought an ionisation smoke alarm, and removed the Americium-241 source from it.
The Americium-241 source is quite radioactive, undergoing approvimately 37,000 disintegrations per second! releasing alpha particles, and some gamma rays.
It was a nice thing to build, but waiting for 10 minutes in the dark, and then straining to see the tiny flashes of light, was somewhat of a pain, though I used to keep it under my bed, so I could watch those tiny scintillations before sleep!
Still, I figured some improvements could be made, to the basic instrument.
I recently aquired one of the first generation image intensifier tubes, and figured I could build a Super-bright Spintharoscope with it.
It is one of the tubes, with the fibre optic windows in it which makes the job extraordinarily simple! The Phosphor screen is simply fixed to the input window, and the radioactive source is placed in front of the phosphor screen. A small cover was made to exclude any external light from the input window, and then this little assembly was powered up.
What a sight it was to behold! Very bright flashes were visible, AND I didnt have to wait in the dark for 10 minutes!
It is even possible to photograph it with an ordinary digital camera!
Americium 241 pressed right up against the screen, you can see the faint star shaped outline of the source.
Americium 241, just over 1cm from the screen.
The same image intensifier tube, was at a later date, fitted with a small portion of fluorescent screen from an x-ray cassette.
The resulting device is now capable of detecting the low energy (60keV) gamma from a 1µCi Am-241 sample, as well as the odd cosmic ray, and of course is capable of imaging x-rays.
In a related experiment,I buit an x-ray pinhole cammera attachment for it.
There is no conventional lens or mirror available that can focus x-rays, however the humble pinhole camera is capable of imaging actual x-ray or gamma-ray sources.
I made a lead shield for the front of the device, into which a very tiny pinhole was made. when pointed at an x-ray tube, an image of the x-ray focal point on the anode is quite visible, and it is even possible to see its shape quite clearly.
This is still a work in progress at the moment, but hopefully I will have some pictures from this "gamma camera" up soon, as well as some of the actual device itself, with diagrams, and other notes etc.
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Copyright © 2007 - 2008, Leslie Wright, All Rights Reserved.