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Saturday, April 05, 2014

Nuclear Research Reactors - MK 1 - Neutron Radiography

The TRIGA MK 1 was most often used to do Neutron Activation Analysis when I was at the facility.  However there were some other uses to which it could be put.  From time to time the MK 1 was used for neutron radiography, which is also an interesting subject!

Most people are familiar with X-ray photography, and have seen pictures of broken bones, devices used to knit bones together, and pacemakers.  Everyone has seen X-ray pictures.  See below for a few examples.

Broken Leg

 Broken Big toe, with plate and a drywall screw

 Pacemaker with stimulating wires

X-rays are ideal for taking pictures of things that absorb X-rays, such as bone and metals.  But if you need to look inside something made of metal... what then?  The X-rays won't make it through to the other side because they will be absorbed.  Let's use neutrons instead!

Neutrons will go right through metal, and let you look at what's inside!  Neutrons are scattered and absorbed by materials far differently than X-rays are.  If you wanted to image a rubber band inside a lead brick, neutrons could do that, but X-rays would be useless.  Let's look at a couple of examples of neutron radiographs.

X-Ray:  Note the lack of detail and large black areas caused by heavier metals absorbing the X-rays
Neutron Radiograph.  Many more of the details of the internals are visible.  Darker items in this image will tend to be plastics.

Below is an air compressor.  Note that you can make out the crankshaft, connecting rod, wrist pin, valves, and even the roller bearings.  Try doing that with X-rays...

 My favorite.  A motorcycle engine.  You can see the intake valve to the right, exhaust valve to the left, valve springs, the sparkplug, the piston rings, the piston and wrist pin, the connecting rod, and the bolts holding the thing together.  You can even see carbon buildup in the exhaust port, and specks of buildup on the piston skirt!  Amazing stuff, really.
The only problem with neutron radiography is that you can't use it on living tissue.  Neutrons are pretty lethal, and tend to make things radioactive.  Also, people, being mostly water, are opaque to neutrons.  Nevertheless, neutron radiography is a very useful imaging tool for metallic objects.
The way neutron radiography was accomplished is thus:  A 12"x12" square aluminum dry tube was placed in the reactor pool, next to the core.  The dry tube had to be 25 foot long to reach the bottom of the pool and the reactor.  This dry tube gave neutrons a path to escape the core without being absorbed in water.  The neutrons would exit the top of the, which also served to "collimate" the neutrons, to help them all travel in the same direction as they exited the top.  On top of the tube would be an aluminum plate, on which the item to be radiographed would sit.  Above that would be the film.  Don't recall if it was special film or just X-ray film. 
The process called for the receptionist to leave the building (whe was not a Radiation Worker), so it wasn't something I wanted to do very often either.  I was a lot closer to the apparatus than she was!  Nevertheless, a very cool process, with some useful applications.

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