The Nuclear Camera Designed to Spot Hidden Radiation Sources

It’s not hard to detect nuclear radiation. A Geiger Muller counter will usually do the trick.

These consist of a small chamber filled with gas. When a particle with high enough energy enters the chamber, it ionises the gas, creating a shower of electrons. A pair of conducting plates can easily pick up this shower and generate one the “clicks” these detectors are famous for.

What is difficult, however, is working out what kind of particle triggered the shower and where it came from. There are various ways of doing this but the detectors tend to be huge; think CERN-sized. What’s needed is an accurate machine that is also portable.

Enter COCAE, a European project to develop a camera capable of imaging sources of nuclear radiation. Today, Kostas Karafasoulis at the Greek Atomic Energy Commission in Athens and pals describe how their device will work.

The basic idea is to reconstruct the trajectory of every particle that hits the detector. To that end, the device consists of a stack of ten pixelated cadmium telluride crystals. Each crystal is 4x 4cm in size and placed 10cm apart. So in theory, any particle passing through the device will hit several pixels in different parts of the detector. It’s then straightforward to see where it came from.

Except for an effect called Compton scattering. This happens when an X-ray or gamma ray strikes an electron, sending both careering off in different directions, like snooker balls. A gamma ray can ricochet several times likes this before finally giving up its energy to a material.

The trick that COCAE hopes to achieve is to use matrix of cadmium telluride crystals to measure the the position and energy of the liberated electron (which tells you the energy of gamma ray), while also keeping track of the ricocheting gamma ray.

Even then, it’s not possible to say exactly where the gamma ray came from. All you can do is confine its origin to with a cone of a certain angle.

However, there is a way to do better: by recreating the trajectories of several different particles from the same origin and seeing how their cones overlap. The region tells you much more accurately where they all came from, to within 10 degrees or so, say Karafasoulis and co.

Their paper today describes the simulated performance of the device in which they work out its energy and angular resolution as well as its detection efficiency.

This kind of device has obvious application in the security world. The ability to accurately identify and locate the position of radioactive materials could be hugely useful in counter terrorism operations and for customs officials.

It would also be useful in tracking down nuclear materials that have been lost and perhaps mixed with scrap metal.

And of course, during nuclear accidents. For example, one of the big problems after Chernobyl was working out exactly what had happened to the core and where the nuclear material had ended up.

So it looks as if Karafasoulis and co have a useful idea on their hands. All they have to do now is build one.

Ref: arxiv.org/abs/1101.3881: Simulated Performance Of A Position Sensitive Radiation Detecting System (COCAE)