• Question: How do you use neutrons to look inside containers and find drugs etc? (It sounds cool!!)

    Asked by to Simon on 12 Mar 2014. This question was also asked by .
    • Photo: Simon Albright

      Simon Albright answered on 12 Mar 2014:

      Neutrons interact in very strange ways, X-rays are easy, the higher the atomic number the faster they stop. Lead will stop X-rays almost immediately, water does almost nothing. Neutrons are way more complicated though, if you wanted to block neutrons you’d be better off with a block of polythene than a block of lead.

      So this means you can get neutrons into things that otherwise would be impossible to see inside, like a car engine or something. The other benefit of neutrons is they do something called “inelastic scattering”. Elastic scattering is when they bounce off and keep their energy, inelastic scattering means some energy is lost. One of the ways the energy can be lost is by the emission of a gamma ray and the energy of that gamma ray is unique to the nucleus the neutron bounced off.

      What all this means is we can get neutrons to go places and get them to send out gamma rays from the things they pass through. By looking at the energies of the gamma rays, and how many you get with each energy, you can work out the ratios of different elements and therefore what the material is.

      For example:
      cocaine has the ratio: C17:H21:N:O4
      RDX (plastic explosive) has the formula: C3:H6:O6:N6

      So you can look at your gamma spectrum, see how the heights of the energies for Carbon, Nitrogen, Oxygen and Hydrogen compare to each other and work out which is more likely.

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