A scintillator is a material which converts the energy from incoming radiation into a form where it is more detectable. Humans cannot see X-rays or neutron beams; however, detecting their presence is incredibly important – for everything from security applications to instruments at CERN. Humans have a certain ‘umwelt’. This is a German word describing the perception that an animal has on the external world, and for humans it is a surprisingly limited slither of reality. For example we only see one-five-hundred-thousandth of the electromagnetic spectrum: every second, radio waves, gamma rays and a whole panoply of other forms of radiation pass through our bodies without our knowledge. This is why we have a fundamental need to expand our umwelt – to access other portions of the world.
A scintillator is a perfect example of the human quest to expand perception. Here, a substance such as a noble gas or inorganic crystal emits a flash of visible light when other non-visible particles, such as high frequency photons, pass through it. This light can then be converted into an electron and the number of electrons can be ‘multiplied’ using a photomultiplier tube. The end result is that the presence of particles (and even some of their properties) can be converted into a measurable voltage. This in turn can be stored, manipulated and analysed.
One interesting and important application of scintillators is a medical technology called Positron Emission Tomography (PET). Invented in the 1950s, the equipment, which consists of a large ring of scintillators attached to photomultiplier tubes, is vital in modern medicine. PET scans are used for everything from analysing the passages of new and developing drugs in the body to diagnosing early brain tumours. The patient is first injected with a radiotracer. This compound is designed to be taken up by the human body in the same way as if the compound were not radioactive – for example, a common radiotracer for PET applications is FDG, which is a modified form of glucose. As the radioactive glucose is distributed and used by the body, it emits electrons with a positive charge (called ‘positrons’). However, these particles are short-lived, soon annihilating and producing two gamma rays oriented in a polar fashion. What does this mean? Imagine that the particles were much larger than they really are, and imagine that they crash from opposite sides of the earth into the equator. One could draw a straight line between them through the centre of the earth. As a result, a gamma ray photon is emitted from the North and South Poles. This is what happens, without the earth as a guide, on a tiny scale inside the patient. These photons, since they of high energy, travel straight through and out of the patient.
Here, they each strike a different scintillator since they come out in opposite directions. the flash of light that they produce is converted into an electrical signal and amplified. By considering that the energy of the two photons should be exactly the same and with the knowledge that the two flashes happen at the same time if their origin was in the centre of the ring, a computer can piece together the location at which they were produced, and therefore the location of the molecule of radioactive glucose.