How Do We Measure Radiation?

Space radiation consists of countless tiny particles—smaller than the atoms in your body—whizzing around at close to the speed of light. How can we detect things so small that no microscope on Earth could possibly reveal them?

Most radiation particles carry an electrical charge. Electrons and protons have an intrinsic charge with equal absolute value but opposite sign. The elementary charge e is defined as the charge of a proton, so the charge of an electron is -e. The charge of any free particle can only be a multiple of this elementary charge.

Space radiation consists mostly of electrons, protons, and atomic nuclei, i.e. atoms that have been stripped of all their shell electrons. The nuclei themselves consist of protons and neutrons and thus carry a charge equal to the number of protons they contain (also called the atomic number) times e.

To detect radiation particles, we make use of the fundamental interactions of nuclear and particle physics: The electromagnetic interactions between charged particles. Or rather, between the charged radiation particles and the atoms of any material they traverse. Because they carry charge, particles interact with the (charged) electrons and nuclei of matter. The many interactions along their path through the material cause them to lose some of their energy, slowing them down until they are either stopped or leave the material.

The energy that is transferred to the atoms in these interactions is detectable. In some materials (e.g. silicon), it causes electrons to break free from their atoms and freely move about. With appropriate treatment of the material and the right set of electronics, these electrons can be amplified into a measurable electrical signal. In others (so-called scintillators, for example made from certain plastics), the energy transferred to an electron does not completely free it of its atom but instead moves it to a state with higher energy (called an excited state). During de-excitation, the excess energy is converted into a photon (light). Since this interaction happens many times, the particle generates a flash of light that can be detected by a photosensor.

We use a number of different silicon- and scintillator-based detectors to build the three dosimeters of the 3D-DOS experiment. Since the amount of energy a particle loses while traversing the detector material depends on its characteristic parameters (mass, charge, and velocity), taking multiple measurements while it is slowing down allows us to uniquely identify it. That gives us the capability to not only detect radiation, but also characterize it.

At the fundamental level, ignoring the nitty-gritty of turning these principles into reality, that’s all the magic (or physics) there is to our detectors.

 

Photo credit: Eljen Technology