Detector

A detector, in the sense of a particle detector, is a tool whose purpose is to reveal the presence or activity of a particle, such as a Photon or muon. Detector physics is the field that deals with the mechanism behind particle detection and the methods that can be employed to this end. Since different particles behave in different ways, there's a wide variety of detector types. For example, photons are generally detected through the use of a scintillator, which is a material that glows visible or UV light when excited by high-energy photons.

In many ways, detectors are just applications of material physics and specifically matter-radiation interaction, since individual particles are radiation colliding with the detectors material.

Operation#

The exact mechanism underlying a detector depends on the specific type of sensor being used, but some concepts are universal. For one, detectors are statistical objects: we don't speak of "detection" so much as the "Probability of detection". This is for a variety of reasons, in part because much of microscopic space is empty, in part because quantum mechanics is fundamentally a probabilistic theory. See Cross section for quantification. We also know that the probability of detection is inversely proportional to its energy: high-energy particles are more likely to phase through the detector without triggering anything. This might come as a surprise since the common conception is that high-energy phenomena are always the flashiest, but in this case it's lower-energy particles are much easier to detect.

Detectors rely on essentially any property that a particle has, but some properties are more useful than others. The most useful property of any is probably the electric charge: its conserved, it's invariant, electromagnetism is very well understood, its very common, its effects are strong enough to be noticeable and have infinite range. Generally, charge is as good as it gets. When the particle lacks electric charge, as is the case with neutrons and neutrinos, the backup is either the strong force (for hadrons) or the weak force (for non-hadrons). In the dreaded case of a particle that does not interact with any of these, one must fall back to gravity, which we don't have a good quantum understanding of, or some indirect means like conservation laws. Fortunately, the only instance of such particles are sterile neutrinos, which are purely hypothetical for now. Unfortunately, these particles are one proposed path to quantum gravity and dark matter.

Detectors fundamentally work by transferring energy from the incident particle into its components, but this isn't always directly possible. It is useful to divide particles in three types based on how they are detected:

• Charged particles are affected by electromagnetism and they go through a lot electromagnetic forces when traveling in a medium. Ionization and collisions with electrons in matter are the primary source of energy loss for these, though in the case light particles (electrons and positrons) bremmstrahlung plays a more important role. This energy loss is formalized by the medium's stopping power. These are the only particles that are reliable to detect; detecting the other two types boils down to making them decay or scatter into charged particles and measuring those instead.
• Non-photon neutral particles can only interact strong or weak, so all techniques relying on ionization, which are the most effective, don't work. They are the hardest to measure and generally require specialized methods that involve detecting specific decay products and inferring their existence from those.
• Photons are purely electromagnetic, but they're also massless, neutral and tend to be absorbed. Their number decreases as per the Beer-Lambert law. Despite this, they are easier to measure than other neutral particles because there's an entire set of understood phenomena responsible for their absorption or deflection, namely the photoelectric effect (low energies), Compton scattering (intermediate energies) and pair production (high energies). These effects stack to form the total absorption coefficient of the Beer-Lambert law: $\mu_\gamma=\mu _\text{PE}+\mu_{\text{CS}}+\mu _\text{PP}$. As a brief summary, these are related to cross section as $\mu=n_{0}\sigma$, where $n_{0}$ is the density of scattering centers, and the $\sigma$ of these three goes like $Z^{5}$, $Z$ and $Z^{2}$ respectively, which gives an idea of how important the chemical element of the material is.

Due to the randomness of interaction, many detection parameters are measured statistically with a Probability distribution.