Particle Detector Technology for Imaging

Abstract

In this article, “particles” refer to charged particles that have enough energy to pass some significant distance through matter. The interaction of particles with matter forms the basis on which particle detectors are designed and operated. The principal process for detection is ionization of matter by the electromagnetic interaction of the moving particle. Ionization produces free charge that can be collected and detected. Electromagnetic radiation from the charged particle moving through matter may also be used to create a detectable signal. Neutral particles can be observed following their interaction with matter, producing one or several charged particles, that are subsequently detected. The development of particle detectors has occurred within the fields of nuclear and high‐energy particle physics. In this regime, the interest lies in reconstructing particles emanating from a particle interaction or from the decay of unstable particles or nuclei. The trajectories of the detected particles are recorded to reconstruct the kinematics (momentum and energy of the particles) of the reactions being observed. The particle trajectories are sampled as the particles pass through the detector elements. The sampled track positions are used to reconstruct the full trajectory. In addition to the kinematics of such reactions, spatial information is also important, especially when determining the distance that an unstable particle travels before decaying. The decay time distributions of unstable particles follow the exponential statistical time distributions of radioactive decays whose half‐lives or lifetimes are characteristic of the particle or the nuclear state being studied. For sufficiently long lifetimes, the combination of track length before decay and the reconstructed kinematics of the decay products are used to determine the decay time of individual decays. The ensemble of data is then used to extract the characteristic lifetime of the particle from the distribution of the individual decays. The most common particles that can be tracked and imaged include electrons, protons, some nuclei, muons, pions, kaons, and neutrinos. High‐energy photons (X rays and gamma rays) also have particlelike properties and provide great utility in imaging applications. The source of the particles can be radioactive tracers distributed within the object being imaged or an external source such as a radioactive source, a reactor, an accelerator, or cosmic rays.