Particle Detection
After reading this chapter, you should be able to manage the basics of particle detection, and to understand the sections describing the detection technique in a modern article of high-energy particle or astroparticle physics.
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Particle Detection
After reading this chapter, you should be able to manage the basics of particle detection, and to understand the sections describing the detection technique in a modern article of high-energy particle or astroparticle physics.
Particle detectors measure physical quantities related to the result of a collision; they should ideally identify all the outcoming (and the incoming, if unknown) particles and measure their kinematic characteristics (momentum, energy, velocity). In order to detect a particle, one must make use of its interaction with a sensitive material. The interaction should possibly not destroy the particle that one wants to detect; however, for some particles this is the only way to obtain information. In order to study the properties of detectors, we shall thus first need to review the characteristics of the interaction of particles with matter.
4.1 Interaction of Particles with Matter 4.1.1 Charged Particle Interactions Charged particles interact basically with atoms, and the interaction is mostly electromagnetic: they might expel electrons (ionization), promote electrons to upper energy levels (excitation), or radiate photons (bremsstrahlung, Cherenkov radiation, transition radiation). High-energy particles may also interact directly with the atomic nuclei.
© Springer International Publishing AG, part of Springer Nature 2018 A. De Angelis and M. Pimenta, Introduction to Particle and Astroparticle Physics, Undergraduate Lecture Notes in Physics, https://doi.org/10.1007/978-3-319-78181-5_4
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4 Particle Detection
Ionization Energy Loss
This is one of the most important sources of energy loss by charged particles. The average value of the specific (i.e., calculated per unit length) energy loss due to ionization and excitation whenever a particle goes through a homogeneous material of density ρ is described by the so-called Bethe formula.1 This expression has an accuracy of a few % in the region 0.1 < βγ < 1000 for materials with intermediate atomic number. Z (zp )2 1 2me c2 β 2 γ 2 δ(β, ρ) dE 2 ρD ln − β , (4.1) − − dx A β2 2 I 2 where • • • • • •
ρ is the material density, in g/cm3 ; Z and A are the atomic and mass number of the material, respectively; zp is the charge of the incoming particle, in units of the electron charge; D 0.307 MeV cm2 /g; me c2 is the energy corresponding to the electron mass, ∼0.5 MeV; I is the mean excitation energy in the material; it can be approximated as I 16 eV × Z 0.9 for Z > 1; • δ is a correction term that becomes important at high energies. It accounts for the reduction in energy loss due to the so-called density effect. As the incident particle velocity increases, media become polarized and their atoms can no longer be considered as isolated. The energy loss by ionization (Fig. 4.1) in first approximation is: • independent of the particle’s mass; • typically small for high-energy particles (about 2 MeV/cm in water; one can roughly assume a proportionality to the density of the material); • proportional to
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