3.5 Light particles in matter

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Do curso por University of Geneva

Particle Physics: an Introduction

216 classificações

University of Geneva

Particle Physics: an Introduction

216 classificações

This course introduces you to subatomic physics, i.e. the physics of nuclei and particles. More specifically, the following questions are addressed: - What are the concepts of particle physics and how are they implemented? - What are the properties of atomic nuclei and how can one use them? - How does one accelerate and detect particles and measure their properties? - What does one learn from particle reactions at high energies and particle decays? - How do electromagnetic interactions work and how can one use them? - How do strong interactions work and why are they difficult to understand? - How do weak interactions work and why are they so special? - What is the mass of objects at the subatomic level and how does the Higgs boson intervene? - How does one search for new phenomena beyond the known ones? - What can one learn from particle physics concerning astrophysics and the Universe as a whole? The course is structured in eight modules. Following the first one which introduces our subject, the modules 2 (nuclear physics) and 3 (accelerators and detectors) are rather self contained and can be studied separately. The modules 4 to 6 go into more depth about matter and forces as described by the standard model of particle physics. Module 7 deals with our ways to search for new phenomena. And the last module introduces you to two mysterious components of the Universe, namely Dark Matter and Dark Energy.

Na lição

Accelerators and detectors

In this module, we treat the basic facts about particle acceleration and detection. This is a rather self-contained module. If your main interest is particle acceleration and detection, you will be well served. You will notice that this is rather substantial module, we recommend that you take two weeks to digest it. We introduce electromagnetic acceleration and focalisation of particle beams and show how they are used in the accelerator complex of CERN. We describe how charged particles and photons interact with matter and how these interactions are used to detect particles and measure their properties. And we show how modern particle detectors use the synergies between different detection methods to get exhaustive information about the final state of particle collisions.

3.5 Light particles in matter4:47

Conheça os instrutores

Martin Pohl
Professeur ordinaire
Département de physique nucléaire et corpusculaire
Mercedes Paniccia
Collaboratrice scientifique
Département de Physique Nucléaire et Corpusculaire
Anna Sfyrla
Assistant Professor
Nuclear and Particle Physics

[MUSIC]

In this module, we are touching the basics of particle acceleration and

detection techniques.

In this video, we show how light charged particles interacts with matter.

These interactions are used to detect these particles.

At the end of this video, you will know the energy loss by ionization and

excitation for electrons.

And the energy loss by radiation, specific to light particles.

For light particles like electrons and positrons,

things get slightly complicated.

The formula for dE/dx is similar to that for heavy particles.

But the special properties of scattering between electrons, the Møller

scattering, and between electrons and positrons, Bhabha scattering, come in.

The difference between dE/dx for electrons and

positrons is however not very large.

But there is an important difference between light and heavy particles.

Since light projectiles and atomic electrons are of comparable mass,

interaction with atomic electrons causes bremsstrahlung.

The energy loss, dE/dx

by this process, is proportional to the energy of the projectile.

The material properties that come into play can be combined

into a single constant, the radiation length X_0.

By integrating the energy loss,

we can calculate the remaining energy, E(x) of the electron.

It is a function which decreases exponentially

with the penetrated material depth x.

The radiation length is then given as the path length after which the energy

of the projectile is reduced by a factor of e.

To obtain the energy loss, dE/dx, or

the radiation length in terms of the target surface density,

one must take into action the volume density of rho of the material.

Many material properties are tabulated by the Particle Data Group,

particularly the volume density and the radiation length,

that you can find on the web page quoted here at the bottom of the page.

A property specific to positrons is that they can annihilate with

atomic electrons of the material.

This process is important at low energy that is at the end of the path

just before the positron stops.

From annihilation at rest, two photons are emitted back to back, each with

an energy equal to the mass of the electron, which is 511 keV.

At energy above about 10 MeV, electrons and

positrons lose energy mainly by bremsstrahlung,

which dominates over ionization and excitation.

This means that many photons are produced when high energy electrons go through

matter.

Those are mostly converted into electron-positron pairs,

as discussed in the next video.

And these pairs will emit photons in turn.

In this way, high energy electrons generate electron photon showers in

matter, which are populated by a large number of particles.

The strong interaction of hadrons with the nuclei of the material can also

cause showers called hadronic showers.

These are typically less populated at comparable energies because

the lightest hadron, the pion, has a mass of about 100 MeV.

For the same reason, they are wider and less regular than electromagnetic showers.

Again, the table provided by the Particle Data Group

summarizes the properties or materials often used in particle detectors.

The characteristic lengths for

nuclear interaction are typically comparable to the radiation length for

lighter materials but much higher for heavy materials.

In the next video, we will discuss in more detail how photons interact with matter.

[SOUND]

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