8.14.1.2. Excitation

Table of Contents

What Excitation Means in Particle Detection

When a charged particle passes through matter, it interacts with the atoms and molecules of the detector material. One possible result is excitation. Excitation happens when the particle transfers some of its energy to an atom or molecule, raising one of its electrons to a higher energy state without removing that electron completely.

This is different from ionization, where an electron is knocked out of the atom. In excitation, the atom remains electrically neutral, but it is left in an excited state. That excited state usually lasts only a short time before the atom returns to a lower energy state.

Excitation means energy transfer to an atom or molecule that raises it to a higher internal energy state, but does not remove an electron from the system.

How a Passing Particle Causes Excitation

A moving charged particle, such as an electron, proton, alpha particle, or muon, creates electric forces around it. As it travels through the detector, these forces disturb nearby atomic electrons. If the transferred energy matches an allowed energy difference in the atom or molecule, the electron can jump to a higher energy level.

If the transferred energy is smaller than the ionization energy, excitation can occur instead of ionization. In real detector materials, both processes often happen together.

The general idea is

$$
\text{incident particle} + \text{atom} \to \text{incident particle with less energy} + \text{excited atom}
$$

and then later

$$
\text{excited atom} \to \text{atom} + \text{emitted energy}
$$

The emitted energy may appear as visible light, ultraviolet light, heat, or further secondary processes inside the material.

Excited States and De-Excitation

Atoms and molecules can only have certain allowed energy levels. Because of this, excitation is quantized. The transferred energy must correspond to one of the possible internal transitions.

After excitation, the system tends to return to a lower energy state. This return is called de-excitation. During de-excitation, the stored energy can be released in several ways. A photon may be emitted, or the energy may be passed to neighboring atoms through collisions.

In many detectors, this released energy is the useful signal. For example, in scintillators, excitation leads to light emission that can be measured.

Excitation stores part of the particle's energy temporarily inside the detector material. The detector signal often comes from the later de-excitation of that material.

Excitation Compared with Ionization

Excitation and ionization are closely related, but they are not the same. The difference is important because different detector types rely on one process more than the other.

Process	What happens to the atomic electron	Charge left behind	Typical detector consequence
Excitation	Electron moves to higher bound state	No net ion created directly	Light emission, delayed energy release
Ionization	Electron is removed from atom	Positive ion and free electron produced	Electric charge collection

Excitation often contributes strongly in materials that produce light, while ionization is especially important in gas detectors and semiconductor detectors. Still, both can occur in the same material.

Why Excitation Matters in Detectors

Excitation is one of the main ways radiation deposits energy in matter. Even when it does not directly create free charge, it can still produce a measurable effect. This makes excitation essential in radiation detection.

In scintillation detectors, excitation is the key first step. The incoming particle excites atoms or molecules in the scintillator. As they de-excite, they emit photons. These photons are then detected by devices such as photomultiplier tubes or silicon photomultipliers.

In some materials, excitation energy may also migrate through the medium before being emitted. This can affect the brightness, timing, and efficiency of the detector signal.

Energy Levels in a Simple Picture

A simple way to picture excitation is with an energy-level diagram. The particle gives energy to the atom, pushing it to a higher level. Later, the atom returns and emits a photon.

Rendering scientific sketch…

Simple excitation and de-excitation

The energy difference between the two states is

$$
\Delta E = E_2 - E_1
$$

If a photon is emitted during de-excitation, its energy is

$$
E_\gamma = h f = \Delta E
$$

where $h$ is Planck's constant and $f$ is the photon frequency.

For radiative de-excitation, the emitted photon energy equals the difference between the two energy levels,
$$
E_\gamma = \Delta E
$$

Excitation in Different Detector Materials

Excitation appears in many detector media, but its role depends on the material.

Detector material	Role of excitation
Gas	Excited atoms can emit light or transfer energy through collisions
Organic scintillator	Molecular excitation produces fast light pulses
Inorganic scintillator	Crystal excitation leads to characteristic scintillation light
Semiconductor	Excitation exists, but charge creation is usually the main measured effect

In molecular materials, such as organic scintillators, excitation often involves molecular energy states rather than simple atomic levels. In crystals, the process may involve the solid's electronic structure and luminescent centers.

Radiative and Non-Radiative De-Excitation

Not every excitation produces visible light. Sometimes the excited state releases energy by emitting a photon. This is called radiative de-excitation. Other times, the energy is transferred away through collisions or vibrations in the material. This is called non-radiative de-excitation.

This distinction matters because only part of the deposited energy may become a detectable signal. If much energy is lost non-radiatively, the detector produces less light.

A simple efficiency idea is

$$
\text{light yield} \propto \text{fraction of excitation energy that becomes photons}
$$

The exact treatment of efficiency belongs to a different topic, but the main point here is that excitation does not automatically guarantee a strong signal.

Excitation and Scintillation

One of the most important applications of excitation is scintillation. A charged particle excites the detector material, and the material emits a brief flash of light as it relaxes.

This can be pictured as a sequence:

$$
\text{particle} \to \text{excitation} \to \text{de-excitation} \to \text{light signal}
$$

The amount of light and the time profile of the flash depend on how excitation and de-excitation occur in the material. Fast and efficient light production is one reason scintillators are widely used.

Rendering scientific sketch…

Excitation leading to a scintillation signal

A Simple Energy View

As a particle moves through matter, its kinetic energy decreases. Part of that lost energy goes into excitation. If many atoms are excited, the total deposited energy can be significant even though each individual excitation involves only a small amount of energy.

If one excitation requires energy $\Delta E$, then in a very simple estimate the number of excitations is roughly

$$
N \approx \frac{E_{\text{deposited}}}{\Delta E}
$$

This is only an approximate picture, because in practice energy is shared among excitation, ionization, and other processes.

The energy lost by the particle in the detector is generally divided among several channels, including excitation and ionization. Excitation is one major channel of energy deposition.

Summary

Excitation is the process in which a passing particle transfers energy to an atom or molecule and raises it to a higher energy state without removing an electron. The excited system later returns to a lower state, often emitting light or transferring energy in other ways. This makes excitation a central mechanism in particle detection, especially in scintillation-based detectors, where the detectable signal comes from de-excitation light.

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