8.4.3.2. Organic Scintillators

Table of Contents

Basic idea

Organic scintillators are scintillation materials made from carbon based molecules. When ionizing radiation passes through them, it excites the molecules. As these molecules return to lower energy states, they emit visible or near visible light. This light can then be detected by a photomultiplier tube or a silicon photomultiplier.

They are widely used because they respond very quickly, can be made in many shapes, and are especially useful for detecting fast charged particles and neutrons. Compared with many inorganic scintillators, they usually produce less light and have poorer energy resolution, but they are much faster.

What makes them "organic"

The word organic here means that the scintillator is based on organic chemical compounds, usually containing aromatic rings with delocalized electrons. These electron systems are easy to excite and de excite, which makes light emission possible.

In practice, organic scintillators are commonly found in three forms.

Form	Typical description	Common use
Plastic scintillator	Organic molecules embedded in a plastic base	Fast timing, particle detectors
Liquid scintillator	Scintillating molecules dissolved in a liquid solvent	Large detector volumes, neutrino and radiation measurements
Crystal organic scintillator	Pure organic crystalline material	Specialized applications

Plastic and liquid scintillators are the most common.

How light is produced

When a charged particle moves through an organic scintillator, it loses energy mainly by exciting the molecules of the material. Some of this excitation energy is converted into light. The emitted photons are usually in the ultraviolet range at first, so wavelength shifting additives are often used to move the light into the visible range, where photosensors are more efficient.

The process is fast because the relevant molecular transitions occur on very short timescales. A simple picture is:

$$
\text{radiation energy} \rightarrow \text{molecular excitation} \rightarrow \text{light photons}
$$

A small fraction of the deposited energy becomes detectable light.

Organic scintillators do not measure radiation by collecting electric charge directly. They measure it by converting deposited energy into flashes of light.

Fast response

One of the most important features of organic scintillators is their short decay time. The scintillation pulse rises and falls very quickly, often in a few nanoseconds. This makes them excellent for timing measurements and for counting events that occur close together in time.

If the light output as a function of time is modeled simply, it is often written as

$$
I(t) = I_0 e^{-t/\tau}
$$

where $I(t)$ is the light intensity, $I_0$ is the initial intensity, and $\tau$ is the decay time constant. For organic scintillators, $\tau$ is usually much smaller than for inorganic scintillators.

A key advantage of organic scintillators is fast timing, not high energy resolution.

Types of organic scintillators

Plastic scintillators

Plastic scintillators are made by dissolving scintillating molecules in a transparent plastic base, often polystyrene or polyvinyltoluene. They are mechanically strong, easy to machine, and can be manufactured as slabs, bars, fibers, or sheets.

They are commonly used in particle physics experiments, cosmic ray detectors, and timing counters.

Liquid scintillators

Liquid scintillators contain scintillating solutes dissolved in an organic solvent. They are useful when very large detector volumes are needed. Because liquids can fill large tanks, they are widely used in low background experiments and neutrino detection.

They can also be loaded with other substances to improve sensitivity to specific types of radiation.

Organic crystals

Some pure organic crystals, such as anthracene, are very efficient organic scintillators. They played an important historical role and are still used in some specialized applications. However, plastics and liquids are usually more practical.

Role of primary fluor and wavelength shifter

In many organic scintillators, energy is not emitted directly by the base material. Instead, the energy is transferred to special molecules called fluors. A primary fluor emits light after being excited. Sometimes a second additive, called a wavelength shifter, absorbs that light and re emits it at a longer wavelength.

This helps in two ways. It improves transparency inside the material, and it matches the emission better to the sensitivity of the photosensor.

A simplified chain is

$$
\text{base material} \rightarrow \text{primary fluor} \rightarrow \text{wavelength shifter} \rightarrow \text{detected light}
$$

Response to different kinds of radiation

Organic scintillators are especially sensitive to charged particles because these particles deposit energy directly in the material. Gamma rays are usually detected indirectly, after they produce secondary electrons. Neutrons are often detected through recoil protons, especially in hydrogen rich plastic or liquid scintillators.

Because many organic scintillators contain a lot of hydrogen, they are useful for fast neutron detection. A neutron can collide elastically with a proton in the material, and the recoiling proton then produces scintillation light.

Light output and quenching

The light output is not always exactly proportional to the energy deposited. For heavily ionizing particles, the scintillation efficiency decreases. This effect is called quenching.

A commonly used empirical description is Birks' law:

$$
\frac{dL}{dx} = S \frac{dE/dx}{1 + k_B \, dE/dx}
$$

where $\frac{dL}{dx}$ is the light produced per unit path length, $\frac{dE}{dx}$ is the energy loss per unit path length, $S$ is a scaling constant, and $k_B$ is Birks' constant.

This means that particles with large energy loss per unit distance do not always produce proportionally more light.

In organic scintillators, light output can be non linear because of quenching. Equal deposited energies from different particles may not produce equal light signals.

Pulse shape discrimination

An important feature of some organic scintillators, especially liquids, is pulse shape discrimination. Different types of radiation can produce pulses with slightly different time profiles. For example, neutron induced events and gamma induced events may have different fractions of fast and slow light components.

By analyzing the pulse shape, it is possible to distinguish one type of radiation from another, even if the total pulse height is similar. This is very useful in mixed radiation fields.

Advantages and limitations

Organic scintillators are popular because they are practical and fast, but they also have important limitations.

Property	Organic scintillators
Response speed	Very fast
Light yield	Moderate, usually lower than many inorganic crystals
Energy resolution	Usually poor to moderate
Mechanical shaping	Easy for plastics, flexible for liquids
Large volume construction	Excellent, especially liquids
Neutron detection	Good, especially hydrogen rich materials
Density and atomic number	Usually low
Gamma ray stopping power	Relatively low

Their low average atomic number means they are less effective than dense inorganic crystals for stopping high energy gamma rays. This is why they are often chosen for timing or large area coverage rather than precise gamma spectroscopy.

Typical applications

Organic scintillators are used in many areas of nuclear and particle physics. Plastic scintillators are common in trigger counters, cosmic ray telescopes, and time of flight systems. Liquid scintillators are used in neutrino detectors, environmental radiation monitoring, and neutron measurements.

They are also used in medical physics and radiation protection when fast timing or large sensitive volumes are important.

Simple detector geometry

A typical plastic scintillator detector has a scintillator block connected to a photosensor. Radiation enters the scintillator, light is produced, and the light travels to the sensor.

Rendering scientific sketch…

Simple plastic scintillator detector

Comparison with inorganic scintillators

Organic scintillators and inorganic scintillators both produce light when radiation deposits energy, but they are optimized for different purposes.

Feature	Organic	Inorganic
Decay time	Very short	Usually longer
Density	Lower	Higher
Gamma stopping power	Lower	Higher
Energy resolution	Usually worse	Usually better
Mechanical fabrication	Easier for plastics and liquids	Often harder, crystal growth needed

This is why organic scintillators are often chosen for fast counters, while inorganic scintillators are often chosen for gamma ray spectroscopy.

Key points to remember

Organic scintillators are carbon based scintillation materials that convert radiation energy into light through molecular excitation and de excitation. Their most important strengths are fast timing, easy shaping, and usefulness for charged particle and neutron detection. Their main weaknesses are lower light yield, lower gamma stopping power, and poorer energy resolution than many inorganic scintillators.

Remember these core ideas: organic scintillators are fast, hydrogen rich materials are useful for neutron detection, and quenching makes the light output not perfectly proportional to deposited energy.

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