How to Choose Your Fast Timing Scintillators

Fast Scintillators, or Fast Timing Scintillators, are scintillators with very short decay times (such as <30 nanoseconds or sub-nanoseconds) for applications requiring high time resolution, including positron Time-of-Flight (TOF) Spectrometer, TOF PET imaging, TOF-LiDAR, and other applications in particle physics.

In this guidance to fast timing scintillators, Shalom EO will use our professional knowledge to help you learn about how to choose the best fast timing scintillators that suits your uses, by facilitating you to understand what is decay time and its implications in the application senario, and offering detailed explanation of the critical performance parameters that you should consider when selecting your scintillator materials, and the application of fast timing scintillators.

What is Decay Time and Why Does It Matter?

A scintillator is a special material that emits light (luminescence) when bombarded by high-energy particles or radiation. The luminescence process of a scintillator consists of two phases: pulse rise and decay. The rise time is mainly affected by the energy dissipation rate of charged particles in the scintillator, which is usually below the microsecond level. Also, it includes a brief process of electron excitation in the scintillator, which is usually negligible. 

After the scintillator is excited, the electron de-excitation process follows an exponential decay law. The number of photons emitted per unit time decreases with time until the pulse drops to 1/e of its maximum value, a period known as the luminous time or decay time of the scintillator. It should be noted that different scintillator materials have different decay characteristics and therefore need to be analysed on a case-by-case basis in practical applications.

For most organic scintillators and some inorganic scintillators, the luminous decay consists of both fast and slow components. In this case, the decay law can be expressed as a double exponential function, in which τ1 and τ2 represent the luminous decay times of the fast and slow components, which range from nanoseconds to hundreds of nanoseconds, respectively. It is also necessary to consider the fluorescence intensities I1 and I2 corresponding to the fast and slow components.

A short, fast decay time is the core property of fast timing scintillators, as it has direct influence on the timing resolution, event separation, and signal clarity in time-sensitive scintillator detectors. A shorter decay time means:

- Improved Timing Resolution

The faster the speed of the light pulses from the scintillator, the higher the timing precision. This is essential for applications like Time-of-Flight (TOF) measurements, where the detection time determines particle velocity or event position.

- Less Piling Up at High Count Rates

When the count rates are high, signals from different events can overlap. Short decay times reduce the signal tail, thus minimizing signal confusion.

- Enable Fast Pulse Processing

Scintillators with short decay times produce shorter, sharper light pulses. These pulses are easier to digitize and process in real time with fast electronics, allowing for better event timing, energy discrimination, and pulse shape analysis.

- Enhance Spatial Resolution in TOF Applications

In TOF-PET, for example, better timing resolution allows for more accurate reconstruction of the annihilation point, which improves image clarity and reduces noise.

Figure 1. Shalom EO's 2.4ns decay time SP101 plastic scintillator (equal to EJ212/BC400)

Other Important Factors for Fast Scintillators

Light Yield: 

Light yield is one of the most important performance indicators of scintillators and refers to the number of photons produced per unit of energy deposited in the scintillator. It is usually assessed by illuminating the scintillator and measuring the number of photons it emits. A higher optical yield means that the scintillator provides a stronger signal output when detected, which contributes to the sensitivity of the detector.

Spectral Response of The Scintillator:

The spectral response determines how well the scintillator is matched to a detector such as a photomultiplier tube or photodiode.

Stopping Power:

The scintillator should be as dense as possible (i.e., have a large effective atomic number, which is often written as the Z number of Zeff) to intercept incoming radiation and give off high light output.

Energy Resolution:

Energy resolution represents the ability of a scintillator to resolve radiation at different energies and is usually characterised by the full width at half height (FWHM) of the photoelectric peaks. The width of the photoelectric peak of a scintillator is measured, and the resolution is calculated by irradiating a source of known energy (e.g., 137Cs or 60Co). A better energy resolution helps to distinguish between particles or rays of different energies in a complex radiation environment.

Radiation Type and Application Environment

Different scintillators respond better to different types of radiation (gamma rays, X-rays, neutrons, etc.). For example:

LYSO: Ce is often used in commercial PET scanners. Hangzhou Shalom EO can provide 30ns decay time LYSO:Ce.

CeBr3 is a good alternative to LaBr₃ without intrinsic radioactivity.

Plastic scintillators are ideal for charged-particle TOF setups where cost and speed matter more than energy resolution.

You also need to consider:

- Optical transmission and refractive index of the scintillator

- Radiation length of the scintillator

- Mechanical durabilities and thermal stabilities of the scintillator

- Linear Response

- Radiation resistance

- Higroscopicity, chemical inertia

Application of Fast Timing Scintillators

 Time of Flight (TOF)

The application of fast decay time scintillators in Time of Flight (TOF) measurement is manifested in their abilities to quickly respond to particle impacts and generate light signals, which are converted into electrical signals through photomultiplier tubes (PMTs) to accurately measure the flight time of particles. Some scintillators have a fast luminescence decay time and high light yield, which makes them often used as the main component of time-of-flight detectors (TOF) in high-energy physics experiments.

TOF-PET: Positron emission tomography (PET) is a molecular imaging technique that provides information at the molecular level. The technique is widely used in medical imaging. The technique is based on radiation detectors to detect incoming coincident annihilation gamma photons emitted from the radiopharmaceutical injected into a patient’s body and uses these data to reconstruct images. Compared with traditional PET scanners, the time-of-flight PET (TOF PET) technique provides better image quality. This is because in traditional PET imaging, when a positron emitted by a radiotracer in the body annihilates with an electron, it produces two gamma photons traveling in opposite directions (180° apart). These are detected almost simultaneously by detectors placed around the patient. Conventional PET can only determine that the annihilation occurred somewhere along the line connecting the two detectors that registered the photons — this is called the line of response (LOR).

TOF-PET not only detects which detectors the gamma photons hit (i.e., the endpoints of the LOR), but also measures the difference in arrival times of the two photons. Because gamma photons travel at the speed of light, even a small difference in arrival time translates to a specific offset along the LOR. This means the scanner can estimate more precisely where along the LOR the annihilation event occurred.

Radiation scintillator detectors are a promising means to develop TOF-PET systems. Scintillators for TOF-PET use are generally required to have decay times shorter than 100ns, while decay times between 20ns to 50ns are the optimal choices. The industrial-standard TOF-PET scintillator detectors are made of LYSO:Ce scintillators. 

TOF-PAS (Positron Time-of-Flight Spectrometers): 

Positron Time-of-Flight Spectrometer (TOF-PAS) is a powerful non-destructive research tool that uses the annihilation radiation of positrons in condensed matter to reveal information such as the microstructure, electron momentum distribution, and defect states inside the material. This technique has been widely used in many fields, including solid state physics, semiconductor physics, metal physics, atomic physics, surface physics, superconducting physics, biology, chemistry, and medicine.

Based on the positron annihilation time measurement method, the main process of positron time of flight spectrometers includes: 

1. Use a ²²Na radioactive source to generate positrons

2. Capture 1.28 MeV start signal and 0.511 MeV stop signal through scintillator detector (such as BaF₂ or LYSO) [3] [5]

3. The time-amplitude conversion unit converts the time difference into an electrical signal, and the multi-channel analysis module generates a lifetime spectrum

Other Fast-timing Applications of Scintillators Besides Time of Flight

In particle physics experiments, such as those at CERN or Fermilab, detectors must process extremely large volumes of events in very short periods, therefore, high count rate scintillator detectors are needed. Fast scintillators help avoid signal overlap (pulse pile-up) and maintain precise timing between successive particle interactions. This is crucial for triggering systems and precise event reconstruction in complex detector arrays. 

In the area of particle accelerators, fast scintillators are used for beam diagnostics and bunch timing. They provide ultrafast detection of beam pulses, which can arrive at GHz frequencies. Accurate timing here is essential for controlling beam parameters, optimizing accelerator performance, and ensuring synchronization between subsystems.

Another area is fast neutron detection. Some plastic scintillators, which have fast response and good pulse shape discrimination (PSD ) capabilities, are used to distinguish between neutrons and gamma rays based on differences in signal decay profiles. These materials are essential in nuclear reactor monitoring, homeland security applications, and scientific neutron spectroscopes.

Finally, fast decay scintillators find use in security inspection scanners, such as cargo or baggage scanning, where rapid X-ray imaging is required. The fast decay ensures quick signal recoveries between successive exposures, enabling high frame rates and better discrimination of different materials, even when scanned at speed.

Scintillators Comparison for Fast Timing Applications

Scintillator crystals: The most common inorganic scintillator crystals for time of flight are BaF2, LYSO:Ce, LaBr3:Ce, CeBr3, YAP(Ce), LaCl3(Ce), and CsI.

Plastic scintillator: Organic scintillators like Shalom EO’s SP101 are also ideal for fast-timing applications.

With a thorough understanding in the professional field and leading-edge techniques, Hangzhou Shalom EO supplies a series of scintillator materials and scintillator detectors suitable for fast timing applications, the scintillator material types include scintillator crytals like BaF2, LYSO: Ce, LaBr3:Ce, CeBr3, YAP(Ce), LaCl3(Ce), CsI and plastic scintillators SP101 (which is equivalent to Eljen's EJ212, and Luxium's BC400). 

Below is a chart listing the performance parameters of Shalom EO's fast timing scintillator materials for comparison:

Fast Scintillator Comparison Chart:

Scintillator Material	Decay Time (ns)	Light Yield (photons/MeV)	Effective Atomic Number/Density	Emission Peak Wavelength (nm)	Refractive Index	Remarks
BaF2	630(slow)/0.6-0.8(fast)	10000(slow)/1800(fast)	Density  4.88 g/cm3	310, 220	1.54@220nm	Very fast, sub-nanosecond decay time; slow component must be filtered out
LYSO(Ce)	<42	≥32000	Zeff 66, Density 7.2 g/cm3	428	1.82	Widely adopted in commercial TOF-PET; excellent comprehensive properties
LaBr3(Ce)	25	63000	Density 5.2 g/cm3	380	1.9	Very bright; excellent time resolution
CeBr3	<20	60000	Density 5.1g/cm3	360~385	/	Similar to LaBr₃:Ce; lower intrinsic background
Plastic Scintillator SP101 (Equal to EJ212/BC400)	2.4	64% of Anthracene Crystal	Density 1.02 g/cm3	423	1.58	Very fast; cost-effective, easy to be processed into different shapes
LaCl3(Ce)	<28	40000	Density 3.85 g/cm3	350	1.9	Decent light output;durable mechanical properties;and good energy resolution maintained against temperature changes
YAP(Ce)	40	18000	Density 5.4g/cm3	325-425	1.95	Well suited for imagining applications due to its ultra-low energy secondary X-ray emissions
CsI	16	2x10^3	Density 4.51g/cm3	315	1.95	Fast decay time; can handle severe shock conditions

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