CLLB vs CLYC: Key Differences in Pulse Shape Discrimination (PSD)
What is pulse shape discrimination (PSD)?
Pulse shape discrimination (PSD) is a signal-processing technique used in scintillation detectors to distinguish between different types of radiation—most commonly gamma rays and neutrons—based on differences in scintillation decay characteristics. When radiation interacts with a scintillator crystal, the resulting light pulse can contain multiple decay components. By analyzing the relative contribution of fast and slow components, neutron-induced and gamma-induced events can be separated within a single detector volume.
PSD-capable scintillators are particularly important in radiation monitoring, safeguards, and neutron detection systems where both gamma and neutron fields are present.
Overview of CLLB and CLYC scintillators
CLLB (Cs₂LiLaBr₆:Ce) and CLYC (Cs₂LiYCl₆:Ce) are both cerium-doped elpasolite scintillator crystals widely studied for combined neutron and gamma detection. Although they share a similar crystal structure and detection concept, their PSD behavior differs significantly due to differences in chemical composition, decay time constants, and interaction mechanisms.
Key material and scintillation differences
| Property | CLLB (Cs₂LiLaBr₆:Ce) | CLYC (Cs₂LiYCl₆:Ce) |
|---|---|---|
| Halide type | Bromide | Chloride |
| Density (g/cm³) | ~4.2 | ~3.3 |
| Effective atomic number | Higher | Lower |
| Gamma efficiency | Higher | Moderate |
| Neutron sensitivity | Yes (⁶Li) | Yes (⁶Li) |
| Dominant PSD mechanism | Mixed fast/slow components | Strong slow neutron component |
The bromide-based lattice of CLLB leads to higher density and gamma-ray interaction probability, while the chloride-based lattice of CLYC produces more pronounced differences between neutron and gamma scintillation decay profiles.
PSD performance and figure of merit (FOM)
The effectiveness of PSD is commonly quantified using the figure of merit (FOM), defined as:
FOM = |μₙ − μᵧ| / (FWHMₙ + FWHMᵧ)
where μₙ and μᵧ are the mean PSD parameters for neutron and gamma events, respectively.
Typical PSD behavior
- CLYC often exhibits a larger separation between neutron and gamma pulse shapes, resulting in higher FOM values, especially under low-count-rate and low-background conditions.
- CLLB typically shows moderate PSD separation, but with greater stability as count rate and gamma background increase.
In practice, CLYC frequently achieves higher peak FOM values in laboratory measurements, while CLLB maintains usable PSD performance across a broader range of radiation environments.
Time structure of scintillation pulses
The difference in PSD behavior between CLLB and CLYC can be traced to their scintillation decay components:
CLYC
- Gamma interactions: dominated by relatively fast decay components
- Neutron interactions: strong slow decay component (microsecond scale)
This large difference in decay constants makes neutron–gamma separation straightforward using standard PSD algorithms.
CLLB
- Gamma interactions: stronger fast-component contribution
- Neutron interactions: slower components are present but less dominant
As a result, CLLB requires more careful optimization of integration windows and electronics but benefits from faster overall signal response.
Performance in high count-rate and mixed radiation fields
Advantages of CLLB
- Higher density improves gamma-ray detection efficiency
- Faster scintillation response reduces pulse pile-up
- PSD remains stable under high gamma backgrounds
These characteristics make CLLB well suited for field-deployable systems, RIID instruments, and safeguards applications where radiation conditions are complex and variable.
Advantages of CLYC
- Excellent neutron–gamma separation at low to moderate count rates
- Cleaner PSD distributions in controlled environments
- Lower intrinsic background in many implementations
CLYC is therefore often preferred in laboratory measurements, educational systems, and applications prioritizing maximum neutron discrimination clarity.
Intrinsic background considerations
Intrinsic radioactivity can influence low-count-rate neutron measurements:
- CLLB may exhibit internal background associated with lanthanum isotopes or trace actinium contamination, depending on crystal growth and purification.
- CLYC generally shows lower intrinsic background, which can improve sensitivity in low-flux neutron environments.
This difference can be a deciding factor for applications operating near detection limits.
Selection guidance: when to choose CLLB or CLYC
- Choose CLYC when the primary goal is maximum PSD separation and measurements are performed under controlled or low-background conditions.
- Choose CLLB when system robustness, high gamma efficiency, and stable PSD performance are required in mixed or high-count-rate radiation fields.
- For compact field systems where electronics simplicity and stability are critical, material selection should consider both PSD performance and operational environment.
Summary
Although both CLLB and CLYC enable combined neutron and gamma detection using pulse shape discrimination, their PSD characteristics differ in meaningful ways. CLYC typically offers higher PSD separation under ideal conditions, while CLLB provides more stable performance across a wider range of practical radiation environments. Understanding these differences is essential for selecting the appropriate scintillator material for a given detector system
