## Definition of Spectral Tailing
Spectral tailing in a CZT planar detector refers to the asymmetric broadening of photopeak signals toward lower energies. This phenomenon reduces peak sharpness, increases the full-width at half-maximum (FWHM), and can obscure low-energy spectral features. Understanding the underlying causes of tailing is essential for optimizing detector design, improving energy resolution, and enabling accurate gamma-ray spectroscopy.
## Hole Trapping Effects
One of the primary sources of spectral tailing is the incomplete collection of holes due to their relatively low mobility-lifetime (μτ) products in CZT. In a planar detector, electrons are the dominant charge carriers contributing to the signal, but holes drift toward the cathode and can be trapped at defect sites. Trapped holes result in partial signal formation, producing a continuous low-energy tail beneath the photopeak. The effect is more pronounced in thicker detectors, where hole drift distances are longer.
## Depth-Dependent Charge Collection
The induced signal in planar detectors depends on the interaction depth because weighting potential is non-uniform. Events occurring near the cathode contribute less to the anode signal if holes are trapped or recombine. This depth dependence creates variation in measured pulse heights for identical photon energies, manifesting as low-energy tailing in the spectral response.
## Polarization and Electric Field Distortion
Long-term biased operation can lead to polarization, where trapped charges locally modify the internal electric field. Polarization effects distort charge drift trajectories and reduce collection efficiency, particularly for interactions occurring far from the anode. This time-dependent degradation introduces additional tailing as a fraction of charge fails to reach the electrode.
## Electronic Noise Contributions
Electronic noise, including thermal noise, shot noise from leakage current, and low-frequency (1/f) noise, contributes to baseline fluctuations. While noise alone broadens peaks symmetrically, in combination with partial charge collection it accentuates low-energy tailing by producing under-amplified pulses that blend with true low-energy signals.
## Surface and Contact Imperfections
Surface leakage currents and imperfect electrode contacts create localized charge loss paths. Charges generated near the surfaces or imperfect contacts may recombine before collection, contributing to low-energy pulse formation. Additionally, non-uniform Schottky barriers or residual oxides at the electrode interface can locally suppress signal amplitude, adding to tailing.
## Compton Scattering and Multiple Interactions
Photons that undergo Compton scattering within the detector before full absorption deposit only part of their energy at a given interaction site. If the scattered photon escapes the detector or deposits residual energy elsewhere, the measured pulse is lower than the incident energy, producing an asymmetric low-energy tail in the spectrum.
## Summary
Spectral tailing in CZT planar detectors arises primarily from hole trapping, depth-dependent charge collection, polarization-induced field distortions, electronic noise, surface leakage, contact imperfections, and partial-energy deposition due to Compton scattering. These effects combine to produce low-energy asymmetry in the spectral response, reducing energy resolution and requiring correction or mitigation strategies for high-precision gamma spectroscopy.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/what-are-the-primary-sources-of-spectral-tailing-in-the-response-function-of-a-czt-planar-detector.html
