Proton Energy Dependence of Radiation Induced Low Gain Avalanche Detector Degradation

TL;DR

This work investigates how proton energy affects radiation-induced degradation of the LGAD gain layer via acceptor removal, comparing HPK and CNM-CNM devices across 18–24 MeV, 400 MeV, and 23 GeV protons. Electrical I–V/C–V measurements yield $V_{gl}$ and acceptor-removal coefficients $c$, while IR TCT laser measurements provide gain evolution with fluence, revealing that 18–24 MeV protons cause the most degradation and 400 MeV the least, even after converting fluences to $n_{eq}$ with $\Phi_{eq}=\kappa\,\Phi$. The observed non-monotonic energy dependence persists after $n_{eq}$ normalization, indicating that standard $NIEL$ scaling does not fully capture energy-dependent defect formation, particularly in mixed-energy irradiation fields. Carbon enrichment in CNM devices mitigates degradation, and laser data corroborate slower gain loss at intermediate energies, highlighting the need for revised damage models and continued timing-focused studies for HL-LHC LGADs.

Abstract

Low Gain Avalanche Detectors (LGADs) are key components for precise timing measurements in high-energy physics experiments, including the High Luminosity upgrades of the current LHC detectors. Their performance is, however, limited by radiation induced degradation of the gain layer, primarily driven by acceptor removal. This study presents a systematic comparison of how the degradation evolves with different incident proton energies, using LGADs from Hamamatsu Photonics (HPK) and The Institute of Microelectronics of Barcelona (IMB-CNM) irradiated with 18 MeV, 24 MeV, 400 MeV and 23 GeV protons and fluences up to 2.5x10^15 p/cm2. Electrical characterization is used to extract the acceptor removal coefficients for different proton energies, whereas IR TCT measurements offer complementary insight into the gain evolution in LGADs after irradiation. Across all devices, lower energy protons induce stronger gain layer degradation, confirming expectations. However, 400 MeV protons consistently appear less damaging than both lower and higher energy protons, an unexpected deviation from a monotonic energy trend. Conversion of proton fluences to 1 MeV neutron-equivalent fluences reduces but does not eliminate these differences, indicating that the standard Non-Ionizing Energy Loss (NIEL) scaling does not fully account for the underlying defect formation mechanisms at different energies and requires revision when considering irradiation fields that contain a broader spectrum of particle types and energies.

Figures (6)

• Figure 1: Exemplary I--V characteristics of HPK W25 LGADs after 23 GeV proton irradiation with increasing particle fluences. The steep rise in leakage current, attributed to the depletion of the gain layer, shifts toward lower applied bias voltages with increasing fluence due to degradation of the gain layer.
• Figure 2: The two plots show the degradation of HPK LGAD gain layers in terms of the reduction of $V_{gl}$ with increasing fluence for various proton energies. The fitted acceptor removal coefficients indicate a strong energy dependence of the damage, with 400 MeV protons consistently leading to the weakest degradation.
• Figure 3: The two plots show the acceptor removal behavior in CNM LGADs, comparing proton energies (left) and the three wafers with different carbon enrichment for one energy (right). The results illustrate both the weaker damage at 400 MeV compared to 23 GeV and the mitigation of acceptor removal with carbon in the gain layer.
• Figure 4: $V_{gl}$ of HPK2 W25 LGADs plotted versus $\mathrm{n_{eq}}$ fluence, showing still energy dependent acceptor removal behavior after the fluence conversion but with changed relative order compared to \ref{['subfig:HPK_W25']}.
• Figure 5: Gain measurements for HPK W25 LGADs irradiated with 400 MeV and 23 GeV protons show similar behavior at lower fluences. While 400 MeV devices retain measurable gain up to about $10^{15}\,\mathrm{n_{eq}}$ fluence, faster loss of gain is observed for 23 GeV irradiated devices at higher fluences.