# Deep Diffused APDs for Charged Particle Timing Applications: Performance after Neutron Irradiation

## TL;DR

This paper investigates deep diffused Avalanche Photo Diodes (APDs) as fast timing detectors for high-energy physics, assessing their performance and radiation tolerance after neutron irradiation, with potential applications in HL-LHC experiments.

## Contribution

It provides the first detailed characterization of deep diffused APDs' timing performance and radiation effects, demonstrating their potential and limitations for HL-LHC timing applications.

## Key findings

- Achieved 27 ps time resolution with an 8x8 mm² APD in beam tests.
- Radiation damage reduces gain, but increased bias can compensate up to certain fluences.
- Performance may be limited for the highest expected HL-LHC neutron fluences.

## Abstract

Recent interest in pile-up mitigation through fast timing at the HL-LHC has focused attention on technologies that now achieve minimum ionising particle (MIP) time resolution of 30 picoseconds or less. The constraints of technical maturity and radiation tolerance narrowed the options in this rapidly developing field for the ATLAS and CMS upgrades to low gain avalanche detectors and silicon photomultipliers. In a variety of applications where occupancies and doses are lower, devices with pixel elements of order 1 cm$^2$, nevertheless achieving 30 ps, would be attractive. In this paper, deep diffused Avalanche Photo Diodes (APDs) are examined as candidate timing detectors for HL-LHC applications. Devices with an active area of $8 \times 8$ mm$^2$ are characterised using a pulsed infrared laser and, in some cases, high energy particle beams.The timing performance as well as the uniformity of response are examined. The effects of radiation damage on current, signal amplitude, noise, and timing of the APDs are evaluated using detectors with an active area of $2 \times 2$ mm$^2$. These detectors were irradiated with neutrons up to a a 1-MeV neutrons fluence $\Phi_{eq} = 10^{15}$ cm$^{-2}$. Their timing performance was characterised using a pulsed infrared laser. While a time resolution of $27 \pm 1$ ps was obtained in a beam test using an $8 \times 8$ mm$^2$ sensor, the present study only demonstrates that gain loss can be compensated by increased detector bias up to fluences of $\Phi_{eq} = 6 \cdot 10^{13}$ cm$^{-2}$. So it possibly falls short of the $\Phi_{eq} = 10^{14}$ cm$^{-2}$ requirement for the CMS barrel over the lifetime of the HL-LHC.