Threshold bounce -- occupancy-dependent modulation of the discriminating threshold in silicon detectors

TL;DR

This paper investigates the phenomenon of threshold bounce in silicon detector front-end electronics, measuring its effects in ATLAS upgrade modules and modeling it with simulations to understand its impact on detector efficiency.

Contribution

It provides the first measurement of threshold bounce in silicon detectors and develops a Monte Carlo model to predict its influence on detector performance.

Findings

Threshold bounce magnitude depends on photon flux and discriminator threshold.

Monte Carlo simulation accurately predicts hit efficiency affected by threshold bounce.

Experimental measurements align well with simulation predictions.

Abstract

The front-end electronics of silicon detectors are typically designed to ensure optimal noise performance for the expected input charge. A combination of preamplifiers and shaper circuits result in a nontrivial response of the front-end to injected charge, and the magnitude of the response may be sizeable in readout windows subsequent to that in which the charge was initially injected. The modulation of the discriminator threshold due to the superposition of the front-end response across multiple readout windows is coined "threshold bounce". In this paper, we report a measurement of threshold bounce using silicon modules built for the Phase-II Upgrade of the ATLAS detector at the Large Hadron Collider. These modules utilize ATLAS Binary Chips for their hit readout. The measurement was performed using a micro-focused 15 keV photon beam at the Diamond Light Source synchrotron. The effect of the choice of photon flux and discriminator threshold on the magnitude of the threshold bounce is studied. A Monte Carlo simulation which accounts for the front-end behaviour of the silicon modules is developed, and its predicted hit efficiency is found to be in good agreement with the measured hit efficiency.