Space charge in ionization detectors and the NA48 calorimeter

TL;DR

This paper investigates how space charge effects from drifting ions impact the performance of ionization detectors, specifically analyzing the NA48 liquid krypton calorimeter, and presents a model to predict these effects under various conditions.

Contribution

It introduces a computational model that accurately reproduces space charge effects in ionization detectors, adaptable to different geometries and media.

Findings

Space charge causes response dependence on shower position.

The model accurately predicts the amplitude reduction.

Ion mobility is the key free parameter in the model.

Abstract

The effects of space charge due to slowly drifting ions can be relevant for detectors operated at high intensity, or for relatively low values of the bias voltage. Accurate measurements have been obtained with the liquid krypton calorimeter of the NA48 experiment, from data collected in 1997. The build-up of space charge takes place during the first part of the beam extraction burst, and causes a dependence of the response on the transverse coordinate of the axis of electromagnetic showers, and a small reduction of average amplitude. The effects are well reproduced by a computation, where the only free parameter is the value of the ion mobility. The model can be applied a wide range of operating conditions, and generalized to detectors with different geometry and active medium.