Effects of gamma irradiation on DEPFET pixel sensors for the Belle II experiment

TL;DR

This study evaluates the radiation hardness of DEPFET pixel sensors used in the Belle II experiment by irradiating a full detector half-ladder with X-rays to simulate 7-10 years of operation, assessing performance impacts.

Contribution

First comprehensive full-system irradiation of a DEPFET-based pixel detector half-ladder, demonstrating its suitability for long-term operation in high-radiation environments.

Findings

DEPFET sensors maintain functionality after high-dose irradiation

Performance of front-end electronics remains stable post-irradiation

Efficiency studies show minimal degradation after exposure

Abstract

For the Belle II experiment at KEK (Tsukuba, Japan) the KEKB accelerator was upgraded to deliver a 40 times larger instantaneous luminosity than before, which requires an increased radiation hardness of the detector components. As the innermost part of the Belle II detector, the pixel detector (PXD), based on DEPFET (DEpleted P-channel Field Effect Transistor) technology, is most exposed to radiation from the accelerator. An irradiation campaign was performed to verify that the PXD can cope with the expected amount of radiation. We present the results of this measurement campaign in which an X-ray machine was used to irradiate a single PXD half-ladder to a total dose of 266 kGy. The half-ladder is from the same batch as the half-ladders used for Belle II. According to simulations, the total accumulated dose corresponds to 7-10 years of Belle II operation. While individual components have been irradiated before, this campaign is the first full system irradiation. We discuss the effects on the DEPFET sensors, as well as the performance of the front-end electronics. In addition, we present efficiency studies of the half-ladder from beam tests performed before and after the irradiation.