Physics of reverse annealing in high-resistivity Chandra ACIS CCDs

TL;DR

This paper investigates the reverse annealing process in CCDs damaged by radiation, using lab experiments to understand how heating affects charge transfer inefficiency after proton irradiation.

Contribution

It provides new experimental data on reverse annealing in CCDs, comparing lab results with in-flight and ground observations to better understand damage recovery mechanisms.

Findings

Reverse annealing increases CTI after warming CCDs.

Proton energy influences annealing behavior.

Derived limits on annealing time constants.

Abstract

After launch, the Advanced CCD Imaging Spectrometer (ACIS), a focal plane instrument on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earth's radiation belts. An effect of the damage was to increase the charge transfer inefficiency (CTI) of the front illuminated CCDs. As part of the initial damage assessment, the focal plane was warmed from the operating temperature of -100C to +30C which unexpectedly further increased the CTI. We report results of ACIS CCD irradiation experiments in the lab aimed at better understanding this reverse annealing process. Six CCDs were irradiated cold by protons ranging in energy from 100 keV to 400 keV, and then subjected to simulated bakeouts in one of three annealing cycles. We present results of these lab experiments, compare them to our previous experiences on the ground and in flight, and derive limits on the annealing time constants.