Investigating Artefacts and Repair Mechanisms Associated with α-particle Interactions in Charge Coupled Devices

Abstract

Charge coupled devices (CCDs) have been shown to have potential for detecting charged particles and other forms of ionising radiation. In particular, the clusters in the pixel images produced are distinctive for α and β radiations, with α particles causing large, symmetrical clusters or long, vertical tracks often referred to as blooming, and β particles causing long, curved tracks. The size and shape of these tracks are also related to the energy of the incident radiation, giving the potential for spectroscopy of these types of radiation. This could be used, for example, to realise a hand-held, portable device for in-situ detection and identification of radioactive contamination. Images have been taken of exposures to a 210Po α-particle source using Sony ICX825AL interline transfer CCDs, covered with a thin, aluminised Mylar film to prevent inadvertent exposure to light. Both vertical streaks and round clusters have been observed in each of the three CCDs used. Increased levels of noise have been seen after several hours of exposure to the 210Po source, found to be caused by damage from α radiation. Three methods have been investigated to reduce this noise: 1) Annealing at 100°C for 24 hours repairs a significant proportion of the damage caused by the α radiation, though the effectiveness is reduced with each subsequent exposure and annealing cycle; 2) Cooling is performed during exposures by using a heat pipe to move heat away from the back of the CCD and a fan to cool the heat pipe and electronics associated with the CCD.This reduces the dark current during CCDmeasurements, and the method of cooling used allows thedevice to retain its small, hand-held size; 3) Anyremaining noise can be removed later using imageprocessing. These techniques are combined to extend thelifetime of the device. The rate of damage from the 210Posource is similar whether the device is cooled or not,however the noise evident in the images produced whencooled is significantly less, allowing the CCD to be usedfor a longer time between annealing operations.