Investigations of degradation and encapsulation of plastic scintillator

摘要

Studies have revealed that plastic scintillators such as polyvinyl toluene and polystyrene can undergo environmentally related material damage ("fogging") that adversely affects detection performance under certain conditions and histories. A significant decrease in sensitivity has been seen in some gamma-ray detectors as they age as a result of this damage. Performance degradation due to such damage is characterized by a signal change from the detector, which shifts to lower energy, and ultimately a reduction in the ability to detect gamma radiation. This degradation is due to the permeation of water into the plastic, which can then cause temporary fogging and permanent damage to the material. As an example, a 1-mm thick barrier of high-density polyethylene with an area of 1 m2 would allow 5 g per year of water transmission at 100% relative humidity. Thus, significant amounts of water can penetrate plastics over time and provide the potential for damage to the plastic. Such damage to plastic scintillator can cause problems for radiation detection applications in uncontrolled environments. Mitigation approaches that have been proposed to prevent damage to plastic scintillator include encapsulation of the plastic scintillator to prevent water intrusion that leads to damage during cold cycles. It is concluded that an encapsulation material with a moisture vapor transmission rate on the order of 10~(-4) gm~(-2)d~(-1)or better is needed for protecting large pieces of plastic scintillator from fogging over years of use. This paper presents information on testing of bare and encapsulated plastic scintillator samples and on diagnostic approaches to measure the nature and progression of the fogging condition. It is shown that several encapsulation approaches fail to prevent water intrusion, while a couple methods can successfully encapsulate plastic and prevent water from penetrating the plastic and producing fogging.