A critical property of radiation detectors is stopping power, which describes the efficiency of the detector in collecting radiation. In order to minimize patient dose, it is helpful to increase stopping power, since stopping power is inversely related to the dose required for obtaining high-quality patient images. Stopping power is strongly related to the effective atomic number of the detector material. Designers of radiological devices would prefer to build radiation detectors from materials with very high effective atomic numbers, such as lead. Unfortunately, most elements with higher stopping powers are not good candidates as radiation detector materials, because electrical charges (created by radiation in the detector) cannot be easily moved out of the detector material. As a result, instruments placed outside the detector are unable to measure the charges caused by radiation in the detector material. The process of removing charge from the inside of the detector into the outside world (where that it can be measured) is called "charge transport". In Phase I, we showed that it was possible to cut the Gordian knot of detector specification, by separating the goal of creating high stopping power from the challenge of favorable charge transport. This separation was accomplished by creating a matrix in which quantum dots made of high-atomic- number material (i.e., lead sulfide) were interspersed within a silicon matrix. Quantum dots are small collections of atoms that have different electrical properties than the bulk versions of these atoms. We used the high stopping power property of the lead sulfide, and the favorable electrical properties of the quantum dots, to convert radiation into charge with high efficiency. We used the silicon matrix to transport the charges effectively from the quantum dots to the surface of the silicon matrix, so that the charges could be amplified and provide strong signals to outside instruments. An additional benefit of using silicon as the host matrix was that we showed we could build circuit elements (e.g., amplifier components) on the same material as the radiation detector. Having demonstrated feasibility, our next goal is to ready the product for commercialization by optimizing detector quality. This project represents the first successful application of nanotechnology to direct-conversion radiation detection. It promises to reduce radiation dose to patients and to lower health care costs. In addition to the diagnostic radiology market, the platform technology will be useful for homeland security and broad-spectrum surveillance for the consumer and defense markets, with potential cross-fertilization to the solar power industry.
This project represents the first successful application of nanotechnology to direct-conversion radiation detection. It promises to reduce the radiation doses required to collect high quality images, and to reduce the costs of radiology instrumentation. In addition to the diagnostic radiology market, the platform technology will be useful for homeland security and broad- spectrum surveillance for the consumer and defense markets, with potential cross-fertilization to the solar power industry.
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