The sensor, developed by Scandinavian research group, SINTEF, uses a silicon drift diode (SDD), which is the basic component of a number of instruments that are used in everything from medical X-ray systems to monitoring experiments at CERN. Another application is in art and archaeology, where the detector can identify which materials have been used and what they consist of.
‘The sensor consists of a double-sided microstructure that is fabricated on silicon wafers. Such structures are complex and difficult to produce. Today, we are one of only two or three suppliers of such sensors in the whole world,’ said Niaz Ahmed, a research scientist at SINTEF.
Although the tiny device measures no more than 8 x 8mm it takes eight weeks to produce, and the entire fabrication needs to take place in a super-clean environment.
The sensor uses spectroscopy, which involves sending light through a transparent object. When the light beam emerges from the other side of the object, the sensor reads off changes in its characteristics.
‘To put it simply, we can say that the sensor sorts the light into its individual energy levels by counting the photons and calculating their energy,’ said Ahmed.
Unlike standard silicon-based sensors, the way that silicon drift diodes work requires them to have structures on both surfaces of the sensor chip.
‘This can only be done with the help of advanced equipment and extremely high levels of accuracy,’ said Ahmed.
Ahmed explained one side of the sensor is called the ‘window side’, and is turned towards the source of radiation. It absorbs the X-ray beam almost without loss. The other side is known as the ‘ring side’ and has a concentric annular structure; i.e. the rings have the same centre but increase in radius, something like a parabolic aerial in microformat. This means that the electrons generated by the radiation source are captured by the central electrode, which, in turn, enables the X-ray sensor to discard all the irrelevant electronic signals, or ‘noise’.
‘Because it easily distinguishes between different materials by registering differences in the absorption energy of their component elements, the chip can be used to identity forbidden materials such as lead, cadmium and mercury,’ explained Ahmed.
Due to its unique sensitivity, the Norwegian-developed sensor is in very high demand on the world market. The researchers have also managed to make it so efficient that it uses very little energy, which is important when the sensor is connected to other electronics.