TETRIS & MACHINE LEARNING | RADIATION MONITORING


Above: Geiger counters simply provide a click when receiving radiation.


Finding a source thus requires moving the detector to locate the maximum intensity Photo credit: IN Dancing Light/Shutterstock.com


beam to only come from a limited range of angles to pass through the aperture of the first layer and to interact with the second layer. An algorithm interprets the different timings and intensities of the signals received by each individual detector or pixel to provide a direction but this approach often leads to complex detector design. Typically, such detector arrays capable of sensing the direction of a radiation source are large and expensive with a 10 by 10 array and at least 100 pixels. Not only are the individual detector elements expensive, but all of the interconnections carrying information from those pixels also become much more complex. The traditional reconstruction algorithm also requires


all the incident beams to come within the field of view. Accuracy will be affected if the radiation is incident from another direction (especially for near-field radiation). In addition, this type of system can only conditionally detect multiple sources, usually when the sources come from different isotopes and can be distinguished by energy. In this scenario, the detection with multiple sources can be reduced to single-source detection by only considering the count of events within an energy range. However, in real- world applications, different sources are not necessarily distinguishable in the energy spectrum. Another approach uses detectors separated by padding shielding material. Radiation sources from different directions and distances can result in different intensity distribution patterns over detector arrays. However, because of the inaccuracy of the model caused by misalignment and manufacturing errors of detector and shielding material, it is challenging to extract information from detector data via these traditional methods. Thus while there have been versions of simplified arrays


for radiation detection, many are only effective if the radiation is coming from a single localised source and they can be confused by multiple sources or sources that are spread out.


An alternative approach to detection As an alternative, the combined MIT/LBNL team developed a radiation mapping framework using detector pixels that have been inspired by the video game Tetris. According to a recent paper from the team and published by Nature Communications, Okabe and MIT professors Mingda Li, and Benoit Forget, senior research scientist Lin-Wen Hu, and principal research scientist Gordon Kohse; graduate student Shangjie Xue; research scientist Jayson Vavrek at LBNL, and a number of others at both institutions, found that using as few as four pixels arranged in the tetromino shapes of the figures in the “Tetris” game can come close to matching the accuracy of the large, expensive systems. Shapes from the Tetrominoes family are composed of four squares. The author’s radiation detection framework uses a minimal number of detectors, combining Tetris-shaped detectors with inter-pixel padding, along with a deep-neural-network- based detector reading analysis. The inter-pixel padding material is included to intentionally increase contrast. One of the major breakthroughs to make the system work is placing an insulating material between the pixels to increase the contrast between radiation readings coming into the detector from different directions. The contrast between pixels is thus not only created by incident angles but enhanced by padding layers that are good absorption layers of radiation. The inter-pixel padding material is chosen to be 1 mm-thick lead, which is thick enough to create contrast and with quite a low photon absorption in the γ-ray range. The lead serves the same function as the more elaborate shadow masks used in the larger conventional systems. The detector pixels are composed of CZT with a size


of 1 cm² each, slightly larger than current crystals used in detectors but still much smaller than the 5 metre source to detector distance. The detector grid is placed horizontally within the plane instead of vertically facing the source.


www.neimagazine.com | July 2024 | 27


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