HIGH-SPEED IMAGING: Detector pinpoints platoons of particles

The first prototype of a patented particle detector, designed and developed by investigators in the Particle Physics department at the Weizmann Institute of Science (Rehovot, Israel), has been used to simultaneously measure the position with time for large numbers of particles and photons.

With the new method, depth-position information is obtained by transforming the time of arrival into a position using a calculation based on the speed of light. For particle streams, a fast-pulsed flashlamp light source illuminates the stream passing through its field of regard. Reflected light is imaged on the rapid phosphor screen, with a decay time on the order of 80 ns, of an image intensifier radiation converter. This screen image is beam-split for detection by two CCD cameras—one functions as a two-dimensional imager for intensity normalization and the other is coupled to a fast optical-gating device that includes an electronic shutter. The gate is normally open, but it can be closed in about 1 ns once the desired particle field is displayed within the 80-ns phosphor decay time after the first photons in the group have been detected on the screen.

The image from the first camera contains the integrated intensities of all light emitted from the screen, while that from the second camera has the light produced only up to the closing of the fast gate. Because the phosphor decay time, the decay-calibration curve, and the gating time are known, dividing the intensities of the two images, pixel by pixel, provides the absolute time of arrival of the light on the screen for each pixel. Multiplying this time by the speed of light gives the distance from the image intensifier to each of the particles in the light-source field of regard.

With a 1-ns optical gate, the depth resolution is about 30 cm for each pixel. Researcher David Zajfman says that with a higher-cost 100-ps gate, resolution can be refined to 3 cm. An increase in the number of pixels will give increased localization of the particles as well. He also notes that the depth resolution is independent of the distance between the objects and the camera system. The system may also have application in three-dimensional imaging for robotics.