A three-dimensional spatial reconstruction is necessary to understand molecular collisions. A CCD camera can recover two-dimensional information, but not three-dimensional, unless the time of impact is also known. Now researchers have developed a unique imaging technique that converts this 2-D measurement scheme into 3-D multiparticle imaging. Daniel Zajfman, an associate professor of physics at the Weizmann Institute, and his colleagues have constructed a system that uses two CCD cameras to image a microchannel plate. One camera focuses on the detector's phosphor screen; the second images an intensifier that is focused on the screen. When the intensifier is on, the cameras provide two-dimensional position information for the luminous spots on the screen. If two particles strike the screen while the intensifier is on, the CCDs will collect photons from the pixels corresponding to their impact position. The phosphor continues to emit light with a well-defined functional form and a well-defined decay time after the impact of a particle. If the intensifier is turned off before the phosphor emission has completely decayed, the recorded intensities for the two particle impacts will differ on the gated camera. That is, the CCD will collect the emission related to the initial particle strike longer than that related to the second. With straightforward signal processing, the researchers have used this difference in intensity to reconstruct 3-D snapshots of many particles at a given moment. Results in the August 2000 issue of The Review of Scientific Instruments show that the 2-D positional accuracy is about 50 µm. The results also show that the temporal accuracy is 0.4 ns. Improved Imaging This is a significant improvement over previous approaches, Zajfman said. Typically, imaging systems detect only one particle at a time, and the delay between two subsequent events must be greater than 10 ns. The team is rebuilding the system, replacing the 7-bit CCDs with 12- to 14-bit ones for even better results. The principles behind the imaging system also apply to visible light. "Although we have used the system for molecular imaging experiments, the same method is applicable to photon measurements, which could be produced by a short laser pulse toward a scene to be imaged," Zajfman explained. "It is now possible to reconstruct a full 3-D image with a single laser shot.