The velocity measurement in the KDTLI is accomplished by a time of flight measurement. A rotating disc with two slits chopps the continuous molecular beam into many small bunches. Since every bunch contains molecules of all velocity classes it extends on its way to the detector. Each time the chopper unblocks the molecular beam, the time measurement is started. As the distance between chopper and detector is known, the velocity can be calculated by analyzing the time of flight of each individual molecule.
The chopper disc has two opposite openings that, together, cover about 10% of the circumference. When the chopper rotates, on average, only one tenth of the particles reach the detector.
The fastest particles (red) hit the detector at first the slowest particles (blue) last.
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Experimental challenge: Velocity measurement
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