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Neutrons for Science
of the beam only affects the resolution in energy to second order. A fairly large beam divergence can be used while having good energy resolution, which can attain 0.62 μeV90. The scanning is performed by the oscillating motion of the monochromator along the beam direction, shifting the energy by the Doppler effect.
The construction of this instrument launched by Maier-Lebnitz was almost stopped because, in 1972, a Hungarian physicist, Ferenc Mezei joined the ILL bringing with him a technique he had invented, neutron spin echo, described earlier, which offered even better energy resolution. Finally it was decided to build both. The two instruments have different applications. The spin echo spectrometer is well adapted to study systems where there are several relaxation times, but it could not easily distinguish sets of spectral lines. Over the following years the success of these two techniques has led to the construction of several versions of these two instruments.
The table includes reference to the first problems encountered with the “Igel” (Hedgehog) diffractometer D6. These led
to the project finally being abandoned. The high resolution diffractometer D1A could attain the desired resolution, but the measured intensities were so low as to be unusable. After the arrival of the British, Alan Hewat modified the collimators using the expertise of the SRC-Rutherford Laboratory, and together with additional detectors it became fully operational and was heavily used.
90 2018 addition:This is about 100 to 1000 times less than the energy of molecular vibrations. With such a small energy resolution it became possible to observe atomic quantum tunneling effects in matter, as well as the cou- pling between electronic and nuclear spins. These may seem exotic topics but we should not forget that, for example, atomic clocks which are so important for satellites and, in particular, the GPS technology, are based on the latter phenomenon.
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