HIGH-RESOLUTION NEUTRON SPECTROMETER LAM-80ET AND ROTATIONAL TUNNELING IN 4-METHYLPYRIDINE N-OXIDE

A high energy-resolution neutron spectrometer with large mica mirrors, LAM-80ET, has been constructed at National Laboratory for High Energy Physics in Japan (KEK). The highest resolution achieved with this spectrometer is 1.2-mu-eV. In order to examine the capability of the spectrometer, the rotational tunnelling of the methyl group in 4-methyl pyridine N-oxide (N-oxy gamma-picoline) has been measured. Using the (004) reflection of the mica mirrors, we have clearly observed four peaks at epsilon = 60, 115, 190 and 274-mu-eV. Measurements of the Q-dependence of these tunnelling peaks show that the tunnelling potential may be three-fold. It is also shown that the line width of tunnelling modes and the spin conversion at T > 5 K are derived by the libron-phonon process with E(L) = 30-40 K.