Development of Thermal Neutron Spin-Echo Spectroscopy

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The aim of this development is to improve the energy resolution of a conventional thermal neutron triple axis spectrometer to relative values of 10^<-4> of the incident neutron energy E_i or better by means of the Neutron Spin Echo method.For this purpose special Larmor precession magnets with "Optimal Field Shape" (OFS) have been developed and constructed.Following the analytical solution of the variational field-inteqral homogeneity problem, such magnets are the best compromise for homogeneous line integrals and low fringe-fields, The residual line integral inhomogeneities of the magnets and the path length effects due to the relatively large beam divergence are corrected by in-beam spiral coils, spark-erosion cut from Aluminum plates.The OFS coils are designed to reach a maximum field integral of 0.06Tm corresponding to a Fourier time of 0.16 nanosec at an incident neutron energy of 14.7meV.Apectral resolution of -0.2 mueV is obtained, to be compared to 2.2 meV for the same TAS conf … More iguration without Spin Echo.This Triple Axis Spectrometer with Spin Echo (TASSE) option can be installed on the conventional triple axis instrument in one and half day time.The spin echo polarisation is fully maintained when the momentum transfer is changed up to the maximum spectrometer values of Q-3.6 *^<-1>.The intensity loss factor of 50 due to the introduction of polarisers and increased distances is to be compared with the resolution gain factor of approximately 10^4.The performance of this new, high resolution TASSE option is demonstrated using the experimental data obtained on the quasielastic diffuse scattering near the order-disorder phase transition in KDCO_3.Here the first evidence for a dynamical Huang scattering has been obtained which may be viewed as a characteristic of a strain field mediated order-disorder phase transition.Furthermore the spin density wave in Chromium have been investigated as an example of antiferromagnetic spin echo experiment.The experiment on SrTiO_3 has been performed, where the problem of the central component' is reinvestigated using the thermal neutron spin echo spectroscopy.Preliminary results on finite energy transfer spin echo at DELTAE-6meV phonon mode in SrTiO_3 have been also obtained. Less