J-NSE “PHOENIX”

This instrument is focussed on cold neutrons. All parameters given here are valid during the current operation of FRM II. Please get in touch with the instrument team well in advance for all further details (length of experiment etc.).

Insrumentscheme J-NSE

In the last cycle of 2017, the refurbished J-NSE (“PHOENIX”) started first operation with new field shape optimised superconducting main solenoids, new powersupplies and associated reconfiguration. Thereby the resolution is increased by a factor of 2.5.

Description:
The neutron spin echo technique uses the neutron spin as an indicator of the individual velocity change the neutron undergoes when scattered by the sample. Due to usage of the spins as individual timekeepers for each neutron the instrument accepts a broad wavelength band (incoming velocity spread) while at the same time maintaining sensitivity to the individual scattering induced velocity changes down to 10^-5. However the information is carried by the spins as intensity modulation proportional to the cosine of a precession angle difference. Thus the measured signal is the intermediate scattering function, i.e. the cosine transform S(Q,τ) of the scattering function S(Q, ω) for small velocity (i.e. energy) changes and a symmetric scattering function. All spin manipulations only serve to establish this special type of velocity analysis. For details see “Neutron Spin Echo”, ed. F. Mezei, Lecture Notes in Physics, Vol. 128, Springer Verlag, Heidelberg, 1980.

Due to the intrinsic Fourier transform property of the NSE instrument it is especially suited for the investigation of relaxation-type motions that contribute at least several percent to the entire scattering intensity at the momentum transfer of interest. In those cases the Fourier transform property yields the desired relaxation function directly without numerical transformation and tedious resolution deconvolution. The resolution of the NSE may be corrected by a simple division.

For a given wavelength the Fourier time range is limited to short times (about 2 ps for the MLZ-setup) by spin depolarisation due to vanishing guide field and to long times by the maximum achievable field integral J. The time is proportional to J x λ³. The new superconducting main solenoids are able to create field integrals up to 1.5 Tm, currently the available correction scheme applied on the new optimised field shape coils allows “real-world” experiments with τ = 100 ns at λ = 8 Å corrsponding to J = 1 Tm.

In a major upgrade in 2017, the instrument has been equipped with superconducting main precession coils which increased the achievable field integral by a factor of 3 compared to the previously used normal conducting copper coils. The field shape of the main solenoids has been optimised in order to guarantee the required field homogeneity of the different neutron paths passing through the instrument thereby extending the regime with acceptable field integral homogeneity by a factor of 2.5 compared to the previous coils.