As part of the MONOPOL project in the neutron and quantum physics group at the Atomic Institute, a traveling wave neutron spin resonator is being developed for the first time to generate almost any shape of wavelength-selected, polarized neutron pulses.

The basic principle for this, spatial magnetic spin resonance, was already proposed in 1968 by Drabkin et al. presented. The polarized neutrons are exposed to a spatially alternating magnetic field that is applied normal to the direction of polarization, i.e. normal to their guiding field. In the reference frame of an individual neutron, the frequency of this alternating magnetic field depends on its velocity and the spatial period of the resonator. If this frequency agrees with the Larmor frequency of the neutrons, which is determined by the orthogonally applied static guide field (selector field), a π-spin flip takes place. A specific wavelength can thus be selected by suitably adjusting this selector field. The simplest, classic structure of this type consists of a current-carrying, meander-shaped folded aluminum strip within an orthogonal guide field. Theoretically, neutron pulses can also be generated by switching such an arrangement on and off, although the shortest possible pulse duration is defined by the length of the resonator, which prevents the generation of shorter pulses. A so-called traveling wave mode is implemented in the new development.The structure consists of a large number of individually controllable and adjustable aluminum coils, in which a "wandering" magnetic field accompanies the neutron pulses to be generated through the resonator. In such an arrangement, the shortest possible time structure of a single neutron pulse is defined by the width of an aluminum coil. In the planned implementation, pulse times that are 2 orders of magnitude shorter than can be realized with a classic resonator appear. The construction of individual elements compared to the aluminum meander also has the significant advantage that the magnetic fields generated can be shaped as desired and thus secondary maxima in the selected wavelength spectra, which are very disruptive to operation, can be eliminated. In addition, an undesired selection of neutrons whose wavelengths correspond to higher orders of those to be selected can also be avoided by appropriate shaping of the individual fields. The first step towards implementation is a design study in which the optimal geometry of the aluminum coils is defined with the help of magnetic field simulations. Based on the computer simulations, a first prototype was built and tested at the TRIGA reactor in Vienna. Based on the knowledge gained in this way, a further developed resonator, optimized for a specific application, was finally constructed, which will soon be tested on a white, polarized neutron beam. Possible applications of this new neutron resonator are, on the one hand, beam preparation in the PERC beta decay project, the construction of a novel 3-axis spectrometer, or the generation of wavelength-selected sub-pulses in the planned long-pulse spallation neutron source ESS.

Publications

Ch. Gösselsberger, H. Abele, G. Badurek, E. Jericha, W. Mach, S. Nowak, T. Rechberger, "Neutron beam tailoring by means of a novel pulsed spatial magnetic spin resonator", Journal of Physics: Conference Series 340 (2012) 012028.

Ch. Gösselsberger, H. Abele, G. Badurek, E. Jericha, S. Nowak, G. Wautischer, A. Welzl, "Design of a novel pulsed spin resonator for the beta-decay experiment PERC", Physics Procedia 17 (2011) 62.

G. Badurek, Ch. Gösselsberger, E. Jericha, "Design of a pulsed spatial neutron magnetic spin resonator", Physica B 406 (2011) 2458. 

G. Badurek, E. Jericha, "Upon the versatility of spatial neutron magnetic spin resonance ", Physica B 335 (2003) 215.