Advanced tailoring of neutron intensity and polarisation

Controlling the incident neutron spectrum is essential for almost every neutron experiment. We propose the development of a neutron optical component which allows not only to control the neutron spectrum but to produce polarised neutron pulses of nearly arbitrary time structure. The concept is based on the Drabkin neutron resonator combined with travelling magnetic fields. This device should allow an almost instantaneous variation of key instrumental parameters by electronic means only. This development is motivated by the upcoming PERC project which requires time definition and spectral control of the incident polarised neutron beam. The experimental test of polarisation preserving neutron guides, which define the neutron decay volume in the PERC instrument, is essential for ist ultimate performance. The neutron resonator will enable to define the time structure of the pulsed PERC beam with regard to optimum relation of data taking and background periods which is particularly essential in high precision experiments. Assessment and minimisation of background events is an important aspect of this work. We plan the development of advanced shielding concepts by Monte Carlo simulations and experimental tests with general impact in neutron instrumentation. Another key component of the PERC instrument is the magnetic field which guides the charged neutron particles towards the detector, and the instrument performance will rely on its optimised design. Experiments will utilise the Triga Reactor in Vienna in continuous and pulsed mode.

With this project we have demonstrated that multi-stage resonance systems with individual resonator elements may conveniently influence neutron beams by defining their spectral and temporal properties and thus provide, as planned, custom-tailored neutron beams. Although the characteristics of the complete resonator determine the result it is not required that the resonator is active in total during the passage of the neutrons. Basis of the resonator is spatial spin resonance of the neutrons in a configuration of magnetic fields that consists of a vertical selector field which defines the resonant wavelength of the neutrons, and a horizontal resonator field which determines the shape and width of the spin-manipulated wavelength distribution. In realizing the resonator field as travelling magnetic wave that accompanies the resonant neutrons through the resonator, we achieve high time resolution and extremely variable temporal pulse formation. As a direct consequence of these properties, the time and energy resolution of the neutron beam may be decoupled by the resonator. We have developed several prototypes of such a resonator system and tested them experimentally. Thereby we could verify the characteristics detailed above regarding spectral distribution and temporal pulse structure for the first time ever with pulsed travelling magnetic waves. The resonant neutron spectrum corresponds to a spectrum obtained with a resonator that has a constant applied resonator field, albeit the magnetic travelling wave is active in a limited number of resonator elements, only. The prototypes were tested successfully with thermal neutrons that exhibit a room temperature velocity distribution and may be formed into micro- second pulses as well as with very cold neutrons whose velocity distribution corresponds to a few Kelvin temperature. For these neutrons no equivalent methods for manipulation exist today. The resonator concept has been developed in the framework of the PERC collaboration. The PERC project aims at a most modern instrument for the investigation of the free neutrons beta decay. The measurement of its key quantities should allow us a high precision characterization of the standard model of particle physics. The resonator MONOPOL may enable us to define and vary the spectral and temporal parameters of the neutron beam employed and by that to help reduce the related systematic uncertainties. Moreover, the resonator allows us to flexibly and specifically tailor polarized neutron beams at continuous neutron sources as well at the European Spallation Source ESS which is currently under development. There it permits to extract arbitrary temporal substructures with characteristic spectral distribution from the several milliseconds long initial neutron pulse. This was also already demonstrated with the neutron resonator MONOPOL at a very cold neutron beam experimentally.