Ultracold Neutron Time Focusing Experiment and Performance Evaluation of an Improved UCN Rebuncher at J-PARC/MLF

Abstract

We have developed and improved an ultracold neutron (UCN) decelerator named "UCN rebuncher." The rebuncher can give continuously changing deceleration to pulsed polarized UCNs by the combination of an adiabatic fast passage spin flipper, a frequency sweeping resonator, and a gradient magnetic field. By using this apparatus, we can control the longitudinal velocity of UCNs and time-focus UCNs on an arbitrary point. In a neutron electric dipole moment measurement at a spallation pulsed superthermal UCN source with a solid deuterium converter, the apparatus makes it possible to recover the spatial density of diffusing pulsed UCNs and store them in a cell efficiently. We carried out the UCN focus experiment with the improved rebuncher at BL05 (NOP) in J-PARC/MLF. We succeeded in decelerating 5.5-m/s UCNs, focusing them on the detection timing of 4.8-m/s UCNs, and confirming an obvious peak in the time-of-flight spectrum. The count rate at the peak was 2.2 times higher than without the rebuncher.

On the possibility of creating a UCN source on a periodic pulsed reactor

“…Later on, the non-stationary diffraction of UCNs by a moving grating was observed in the experiment described in [22], and after some time, using aperiodic moving grating, the effect of focusing in time was demonstrated in experiments [23,24]. The possibility of time focusing based on the resonant spin-flip of the neutron has also been experimentally confirmed [25,26].…”

Production of ultracold neutrons in a decelerating trap

“…An alternative focusing upon the interaction with a moving material wall (grating) is studied, for example, in a series of works [15,16], or with a neutron spin-flip in the intense magnetic field [6,20]. Other methods of focusing can also be proposed, for example, using the reflection of VCNs from a moving wall.…”

Neutron focusing in time and magnification of the time lens

“…without commenting on this result. The question of the time magnification was also left unmentioned in the publications devoted to the results of the conducted experiments on the time focusing of UCNs [13][14][15][16]. At the same time, the issue becomes especially relevant in connection with the discussion of the possibility of creating a UCN source based on the principle of time focusing on the periodic action pulsed reactors in Dubna [7,8].…”