Experimental Limit on Neutron Orbital Angular Momentum Detection Using Polarized 3He

TL;DR

This study experimentally tests the detection of neutron orbital angular momentum via polarized 3He absorption, finding no observable effect and highlighting the importance of spatial considerations in OAM detection methods.

Contribution

It provides the first experimental constraints on neutron OAM detection using polarized 3He, emphasizing the role of spatial wavefunction structure in such measurements.

Findings

No OAM-dependent absorption change observed within statistical limits.

Neutron OAM's spatial structure reduces overlap with 3He nuclei, limiting detection.

Results set constraints on nuclear absorption-based neutron OAM detection methods.

Abstract

A recent proposal suggested that neutron orbital angular momentum (OAM) states could be detected via spin-polarized absorption in polarized 3He, with predicted cross-section variations linked to the neutron's OAM. We experimentally tested this hypothesis using spin-polarized neutron beams with OAM =-2 to 2, generated by fork-dislocation phase-gratings, and transmitted through a polarized 3He cell. Within statistical precision, no OAM-dependent change in the absorption cross section was observed. This null result places stringent constraints on polarized 3He-based OAM detection schemes. The absence of an effect in the given regime is traced to the proposal's disregard of the spatial character of neutron OAM: unlike spin, OAM arises from the transverse phase structure of the wavefunction and couples only through spatial gradients and overlap. The transverse extent of neutron OAM modes expands rapidly, producing a doughnut-shaped intensity profile with negligible overlap with on-axis 3He nuclei, while off-axis capture samples only a locally uniform phase and reduces the interaction to the known spin dependence. These results clarify the limits of absorptive nuclear methods for probing neutron OAM and emphasize the necessity of spatially resolved interactions in any viable detection scheme.