Neutron interference from a split-crystal interferometer

Abstract

The first successful operation of a neutron interferometer with a separate beam-recombining crystal is reported. This result was achieved at the neutron interferometry setup S18 at the ILL in Grenoble by a collaboration between TU Wien, ILL, Grenoble, and INRIM, Torino. While previous interferometers have been machined out of a single-crystal block, in this work two crystals were successfully aligned on nanoradian and picometre scales, as required to obtain neutron interference. As a decisive proof-of-principle demonstration, this opens the door to a new generation of neutron interferometers and exciting applications.

Keywords: atomic scale positioning; neutron interferometry; silicon crystals; split-crystal interferometers.

Figure 1

(a) Symmetric and (b) skew-symmetric crystal interferometers. The neutrons are coherently split, reflected and recombined by the crystal lamellas. The two crystals of a skew-symmetric interferometer can be placed far apart.

Figure 2

A rocking curve of the analyser lamella with path 1 of the interferometer blocked, cf. Fig. 3 ▸. The error bars lie within the bullets. The theoretical prediction (solid line) is calculated by convolving the product of three Bragg reflections (the triple-bounce Bragg monochromator) and two Laue reflections (the splitter and mirror lamella) with the final Laue reflection by the analyser. The overall agreement is very good. We attribute the remaining discrepancies to imperfections of the crystal geometries on the level of a few micrometres.

Figure 3

The crystal interferometer with separate analyser lamella (A). The neutron beam (N) enters from the right side, is split into two paths (1, 2), and exits through the output ports O and H. Polished surfaces on the crystal’s sides allow pre-alignment by an optical autocollimator on the front (P) and monitoring of the analyser’s coordinates (θ, ρ and x) by an optical interferometer (M) on the rear. A piece of cadmium (Cd) can be inserted from the top to block one or the other interferometer path.

Figure 4

(a) Theoretical and (b) observed interference generated during the ρ angle alignment. For each ρ value, a neutron-camera image is horizontally integrated yielding a vertical intensity distribution, which is shown here by each pixel column. Concatenating the columns of all ρ values generates the complete image (b). The theoretical pattern (a) perfectly reproduces the observed pattern, although the vertical fringes fluctuate in position and spacing due to phase drifts during the measurement. (c) If the full-beam height is used, the interference is visible only in the vicinity of perfect ρ alignment. The bottom figures in (b) and (c) show the total camera counts. (d) An interferogram created with an auxiliary phase shifter. The red curves are the best sinusoids fitting the data. The fringe visibility is ∼40%.

Figure 5

Amplitude spectral density of the analyser position noise. The abscissa is gauged in one-third of octave frequency bands.