Neutron Interferometry and Neutron Schrodinger Wave Optics

Tulane University, New Orleans LA

Investigators

Abstract

The neutron interferometer is a device that uses the quantum mechanical (QM) wave-like properties of free neutrons to measure certain properties of nature very precisely. It functions much like an optical interferometer, except that it detects the interference of neutron "waves" rather than light waves. A slow neutron (speed less than about 2000 m/s) has an associated QM wavelength comparable in size to the spacing of atoms in a solid, so it interacts with matter coherently, i.e. like a wave rather than a particle. When a slow neutron enters the interferometer, its QM wave is diffracted in a perfect silicon crystal and splits into two sub-waves, each taking a different path. These two sub-waves then strike a second silicon crystal and are diffracted again, into four sub-waves, all still associated with the original single neutron. Two of these four sub-waves meet again in a third silicon crystal and can interfere with each other. The degree of interference determines the probability of detecting the neutron in a neutron detector placed behind the interferometer. We can place a piece of matter, a magnetic or electric field, or even a gravitational field inside the interferometer to shift the phase of one sub-wave relative to another and change this interference. By noting the resulting change in the neutron detection rate we precisely measure the amount of phase shift. This gives important and often unique information about the strength and nature of the neutron's interaction in the material or field. In one experiment, we single out the neutron phase shift caused by its interactions with electrons in a silicon crystal. Overall the neutron is electrically neutral, but it does feel a force from charged particles such as electrons because of its magnetic moment, and because it contains charged quarks. Our measurement will lead to a much better understanding of the internal charge distribution in the neutron. In another experiment, we precisely measure the phase shift due to the neutron's interaction with He-3, a rare light isotope of helium, and how it changes when the neutron's spin direction is made the same or opposite to the spin direction of the He-3 nuclei. This will provide important details about the nature of the force between neutrons and protons inside atomic nuclei. A third experiment measures the neutron's phase shift in the Earth's gravitational field using a special neutron interferometer that floats in a zinc bromide solution. We will try to resolve a long-standing discrepancy between past neutron experiments of this type and Einstein's Weak Equivalence Principle, which states that a particle's gravitational and inertial masses are equal.