Neutron reflectometry

The neutron is an analytical method for the study of interfaces and thin films. Neutrons to be irradiated on an interface and dispersed therein. The scattered neutrons are detected and evaluated. The basic principle is comparable to other reflectometric techniques, such as the X-ray reflectometry and ellipsometry, which, however, are based on the reflection of electromagnetic radiation.

The method is suitable for the extraction of surface and interface information of a solid layer or layer system to a depth of 150 nm Especially materials with magnetic properties can be studied well. By depth dependence of the average neutron scattering length density results in a high resolution of approximately one nanometer, so that it is possible, among other things interdiffusion between adjacent layers of different isotopic evidence. This " isotope sensitivity " is based on the fact that neutrons preferentially interact with atomic nuclei and not as electromagnetic radiation with the atomic shell.

Operation

For the measurement, a collimated beam of neutrons having a kinetic energy of a few hundredths of a volt electrons to a ( very smooth ) interface is irradiated and the intensity of reflektrierten neutrons measured by the angle of reflection ( specular reflection, reflection angle corresponds to the angle of incidence). A corresponding neutron source, such as spallation and neutron guide is necessary. The shape of the intensity profile of various information on the measured surface, such as the thickness, density or interface roughness.

According to the theory of Louis de Broglie microscopic particles such as neutrons can be described as matter wave and assigned them a characteristic wavelength. This wavelength depends on the momentum of the neutron:

Where the Planck constant is. For neutrons with a kinetic energy of a few hundredths of electron volt is the de Broglie wavelength, and thus the theoretical resolving power of a few tens of nanometers.

Mathematically, the reflection of neutrons to the reflection of electromagnetic radiation can be described inter alia by these connections only. That is, one defined for the material has a complex refractive index and uses the known laws of optics ( cf. law of refraction, Fresnel's equations, Abeles matrix formalism and Parratt - Rekusionsformel ). The form of presentation makes sense because, as in the X-ray range, the real part of the refractive index is very close to 1. In the literature, therefore, is often called the dispersion indicated. It is usually of the order of 10-6. The absorption coefficient can be neglected in many cases, since it is of the order of 10-12 except for strongly absorbing, such as isotopes of boron or lithium.

Than for air / vacuum even when neutrons similarly to X-radiation occurs because of the minimum lower real part of refractive index of total external reflection on if the neutron very shallow, that is, at angles of incidence close to 90 ° ( from the vertical ), incident on the smooth sample, known as grazing incidence. This measurement setup makes sense, because otherwise the intensity of the reflected neutrons is too low or the losses would be too high for an evaluation.

Variants

In addition to the specular reflection, there are two other assay techniques under grazing incidence:

The methods differ not only in the way in which the neutron spectrum is recorded and what scattering mechanisms act, but also in the depth of information. as mentioned above, the depth information is the specular reflectance in the range of 3 nm to 100 nm (sometimes referred to 150 nm). Wherein the dispersion perpendicular to the plane of incidence Neutronenreflektrometrie provides similar depth information ( 3 nm to 100 nm). In contrast, however, the off- specular scattering - technique that provides information from the range 600 nm to 60 microns depth.

Representation

In contrast to " optical " reflectometry the representation of the measurement results is not normally in the form of reflection, absorption or transmittance as a function of the angle or the wavelength, but it is the degree of reflection ( reflectance) as a function of the momentum transfer ( in the z direction, perpendicular to the interface ) is shown. Momentum transfer vector describing the change of the neutron pulse at the reflection on the material and can be detected as follows mathematically.

Here is the de Broglie wavelength and the angle of incidence of the neutrons.