Sample Environments for Neutron Experiments

In some experiments it is important to control the environment of a sample. For instance if a sample reacts chemically with air it would need to be kept in a vacuum. In many cases in materials research we want to examine how a sample behaves under specific conditions such as applied pressure, magnetic field, or temperature. In order to make a proper scientific study and observe any changes in a sample, we need to control exactly what the conditions are. For investigations where low temperatures are needed a cryostat is an excellent tool. The name cryostat says what is does, cryo means cold and stat is for static or stable, so it is a machine that keeps a sample stably cold at a specific temperature which we define.

Other sample environments are often used in materials science, for example cryomagnets which can control the temperature and apply a magnetic field to the sample at the same time by combining a cryostat with a powerful superconducting magnet. Sometimes the sample is inserted in a pressure cell which is then loaded into the cryostat in order to control the temperature and pressure of the sample at the same time. Sometimes you need a furnace to heat the sample rather than cooling it and sometimes you need a system to feed a chemical to the sample during the experiment. Which sample environments are used in the neutron experiment depends on which characteristics of the sample you want to measure.

Using cryostats in neutron experiments

There are many types of cryostat system, each specialized for the temperature range that is needed and the precision that is required in the given experiment, but we will not go into the technical details on how they work. The overall concept is like a refrigeration unit; it has a system that extracts heat by evaporating a liquid which is in thermal contact with the sample chamber that needs to be cooled. Cryostats usually use liquid helium as a coolant in order to cool close to absolute zero temperature. They are constructed as several concentrical metal cylinders for separating the coolant from the sample chamber and the pre-coolant chamber which is usually filled with liquid nitrogen.

Cryostats for neutron experiments have some specific features. They are usually a vertical tank as shown in the picture. The sample is mounted on a sample stick that is then lowered into the cryostat sample chamber from above. The sample stick needs to have the right length so the sample will reach the sample zone where the neutron beam will hit. The material of the cryostat at the sample zone is aluminum which almost entirely transparent to neutrons due to a low total cross-section. Using aluminum means we don’t lose the beam intensity and have a minimum of scattering events from the cryostat. We cannot entirely disregard the cryostat interaction with neutrons, since it will leave a fingerprint in the data recorded on the neutron detector, but there are ways to subtract this so-called background.