IN22

CRG - Thermal neutron three-axis spectrometer with polarization analysis

IN22 at work

The aim of the experiment presented here was to measure the neutron scattering from liquid indium. This was never measured before since the expected scattering signal is nearly zero for such a strong neutron absorber. Thus an advanced sample environment setup was required with both both aerodynamic levitation and laser heating of the sample.

Why aerodynamic (gas flow) levitation of the sample ?

• No container windows in the neutron beam and hence no additional background.

Why laser heating ?

• No sample contamination by hot vapor from electrodes.

Why such a simple sample, why this specific metal?

• To answer this question we must first emphasize the importance of computer simulations in science. This is true for chemistry, astrophysics, etc. and also for neutrons. A new simulation tool is currently being developed making it possible to fully simulate a neutron experiment, i.e. to simulate the neutron source, the instrument optics, the sample and the resulting spectra. However this tool still has to be validated through comparisons with real experiments.

An experiment on molten indium was selected for several reasons:

• a simple material is required to have a reasonable chance that discrepancies between real and simulated data can be fully understood.
• a molten metal can be easily simulated using ab-initio techniques while a solid material is more difficult.
• indium has never been successfully measured before since the expected scattering signal is nearly zero for such a strong neutron absorber.

Defining the optimal parameters for such a difficult experiment is in itself a first proof of the value of simulated experiments. A second came during the real experiment itself. Some features of the recorded spectra seemed strange to the measuring team. Simulations helped understand which part of the signal was the residual background from the support and which part was the very weak neutron scattering from indium.