Levitating samples in neutron beams
P. Martin, F. Marchal, N. Belkhier, A. Filhol, H. Fischer, M. Koza, J. Kozaily, E. Lelièvre-Berna
Institut Laue Langevin, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France
CNRS-CRMHT, 1d av. de la Recherche Scientifique, 45071 Orleans Cedex 2, France
Motivated by the results of recent experiments performed in a furnace exploiting the aerodynamic levitation technique, we have investigated the possibility to levitate samples well above nozzles. Indeed, with the present technique, only two thirds of the sample escape the nozzle and scatter neutrons. Even with a good collimation, one cannot prevent some neutrons to be scattered by the nozzle, leading to spurious signal in the detector.
When increasing the flow of gas exiting the nozzle, the force applied on the sample increases and pushes it out. The forces applied on the sample are maximum on the vertical axis of the nozzle and lower around the axis, leading to the immediate ejection of the sample. To circumvent that problem, we have decided to add a secondary jet parallel to the main one but directed in the opposite direction and off-centred by the size of the sample. This technique creates a twist at the sample position which may be set as high as several sample diame- ters above the bottom nozzle. This leads to an excellent stabilisation of the sample in the horizontal (scattering) plane. However, because of the non-laminar character of the jets, there remains some vertical oscillations which can be reduced with a third jet.
We present this technique and the results obtained with various gases, flow rates, sample sizes and densi- ties. We also summarise the attempts to levitate melted samples.