REF - Neutron Reflectometer
Instrument responsible: Tamás Veres
The neutron reflectometer (REF) located in the cold neutron hall, at guide 1 is under reconstruction. At present time reflectivity can be measured down to 5*10-5 in the case of large samples. The instrument operates at constant wavelength l = 4.28 Å.The schematic view of the reflectometer is shown in Fig. 1. below.
Figure 1. The schematic view of the reflectometer (REF) at the guide No.1.
Due to the geometrical restrictions the neutron path had to be aligned parallel with the guide. A double PG crystal monochromater is used for this purpose. The elements of the focusing monochromators have mosaicity of 0.4 o. The inner monochromator has 5x5 elemts, the outer one consists of 5. The sketch of the focusing arrangement is displayed on Fig. 2.
Figure 2: The focusing arrangement of the new monochromators.
The relative intensity is about 3.7 times compared to the previous one. The height of the beam is 8 cm at the sample position, the middle part is the brightest.
The higher harmonics of the l = 4.28 Å are filtered out using a Be filter positioned between the two monochromators. The Be filter can be cooled by liquid N2.
The usual slit width of the collimator is 1 mm, but it can be changed between 0.1 and 3 mm, the slit distance is 2 m.
The smallest possible angular step of the sample holder is 0.0055 grad.
The reflected neutrons are detected using a position sensitive 3He detector, with active area 20x20 cm2. Our new measurement control software allows to save the 2D picture for every measurement point and to count only neutrons arrived to specular position.
The sample-detector distance is 160 cm, it gives the angular limit (according to the detector size) of the usual measurement, but by manual moving of the detector the angular limit can be increased.
Flux of the neutrons at sample position: 780 n/cm2 sec
In order to illustrate the ability of the instrument several typical measurements are displayed below.
Figure 3. The reflectivity of two Ni-Ti bilayer (dNi=84 Å, dNi=70 Å) on glass substrate.
Figure 4. The reflectivity of a glass sample
Figure 5: Off-specular neutron reflectograms of reverse layer sequence m=2.5 SM at different incidence angles. The first and second order of Resonant Diffuse Scattering are observable beside the specular peak.
Necessary and recommended improvements
- It is essential to suppress the background. For that both the shielding of the guide No.1 and that of the reflectometer has to be essentially improved. It means that relatively large amount of shielding material (e.g. boron-carbide) has to be procured.
- It is important for the solid operation of the instrument to build new control electronics.
Utilization of the reflectometer
- quality control of supermirror multilayers;
- thin layer studies;
- offspecular scattering experiments;
- study of standing waves;
T. Veres, L. Cser, Sz. Bálint, Sz. Sajti (2016) Roughness replication in neutron supermirrors and periodic multilayers, submitted to Journal of Applied Crystallography
T. Veres, L. Cser, V. Bodnarchuk, V. Ignatovich, Z. E. Horváth, B. Nagy (2013) Investigation of periodic Ni-Ti multilayers, Thin Solid Films 540, 69-72
T. Veres and L. Cser (2010) Study of the reflectivity of neutron supermirrors influenced by surface oil layers, Review of Scientific Instruments 81, 063303
V. Bodnarchuck, L. Cser, V. Ignatovich, T. Veres, S. Yaradaykin (2009) Investigation of periodic multilayers. JINR Е14-2009-127