Page 33 - ILL Annual Report 2019

100 100 -1 -1
R++ R++ IIncoh+ IIncoh+ R-- R-- Iincoh- Iincoh-
1.6 1.6 R2' R2' SVH0.5 SVH0.5
Incoherent (10-3cts/mon)
Incoherent (10-3cts/mon)
Reflected (cts/mon)
Reflected (cts/mon)
Inc. [arb. units] Inc. [arb. units]
10 10
SV SV
R2 R2 R 1R.4 1.4
Mg
inMcgohin.c(ohv.e(ro-ivlleurm-i.l)lum.)
Mg inMcgohin.c(othra. n(stmrainssmioins)sion)
R2' R2'
RR33 0.8 0.8
10-2 10-2
-3 -3 10 10
10-4 10-4 10-5 10-5
0.0050.0050.010.010.0150.0150.020.020.0250.0250.030.030.0350.0350.040.040 -1 -1
Figure 2
Intensity reflected from the sample surface (left axis) for + and – polarised neutrons plotted together, with the signal collected at a detector mounted directly behind the sample (photograph on the right-hand side) to measure the incoherent scattering. The insert to the upper right shows a comparison of a sample loaded with hydrogen and an empty one.
specific incident wave numbers, the neutron wave field (shown by the colour map) is enhanced (red colour), resulting in an increased scattering probability from nuclei inside the resonator.
Tracer diffusion is measured by neutrons via nuclear incoherent scattering. Since neutrons scatter from the nucleus, which is much smaller than the wavelength, this scattering is isotropic. To capture as much of it as possible we glued the sample onto a well-shielded neutron detector, leaving only a small window towards the sample open (see Figure 2 photo on the right-hand side).
Figure 2 depicts the reflected neutron intensity from the sample, plotted together with the signal registered by the detector directly behind the sample. The region of total external reflection is visible for Qz values below 0.018 Å-1. The dips in the signal are at the positions where the evanescent wave enters the resonator. At exactly these positions, the detector catching the incoherent scattering registers neutrons. To prove that the detected signal is indeed originating from the resonator, we magnetised
the iron layers and used spin-polarised neutrons in the experiment to change the height of the resonator walls and shift the position of the resonances for the different polarisations of the incident neutron beam. As a second proof we compared the signal of a film loaded with hydrogen with an empty vanadium layer (Figure 2, panel top right). Clearly, only for the hydrogenated film significant nuclear spin-incoherent scattering is detected. Note the nuclear spin-incoherent cross section of vanadium is much smaller than that of hydrogen and the overall incoherent background results from the incoherent cross section of magnesium, which is of much larger volume.
Our measurements demonstrate that the incoherent signal from a thin film of 100 nm can be measured [5], paving the way towards experiments probing tracer diffusion and dynamics in thin films and at interfaces.
2
R2 ' R ' 3 3
R'RR' R 4444
-1 -1
Qz[ÅQ]z[Å] 0.60.6
R R
1.0 1.0 0.0100.0100.0150.015
Q(ÅQ)(Å ) zz
33
R3' R3' 1.2 1.2
0.4 0.4 0.2 0.2
SCIENTIFIC HIGHLIGHTS
30-31
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