Wavelength multiplexing for very cold neutrons in a double holographic-grating spectrometer
J. Klepp 1 , M. Fally 1 , C. Pruner 2 , Y. Tomita 3 , P. Geltenbort 4
Faculty of Physics, University of Vienna, A-1090 Vienna, Austria
2
Department of Materials Science and Physics, University of Salzburg, A-5020 Salzburg, Austria
3
University of Electro-Communications, Department of Engineering Science, 1-5-1 Chofugaoka, Chofu, Tokyo 182, Japan
4
Institut Laue Langevin, Boı̂te Postale 156, F-38042 Grenoble Cedex 9, France
1
We proposed to test a wavelength-multiplexing technique using a double-grating spectrometer for very cold neutrons based on neutron-diffraction gratings recorded in nanoparticle-polymer composites by holographic means.
Light-sensitive materials combined with holographic techniques can be used to produce diffraction gratings for
neutron-optics [1]. The treated materials exhibit a periodic
neutron refractive-index pattern, arising from a light-induced
density modulation. In recent years, we have shown that
such holographic nanoparticle-polymer composite gratings
can be used as 50:50 beam-splitters, mirrors or three-port
beam splitters (30:30:30) for cold- and very-cold neutrons
(VCN) [2].
In our last experiment, we successfully tested a double grating spectrometer consisting of two successive (distance about
20 cm) holographic gratings. For the presently described
experiment, the first grating was tilted by 90◦ about the
incident-beam axis, the grating vector pointing in vertical direction. This trick lets Bragg-diffraction and gravity ‘join
forces’, as shown in Fig. 1 (1): The diffraction angles of the
broad-spectrum incident beam depend on the wavelength λ
via θB ≈ λ/(2Λ), where Λ is the grating spacing. Furthermore, the longer λ the smaller the horizontal momentum component of the neutrons, and the lower the neutrons
hit the 2D-detector situated at a certain distance from the
source due to earth’s gravity. Both effects add up constructively. The resulting rainbow-like beam is used as a
source for the second grating, which remains with its grating vector parallel to the horizontal direction. For the analysis, the detector image showing the diffraction spots [see
Fig. 1 (2)] was divided into detector-pixel lines. In each
line, a different wavelength distribution is detected to exploit
the broad wavelength-distribution of the VCN-beam at PF2
(wavelength-multiplexing). We found that the intensity loss
by absorption/incoherent scattering by both gratings together
is about 35%.
We used a collimated beam with divergence limited – both
horizontally and vertically – to 0.001 rad. The latter allows to separate the wavelengths on the 2D detector at a
sample-detector distance of about 2 m. For the first grating of
the double-grating spectrometer, the free-standing film ‘mirror’ grating (periodicity 500 nm, thickness of about 100 µm,
20 vol % nanoparticles [3]) was chosen. Although the sample
had been stored under ambient conditions for almost three
years, its quality and properties have apparently not changed.
Rocking curves of the first grating were recorded to arrive at
maximum reflectivity for a wavelength approximately in the
center of the broad VCN spectrum (a situation also shown
in Fig. 2 of [3]). For the second grating, the best of the recently investigated types [4] was chosen, which exhibited an
increase in reflectivity of 0.1, about 40% more as compared
to earlier experiments.
The concept worked out in principle. Subtle details concerning experiment and analysis will be described in dedicated
publications [5, 6].
[1] R. A. Rupp, J. Hehmann, R. Matull, and
K. Ibel.
Phys. Rev. Lett. 64, 301 (1990).
doi:10.1103/PhysRevLett.64.301.
[2] J. Klepp, C. Pruner, Y. Tomita, P. Geltenbort,
I. Drevenšek-Olenik, S. Gyergyek, J. Kohlbrecher,
and M. Fally.
Materials 5, 2788 (2012).
doi:10.3390/ma5122788.
[3] J. Klepp, C. Pruner, Y. Tomita, K. Mitsube, P. Geltenbort,
and M. Fally. Appl. Phys. Lett. 100, 214104 (2012).
doi:10.1063/1.4720511.
[4] Y. Tomita, E. Hata, K. Momose, S. Takayama, X. Liu,
K. Chikama, J. Klepp, C. Pruner, and M. Fally (2016).
Accepted for publication in J. Mod. Opt.
[5] J. Klepp, C. Pruner, Y. Tomita, P. Geltenbort,
J. Kohlbrecher, and M. Fally (2016). Submitted.
[6] J. Klepp, C. Pruner, Y. Tomita, P. Geltenbort, and
M. Fally (2016). In preperation.
Proposal-number: 3-14-354
Instrument: PF2 VCN
(1)
(2)
c
a
d
b
e
θB
F=mn g
f
θB (λ)
Figure 1: (1) Holographic-grating diffracting a ‘pink’ incident neutron-wavelength spectrum downwards. For clarification of
the concept, all angles are exaggerated (θB ≈ 0.1◦ ) and the transmitted (forward-diffracted) beam was omitted. (2) Detector
image of the diffraction spots a, b, c, d, e, f after the double holographic-grating spectrometer. a: Transmitted by both gratings.
b: Diffracted by 1st , transmitted by 2nd grating. c: Transmitted by 1st , diffracted by 2nd grating. d: Transmitted by 1st , diffracted
by 2nd grating. e: Transmitted by 1st , diffracted by 2nd grating. f: Transmitted by 1st , diffracted by 2nd grating. The data analysis
was done line by line, assuming different wavelength distributions.