Utilizing an optical holographic setup thick refractive index gratings are written in poly (methyl methacrylate) (PMMA). We show experimentally that these gratings diffract not only visible light but also cold neutrons.PACS numbers: 61.12. Gz, Holographic methods are a well established tool for the investigation of photorefractive materials, i.e., materials which change their refractive index upon illumination. 1 Certain photorefractive processes like thermal fixing 2 or photopolymerization 3 involve mass transport. As a consequence, not only the optical but also the neutron refractive index may be changed. We report here, for the first time, on neutron diffraction at thick gratings produced by optical holography.Our sample was a 1.93-mm-thick plate (approximately 25x25 mm 2 ), consisting of a poly (methyl methacrylate) (PMMA) matrix, residual monomer, and the photoinitiator bis(cyclopentadienyl)-titaniumdichloride. When illuminating the sample with the light interference pattern of two coherent plane waves of optical wavelength X =514.5 nm, a photochemical reaction is started. In the course of the chemical reaction diffusion processes take place which build up a mass-density modulation in conformity with the intensity modulation of the light interference pattern. This mass-density modulation is the origin of the optical and neutron refractive-index grating. By an optical diffraction experiment with X =632.8 nm the grating period A has been determined to be 361.7 dz 0.1 nm. The grating-containing area of the sample has an elliptical form with extensions 10x18 mm 2 .The neutron diffraction experiment was performed at the Dll small-angle scattering instrument of the Institute Lau-Langevin (ILL) at Grenoble, France. For our test experiment two hours of beam time were provided.Wavelength selection at the instrument Dl 1 is done by setting the rotary speed of a mechanical monochromator drum. 4 A speed of 1832 rpm, as used in our experiment, yields a wavelength distribution with a full width at half maximum of 0.09 nm. According to Ref. 4 the neutron wavelength at the maximum is theoretically X" = 1.025 and experimentally X n = 1.01 ±0.01 nm; according to Ref. 5, X"=1.00 and 0.98 nm, respectively. The uncertainty is partly due to a slight misalignment of the drum's rotary axis.The sample was placed in the collimated neutron beam (beam divergence -0.05°) behind a cadmium dia-phragm with a clear aperture of 8x11 mm 2 . The diaphragm ensured that neutrons could pass through only the grating-containing area of the sample. The distance between the detector (64x64 10 BF 3 gas-containing cells of 100-mm 2 cross section) and the sample was 35 m.In order to eliminate the effect of gravity on the diffraction pattern the sample was mounted such that the Bragg peaks were to be expected in the horizontal plane. Therefore, the grating vector K of the refractive-index grating also has to be in the horizontal plane (x-y plane in Fig. 1). By turning the sample around a vertical axis (z axis in Fig. 1), the inclination angle a between t...

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Light-induced changes of the refractive index are referred to as photorefractive effect. Thereby it does not matter whether the light causing and the light sensing these changes have different wavelengths—from the infrared to the 7-ray spectral region— or not. In this paper we intend to generalize the notion of a photorefractive effect to cover also light-induced changes of the refractive index for neutron waves (photorefractive effect of a second kind). The scope of possible applications of photorefractive materials is extended towards holographic optical elements for neutron optics.

We succeeded in characterizing by long-wavelength neutron diffraction light-induced volume phase holograms which have been recorded in photopolymers. The samples were prepared from protonated and deuterated poly-(methylmethacrylate). They initially contained about 10 -15 % monomers and a photo-initiator.Discs of about 2 mm thickness and 2 to 3 cm diameter were exposed, in a conventional two-beam interference set-up, to a periodic light intensity pattern, with fringe spacings between 120 and 390 nm. The illumination modulated the neutron optical refractive index. This was due to a density variation which was introduced by a diffusion-controlled photopolymenzation, within the light regions, of the residual monomers.The neutron djffraction has been observed using the full length of the 80 m small-angle camera Dl 1 at the High-Flux Reactor, Grenoble. The primary neutron beam was monochromatized at around = 1 nm within AX/? = 18 % full width and collimated within 0.75 mrad. The amplitude of the neutron refractive index modulation of the protonated samples was by i05 smaller than the light refractive index modulation if using a helium-neon laser beam of X = 633 nm (R.A. Rupp, J. Hehmann, R. Matull, K. Ibel, Phys. Rev. Lett. 64 (1990) 301). The neutron reflectivity of the fully deutcrated samples, 1.4 %, was by a factor 40 larger than that of the protonated samples (R. Matull, P. Eschkotter, R.A. Rupp, K. Ibel, Europhys. Lett. 15 (1991) 133); this makes these gratings to valuable etalons for neutron small-angle cameras. Possible further applications of our research include: the study of light-induced neutron refractive index changes and of photochemical processes in situ using a radiation which does not alter the structure; the investigation of the factors which influence the regularity and stability of the holographic gratings; the development of neutron optical devices, as beam splitters, mirrors, lenses; and, in a more distant future, the construction of instruments, e.g., neutron interferometers and neutron microscopes. 0-8194-0687-2/91 /$4.0O SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, arid Applications II (1991) / 393 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/27/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx