Neutron diffraction from photoinduced gratings in a PMMA matrix

Abstract

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...

Neutron Optical Beam Splitter from Holographically Structured Nanoparticle-Polymer Composites

“…A few attempts have been conducted to manufacture diffractive elements for cold neutrons. However, the results on the efficiency were rather discouraging [2,[8][9][10].…”

“…This is the so-called photo-neutron refractive effect (PNRE) [8,11]. Upon illumination with a spatial light pattern a corresponding refractive-index modulation Án ¼ ð 2 =2ÞÁB occurs that is proportional to the local change ÁB of the coherent scattering length density B.…”

“…In particular, the groups of Tomita et al [14][15][16][17] and Stumpe et al [18,19] have intensively investigated inorganic nanoparticles (SiO 2 and ZrO 2 ) embedded in a photopolymer matrix. They serve as proof of principle for the essential advantages offered by this class of materials for neutron optics over previously used pure photopolymer systems [8,11,20]: the refractive-index modulation can be tuned by the type of nanoparticles, their size, and volume ratio. An additional benefit of including nanoparticles constitutes the increased mechanical stability because shrinkage, which is typical for polymerization processes, is strongly reduced [14].…”

A Review of the Optimisation of Photopolymer Materials for Holographic Data Storage

“…As a consequence, not only the optical but also the neutron refractive index may be changed in these cases [86]. The light-induced change of the refractive index for neutrons in materials (the photoneutron-refractive effect) has been exploited to produce diffraction gratings for neutrons by structuring suitable recording materials using holography [87].…”

The photo-neutronrefractive effect

“…A typical holographic two-wave mixing setup was used to record a refractive-index pattern, a grating, in PMMA which was reconstructed not only with light, as usual, but also with neutrons [9]. Evidently, the illumination induced refractive-index changes for both, light and neutrons!…”