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First experiments on flipping ratio mapping with the “multi-PM” position sensitive detector
Physica B 156 & 157 (1989) 581-583 North-Holland, Amsterdam
FIRST EXPERIMENTS ON FLIPPING RATIO MAPPING WITH THE “MULTI-PM” POSITION SENSITIVE DETECTOR D. SILLOU’, J. BARUCHELzt3, K. KURODA’, J.P. GUIGAY3 and M. SCHLENKER3 ‘L. A. P. P., B.P. 709, 74019 Annecy-le-Vieux, France ‘Institut Laue-Langevin, B.P. 156, 38042 Grenoble, France ‘Laboratoire Louis Niel, CNRSIUJF, BP 166, 38042 Grenoble,
A. MICHALOWICZ’,
France
The “multi-PM” was used in tests of a topographic (local) quantitative approach in polarized neutron diffraction. It provides a unique possibility for measuring flipping ratios with a decent resolution (0.2mm) without biasing the measurement in favor of the more highly reflecting, less perfect or better oriented, parts of a inhomogeneous specimen.
1. Introduction The “multi-PM” position-sensitive detector system was recently shown [l] to hold promises as a tool for neutron diffraction topography. The intrinsic ability to measure and numerically store intensity distributions, which can be processed off-line, made it tempting to try mapping intensity ratios, i.e., in the case of polarized neutron diffraction, flipping ratios. This information can be of high interest in the case of very good crystals, because it can provide a way of testing extinction theories, which have recently become capable, in principle, of dealing with nearly perfect crystals [2-51. Although the quantity of basic interest is the absolute integrated reflectivity, measurements of the flipping ratio, based on identical geometrical conditions, eliminate several of the experimental uncertainties and are therefore somewhat more reliable. The work described below is a preliminary test of this possibility. Two main difficulties arose: the low detective efficiency, and a high background level; the origin of the latter was found. 2. Experimental procedure The multi-PM system is described in ref. [ 11; it consists of only four by four anodes, with a processing system for discrimination and determination of the center of the light distribution
produced by an incoming neutron in a 6LiF-ZnS scintillator, 0.25 mm thick in our experiments. The spatial resolution was measured, using test slits and inholes, to be about 0.2 mm over the p 8 x 8 mm sensitive area. The photomultiplier in its present form requires an axial magnetic field, which had to be prevented from interfering with the magnetic field applied to saturate the specimens or to retain adequate neutron beam polarization. Furthermore, in order to keep image blurring low, the converter had to be as close to the specimen as possible. For these two reasons, the converter was set some 20 cm from the photomultiplier entrance, and connected to it by a bundle of plastic light-fibers. Eventually it became clear that they were the culprit, through the incoherent scattering of the initially undetected neutrons, for the annoyingly high background level, which was nearly uniform over the entrance window of the multi-PM. Two single crystal samples were used in the experiments described here: a (100) plate of terbium, 0.3 mm thick, grown by solid state electrotransport at the Centre for Materials Science in Birmingham, G.B., from material purified at the Rare Earth Center in Ames, Iowa, USA, featuring a resistivity ratio of 200 and a mosaic spread of 21’, mechanically polished and etched; and a (112) plate of yttrium iron garnet (YIG), grown at LET1 Grenoble, cut out so that its central part, corresponding to a (112) growth
0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
582
D. Sillou et al. I Flipping ratio mapping with multi-PM PSD
sector, had its growth striations parallel to the surface and therefore inoperative, in a first approximation, for symmetrical Laue diffraction geometry since the displacement is then perpendicular to the scattering vector [6], 0.49 mm thick, mechano-chemically polished. The magnetic field was applied in the vertical direction, perpendicular to the scattering vector. For the experiments on terbium, low temperatures were provided by a closed-circuit (Displex type) helium refrigerator. The experiments were performed on the special instrument S20, dedicated to neutron topography, at ILL. The polarized neutron beam was obtained from a CUzMnAl single crystal, saturated in a field of 0.27T. The guide-field was provided by strips of ferriteloaded rubber (Ferriflex from Aimants Ugimag), magnetized perpendicular to their surface. Polarization flipping involved a Majorana sheet of current. Tests of the flipping ratio with a CU,MnAl analyzer gave a value of about 20. The data processing involved background correction, a big problem here which should be enormously reduced in future experiments, projections of the intensities or flipping-ratio data into slices displayed versus x or y with usually 5 channels grouped in the other direction, chisquare tests, and the determination of standard deviation bars. 3. Results and discussion Fig. 1 shows a one-dimensional representation of the diffracted intensity Z(X) distribution for the YIG sample, magnetically saturated in the easy lli] direction in a 0.1 T field, using reflection 220 at room temperature. The y position of the 5-channel section corresponding to this position is shown on the inset, where diffracted intensity is represented like in ref. [l], i.e. topographically from the multi-PM output. Highly inhomogeneous reflectivity is related to the different orientations of the growth striations, which are (llO)type, hence not parallel to the surface, in the edge parts of the sample; these obviously feature higher reflectivity by almost an order of magnitude, or reduced extinction. The flipping-ratio variation R(x) at the same y shown on the same
Fig. 1. Spatial distribution of diffracted intensity (dots and solid line) and of the corresponding flipping ratio R (squares) from a YIG single crystal, at the position shown by the band on the image in the insert, which resulted from the multi-PM output. 2jO reflection, A = 1.6 p\.
figure reveals appreciable differences, with the value corresponding to the higher reflectivity, about 0.7, somewhat lower than that for the more perfect region. Independent measurements, using a standard detector, of the flipping ratio from the whole specimen (intensity mainly contributed by the edge regions) gave R = 0.66, consistent with the edge value. However, when the standard measurement was performed with only the low reflectivity region illuminated through a 2 mm diameter mask, we obtained a value of 0.83, inconsistent with the present result. We believe the reason for this discrepancy is the difficulty in correctly estimating, in the present data, the background level, mostly due (see above) to the diffracted beam itself. This estimation had to be performed over a rather small area because of the small distance between the edges of the crystal image and of the detector, where noise piles up anyway. This problem should largely disappear when the plastic fiber is disposed of. The rough general conclusion from the measurements on this very perfect sample is that the
D. Sillou et al. I Flipping ratio mapping with multi-PM PSD
less perfect, more strongly diffracting, parts exhibit a larger l/R value (R further from 1). Fig. 2 shows the diffracted intensity distribution vs position in the c-axis direction, corresponding again to a 5-channel high slice, but for the 002 reflection of the much less perfect Tb sample, saturated by 0.2T, at 160 K (dots and solid curve), as well as the corresponding flipping ratio. In contrast with YIG, values of R deviating further from 1 are seen here to correspond to a lower diffracted intensity. The experiment was performed at several temperatures in the range
12-4
229-160K: the general shape of the intensity and flipping-ratio distribution remains, although their values vary considerably because the magnetic structure factor does. This behavior is consistent with the secondary extinction model, as discussed e.g. in the pioneering paper on polarized neutron diffraction of Nathans et al. [7]; the same explanation holds for the two edges of the sample. Our experiment, performed with the sample set for maximum diffracted intensity and a PSD, gives information that is in this case similar to that obtained by measuring the flipping ratio across the rocking-curve. The multi-PM PSD offers very promising features in polarized neutron diffraction, particularly that of revealing the behavior of low reflectivity regions in inhomogeneous samples. It will become very handy when its efficiency is improved through new scintillators and the background level is drastically reduced through the use of a 45 x 45 mm* detector which does not require a magnetic field, nor therefore optical fibers; these improvements are currently under way. References
6-
.... ._..Or
583
lmm
...._.._. ....
Fig. 2. Spatial distribution of diffracted intensity (dots and squares) and corresponding flipping ratio R (squares) from a ‘lb crystal with a mosaic spread of 21’ at T = 160 K. 002 reflection, A = 1.8 A.
[l] J. Baruchel, K. Kuroda, P. Liaud, A. Michalowicz and D. Sillou, J. Appl. Cryst. 21 (1988) 28. [2] N. Kato, Acta Cryst. A 36 (1980) 763, 770. [3] M. Al Hadded and P.J. Becker, Acta Cryst. A 44 (1988) 262. [4] J. Kulda, these Proceedings, Physica B 156 & 157 (1989) 671. [5] J.P. Guigay, Acta Cryst. A, submitted. [6] J. Baruchel, J.P. Guigay, C. Mazurt, M. Schlenker and J. Schweizer, J. de Phys. 43 (1982) C7-101. [7] R. Nathans, C.G. Shull, G. Shirane and A. Andresen, J. Phys. Chem. Sol. 10 (1959) 138.
FIRST EXPERIMENTS ON FLIPPING RATIO MAPPING WITH THE “MULTI-PM” POSITION SENSITIVE DETECTOR D. SILLOU’, J. BARUCHELzt3, K. KURODA’, J.P. GUIGAY3 and M. SCHLENKER3 ‘L. A. P. P., B.P. 709, 74019 Annecy-le-Vieux, France ‘Institut Laue-Langevin, B.P. 156, 38042 Grenoble, France ‘Laboratoire Louis Niel, CNRSIUJF, BP 166, 38042 Grenoble,
A. MICHALOWICZ’,
France
The “multi-PM” was used in tests of a topographic (local) quantitative approach in polarized neutron diffraction. It provides a unique possibility for measuring flipping ratios with a decent resolution (0.2mm) without biasing the measurement in favor of the more highly reflecting, less perfect or better oriented, parts of a inhomogeneous specimen.
1. Introduction The “multi-PM” position-sensitive detector system was recently shown [l] to hold promises as a tool for neutron diffraction topography. The intrinsic ability to measure and numerically store intensity distributions, which can be processed off-line, made it tempting to try mapping intensity ratios, i.e., in the case of polarized neutron diffraction, flipping ratios. This information can be of high interest in the case of very good crystals, because it can provide a way of testing extinction theories, which have recently become capable, in principle, of dealing with nearly perfect crystals [2-51. Although the quantity of basic interest is the absolute integrated reflectivity, measurements of the flipping ratio, based on identical geometrical conditions, eliminate several of the experimental uncertainties and are therefore somewhat more reliable. The work described below is a preliminary test of this possibility. Two main difficulties arose: the low detective efficiency, and a high background level; the origin of the latter was found. 2. Experimental procedure The multi-PM system is described in ref. [ 11; it consists of only four by four anodes, with a processing system for discrimination and determination of the center of the light distribution
produced by an incoming neutron in a 6LiF-ZnS scintillator, 0.25 mm thick in our experiments. The spatial resolution was measured, using test slits and inholes, to be about 0.2 mm over the p 8 x 8 mm sensitive area. The photomultiplier in its present form requires an axial magnetic field, which had to be prevented from interfering with the magnetic field applied to saturate the specimens or to retain adequate neutron beam polarization. Furthermore, in order to keep image blurring low, the converter had to be as close to the specimen as possible. For these two reasons, the converter was set some 20 cm from the photomultiplier entrance, and connected to it by a bundle of plastic light-fibers. Eventually it became clear that they were the culprit, through the incoherent scattering of the initially undetected neutrons, for the annoyingly high background level, which was nearly uniform over the entrance window of the multi-PM. Two single crystal samples were used in the experiments described here: a (100) plate of terbium, 0.3 mm thick, grown by solid state electrotransport at the Centre for Materials Science in Birmingham, G.B., from material purified at the Rare Earth Center in Ames, Iowa, USA, featuring a resistivity ratio of 200 and a mosaic spread of 21’, mechanically polished and etched; and a (112) plate of yttrium iron garnet (YIG), grown at LET1 Grenoble, cut out so that its central part, corresponding to a (112) growth
0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
582
D. Sillou et al. I Flipping ratio mapping with multi-PM PSD
sector, had its growth striations parallel to the surface and therefore inoperative, in a first approximation, for symmetrical Laue diffraction geometry since the displacement is then perpendicular to the scattering vector [6], 0.49 mm thick, mechano-chemically polished. The magnetic field was applied in the vertical direction, perpendicular to the scattering vector. For the experiments on terbium, low temperatures were provided by a closed-circuit (Displex type) helium refrigerator. The experiments were performed on the special instrument S20, dedicated to neutron topography, at ILL. The polarized neutron beam was obtained from a CUzMnAl single crystal, saturated in a field of 0.27T. The guide-field was provided by strips of ferriteloaded rubber (Ferriflex from Aimants Ugimag), magnetized perpendicular to their surface. Polarization flipping involved a Majorana sheet of current. Tests of the flipping ratio with a CU,MnAl analyzer gave a value of about 20. The data processing involved background correction, a big problem here which should be enormously reduced in future experiments, projections of the intensities or flipping-ratio data into slices displayed versus x or y with usually 5 channels grouped in the other direction, chisquare tests, and the determination of standard deviation bars. 3. Results and discussion Fig. 1 shows a one-dimensional representation of the diffracted intensity Z(X) distribution for the YIG sample, magnetically saturated in the easy lli] direction in a 0.1 T field, using reflection 220 at room temperature. The y position of the 5-channel section corresponding to this position is shown on the inset, where diffracted intensity is represented like in ref. [l], i.e. topographically from the multi-PM output. Highly inhomogeneous reflectivity is related to the different orientations of the growth striations, which are (llO)type, hence not parallel to the surface, in the edge parts of the sample; these obviously feature higher reflectivity by almost an order of magnitude, or reduced extinction. The flipping-ratio variation R(x) at the same y shown on the same
Fig. 1. Spatial distribution of diffracted intensity (dots and solid line) and of the corresponding flipping ratio R (squares) from a YIG single crystal, at the position shown by the band on the image in the insert, which resulted from the multi-PM output. 2jO reflection, A = 1.6 p\.
figure reveals appreciable differences, with the value corresponding to the higher reflectivity, about 0.7, somewhat lower than that for the more perfect region. Independent measurements, using a standard detector, of the flipping ratio from the whole specimen (intensity mainly contributed by the edge regions) gave R = 0.66, consistent with the edge value. However, when the standard measurement was performed with only the low reflectivity region illuminated through a 2 mm diameter mask, we obtained a value of 0.83, inconsistent with the present result. We believe the reason for this discrepancy is the difficulty in correctly estimating, in the present data, the background level, mostly due (see above) to the diffracted beam itself. This estimation had to be performed over a rather small area because of the small distance between the edges of the crystal image and of the detector, where noise piles up anyway. This problem should largely disappear when the plastic fiber is disposed of. The rough general conclusion from the measurements on this very perfect sample is that the
D. Sillou et al. I Flipping ratio mapping with multi-PM PSD
less perfect, more strongly diffracting, parts exhibit a larger l/R value (R further from 1). Fig. 2 shows the diffracted intensity distribution vs position in the c-axis direction, corresponding again to a 5-channel high slice, but for the 002 reflection of the much less perfect Tb sample, saturated by 0.2T, at 160 K (dots and solid curve), as well as the corresponding flipping ratio. In contrast with YIG, values of R deviating further from 1 are seen here to correspond to a lower diffracted intensity. The experiment was performed at several temperatures in the range
12-4
229-160K: the general shape of the intensity and flipping-ratio distribution remains, although their values vary considerably because the magnetic structure factor does. This behavior is consistent with the secondary extinction model, as discussed e.g. in the pioneering paper on polarized neutron diffraction of Nathans et al. [7]; the same explanation holds for the two edges of the sample. Our experiment, performed with the sample set for maximum diffracted intensity and a PSD, gives information that is in this case similar to that obtained by measuring the flipping ratio across the rocking-curve. The multi-PM PSD offers very promising features in polarized neutron diffraction, particularly that of revealing the behavior of low reflectivity regions in inhomogeneous samples. It will become very handy when its efficiency is improved through new scintillators and the background level is drastically reduced through the use of a 45 x 45 mm* detector which does not require a magnetic field, nor therefore optical fibers; these improvements are currently under way. References
6-
.... ._..Or
583
lmm
...._.._. ....
Fig. 2. Spatial distribution of diffracted intensity (dots and squares) and corresponding flipping ratio R (squares) from a ‘lb crystal with a mosaic spread of 21’ at T = 160 K. 002 reflection, A = 1.8 A.
[l] J. Baruchel, K. Kuroda, P. Liaud, A. Michalowicz and D. Sillou, J. Appl. Cryst. 21 (1988) 28. [2] N. Kato, Acta Cryst. A 36 (1980) 763, 770. [3] M. Al Hadded and P.J. Becker, Acta Cryst. A 44 (1988) 262. [4] J. Kulda, these Proceedings, Physica B 156 & 157 (1989) 671. [5] J.P. Guigay, Acta Cryst. A, submitted. [6] J. Baruchel, J.P. Guigay, C. Mazurt, M. Schlenker and J. Schweizer, J. de Phys. 43 (1982) C7-101. [7] R. Nathans, C.G. Shull, G. Shirane and A. Andresen, J. Phys. Chem. Sol. 10 (1959) 138.