Multilayer Fe-Co mirror polarizing neutron guide

NUCLEAR

INSTRUMENTS

AND

METHODS

133

(1976) 453-456;

©

NORTH-HOLLAND

PUBLISHING

CO.

M U L T I L A Y E R Fe-Co M I R R O R P O L A R I Z I N G N E U T R O N G U I D E G. M. D R A B K I N , A . I . OKOROK.OV, A. F. S ' C H E B E T O V , N . V . B O R O V I K O V A , A . G . G U K A S O V , V.A. KUDRIASHOV, V.V. RUNOV

Leningrad Nuclear Physics Institute, Gatchina, Leningrad 188350, U.S.S.R. and D.A.K.ORNEEV

Joint Institute for Nuclear Research, Laboratory of Neutron Physics, Dubna, U.S.S.R. Received 27 October 1975 In this paper a polarizing neutron guide made from 50Co50Fe mirror with an absorbing underlayer is described. The neutron guide design and results o f its test on the reactor V V R - M (Leningrad Nuclear Physics Ihstitute, Gatchina) are reported. The flux at the end of the 1.6 × 30 m m 2 neutron guide was equal to 1.7 × 107 n/cm2s with a reactor power of 16 MW. Polarizations averaged over the spectra of the outgoing b e a m for n a r r o w and broad incoming collimations were 0.95 and 0.97, respectively.

Neutron guides were extensively developed during the last decade. Now they allow to separate the thermal neutron beam from the fast neutron and y-ray background and conduct it at a fairly large distance from the reactor. The intensity of the transmitted beam does practically not depend on the guide length. This yields a significant gain in neutron flux on the sample in comparison with the conventional beam experiment realized at the same distance from the reactor. The physical principle of the guide tube is the total reflection of neutrons from the polished surface. The transmitted beam is polarized if the neutron guide walls consist of magnetized ferromagnetic material. At present there exist the following polarizing neutron guide systems: the Berndorfer neutron guide [Munich J)], the neutron guide of the Atomic Institute [Moscow2)], the Stecher-Rasmussen multislit focusing system [RisS3)], the Shaerpf neutron guide6)]. The Berndorfer neutron guide with 50Co48Fe2V reflecting plates gives a maximum polarization of Pmax = 0.91, the polarization in the maximum of the spectrum (2 = 3.1 A) was 0.85, the total exit flux being 3 x 10 6 n/cm2s. Moreover, a strong decrease of polarization to P = 0.5 at ~. = 6.3 A was observed. The polarization attainable by reflection mirrors depends on the reflecting alloy used, and on the quality of the mirror surface. Multilayer polarizing FeCo mirrors developed in the Laboratory of Neutron Research 7) (glass plates coated with a neutron absorbing film and with 50Co50Fe alloy) give a high polarization: in the maximum of the spectrum of the reflected beam (2 = 4 A) the polarization is close to 100% and the mean polarization of the whole spectrum of the

outgoing beam is P = 0.97. The critical glancing angle per 1 A is ct = 7 ¢ / 2 = 1.85x 10 - 3 rad/A. The neutron guide made of the above mentioned mirrors was expected to give a high polarization ratio. The purpose of this work was to investigate the effectiveness of a neutron guide built up from multilayer polarizing mirrors. The neutron guide under description has been made as a trial model of the future neutron guide for the polarized neutron spectrometer at the pulsed reactor IBR-2 of J I N R (Dubna). It is worth noting that in the case of a pulsed neutron source it is necessary to have a high degree of polarization in a wide range of neutron wavelengths. The geometrical dimensions of the neutron guide were calculated using the characteristic wavelength 2* of neutrons transmitted through the bent guide channel and the breadth a of the bent neutron guide. We had accepted ~.*= 2.7 A, a = 1.6 m m (fig. 1). Making use of the known formulae (see ref. 4) the expressions for the characteristic angle 7", the radius of curvature R and for the length of direct sight L 1 are given by: 7* = ~;~* = 5 × 10-3 rad, R

= 2 a / ~ .2 = 131 × 103 r a m ,

L1 = 4 a / y * = 1.3 x 103 r a m .

The full length of the neutron guide L has been taken slightly larger than L 1 (L = 1470 mm). The construction of the neutron guide and the adjustment scheme are presented in fig. 1. A solid aluminium tube (5) served as a basis for the adjustment mechanism. The neutron guide channel consisted of seven sections

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DRABKIN

210 mm in length. Each section was formed by two mirrors (6) with two copper spacers (7) between them. The polished surface of the spacers [(1.6_+0.025) mm thick] faced the inner part of the mirror channel. The mirrors and spacers were clamped together with cramps (8). Each section was fastened between the adjustment screws (3) and the spherical rest on the frame with a rod (4). The screw (3) enables us to adjust the section in the vertical plane. The whole guide tube was adjusted according to the accepted radius of curvature by turning sections with respect to each other by rod (4) at angle ,/. The angle 7 was measured with respect to the basal mirror surface (10) using a theodolite with autocollimation (9). The protruding spacers of each following section were I

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inserted into the slots of the previous one which allowed to exclude the formation of steps in the horizontal plane within an accuracy of 0.025 ram. To avoid the vertical shift of sections, screws (l l) were applied. The whole device was made from nonferromagnetic materials. The mounted and adjusted neutron guide was covered by a vacuum cap and evacuated to 1 x l 0 - 2 torr. Then the mirror channel of the neutron guide was moved into the pole gap (130 mm high, 1050 Oe) of permanent magnets (2). Magnets (2) and carrier tube (5) were fastened with bolts to the aluminium plate (12). Then the assembly was mounted at the reactor VVR-M for testing. Fig. 2 shows the testing geometry of the neutron guide. The reactor power was 16 MW. For the first run we have used the collimator (1) window (3 x 10 mm 2) which formed the neutron beam with a horizontal angular divergence of 7 min of arc. The neutron guide entrance was at a distance of 500 mm from the collimator exit. Three supports allowed to turn the guide tube as a whole around the vertical

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Fig. 1. T h e n e u t r o n guide. (a) Section o f the built-up n e u t r o n guide, (b) mirror channel section, (c) scheme o f adjustment, (d) a scheme illustrating the m e a n i n g o f the m a i n n e u t r o n guide parameters.

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Fig. 3. The shape o f the polarized neutron beam in the horizontal plane in the case o f 7' collimation o f the ingoing beam. Curves a-j are the intensity distributions m e a s u r e d at different angles o f the n e u t r o n guide with respect to the axis o f the collimator, Z is the s u m curve.

MIRROR

POLARIZING

axis located at the guide entrance. It permitted to check the neutron guide for all possible angles q~ between the collimator and guide axis. The disc cadmium chopper was placed at the exit allowing to measure the polarized neutron spectra by the time-of-flight method. The path was 7.1 m and the resolution 0.25/~. The analysis of polarization was performed using the spin-flipper (4) with efficiency f ~ l s), detector (6) and 4 2 0 m m analyser-mirror (5). The polarization efficiency of the analyser mirror PA(2) was known. It was possible to adjust the analyser mirror and detector to any angle within the polarized beam. We measured the ratio e(2) = (N1 --N2)/(NI + N 2 ) , where NI and N a are the detected intensities for neutron spins in the upward and downward directions, respectively. The beam polarization at the exit of the neutron guide is then given by: P(2) = e(2)/PA(2 ). The angular distribution of the polarized beam was measured moving the detector with a narrow vertical slit in front of it. Fig. 3 represents such distributions for ten different angles (p (the scanning step A(p was 4.5 min of arc). The peaks in the angular distributions appeared due to the fact that each of the seven mirrors contributed at different angles to the intensity at the exit. The total intensity (curve X) obtained by summing up the distributions in the geometry from " a " to " j " conveyed an idea of the angular distribution of the polarized beam intensity in the horizontal plane if the collimation of the incident beam is 45 min of arc. The halfwidth of the outgoing beam in this case is equal to 27 min. Absolute values of neutron fluxes at the entrance and at the exit of the neutron guide in the " e " geometry (fig. 3) were measured by the method

NEUTRON

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of gold foil activation. The measured values of flux were a) at the entrance of the neutron guide (8___0.8)x 107 n/cm2s, b) at the exit of the neutron guide in the centre of the window (6.7_+0.7) x 106 n/cmZs, c) at the exit 10 m m below the centre of the window (6___0.6) x 106 n/cmZs, d) at the exit 10 mm under the centre of the window (6+__0.6) x 106 n/cm2s. The neutron spectra measured before and after polarization (curves 1 and 2 in fig. 4) were taken in the " e " geometry of the neutron guide as well as the polarization at different wavelengths and averaged polarization P. The latter was found to be constant across the beam. The P(2) curve (fig. 4, curve 3) was obtained with the detecting system adjusted on the central peak of the angular distribution (see fig. 3e). As should be expected the neutron guide gives a considerable softening of the spectrum. We have observed a slow decrease of P with increasing 2. However, this fall was remarkably reduced in the measurements performed with a broad beam (see in the following). The average value of polarization for the measurements with narrow input collimator was 0.95. The estimation of the Cd-ratio in the polarized beam gave Rca t> l0 sThe second run of measurements was performed with incident beam collimation increased up to 25 rain of arc. In this run the time-of-flight path and the field strength were 3.42 m and 500 Oe, respectively. The polarized neutron flux was found to be 1.7 x

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456

G . M . DRABKIN et al. P

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Fig. 6. Some results obtained in the case of 25' collimation of the ingoing beam. (1) The spectrum of incident neutrons, (2) the spectrum of polarized neutrons, (3) the P(2) dependence obtained for our neutron guide, (4) the P(2) dependence obtained in ref. 1. 107 n/cm2s at a reactor p o w e r o f 16 M W . The a n g u l a r distribution was m e a s u r e d by a n a r r o w slit detector (fig. 5). A value 25 min o f arc was f o u n d for the a n g u l a r halfwidth. The spectrum o f p o l a r i z e d neutrons a n d P(2) dependence (fig. 6, curves 2 a n d 3) were m e a s u r e d at the m a x i m u m o f the a n g u l a r d i s t r i b u t i o n with analyser a n g u l a r aperture being 3 min o f arc. The p o l a r i z a t i o n averaged over the whole spectrum was P = 0.97 in this case. The P(2) dependence is m o r e fiat as c o m p a r e d with the n a r r o w b e a m geometry. A visible decrease in P ( 2 ) is observed only outside the 2 - 6 A region which seems to be quite satisfactory for the experiments with t h e r m a l a n d s u b t h e r m a l neutrons.

A c o m p a r i s o n with a similar d e p e n d e n c e o b t a i n e d for the Berndorfer n e u t r o n guide (fig. 6, curve 4) shows clearly the i m p r o v e m e n t o f the m a i n characteristics achieved using the m u l t i l a y e r m i r r o r neutron guide. The r e p o r t e d results illustrate the a d v a n t a g e s o f utilization o f the multilayer F e C o p o l a r i z i n g m i r r o r s for the f o r m a t i o n o f a p o l a r i z e d n e u t r o n b e a m in a fairly large wavelength range. The a u t h o r s are indebted to D. M. K a m i n k e r , A. Bajorek a n d Yu. M. Ostanevich for their p e r m a n e n t interest in the w o r k a n d valuable discussions, to V. B. S ' c h e b e t o v a a n d V. A. Priemyshev for their help in designing a n d construction o f the neutron guide, to G. Ya. Vasil'ev a n d the V V R - M reactor team for the assistance d u r i n g the measurements.

References

1) K. Berndorfer, Z. Physik 243 (1971) 188. 2) B.G. Erozolimsky, Yu. A. Mostovoj, V.P. Fedunin, A. I. Frank and O. V. Khakhan, Lett. Soy. J. Exp. Theor. Phys. 20 (1974) 745. a) K. Abrahams, W. Ratynski, F. Stecher-Rasrnussen and E. Warming, Nucl. Instr. and Meth. 45 (1966) 293. 4) H. Maier-Leibnitz and T. Springer, J. Nucl. Energy A/B 17 (1963) 217. 5) G.M. Drabkin, E.I. Zabidarov, Ya. A. Kasman, A.I. Okorokov and V. A. Trunov, Preprint FTI, no. 183, Leningrad (1969). 6) O. Schaerpf, Abstracts of Neutron Diffraction Conf. 1975, Petten, The Netherlands (1975). 7) G.M. Drabkin, A.I. Okorokov, A.F. S'chebetov, N.V. Borovikova, A. I. Egorov, A. G. Gukasov and V. V. Runov, Sov. Phys. JETP 12 (1975) (to be published).