JOURNAL OF APPLIED PHYSICS
VOLUME 85, NUMBER 8
15 APRIL 1999
The ultrahigh flux polarized cold neutron spectrometer PANDA of the FRM-II reactor M. Loewenhaupt Institut fu¨r Angewandte Physik, TU-Dresden, D-01062 Dresden, Germany
N. M. Pykaa) Forschungsheutronenquelle FRM-II, TU-Mu¨nchen, D-85747 Garching, Germany
At present the high flux research reactor FRM-II is under construction in Garching, Germany. As part of the first set of instruments, a triple axis spectrometer is under construction at the cold source in the reactor hall. This instrument will be mainly used to study inelastic magnetic phenomena in condensed matter physics. Besides an unpolarized beam setup, PANDA is equipped with devices allowing full polarization analysis. This instrument largely profits from its vicinity to the reactor, and uses state of the art neutron optics. Thanks to the large focusing Heusler monochromator and analyzer the neutron flux of the polarization setup will be of the same magnitude as that of an unpolarized beam at a high flux reactor. The accessible energy and Q ranges will be extremely large due to an advantageous flux distribution of the cold source and a particular construction of the beam tube and the biological shielding which contain supermirror neutron guides. A resonant spin echo device, used to perform line shape analysis of dispersive excitations, will complete the extensive array of equipment, available on PANDA. © 1999 American Institute of Physics. @S0021-8979~99!21708-2# INSTRUMENT DESIGN
of a cold triple axis spectrometer. Because of its exceptional flux, it will certainly open the door to novel research fields in neutron spectroscopy. The monochromator shielding extends out from the reactor4 wall permitting the installation of the large horizontal diaphragm ~HD! and the first collimator ~C1! outside the reactor. Such a construction has several advantages. ~1! It will allow access to the in-pile supermirror ~SM! guide, which will be replaced periodically in order to maintain the SM coatings in good condition. ~2! The primary shutter is flexible, see below. ~3! The accessible monochromator and sample scattering angle is rather large, about 140°, permitting high resolution ~20 meV! and large Q ~6 Å21! measurements, respectively. Finally, the distance to the neighboring instrument SR-3 has been increased. The spectrometer can be used in a high ~q,v!-resolution mode with a fully collimated beam, in an ultrahigh neutron flux/high v-resolution mode with doubly focusing monochromators @PG~002!, Cu~111!, Heusler(111)( v )# and analyzers @PG~002!, Si~111!, Heusler(111)(h)#, or in a mixed mode depending on users demands. Lifetime measurements of dispersive excitations using the neutron resonant spin echo ~NRSE! technique will be possible in a reasonable time thanks to the high brilliance of PANDA. The neutron optical principles on which the high neutron flux of this spectrometer is based are described elsewhere.4 Initially PANDA will use single detectors while we keep a multidetector option for the future. The same holds for the addition of a velocity selector, which could be superior to filters, since at present our requirements on beam size and cutoff efficiency for higher-order wavelengths at high energies cannot be fulfilled by the market. Three examples will demonstrate applications in the field of magnetism which will profit from superior properties of PANDA. In the low-energy region, a better energy resolution at high neutron flux can be achieved to study, e.g., the quasielastic
The construction of the neutron research facility FRM-II in Garching,1–3 Germany, presents an exciting opportunity to scientists to implement new ideas and state of the art techniques in the field of neutron scattering. This article describes a novel cold-neutron triple-axis spectrometer ~TAS! named PANDA with many outstanding features: ~1! The PANDA spectrometer is designed to deliver a neutron flux on the sample of about an order of magnitude more intense than any other existing cold TAS. At the same time the background can be kept low using variable-size in-pile diaphragms with which it is possible to reduce the image size of the source at the sample position as needed. ~2! The largest energy transfers achievable on present day cold TASs are restricted to 10 meV. PANDA will exceed this limit: Using the Cu~111! monochromator even experiments beyond 20 meV energy transfer will be possible with reasonable flux. This high energy option is significant because it gives experimenters access to energies between those in the optimum ranges of conventional cold and thermal TASs, respectively. ~3! The design of PANDA takes advantage of the considerable improvement in current efficiencies for producing polarized neutrons. ~4! There is an increasing demand for the ability to analyze the linewidth of dispersive excitations as is needed for the study of phonon lifetimes or couplings in magnetic systems. A resonant spin echo device with an energy resolution of about one meV is planned to be installed which permits such investigations. ~5! The spectrometer is equipped with strong cryomagnets ~15 T vertical, 5 T horizontal! which will give users access to the growing area of high field spectroscopy. In conclusion, one can state that PANDA will be able to meet todays and tomorrows demands a!
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© 1999 American Institute of Physics
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J. Appl. Phys., Vol. 85, No. 8, 15 April 1999
M. Loewenhaupt and N. M. Pyka
FIG. 1. Calculated neutron flux distribution for hot, thermal, and cold neutrons. Bold: accessible flux for PANDA.
response near the magnetic instability of the non-Fermiliquid compound CeCu62x Aux much closer to the instability,5 to figure out better parameter. The access of the overlap region from a cold to a thermal triple-axis spectrometer will allow to study, e.g., excitations of the itinerant magnet UFe2 which shows interactions of modes between 8 and 20 meV,6 within one experiment. Corrections for different instruments do therefore not apply. The simultaneous high flux, energy resolution, and polarization analysis of PANDA could be used to study, e.g., the f-electron dynamical response in the heavy-fermion metal UPd2Al3 that is connected with superconductivity,7 in more detail. The following subsections describe the main features of PANDA. NEUTRON FLUX DISTRIBUTION
position. The vertical openings are such that the different heights of the PG~002!, Cu~111!, and Heusler monochromators are fully illuminated, and that background neutrons are blocked as much as possible. The gain in intensity due to horizontal focusing is limited by the beam divergence that can be accepted by the monochromator. For PANDA the lateral walls of P1 are straight supermirror neutron guides with three times the critical angle (M 53) of natural Ni. The gain in useful neutrons obtained by introducing an SM guide inside the beam tube is shown in Fig. 2. M 53 is a compromise between a long lifetime of the coating and a large critical angle. Wavelengths smaller than l52.6 Å profit from the large horizontal aperture of P1 ( d 51.6°). P2 has a small horizontal parallel opening ~40 mm! and serves as a collimated mode of operation only. The P3 insert serves the large Heusler monochromator which permits monochromatic focusing in the vertical direction while maintaining a high degree of polarization. If necessary the flux can be increased by about a factor of three for small samples by using a convergent SM guide between monochromator and sample and the larger horizontal beam width of 50 mm. The ‘‘natural’’ flux gain obtained by simply removing the C2 collimator ~typically 40 arc min! is nearly a factor of 1.6, which is due to a finite probability of adjacent wavelengths being reflected by the monochromator in the sample direction. However, this gain in flux is accompanied by a loss in energy resolution of the same order of magnitude. The ratio of intensity gain to energy resolution loss is much better if a convergent SM guide is used. However, the instrumental resolution function becomes more complicated. The features of a convergent neutron guide are described elsewhere.9 RESONANT SPIN ECHO DEVICE
8
The cold source, which was developed in Garching represents, in conjunction with the novel compact fuel element design, the current state of the art. The SR-2 beam tube of PANDA looks into the cavity of the cold source, thus profiting from its advantageous properties. Monte Carlo simulations of the neutron flux distribution of the compact fuel element and the cold source8 ~see Fig. 1! show that the cold neutron flux is nearly constant over a large energy range and will reach approximately the highest neutron flux available at present, despite the low thermal power of 20 MW. PANDA will have access to high energy neutrons due to an undermoderated cold source. This will allow experimenters to cover the interesting energy range between the limits of conventional cold and thermal TASs, and will be a unique option of a cold TAS. The use of uranium silicide as fuel element has the advantage of not heating up the cold source, and of transferring less heat to the supermirror guide sections in the beam shutter. This helps to maximize the lifetime of the coatings. SHUTTER DESIGN SR-2 AND IN-PILE SUPERMIRROR NEUTRON GUIDE
The primary shutter is divided into a stationary part which is close to the cold source, and a ‘‘revolver’’ part which can rotate into three open ~P1 to P3! and one closed
The conventional neutron spin echo technique ~NSE! is rather successful,10 but fails in the investigation of dispersive excitations. In order to determine the line width of dispersive excitations one can combine NSE with a conventional triple axis spectrometer ~see Fig. 3 top!.11 In this case one uses the TAS resolution function to separate the inelastic event in
FIG. 2. Gain factor for horizontal focusing of an in pile SM guide compared to a simple beam tube. M 53 will be installed when the reactor starts, M 54.8 is, at present, the top value for supermirrors, but inapplicable for in-pile use. The SM reflectivity is not taken into account in the figure. PANDA: one reflection for M 53, up to two reflections for M 54.
J. Appl. Phys., Vol. 85, No. 8, 15 April 1999
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dependent spin echo equation which describes the relation between the precession angle and the energy transfer ~v d 5energy on the dispersion curve! are B NSE~ k1 ,k2 ! 5 g mh 21 ~ B 1 L 1 cos u 1 /k1 n1 2B 2 L 2 cos u 2 /k2 n2 ! , B NSE2B NSE5 t @ v 2 v d ~ q!# .
~1.1! ~1.2!
The left- and right-hand side of the latter equation are to a first approximation B NSE2B NSE52 g mB 1 L 1 ~ hkO 21 cos u 1 ! 21 d k1 n1 1 g mB 2 L 2 ~ hkO 22 cos u 2 ! 21 d k2 n2
~1.3!
v 2 v d ~ q! 52 @ hkO 1 /m2grad v d ~ q0 !# d k1 1 @ hkO 2 /m2grad v d ~ q0 !# d k2 .
FIG. 3. Top: Side view of the neutron resonant spin echo setup. Note the tilted bootstrap coils with N52. Bottom: Principle of NRSE, top view. L 1,2 denotes the separation of the pairs of bootstrap coils and the spaces which are field free, u 1,2 and k 1,2 denote the tilt angle of the coils and the momentum wave vectors of the incoming/outgoing beam, respectively. n 1,2 are unit vectors of the bootstrap coils.
~1.4!
Equation ~1.2! should be fulfilled for any d k1,2 therefore d k1 5 d k2 . The final equations for the ratio of the field integrals and the spin echo time for tilted coils are then B 1 L 1 /B 2 L 2 5 $ hkO 31 /m2kO 1 @ kO 1 grad v d ~ q0 !# % /
$ hkO 32 /m2kO 2 @ kO 2 grad v d ~ q0 !# % ,
~2.1!
t 5 g ~ m/h ! 2 B 1 L 1 / $ kO 31 2mkO 1 /h @ kO 1 grad v d ~ q0 !# % . ~2.2! ~q,v!, whereas the high resolution measurement takes place in the NSE part of the spectrometer. A technically brilliant solution is the neutron resonant spin echo technique12 ~NRSE!. Unlike NSE, NRSE uses compact high frequency flipping coils separated by a distance L 1,2 to simulate NSE ~see Fig. 3 bottom! combining a static magnetic field with a superposed HF field ~p coils! with a spin echo time t 5 g hm 21 (L 1 B 1 / vI 31 )5 g hm 21 (L 2 B 2 / vI 32 ). The effective NRSE field can be 2N times the NSE field, i.e., 2N times the energy resolution, using bootstrap coils12 composed of N p coils with opposite fields, respectively. The important advantage of NRSE for a TAS experiment is that the bootstrap coils can be tilted without depolarization, keeping a large cross section for the passing beam. Large tilt angles are required to align the q dependence of the spin echo signal along the particular dispersion being investigated. A typical NSE experiment determines the polarization P( t ) at different t, with t varied via different fields B 1,2 with B 1 /B 2 5const. A semilogarithmic plot of P( t ) vs t yields a straight line with a slope of 2G, equal to the width of the Lorentzian scattering function S( v ). The polarization P( t ) is the integral over all energies with P( t )5e 2G t , and the energy resolution is the value of G at the maximum spin echo time, i.e., at maximum field, where the signal is P( t max)/P0 5e21, G5 t 21 max. We now need to introduce an energy and k dependence to the formalism in order to describe dispersive inelastic events. The derivation of the following formulas for the NRSE technique are given elsewhere13 ~see bottom Fig. 3 for notations!. The precession angle B and the momentum
The polarization then follows as the cosine-Fourier transform of the scattering function. The NRSE technique has recently proven its utility at a TAS experiment,14 and in quasielastic measurements.15 1
Web page of FRM-II: http://www.frm2.tu-muenchen.de A. Axman, K. Bo¨ning, U. Hennings, and E. Steichele, Jahrbuch der Atomwirtschaft 1997, Verlagsgruppe Handelsblatt GmbH. 3 A. Axman, K. Bo¨ning, and J. Blombach, 2nd International Conference on Advanced Reactor Safety, Orlando, FL, 1–4 June 1997. 4 M. Loewenhaupt and N. M. Pyka, Proceedings of the Workshop of Polarized Neutrons for Condensed Matter Investigations, Grenoble, France, 20–23 September 1998; Physica B ~to be published!. 5 O. Stockert, H. V. Lo¨hneysen, A. Rosch, N. Pyka, and M. Loewenhaupt, Phys. Rev. Lett. 80, 5627 ~1998!. 6 L. Paolasini, G. H. Lander, S. M. Shapiro, R. Caciuffo, B. Lebech, L.-P. Regnault, B. Roessli, and J. M. Fournier, Phys. Rev. B 54, 7222 ~1996!; and ILL Annual Report 96, Scientific Highlights ~1997!, p. 66. 7 N. Bernhoeft, A. Hiess, N. Sato, N. Aso, Y. Endoh, T. Komatsubara, B. Roessli, and G. H. Lander, Phys. Rev. Lett. 81, 4522 ~1998!. 8 K. Gobrecht and W. Gaubatz in The International Workshop on Cold Neutron Utilization, KAERI, Taejon, Korea December 1997; Report No. KAERI/GP-119/98, pp. 41 and 213. 9 Neutron beam focusing using supermirrors, I. S. Anderson, Proc. SPIE 983, 84 ~1988!. 10 F. Mezei, Neutron Spin Echo, Lecture Notes in Physics ~Springer, Berlin, 1980!, Vol. 128. 11 Figure from R. G. Heigl, Diploma thesis ~1998!, E13-Bibliothek, TuMu¨nchen, D-85747 Garching. 12 R. Golub and R. Ga¨hler, Phys. Rev. Lett. A 123, 43 ~1987!; T. Keller, Ph.D. thesis ~1993!, E13-Bibliothek, Tu-Mu¨nchen D-85747 Garching. 13 R. Ga¨hler, R. Golub, K. Habicht, T. Keller, and J. Felber, Physica B 229, 1 ~1996!. 14 P. Vorderwisch et al. ~unpublished!. 15 M. Ko¨ppe, P. Hank, J. Wuttke, W. Petry, R. Ga¨hler, and R. Kahn, J. Neutron Res. 4, 261 ~1996!. 2
