Nuclear Instruments and Methods m Physics Research A270 (1988) 95-98North-Holland, Amsterdam

BISTABLE NEUTRON SPIN FLIPPERS

D. DUBBERS and R. VLAMINGInstitut Laue-Langevrn, F-38042 Grenoble Cedex, France

E. KLEMTPhysikahsches Institut der Uneversudt Heidelberg D-6900 Heidelberg, FRG

Received 18 January 1988

We have built a bistable spin-flipping device for slow neutrons : it very efficiently flips the spin polarization of neutrons over awide range of neutron velocities, while leaving unaffected the spins of neutrons in other velocity intervals . Such bistable broad-bandflippers can be designed both for 1T and 17/2 spin flips, and also for refocussing flips of the kind used in neutron spin-echoinstruments . We further built a neutron velocity filter for polarized neutrons based on this principle, together with a simple reflectingfoil neutron velocity high-pass.

1. Introduction

Polarized slow neutrons find widespread use in con-densed matter research [1] as well as in a number ofexperiments on elementary particle physics [2,3] . Allthese experiments use some kind of neutron spin-flip-ping device to isolate spin-dependent effects . The vari-ous types of neutron spin flippers in use are listed in ref .[4] .

One popular type of spin flipper, with main applica-tions in neutron spin-echo (NSE) spectrometry [5], con-sists of a simple single layer rectangular do coil. Thepolarized neutrons enter and leave the static magneticfield region nonadiabatically through the layers of thecoil, as shown in fig . 1 . Under the action of the mag-netic field B their polarization is turned by an anglea =yBT, where y is the neutron's gyromagnetic ratioand T the neutron's time of flight through the coil.Usually magnetic field strength and neutron velocity arechosen such that a = 90' ("w/2-flipper") or a = 180'("77-flipper") . These devices work properly only for arather narrow band of neutron velocities .We have tested a new kind of bistable do neutron

spin flipper with broad-band response : it flips the spinsover a wide range of neutron velocities, while leavingthe spins of other velocity classes unaffected . Theseflippers exist both in ar and v/2-flip versions . Possibleapplications are in neutron spin-echo instruments and

' Work performed during a summer studentship at ILL; pre-sent address : University of Technology, Delft, The Nether-lands.

0168-9002/88/$03.50 C Elsevier Science Publishers B.V .(North-Holland Physics Publishing Division)

Fig. 1. Ordinary one coil do neutron spin flipper.

in velocity filters for polarized neutrons . The latter wastested, together with a simple reflecting foil neutronvelocity high-pass.

2. Principle of the method

95

The broad-band neutron spin flippers described hereare composed of a number of n successive do spin-flipcoils of the type shown in fig . 1. The coils producemagnetic field B, perpendicular to the neutron beamaxis z, and with angles (p, (i = 1, - - - , n) with respect tothe x-axis . Fig. 2 shows a simple example with n = 3successive it-flip coils placed under ¢1 = 0 °, 02 = 120 °and 03 = 0 ° . We call such a device a woo 7T120o ao0flipper .

To explain the response of neutron polarization tothis flipper we follow the movement of the tip of theneutron's polarization vector on a unit sphere. Fig. 3ashows the case where each of the three coils induces aperfect 77-flip : the neutron polarization is flippedsuccessively from +z to -z to +z to -z . Fig. 3bshows the case when the system is detuned (via B or T)such that each coil rotates the neutrons only by e.g .

96

Fig. 2 . Example of a three coil broad-band neutron spin flipperin a ITo ° 7 120° 'To ° sequence .

a = yBT = 120 ° , instead of a =180 ° : still, the neutronpolarization finally is flipped into the -z direction withrather high precision. Fig. 4 shows the total theoreticalresponse curve of such a flipper as a function of a.

The magic sequences 00,102),,, . . .(an)¢that weemployed in our neutron flippers were discovered invery recent years in the field of multipulse nuclearmagnetic resonance (see ref. [6] and references therein) .The connection with polarized neutron work isstraightforward: polarized neutrons flying throughsuccessive, uniform and static, transversal magneticfields see the same fields as nuclei in a NMR probe seein the rotating frame when irradiated by a sequence ofmagnetic radiofrequency pulses. In the NMR case B�T, and 0, are the strength of the rotating field, theduration, and the relative phase of the i th radio-frequency pulse. The larger the number n of pulsesemployed, the sharper the transition between flip andno-flip intervals will be .

With neutrons a reasonable number of successivecoils for such a flipper is three or four ; when the coils

Y

Fig. 3 . Response of neutron polarization to the ITo° 7r120' 'To °flipper of fig. 2: (a) for optimized neutron velocity, such thata = yBT =180 ° for each coil ; (b) for neutron velocity increased by 50%, such that the system is detuned to a= 120 °

the apparatus still works as a perfect i7-flipper .

D. Dubbers et al. / Blstable neutron spin flippers

2na=yBT

Fig. 4. Theoretical response curve of the gyro. 7T120 - 'TO' flipperof fig . 2. The curve is periodic in a. Also indicated (dashedline) is the sinusoidal response curve of the one coil flipper of

fig . 1 .

are wound from 0.5 mm thick aluminium wire or stripthen the neutron intensity loss in such a flipper will beonly 3-4% .

3. Test of wide-band neutron spin flippers and velocityselectors

We have assembled several such wide-band flippers .The coils had dimensions of 100X 35 X 5 mm each,were made from 1 mm aluminium wire, and were placedwithin a mu-metal cylinder to shield the polarized neu-trons from external fields and to provide a yoke for thereturn field of the coils.

The test of the flippers was done with a monochro-matic polarized neutron beam of 1 .5 cm diameter fromone of ILL's neutron guides . The Bragg-reflected beamhad a neutron wavelength of A = 7 .2 A, which corre-sponds to a velocity of v = 550 m s-1 . The neutronpolarization P = 97% was obtained with a neutron su-permirror polarizer [7,8]. The neutron time of flightthrough one coil was thus fixed to T- 10 fts, and themagnetic field B in the coils was varied to obtain theresponse curves of the flippers . A i7-flip in one coilrequired a field of B = 1.6 mT.

Fig. 5 shows the result for a four coil ir-flipperarranged as IT,° 17105° iT210° ( 2 iT)59 . . This sequence was

1cC o

0 0L N

0LaN

0

1

2

3

4 A

5Coil Current I

Fig . 5 . Theoretical and experimental response of a moo' X105°q7zio° (21r)s9° broad-band 7r-flipper, measured with mono-chromatic neutrons as a function of the flippers magnetic fieldB . A neutron time-of-flight transmission curve for fixed B

would look identical .

0Coil Current

Fig. 6 . Theoretical and experimental response of a (7r/2) o o

' T 105' g315, broad-band 7r/2-flipper, measured with crossedpolarizers .

originally proposed for NMR spin-echo applications [9] .The initial neutron polarization is along the beam axisz, as is the polarization component analyzed . The samecurve would, of course, be obtained if not the field Bwas varied but the neutron time of flight T, i.e . theneutron wavelength a . One finds that the theoreticalflip efficiency remains practically 100% even for anincoming neutron wavelength band of width f 40% .A broad-band 7r/2-flipper is obtained with a (7T/2),.

7T105 ° 7T315 o sequence [9] . Fig. 6 shows the response ofthis flipper measured with crossed polarizers. Bothflippers can also be used for spin-echo flips, because theinduced total phase spread remains below 16 ° over thewhole ±40% wavelength interval . A 7r/2-flipper withfour coils in a (37r/2) o o (210 169° 7x33° -178° sequenceshows practically no phase distortions [9] .

When the setup polarizer-broad-band flipper-ana-lyzer is used with a white neutron beam it acts as asimple velocity filter for polarized neutrons . Fig . 7shows a time-of-flight spectrum, measured over a flightdistance of 6.4 m, with and without our four coil7-flipper inserted between two parallel supermirrorpolarizers. The window of transmission can be shiftedby varying the current in the coils . The relative width ofthe window, however, is fixed for a given type offlipper .

As the response of the flippers is periodic in T (orB), once can see another peak at twice the wavelength

r

z ,_W _N

C

zL~

wz

4 A

2 4 6 81 10 12 14 161 18 msX=5A

X=10 ,$NEUTRON TIME OF FLIGHT

Fig . 7. Neutron time-of-flight spectrum with a go . 'T105' 17210

(2059° flipper used as a velocity filter for polarized neutrons .

D. Dubbers et al / Bistable neutron spin flippers

c0 10

1 000C0L

16080 40

20

16080

40

2 0 ms-11000

1000

100

10

m 100z

10

4. Conclusion

Neutron velocily

8-92'

e=4.6'

Neutron wavelength

100

10

1n1000

100

1 I

i 1 i

i

il I

i

i

i

il l0

50

100 150 200

0

50

100 150 200

97

Fig . 8 . Slow neutron transmission curve of a nickel coated thinfoil under glancing angle 0 . It can be used to cut off higherorder transmission peaks in the velocity spectrum of fig . 7, if

necessary .

of the first peak . These higher order peaks can be cutoff very efficiently with a neutron velocity high-pass,based on a well known principle : the high-pass consistsof a 20 lum Mylar foil, coated with some 100 nm ofnickel, spanned under a small glancing angle k acrossthe beam : neutrons with wavelengths X longer than thecritical wavelength A . for total reflection will be re-flected out of the beam (where Bc[degree] - 0.1X c[A] fornatural nickel) .We have built such a neutron velocity high-pass for

other purposes (in a neutron-antineutron oscillationproject under construction at ILL it will cut off veryslow neutrons which otherwise would fall onto the innerwall of the 100 m long drift vessel) . This high-passconsists of two such foils, one under +0 and one under-0 with respect to the beam axis, in order to get asymmetric velocity cutoff also from a diverging beam.Such a device was tested with a beam of very coldneutrons . Fig. 8 shows that long neutron wavelengthsare indeed completely suppressed with this procedure,which can also be used in any experiment where higherorder wavelength transmission must be suppressed .

We have applied a spin-flip scheme recently inventedin pulsed NMR spectroscopy to polarized neutron work,and have indicated some possible applications of this

98

scheme in the growing field of polarized slow neutronresearch. We further have tested a simple device actingas an efficient neutron velocity high-pass.

Acknowledgements

We thank K. Zapfe, M. Kessler, O. Scharpf, W.Mampe, W. Just and B. Sarkissian for help and lendingof equipment.

This work has been funded by the German FederalMinister of Research and Technology under contractnumber 06HD983I .

References

[11 Int . Conf . on The Impact of Polarized Neutrons on Solid-State Chemistry and Physics, Grenoble (1982) J. de Phys .43 (1982) C7 .

D. Dubbers et al. / Bistable neutron spin flippers

[21 Workshop on Reactor Based Fundamental Physics, Greno-ble, (1983) J. de Phys . 45 (1984) C3 .P.A . Krupchitsky, Fundamental Research with PolarizedSlow Neutrons (Springer, Heidelberg, 1987) .G. Badurek, Nucl . Instr . and Meth . 189 (1981) 543.F. Mezei (ed.), Neutron Spin Echo (Springer, Heidelberg,1980) .

[61 H. Cho, J. Baum and A. Pines, J. Chem. Phys . 86 (1987)3089 .F. Meze, Commun. Phys. 1 (1976) 81 ;F. Mezei and P.A. Dagleish, Commun . Phys. 2 (1977) 41 .

[8] O. Scharpf, in : Neutron Scattering - 1981, ed. J. Faber Jr .,American Inst . Phys . Proc. no . 89 (1981) p. 182.R. Tycko, H.M . Cho, E. Schneider and A. Pines, J . Magn .Res. 61 (1985) 90 .

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