Polarized nitrogen beams at LISOL

Nuclear Instruments and Methods in Physics Research North-Holland

B66(1992) 267-273

N rebarInstnansnlts a Methods in PhysicsResearch Section 8

Polarized nitrogen beams at LISOL W. Vanderpoorten a, J. Wouters b, P . De Moor a, P. Schuurmans a, N . Severijns b,*, J . Vantiaverbeke a, L. Vanneste a and J . Vervter b Instituut voor Kern- & Stralingsfysika, KUL, Célestijnenlaan 200D, B-3001 Heverlee, Br!gium "Institut de Phy,,ique Nucléaire, UCL, Chemin du Cyclotron 2, B-1348 Louvain-la-Neuve, Belgium

A setup has been constructed which enables to produce nuclear polarized, mass separated beams at the Leuven Isotope Separator On Line (LISOL). First experiments demonstrate the existence of substantial atomic orientations, inducedwith both the tilted foil (TF) and the grazing surface scattering (GSS)technique. The results of the TF experiments beingas good as optimal for our situation, the GSS results can still be enhanced . The setup could be used as a complementary method in material characterization studies and first experiments to use apolarized '3N beam to determine site distributions of Nimplanted in Fe are being planned. 1. Introduction During the past two years, a great deal of the on-line nuclear orientation activities at the Cyclotron Research Center in I.otwain-la-Neuve has been devoted to the development of an in-beam nuclear orientation setup. The origi-nal motivation for this work was the orientation of short-lived radioactive nuclei which could not be oriented by the combination of very low temperatures and high electromagnetic fields (cf. conventional on line low temperature nuclear orientation (OL-LTNO) [1]) because of the too snort half-life with regard to the spin lattice relaxation time and/or the too small hyperfine interaction after implantation in a host lattice. Two polarizing methods were incorporated in the already existing OL-LTNO setup, i .e. the tilted foil method (TF) [2] and the grazing surface scattering method (GSS) [3]. Both rely on the transfer of an atomic orientation to the nucleus of the beam ions via the hyperfine interaction. The atomic orientation is generated by an anisotropic Coulomb interaction at the surface of (a) a carbon foil which is tilted with respect to the beam axis in the TF geometry or (b) a Si(111) surface in the GSS geometry. Both geometries are drawn in fig . 1 and the preferential direction of the atomic angular momentum, which is generated, is indicated. These particular polarization methods were chosen because they are very polyvalent: no a-priori selection is made with regard to the atoms that can be polarized because the atomic orientation is purely a consequence of the dynamics of the beam-surface inters, ., ,a. * Presently research associate forthe NFWO/FNRS. 0168-583X/92/$05.00 0 1992 - Elsevier

During the development it became clear that the commonly available beam intensity at the Leuven Isotope Separator On Line (LISOL)would be the limiting factor with regard to the applicability of the technique. Indeed, although a rich variety of mass separated radioactive beams is available, the intensity of the beams is limited to typically 10° atoms/s, when the existing target-ionization systems are applied (FEBIAD source [4] or the Ion Guide based mass separation [5]). This beam intensity is the commonly accepted lower limit for nuclear magnetic resonance measurements on oriented nuclei (NMR/ON), which will be the first measurements to be performed. With the advent, however, of the radioactive ion beam (RIB) facility and the development of an electron cyclotron resonance (ECR) ion source at the separator - which is coupled to the RIB production area - this problem seems to be, at least partially, solved . In the present stage of development several light ion beams are available at intensities up to 10'° atoms/s (see below) and can be used for polarization experiments. The tests of the polarization equipment were performed with a stable 14N beam and will be described in the next section. The first radioactive beam which was successfully produced in the framework of the RIB project was a '3N beam (I= i, t1/2= 10 in) [12] and this beam will be used in a first polarization experiment as well. This will permit to use a great deal of the test results, obtained with ' °N, in the analysis . The final goal of the first experiments is then to implant a polarized ' 3N beam in a metal host (e.g. AI) and to study site occupancies of N in this metal as a function of dose with NMR/ON . Nitrogen implantation in Al has been studied quite extensively by Bodart and coworkers at the LARN

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W. lianderpoorten et al. / Polarized nitrogen beams at LISOL vt

Q

Fig. 1 . Geometry of both the tilted foil (a) and grazing surface scattering (b) method . x is the beam direction and (L) = n X v is pointing in the - z direction (see text). ip is the angle between n and v. laboratory in Namur !5] . They developed the IMPL code which calculates the ion distribution after implantation at high doses, taking into account the modification of the ion distribution during the implantation process [7]. The site occupancy, however, of N in Al is not known until now and NMR on the polarized t 3 N nuclei probing the environment will clarify this situation . The same method has already proven to be very successful in earlier experiments with polarized 12 B in Al and Cu [8]. Besides NMR/ON, also cross relaxation experiments have yielded the same information, even somewhat more reliable [9]. The latter method consists in observing the changes of the polarization as a function of external magnetic field . These changes are introduced by the dipole-dipole interaction between the impurity nucleus and the host spins. Such measurements could, in principle, be carried out as well . In the following, the experimental situation will be described in somewhat more detail together with the already obtained results.

gases in an efficient way. Thus, to transport the radioactive nuclei a suitable carrier gas has to be found. Up to now already some very interesting activities are being produced : '3N, t9 Ne. "0. Other are being tested and will be available in the near future (like e.g. t t C). The ECR source will enable to ionize all these elements and the mass-separated beams can be used for polarization experiments. At present, the developments are concentratingon t3N because this will be the first beam to be used. A gas handling system is incorporated as well to produce intense stable beams of several WA . After the ionization, the beams are extracted and accelerated up to 50 keV. Finally they are mass-analyzed and sent into the central beam line . 2.2. The central beam line The beam line has a modular design to allow a certain flexibility, which is necessary because it is used for different experiments. Several beam manipulation and beam monitoring devices are incorporated [10].

2. The experimental setup Fig. 2 shows an overview of the experimental area connected to the mass separator. The polarization setup is localized in the central beam line. At the end a 3He-'He dilution refrigerator is connected, which contains the sample . The technical details of the beam line are published elsewhere [10] and the discussion further will be limited to a short description of the experimental peculiarities. 2.1. The ion source Recently, an ECR source has been constructed at the LISOL mass separator [11] to ionize the large number of radioactive nuclei which can be produced with the Cyclone 30 production facility [12] . The connection between the RIB production area and the ion source is made by a ±30 m long tube constructed in 316 stainless steel, allowing th, transport of several

Fig. 2. Layout of theexperimental area at the mass separator, LISOL; (1) ion source area, (2) separator magnet, (3) beam switch, (4) central beam line, (5) 3He-°He refrigerator.

W. Vanderpoorten et al. / Polarized nitrogen beamsat LISOL

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directly the nuclear polarization of the radioactive beams by means of the anisotropically emitted nuclear radiation. Since ß-decay will be the main decay mode, 0-particles can be detected at 0 and 180°, with respect to the external field simultaneously, ensuring a high sensitivity to the first order nuclear polarization. 3. Generation and detection of atomic orientation Fig. 3. Schematical drawing of the geometry of the optical detection device. In the complete system, some lenses and an interference filter are incorporated as well . The light is de tected with a cooled Burle C31034 photomultiplier. For details see ref. [10]. The actual design is sufficient for our needs as was also shown by calculations with OOTRAN [13]. The beam line contains two interaction chambers, where the polarization is induced. This will be discussed in the next section. Each chamber can be connected to the 3He'He dilution refrigerator . To reduce the amount of heat radiation from the beam line and to obtain the shortest possible distance between the interaction zone and the implantation zone, a very short side access (92 mm at room temperature) has been constructed [10]. The side access still allows an implantation temperature of about 15 mK . This is established by a s:crics of diaphragms which reduce substantially the amount of heat radiation onto the cooled sample. The reduction of the distance between interaction region and implantation site is necessary for the most important factor of loss of intensity in the transport being the beam divergence after the interaction, due to the low energy of the beam . Since plans are being made to build a postaccelerator, this problem might be solved in the future . 2.3. The refrigerator A dilution refrigerator is placed at the end of the beam line. Samples can be top loaded during its operation. At the sample site a static magnetic field up to 1.5 T can be applied with a superconducting magnet . An NMR coil is incorporated as well. Several side accesses have already been designed to allow continuous implantation in a cooled sample. The usual side access, with a length at room temperature of t 1 m and which enables to reach implantation temperatures < 10 mK had to replaced by the very short side access, already mentioned before. Further, solid state detectors, i.e . 1 mm thick Si-PIPSparticle detectors are mounted, which can operate at 4 K without window between source and detector. These detectors will be used to measure

3.1 . Generation of the atomic orientation 3.1 .1. General aspects In the in-beam methods that will be discussed here, the atomic orientation is generated by an anisotropic Coulomb interaction of the beam ions with a surface which is tilted with respect to the beam axis. This tilting of the surface introduces a noncylindrical symmetry and is as such responsible for the generation of mainly a first order orientation of the electronic shell . A net angular momentum (L) = n x v is created, with n pointing parallel to the normal on the surface and v the beam velocity. Several authors have already tried to explain this effect and a first explanation was given by Schr6der and Kupfer [14] who introduced the socalled e--density gradient model, which describes the orientation as originating from the interaction of the beam ions with the e--density gradient at the solid's surface. More specifically for the GSS geometry, Winter and coworkers [15] adopted current charge exchange models [16] to explain the observed orientation. They can explain the observed charge state distributions and polarizations fairly well, taking into account resonant electron transfer between the atom and the metal of which the Fermi sphere is Galilean-transformed. They show that long range interactions are very important for the production of the observed effects . This is an important difference compared to the TF geometry in which the velocity component, perpendicular to the surface, is generally larger (v 1 a v sin gyp, in fig. 1). A detailed study, however, is not available for the TF case although it would be very interesting to see whether these long range interactions play a role in the TF geometry as well. With their model, also the energy dependence of the atomic orientation - in the case of N - could be explained, the result being that the polarization increased with the beam velocity up to a saturation value and the maximum of the nuclear polarization was reached at an energy of 50 keV [17] . 3.1.2 The tilted foil setup The TF method consists in directing a collimated beam through a foil which is tilted with respect to the beam axis. A major problem in our situation was that to transmit in a reasonable way a 50 keV beam of mass III. CONTRIBUTED PAPERS

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=15 through a carbon foil, the thickness of this foil is limited to f150 Â which is about 4 ,,g/cm2. Self supporting C"foils of 2-3 pg/cm2 can be manufactured now and are mounted on stainless steel frames. The self supporting area is 20-50 mm2. The foils stay intact for several hours in beams of 20-50 nA . The fragility of the foils is a major obstruction to use more intense beams and this is a drawback for the optical measurements (see further) . When polarizing radioactive beams, however, the intensities are smaller and the problem of fragility should be less important. The foils are subsequently mounted in a rotatable foil holder for normalisation purposes . In the actual construction including the small side access, the distance between the foil and the implantation zone is t 170 mm. Transmission tests showed that a transport efficiency of a few percent is possible in this configuration. Finally a diaphragm in front of the foil should enable to avoid as much as possible instrumental asymmetries. 3.1.3. The grazing surface scattering setup

Forpractical reasons the tilt angle in the TF geometry is limited to 60-70 degrees, although the first experiments in this regard showed that the degree of atomic polarization increases with the the sine of the tilt angle [18], favouring large tilt angles. As a consequence of this result, Andrd proposed the scattering technique as an extension to larger angles and indeed larger polarizations were observed [3]. The method consists in the specular reflection of the beam at graz-

ing incidence on very flat surfaces. The technique is slightly more complicated than the TF method and more extreme experimental conditions are required to obtain the best results . The considerable gain in polarization, however, makes it worthwile to try the effort. A schematic drawing of the interaction region in this geometry is shown in fig. 4, together with the angular distribution of scattered t°N' ions. The latter was measured with a 1 mm 0 needle at 60 mm behind the crystal. The measurements with the ECR source were performed at 40 kV . The angular distribution can thus still be somewhat enhanced going to 50 kV. The scattering crystal is attached to a high precision xy-z positioner and the scattering angle can be adjusted with a precision of 0.25°, which is satisfying for our purposes . The first experiments were performed with a Si(100) crystal [10] but with a new Si(III) crystal having abetter surface granularity (< 10 nn), higher polarizations were obtained (cf. next section) . The positioner is mounted in an UIiV chamber and during the experiments the on-line vacuum was 5 x 10 -9 mbar. The improvement of the differential pumping should still lower this value since off-line a vacuum of 3 x 10 -to mbar is easily obtained . 3.2. Detection of atomic orientation

Thie atomic orientation is measured via the Stokes parameters of the fluorescence light of the excited atoms after the interaction. The Stokes parameters S, C, M, I define in a complete way the state of a light

15

io

FWHM 50

-5

Ilou)

Fig. 4. Drawing of the interaction zone for the GSS geometry; (1) diaphragm, (2) deflection plates, (3) two-dimensional wire grid, (4) beam "skimmer", (5) scattering crystal, (6) 1 mm 0 needle. The angular distribution of the scattered ions, measured with the needle, is also indicated.

W. Vanderpoorten et a/. / Polarized nitrogen beams at LISOL

beam . The parameter S is sensitive to the amount of circular polarization in the light beam and hence to the amount of first order polarization of the light source [19]. An optical detection system has been designed for this reason (fig. 4). It consists essentially of a quarter wave plate and a linear analyzer. The intensity of the light, measured as a function of the positions of these two optical instruments, can be written as (cf. fig. 3 for the definition of the angles) [20] : M

sin 2a) Ma, 0)= 12 II+~ 2 cos2a+Ç 2 J S sin(2a - 29)

+2 + î[(M cos 2a - C sin 2a) cos 4ß

(1) +(M sin 2a + C cos 2a) sin 4ß]. The second term contains the parameter S and has a periodicity 20. This means that circularly polarized light, when measured with this system, will give rise to a ar-periodic signal as a function of the position of the .1/4-plate. A -rr/2 periodicity is significant for linear polarization. The relative Stokes parameter, S/1, can be easily measured as I(a )-I(o,+ ) S/II(o,_)+1(0"), where I(tr *) are the intensities with positive and negative helicity. When the detection geometry is as described in fig. 3 (x is beam axis), S/I is proportional to the atomic orientation (1-poll r p$, as defined in ref. [21]), neglecting a small alignment contribution. To select the proper optical transition no monochromator

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was used for intensity reasons. Narrow band interference filters were used instead. As a consequence, the measured signal is an averageover the hyperfine multiplets. The first experiments were done with a t° N' beam from a FEBIAD source. Recently the ECR source has become operational and this source has been used from then on . The circular polarization of the NII2s22p3p 3 D-2s22p3d 3F° and of the NII2s22p3s 3P2s22p3p3D optical transitions was measured for both methods with filters, having a FWHM of 10 run, centered respectively at 500 nm and 570 nm . The light was detected with a cooled RCA-31034 photomultiplier operating in single photon counting mode. After background subtraction, the light intensity was normalized in br " h cases to the beam current measured immedia.ely behind the interaction zone . 3.2.1. The tilted foil result

Fig. 5 shows the intensity variation of the 500 nm fluorescence light after the interaction of a 50 keV r4N beam with a 2 wg/cm2 C-foil . From these data a value of S/1=10 .2(1.9) is extracted. This result has to be compared with previous results polarizing a 15 N beam at 300 keV [22] with a single foil and it is seen that our value is in good agreement. This indicates that the optimum is practically reached unless more foils can be used but this is mainly an energy problem . The fragility of the foils prohibited to use more than 50 nA of primary beam, resulting in a relatively small signal to background ratio (= 1:1). This explains part of the large error bars. The other part was due to an

Fig. 5. Intensity as a function of quarterwave plate position for theTF case, as measured with the optical detection device of fig. 3. A value S/I=10.2(1 .9) is deduced. III. CONTRIBUTED PAPERS

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inaccurate normalization procedure at that time, a problem being solved at the moment. The use of 90% transparent wire grids to mount the foils should increase their strength . Tests to enhance the signal/noise ratio this way are now being carried out. 3.22. Thegrazing surface scattering result Fig. 6 shows the result of the most recent scattering experiments. The three different curves were measured at three different positions (cf. fig . 4) . Position C is relevant for the beam which is implanted, since at positions A and B there is a large contribution to the light intensity from atoms which have made violent collisions with a.o. adsorbates on the surface and which are consequently scattered at higher angles [23] . The degree of polarization of these atoms is in general smaller compared to the atoms which are specularly scattered . The value of 20% which has been obtained is already a good result since the measurements were done without cleaning the surface. This value is then to be compared with other measurements on "dirty" surfaces, obtaining, about the same value [24] . Cleaning of the surface enhances considerably the amount of atomic orientation after GSS. S/1 values of 63% are reported for the 567 nm line after sputtering of the surface. For cleaning purposes, a heating coil is installed under the crystal, to bake out the crystal in situ . A procedure of sputtering and heating is known to give good results. These tests will be done in the near future . 4. The nuclear polarization The final goal of the development is the production of a nuclear polarized beam . The nuclear polarization is established by the hyperfine interaction . In low external field, the atomic angular momentum J and the nuclear spin I couple to a total angular momentum F-1 +J. F being conserved during the motion of the atom, I and J process around F and this induces a polarization of I if J has been polarized before. The transfer efficiency depend on the angular momenta involved and the amount of nuclear orientation, P, = (1Z)/I is proportional to the induced atomic orientation and one can write [21] Pi = (1+1)131--C

Pô'

in terms of the tensorial components of the atomic and nuclear density matrices . The factor can be calculated for a given (LS)J term and a given nuclear spin [25]. Table 1 lists some of these factors . As can be seen is the transfer process most efficient when I= J and no transfer occurs from a J= 0, atomic S-state.

Fig. 6. Intensity as a function of quarter wave plate position for the GSS geometry, measured at three different positions relative to the crystal ;cf. also fig. 4. The transfer of polarization goes via the stable and metastable terms of the atom. In the case of nitrogen, at low excitation energies, the relevant terms are the 2p 3 °S, ZP and ZD terms. As can be seen from table 1, transfer coefficients of ±0.3 can be expected. The

W. Vanderpoorten et al. / Polarized nitrogen beams at LISOL Table 1 Time-integrated transfer coefficients for transfer of atomic polarization to the nucleus [25) (LS)

xS 'P 'D 2P 2D 3P 3D 4P

0 0.363 0.226 0.306 0.217 0.170 0 .198 0 .080

0 0.5 0.353 0.356 0 .335 0 .252 0 .299 0.152

transfer is slightly less efficient for t3N (1=1/2) compared to t4 N (I= 1). This means that at present we are, in principle, capable of producing beams with Pt = 6-7% with the GSS method, which could be enhanced after a proper surface treatment. Tests to detect this polarization are now being performed . The potentiality to use the present technique in material characterization studies is then to be compared with other techniques . The degree of polarization, especially in the GSS case, should be high enough to perform reliable NMR/ON measurements. It can be remarked in this regard that NMR measurements have already been reported using degrees of polarization of the order of 1% [26] . Further, a big advantage is, as already stated, the large variety of possible probe nuclei . This could stimulate systematic studies of materials, doped with impurities as a function of valence number . This way, the present technique could be a nice enrichment in this field of research . Acknowledgments We acknowledge motivating discussions with H. Winter and we are very grateful for the Si(111) crystal he provided us with. The evaporator assistance of P. Demaret is also very much appreciated. Finally we want to thank the separator people for the help in preparing the beams. This work is financially supported by the Nationaal Fonds voor Wetenschappelijk

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Onderzoek and the interuniv::,titair Instituut voor Kernwetenschappen. References [1] D. Vandeplassche et al ., Nucl .Instr. and Meth. 186 (1981) 211 . Nuclear Orientation and Nuclei far from Stability, eds. B.1 . Deutch and L. Vanneste U .C. Baltzer A.G ., Basel, Switzerland, 1985). [2) H .G . Berry et al . Phys. Rev. Lett. 32 (1974) 751 . [3] H.J . Andrä, Phys . Lett. 54A 1975) 315 . [4] M . Huyse et al., Nucl . Instr . and Meth. B26 1987) 105. [5) K. Deneffe et al., Nucl. Instr. and Meth . B26 (1987) 399 . [6] a .o. S. Lucas et al., Nucl. Instr. and Meth . B50 (1990) 401 . [7) M. Piette et al., J. Mater. Sci . Eng . B 2 (1989) 189. [8] R .E . Mc Donald and T.K . Mc Nab, Phys . Lett. 63A (1977) 177. T.K . Mc Nab and R.E. Mc Donald, Phys . Rev. B13 (1976) 34 . [9) E . Mger et al., Phys. Lett. A123 (1987) 39. [10] W. Vanderpoorten et al ., Vacuum 42 (1991) 789 . J. Wouters et al ., Nucl . Instr. and Meth. B61 (1991) 348. [11] P. Decrock et al., Rev . Sci . Instrum. 61 (1990) 279 . [12] M. Arnould et al., Nucl . Instr. and Meth. B40/41 (1989) 489 . [13] G . Vancraeynest, Master's Degree Thesis, K.U. Leuven, 1989, not published . [14) H . Schr8der and E. Kupfer, Z. Phys. A279 (1976) 13. [15] H.J . Andrä et al ., Nucl. Instr. and Meth. B9 (1986) 572 . [16] J.N .M. Van Wunnik et al., Surf. Sci. 108 (1981) 253 . [17] H. Winter, private communication . [18] K.G .Berry, L.J . Curtis and R.M. Schektman, Phys. Rev. Lett. 34 (1975) 509 . [19] M. Born and W. Wolf, Principles of Optics (Pergamon, Oxford, 1970). [20] H .G. Berry, G . Gabrielse and A.E. Livingston, Appl. Opt . 16 (1977) 3200. [21] H . Winter, R. Zimny, Coherence in Atomic Collision Physics, ed . H.J. Beyer (Plenum, New York, 1988). [22) B.I . Deutch, Hyperfine Interactions 24-26 (1985) 251 . [23] C . Marsch and H . Winter, Z. Phys. D18 (1991) 25 . [24] H. Winter, P.H. Heckman and B. Raith, Hyperfine Interactions 8 (1980) 261 . [25] H. Winter, Habilitationsschrift, Westfdlischen WilhelmsUniversität, Miinster (1987). [26) W .F. Rogers et al., Phys. Lett. B177 (1986) 293.

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