Important micro- and macroscopic details of heavy-ion reactions may be explicitly determined when nuclear spin aligned (polarized) targets are used. For deformed nuclei, the orientation of the symmetry axis of the nuclear density distribution is determined by the nuclear spin orientation. Polarized targets would thus allow experiments to be performed as a function of the orientation of the symmetry axis of the nuclear density distribution. A polarized target of /sup 151/ /sup 153/Eu is being developed at Oak Ridge and is based on laser depopulation optical pumping. A spatially defined target is provided by a supersonic gas jet and consists of Eu atoms seeded into an inert carrier gas. Detailed time-dependent optical-pumping calculations predict approx. = 90% nuclear spin polarization in a Eu target with an expected thickness in excess of 10/sup 15/ atoms/cm/sup 2/. We present some of the effects that will be observable in heavy-ion reactions when deformed polarized targets are used

Beene, J.R.

Bemis, C.E. Jr.

Erb, K.A.

Ford, J.L.C. Jr.

Shapira, D.

Shivakumar, B.

UNT Digital Library

CONF-62042 8—3DE82 013434DEVELOPMENT OF A LASER OPTICALLY PUMPED POLARIZEDTARGET FOR USE IN HEAVY-ION PHYSICSB. SHIVAKUMAR, J. R. BEENE, C. E. BEMIS, JR.,K. A. ERB, J. L. C. FORD, JR.^ and D. SHAPIRAPhysics Division, Oak Ridge National Laboratory,Oak Ridge, TN 37830Abstract Important micro- and macroscopic details ofheavy-ion reaction? may be explicitly determined whennuclear spin aligned (polarized) targets are used. Fordeformed nuclei, the orientation of the symmetry axisof the nuclear density distribution is determined bythe nuclear spin orientation. Polarized targets wouldthus allow experiments to be performed as a function ofthe orientation of the symmetry axis of the nucleardensity distribution. A polarized target of 151>153Euis being developed at Oak Ridge and is based on laserdepopulation optical pumping. A spatially definedtarget is provided by a supersonic gas jet and consistsof Eu atoms "seeded" into an inert carrier gas.Detailed time-dependent optical-pumping calculationspredict ra 90% nuclear spin polarisation in a Eu targetwith an expected thickness in excess of 10 l s atoms/cm2.We present some of the effects that will be observablein heavy-ion reactions when deformed polarized targetsare used.In deformed nuclei, the orientation of the nuclear groundstate spin generally coincides with the orientation of thesymmetry axis of the nuclear density distribution (i.e.K=I). For projectile energies near the coulomb barrier,a polarized target permits us to change the amount of over-lap of the target and projectile wavefunctions, keeping theGraduate student on assignment from Yale University.By acceptance oi thit article, thepublisher or recipient acknowledgesthe U.5. Government's right toretain a nonexclusive, royalty-freelicense in and to any copyrightcovering the article.total energy in the center-of-mass system fixed. Theeffects of target deformation can be quite pronounced, asinferred from the magnitude of measured fusion yields, ashave been demonstrated in the sub-coulomb fusion of 160 withl^-is^m (refs. 1,2) and for the fusion-fission reactionsof 32S with 1'*'4»15l*gm (ref. 3). Since these experimentswere performed with unpolarized targets, the measured crosssections correspond to an average over all possible orien-tations of the deformed target nucleus, so that such detailsas the strong orientation dependence of the fusion crosssections were lost. A polarized target is needed toinvestigate such effects.A polarized Eu target is currently being developed(ref. 4) at Oak Ridge for use in the heavy-ion physicsresearch program. Although polarized targets of the moreconventional variety have been used in at least two casesfor reaction studies with charged particle projectiles(refs. 5,6), the rigors of such bombardment preclude theiruse with heavy-ion projectiles. The Oak Ridge polarizedtarget assembly (fig. 1) is based on the supersonic gas jettarget recently developed at ORNL (ref. 7) for use with the25 MV tandem electrostatic accelerator. A convergent-divergent nozzle exhausts into a differentially pumpedtarget chamber producing a spatially defined supersonic jet.This windowless jet provides a target in which the effectsof straggling and energy loss are minimized, thus permittinghigh resolution measurements with heavy-ion beams. When theapparatus is operating as a polarized target, the jet willbe an inert carrier gas, most probably N2 or He, seeded withatoms of 151Eu (1=5/2) and/or 153Eu (1=5/2). Target thick-nesses of europium in excess of 10*5 atoms/cm2 are expected.ARtfflTlCWJ OPTICS *iCCElFRi'OOBf £••' L i ! ! , U *p- ; .; '. a-."-FIGURE 1. Polarized target apparatus. The details oflaser diagnostics, thiee axis degaussing and "keeper"field coils and nuclear reaction detection devices arenot shown»Nuclear reaction products can be observed using a high reso-lution magnetic spectrometer. Because the target is formedfrom a jet of essentially free atoms, very large beamcurrents can, in principle, be tolerated without major dele-terious effects. However, beam induced depolarizationmechanisms may pose practical limitations for its use as apolarized target.The europium is aligned via depopulation opticalpumping using the LINUP (Laser Induced NUclear Polarization)technique. This method has been used previously to studyspontaneous fission isomerism (ref. 8). Europium is a par-ticularly favorable case for optical pumping (refs. 9, 10)as the atomic ground state, ^7/2, n a s near sphericalsymmetry. This reduces the relaxation, and hence, a loss cfalignment due to collisions. Other attractive features ofEu are the presence of only a few low lying excited states,and the small hyperfine splitting of the ground state. Ourintent is to pump the entire hyperfine structure multipletof the 8S7/ 2 ~ y 8?5/2 resonance transition in Eu at466.3 nm ( T ~ 5 nsec) with linearly (it) polarized light.The UV output from a commercial argon-ion laser is used topump a coumarin dye in a broadband dye laser (linewidth=30 GHz). The laser is frequency stabilized, by comparingits frequency with the 466.3 nm transitions in a Eu hollow-cathode discharge lamp, using the optogalvanic effect toprovide the necessary error signals (ref. 11).Extensive optical pumping calculations have been donefor the Eu system using the computer program RATES (ref.12). The complete set of coupled differential equations aresolved to obtain the detailed time evolution of the coupled(F = I + J) density matrix. The rate equations include theeffects due to both ground and excited state relaxation,laser linewidth and a realistic atomic absorption profile.The nuclear density matrix can be projected from the coupledmatrix at any time. Although the effects of the beam plasmadue to the passage of the heavy-ion beam remain unaddressed,ground state relaxation effects are expected to be unimpor-tant in the near collisiociless domain expected in the. gasjet. The time evolution of the nuclear orientation in thetarget is shown in figs. 2 and 3, for optical pumping of they P5/2 and y 8P7/2 levels with linearly (IT) and circularly (o)Eul PUMPINGI0.5 = .SATURATION 6 a0 *- J I j I<C '5 20PUMP TIWC (US21FIGURE 2. Nuclear orientation parameters for the euro-pium ground state as calculated by RATES (ref. 12), forthe transition 8Sy/2 -y 8P5/2- Te i s t h e excited staterelaxation rate.Eul S7/—*y 8P. PUMPING'2 >z( 00 500 51 1/ ^ i " 8 > I O f s " '- • » _ 7r P U M P1 11 1^ — ' — " ~ r4 T'JMP•—1 1FIGURE 3. Same as Fig. 2, for the transition8 S 7 / 2 " y 8Ppolarized laser light, to illustrate the effects. Theorientation parameters, 3% and B 4 ) have been projected fromthe alignment tensors of the coupled nuclear-electronicsystem (done by RATES). The orientation parameters arerelated to the populations W(Mj) of the nuclear magneticsubstates Mj by (ref. 13):BK(I) = I W(MI)(-1)I~MI (2I+1)1/2 <I I MJ-MJ. K0>These parameters are the Q=0 components of the normalizedstatistical tensors or state multipoles, BJ^Q(I), commonlyused to specify alignment in the general case. We havechosen the coordinate system and pumping geometry such thatonly the Q=0 components are nonvanishing. One notes fromfigs. 2 and 3 that nearly ideal alignment conditions arereached in about 15 us for two of the four cases. Figure 4shows the associated expected steady state populations ofthe magnetic substates in the F=6 ground state hyperfinelevel, the absolute population of which has also beenindicated. The quantization axis, chosen in the "photonframe," is the polarization vector (E) in the case of opti-cal pumping with linearly (TI) polarized light and the propa-gation direction of the laser in the case of optical pumpingwith circularly (o) polarized light. The use of linearly(IT) polarized light to pump the 8 S 7 / 2 ~ v 8p5/2 transitioncan achieve near ideal alignment conditions. By rotatingthe polarizi 'ion (E) vector we can rotate the axis ofnuclear spin alignment in the horizontal nuclear reactionplane to any desired orientation. It is clear that the useof circularly (a) polarized light on the 8Sj/2 ~ y 8?7/2transition also produces a target with an equally largeLINEAR (TT)f.- 00.60.40.2 -1 1 1 1 "P | F = 6 )= ( .O5/ ,0 H f h - 1 — 1 - - 1 — l - - t - l l0(2ID 0.08„= 004 -"i ! i—i—rP(F=6) = 0.29111 II- 6 0MFCIRCULAR 1(7*)"i—i—i—i—i—r0.604020 hi--t--t--l--I 208~i—i—i—rP(F = 6 ) = 1 . 0o 1—t— i— 4—i-—t— *--! •!--6 0 +6FIGURE 4. Final populations of the magnetic substatesof the F=6 hyperfine level in the europium ground stateafter laser excitation by polarized photons. The abso-lute population of a hyperfine substate in a randomlyoriented europium system is P(M) = 1/48 = 0.02083. Thefour figures pertain to excitations with linear (IT) andcircular (o) polarized light through the two excitedlevels 8P5/2> 7/2 respectively. The 8S7/ 2 - y 8Ps/2transition with linear (TI) pumping is selected for thedevelopment of the polarized Eu target.alignment. In this mode, however, with a fixed opticalpumping geometry (as shown in fig. 1), we do not have theability to rotate the axis of alignment of the targetnuclear spin. Including the effects of radiationimprisonment, we believe a target nuclear spin polarizationof ~ 90% can be achieved.Elastic- and inelastic-scattering experiments will beextremely sensitive to target orientation. Dynamic defor-mation effects in nuclear reactions (ref. 14) have not beenisolated satisfactorily in past experiments, and our abilityto control the orientation of the target gives us thepossibility to disentangle the effects of dynamic defor-macion from those of static deformation. Static calcula-tions of fusion reactions, such as l60 + 15l*Sm, haveestimated the fusion cross section to vary by a factor ofabout 3000 for a 90° rotation of the target alignment axis(rer. 2). Such dramatic effects can be studied with ourapparatus. The role of angular momentum in fusion-fissionreactions also needs systematic investigation. By changingthe orientation of a deformed target nucleus, we can, inprinciple, vary the maximum amount of angular momentum goinginto a fusion reaction without changing the excitationenergy of the fused system, and as a consequence, we canevaluate its effect on the subsequent fission. We may learnmore about the mechanisms of heavy-ion transfer reactionsby studying such processes as a function of targetorientation. Other studies could probe any possible orie~_-tation dependence of optical model parameters.In conclusion, we would like to emphasize that thisscheme to polarize targets is not specific to europiumalone, and we intend to use this assembly to producepolarized targets of other species that are as equallyamenable to optical pumping.This research was sponsored by the Division of HighEnergy and Nuclear Physics, U.S. Department of Energy, undercontract W-7405-eng-26 with the Union Carbide Corporation.Partial support for B. Shivakumar was provided undercontract DE-ACO2-76ERO3O74 with Yale University.REFERENCES1. (a) R. G. Stokstad, Y. Eisen, S. Kaplanis, D. Pelte, U.Smilansky, and I. Tserruya, Phys. Rev. Lett. 41, 465(1978); (b) id. Phys. Rev. C21, 2427 (1980);~Tc) R. G.Stokstad, W. Reisdorf, K. D. Hildenbrand, J. U. Kratz,G. Wirth, R. Lucas, and J. Poitiou, Z. Phys. A295, 269(1980).2. R. G. Stokstad and E. E. Gross, Phys. Rev. C 23_, 281(1981).3. B. B. Back, R. R. Betts, W. Heming, K. L. Wolf, A. C.Mignerey, and J. M. Lebowitz, Phys. Rev. Lett. _45, 1230(1980).4. J. R. Beene, C. E. Bemis, Jr., J. L. C. Ford, Jr.,K. A. Erb, and D. Shapira, Phys. Div Prog. Rep. ORNL5787, p. 132 (1981).5. T. R. Fisher, S. L. Tabor, and B. A. Watson, Phys. Rev.Lett. 27_, 1078 (1971).6. D. R. Parks, S. L. Tabor, B. B. Triplett, H. T. King,T. R. Fisher, B. A. Watson, Phys. Rev. Lett. 2S_, 1264(1972).7. (a) J. L. C. Ford, Jr., J. Gomez del Campo, J. W.Johnson, D. Shapira, J. E. Wiedley, S. T. Thornton, andR. L. Parks, Phys. Div. Prog. Rep. ORNL 5498, p. 45(1979); (b) J. L. C. Ford, Jr., R. Novotny, D. Shapira,R. L. Parks, and S. T. Thornton, Phys. Div. Prog. Rep.ORNL 5787, p. 128 (1981) and Bull. Am. Phys. Soc. lb_,595 (1981).8. C. E. Bemis, Jr., J. R. Beene, J. P. Young, and S. D.Kramer, Phys. Rev. Lett. 43, 1854 (1979).9. A. Sahm, J. Kowalski, and G. zu Putlitz, Z. Phys. A281,317 (1977).10. B. Bohmer, Ph.D. Thesis, Univ. of Heidelberg,unpublished (1972).11. D. S. King, P. K. Schenck, K. C. Smyth, and J. C.Travis, Appl. Optics 16_, 2617 (1977).12. J. R. Beene, "RATES:' A system of computer programs toperform optical pumping calculations, unpublished(1980).13. D. H. Brink and G. R. Satchler, "Angular Momentum,"Oxford (1962).14. A. S. Jensen and C. Y. Wong, Nucl. Phys. A171, 1 (1971).

Development of a laser optically pumped polarized target for use in heavy-ion physics. [/sup 151/ /sup 153/Eu]

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