The development of a laser optically pumped nuclear spin aligned target of /sup 151/ /sup 153/Eu is outlined. The current status of this project, together with the unique macroscopic heavy-ion reaction physics may be addressed using this target, is given. 14 references

Beene, J.R..

Bemis, C.E. Jr.

Ford, J.L.C. Jr.

Shapira, D.

Shivakumar, B.

UNT Digital Library

DEVELOPMENT OF OPTICALLY PUMPED POLARIZED JET TARGETSFOR USE IN HEAVY-ION REACTION STUDIES*C. E. Bemis, Jr., J. R. Beetle, J.L.C. Ford, Jr.,tD. Shapira, and B. Shivakumar§Physics Division,Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831C O H F - 8 4 0 5 1 1 3 - - 3DEG5 0018G3— Invited Paper —Workshop on Polarized Targets in Storage Rings,Argonne, Illinois, May 17-18, 1984By acceptance oi this article, thepublisher or recipient acknowledgesthe U.S. Government's right toretain a nonexclusive, royalty-freelicense in and to any copyrightcovering the article.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.*Research sponsored by the Division of High Energy and Nuclear Physics, U.S.Department of Energy, under contract DE-AC05-84OR21400 with Martin MariettaEnergy Systems, Inc.DISTRIBIJTIOK OF THIS M O N O ? IS WUWIEDDEVELOPMENT OF OPTICALLY PUMPED POLARIZED JET TARGETSFOR USE IN HEAVY-ION REACTION STUDIES*C. E. Bemis, Jr., J. R. Beene, J.L.C. Ford, Jr.,tD. Shapira, and B. Shlvakumar§Physics Division,Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831The development of a laser optically pumped nuclear spin alignedtarget of 1 5 1»^ 5 3Eu is outlined. The current status of this project,together with the unique "macroscopic" l;eavy-ion reaction physicsthat may be addressed using this target, is given.I. INTRODUCTIONPolarized beams and/or targets can provide sensitive tests of heavy-ionreaction mechanisms, and as demonstrated with polarized ^Li and 7Li beams,1'2the polarization observables are dominated by quadrupole effects. The mostdramatic polarization effects involve entrance-channel phenomena which areconnected to the change in spatial overlap between target and projectile as afunction of alignment for a given impact parameter or trajectory. Althoughnot performed with polarized targets/projectiles, subbarrier and near-barrierheavy-ion reaction studies have clearly demonstrated the influence of targetstatic quadrupole deformation on fusion cross sections3 »** a ^ on fusion-fission cross sections.5 These particular experimental examples (Refs. 3-5)used near-spherical heavy-ion projectiles (160, 3 2 S , 1*°Ar) and targets of theSm isotopes, which range from spherical 141*Sm to deformed 15**Sm, to investi-gate the effects. Near-barrier fusion reactions depend upon the subtle inter-play between the repulsive point Coulomb interaction and the attractivenuclear interactions which, in turn, depend on the spatial jverlap of thetarget and projectile nuclear density distributions. For fixed projectileenergy, thus fixed distance of closest approach from Coulomb interactions, thespatial overlap, averaged over all target orientations, Is largest for themore deformed target. This simple classical idea explains to first order thelarger fusion cross sections observed for the more deformed target (151|Sm), asillustrated in Fig. 1. Stokstad and Gross6 have performed an analysis of the1 60 + Sm results and have estimated the large increase in sensitivity thatwould be realized if the *60 + 154gm experiment could be performed at fixedenergy, but as a function of target orientation. This dramatic example, shownin Fig. 2, illustrates one of the advantages in "macroscopic" heavy-ion reac-tion studies if polarized targets could be used. Subbarrier fusion crosssection calculations for the ^ 0 + -^5JJO reaction, which is similar to the16Q + 15<*sm reaction, have recently been performed by Jacobs and Smilansky7 asa function of polarization. The effects predicted by Jacobs and Smilanskyare smaller than those predicted by Stokstad and Gross, although approxima-tions were made in both calculations. Further theoretical work is required.Effects, such as those illustrated in Fig. 2, have prompted us to develop apolarized target for use in the heavy-ion physics program at ORNL. Designconsiderations, laser development, mechanical construction, and implementa-tion, together with some of the additional physics addressable when polarizedtargets are vsed, are given below.II. THE ORNL SUPERSONIC GAS-JET TARGETThe polarized target developmental efforts are based on a windowlesssupersonic gas-jet target which had previously been constructed to facilitatehigh resolution heavy-ion physics experiments with gaseous targets.8*9 Thistarget assembly has been completed and installed as a replaceable unit of thescattering chamber of the Enge split-pole magnetic spectrometer located onone of the tandem accelerator beam lines at the Holifield Heavy Ion ResearchFacility (HHIRF) at ORNL. The target incorporates four stages of differen-tial pumping which allows tandem beam line pressures to be maintained below3 x 10~8 torr under experimental conditions, while the pressure in the targetregion is ~50 torr. For gaseous targets (N2, Ar, Xe, etc.), target thicknessesranging from fractions of a ug/cm2 to ~30 ug/cm2 have been achieved in highresolution heavy-ion reaction experiments at HHIRF.10 Sliding seals on theinner and outer sections allow the target exit aperture and the Enge split-pole spectrometer entrance slits to be positioned within the angular range0° = 9 < 35°. Schematic views of this gas-jet target are shown in Figs. 3and 4. Additional details may be found in Refs. 8 and 9.III. THE ORNL OPTICALLY PUMPED POLARIZED TARGETBased on the supersonic gas-jet target described briefly above in Sec. II,plans to implement a polarized version at ORNL were developed in 1982. Onlytwo experiments have ever been performed with polarized targets,11 other thanfor hydrogen, in low-energy nuclear and heavy-ion physics with charged—particle beams. Charged-particle beams, especially heavy-ion beams, havelarge stopping powers in target matter, and the relatively large energydeposit poses severe limitations for polarized targets oriented by cryogenicmethods. As an illustration, 3He-4He dilution refrigerators have typical"refrigeration powers" of ~1 (iW at 7 mK; this is equivalent "in heating power"to the passage of ~70-pA beam of 100-MeV ^ 0 ions through a self-supported10-ug/cm2 rare-earth target. Polarized gas-jet targets, with nuclear orienta-tion achieved dynamically via laser optical pumping, overcome the beam-heatingproblems associated with targets oriented by low-temperature methods.Europium was chosen as the target species for development of the opti-cally pumped polarized target and is a particularly favorable case for opticalpumping12 since the atomic ground state, 8S7/2» has near-spherical symmetry.This tends to reduce the relaxation and, hence, a loss of alignment due tocollisions. Other attractive features of Eu are the presence of only a fewlow-lying excited atomic states and the small hyperfine splitting of theground state. Our intent is to pump the entire hyperfine structure multipletof the 8S7/2~y 8p5/2 resonance transition in Eu at ^66.3 nm (T ~ 5 nsec) withlinearly (u) polarized light. Nuclear orientation is achieved via "depopula-tion pumping." We choose the quantization axis, upon which the orientationis measured, to be in "photon frame" and is the polarization vector (E) in thecase of optical pumping with linearly polarized (TI) light. In our experimen-tal geometry (see Figs. 5 and 6), the polarization vector * lies in the hori-zontal nuclear reaction plane. Rotation of the polarization £ vector in thisplane, and hence the quantization axis for the nuclear spin alignment, iseasily accomplished using a half-wave polarization rotator appropriate for466 nm photons.The nuclear spin alignment is conveniently described by orientationparameters,^ Bj^(I), which are related to the the populations W(Mj) of thenuclear magnetic substates by:H'l WCh^X-l)1""1 (21 + 1) 1 / 2 <I I MX - M!|K0> .MIThese parameters are the Q = 0 components of the normalized statistical ten-sors or state multipoles, B K Q ( I ) , commonly used to specify alignment in thegeneral case. In our coordinate system and pumping geometry, only the Q = 0components are nonvanishing. For both stable isotopes of Eu, l^Eu andthe nuclear spin, I, is 5/2; only the even rank orientation parameters B2 andB4 are needed to completely describe the alignment (K^x =» 21), since the sys-tem is axially symmetric and invariant to the reversal of the quantizationaxis.Extensive optical pumping calculations have been done for the Eu systemusing the computer program PATES.11* The complete set of coupled differentialequations i~ solved to obtain the detailed time evolution of the coupled(? = t + 5) density matrix. The rate equations include the effects due toboth ground— and excited-state relaxation, laser linewidth, and a realisticatomic absorption profile. The nuclear density matrix can be projected fromthe coupled matrix at any time. Although the effects of the beam plasma dueto the passage of the heavy-ion beam remain unaddressed, ground-state relaxa-tion effects are expected to be unimportant in the near-collisionlesi domainexpected in the gas jet. The time evolution of the nuclear orientation in thetarget is shown in Fig. 7, for optical pumping of the y "P5/2 level with bothlinearly (it) and circularly (a) polarized laser light, to illustrate theeffects. The orientation parameters, B2 and Bî , have been projected from thealignment tensors of the coupled nuclear-electronic system (done by RATES).One notes from Fig. 7 that nearly ideal alignment conditions are reached inabout 15 us for optical pumping of the 8S7/2 "* v 8p5/2 transition withlinearly polarized (it) laser photons for our choice of pumping conditions.Figure 8 shows the associated expected steady state populations of the mag-netic, substates in the F » 6 ground-state hyperfine level, the absolute popu-lation of which has also been indicated. Again, the quantization axis ischosen in the "photon frame," i.e., the polarization vector (E) in the case ofoptical pumping with linearly (it) polarized light and the propagation direc-tion (k) of the laser in the case of optical pumping with circularly (a)polarized light.Considerable laser development to efficiently pump the relatively broadhyperfine structure manifold for both isotopes (Af «* 5 GHz) with one broadbandCW laser has been completed. These developments include tailoring the laseroutput bandwidth to match the approximately Gaussian absorption profile forthe transition without "holes" in the output spectrum. A magneto-opto gal-vanic lock and stabilization technique is used to provide laser stability atthe center of the absorption profile.Most polarized target hardware has been designed and constructed, and ourdevelopmental efforts are currently centered on the construction of the high-teirperature components of the gas-jet target that are required for vapor Eu.REFERENCES*Research sponsored by the Division of High Energy and Nuclear Physics, U.S.Department of Energy, under contract DE-ACC5-84OR21400 with Martin MariettaEnergy Systems, Inc.tDeceased.§Graduate student on assignment from Yale University.1D. Fick, Polarization in Heavy-Ion Reactions (Lecture Notes), Oak RidgeNational Laboratory Report, ORNL/TM-8816, August 1983 (unpublished).2D. Fick, Ann. Rev. Nucl. Part. Sci. 31.. 5 3 (1984); Comments Nucl. Part.Phys. _L2, 29 (1983).3R. G. Stokstad, Y. Eisen, S. Kaplanis, D. Pelte, U. Smilansky, and I.Tserruya, Phys. Rev. Lett. Ul_, 465 (1S09); R. G. Stokstad, W. Reisdorf,K. D. Hildenbrand, J. V. Kratz, G. Wirth, R. Lucas, and J. Poitou, Z. Phys.A295, 269 (1980).••R. G. Stokstad, Y. Eisen, S. Kaplanis, D. Pelte, U. Smilansky, and I.Tserruya, Phys. Rev. C21., 2427 (1980).5B. B. Back, R. R. Betts, W. Henning, K. L. Wolf, A. C. Mignerey, and J. M.Lebowitz, Phys. Rev. Lett. U5_, 1230 (1980).6R. G. Stokstad and E. E. Gross, Phys. Rev. C 23, 281 (1981).7 P . Jacobs and U. Smilansky, "Polarization in Sub-Barrier Fusion Reactions,"March 1984 preprint, Weizmann Institute of Science, Rehovot, Israel.8J.L.C. Ford, Jr., J. Gomez del Campo, J. W. Johnson, D. Shapira, J. E.Weidley, S. T. Thornton, and R. L. Parks, Phys. Div. Ann. Prog. Rep.,December 31, 1978, Oak Ridge National Laboratory Report, ORNL-5498, p. 45(1979); J.L.C. Ford, Jr., D. Shapira, R. Novotny, R. L. Parks, and S. T.Thornton, Phys. Div. Ann. Prog. Rep., June 30, 1981, Oak Ridge NationalLaboratory Report, ORNL-5787, p. 128 (1981); J.L.C. Ford, Jr., D. Shapira,B. Shivakumar, and S. T. Thornton, Phys. Div. Ann. Prog. Rep., September 30,1983, Oak Ridge National Laboratory Report, ORNL-6004, p. 153 (1983).9"The HHIRF Supersonic Gas-Jet Target Facility," D. Shapira, J.L.C. Ford,Jr.,R. Novotny, B. Shivakumar, R. L. Parks, and S. T. Thornton (submitted forpublication, May 1984).1 0B. Shivakumar, D. Shapira, J. Gomez del Campo, J.L.C. Ford, Jr., P. H.Stelson, B. A. Harmon, and S. T. Thornton^ Bull. Am. Phy. Soc. j£9_, 626(1984); D. Shapira> B. Shivakumar, J. Gomez del Campo, J.L.C. Ford, Jr.,P. H. Stelson, B. A. Harmon, and S. T. Thornton, ibid.n T . R. Fisher, S. L. Tabor, and b. A. Watson, Phys. Rev. Lett. Z7_, 1078(1971); D. R. Parks, S. L. Tabor, B. B. Triplett, H. T. King, T. R. Fisher,and B. A. Watson, Phys. Rev. Lett. ̂ 9_, 1264 (1972).1 2A. Sahm, J. Kowalski, and G. zu Putlitz, Z. Phys. A281, 317 (1977); B.Bohmer, Ph.D. Thesis, University of Heidelberg, unpublished (1972).1 3D. M. Brink and G. R. Satchler, Angular Momentum, Oxford University Press,Oxford (1968); K. Blum, Density Matrix Theory and Applications, PlenumPress, New York, 1981.R. Beene, "RATES: A System of Computer Programs to Perform OpticalPumping Calculations," unpublished (1980)ORNL-DWG 79-2074910*b =10° -10,-tTTTTFusion—-- ftz- In\iiiofjft/•//1I iAv/////11 11 1 1ASm1 1 11 1•••1 11A154152150148|1f.000.1 1 1 1 1 =-INI? 2 —2018 ~140.001-I I I I60 65 70 75(MeV)Fig. 1. Fusion cross sections for the 1 60 + 1 4 8» 1 5 0» 1 5 2» 1 5 4Smreactions as a function of 1 60 bombarding energy (from Stokstad etal., Refs. 3, A, and 6). For fixed bombarding energy (distance ofclosest approach), cross sections for 15l*Sm are largest due to thelarger nuclear density spatial overlap between target and projec-tile due to quadrupole deformation.ORNL-DWG 79-2O755H100to3b0.1n n i11 h 111 i i ir—1\11 1 1Cross Sections forE(0b=60MeV,Ecn\V\1 1 1IFusion11 1of 0 + Sm,MeV. :-ii111 i ir-1 I10 20 30 40 506 (deg)60 70 80 90Fig. 2. Calculated fusion cross sections for 1 60 + 15l)Sm atfixed 1 60 energy (Ej_aj, = 60 MeV) as a function of the orientationof the symmetry axis of the nuclear density distribution for(Ref. 6).ORNLDWG 83-10642LASERCRYOPUMP2500 L/sLEYBOLD HERAEUSWAU 2000600 L/sKINNEY KMBD*401140 L/sDIFFERENTIALPUMPING SLITSTANDEM BEAM- ^ DIFFERENTIALLY^ | \ PUMPED VOLUMESIVSUPERSONICI GAS JET-—DETECTORSPRESSURESENSORSFig. 3. Schematic side view of the 0RNL supersonic gas-jet target.ORNL-DWG 8310644TANDEMBEAM LINEOPTICALWINDOW^-?HIGHQUALITYOPTICALWINDOWPUMPING VOLUMESIENGE MAGNETENTRANCESLITOUTER SLIDING SEALFig. 4. Schematic top view of the ORNL supersonic gas-jet target.OSNL UWGB3 1??9?TUNABLE CONTINUOUSWAVE DYE LASERTO DIFFERENTIALVACUUM PUMPSTAGEGAS HANDLINGSYSTEMHEAVY ION BEAMENTERS NORMAL TO THEPLANE OF THE PAPERSPOOL PIECE- HALF WAVEPLATE• GAS JETTARGETAPPARATUSHEAVY ION BEAMLASER BEAM. GAS JETINTERACTION REGIONOPTICAL WINDOWU>*-— HIGH TEMPERATURENOZZLE ASSEMBLYTEMPERATURECONTROLLEDHEATERSOVEN/SEEDERFig. 5. Experimental arrangement and geometry of the polarizedtarget apparatus.ORNL-DWG 83-10643PROBE BEAMIff, <7+ OR O lION BEAMFig. 6. Schematic illustrating the coordinate system and quan-tization axis in the "photon frame" for the ORNL polarized target.ORNL-DWG 81-23495Eul 8S 7/2P5/ PUMPINGTENSOR CDMENTO_J1.51.00.50! I I I— SATURATION 8 2re = io?s-< -^^^y ^ T e = Zx 1O8S~41 1 1 11—_—7T———1SATURATION 8 45 10 15 20 .5PUMP TIME (/is)<6 W/cm2 AT 0.5 A BANDWIDTH = Tup = 6 x 106 s"1Fig. 7. Evolution of the nuclear orientation parameters, B2and B4, for Eu for optical pumping of the 8 S 7 / 2 * y 8p5/2 transi-tion at 466 nm for the assumed pumping parameters given in the bot-tom of the figure. Here, re is the excited-state relaxation rate;r u p is the induced transition rate; and rd o w n is the spontaneousradiative decay rate for the y 8Ps/2 state.ORNL-DWG 8f-234970.60.40.2 hEul 8 S. 8rP0(MF)= ± =0.0208LINEAR (77-) CIRCULAR (cr+)0.60.40.20-III I I I IP(F = 6)=1.05/2—I—t — I 1— -I—II I! I I il iI I I 1 I IP(F=6)=0.58--I—t--t-i-^4r-TTr -1\1- 6 0My+ 6 - 6 0MF+ 6Fig. 8. Steady state magnetic substate (Mp) populationsfor the hyperfine state F = 6 (I + J) of the Eu ground state,for pumping with % or o + light. Here, Mp is with respect tothe quantizatio . axis in the "photon frame."

Development of optically pumped polarized jet targets for use in heavy-ion reaction studies

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Development of optically pumped polarized jet targets for use in heavy-ion reaction studies

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