Calibration of the polarimeter ROMME at deuteron energies ... - JINR

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Calibration of the polarimeter ROMME at deuteron energies ... - JINR

~fCALIBRATION OF ÒÍÅ POLARIMETER ÐÎÌÌÅ ÀÒ DEUTERONENERGIES BETWEEN 0.7 AND 1.6 GEV~"(~Ó.Ð. Ladygin!t, J. Áàll!, Ì. Áîivin!, R. Êènnå!, Å. Tomasi-Gustafsson!,L. ÁimÜît2, Ó. Áissîn2, J.L. Áîóàrd2, Ph. Courtat2, R. Gacougnolle2,Ò. Hennino2, Ì. Morlet2, R. Skowron2, N.E. Cheung3, Ì.Ê. Jones3,C.F. Perdrisat3, N.M. Piskunov4, I.M. Sitnik4, Å.À. Strokovsky4, Ó. Punjabi5fr(1) CEAjDSM CNRSjlN2P3 LaÜîratoire National Satème, ÑÅ Saclay, France(2) CNRSjlN2P3 IPN, 91400 Orsay, France(3) The College î! Williaò and Mary, Wil/iaòsbèry, Viryinia 23185, USA(4) LHE-JINR, 141980 Dèbna, Moscow region, Rèssia(5) Norfolk State University, Norfolk, Viryinia 2,'/504, USAt Permanent address: LHE-JlNR, 141980 Dèbna, Moscow region, RèssiaABSTRACTWe report the results of à new calibration of the polarirneter ÐÎÌÌÅ forenergies between 0.7 and 1.6 ÑñÓ I\sing the polarizcd del\tcron Üåàm of theLaboratoire National Saturne. It is shown that the figure of merit Ð11 is stillvery high at these energies.The fits of the vector analyzing power and efficiency as à function of energyand scattering angle, based îï empirical formulas are given betwecn 0,7 and1.8 ÑåÓ incident deuteron energy.~,İ,t;,,l""I;I!The focal plane polarimeter ÐÎÌÌÅ with 27Ò azimuthal angular acceptance basedîï inclusive scattering îï à carbon target, was built to measure the polarization ofthe secondary particles in order to provide the information concerning the nuclearstructure and mechanisms of the reactions. Since few years ÐÎÌÌÅ was I\sed in severalexperiments at SATURNE as à deuteron polarimeter to study the isoscalar spin transitionsin nuclei for low-lying levels, as well as in the continuum Üó inela.,;ti(~ (d~ Ji) scattering îïnuclei [1] and to measure the polarization of the deuteron from the reaction ðð -+ d;+[2].ÐÎÌÌÅ has Üåån previously calibrated for incident dcutcron kinctic cnergies upto 0.7 GeV [3]. Using àn iron absorber and trigger counters behind thc coordinat('detectors allowed to stop protons from the deuteron breakl\p in the carbon with smallanalyzing power and, therefore, to increase the performance of ÐÎÌÌÅ Üó the sclectiollof quasielastic deuterons. But at high energies this principle do(~s !lot work b(~('aus(' of \,h('small difference between the proton and deuteron energy losses i!l thc iron absorbcr.Here we report thc ncw results of calibration of ÐÎÌÌÅ with the polarizcd dcuterollÜåàò at four energies: 0.7,1.1,1.35 and 1.6 GeV performed without a!l iron absorvff andscintillator counters behind the chambers in the same way as the (~alivratiOll [m jJrot!ms[5, 6].À detailed description of the polarimeter ÐÎÌÌÅ is giv(,n ('\s('whcr(' [3, 5]. TII(,basic pri!l(~iple of ÐÎÌÌÅ is the analysis of thc azimuthal distributioll of »Hr\'i(.h's fr(mlàn inclusive reaction d + ñ -+ înå char,lJed particu' +.\. Thrcc fnm\, »n»>ortiO!lH.)50 õ 50 C11l2 wire chambers are used to deternlinc thc trajc.(~torics of thc ucut!'nms iI1l-iU('nlîï the (~arbon analyzer. The scattered particles arc dcte(~ted Üó threc rcar ('hamb('rs whi('h291


have dimensions of 100 õ 100 ñò2. The resolution is approximately 1.7 òò and 3.2 òòfor front and rear chambers, respectively. The chambers are mechanically aligned at thelevel îÑ ~ 1 òò, however à software procedure îï particle trajectories allows to alignto increase the precision to 0.1 òò. Carbon bIock îÑ à 30 ñm thick with à density îÑ1.7 9 / ñò3 W3.× used as an analyzer at all energies.The trigger îÑ the polarimeter is defined Üó an ensembIe îÑ overlapping scintillators Fiplaced close to the Ñîñà! plane in coincidence with the counter Ð, covering the full size îÑthe front chambers in ,Õ" -direction, placed upstream îÑ the analyzer. In this experiment theplane îÑ scintillators placed behind the last rear chamber was not included in the trigger.in order to avoid the false asymmetries arising from any dependence îÑ the efficiency ofthe counters due to the light collection. No fast rejection îÑ events with small scatteringangles due to Coulomb interaction was done at the trigger level.The polarimeter was calibrated using polarized deuteron Üåàò îÑ Laboratoire NationalSaturne, which was incident directly îï the polarimeter. The lowest possible intensityfrom the machine ~ 108 particles per Üåàò burst was further reduced to approximately104 - 2 . 104 with the help îÑ Üåàò slits and Üó defocalizing the quadrupoles îÑ thespectrometer.The polarized deuteron Üåàò îÑ Saturne has vector and tensor polarizationcomponents PIO and Ð20 along the vertical symmetry axis [7]. Different radio frequencytransitions ofthe ion source are combined to obtain either 2 states (only vector polarized),or 4 states îÑ polarization (vector and tensor polarized) Üó changing the sign îÑ thecomponents at each Üåàò burst. The polarization îÑ the deuteron Üåàò was measuredevery eight hours Üó à low energy polarimeter, located just at the exit îÑ the ion source [7].With the four state Üåàò the vector and tensor polarizations were PIO = -0.40:tO.02 andÐ20 = 0.65 :f: 0.02, respectively. For the purely vector polarized Üåàò PIO = 0.80 :f: 0.02.During the calibration, we used 4 states Üåàò at energy 1.6 GeV, and two states îÑ theÜåàò at all four energies. The changing îÑ the sign îÑ the polarization at every burstallows to cancel the instrumental asymmetries.During the off-line analysis, the incoming and outgoing particle trajectories werereconstructed using the information from multiwire proportional chambers. Conditionsîï the vertex coordinate insured that the scattering takes place in the target. À ñàïå test,which requires that for an event with given polar and azimuthal angles, 8 and ô, anyotherchoice îÑ Ô would lie within the acceptance îÑ the detection was also applied to remove theinstrumental asymmetry introduced Üó the finite size îÑ the chambers downstreams îÑ theanalyzer. Events which passed the software tests were used to determine the analyzingpower and efficiency îÑ the polarimeter filled à two-dimensional spectrum, N(i) (8, ô), foreach spin orientation îÑ the Üåàò (i). The angular ranges were 00 ~ 8 ~ 200 and-1800 ~ Ô ~ 1800.In the general case with vertical symmetry axis îÑ the deuteron polarization, thenumber îÑ events in each (8, ô) bin ñàï Üå written as:.~"....N(8, N(fJ, ô) = No(8) No(fJ) [1 + V2Pl0iT11 (fJ)ñîs(ô) - ~fJ20T20(fJ) - ËfJ20Ò22 (fJ)cos(2Ô))which reduces in ñàâå of absence of tensor polarization(fJ2o = Î) to:N(8,ô) = No(8) [1 + J2ÐlîiÒII(8)ÑÎS(ô)],(2)292


~[~;~:~;'iJ":1 ;."':1:I~.where Tij àñå the analyzing powers and No(8) is à normalization factor which depends îïthe cross section, and îï the detection efficiency.Combinations of the events from N(i)(8,ô) distributions with different polarizationstates give purely vector and purely tensor distributions, which allow to reconstruct thevector analyzing power iT11 or tensor analyzing powers Ò20 and Ò22 from the values ofthefit coefficients.I~,,~t,Ị!',j(j..vIiT ..Fig.l. Vector analyzing power iT11 Fig.2. Efficiency îÑ the polarimeterfor 4 energies. Solid and dashed corrected for the track reconstructionlines àãå the results îÑ the 6 and efficiency. Results îÑ à 17-parameter12 parameter fit, respectively. fit àãå given Üó the solid lines.The results îÑ the calibration for the vector analyzing power iT11 at 4 energies àãåshown as à function îÑ the polar angle in Fig.l. The measurements îÑ iT11 and efficiencyat 1.6 GeV using the two and four state beams were in good agreement and, thereforewere averaged to obtain the final results. Tensor analyzing powers Ò20 and Ò22 are bothcompatible with zero at this energy, what is in agreement with the previous calibrationat 1.8 GeV [4].We calculate the efficiency as à ratio between number îÑ events which passed allsoftware tests to the number îÑ events having à good trajectory before the analyzer.However, the detection efficiency depends îï the counting rate (dead time..) and îïgood response îÑ the chambers and scintillators, which is strongly dependent îï theexperimental conditions. The typical efficiency îÑ the track reconstruction after theanalyzer was in the range ""' 60% - 90% depending îï the Üåàø intensity. The angulardependence îÑ the efficiency îÑ ÐÎÌÌÅ corrected for the above factors for four energiesis shown in Fig.2.The performanceof à polarimeter is usually expressed in terms îÑ the figureof merit,:F;j (where(ij) àãå the tensorial indices). It depends îï the polar angle Î and it is functionîÑ the efficiency, Å.-, and îÑ the analyzing powers, 1';j. It is defined as:F;~ = i Å.-(Î) Ò;~(O) d(J = i g?j(O) d(J, (3)-293


where g;j((J) is the differential figure îÑ merit and the integration is done over theangular range where the polarimeter is efficient. The figur~ îÑ merit allows to determinethe number îÑ events, N;nc, necessary to obtain à given uncertainty in à polarizationmeasurement. The differential figure îÑ merit gll ((J) is presented in Fig.3 Cor all measuredenergies.The Craction îÑ events having à scattering angle smaller than 1.50, mostly due tomultiple scattering, is ,..., 40%. These events are picked at 10 and 0.50 Cor energies at 0.7and 1.6 GeV, respectively.1\~,..F' 3 D' fii t . 1fi f .t Fig, 4, Analyzing power iT\\ and\g" \ eren \à gère î òån 9\\ ,. ." 4 . S l ' d d d h d d\fferent\al figure îÑ ment 91\ atlor energ\es. î \ àn as å . ,1. th lt f th 6 d 700 MeV from present cailbrat\onøes are å resè s î å àn. .12 t fit t . 1 (ðîøts) and from ref[3] (ilnes).parame er , respec \óå ó.As we stressed in the introdèction, in the previoès calibration at low energy [3], aniron absorber allowed to select qèasi-elastic (d, d') events, which Üàóå larger analyzingpowers [8], We ñàï ñîòðûå the present resèlts (withoèt absorber) at oèr lowest energy,0.7 GeV, with the previoès ones [3] (Fig,4), where the carbon target and àn iron absorberwere 25,2 cm and 9 cm thick, respectively, One ñàï âåå rather sharp pick in the analyzingpower, placed at '" 60, in the data previoèsly obtained [3] (solid line), whereas the presentdata from oèr calibration show à smooth bchavioèr. Both calibrations give high figèreîÑ merit, F1\ '" 5% from the previoès calibration to Üå compared to the actèal valèe îÑ'" 7%, what gives à gain in data taking time îÑ aboèt factor 1,4 for an experiment toprovide the same statistical errors.An analytical parametrization îÑ analyzing powers and efficiency as à fènctions ofscattering angle and momentèm in the middle of the carbon analyzer is very èsefèl forànó experiments performed with incident energy in the range of the present calibration,The data îï the analyzing power iT\\ from present calibration and obtained in previoèsexperimcnt at 1,8 GeV [4] were fitted Üó the 12 parameter fènction ofthe scattering angletaken in the following analytical form:iT11 = àõ .1 + Üõ2 + ñõ4 + dpmids~n(58), (4)~"-294


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