An experiment demonstrating a new method for producing polarized positrons has been performed at the CEBAF accelerator at Jefferson Laboratory. The PEPPo (Polarized Electrons for Polarized Positrons) concept relies on the production of polarized e+/e− pairs originating from the bremsstrahlung radiation of a longitudinally polarized electron beam interacting within a 1.0 mm tungsten pair-production target. This paper describes preliminary results of measurements using an 8.2 MeV/c electron beam with polarization 84% to generate positrons in the range of 3.1 to 6.2 MeV/c with polarization as high as ∼80%

Adderly, P.A.

Adeyemi, A.

Ali, M. M.

Areti, H.

Benesch, J. F.

Cardman, L. S.

Clark, J.

Covert, S.

Golge, S.

Gueye, G.L

Hyde, C.

Stutzman, M. L.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsPhysics Faculty Publications Physics2015PEPPo: Using a Polarized Electron Beam toProduce Polarized PositronsA. AdeyemiG.L GueyeP.A. AdderlyM. L. StutzmanM. M. AliSee next page for additional authorsFollow this and additional works at: https://digitalcommons.odu.edu/physics_fac_pubsPart of the Engineering Physics Commons, and the Plasma and Beam Physics CommonsThis Conference Paper is brought to you for free and open access by the Physics at ODU Digital Commons. It has been accepted for inclusion inPhysics Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contactdigitalcommons@odu.edu.Repository CitationAdeyemi, A.; Gueye, G.L; Adderly, P.A.; Stutzman, M. L.; Ali, M. M.; Areti, H.; Benesch, J. F.; Cardman, L. S.; Clark, J.; Covert, S.;Golge, S.; and Hyde, C., "PEPPo: Using a Polarized Electron Beam to Produce Polarized Positrons" (2015). Physics FacultyPublications. 312.https://digitalcommons.odu.edu/physics_fac_pubs/312Original Publication CitationAdeyemi, A. H., Gueye, P. L., Adderley, P. A., Stutzman, M. L., Ali, M. M., Areti, H., . . . Variola, A. (2015). PEPPo: Using a PolarizedElectron Beam to Produce Polarized Positrons. Paper presented at the 6th International Particle Accelerator Conference, Richmond, VA,May 3-8, 2015. http://accelconf.web.cern.ch/AccelConf/IPAC2015/papers/tupma020.pdfAuthorsA. Adeyemi, G.L Gueye, P.A. Adderly, M. L. Stutzman, M. M. Ali, H. Areti, J. F. Benesch, L. S. Cardman, J.Clark, S. Covert, S. Golge, and C. HydeThis conference paper is available at ODU Digital Commons: https://digitalcommons.odu.edu/physics_fac_pubs/312PEPPo: USING A POLARIZED ELECTRON BEAM TO PRODUCEPOLARIZED POSITRONSA. Adeyemi∗, Hampton University, Hampton, VA, USAon behalf of the PEPPo CollaborationAbstractAn experiment demonstrating a new method for produc-ing polarized positrons has been performed at the CEBAFaccelerator at Jefferson Laboratory. The PEPPo (Polar-ized Electrons for Polarized Positrons) concept relies onthe production of polarized e+/e− pairs originating fromthe bremsstrahlung radiation of a longitudinally polarizedelectron beam interacting within a 1.0 mm tungsten pair-production target. This paper describes preliminary resultsof measurements using an 8.2 MeV/c electron beam withpolarization 84% to generate positrons in the range of 3.1 to6.2 MeV/c with polarization as high as ∼80%.INTRODUCTIONPolarized positrons are a powerful probe for the investiga-tion of numerous physics phenomena. At thermal energies,polarized positrons may be used to study spintronic prop-erties of materials [1]. Accelerated to GeV energies, thecomparison between polarized electron and positron scatter-ing cross-sections tests the precision of the electromagneticinteraction for the investigation of nucleon and nuclear struc-ture [2]. At TeV energies, polarized positrons are essentialfor testing the Standard Model in the context of the Interna-tional Linear Collider (ILC) [3].Polarized positrons are produced in radioactive beta-decay [4], or by man-made methods such as the Sokolov-Ternov self-polarization of unpolarized positrons in a storagering [5] or resulting from pair-production following the irra-diation of nuclei with circularly polarized gamma rays. Forthe latter method, circularly polarized gamma rays can beproduced via Compton back-scattering [6] or using a helicalundulator [7]. Both techniques produced positrons with ahigh degree of positron spin-polarization and demonstratedthe potential for high yield. However, both methods requirevery high electron beam energies 10-100 GeV and elaboratetechnologies.In this work, we demonstrate a new technique for theproduction of polarized positrons, and with very low beamenergy ∼10 MeV. The idea, suggested in the late 1990’sin the context of the ILC [8, 9], relies on the productionof circularly-polarized gamma rays by the bremsstrahlungproduced using polarized electrons; these photons then cre-ate polarized positrons via pair-production on nuclei. Bydemonstrating the technique at low energy, we avoid issuesassociated with high power targets and activation, illustratingthe applicability of this technique to the design of future po-larized positron sources, whether at similar or much higherenergy where yield may be significantly increased.∗ adeyemi@jlab.orgEXPERIMENTThe PEPPo experiment [10] was installed at the Jeffer-son Lab CEBAF injector where a highly spin-polarized 84%electron beam with energies up to 10 MeV could be provided.The electron beam could be delivered for characterization toeither a Mott scattering electron polarimeter or a precisionelectron spectrometer; alternatively it could be sent to thePEPPo apparatus (see Fig. 1) which included: the e+/e− pro-duction target, a quarter-wave solenoid to collect positionswithin a large divergence angle, a combined-function spec-trometer to select and focus discrete positron momenta slices,a pair of coincidence positron annihilation detectors, and asecond solenoid to transport and focus positrons through an8 mil Al window to the PEPPo polarimeter.Conversion Target Collection Solenoid Collection Spectrometer Annihilation Detectors Transport Solenoid To Polarimeter Figure 1: Vacuum beam line includes the insertable pro-duction target, a solenoid to collect e+, a double-bend spec-trometer to define the momentum acceptance, a positronannihilation detector, and a second solenoid to transport e+to an exit vacuum window in front of the polarimeter.PEPPO POLARIMETERThe PEPPo polarimeter (see Fig. 2) is a Compton trans-mission type polarimeter that relies on the bremsstrahlungspectrum produced by longitudinally polarized positrons(or electrons) having small energy and angular distributionsinteracting with a reconversion target at the entrance of thepolarimeter. The polarimeter takes advantage of the differ-ential cross section for the Compton scattering of circularlypolarized bremsstrahlung photons with a longitudinally po-larized electron target (PT ) [11]. The measurement of theTUPMA020 Proceedings of IPAC2015, Richmond, VA, USAISBN 978-3-95450-168-71878Copyright©2015CC-BY-3.0andbytherespectiveauthors2: Photon Sources and Electron AcceleratorsT02 - Electron SourcesPolarizing Magnet Reconversion Target Polarized Target 3x3 CsI  Array PMT Readout Lead Hut Figure 2: PEPPo Compton transmission polarimeter in-cludes a positron (or electron) to gamma conversion tar-get, an iron core target polarized by a magnet, and a CsIcalorimeter to measure the flux and energy of transmittedphotons.beam (positron or electron) polarization is essentially ob-tained from the transmission asymmetry (AT ) of the numberof transmitted bremsstrahlung photons (N±γ) for oppositelypolarized target or beam polarization orientations accordingtoAT =N+γ− N−γN+γ + N−γ= Pe±PT Ae± (1)where Ae± is an effective analyzing power determined eitherby simulation or experimentally (e.g. by using the polarizedelectron beam with a measured polarization directly). In thePEPPo polarimeter, bremsstrahlung photons generated in a 2mm thick tungsten reconversion target interact with a 7.5 cmlong and 5 cm diameter iron target longitudinally polarizedby a magnetic field close to saturation (7.06%± 0.05%sys ±0.07%sys). Transmitted photons are detected in a segmented3x3 array CsI calorimeter following the polarized target.0 1 2 3 4 5 6 7 800.020.040.060.080.10.120.140.160.180.2) [MeV/c]e-Electron Momentum (pAnalyzing Power (Ae-)Measured e- Analyzing power (Statistics only) Simulated e- Analyzing power Figure 3: The comparison of the Geant4 simulation andmeasured analyzing powers for electrons in the PEPPo po-larimeter.A model of the polarimeter was constructed usingGeant4 [12] v8.3 with the ultimate purpose of simulatingthe analyzing power for positrons. However, the a priorknowledge of the electron beam polarization via a Mottscattering polarimeter provided a pathway to gain a highlevel of confidence by first benchmarking the simulationfor electrons with a directly measured analyzing power forelectrons. To accomplish this an electron beam of measuredpolarization 83.7% ± 0.6%stat ± 2.8%syst was acceleratedto varying momenta in the range of 3.1 to 7.3 MeV/c andmeasured using the PEPPo polarimeter. At each momen-tum the transmission asymmetry was computed in termsof the energy weighted integral of the photon flux, whileaccounting for the reversal of the beam helicity or targetpolarization. The Geant4 model and simulation were tested;the major obstacle of which was learning of a bug in thepolarized transportation process for releases later than v.8.3.The comparison (statistics only) between the simulated andmeasured analyzing power for electrons is shown in Fig. 3for the central crystal of the array. Initial assessment of thesimulated analyzing power appears dominated by the sys-tematic uncertainty of the measured analyzing power, i.e.,the beam polarization, target polarization and data analysisof the transmission asymmetry (pedestal subtraction, lin-earity, false asymmetry), and then to a lesser extent on thesimulated dependencies of the electron beam distribution.Preliminary results suggest the simulation is in agreementwith the measurement at the ∼5% level.POSITRON MEASUREMENTSThe electron beam was initially used to calibrate the set-tings of the collection spectrometer (with solenoids off) thatefficiently transported discrete slices of beam momenta tothe PEPPo polarimeter. After obtaining suitable transportthe beam momentum was fixed at 8.2 MeV/c and the 1.0 mmtungsten conversion target was inserted, thus generating pair-produced e+/e− pairs, gamma photons and energy-degradedbeam electrons. At a series of four successive momenta inthe range of 3.1 to 6.2 MeV/c the collection and transportsolenoids were optimized for degraded electrons by opti-mizing the yield after the collection spectrometer and at thepolarimeter reconversion target. Once the solenoids wereoptimized for the collection of electrons the polarity of thespectrometer was reversed to efficiently transport the desiredmomentum slice of positrons to the reconversion target ofthe polarimeter. The transmission asymmetry of positrons(AT ) was measured while accounting for the reversal of ei-ther the positron (via the original electron) helicity or thetarget polarization.The positron polarization is computed as Pe+ =ATPT ·Ae+where Ae+ is the simulated analyzing power of the polarime-ter for positrons. A comparison of the benchmarked ana-lyzing power for positrons and electrons for the PEPPo po-larimeter is shown in Fig. 4. The enhanced analyzing powerof the positrons is expected, primarily as a consequence ofin-flight annihilation of positrons in the reconversion target.Finally, our preliminary result for the positron polariza-tion (statistics only) is shown in Fig. 5. We are evaluatingProceedings of IPAC2015, Richmond, VA, USA TUPMA0202: Photon Sources and Electron AcceleratorsT02 - Electron SourcesISBN 978-3-95450-168-71879 Copyright©2015CC-BY-3.0andbytherespectiveauthors0 1 2 3 4 5 6 7 800.020.040.060.080.10.120.140.160.180.2Beam Momentum [MeV/c]Analyzing Power (Ae)Simulated e- Analyzing power   Simulated e+ Analyzing power Figure 4: Comparison of the Geant4 simulation of the PEPPopolarimeter analyzing power for electrons and positrons.the contributions of systematic uncertainties, in the targetpolarization (<2%), the data analysis of the transmissionasymmetry (<7%), and the accuracy of the Geant4 modelof the polarimeter (<5%). Dependence of the simulatedanalyzing power on the positron distribution have begun.0 1 2 3 4 5 6 7 80102030405060708090100)  [MeV/c]e+Positron Momentum (p) [%]e+ PRELIMINARYStatistics onlyFigure 5: Preliminary results of the polarization of positronsproduced in the PEPPo apparatus.In addition to positron polarimetry, a pair of NaI detect-ors (energy calibrated with a 22Na source) were locatedfollowing the collection spectrometer to detect and measurethe yield of positrons annihilated in a 11 mil target (99.5%Al2O3, 0.5% Al2O3Cr2O3) oriented at 45◦ to the beam. Thecoincidence and deposited energy of two 511 keV annihil-ation X-rays indicated the presence of positrons. The raw(upper) and background subtracted (lower) energy spectraare demonstrated in Fig. 6. Although shielded, the acci-dental rate of both detectors suffered from proximity to thepositron production target. When corrected for solid angle(0.1 sr), annihilation probability (0.25%) and detectorefficiency (49% at 511 keV) the measured coincidence rateindicates a yield of 1 positron per 106 electrons, consistentwith the expected efficiency of the experiment.CONCLUSIONWe have demonstrated a new means to produce polar-ized positrons. This paper described preliminary results ofmeasurements using an 8.2 MeV/c electron beam with po-larization 84% to generate positrons in the range of 3.1 to6.2 MeV/c with polarization as high as ∼80%.Figure 6: The coincidence detection of positrons with(upper) and without (lower) background subtraction.ACKNOWLEDGMENTWe are grateful to the SLAC E-166 Collaboration, partic-ularly the support of K. Laihem, K. McDonald, S. Riemann,A. Schälicke, P. Schüler, J. Sheppard and A. Stahl. We thankN. Smirnov for delivery of critical hardware. This work wassupported in part by the U.S. Department of Energy, theFrench Centre National de la Recherche Scientifique, theInternational Linear Collider project and the Departmentof Energy National Nuclear Security Administration underAward Number DE-NA000097. Jefferson Science Asso-ciates operates the Thomas Jefferson National AcceleratorFacility under DOE contract DE-AC05-06OR23177.[1] H.J. Zhang et al, Scientific Reports 4 (2014) 4844.[2] Proceedings of the International Workshop on Positrons atJefferson Lab, Edts. L. Elouadrhiri, T.A. Forest, J. Grames,W. Melnitchouk, E. Voutier, AIP Conf. Proc. 1160 (2009).[3] International Linear Collider Technical Design Report(2013).[4] L.A. Page and M. Heinberg. Phys. Rev. 106(6):1220-1224(1957).[5] D. Barber, AIP Conf. Proc. 588, 338 (2001).[6] T. Omori et al, Phys. Rev. Lett. 96 (2006) 114801.[7] G. Alexander et al, Phys, Rev, Lett. 108 (2008) 210801.[8] E.G. Bessonov A.A. Mikhailichenko, EPAC(2006).[9] A.P. Potylitsin, NIM A398 (1997) 395.[10] J. Grames, E. Voutier et al., JLab Experiment E12-11-105(2012).[11] W.H. McMaster, Rev. Mod. Phys. 33 (1961) 8.[12] S. Agostinelli et al., Nuclear Instruments and Methods inPhysics Research A 506 (2003) 250-303.TUPMA020 Proceedings of IPAC2015, Richmond, VA, USAISBN 978-3-95450-168-71880Copyright©2015CC-BY-3.0andbytherespectiveauthors2: Photon Sources and Electron AcceleratorsT02 - Electron Sources• • • • • D D D D D D • + + + t NalDc1«1orAl 20.12r-----== ==------~ ?0·14 " ;0.12 J O.J ~ 0. 1 0.08 0.06 go.OS • ~ 0.06 0.04 0.02 Nal Dct~rorA3 ~••,.,-?O,I _.,,.,  7i' , .. ,., .... ~ 0.08 "'l!el Om 0.()6 0,0, 0.02 0 -0.02 T•rg,..1 ln ln1cgral•0.Slbct /05nA c-.o.oib,,._~,ooc,.....~,.Loo~ooo_L_~,-'-oo~,...Jooo~-,,c......oo~,,Loo.J :..-,,""'""-±,-..,±c-"-,,±oo,-..,,"'ooo,,_,,,.,oo,....,,'"",oo=1 Encrgy(kcV) Encrgy(kcV) 

PEPPo: Using a Polarized Electron Beam to Produce Polarized Positrons

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PEPPo: Using a Polarized Electron Beam to Produce Polarized Positrons

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