International Workshop on Physics with Positrons at Jefferson Lab (JPos17)

Sep 12, 2017, 8:00 AM → Sep 15, 2017, 6:00 PM US/Eastern

Room F113 (CEBAF Center)

Eric Voutier (CNRS/IN2P3/IPNO - UPS) , Joe Grames (JLab)

This 4 day workshop plans to explore the scientific basis for polarized and unpolarized positron beams at JLab in the context of 12 GeV CEBAF, the proposed Jefferson Lab Electron Ion Collider (JLEIC), and also for low energy applications. An accelerator session dedicated to positron production and beam formation for each scenario completes the scientific program. Collecting the motivation and requirements for a positron physics experimental program within the workshop proceedings as a basis for a Jefferson Lab Positron Physics white paper is a key goal of the meeting.

Poster conference_poster.pdf

Tuesday, September 12

8:00 AM → 8:30 AM Registration and Continental Breakfast

8:30 AM → 8:40 AM Welcome - Workshop Organization and Goals

8:40 AM → 10:15 AM Plenary 2

Convener: John Arrington (Argonne National Laboratory)

8:40 AM Measurement of TPE with electron/positron elastic scattering off the proton

Since the discovery of the discrepancy between Rosenbluth and polarization-transfer measurements of the proton form-factor ratio $G_E/G_M$ there has been a renewed interest in measuring contributions to the elastic cross section from two-photon exchange (TPE). Three recent experiments conducted at CLAS, VEPP-3, and OLYMPUS directly determined the real part of the TPE amplitude by measuring the ratio of elastic $e^+p$ and $e^-p$ cross sections at $Q^2$ up to about 2 GeV$^2$. I will discuss these results and present the results of an analysis that compared them to recent models of TPE that largely reconcile the two types of form-factor measurements. I will also discuss the possibility of extending the TPE measurements at Jefferson Lab to higher $Q^2$ where the form-factor discrepancy suggests a large TPE contribution.

Speaker: Brian Raue (Florida International University)

Paper AIP_Conference_Proceedings_License_Agreement.pdf TPERaue.pdf

Slides Brian_Raue_2017-12-09.pdf

9:15 AM TPE Theory: Partonic Approach

Speaker: Andrei Afanasev (GWU)

Slides Andrei_Afanasev_2017-09-12.pdf

9:50 AM Overview of recent theoretical work on two-photon exchange

In this talk I will give an overview of recent progress in theoretical calculations of two-photon exchange (TPE) effects in elastic electron-proton scattering. This will include a survey of existing models and theoretical frameworks. TPE effects are relevant for extractions of proton form factors at high $Q^2$, and of the proton radius at low $Q^2$. Recent experiments to directly measure hard TPE effects by comparing positron and electron scattering will be discussed from a theoretical perspective.

Speaker: Peter Blunden (University of Manitoba)

Slides Peter_Blunden_2017-09-12.pdf

10:15 AM → 10:35 AM Coffee Break

10:35 AM → 12:25 PM Plenary 1

Convener: Eric Voutier (CNRS/IN2P3/IPNO - UPS)

10:35 AM Vision of polarized positron in Hall B

Speaker: Volker Burkert (Jefferson Lab)

Slides IntroTalk.pdf

11:20 AM The PEPPo method for polarized positrons and PEPPo II

The Polarized Electrons for Polarized Positrons (PEPPo) experiment at the injector of the Continuous Electron Beam Accelerator Facility demonstrated for the first time the efficient transfer of polarization from electrons to positrons via a two-step process: polarized bremsstrahlung radiation is induced by a polarized electron beam in a high-Z target; and then the polarized bremsstrahlung produces polarized positrons via the pair-production process in the same target. Positron polarization up to 82% was measured for an initial electron beam momentum of 8.19 MeV/c, limited only by the electron beam polarization. This technique extends polarized positron capabilities from GeV to MeV electron beams, and opens access to polarized positron beam physics to a wide community. We will present the results of the PEPPo experiment and outline tentative plans for a follow-up experiment that would investigate key aspects of an approach based on PEPPo as a polarized positron source for the 12 GeV Upgrade of CEBAF.

Speaker: Larry cardman (Jefferson Lab)

Slides The_PEPPo_method_for_polarized_positrons_and_PEPPo_II_As_Presented.pptx

11:50 AM DVCS with Electron and Positron Beams

Speaker: Maxime DeFurne

Slides Maxime_Defurne_2017-09-12.pdf

12:25 PM → 1:25 PM Lunch Break

1:25 PM → 2:40 PM Plenary 3

Convener: Andrei Afanasev (GWU)

1:25 PM Precise extraction of elastic scattering for two-photon exchange measurements

Two experimental techniques, Rosenbluth separation and recoil polarization transfer, used to extract proton's electromagnetic form factors ratio $\frac{G_E}{G_M}$ yield markedly different results. Modern theoretical calculations suggest that two-photon exchange (TPE) might be responsible for the observed discrepancy and that it is $\varepsilon$ dependent. Jefferson Lab Experiment E05-017 was designed to measure the TPE contribution over a wide range of $\varepsilon$ and $Q^2$. In contrast with the conventional Rosenbluth method, E05-017 detected the elastically scattered proton rather than the electron. At a fixed $Q^{2}$, proton momentum is constant for all $\varepsilon$ while the proton rate and the radiation corrections have a smaller dependence on $\varepsilon$ compared to ($e,e{\textprime}$) detection. This allows a much more precise extraction of the form factor ratios. This approach can be exploited to significantly improve precision for experiments comparing $e^{+}p$ and $e^{-}p$ scattering which have been used to directly constrain TPE at lower $Q^2$. Positron production development at JLab in the context of the 12 GeV program opens up an opportunity to directly explore TPE effects at low $\varepsilon$ and much higher values of momentum transfer where the disagreement between the Rosenbluth separation and polarization transfer methods is largest. The current status of E05-017 analysis along with preliminary results will be presented. The kinematic coverage and the effects on TPE extraction that could be obtained from measurements with a 5uA positron beam combined with proton detection will be outlined.

Speaker: Mikhail Yurov (UVA)

Paper

• jpos_myurov.pdf
• license_agreement_anl.pdf
• license_agreement_myurov.pdf

Slides jpos_2017_09_m_yurov.pdf

1:50 PM Deeply Virtual Compton Scattering with a Positron Beam

The exclusive electroproduction of a hard photon off a nucleon $eN\rightarrow eN\gamma$ provides three-dimensional information on the nucleon structure. This reaction proceeds via the Bethe-Heitler (BH) process (with the photon emitted by the electron), and the Deeply Virtual Compton Scattering (DVCS) process (with the photon emitted by the proton). BH and DVCS are indistinguishable, and interfere at the amplitude level. In the Bjorken regime of large $Q^2$ at fixed $x_B$, and for $-t/Q^2<1$, the DVCS amplitude factorizes with the non-perturbative part described in terms of Generalized Parton Distributions (GPDs). The imaginary part of the amplitude parameterized by GPDs provide three-dimensional imaging through the spatial distributions of partons in the transverse plane at fixed longitudinal momentum fraction $x_B$. The real part of the amplitude is sensitive to the Form Factors parameterizing the Energy Momentum Tensors of partons in the nucleon, in particular the elusive D-term. Existing strategies to access the D-term include dispersion relations between the imaginary parts of the amplitude accessed through spin asymetries, and the real parts of the amplitude accessed through cross-sections and double spin asymetries. These dispersion relation strategies are difficult to implement, and include several forms of systematical uncertainties. The availability of a positron beam gives a more direct access to the real part of the amplitude through beam charge asymetry measurements and would be an invaluable tool to fully realize the potential of the exclusive reaction program at Jefferson Lab.

Speaker: Francois-Xavier Girod (JLab)

Slides girod_JPos17.pdf

2:15 PM Polarization Observables using Positron Beams

The discrepancy between polarized and unpolarized measurements of the proton's electromagnetic form factors is striking, and suggests that two-photon exchange (TPE) may be playing a larger role in elastic electron-proton scattering than is estimated in standard radiative corrections formulae. While TPE is difficult to calculate in a model-independent way, it can be determined experimentally from asymmetries between electron-proton and positron-proton scattering. The possibility of a polarized positron beam at Jefferson Lab would open the door to measurements of TPE using polarization observables. The epsilon-dependence of polarization-transfer to the proton is an excellent test of our understanding of TPE in the context of electron-proton radiative corrections, and the ability to form a "super-asymmetry" between electrons and positrons would lead to a drastic reduction of systematics. In addition, a measurement of the target single-spin asymmetry, which is sensitive to the imaginary part of the TPE amplitude, can be improved through the simultaneous measurement with electron and positron beams. In this talk, I will discuss the prospects and limitations for future measurements at Jefferson Lab.

Speaker: Axel Schmidt (MIT)

Paper license_schmidt.pdf proceedings_schmidt.pdf

Slides schmidt_jpos17_v2.pdf

2:40 PM → 3:00 PM Coffee Break

3:00 PM → 5:30 PM Plenary 4

Convener: Vasiliy Morozov (Jefferson Lab)

3:00 PM Operational Experience of Positron Beam at DESY

Speaker: Ferdinand Willeke (Brookhaven National Lab)

3:30 PM Using CEBAF as a positron machine, prospects and limitations

We will discuss the possibility of using CEBAF as a positron machine. After a review of the current machine capabilities, we will examine one particular scenario for generating, accelerating and transporting positrons in CEBAF. Modifications to the existing CEBAF machine will be discussed as well as expected performance and impact on the regular program.

Speaker: Yves Roblin (Jefferson Lab)

Slides jpos17roblinpdf.pdf jpos17roblin.pptx

4:00 PM Bipolar Operation of CEBAF Magnets -- Considerations and Implications

Operation of CEBAF with positrons using the standard electron beam direction (clockwise as viewed from above) requires inverting the polarity of the recirculation arc and Spreader/Recombiner dipoles. Retaining the focusing optics used for electrons requires inverting the quadrupole gradients. The hardware readily supports reversed polarity, and no observations are known to suggest any change of field vs. current calibration after restoring the (unipolar) dipoles to electron polarity. Depending upon residual calibration errors and beam diagnostic capabilities, it may be useful to reverse the leads not only of the dipoles, but also of the quadrupoles. These and related operational issues will be discussed.

Speaker: Michael Tiefenback (Jefferson Lab)

Slides 2017-09-12_JPos17_MagnetPolarity.pdf

4:30 PM An Operational Diagnostic Complement for Positrons at CEBAF/JLab

The CEBAF accelerator has a demonstrated capacity for delivery of electron current to Hall B at the ~100 nA level, on par with the current anticipated for future positron beams. However, machine configuration requires macropulse current levels of a few micro-Amperes due to limited sensitivity of many installed diagnostics. Informed by this operational experience, we outline a set of diagnostic extensions leading to operationally reliable delivery at JLab of a low-current beam of positrons. Alternate diagnostic choices are listed, as well.

Speaker: Michael Tiefenback (JLab)

Slides 2017-09-12_JPos17_PositronOperation.pdf

5:00 PM High Energy Polarimetry of Positron Beams

Speaker: David Gaskell (Jefferson Lab)

Slides

• gaskell_copyright.pdf
• gaskell_polarimetry_jpos.pdf
• gaskell_positron_polarimetry_proceedings.pdf

5:30 PM → 7:30 PM Reception/Poster Session

Wednesday, September 13

8:00 AM → 8:30 AM Continental Breakfast

8:30 AM → 10:10 AM Plenary 5

Convener: Wally Melnitchouk (Jefferson Lab)

8:30 AM Review of structure function measurements at the HERA collider

Structure functions and parton densities determined in ep collisions at HERA are reviewed. At HERA, about 0.5 fb^-1 of data were recorded at a centre-of-mass energy of about 320 GeV by each of the two collider experiments H1 and ZEUS. The data samples are about equally shared between operation with e⁺p and e^-p beams. In e⁺p collisions further small samples were collected at reduced centre-of-mass energies. These datasets were used to determine the structure functions F₂, xF₃ and F_L in neutral-current reactions. Together with data from charged current reactions, parton densities of the proton have been determined in a NLO QCD fit. In this talk, the experimental methods applied at HERA and the results are reviewed.

Speaker: Stefan Schmitt (DESY)

Slides jpos17_hera_170913.pdf

9:10 AM Constraints on PDFs from Charged Current DIS

In order to better understand the behavior of parton distribution functions in x and Q, it is necessary to be able to separate the contributions from various flavors. Such flavor differentiation can be greatly assisted by charged current deep inelastic scattering. The status of flavor differentiation will be discussed and some useful examples utilizing charged current interactions will be given.

Speaker: Jeff Owens (Florida State University)

Slides owens_jpos2.pdf

9:40 AM Possibilities for learning about the nucleon spin structure with positrons and electrons

The study of the nucleon spin structure has been approached from two complementary perspectives. From the one hand, generalized parton distributions (GPDs) give information about the distribution of quarks and gluons inside the nucleon as a function of the fraction of their longitudinal momentum with respect to the longitudinal nucleon momentum and as a function of their transverse position inside the nucleon. Different distributions are of importance depending on the nucleon and parton polarization state. In addition, some of these distributions provide access to the partons' angular momenta. From the other hand, the transverse-momentum-dependent (TMD) parton distribution functions describe the distribution of quarks inside the nucleon as a function of their longitudinal and transverse momenta. Also here, depending on the polarization state of the partons and nucleon, different distributions need to be considered. Both GPDs and TMD parton distribution functions reduce to the standard parton distribution functions when integrating over the respective transverse kinematic variables. How to access the observables with electrons and positrons as well as the possible advantages of using both types of lepton beams will be presented and discussed.

Speaker: Charlotte Van Hulse (University of the Basque Country UPV/EHU)

Slides jpos_charlotte.pdf

10:10 AM → 10:20 AM Group Photo

10:20 AM → 10:40 AM Coffee Break

10:40 AM → 12:10 PM Plenary 6

Convener: Yulia Furletova (Jefferson Lab)

10:40 AM Pion and kaon structure with tagged DIS

Speaker: Tanja Horn (Catholic University of America)

Slides Tanja_Horn_2017-09-13.pdf

11:10 AM Probing strangeness via charm production in CC DIS

Speaker: Jae Nam

Slides Jae_D._Nam_talk_slides.pdf

11:40 AM Photon photon and photo nucleus physics at the LHC

The unprecedented energies, luminosities and experimental capabilities of the LHC have given physicists opportunities to search for new physics over a very wide mass range using photon induced reactions. This talk will summarize recent LHC results on photon-photon, photon proton and photon-proton reactions at both the electroweak and QCD scales. These results will be placed in the context of earlier measurements at HERA and future measurements at the Electron Ion Collider.

Speaker: Michael Murray (University of Kansas)

Slides Michael_Murray_2017-09-13.pdf

12:10 PM → 1:30 PM Lunch Break

1:30 PM → 2:40 PM Plenary 7

Convener: Farida Selim (Bowling Green State University)

1:30 PM Development and application of spin-polarized positron beams using radioisotopes

Positrons emitted from radioisotopes are longitudinally spin-polarized due to parity non-conservation in the weak interaction. In 1960's, the angular correlation of annihilation radiation measurement with spin-polarized positrons was demonstrated to be useful in studying the ferromagnetic band structures. In 1982, the first experiment on surface ferromagnetism with spin-polarized positron beam was reported by the Michigan group. This work demonstrated the existence of ferromagnetism on the Ni first surface layer against the magnetic dead layer hypothesis. There might be many potential applications of spin-polarized positron beam in rapidly progressing spintronics field. However, after the Michigan group only a limited number of works have been carried out. We have been developing radioisotope (22Na, 68Ge)-based polarized positron beams. In this talk, we introduce the current status of our beam development and its applications to (i) ferromagnets (both bulk and surface) [1-3], (ii) strong spin-orbit interaction systems [4-6] and (iii) high-spin state vacancy systems [7]. [1] A. Kawasuso et al., Phys. Rev. B83 (2011)100406(R), Phys. Rev. B85 (2012)024417(6). [2] H. Li et al., J. Phys. Condens. Matter 27(2015)246001-1-5. [3] H. Li et al, Defect and Diffusion Forum, 373(2017)65-70. [4] A. Kawasuso et al., J. Mag. Mag. Mater. 342(2013)139-143. [5] H. J. Zhang et al., Scientific Reports 4(2014)04844. [6] H. J. Zhang et al., Phys. Rev. Lett. 114(2015)166602-1-5. [7] M. Maekawa et al., Appl. Phys. Lett., 110(2017)172402-1-5.

Speaker: Atsuo Kawasuso (QST)

notes JPos2017_CRA_Kawasuso.pdf

Paper JPos2017_Kawasuso.pdf

Slides JPos2017_Kawasuso.pdf

2:05 PM Jefferson Laboratory’s role for research with high intensity, high brightness polarized slow positrons

One goal of the JPos-17 International Workshop on Physics with Positrons is to ascertain whether it would be a good idea to expand the mission of the Thomas Jefferson National Accelerator Facility to include science with low energy (i.e. “slow”) spin polarized positrons. It is probably true that experimentation with slow positrons would potentially have wide-ranging benefits comparable to those obtained with neutron and x-ray scattering, but it is certain that the full range of these benefits will never be fully available without an infrastructure comparable to that of existing neutron and x-ray facilities. The role for Jefferson Laboratory would therefore be to provide and maintain (1) a dedicated set of machines for making and manipulating high intensity, high brightness beams of polarized slow positrons; (2) a suite of unique and easily used instruments of wide utility that will make efficient use of the positrons; and (3) a group of on-site positron scientists to provide scientific leadership, instrument development, and user support. Some examples will be given of the science that might make a serious investment in a positron facility worthwhile. At the same time, the lessons learned from various proposed [1-4] and successful [5,6] positron facilities will be presented for consideration. Work supported by the US National Science Foundation under grants PHY 1505903, PHY 1404576, PHY MRI 1532300 and PHY MRI 1429718. [1] W. J. Kossler, A. J. Greer, and L. D. Hulett Jr., “Positrons at CEBAF”, in Slow positron beam techniques for solids and surfaces : Fifth International Workshop, Jackson Hole, WY, August 1992 / editors, Eric Ottewitte, Alex H. Weiss (New York: American Institute of Physics, 1994) AIP Conference Proceedings 303, 296 (1994) http://doi.org/10.1063/1.45513. [2] K. G. Lynn and F. M. Jacobsen, “Intense positron beams”, Hyp. Int. 89, 19-29 (1994), Table I. [3] L. D. Hulett and C. C. Eberle, “A high intensity slow positron facility for the Advanced Neutron Source”, 10th International Conference on Positron Annihilation, Beijing, China, 23-29 May 1994. [4] S. Okada, H. Sunaga, H. Kaneko, H. Takizawa, A. Kawasuso, K. Yotsumoto, and R. Tanaka, “The Japanese Positron Factory”, AIP Conference Proceedings 475, 349 (1999); doi: http://dx.doi.org/10.1063/1.59250. [5] C. Huggenschmidt, “Positrons in surface physics”, Surf. Sci. Rept. 71, 547 (2016). [6] T. Hyodo, et al., “Research progress at the slow positron facility in the institute of materials structure science, KEK”, IOP Conf. Series: Journal of Physics: Conf. Series 791, 012003 (2017).

Speaker: Allen Mills, Jr. (University of California Riverside)

Slides Allen_Mills_2017-09-13_Rev.pptx

2:40 PM → 3:05 PM Coffee Break

3:05 PM → 5:50 PM Plenary 8

Convener: Tony Forest (Idaho State University)

3:05 PM Opportunities and Challenges of a Low-energy Positron Source in the LERF

Though there are many applications of low energy positrons, many experiments are source limited. Using the LERF accelerator at the Thomas Jefferson National Accelerator Facility, it is possible to produce a high brightness source of very low-energy positrons. The accelerator requirements are well within the capabilities of the installed hardware. The accelerator can easily produce 50 kW of beam with a beam energy of up to 170 MeV. For these experiments, we only need run at up to 100 MeV. The gamma converter must be able to absorb the 50 kW of beam power that the linac delivers. At this low an energy the converter, though challenging, is possible. The transport of the low energy positrons from the production target to the next stage, where the energy is reduced even further, must have a very large acceptance to be able to efficiently transport the flux of positrons from the positron production target to the moderator. We propose to accomplish such a transport by means of a guiding solenoidal field with a novel endcap design. In this presentation, we will present the proposed schemes necessary to realize such a high brightness positron source.

Speaker: Stephen Benson (Jefferson Lab)

Slides Low-energy_Pos_Source_in_the_LERF.pdf Low-energy_Pos_Source_in_the_LERF.pptx

3:40 PM Positron Annihilation Studies using a Superconducting Electron LINAC

The Helmholtz-Center at Dresden-Rossendorf operates several user beamlines for materials research using positron annihilation energy and lifetime spectroscopy. Two beamlines are being operated at a superconducting electron linear accelerator [1] producing hard X-rays from electron-bremsstrahlung and in turn generating positrons from pair production. Both installations employ bunched continuous-wave (CW) electron beams with energies between 15 MeV and 30 MeV. The CW-operation results in significantly reduced pile-up effects in the detectors in comparison to normal conducting accelerators. Electron bunch lengths below 10 ps FWHM allow positron annihilation lifetime spectroscopy measurements with high timing resolutions. The bunch repetition rate is adjustable to 26 MHz / 2n, n=0, 1, 2 ... 16 matching wide spans in positron or positronium lifetimes. The GiPS (Gamma-induced Positron Source) generates energetic electron-positron pairs inside the sample under investigation from hard x-rays impinging onto the sample [2]. Therefore, the source is especially suited for materials which are not qualified for vacuum conditions or because they are imposing hazardous conditions or intrinsic radioactivity. Exemplary defect studies on the skyrmoin-lattice compound MnSi [3] will be presented. MePS (the Monoenergetic Positron Source) utilizes positrons with fixed energies ranging from 500 eV to 16 keV [4]. A magnetic beam transport system guides positrons to the samples under investigation. A dedicated chopper/buncher system is used to maintain a high timing resolution for depth-dependent annihilation lifetime studies in thin films. The signal-to-noise ratio is beyond 104 while lifetime resolutions of around 280 ps FWHM have been obtained. Applications of porosimetric studies in low-k dielectrics [5] and polymer brushes [6] will be presented. The MePS facility will be extended by an end-station called AIDA2 (Apparatus for in-situ Defect Analysis) where defect studies can be performed in a wide temperature range during thin film growth and ion irradiation. A similar setup named AIDA-1 is already in operation at a 22Na-based mono-energetic continuous positron beam [7] used for Doppler-broadening spectroscopy experiments [8,9]. The MePS facility has partly been funded by the Federal Ministry of Education and Research (BMBF) with the grant PosiAnalyse (05K2013). The initial AIDA system was funded by the Impulse- und Networking fund of the Helmholtz-Association (FKZ VH-VI-442 Memriox). The AIDA facility was funded through the Helmholtz Energy Materials Characterization Platform.

Speaker: Andreas Wagner (Helmholtz-Zentrum Dresden-Rossendorf)

Paper Wagner-AIP_Conference_Proceedings_License_Agreement.pdf Wagner-JPOS17-V3.pdf

Slides Wagner-JPos17-short.pptx

4:15 PM Slow positron applications at Slow Positron Facility of Institute of Materials Structure Science, KEK

Slow Positron Facility at KEK (High Energy Accelerator Research Organization) is a user dedicated facility with an energy-tunable (0.1 - 35 keV) slow positron beam created by a dedicated 50MeV linac [1]. It operates in a short pulse (width 1-12 ns, variable, \5\times 10 ^6\ e\^+\/s) and a long pulse (width 1.2 \mu s, \5\times 10 ^7\ e\^+\/s) modes of 50 Hz. High energy positrons from pair creation are moderated by reemission after thermalization in W foils, which have negative positron work function of -3 eV. The positrons emitted at 3 eV are then electrostatically accelerated to a desired energy, which is variable from 50 eV to 35 keV depending on the requirement of each individual experiment. The accelerated beam is then magnetically transported. A pulse-stretch section is installed in the middle of the beamline. It stretches the slow positron pulse of width 1.2 \mu s to a variable width from 10 \mu s up to 20 ms at beam energy of 5 keV. The stretched beam is used for the position-sensitive delay-line detector of the LEPD (low-energy positron diffraction) system. It will also be used for the future spectroscopy of the 511 keV annihilation \gamma rays and for the future construction of a short pulse producing system for the positron lifetime study. Four beamline branches, SPF-A3, SPF-A4, SPF-B1 and SPF-B2, are available in the measurement area. (1) A TRHEPD (total-reflection high-energy positron diffraction) station is connected to SPF-A3. TRHEPD is the positron counterpart of RHEED (reflection high-energy positron diffraction). The exceedingly high surface sensitivity of TRHEPD [2] determined the structure of Ge(001)-(4×2)-Pt nanowire after a decade from its discovery [3], rutile-TiO2(110)-(1×2) surface which had been under debate over the past 30 years [4], buckling and sheet-substrate distance of atomic sheet materials such as graphene [5], silicene [6], germanene [7], etc. synthesized on solid substrates. (2) A LEPD station is recently constructed on SPF-A4. (3) SPF-B1 is a general-purpose branch. A Ps– (positronium negative ion) station is currently connected. Ps– is a three-lepton (one positron and two electrons) bound system. Emission of Ps– from W surface is greatly enhanced by coating the surface with sub-monolayer of alkali metal. Laser photo-detachment of one of the electrons makes the ion neutral Ps [8]. By accelerating the ion before the photo-detachment energy-tunable Ps beam is created [9]. A spectroscopic study of the photo-detachment process verified a theoretically predicted shape resonance of 1Po symmetry near the Ps (n=2) formation threshold [10]. (4) Ps-TOF (positronium time-of-flight) station is connected to SPF-B2. It was found that alkali-metal-coating on W surface remarkably enhances Ps emission also [11]. The TOF spectroscopy of the energy distribution of the emitted Ps will give information the electronic states of the surface [12]. References [1] T Hyodo, et al., J. Phys.: Conf. Ser. 262, 012026 (2011). [2] Y. Fukaya, et al., Appl. Phys. Express 7, 056601 (2014). [3] I. Mochizuki et al., Phys. Rev. B 85, 245438 (2012). [4] I. Mochizuki et al., Phys. Chem. Chem. Phys. 18, 7085 (2016) [5] Y. Fukaya, et al., Phys. Rev. B 88, 205413 (2013). [6] Y. Fukaya, et al., Carbon 103, 1 (2016). [7] Y. Fukaya, et al., 2D Materials, 3, 035019 (2016). [8] K. Michishio, et al., Phys. Rev. Lett. 106 153401 (2011). [9] K. Michishio, et al., Appl. Phys. Lett. 100 254102 (2012). [10] K. Michishio, et al., Nature Communications 7, 11060 (2016). [11] H. Terabe, et al., Surf. Sci., 641, 68 (2015). [12] S. Iida, et al., J. Phys.: Condens. Matter 28, 475002 (2016).

Speaker: Toshio HYODO (Slow positron applications at Slow Positron Facility of Institute of Materials Structure Science, KEK)

Slides Toshio_Hyodo_2017-09-13.pptx.pdf

4:50 PM Development of Slow Positron Beam lines and Applications

Speaker: Nagendra Nath MONDAL (Techno India Batanagar (Techno India Group))

Copyright NNM_copyright.pdf

Paper

• NNM_Jpos2017_manuscript.pdf
• NNM_Jpos2017_manuscript.pdf
• NNM_Jpos2017_manuscript.pdf

Slides Nagendra_Mondal_2017-09-13.pdf

5:20 PM Surface Acoustic Wave Enhancement of Photocathode Quantum Efficiency

Speaker: Rolland Johnson (Muons, Inc.)

Slides Rol_Johnson_2017-09-13.pdf

Thursday, September 14

8:00 AM → 8:30 AM Continental Breakfast

8:30 AM → 9:50 AM Plenary 9

Convener: Robert Michaels (Jefferson Lab)

8:30 AM Using positron to measure the speed of light anisotropy

Speaker: Bogdan Wojtsekhowski (Jefferson Lab)

Slides Bogdan_Wojtsekhowski_2017-09-14.pdf

8:50 AM Leptoquark, SUSY, excited leptons, double charge Higgs

Speaker: Sonny Mantry

Slides Sonny_Mantry_talk_slides.pdf

9:25 AM Weak Neutral Current Studies with Positrons

Weak neutral current interactions with charged leptons have offered unique opportunities to study novel aspects of hadronic structure and search for physics beyond the standard model. These studies in the medium energy community have been primarily through parity-violating processes with electron beams, but with the possibility of polarized positron beams, new and complementary observables can be considered in experiments analogous to their electron counterparts. Such studies include elastic proton, deep inelastic, and electron target scattering. In this talk, potential positron neutral current experiments will be discussed along with their potential physics reach, requirements, and feasibility.

Speaker: Seamus Riordan (Stony Brook University)

Slides

• riordan_jpos17_proc.pdf
• riordan_jpos_nc.pdf
• Riordan_proc_11-17_copyright.pdf

9:50 AM → 10:10 AM Coffee Break

10:10 AM → 11:35 AM Plenary 10

Convener: Seamus Riordan (Argonne National Laboratory)

10:10 AM Dark forces searches with positrons: experiments and facilities

Dark Matter elusiveness could be explained by speculating that it lives in a separate sector with respect to the Standard Model (SM) and that interacts with it only by means of messengers. The simplest model foresees just one messenger: a possibly massive vector boson given by a new U(1) symmetry. This mediator can faintly mix with the photon and, hence, interact with SM charged particles, seeing an effective charge equal to epsilon*e, but also the production of axion-like particles or dark scalars can be explored. In searching such mediators at accelerators the fixed-target approach is favoured over head-on collisions because of the higher luminosity; among the different classes of experiment the e+e- annihilation is the less model-dependent approach, and has the potential of positively identifying new particles, regardless from its final state. Producing high-energy, high-intensity positron pulses from a LINAC or extracting them from a e+ ring have been both considered: the different available time structure, repetition rate, maximum energy and beam intensity reflect in different sensitivities for dark sector searches, a panorama of the available facilities in Italy and USA is given.

Speaker: Paolo Valente (INFN Rome)

Slides jpos17_valente_compressed.pptx

10:45 AM Searching for Dark Photons with Positrons at JLab

The interest in the Dark Photon (A' or U) has recently grown, since it could act as a light mediator to a new sector of Dark Matter particles. In this paradigm, the electron-positron annihilation can rarely produce a $\gamma $ U couple. Various experiments (e.g. PADME@LNF, Adv. High Energy Phys. 2014:959802; VEPP-3, arXiv:1207.5089[hep-ex]) have been proposed to detect this process using positron beams impinging on fixed targets. In such experiments, the energy of the photon from the $e^+ e^- \rightarrow \gamma U$ process is measured with an electromagnetic calorimeter and the missing mass is computed (the U interacts weakly with Standard Model matter so it can't be detected). However, the U mass range that can be explored with this technique is limited by the accessible energy in the center of mass frame, which goes as the square root of the beam energy. The realization of a 11 GeV positron beam at Jefferson Lab would allow to search for U masses up to $\sim$100 MeV, reaching unexplored regions of the U parameter space. A preliminary study on the feasibility of a PADME-like experiment at Jefferson Lab has been carried out, assuming a 11 GeV positron beam with a $\sim \mu$A current. The achievable sensitivity was estimated, studying the main sources of background (positron Bremsstrahlung, annihilation into 2 gammas) using CALCHEP and GEANT4 simulations.

Speaker: Luca Marsicano (INFN Genova)

Paper AIP_Conference_Proceedings_License_Agreement.pdf Luca_Marsicano_JPOS17.pdf

Slides Luca_Marsicano_2017-09-14.pdf

11:10 AM Constraints on CPT and Lorentz violation from deep inelastic scattering at a future EIC

Proposed theories beyond the Standard Model (SM) can support the breaking of Lorentz and CPT symmetry. Searches for violations of these symmetries are often performed within the framework of the Standard Model Extension (SME), the most general effective field theory parametrizing CPT and Lorentz violation. The breaking of CPT and Lorentz symmetries in the SME is characterized by coefficients that couple to the usual SM fields. Thus far, searches within many areas of physics have resulted in numerous constraints on coefficients across all sectors of the SME. However, coefficients in the quark- and gluon-sectors remain largely unconstrained. This is in part due to the difficulty in connecting quark-level interactions to hadronic observables. Deep inelastic scattering (DIS) provides a setting where quark-sector coefficients can be directly accessed. In this work, we focus on the observable effects induced by a particular subset of CPT-even coefficients on the unpolarized electron-proton DIS cross-section. Bounds on the coefficients are extracted using pseudo-data mimicking the experimental probes attainable by the proposed EIC configuration at Jefferson Lab.

Speaker: Nathan Sherrill (Indiana University Bloomington)

Paper

• AIP_Conference_Proceedings_License_Agreement.pdf
• LV_EIC_sherrill.pdf
• LVtalk_08_28_17.pdf

11:35 AM → 12:00 PM Proceedings and Roundtable Discussion

Convener: Eric Voutier (CNRS/IN2P3/IPNO - UPS)

12:00 PM → 1:00 PM Lunch Break

1:00 PM → 2:40 PM Plenary 11

Conveners: Farida Selim (Bowling Green State University) , Tony Forest (Idaho State University)

1:00 PM Positron annihilation induced Auger electron spectroscopy (PAES) to investigate the Auger relaxation of deep valence holes in single layer graphene

We discuss our recent report on the direct observation of a low-energy Auger electron emission process initiated by the creation of a deep valence hole in single layer graphene through positron annihilation. Here, an electron from a higher energy level in the valence band fills the valence hole created by positron-electron annihilation. The energy released because of this relaxation is transferred to a third electron, which may have enough energy to escape into the vacuum by overcoming the surface barrier. Emission of electrons into the vacuum through this kind of Auger relaxation of valence hole is possible in single layer graphene because of its wide valence band (~ 20 eV). The time of flight (TOF)-PAES data from single layer graphene showed the presence of a low energy peak at ~ 4 eV as a result of this Auger transition. An empirical model of the Auger process developed with inputs from ab-initio calculations reproduced the low energy Auger line shape. Comparison of experimental and theoretically generated Auger spectrum revealed that more than 80% of the deep valence holes relaxed via an Auger relaxation process. PAES was able to observe this low energy Auger peak because of its ability to measure the Auger spectrum from surfaces without any background from processes unrelated to the Auger relaxation and due to the sensitivity of the technique to the top mono-layer of the sample. The top layer sensitivity comes from the fact that the low energy positrons (< 1 eV) which are deposited on the surface get trapped in an image potential well in the vacuum side of the sample and as a result selectively annihilate with core or valence electrons of the top mono-layer. We will use the present results to motivate how the top layer sensitivity of PAES, when combined with a spin polarized positron beam, can be used to probe the spin density of states in 2D materials and thus, possibly shed light on some of the exotic properties on the surfaces of novel 2 D materials.

Speaker: Varghese Anto Chirayath (Department of Physics, University of Texas at Arlington, 76019)

1:25 PM Electronic structure probed with positronium: Theoretical viewpoint

Positronium can be very helpful when studying the electronic structure of materials. Indeed, the recent experiment [1], where the Ps emission from a copper (110) surface was examined, allowed for the precise determination of the electron chemical potential of copper by means of measuring the Ps affinity. This affinity ($A_{Ps}$) [1] is defined formally via the electron ($\Phi_-$) and positron ($\Phi_+$) work functions as $A_{Ps} = \Phi_- + \Phi_+ - E_{Ps}$, with $E_{Ps}$ ($\doteq$ 6.803 eV) being the Ps ground state binding energy. In the Ps emission experiment, the maximum kinetic energy ($E_K$) of emitted Ps atoms is measured via the Ps time of flight. Since $E_K = -A_{Ps}$, the Ps affinity can be determined. Alternatively, $\Phi_- + \Phi_+$ can be represented via the sum of the electron and positron chemical potentials, which can be obtained using density functional theory for electrons and positrons. The Ps affinity is, therefore, a bulk property. When the accurate correlation functional for positrons [2] is employed, one can check various possibilities for the electron exchange-correlation (XC) functional and compare the resulting electron chemical potential with that deduced from the Ps affinity. Such a procedure was shown to work for Cu [1] where we could find the proper XC functional with a precision of order 10 meV. In this contribution, we investigate computationally other materials, which are expected to show interesting features in their electronic structure like 2D or 3D Dirac cones and Weyl points (half-metals), and check whether the Ps affinity is a negative number so that Ps atoms may escape materials surfaces, allowing thus for the precise measurement of $A_{Ps}$. Several XC functionals are tested for electrons, including the recently introduced meta-generalized-gradient approximation [3]. As for materials, we examine the Heusler alloy/compound Co$_2$MnAl, topological insulator Bi$_2$Se$_3$, and Dirac metal candidate Zr$_2$Te$_2$P. In addition, we inspect the Na$_3$Bi system exhibiting several topological features in its electronic structure; Na$_3$Bi is further considered as a Na-ion battery (anode) material [4]. The possibility of studying the electronic structure of high-entropy alloys via the Ps emission is also discussed. A working positron beam is a necessary condition – but by far not the only one – to perform such experiments. Spin-polarized positron beams may bring further research possibilities (e.g. for spintronics). In this respect, we discuss approaches to the spin-polarized theory of electron-positron correlations [5]. [1] A.C.L. Jones et al., Phys. Rev. Lett. 117, 216402 (2016). [2] B. Barbiellini and J. Kuriplach, Phys. Rev. Lett. 114, 147401 (2015). [3] J. Sun et al., Phys. Rev. Lett. 115, 036402 (2015). [4] J. Sottmann et al., Chem. Mater. 28, 2750 (2016). [5] H. Li et al., J. Phys.: Condens. Matter 27, 246001 (2015).

Speaker: Jan Kuriplach (Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic)

Slides presentation-ESPs-Kuriplach.pdf

1:50 PM Historical survey of beta-particle interaction experiments with asymmetric matter.

Asymmetry is a basic property found at multiple scales in the universe [1]. Asymmetric molecular interactions are fundamental to the operation of biological systems in both signaling and structural roles. Other aspects asymmetry are observed and useful in many areas of science and engineering and have been studied since the discovery of chirality in tartrate salts [2]. The observation of parity violation in beta decay [3, 4] provided some impetus for later experiments using asymmetric particles. Here we survey historical work and experiments related to positron interactions with asymmetric materials in gas liquid and solid forms. Asymmetric interactions may be classified as: 1) stereorecognition, 2) stereoselection and 3) stereoinduction [1]. These three facets of physical stereochemistry are unique but interrelated; examples from chemistry and materials science will be given for illustrative purposes. Experimental positron and electron interactions with asymmetric materials may be classified in like manner. Thus, a qualitative assessment of helical and polarized positron experiments with different forms of asymmetric matter from the past 40 years will be presented, as well as recent experiments with left-hand and right-hand single crystal quartz [5] and organic compounds. The purpose of this classification and review is to evaluate the field for potential new experiments and directions for positron (or electron) studies with asymmetric materials; future directions in bulk and beam positron asymmetric experiments will be discussed. [1] J. M. Hicks, Ed., “Chirality: Physical Chemistry.” Am. Chem. Soc., Washington, DC, 2002. [2] L. Pasteur, CR Hebd. Acad. Sci., 26, 535 (1848). [3] C. S. Wu, E. Ambler, R. W. Hayward, D. D. Hoppes, and R. P. Hudson, Phys. Rev. 105, 1413 (1957). [4] R. L. Garwin, L. M. Lederman and M. Weinrich, Phys. Rev. 105, 1415 (1957). [5] J. D. Van Horn, F. Wu, G. Corsiglia and Y. C. Jean, Def. Diffus. Forum, 373, 221 (2016).

Speaker: J. David Van Horn (University of Missouri-Kansas City)

notes COpyright_Form_Van_Horn-Wu.pdf

Slides 2017-09_JPos-17_Van_Horn_Asymmetric_Beta_Seminar.pdf

2:15 PM Spin Polarized Core Hole Generation in Magnetic Structures using Spin-Polarized Positrons.

The ability of positrons to create polarized core holes stems from the fact that the annihilation rate for spin singlet collisions is 1115 times faster than for spin triplet collisions and thus ~558 times faster with an electron of opposite spin. It is therefore possible to use the inherent polarization of positron beams to create polarized core holes with a net polarization approximately equal in amount and opposite in direction of the incident positron beam. This allows a method for the investigation of magnetism using spin-polarized positron beams due to the preferred annihilation of electrons and positrons when their spins are aligned anti-parallel. As a result of this, by applying an external magnetic field the positrons will annihilate predominantly with electrons from the majority (when magnetic field is parallel to the positron spin-polarization) or the minority (when magnetic field is anti-parallel to the positron spin-polarization) spin directions. At low positron beam energies (less than 20eV) this method can be used to study surface magnetism and also measure element specific magnetization using spin- polarized Positron Annihilation Auger Electron Spectroscopy. Using a variable energy positron beam, spin- polarized positron annihilation spectroscopy and the measured field reversal asymmetry can be used to do magnetic depth profiling in magnetic materials and thin films.

Speaker: Ali R. Koymen (University of Texas at Arlington)

2:40 PM → 3:00 PM Coffee Break

Prepare for Tour

3:00 PM → 5:00 PM Tour

CEBAF, LERF, Hall B

5:00 PM → 6:00 PM Low Energy Appplications Roundtable

Conveners: Farida Selim (Bowling Green State University) , Tony Forest (Idaho State University)

Friday, September 15

8:00 AM → 8:30 AM Continental Breakfast

8:30 AM → 10:20 AM Plenary 12

Conveners: Eric Voutier (CNRS/IN2P3/IPNO - UPS) , John Arrington (Argonne National Laboratory)

8:30 AM JLEIC with Polarized Positrons

Both polarized and unpolarized positron beams with an energy range from a few eV to hundreds of GeV have diverse applications in medicine, material science and nuclear physics. In many cases, they are the unique tools for the study of the physical world. However, the creation of polarized positrons with sufficient intensity is particularly challenging. We propose a dedicated scheme to generate polarized positrons for CEBAF and Jefferson Lab Electron-Ion Collider (JLEIC). Rather than trying to accumulate “hot” positrons after conversion, we will accumulate “cold” electrons before conversion. Charge accumulation additionally provides a novel means to convert high repetition rate (>100 MHz) electron beam from the gun to a low repetition rate (<100 MHz) positron beam for some applications. In this report, we will address the scheme in detail and explain the key areas to reach the parameter requirement.

Speaker: Fanglei Lin (Jefferson Lab)

Slides

• 2017Sep15_JPos17_Lin.docx
• 2017Sep15_JPos17_Lin.pdf
• 2017Sep15_JPos17_Lin.ppt

9:10 AM Positron beams and Two-photon-exchange: The key to precision form factors

One of the puzzles of the proton form factor is the difference between determinations of the form factor ratio using unpolarized and polarized beams. The causal source of this discrepancy is is believed to be two photon exchange (TPE). Recent precision experiments at $Q^2$ below 2.5 $(\mathrm{GeV}/c)^2$ found smaller than expected but clear indication of two photon exchange. However, they could not establish that TPE is indeed the bulk effect driving the discrepancy at larger $Q^2$. This hampers our ability to provide accurate, separated form factors are large $Q^2$, related to the short-range structure of the proton. A multi-GeV positron beam enables a new generation of experiments to study this effect in detail at these larger $Q^2$. In my talk I will discuss the possible reach of experiments at Jefferson Lab using unpolarized positron beams to measure TPE directly.

Speaker: Jan Bernauer (MIT)

Paper copyright.pdf JPOS_Proceedings.pdf

Slides twophoton.pdf

9:45 AM Constraining Coulomb Corrections in DIS with Positrons

Measurements of Deep Inelastic Scattering (DIS) from nuclei, aimed at understanding the nuclear dependence of inelastic structure functions (the EMC effect), are typically performed at high energies, hence effects due to the acceleration of electrons in the Coulomb field of a large $Z$ nucleus are usually ignored. However, there are certain kinematics, in particular at large $x$ and large electron scattering angles (small $\epsilon$) where so-called Coulomb corrections can become significant. This is particularly relevant for experiments that wish to extract information about the nuclear dependence of $R=\sigma_L/\sigma_T$, which need to make measurements over a range of scattered electron momenta and angle at fixed $x$ and $Q^2$. While Coulomb corrections have been studied theoretically and experimentally for quasielastic electron scattering, this is not the case for DIS. Studies of DIS using both electron and positron beams would provide crucial information about the role of Coulomb effects in DIS. This presentation will discuss the impact of Coulomb corrections on existing and future measurements of the nuclear dependence of $R$. The feasibility of measurements that would be maximally sensistive to placing constraints on Coulomb corrections will also be discussed.

Speaker: David Gaskell (Jefferson Lab)

Slides gaskell_cc_dis_jpos.pdf

10:20 AM → 10:40 AM Coffee Break

10:40 AM → 11:50 AM Plenary 13

Convener: Charles Hyde (Old Dominion University)

10:40 AM Measuring the Coulomb Sum Rule at JLab

In order to determine the Coulomb sum in nuclei, a precision measurement of inclusive electron scattering cross sections in the quasi-elastic region was performed at Jefferson Lab. Incident electrons with energies ranging from 0.4 GeV to 4 GeV scattered from $^{4}He$,$^{12}C$,$^{56}Fe$ and $^{208}Pb$ nuclei at four scattering angles ($15^{\circ},60^{\circ},90^{\circ},120^{\circ}$) and scattered energies ranging from 0.1 GeV to 4 GeV. The Rosenbluth separation method is used to extract the transverse and longitudinal response functions at three-momentum transfers in the range 0.55 GeV/c$\leq \mid \overrightarrow{q}\mid\leq$1.0 GeV/c. The Coulomb Sum is obtained for $^{56}Fe$ and $^{12}C$, and compared to predictions. The latest preliminary results will be shown. The impact of a similar positron beam measurement, and its importance in testing coulomb corrections used to extract the Born cross-section, will also be discussed.

Speaker: Michael Paolone (Temple University)

Paper AIP_Conference_Proceedings_License_Agreement.pdf csrJpos.pdf

Slides mpaoloneJPos.pdf

11:15 AM Charge and spin asymmetries in elastic lepton-nucleon scattering

Elastic electron/positron scattering off a nucleus has proved to be an efficient tool to study the structure of nucleons. Modern cross section and asymmetry measurements at Jlab require radiative corrections to be taken into account. In particular, two-photon exchange (TPE) effects beyond the standard model-independent contributions have to be considered at the current level of precision. Experimentally, the real part of the TPE amplitude can be studied through charge asymmetry measurements in elastic electron/positron scattering on a proton target. Besides that, the imaginary part of the TPE amplitude can be assessed through measurements of normal single spin asymmetries (SSA) (e.g., JLab experiment [1]). Corresponding theoretical calculations are in the focus of this report. In my talk, I will present our charge-asymmetry predictions for scattering of unpolarized and massive leptons on the proton target in connection to the MUSE experiment at PSI [2]. These predictions include the contributions coming from hard TPE. I will also discuss our neutron normal SSA calculation at GeV beam energies and compare it with the results of the measurement [1]. [1] Y. W. Zhang et al., PRL 115, 172502 (2015). [2] R. Gilman et al. (MUSE Collaboration), arXiv:1303.2160.

Speaker: Oleksandr Koshchii (George Washington University)

Paper Jpos17.license.koshchii.pdf JPos17.proceedings.koshchii.pdf

Slides Koshchii_PresentationPDF.pdf

11:50 AM → 1:00 PM Lunch Break

1:00 PM → 2:30 PM Plenary 14

Convener: Joe Grames (JLab)

1:00 PM High current polarized electron source + thermal

Speaker: V. Tioukine

Slides JPos17-V-Tyukin.pdf

1:30 PM High Current Polarized Electron Source

Highlights of R&D to improve the performance of polarized electron sources and prolong the lifetime of GaAs Superlattice will be presented.

Speaker: Riad Suleiman (Jefferson Lab)

Slides JPos17_PolElectronSource_suleiman_talk.pptx

2:00 PM High Current polarized electron source for future eRHIC

Speaker: Erdong Wang (bnl)

Slides High_current_polarized_electron_source.pptx

2:30 PM → 2:50 PM Coffee Break

2:50 PM → 3:50 PM Plenary 15

Convener: Matt Poelker (Jefferson Lab)

2:50 PM PEPPo Based Polarized Positron Beam Formation for JLEIC

JLEIC polarized positron program calls for a luminosity no more than one order of magnitude lower than the electron program. In this talk, a PEPPo based positron beam formation scheme will be presented. The combination of PEPPo’s low positron yield, the low duty factor injection bunch train required by JLEIC e-/e+ ring, as well as the mismatch of the JLEIC and CEBAF RF frequency, pose major challenges for this injection scheme, requiring enormous electron charge per bunch at the e-/e+ conversion. We propose a solution with a small 748.5MHz ring accumulating the “cold” electron, and extracting the long bunch train in 17MHz bunch reprate with a stripline harmonic RF kicker. Preliminary study shows that this scheme can achieve the ~0.2A beam current for the ~10^33/cm^2/s luminosity goal.

Speaker: Jiquan Guo (Jefferson Lab)

Slides JPOS17_Jiquan_Guo.pptx

3:20 PM High-flux positron source based on an SRF electron linac and liquid-metal target

High power electron beams from superconducting radio-frequency (SRF) linacs have numerous commercial applications including x-ray sterilization, active interrogation and radiography, industrial and medical isotope production, and free electron lasers. Many of these applications use an x-ray beam generated when the electrons strike a high-Z target. Such a target can be optimized for production of electron-positron pairs by high-energy Bremsstrahlung photons. Conventional x-ray and positron converters are made of solid, high-Z metals which can only dissipate a few kilowatts of beam power without complicated cooling systems. A liquid-metal target, on the other hand, can dissipate large quantities of heat by forced convection of the target material itself. This contribution will discuss a positron production system built at Niowave, Inc. for use with superconducting radio-frequency (SRF) linacs. The target is designed for use with a 10 MeV electron beam at 10 kW beam power in continuous-wave operation – producing a 20 nA positron current at the capture target for a capture field of 0.2 T. Future plans involve upgrading the target design to handle 100 kW at 10 MeV. Details to be presented include the predicted production rates for positrons, the prototype magnetic design for positron collection, design and construction of the liquid-metal targets with the magnetic collection system, testing of target flow in high magnetic fields, and the building and testing of prototype targets with electron beam from an SRF linac.

Speaker: Chase Boulware (Niowave, Inc.)

Slides 170630_High-flux_positrons_from_SRF_linac_(Niowave).pdf

3:50 PM → 4:00 PM Closing Remarks