Positron Beam Research Articles

Investigation on structural and optical properties of ZnSe thin films prepared by selenization

In this paper, ZnSe films were prepared on quartz substrates by selenizing as-sputtered zinc films at different temperatures for 6 h. The crystal structure, morphology, microscopic defects and optical properties of the ZnSe films were characterized by XRD, SEM, Raman spectroscopy, UV–visible spectrophotometer and slow positron beam Doppler broadening spectroscopy, respectively. The results showed that ZnSe thin films had the best structural and optical properties when the selenization temperature was 700 °C . All ZnSe thin films have a hexagonal wurtzite structure. The results of XRD、SEM and Raman spectra all showed that the crystallization and molar ratio of the films were significantly optimized with the increase of temperature. The transmission spectra of the films showed an obvious absorption edge of ZnSe and increased transmittance with the increase of temperature. The variation of defect concentration and types in the ZnSe thin films were released visually by slow positron beam Doppler broadening energy spectra.

Light dark matter searches with positrons

We discuss two complementary strategies to search for light dark matter (LDM) exploiting the positron beam possibly available in the future at Jefferson Laboratory. LDM is a new compelling hypothesis that identifies dark matter with new sub-GeV “hidden sector” states, neutral under standard model interactions and interacting with our world through a new force. Accelerator-based searches at the intensity frontier are uniquely suited to explore it. Thanks to the high intensity and the high energy of the Continuous Electron Beam Accelerator Facility (CEBAF) beam, and relying on a novel LDM production mechanism via positron annihilation on target atomic electrons, the proposed strategies will allow us to explore new regions in the LDM parameters space, thoroughly probing the LDM hypothesis as well as more general hidden sector scenarios.

A new system for real-time data acquisition and pulse parameterization for digital positron annihilation lifetime spectrometers with high repetition rates

We present a new system for high repetition rate and real-time pulse analysis implemented at the Monoenergetic Positron Source (MePS) at the superconducting electron LINAC ELBE at Helmholtz-Zentrum Dresden-Rossendorf. Dedicated digital signal processing and optimized algorithms are employed allowing for high bandwidth throughput, online pulse analysis and filtering. Positrons generated from radioisotopes and from bremsstrahlung pair production by means of highly intense accelerator-based positron beams serve as a microstructure probe allowing material characterizations with respect to chemical, mechanical, electrical, and magnetic properties. Positron annihilation lifetime events with up to 13 MHz repetition rate are being processed online without losses while performing signal selections for pile-up reduction, online energy calibration, and — for radioisotope-based measurements — identification of start and stop events.

Design study of CEPC lower emittance booster

The CEPC booster needs to provide electron and positron beams to the collider at different energy with required injection efficiency. At Higgs energy, only the on-axis injection from booster to collider can be fulfilled in CDR. With a consideration of keeping the off-axis injection scheme for safety and reliability, a new booster design based on TME lattice is considered to reduce the emittance by three times after CDR. The new booster design has reached an emittance of 1.3 nm at 120 GeV and the DA without errors is even better than CDR. The geometry of new booster is designed carefully in order to share the same tunnel with collider. The design status of CEPC new booster including parameters, optics, dynamic aperture and geometry is discussed in this paper.

Design and beam dynamics of CEPC damping ring system

A damping ring system which includes a small 1.1 GeV ring and two transport lines is introduced in CEPC linac in order to reduce the transverse emittance of positron beam at the end of linac and hence reduce the beam loss in the booster. This paper introduces the parameter choice and optics study of damping ring. The corresponding instability effect and IBS effect are also checked to make sure the design current and design emittance can be realized. Except for damping ring, two transport lines are needed to match the parameters between linac and damping ring. Both designs for energy compressor and bunch compressor including beam simulations are discussed in this paper.

Effect of chlorination on positron binding to hydrocarbons: Experiment and theory

Measured and calculated positron binding energies are presented for a range of hydrocarbons with up to six carbon atoms (viz., methane, acetylene, ethylene, ethane, propane, butane, and hexane) and their chlorinated counterparts. Both experiment and theory confirm the large effect that the chlorine atoms have on the positron binding energy and the strong sensitivity of the binding energy to the exact position of the chlorine atoms. The experimental binding energies have been obtained by measuring positron resonant annihilation using a trap-based positron beam. The calculations are performed using the previously developed model-correlation-potential method [A. R. Swann and G. F. Gribakin, J. Chem. Phys. 149, 244305 (2018)]. The overall trends are discussed with regard to the molecular polarizability, dipole moment, and geometry. Good agreement between theory and experiment is found, with the exception of the chlorinated ethylenes and chlorinated hexane. Calculations of the electron-positron annihilation rate in the bound state are also presented.

Ultrashort, MeV-scale laser-plasma positron source for positron annihilation lifetime spectroscopy

Sub-micron defects represent a well-known fundamental problem in manufacturing since they can significantly affect performance and lifetime of virtually any high-value component. Positron annihilation lifetime spectroscopy is arguably the only established method capable of detecting defects down to the sub-nanometer scale but, to date, it only works for surface studies, and with limited resolution. Here, we experimentally and numerically show that laser-driven systems can overcome these well-known limitations, by generating ultra-short positron beams with a kinetic energy tuneable from 500 keV up to 2 MeV and a number of positrons per shot in a 50 keV energy slice \color{black} of the order of $10^3$. Numerical simulations of the expected performance of a typical mJ-scale kHz laser demonstrate the possibility of generating MeV-scale narrow-band and ultra-short positron beams with a flux exceeding $10^5$ positrons/s, of interest for fast volumetric scanning of materials at high resolution.

Search for the anomalous WWγ couplings through the process [formula omitted] at ILC with unpolarized and polarized beams

We investigate the anomalous W+W−γ couplings through the process e−e+→νeν‾eγ for unpolarized and polarized electron (positron) beams at the International Linear Collider. We give the 95% Confidence Level limits on the anomalous couplings with and without the systematic uncertainties for various values of center-of-mass energies and the integrated luminosities. We show that the obtained limits on the anomalous couplings through the process e−e+→νeν‾eγ can highly improve the current experimental limits.

Double deeply virtual Compton scattering with positron beams at SoLID

Double Deeply Virtual Compton Scattering (DDVCS) is the only experimental channel for the determination of the dependence of the Generalized Parton Distributions (GPDs) on both the average and the transferred momentum independently. The physics observables of the electron induced di-muon production reaction $\vv{e}^{\pm}p \to e^{\pm}p\mu^+\mu^-$ off unpolarized hydrogen are discussed. Their measurement with the high luminosity and large acceptance SoLID spectrometer at the Thomas Jefferson National Accelerator Facility, using polarized and unpolarized positron and electron beams at 11 GeV is investigated. This experimental configuration is shown to provide unprecedented access to the GPDs with the determination of the real and imaginary parts of the Compton Form Factor ${\mathcal H}$ in an unexplored phase space, and to enable an exploratory investigation of higher twist effects.

Deeply virtual Compton scattering on the neutron with positron beam

Measuring DVCS on a neutron target is a necessary step to deepen our understanding of the structure of the nucleon in terms of Generalized Parton Distributions (GPDs). The combination of DVCS observables on neutron and proton targets allows to perform a flavor decomposition of the Compton Form Factors (CFFs), which are related to integrals of GPDs. Moreover, neutron-DVCS plays a complementary role to DVCS on a transversely polarized proton target in the determination of the CFF of the GPD E, the least known and constrained GPD that enters Ji’s angular momentum sum rule. A measurement of the beam-charge asymmetry (BCA) in the \(e^{\pm } d\rightarrow e^{\pm }n\gamma (p)\) reaction can significantly impact the experimental determination of the real CFFs of the E and, to a lesser extent, \(\widetilde{H}\) CFFs.

Magnetically collimated relativistic charge-neutral electron–positron beams from high-power lasers

We report the observation of charge-neutral MeV electron–positron beams from magnetically collimated laser-driven pair-production experiments. Relativistic pairs of electrons were generated from laser–solid interactions in an external 13-T mirror field. The pairs were subsequently confined, deflected, or collimated depending on the particle energy and field strength and measured by a magnetic particle spectrometer. Equal quantities of positrons and electrons were measured in the collimated beams with an energy around 13 MeV along the magnetic mirror axis.

Defects introduced by helium irradiation at different temperatures in W and W-5wt%Re alloy

W and W-5wt%Re alloy were irradiated by 50 keV helium ions at RT, 200 °C, 350 °C and 500 °C to investigate the influence of temperature on the microdefects and surface morphology in the sample. The microdefects in the subsurface region and the surface damage caused by irradiation were characterized by Doppler broadening spectroscopy (DBS) of slow positron beam, transmission electron microscope (TEM) and scanning electron microscopy (SEM), respectively. The results show that a large number of vacancy-type defects were produced after helium ions irradiation, especially at 500 °C, the vacancy-type defects of W alloy are higher than that of W-5wt%Re alloy, which indicated that Re restricted the behavior of vacancy-type defects in tungsten to some extent. The surface irradiation damage such as crystal gap enlargement, grain fell off and pores were observed at different irradiation temperatures. Compared with the coral-like structure observed in W, the circular objects and Hilly-gully structure were observed in W-5wt%Re alloy, and both of them increased and grew with the increase of irradiation temperature. It was speculated that the presence of Re led to the difference by analyzing the formation mechanism of the coral-like structure and the Hilly-gully structure.

Open Porosity and Pore Size Distribution of Mesoporous Silica Films Investigated by Positron Annihilation Lifetime Spectroscopy and Ellipsometric Porosimetry.

Tunable mesoporous silica films were prepared though a sol-gel process directed by the self-assembly of various triblock copolymers. Positron annihilation γ-ray energy spectroscopy and positron annihilation lifetime spectroscopy (PALS) based on intense pulsed slow positron beams as well as ellipsometric porosimetry (EP) combined with heptane adsorption were utilized to characterize the open porosity/interconnectivity and pore size distribution for the prepared films. The consistency between the open porosities was examined by the variations of orthopositronium (o-Ps) 3γ annihilation fractions and the total adsorbed volumes of heptane. The average pore sizes deduced by PALS from the longest-lived o-Ps lifetimes are in good agreement with those by EP on the basis of the Barrett–Joyner–Halenda model, as indicated by a well fitted line of slope k = 1. The results indicate that the EP combined with heptane adsorption is a useful method with high sensitivity for calibrating the mesopore size in highly interconnected mesoporous films, whereas PALS is a novel, complementary tool for characterizing both closed and open pores in them.

Beam charge asymmetries for deeply virtual Compton scattering off the proton

The unpolarized and polarized Beam Char\-ge Asymmetries (BCAs) of the $\vv{e}^{\pm}p \to e^{\pm}p \gamma$ process off unpolarized hydrogen are discussed. The measurement of BCAs with the CLAS12 spectrometer at the Thomas Jefferson National Accelerator Facility, using polarized positron and electron beams at 10.6 GeV is investigated. This experimental configuration allows to measure azimuthal and $t$-dependences of the unpolarized and polarized BCAs over a large $(x_B,Q^2)$ phase space, providing a direct access to the real part of the Compton Form Factor (CFF) ${\mathcal H}$. Additionally, these measurements confront the Bethe-Heitler dominance hypothesis and eventual effects beyond leading twist. The impact of potential positron beam data on the determination of CFFs is also investigated within a local fitting approach of experimental observables. Positron data are shown to strongly reduce correlations between CFFs and consequently improve significantly the determination of $\Re {\rm e} [\mathcal{H}]$.

Polarization transfer in $$\varvec{e}^+p \rightarrow e^+ \varvec{p}$$ scattering using the Super BigBite Spectrometer

The effects of multi-photon-exchange and other higher-order QED corrections on elastic electron-proton scattering have been a subject of high experimental and theoretical interest since the polarization transfer measurements of the proton electromagnetic form factor ratio \(G_E^p/G_M^p\) at large momentum transfer \(Q^2\) conclusively established the strong decrease of this ratio with \(Q^2\) for \(Q^2 \gtrsim 1\) \(\hbox {GeV}^2\). This result is incompatible with previous extractions of this quantity from cross section measurements using the Rosenbluth Separation technique. Much experimental attention has been focused on extracting the two-photon exchange (TPE) effect through the unpolarized \(e^+p/e^-p\) cross section ratio, but polarization transfer in polarized elastic scattering can also reveal evidence of hard two-photon exchange. Furthermore, it has a different sensitivity to the generalized TPE form factors, meaning that measurements provide new information that cannot be gleaned from unpolarized scattering alone. Both \(\epsilon \)-dependence of polarization transfer at fixed \(Q^2\), and deviations between electron-proton and positron-proton scattering are key signatures of hard TPE. A polarized positron beam at Jefferson Lab would present a unique opportunity to make the first measurement of positron polarization transfer, and comparison with electron-scattering data would place valuable constraints on hard TPE. Here, we propose a measurement program in Hall A that combines the Super BigBite Spectrometer for measuring recoil proton polarization, with a non-magnetic calorimetric detector for triggering on elastically scattered positrons. Though the reduced beam current of the positron beam will restrict the kinematic reach, this measurement will have very small systematic uncertainties, making it a clean probe of TPE.

Elastic positron–proton scattering at low Q$$^2$$

Systematic differences in the the proton's charge radius, as determined by ordinary atoms and muonic atoms, have caused a resurgence of interest in elastic lepton scattering measurements. The proton's charge radius, defined as the slope of the charge form factor at Q$^2$=0, does not depend on the probe. Any difference in the apparent size of the proton, when determined from ordinary versus muonic hydrogen, could point to new physics or need for the higher order corrections. While recent measurements seem to now be in agreement, there is to date no high precision elastic scattering data with both electrons and positrons. A high precision proton radius measurement could be performed in Hall B at Jefferson Lab with a positron beam and the calorimeter based setup of the PRad experiment. This measurement could also be extended to deuterons where a similar discrepancy has been observed between the muonic and electronic determination of deuteron charge radius. A new, high precision measurement with positrons, when viewed alongside electron scattering measurements and the forthcoming MUSE muon scattering measurement, could help provide new insights into the origins of the proton radius puzzle, and also provide new experimental constraints on radiative correction calculations.

Compton-driven beam formation and magnetization via plasma microinstabilities

Compton scattering of gamma rays propagating in a pair plasma can drive the formation of a relativistic electron positron beam. This process is scrutinized theoretically and numerically via particle-in-cell simulations. In addition, we determine in which conditions the beam can prompt a beam-plasma instability and convert its kinetic energy into magnetic energy. We argue that such conditions can be met at the photosphere radius of bright gamma-ray bursts.

Optimized injector schemes for FCC-ee positron production

Injectors and in particular the positron production systems are a crucial element in linear and circular colliders. The proposed FCC-ee injector linac accelerates both electrons and positrons up to 6 GeV in order to be injected into the Pre-Booster Ring (PBR) with a high intensity of 3.5 nC per bunch for the Z running mode. The current design baseline of the FCC-ee injector linac is based on the SuperKEKB scheme, in which the electron and the positron beams share the same linacs with a fixed target configuration on-axis hole for electron beam passage. One of the main drawbacks of this scheme is the positron production efficiency at the exit of the injector linac. In order to achieve a better performance and a higher positron yield, in this paper we study different bypass injection options. Start-to-end design and optimization for beam production, acceleration and transport from the feeding electron source to the positron damping ring are also being presented.

Beam particle tracking with a low-mass mini time projection chamber in the PEN experiment

The international PEN collaboration aims to obtain the branching ratio for the pion electronic decay π+→e+νe(γ), aka πe2, to a relative precision of 5×10−4 or better. The PEN apparatus comprises a number of detection systems, all contributing vital information to the PEN event reconstruction. This paper discusses the design, performance, and Monte Carlo simulation of the mini time projection chamber (mTPC) used for pion, muon, and positron beam particle tracking. We also review the use of the extracted trajectory coordinates in the analysis, in particular in constructing observables critical for discriminating background processes, and in maximizing the fiducial volume of the target in which decay event vertices can be accepted for branching ratio extraction without introducing bias.

Development of digital BPM front-end conditioning circuit for BEPCII linac

Aiming to control the rising failure rate of electronics system for BEPC II linac’s beam position measurement, considering the physical design parameters of BEPC II and the requirements of band-pass sampling of BPM electronics ADC chip, a digital BPM RF front-end electronics with high isolation degree and good amplitude and phase consistency is designed. The digital BPM electronics system adopts MicroTCA 4.0 system architecture, takes FPGA as the main controller, and is designed based on EDA software. This paper mainly introduces RF power amplifier, digital adjustable attenuator and bandpass filter in RF front-end electronics module, as well as the laboratory and online test results. In the collision mode of BEPC II, positron beam was used to complete the electronic system online test, and the measurement accuracy of x-direction was about 38.46 μm, while that of y-direction was about 26.16 μm. The measurement accuracy and system stability of the proposed method are better than that of commercial analog BPM electronics module, and can meet the beam position measurement requirements of BEPC II linac.