Do unpolarized electrons affect the polarization of a stored beam?

We present a short overview of the PAX physics case for polarized antiprotons. In order to progress towards a stored polarized antiproton beam, it is crucial to understand the interaction of polarized protons with unpolarized electrons. Therefore investigations that address in particular the contributions of electrons to the polarization buildup of a stored proton beam are presented here in more detail. The measurement of the depolarizing p e cross section settled a long-standing controversy about the role of electrons in the polarization buildup of a stored beam by spin-filtering. Instead of studying the buildup of polarization in an initially unpolarized beam, here the inverse situation was investigated by observation of the depolarization of an initially polarized beam. For the first time, electrons in the electron cooler have been used as a target to study their depolarizing effect on a 49.3 MeV proton beam orbiting in COSY. The foreseen spin-filtering experiments at COSY–Jülich and at the AD of CERN are briefly discussed as well.     

Observation of polarized positrons from an undulator-based source

An experiment (E166) at the Stanford Linear Accelerator Center has demonstrated a scheme in which a multi-GeV electron beam passed through a helical undulator to generate multi-MeV, circularly polarized photons which were then converted in a thin target to produce positrons (and electrons) with longitudinal polarization above 80% at 6 MeV. The results are in agreement with GEANT4 simulations that include the dominant polarization-dependent interactions of electrons, positrons, and photons in matter.     

Polarization of neutrons from the d-t reaction and in n-p scattering

The polarization of 32 MeV neutrons from the d-t reaction has been measured using n-He scattering at an angle of 130° (lab). A He gas scintillator was used as a scatterer, and the scattered neutrons were measured by counting the He recoils which were coincident with pulses from the side detector and which were synchronized with the deuteron beam bunch. Polarizations in n-p scattering have been measured using the polarized neutrons at angles of 30°, 40° and 50° (lab).     

Measurements of the electric form factor of the neutron up to Q2=3.4 GeV2 using the reaction 3He(e,e'n)pp

The electric form factor of the neutron was determined from studies of the reaction 3He(e,e'n)pp in quasielastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear polarization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling the Q2 range over which it is known, we find G(E)(n)=0.0236±0.0017(stat)±0.0026(syst), 0.0208±0.0024±0.0019, and 0.0147±0.0020±0.0014 for Q(2)=1.72, 2.48, and 3.41 GeV2, respectively.     

Experimental study on helium optical electron polarimetry

Optical electron polarimetry is suitable for calibration of a spin-polarized electron source, especially for measurement of polarization of spin-polarized electron beam. In this paper, a new optical electron polarimeter is described, which is based on the circularly polarized He radiation induced by the bombarding of He atoms with spin-polarized electrons. The theoretical basis of the optical electron polarimetry and the structure of the optical electron polarimeter are discussed. The measurement of polarization of spin-polarized electrons produced from a new GaAs （100） spin-polarized electron source is carried out. The result of polarization of 30.8% for our spin-polarized electron source is obtained using the He optical electron polarimeter.     

A method to polarize stored antiprotons to a high degree

Polarized antiprotons can be produced in a storage ring by spin-dependent interaction in a purely electron-polarized hydrogen gas target. The polarizing process is based on spin transfer from the polarized electrons of the target atoms to the orbiting antiprotons. After spin filtering for about two beam lifetimes at energies T approximately equal 40-170 MeV using a dedicated large acceptance ring, the antiproton beam polarization would reach P=0.2-0.4. Polarized antiprotons would open new and unique research opportunities for spin-physics experiments in p(-) p interactions.     

Development of a Spin-Exchange Optical Pumping-Based Polarized ~3He System at the China Spallation Neutron Source(CSNS)

Polarized ~3He neutron spin filters(NSFs) can be used as a vital tool for neutron polarization production and analysis.The China Spallation Neutron Source(CSNS),as one of the major neutron facilities in China,has committed resources to the development of a polarized ~3He NSF program to support its growing polarized neutron research.A spin-exchange optical pumping(SEOP)-based polarized ~3He system and other necessary hardware for NSF transport has been recently developed.The performance of the system is benchmarked using an in-house developed cell named "Trident".Neutron beam measurements yield a ~3 He polarization of 77% with over 200 h of on-beam relaxation time.Combining this newly developed SEOP system with the recently reported cell fabrication station,CSNS is now capable of the fully self-sustained production of ~3He NSFs that shall support its future neutron polarization research.     

Spin correlation in pp elastic scattering and in pion production near threshold

An internal polarized gas target in conjunction with a beam of polarized protons stored in the IUCF Cooler storage ring has been used to measure analyzing power and spin correlation parameters between 200 MeV and 450 MeV over a wide range of angles. A consistent absolute beam and target polarization calibration was established by ramping the stored protons up and down in energy. The analyzing power A _y and the spin correlations A _xx , A _yy and A _xz were measured with transverse beam polarization, while determination of A _zz required the development of a longitudinally polarized beam. First experiments on the spin dependence on π₀-production have recently been carried out.     

The d + d reactions at low energies II. Comparison of angular distributions of outgoing nucleon polarizations for the charge symmetric reactions2H(d,p)3H and2H(d,n)3He

Precision measurement of the angular distribution of polarized protons from the²H(d,p)³H reaction at about 1 MeV is compared with experimental polarization data from the mirror reaction²H(d,n)³He at the same entrance and exit channel energies. In both comparisons, differences in polarizations of outgoing nucleons from the d + d charge symmetric reactions were found to be larger than experimental uncertainties.     

Polarization detection of trapped electrons via interaction with polarized atoms

Electrons were trapped in an electrostatic quadrupole trap with superimposed homogeneous magnetic field. The electrons were polarized by spin exchange with a polarized atomic beam. The free trapped electron polarization was converted to a change in the electron translational energy via spin-dependent inelastic collisions with the atomic beam, and the electron translational temperature was monitored. Discussed are the development of this variation of the measurement technique, characteristics of electron storage, and the electron-polarized atom inelastic interaction as a function of electron temperature and time. The method has been applied to the detection of the (g-2) resonance of free, stored electrons.     

Spin-dependence and coherent summations in two-nucleon transfer reactions

The differential cross sections of the ¹⁹F(p, ³He)¹⁷O and ¹⁹F(p, t)¹⁷F reactions leading to the ground and first three excited states have been measured at an incident energy of 42.4 MeV. The polarization analysing powers of two of the (p, ³He) reactions have been measured at a higher energy of 49.5 MeV with a polarized incident beam. The experimental results have been analysed in terms of conventional DWBA theory, and the importance of interference terms arising from the proper coherent summations contained in the definition of the differential cross section and analysing power, have been investigated. It is found that the interference term arising from the coherent summation over the transferred spin S is significant and should not normally be neglected.     

The reaction3He(d,p)4He with a polarized target

The left-right asymmetry for the³He(d, p)⁴He reaction has been measured with a polarized³He target at a bombarding energy of 12 MeV. The experimental results are compared with proton polarization measurements for unpolarized target and beam. The construction of the optically pumped³He target is described. A special design has been chosen to overcome the difficulty that the target contamination caused by the bombarding particles destroys the polarization. The target can easily be purified without removing it from the experimental set-up.     

Q2 evolution of the generalized Gerasimov-Drell-Hearn integral for the neutron using a 3He target

We present data on the inclusive scattering of polarized electrons from a polarized 3He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5 degrees. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference sigma(1/2)-sigma(3/2). We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q2 of 0.1-0.9 GeV2.     

Measurement of spin-dependent total cross-section differenceΔσ T in neutron-proton scattering at 16 MeV

A new measurement ofΔσ _T for polarized neutrons transmitted through a polarized proton target at 16.2 MeV has been made. A polarized neutron beam was produced with the³H(d, n)⁴ He reaction; proton polarization over 90% was achieved in a frozen spin target of 20 cm³ volume. The measurement yields the valueΔσ _T=(?126±21±14) mb. The result of a simple phase shift analysis for the³ S ₁?³ D ₁ mixing parameter ε₁ is presented and compared with the theoretical potential model predictions.     

Measurement of the proton polarization in the 3He(d,p)4He Rection between 2.0 and 6.0 MeV

Angular distributions of the polarization of ³He(d, p)⁴He protons have been measured for unpolarized deuterons with laboratory energies of 1.99, 2.81, 3.94 and 6.00 MeV. The polarizations were determined from the left-right asymmetry of elastic scattering through 60° in ⁴He gas using a vaned polarimeter whose analyzing power was computed by trajectory tracing from the known p-α polarizations. A contour diagram of proton polarization for deuteron energies of 1 to 12 MeV is presented. A Legendre polynomial expansion of four or five terms has been fitted to the products p(θ)σ(θ). The coefficients of the expansions of these and other measurements show resonance-like behaviour at ⁵Li excitations of 20.0 and 20.9 MeV. The vector-polarized beam and polarized-target analyzing powers are compared with the proton polarization. The proton polarizations are in good agreement with the ³H(d, n)⁴He neutron polarizations when compared at the same entrance-channel energy but disagree when compared at equal exit-channel energies or compound-nucleus excitations.     

Charge form factor of the neutron at low momentum transfer from the 2H-->(e-->,e'n)1H reaction

We report new measurements of the neutron charge form factor at low momentum transfer using quasielastic electrodisintegration of the deuteron. Longitudinally polarized electrons at an energy of 850 MeV were scattered from an isotopically pure, highly polarized deuterium gas target. The scattered electrons and coincident neutrons were measured by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The neutron form factor ratio GEn/GMn was extracted from the beam-target vector asymmetry AedV at four-momentum transfers Q2=0.14, 0.20, 0.29, and 0.42 (GeV/c)2.     

Measurement of the electric form factor of the neutron through d-->(e-->,e(')n)p at Q2 = 0.5 (GeV/c)(2)

We report the first measurement using a solid polarized target of the neutron electric form factor G(n)(E) via d-->(e-->,e(')n)p. G(n)(E) was determined from the beam-target asymmetry in the scattering of longitudinally polarized electrons from polarized deuterated ammonia ( 15ND3). The measurement was performed in Hall C at Thomas Jefferson National Accelerator Facility in quasifree kinematics with the target polarization perpendicular to the momentum transfer. The electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle segmented detector. We find G(n)(E) = 0.04632+/-0.00616(stat)+/-0.00341(syst) at Q2 = 0.495 (GeV/c)(2).     

Evidence for a new low-lying resonance state in 7He

Low-lying resonance states in 7He(6He+n), formed after fragmentation reactions of a 227 MeV/nucleon 8He beam on a carbon target, have been studied. Coincidences between 6He nuclei and neutrons, corresponding to the one-neutron knockout channel in 8He, were selected. The relative energy spectrum in the 6He+n system shows a structure, which is interpreted as the 7He ( Ipi = 3/2(-)) ground state, unbound with 0.43(2) MeV relative to the 6He+n system and a width of Gamma = 0.15(8) MeV overlapping with an excited ( Ipi = 1/2(-)) state observed at 1.0(1) MeV with a width of Gamma = 0.75(8) MeV.     

In-situ SEOP polarizer and initial tests on a high flux neutron beam

Polarized ³He has shown its unique characteristics in many areas of polarized neutron scattering, its ability to polarize neutrons at short wavelengths, accept wide-angle and divergent beams and low backgrounds enable new classes of experiments. While polarized ³He is not a steady state solution as commonly applied, the benefits have been shown to offset the drawbacks of polarizing and refreshing the polarization in the neutron spin filter cells. As an extension of this work, in-situ polarization using the spin-exchange optical pumping (SEOP) method was explored as a means to construct a system which could be used to polarize ³He in the state used for an effective neutron spin filter to constant polarization while on the neutron beam. An in-situ SEOP polarizer was constructed. This device utilized many devices and principles developed for neutron spin filters which are polarized off the beam line using either SEOP or metastability exchange optical pumping (MEOP) under the same research program. As a collimation of this work effects of extremely high neutron capture flux density incident on the in-situ polarizer were explored.     

The Ar(p, d)Ar reaction

The ³He spin analysing power of hydrogen (protons) has been measured at a c.m. energy of 20 MeV at 14 c.m. angles from 40° to 160°. The measurement of ³He target polarization was calibrated by measuring the ³He spin analysing power of ⁴He at an energy and angle where it was ?1.00. The proton-³He differential elastic scattering cross section is also reported at a c.m. energy of 20.0 MeV for 36 c.m. angles from 20° to 170°.