Edward J. Stephenson

Measurement of the absolute n p scattering differential cross section at 194 MeV

We describe a double-scattering experiment with a novel tagged neutron beam to measure differential cross sections for np backscattering to better than +/-2% absolute precision. The measurement focuses on angles and energies where the cross section magnitude and angle dependence constrain the charged pion-nucleon coupling constant, but existing data show serious discrepancies among themselves and with energy-dependent partial-wave analyses. The present results are in good accord with the partial-wave analyses, but deviate systematically from other recent measurements.

The Kent State "2π" Neutron Polarimeter

We designed, tested and calibrated a medium‐energy neutron polarimeter of a new design, which we call the ‘‘2π’’ polarimeter because of its symmetric coverage of all 2π of azimuth for double‐scattered neutrons. During calibration tests at the IUCF we observed an over all neutron time‐of‐flight resolution of 360 ps. The measured analyzing power is typically 39% for neutrons of both 130 and 165 MeV for optimum software cuts. The efficiency is typically 0.3%.

Modifications of the Effective Isovector Interaction from Studies of (\left( {\vec{p},\vec{p}'} \right)) Polarization Transfer

A complete set of polarization transfer coefficients (D ij ), as well as differential cross section, analyzing power (A y ), and induced polarization (P), are now available1,2 for the 4- “stretched” T = 0 and T = 1 transitions in 16O((left( {vec{p},vec{p}} right)))16O at E p = 200 MeV. These transitions at 17.79 and 19.80 MeV (T = 0) and at 18.98 MeV (T = 1) can be described within the framework of the distorted wave impulse approximation, which models the transition with an effective t-matrix based on NN scattering. The spin transfer (ΔS = 1) required by the dominant (1p_{3/2}^{ - 1}) 1d5/2 character of these transitions emphasizes their sensitivity to the spin-orbit and tensor parts of the t-matrix. Various interactions3,4 based on free NN scattering (phase shifts or potentials) often agree with each other but not with the measurements, giving several systematic discrepancies1,2 with the polarization transfer observables. Because the “stretched” transitions occur predominantly in the low density of the nuclear surface, interactions4 that correct for Pauli blocking in the nuclear medium have little effect on these calculations.2

Managing Systematic Errors in a Polarimeter for the Storage Ring EDM Experiment

The EDDA plastic scintillator detector system at the Cooler Synchrotron (COSY) has been used to demonstrate that it is possible using a thick target at the edge of the circulating beam to meet the requirements for a polarimeter to be used in the search for an electric dipole moment on the proton or deuteron. Emphasizing elastic and low Q-value reactions leads to large analyzing powers and, along with thick targets, to efficiencies near 1%. Using only information obtained comparing count rates for oppositely vector-polarized beam states and a calibration of the sensitivity of the polarimeter to rate and geometric changes, the contribution of systematic errors can be suppressed below the level of one part per million.

Spin Flipping and Polarization Lifetimes of a 270 MeV Deuteron Beam

We recently studied the spin flipping of a 270 MeV vertically polarized deuteron beam stored in the IUCF Cooler Ring. We swept an rf solenoid’s frequency through an rf‐induced spin resonance and observed the effect on the beam’s vector and tensor polarizations. After optimizing the resonance crossing rate and setting the solenoid’s voltage to its maximum value, we obtained a spin‐flip efficiency of about 94 ± 1% for the vector polarization; we also observed a partial spin‐flip of the tensor polarization. We then used the rf‐induced resonance to measure the vector and tensor polarizations’ lifetimes at different distances from the resonance; the polarization lifetime ratio τvector/τtensor was about 1.9 ± 0.4.

First test of a partial Siberian snake for acceleration of polarized protons

We recently studied the first acceleration of a spin‐polarized proton beam through a depolarizing resonance using a partial Siberian snake. We accelerated polarized protons from 95 to 140 MeV with a constant 10% partial Siberian snake obtained using rampable solenoids. The 10% partial snake suppressed all observable depolarization during acceleration due to the Gγ=2 imperfection depolarizing resonance which occurred near 108 MeV. However, 20% and 30% partial Siberian snakes apparently moved an intrinsic depolarizing resonance, normally near 177 MeV, into our energy range; this caused some interesting, although not‐yet‐fully understood, depolarization.

Spin flipping a stored vertically polarized proton beam with an RF solenoid

A recent experiment in the IUCF cooler ring studied the spin flip of a stored vertically polarized 139 MeV proton beam. This spin flip was accomplished by using an RF solenoid to induce an artificial depolarizing resonance in the ring, and then varying the solenoid’s frequency through this resonance value to induce spin flip. We found a polarization loss after multiple spin flips of about 0.00±0.05% per flip and also losses for very long flip times. This device will be useful for reducing systematic errors in polarized beam‐internal target scattering asymmetry experiments by enabling experimenters to perform frequent beam polarization reversals in the course of the experiment.

A Polarimeter to Determine the Vertical and Horizontal Polarization of Protons in the Indiana Cooler

A cylindrically symmetric detector system has been installed in one of the straight sections of the Indiana Cooler. The apparatus is suitable for measuring the polarization of 80–300 MeV stored proton beams. Fig. 1 shows a schematic view of the detector array consisting of a 1.5 mm thick scintillator (F), 2 wire chambers, a 102 mm thick scintillator (E) and a 6.5 mm thick scintillator (V). The detector system was designed for the first nuclear physics experiment (p+p → p+p+π° near threashold [1]) at the Indiana Cooler and accepts scattering angles between ( vartheta _1 = 5^circ {text{ and }}vartheta _2 = 20^circ ) with respect to the beam axis.