Search

We have devised a technique for measuring some of the most important nuclear reactions in stars which we expect to provide considerable improvement over previous experiments. Adapting ideas from dark matter search experiments with bubble chambers, we have found that a superheated liquid is sensitive to recoils produced from γ rays photodisintegrating the nuclei of the liquid. The main advantage of the new target-detector system is a gain in yield of six orders of magnitude over conventional gas targets due to the higher mass density of liquids. Also, the detector is practically insensitive to the γ -ray beam itself, thus allowing it to detect only the products of the nuclear reaction of interest. The first set of tests of a superheated target with a narrow bandwidth γ -ray beam was completed and the results demonstrate the feasibility of the scheme. The new data are successfully described by an R-matrix model using published resonance parameters. With the increase in luminosity of the next generation γ -ray beam facilities, the measurement of thermonuclear rates in the stellar Gamow window would become possible.

We have measured the nuclear transparency of the incoherent diffractive A(e,e′ρ0) process in 12C and 56Fe targets relative to 2H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced ρ0ʼs on a nucleus relative to deuterium, which is sensitive to ρA interaction, was studied as function of the coherence length (lc), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared (Q2). While the transparency for both 12C and 56Fe showed no lc dependence, a significant Q2 dependence was measured, which is consistent with calculations that included the color transparency effects.

We report on the status of the Novosibirsk experiment on a precision measurement of the ratio $R$ of the elastic $e^+ p$ and $e^- p$ scattering cross sections. Such measurements determine the two-photon exchange effect in elastic electron-proton scattering. The experiment is conducted at the VEPP-3 storage ring using a hydrogen internal gas target. The ratio $R$ is measured with a beam energy of 1.6 GeV (electron/positron scattering angles are $\theta = 55 \div 75^{\circ}$ and $\theta = 15 \div 25^{\circ}$) and 1 GeV ($\theta = 65 \div 105^{\circ}$). We briefly describe the experimental method, paying special attention to the radiative corrections. Some preliminary results are presented., Comment: 5 pages, 6 figures and 1 table; contribution to the Proceedings of the PHIPSI11 Workshop (Novosibirsk, Russia, 19-22 September 2011)

We report a new, high-precision measurement of the proton elastic form factor ratio μ p G E / G M for the four-momentum transfer squared Q 2 = 0.3 – 0.7 ( GeV / c ) 2 . The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio μ p G E / G M from unity observed in previous polarization measurements at high Q 2 continues down to the lowest Q 2 value of this measurement. The updated global fit that includes the new results yields an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q 2 range. We obtain new extractions of the proton electric and magnetic radii, which are 〈 r E 2 〉 1 / 2 = 0.875 ± 0.010 fm and 〈 r M 2 〉 1 / 2 = 0.867 ± 0.020 fm . The charge radius is consistent with other recent extractions based on the electron–proton interaction, including the atomic hydrogen Lamb shift measurements, which suggests a missing correction in the comparison of measurements of the proton charge radius using electron probes and the recent extraction from the muonic hydrogen Lamb shift.

Permanent electric dipole moments (EDMs) in atoms or molecules are a signature of time-reversal and parity violation and represent an important window onto physics beyond the Standard Model. We are developing a next generation EDM search based on laser-cooled and trapped {sup 225}Ra atoms. Due to octupole deformation of the nucleus, {sup 225}Ra is predicted to be two to three orders of magnitude more sensitive to T-violating interactions than {sup 199}Hg, which currently sets the most stringent limits in the nuclear sector. We will discuss progress toward realizing a first EDM measurement for {sup 225}Ra.

We report on the progress towards measuring the charge radius of 6 He and 8 He nuclei by performing laser spectroscopy on these helium atoms in a magneto-optical trap (MOT). First tests to produce neutral 6 He atoms via the 12 C( 7 Li, 6 He) 13 N reaction at the ATLAS accelerator have been successfully conducted. The MOT apparatus including the laser system and the discharge source to populate the metastable level are currently being set up.

We present the results of the first direct measurement of nuclear polarization in an optically pumped spin-exchange polarized deuterium target. Measurements of the nuclear tensor polarization p zz are made in a storage cell as a function of flow of deuterium nuclei. For these measurements, the optical pumping was performed on atoms in magnetic fields of 600 and 3600 G. These measurements verify that the spin-exchange method is capable of producing polarized deuterium nuclei without the use of rf transitions. The electronic polarization, P e , is also measured. The P e and p zz measurements at 600 G indicate that the system is in spin-temperature equilibrium, whereas the 3600 G measurements show signatures of non-equilibrium effects. P e is also measured with the system operated as a source of polarized atoms, with no storage cell. These measurements are compared with previous measurements using a similar source.

A laser-driven source of spin-polarized hydrogen (H) and deuterium (D) that relies on the technique of optical pumping spin exchange has been constructed. In this source, H or D atoms and potassium atoms flow continuously through a drifilm-coated spin-exchange cell where potassium atoms are optically pumped with circularly-polarized laser light in a high magnetic field. The H or D atoms become polarized through spin-exchange collisions with polarized potassium atoms. High electron polarization ( ≈ 80%) has been measured for H and D atoms at flow rates ≈ 2 × 1017 atoms/s. Lower polarization values are measured for flow rates exceeding 1 × 1018 atoms/s. In this paper, we describe the performance of the laser-driven source as a function of H and D atomic flow rate, magnetic field strength, alkali density and pump-laser power. Polarization measurements as a function of flow rate and magnetic field suggest that, despite a high magnetic field, atoms within the optical-pumping spin-exchange apparatus evolve to spin-temperature equilibrium which results in direct polarization of the H and D nuclei.

The first experiment using an active storage cell for polarized atoms to increase the target thickness in an electron storage ring has been performed at the VEPP-3 facility. A clam-shell storage cell, which is opened while injecting electrons into the storage ring and closed during operation, was used to increase the target thickness viewed by the detector system. The average tensor polarization of the target was found to be p zz = 0.59 ± 0.14 and the figure of merit was increased by more than a factor of thirty in comparison with a direct jet thickness. The measured target thickness and polarization are in a good agreement with the expected values.

The first experiment using a storage cell to increase the thickness of an internal polarized gas target in an electron beam storage ring was performed at the VEPP-3 facility. We describe the storage cell technique as applied in this measurement of elastic and inelastic electron scattering from tensor polarized deuterium. An analysis of electron-beam-induced depolarization of the target was performed and experimental tests were carried out which verify the effect. Other effects causing depolarization of the target are discussed as well as the means by which they are overcome. The effective p zz of the target, shown to be stable over 8 months, was 0.57±0.05; the total target thickness was increased over that of a jet target by a factor of fifteen.

A novel laser-driven polarized source of hydrogen and deuterium which operates on the principle of spin exchange optical pumping is described. The advantages of this method over conventional polarized sources for internal target experiments are presented. Technological difficulties which prevent ideal source operation are outlined along with proposed solutions. At present, the laser-driven polarized hydrogen source delivers 8 × 10 16 atoms/s with a polarization (P z ) of 24%.

The development of a polarized internal target at an electron store ring is of great interest for studies in nuclear physics. The deuterium nuclei acquire polarization via the hyperfine interaction after electron spin exchange with optically pumped potassium vapor. A high power laser which covers the entire doppler width and hyperfine splitting of the /sup 2/S/sub 1/2/ - /sup 2/P/sub 1/2/ transition in potassium has been developed. Spin relaxation rates for potassium on various wall coatings have been measured. A dri-film surface has been shown to preserve polarization (>100 bounces), be long-lived at elevated temperatures, and to be efficient at preventing deuterium recombination.

We report on the detailed design and calibration of a novel, high-efficiency polarimeter for tensor polarized deuterons. This device measures the t 20 moment of the deutron, and was built to observe the polarization of recoil deutrons from 180° π−d elastic scattering. Based on the 3 He(d, p) 4 He reaction at low energies and forward angles, the polarimeter detects 1.8×10 −4 protons/incident deuteron with an average analyzing power of T 20 = −0.6.. The performance of the polarimeter in high background fluxes is discussed.

The momentum spectrum for double-charge-exchange of π − on 4 He has been measured at 0° and T π − = 165 MeV. A 1σ upper limit of 22 nb/sr is set on the cross section for transitions leading to bound tetraneutrons. The cross section in the continuum region is shown to be inconsistent with a four-neutron phase space distribution, while a model including final state interaction effects gives better agreement with experiment.

A polarimeter system has been developed which is suitable for use with a continuous energy spectrum of neutrons below 1 MeV. This polarimeter is well-suited for measuring the polarization of photoneutrons near threshold. Analyzing targets of 16 O and Mg were selected for the present study. A spin-precession solenoid, designed for use with a continuous energy spectrum of neutrons, was employed in order to minimize systematic errors. As an example of the method, the photoneutron polarization is observed from resonances near threshold for the 208 Pb(γ,n) 207 Pb reaction at an angle of 135°.

The non-analog pion double charge exchange reaction 16 O (π + ,π − ) 16 Ne has been observed. The ground-state differential cross section at 0° is found to be 0.87 ± 0.21 μb/sr at a mean pion energy of 145 MeV. The mass excess obtained for 16 Ne of 24.4 ± 0.5 MeV is in fair agreement with theoretical predictions. The result indicates that cross sections for pion induced non-isobaric-analog double-charge-exchange transitions can be comparable to those for analog transitions.

A measurement of the charge and quadrupole form factors of the deuteron would address the most fundamental questions in nuclear physics: validity of perturbative QCD at relatively low momentum transfer, effect of isoscalar meson-exchange currents, and the deuteron structure at short range. Polarization methods will be required in order to provide the separation of these form factors. We propose that the high current of electrons in a storage ring be employed in order to scatter electrons from a gaseous, tensor-polarized, deuterium target.

A new technique is being developed at Argonne National Laboratory to produce a beam of polarized deuterium atoms. A part of the apparatus consists of a small cell which contains potassium vapor. The potassium atoms are polarized by circularly polarized light from a high-power cw laser. A port feeds deuterium into the cell and the polarization of the potassium is transferred to the deuterium by spin-exchange scattering. It is important that the potassium and deuterium atoms do not lose polarization by scattering from the walls of the exchange cell. We have tried various coatings of the exchange walls in order to inhibit losses from spin relaxation. Methods used to produce these coatings as well as their success will be described.

A deuteron tensor polarimeter with the highest available figure-of-merit was developed in order to measure the tensor polarization t 20 in electron and pion elastic scattering from the deuteron. The polarimeter was calibrated at the Berkeley 88 cyclotron with the associated polarized ion source. The feasibility of the method was demonstrated by measuring t 20 in pion-deuteron elastic scattering at LAMPF. The application of this method to e-d elastic scattering will be discussed for the momentum transfer range 2 fm −2 2 −2 .