A. M. Sandorfi

High Energy Gamma Ray Beams from Compton Backscattered Laser Light

Collisions of light photons with relativistic electrons have previously been used to produce polarized ¿-ray beams with modest (~10%) resolution, but relatively low intensity. In contrast, the LEGS project (Laser+Electron Gamma Source) at Brookhaven will produce a very high flux (>2×107 s-1) of background-free polarized ¿ rays whose energy will be determined to a high accuracy (¿E =2.3 MeV). Initially, 300(420) MeV ¿ rays will be produced by backscattering UV light from the new 2.5(3.0) GeV X-ray storage ring of the National Synchrotron Light Source (NSLS). The LEGS facility will operate as one of many passive users of the NSLS. In a later stage of the project, a Free Electron Laser is expected to extend the ¿-ray energy up to 700 MeV.

Chiral symmetry and pion polarizabilities

Abstract We use chiral perturbation theory including one-loop contribution to derive formulae needed to deduce pion polarizabilities for γπ→γπ and γγ→ππ data. We deduce for the first time values for the π± and π0 polarizabilities from ππ production data, and compare these new results to chiral symmetry predictions.

First Extraction of a Spin Polarizability of the Proton

A proton spin polarizability characterizing backward Compton scattering has been extracted from a dispersion analysis of data between 33 and 309MeV. This backward spin polarizability, {delta}{sub {pi}}=[27.1{plus_minus}2.2(stat +syst){sup +2.8}{sub {minus}2.4}(model )]{times}10{sup {minus}4} fm {sup 4} , differs significantly from theoretical estimates and suggests a new contribution from the nonperturbative spin structure of the proton. This {delta}{sub {pi}} value removes an apparent inconsistency in the difference of charge polarizabilities extracted from data above {pi} threshold. Our global result, {ovr {alpha}}{minus}{ovr {beta}}=[10.11{plus_minus}1 .74(stat+syst){sup +1.22}{sub {minus}0.86}(model)]{times}10{sup {minus}4} fm{sup 3} , agrees with the previous world average of data below 155MeV. Our value for {ovr {alpha}}+{ovr {beta}}=[13.23{plus_minus}0 .86(stat+syst){sup +0.20}{sub {minus}0.49}(model)]{times}10{sup {minus}4} fm{sup 3} is consistent with a recent reevaluation of the Baldin sum rule. {copyright} {ital 1998} {ital The American Physical Society}

The legs electron spectrometer for tagging backscattered photons

Abstract A magnetic spectrometer is described for tagging a gamma-ray beam produced by Compton backscattering laser light from the electron beam circulating in a storage ring. The spectrometer, as it is now installed, has a design energy resolution of 5.5 MeV, a dispersion of 2.8 MeV/cm, and a dynamic range of 160 MeV. The energy resolution is limited by the energy spread of the stored beam. If dispersion can be introduced into the laser-electron interaction straight section of the storage ring, the resolution can be improved to be as small as 1.5 MeV, depending on the value of this dispersion. Measured properties of the spectrometer are in substantial agreement with the design values.

The characteristics of electric dipole strength built on highly-excited continuum states

Abstract High-energy γ-ray spectra from heavy-ion fusion reactions have been measured with low background and high resolution over a broad mass range. The data have been fitted with statistical-evaporation calculations in which the γ-ray strength associated with each level follows a lorentzian. The extracted resonance widths are generally much larger than the width of the giant dipole resonance built on the ground state of the compound system (gs-GDR). The centroids of these excited-state resonances are shifted from those of the gs-GDRs, but no simple A −1 3 dependence is evident. The most surprising result is that the extracted sum rule strengths of these resonances track those of the gs-GDRs observed in photoabsorption.

Measurements of HD (gamma, pi) and Implications for the Convergence of the Gerasimov-Drell-Hern Integral

We report new measurements of inclusive pion production from frozen-spin HD for polarized photon beams covering the Delta(1232) resonance. These provide data simultaneously on both H and D with nearly complete angular distributions of the spin-difference cross sections entering the Gerasimov-Drell-Hearn (GDH) sum rule. Recent results from Mainz and Bonn exceed the GDH prediction for the proton by 22 microbarns, suggesting as yet unmeasured high-energy components. Our pi0 data reveal a different angular dependence than assumed in Mainz analyses and integrate to a value that is 18 microbarns lower, suggesting a more rapid convergence. Our results for deuterium are somewhat lower than published data, considerably more precise and generally lower than available calculations.

New investigations of polarized solid HD targets

Abstract Polarized solid HD targets in a frozen-spin mode, with superior nuclear physics characteristics and simple operational configurations, have previously been restricted in their deployment due to a disproportionate target production time with respect to utilization time. Recent investigations have yielded frozen-spin polarization lifetimes, at a convenient target temperature of 1.5 K, of nearly a year for both H and D at high holding fields, and of more than a week at sub-Tesla holding fields. These results, taken together with the advent of new interesting spin-physics using relatively weakly ionizing beams, such as polarized photon beams, remove the above impediment and open up the use of polarized solid HD to long duration nuclear spin-physics experiments. Large, multiple targets can be produced, retrieved from the polarization-production apparatus with a cold-transport (4 K) device, stored for very long times in inexpensive (1.5 K, 7 T) cryostats, and introduced “off-the-shelf” into in-beam cryostats via the portable cold-transport apparatus. Various modes for achieving polarized H and/or D, as well as already achieved and expected polarization values, are reported. Experimental results are given on Kapitza resistance between the solid HD and the cooling wires necessary to obtain low temperatures during the heat-evolving polarization process. 15 mK is achievable using gold-plated aluminum wires, which constitute 15% extraneous nucleons over the number of polarizable H or D nucleons. Application to more highly ionizing beams is also given consideration.

Coherent π0 photo-production on 4He at intermediate energies with polarized photons

Abstract The reaction 4 He ( γ ,π 0 ) 4 He has been studied with the LEGS tagged, polarized gamma-ray beam. Data are presented at laboratory angles of 31°, 45°, 72.5°, 90°, 110° and 130° for incoming photon energies of 206, 219, 232, 245, 259, 274, 289, 306 and 322 MeV. The unpolarized cross sections are compared with the Δ-hole theoretical model results available in literature. The measured photon beam asymmetry is −1 as expected from conservation of angular momentum and parity.

FIRST MEASUREMENT OF THE REACTION 3HE(GAMMA ,P)X WITH POLARIZED PHOTONS

The first measurement of the reaction He-3(gamma over arrow pointing right, p)X using linearly polarized photons is reported. Cross sections and beam-polarization asymmetries for theta(p)lab = 60-degrees - 100-degrees and E(gamma) - 195-304 MeV are compared with a microscopic calculation which includes one-, two-, and three-nucleon absorption mechanisms. One- and two-nucleon absorption alone fails to describe the data at low proton momenta. The inclusion of three-nucleon absorption significantly improves the comparison with the measured cross sections. However, some features of the asymmetry distributions are not explained.

Large, mobile frozen-spin polarized solid HD

Abstract A completed system for producing large quantities of highly spin-polarized H and D in solid HD is described. It is of the relaxation-switched frozen-spin type, in which relaxation is turned off after polarization at high B (17 T) and low T (15 mK), and the polarization is retained at high T (4 K) and low B (fractional Tesla). The first application is as targets for polarized gammas at BNL. Presently, three targets of 1.3 mol each are produced in a single run at Syracuse, cold transferred to a mobile storage cryostat with high field, transported to BNL, and cold-transferred there into an in-beam cryostat.