O. Kaul

Positive pion photoproduction from hydrogen at photon energies between 500 MeV and 1,400 MeV in forward direction

The differential cross section of the reactionγ+p→π+ was measured at pion CM-angles of 20° and 30° for photon energies between 500 MeV and 1,400 MeV. The pions were detected in a magnetic spectrometer. By measuring each pion trajectory and by offline calculation of the initial pion parameters an energy resolution of about 2.5% FWHM was achieved. The results complete a set of data which were measured in recent years at the Bonn 2.5 GeV synchrotron. In comparison to photoproduction analyses two effects were revealed: The η cusp appears in the energy dependence of the cross section as a sharp drop atKγ=710 MeV. In the region of the third resonance the data show a greater enhancement than predicted by most of the analyses.

Dynamic deuteron polarization in irradiated D-Ammonia (ND3 and its first use in a high energy photon beam

We have dynamically polarized the deutorons in deuterated ammonia (ND3) at 1 K, 0.5 K and at about 200 mK in a magnetic field of 2.5 T. The paramagnetic radicals were created by irradiating the material under liquid argon at 90 K (‘high temperature’ irradiation) with electrons from the 20 MeV injection linac of the Bonn synchrotron. Electron paramagnetic resonance (EPR) measurements identified the radicals as ṄD2. First target asymmetry measurements of the reaction γd ↑ → pn with ND3 as target material in a dilution refrigerator were performed at the Bonn 2.5 GeV electron synchrotron. Starting with a deuteron polarization of 31% (using‘ high temperature’ irradiated material) the polarization went up to 44% after additional irradiation at about 200 mK with the photon beam. This polarization value corresponds to an overall polarization of 13.2% of all nucleons in ND3. The increase of the polarization could be observed up to a dose of 8 × 1014 equivalent quanta/cm2. The subsequent resistance of the polarization to radiation damage is more than one order of magnitude higher than that of butanol. These results mean a considerable improvement with respect to the deuterated alcohol materials which are currently used in high energy physics experiments with polarized deuteron targets. The dependence on the microwave frequency of the polarizations of deuterons and unsubstituted protons was examined at 1 K in order to gain insight into the prevailing mechanism of dynamic nuclear polarization (DNP). These measurements disagree with the equal spin temperature (EST)-hypothesis. They rather give evidence for a more complex behaviour apparently involving more than one polarization mechanism. It is attempted to explain these observations at 1 K with a differential solid state model. Furthermore, we present a series of DNP-signals as measured with our deuteron magnetic resonance (DMR)-system at various microwave frequencies. An unusual change of the DMR-signal shape was observed.

Photoproduction of neutral pions at backward angles in the resonance region and the η-cusp

At the Bonn 2.5 GeV electron synchrotron we have measured the differential cross section of the reaction γp→π0p at a pion CM angle of 170° and at photon energiesKγ between 0.6 and 1.8 GeV. In comparison to previous measurements the accuracy of the data was improved substantially. For the first time in neutral pion photoproduction a cusp structure at the η-threshold has been confirmed [1].

Observation of a narrow enhancement in the pp mass distribution from the reaction γd → ppπ−

Abstract Studying the reaction γ d → pp π − , a narrow resonance structure in the pp invariant mass distribution has been observed at 2014 ± 2 MeV/ c 2 with a statistical significance of 4.7 standard deviations. Using a bremsstrahlung photon beam ( k 0 = 520 MeV), the kinematics of the reaction were completely determined by detecting both outgoing protons in two large solid angle time-of-flight spectrometers. By accepting only proton momenta greater than ∼300 MeV/ c , the predominant contribution from quasi-free pion production was suppressed.

Angular distribution of the target asymmetry on polarized protons for the reaction γp↑ → π+n at a photon energy of 700 MeV

Abstract At the Bonn 2.5 GeV electron synchrotron the angular distribution of the target asymmetry T = (σ↑ − σ↓) (σ↑ + σ↓) for the reaction γp↑ → π+n was measured at a mean photon energy of 700 MeV and pion CM-angles from 50° to 155°. The combination of a 3He-cryostat, polarizing the free protons in the target up to 65%, with a large acceptance magnet for pion detection led to statistical errors of the target asymmetry comparable with those of cross section measurements.

Elastic Electron Deuteron Scattering on a Tensor Polarized Solid ND3 Target

Elastic electron deuteron scattering has been measured at the Bonn 2.5 GeV electron-synchrotron using a tensor polarized ND3 target. The scattered electrons and deuterons were detected in coincidence by two magnetic spectrometers. Data were taken at an average value of four momentum transfer ofQ2=0.495 (GeV/c)2 in order to separate the electric monopole and quadrupole form factors of the deuteron. To have a sufficient control of systematic effects, both polarized and unpolarized cross sections were measured with an identical experimental set up. In the ratioREXP=dσpol/dσunp systematic effects are expected to cancel. This ratioREXP is related to the theoretical polarized cross section d dσT where the spin quantization axis for the deuterons is orientated in the ed-scattering plane, perpendicular to the virtual photon direction. Our final result for the corresponding recoil tensor polarization ist20=−2.01±1.13.

Tensor polarization of deuterons in irradiated ND3

Abstract We have dynamically polarized the deuterons in deuterated ammonia (ND 3 ) at a temperature of about 200 mK. Maximum vector polarizations of −0.44±0.02 in a magnetic field of 2.5 T and −0.49±0.04 at 3.5 T have been obtained. From these values the deuteron tensor polarization is calculated to be 0.15±0.02 and 0.19±0.03, respectively.

Dynamic deuteron polarization measurements performed in a new type of horizontal dilution cryostat

Abstract We have reached 31% deuteron polarization in D-ammonia (ND 3 ) and 27% in D-butanol (C 4 D 10 O). The dynamic polarization experiments were performed at a magnetic field of 2.5 T in a new type of horizontal dilution cryostat. This dilution cryostat, built for target asymmetry measurements with a photon beam, was developed with special regard to fast cool-down and easy loading of the target material. The cooling power is 5 mW at 0.2 K, 20 mW at 0.3 K and 34 mW at 0.4 K. Starting from room temperature the lowest temperature of 165 mK is reached in about 2 h including the loading of the target material.

Photoproduction of pions on polarized protons and neutrons in the second resonance region

Abstract Measurements of the target asymmetry T = ( σ ↑ − σ ↓)/( σ ↑ + σ ↓) for the reactions γ p → π + n and γ n → π − p at a fixed photon energy of 850 MeV and pion c.m. angles between 70° and 150° are reported. The data are compared to the previously measured angular distribution at 700 MeV.

Measurement of the target asymmetry for the reaction γp↑ → π+n at photon energies between 0.7 and 2.2 GeV and a pion CM-angle of 65°

Abstract At the Bonn 2.5.GeV electron synchrotron the target asymmetry for the photoproduction of positive pions has been measured. Data were taken at photon energies between 0.7 and 2.2 GeV and a pion CM-angle of 65°.