E. Schilling

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.

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.

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.

Irradiated ammonia (NH3) as target material for polarized proton targets

Abstract Dynamic proton polarization measurements in irradiated NH 3 were performed at 1 K and 0.5 K in a magnetic field of 2.5 T. The paramagnetic radicals in the frozen ammonia beads were created by irradiation with 20 MeV electrons from the injection linac of the Bonn 2.5 GeV electron synchrotron. During the irradiation the ammonia beads were cooled in liquid argon at about 90 K. Electron paramagnetic resonance measurements identified the created radicals as NH 2 . Samples of NH 3 yielded a maximum proton polarization of 35% at 1 K and 66% at a temperature of 0.5 K. They exhibited short polarization build-up times of 2 min at 1 K and 9 min at 0.5 K. The radiation resistance of the polarization of NH 3 is better than that of butanol. The radiation damage can be repaired by annealing.

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.

Target asymmetry measurement of deuteron photodisintegration at a photon energy of 550 MeV

The target asymmetry of the deuteron photodisintegration was measured at a photon energy of 550±50 MeV and at proton center-off-mass angles between 25 and 155 degrees.D-butanol andND3 were used as target material yielding a maximum deuteron polarization of 41%. Proton and neutron were detected in coincidence. The data show a structure which cannot be described by the existing analyses.

Deuteron photodisintegration at photon energies between 200 and 700 MeV in backward direction

The differential cross section of the deuteron photodisintegration was measured at a protion c.m. angle of 180 degrees and for photon energies between 180 and 730 MeV. The protons were detected in a magnetic spectrometer. The proton energy resolution varied between 30 MeV and 50 MeV FWHM. Since these are the first data at 180 degrees in this energy range a comparison can only be done with data from other laboratories extrapolated to 180 degrees and with theoretical predictions. The agreement with existing calculations is poor. Contributions of dibaryons to the cross section seem not to improve the situation.

First Dynamic Deuteron Polarization Measurements in Irradiated ND3

Dynamic deuteron polarization has been demonstrated in deuterated ammonia, using paramagnetic radicals created by electron irradiation. During the irradiation, performed in the 20 MeV electron beam of the Bonn injection linac, the solid ammonia beads were cooled in liquid argon at a temperature of 87 K. A deuteron polarization of 11% was obtained in a field of 2.5 T and at a temperature of 0.5 K. These first measurements with an irradiated ND3 sample indicated a rather small electron spin resonance line. The deuteron tensor polarization could be changed by a simple method.