W. Thiel

Photoproduction of positive pions from hydrogen with PHOENICS at ELSA

Abstract The differential cross section of the reaction γ p→ π + n has been measured with the PHOENICS detector at ELSA in Bonn. For the first time this cross section has been determined simultaneously over a large range of photon energies ( E γ = 220−900 MeV) and pion angles ( Θ c . m . pi = 35°−135°) with a tagged photon facility. The experimental set-up allowed a considerable kinematic overdetermination of the investigated reaction. Accordingly, the background contributions have been suppressed to below 1%. The measured differential cross section is in good agreement with existing data. The comparison with different model calculations is presented.

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. n nFirst 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. n nThe 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. n nFurthermore, 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.

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.

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.

Measurement of the vector analyzing power iT sub 11 in. pi. sup + - sup 6 L rvec i scattering

The vector analyzing power {ital iT}{sub 11} was measured for {pi}{sup +}-{sup 6}{rvec L}i elastic and inelastic scattering (3{sup +}, 2.19 MeV) at 100, 134, 164, and 219 MeV, in an angular range between 40{degree} and 125{degree} using a vector polarized {sup 6}LiD target. Sizable spin effects were observed over most of the angular range. The data were compared with a number of theoretical predictions based on different scattering models and nuclear structure input. None of the calculations is able to reproduce the data set; however, it appears that the vector analyzing power {ital iT}{sub 11} is mostly sensitive to the nuclear wave function chosen. The angular distribution of {ital iT}{sub 11} in {pi}{sup +}-{sup 6}Li elastic and inelastic scattering is discussed in terms of nuclear form factors and elementary {pi}{ital N} amplitudes.

Measurement of the vector analyzing power iT 11 in π + − 6 Li→ scattering

The vector analyzing power {ital iT}{sub 11} was measured for {pi}{sup +}-{sup 6}{rvec L}i elastic and inelastic scattering (3{sup +}, 2.19 MeV) at 100, 134, 164, and 219 MeV, in an angular range between 40{degree} and 125{degree} using a vector polarized {sup 6}LiD target. Sizable spin effects were observed over most of the angular range. The data were compared with a number of theoretical predictions based on different scattering models and nuclear structure input. None of the calculations is able to reproduce the data set; however, it appears that the vector analyzing power {ital iT}{sub 11} is mostly sensitive to the nuclear wave function chosen. The angular distribution of {ital iT}{sub 11} in {pi}{sup +}-{sup 6}Li elastic and inelastic scattering is discussed in terms of nuclear form factors and elementary {pi}{ital N} amplitudes.

Measurement of the vector analyzing poweriT11inπ+−6Li→ scattering

The vector analyzing power {ital iT}{sub 11} was measured for {pi}{sup +}-{sup 6}{rvec L}i elastic and inelastic scattering (3{sup +}, 2.19 MeV) at 100, 134, 164, and 219 MeV, in an angular range between 40{degree} and 125{degree} using a vector polarized {sup 6}LiD target. Sizable spin effects were observed over most of the angular range. The data were compared with a number of theoretical predictions based on different scattering models and nuclear structure input. None of the calculations is able to reproduce the data set; however, it appears that the vector analyzing power {ital iT}{sub 11} is mostly sensitive to the nuclear wave function chosen. The angular distribution of {ital iT}{sub 11} in {pi}{sup +}-{sup 6}Li elastic and inelastic scattering is discussed in terms of nuclear form factors and elementary {pi}{ital N} amplitudes.

Polarization Behaviour of NH3 at 3.5 Tesla and in the Frozen Spin Mode

With The the new Bonn Frozen Spin Target we investigated the polarization behaviour of 14NH3 in a magnetic field of 3.54 Tesla and at a temperature of 70 mK. We got a maximum polarization of +94 ±2.5% at a frequency of 98.750 GHz and -100%1 at 99.075 GHz. The polarization build up time r was about 90 minutes. Examining the relaxation time at a holding field of 0.35 Tesla a superradiance effect was observed in the case of high negative polarization. The resulting signals show the same lineshape as signals during the polarization reversal by microwave irradiation. In case of positive polarization a relaxation time greater than 30 days was measured. Finally the maximum polarization of 15NH3 was measured in a slightly different apparatus.

Butanol in the Frozen Spin Mode

Measurements of the polarization build-up in butanol at 3.5 T are presented. Measurements of the proton relaxation times in the frozen spin mode were performed at magnetic fields of 0.5 and 0.35 Tesla, leading to the conclusion that butanol is a suitable material for the Bonn Frozen Spin Target.