M. Schmitz

Two neutral pion photoproduction off the proton between threshold and 800 MeV

Abstract The photoproduction of two neutral pions off the proton has been studied for incident photon energies between E γ = 309 MeV (threshold) and 792 MeV with the TAPS photon spectrometer at the Mainz Microtron MAMI. Both 3γ- and 4γ-events have been analysed in order to deduce the total p ( γ , 2 π 0 ) p cross section. The total cross section is less than 300 nb between threshold and ⋍ 460 MeV, beyond which it starts to rise to a maximum value of about 11 μb at around 750 MeV. For the highest energies it starts to decrease again. The 4γ-sample has been used to reconstruct 2 neutral pions from the above reaction channel by an invariant mass and a missing mass analysis. After reconstruction of the unobserved proton, a Dalitz-plot of m π 0 π 0 2 vs. m pπ 0 2 has been obtained, which indicates that the decay chain includes the Δ(1232)-resonance as an intermediate state.

Total photofission cross section for 238U as a substitute for the photon absorption cross section in the energy range of the first baryon resonances

Abstract The total photofission cross section σγ, F for 238U has been measured as a substitute for the total photon absorption cross section σtot in the energy range of the first baryon resonances (50⩽Eγ⩽800 MeV). The experiments were performed at the tagging facility of the new 855 MeV Mainz Microton MAMI B using monochromatic photons and a sophisticated 4π-arrangement of position sensitive fragment detectors. The measured cross section agrees in the Δ-resonance region with previous results and the so-called universal curve αtot/A. In the energy region of the D13-resonance near 710 MeV resonancelike structure could be observed as is seen for the free proton.

Compton scattering by the proton using a large-acceptance arrangement

Abstract:Compton scattering by the proton has been measured over a wide range covering photon energies 250MeV Eγ 800MeV and photon scattering angles 30°labγ 150°, using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The previously existing data base on proton Compton scattering is greatly enlarged by more than 700 new data points. The new data are interpreted in terms of dispersion theory based on the SAID-SM99K parametrization of photo-meson amplitudes. It is found that two-pion exchange in the t-channel is needed for a description of the data in the second resonance region. The data are well represented if this channel is modeled by a single pole with the mass parameter mσ≈ 600 MeV. The asymptotic part of the spin-dependent amplitude is found to be well represented by π0 exchange in the t-channel. No indications of additional effects were found. Using the mass parameter mσ of the two-pion exchange determined from the second resonance region and using the new global average for the difference of the electric and magnetic polarizabilities of the proton, α - β = 10.5±0.9stat+syst±0.7model×10-4fm3, as obtained from a recent experiment on proton Compton scattering below pion photoproduction threshold, a backward spin polarizability of γπ = -37.1±0.6stat+syst±3.0model×10-4fm4 has been determined from data of the first resonance region below 455 MeV. This value is in a good agreement with predictions of dispersion relations and chiral perturbation theory. From a subset of data between 280 and 360 MeV, the resonance pion photoproduction amplitudes were evaluated leading to a E2/M1 multipole ratio of the p↦Δ radiative transition of EMR(340 MeV) = (- 1.7±0.4stat+syst±0.2model)%. It was found that this number is dependent on the parameterization of photo-meson amplitudes. With the MAID2K parameterization, an E2/M1 multipole ratio of EMR(340 MeV) = (- 2.0±0.4stat+syst±0.2model)% is obtained.

Compton scattering by the proton through θcms = 75° and 90° in the Δ-resonance region

Abstract Differential cross sections for Compton scattering by the proton have been measured in the energy interval between 200 and 500 MeV at scattering angles of θ cms = 75° and θ cms = 90° using the CATS, the CATS/TRAJAN, and the COPP setups with the Glasgow Tagger at MAMI (Mainz). The data are compared with predictions from dispersion theory using photo-meson amplitudes from the recent VPI solution SM95. The experiment and the theoretical procedure are described in detail. It is found that the experiment and predictions are in agreement as far as the energy dependence of the differential cross sections in the Δ-range is concerned. However, there is evidence that a scaling down of the resonance part of the M 1+ 3 2 photo-meson amplitude by (2.8 ± 0.9)% is required in comparison with the VPI analysis. The deduced value of the M 1+ 3 2 - photoproduction amplitude at the resonance energy of 320 MeV is: |M 1+ 3 2 | = (39.6 ± 0.4) × 10 −3 m π + −1 .

Elastic and inelastic photon scattering from 12C

Abstract The elastic and inelastic photon scattering from 12 C was investigated in the energy range of the Δ-resonance ( E γ = 200–500 MeV) at a scattering angle of 40°. The experiment was performed with the tagged photon facility at the cw electron accelerator MAMI ( E e − = 855 MeV). By using a large NaI(Tl) detector having an energy resolution of 1.5%, the elastic scattering could be separated from the inelastic process to the first excited state ( E x = 4.44 MeV of 12 C. The differential cross section for the coherent scattering is compared with a simple static ansatz, that uses the known charge form factor, and with a recent Δ-hole calculation.

Compton scattering by the proton through θcms = 75° in the Δ-resonance region

Using the tagged-photon beam of MAMI (Mainz) differential cross-sections for Compton scattering by the proton through θcms = 75° have been measured for the Δ-resonance region from 200 to 410 MeV. The data put a new constraint on the M1+32 photoproduction amplitude in the resonance, |M1+32| = (39.6 ± 0.4) · 10−3mπ+−1 at 320 MeV, which is (2.8 ± 0.9) % less than given by the recent analysis SM95 of the Virginia group.

Photofission of235U and238U at intermediate energies: absolute cross sections and fragment mass distributions

The total photofission cross section σγ,F for235U and238U has been measured in the energy range 50≤Eγ≤800 MeV at the 855 MeV Mainz Microtron MAMI using energy and time tagged photons (Glasgow Tagger) and a 4π arrangement of position sensitive fragment detectors. Besides the absolute photofission cross section σγF, which almost completely exhausts the total photon absorption cross section for these nuclei, fragment mass distributions in this energy domain were determined via time of flight techniques (TOF). The results for the total photofission cross sections σγ,F normalized to the atomic numberA for both isotopes coincide, and agree in theΔ-resonance region, within the systematic errors, with the socalled“Universal Curve” σγ,T/A of the total photon absorption cross section σγ,T. At higher energies the cross sections exhibit a smooth behaviour. In particular, it is shown for the first time that there isno resonance-like shape near the D13 resonance (at ≈710 MeV) as observed for the free proton. This complete suppression of the D13 resonance in complex nuclei is not yet understood on a microscopic level. The fragment mass distributions show a predominantly mass symmetric fission. However, contributions from mass asymmetric fission at some photon energies may give a hint of an increased mass asymmetric fission after the onset of the pion and two pion channels.

Cross sections and angular distributions for 12C(γ,π0)☆

Abstract Absolute cross sections and angular distributions for the production of neutral pions from 12 C by monochromatic photons have been measured near threshold. The contribution of incoherent pion production is found to be less than 10% for E γ ⩽168 MeV. Both the absolute cross section and the angular distribution indicate that a quantitative analysis requires final-state interactions of the produced π 0 with the surrounding nucleons to be taken into account.

Photoemission of protons from4He in theΔ-resonance region

High resolution proton energy spectra from the4He(γ,p) reaction have been measured with tagged photons in the range Eγ = 130−525 MeV using the large Mainz NaI(T1) spectrometer atΘplab=37.1°. Three separate reaction channels were identified, viz.4He(γ,p)t two-body breakup,4He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi motion is taken into account.