PIP: a large solid angle detector for protons and pions

A ~1 sr solid angle detector for protons of up to ~200 MeV, and for pions of up to ~250 MeV energy is described. The detector has been developed to provide a charged particle trigger for use in (gamma,2N) and (gamma,piN) experiments with tagged photons at the MAMI-B 850 MeV electron microtron in Mainz. The design is based on a smaller detector previously used at lower energies at the 180 MeV MAMI-A accelerator. Many improvements have been made which will increase the performance of PIP with respect to the earlier detector. Positive pions which are stopped in the detector will be identified by looking for delayed pulses from the e+ in the pion decay chain

Eta photoproduction on the proton

Eta photoproduction experiments have proven a useful tool in the search for narrow nucleon resonances. In recent years, there has been much interest in the possible existence of the N*(1685) narrow nucleon resonance. Several experiments have been performed that rely on extracting neutron observables from deuteron target data. These have shown some evidence of narrow structure, however, no structure was observed on the proton channel. Within the A2 collaboration at Mainz, a more detailed study has been undertaken using eta photoproduction on an LH2 target (gamma p -> eta p); it has high resolution and high precision, in an attempt to overcome the predicted low photocoupling between the N*(1685) and the proton. This paper provides an overview of the experiment and detector systems, and shows the current state of the data analysis. The gamma p -> eta p total cross section is presented for the 2 gamma-decay channel. This shows no evidence of the N*(1685) in ´ photoproduction on the proton

Recent γNN Results and Future Plans

This paper reports on: 1. Recent 12C (gamma;,pn) and 12C(gamma,pp) measrements in the Delta-resonance region.2. A proposed experiment to measure Short Range Effects in the 12C(gamma;,NN) reaction, above the Delta-resonance, using detectors close to the photon beamline

Two-nucleon emission with polarised photons

The 12C(gamma,pp) and 12C(gamma,pn) reactions have recently been measured in the Delta-resonance region using linearly polarised tagged photons. The experiments used the Glasgow tagged-photon spectrometer[1] at the Mainz 855 MeV electron microtron MAMI [2]. Data corresponding to the direct emission of nucleon pairs form (1p)2 and (1p)(1s) orbits were selected using missing energy and angular cuts. The experiments investigated the photon energy, missing energy, recoil momentum and angular dependences of the photon asymmetry Sigma and the cross sections sigma_parallel (sigma_perpendicular) for events emitted parallel (perpendicular) to the plane of polarisation of the incident photons. The data are compared with state-of-the-art calculations of direct two-nucleon emission using the model developed by the Gent theory group [3]. The experimental data reveal important features which were not accessible to previous experiments with unpolarised photons. In particular the 12C(gamma,pp) reaction shows a strong (negative) peak in Sigma at missing energies just above the reaction threshold., which is interpreted as new evidence of a direct photoproduction process for this reaction channel. The theoretical calculations reproduce some of the general trends of the measured data, but do not agree in detail with the experimental spectra. At high missing energies, for Egamma> 250 MeV, where two-step processes dominate, Sigma has a substantial negative value for both reaction channels. This is attributed to a strong asymmetry in the initial photon absorption process

Photoinduced two-nucleon emission in near super-parallel kinematics

Electromagnetically induced two nucleon emission reactions are sensitive to the interaction between nucleon pairs in the target nucleus. Contributing mechanisms include Meson Exchange Currents (MEC), Delta currents, Final State Interactions (FSI) and Nucleon-Nucleon Correlations (NNCOR). The roles of MEX and Delta currents in two nucleon emission reactions are well established. The task now confronting experimenters is to devise experiments which are sensitive to the interesting NNCOR while minimising the effects of other contributions. This paper reports experiments carried out to measure (gamma,NN) cross sections on 4He and 16O in near super-parallel kinematics where NNCOR are expected to be relatively large and the contribution from Delta currents is suppressed

Two-nucleon emission with polarised photons

The 12C(gamma,pp) and 12C(gamma,pn) reactions have recently been measured in the Delta-resonance region using linearly polarised tagged photons. The experiments used the Glasgow tagged-photon spectrometer[1] at the Mainz 855 MeV electron microtron MAMI [2]. Data corresponding to the direct emission of nucleon pairs form (1p)2 and (1p)(1s) orbits were selected using missing energy and angular cuts. The experiments investigated the photon energy, missing energy, recoil momentum and angular dependences of the photon asymmetry Sigma and the cross sections sigma_parallel (sigma_perpendicular) for events emitted parallel (perpendicular) to the plane of polarisation of the incident photons. The data are compared with state-of-the-art calculations of direct two-nucleon emission using the model developed by the Gent theory group [3]. The experimental data reveal important features which were not accessible to previous experiments with unpolarised photons. In particular the 12C(gamma,pp) reaction shows a strong (negative) peak in Sigma at missing energies just above the reaction threshold., which is interpreted as new evidence of a direct photoproduction process for this reaction channel. The theoretical calculations reproduce some of the general trends of the measured data, but do not agree in detail with the experimental spectra. At high missing energies, for Egamma> 250 MeV, where two-step processes dominate, Sigma has a substantial negative value for both reaction channels. This is attributed to a strong asymmetry in the initial photon absorption process

Glasgow, Edinburgh, Mainz (GEM) Tagging System

The GEM tagging spectrometer was designed to make best use of the 100% DC MAMI-A beam for doing photoreaction experiments. As the location available for the spectrometer was in the Magnet Hall a non standard design was necessary so that the tagging magnet system could also act as electron beam handling system when the beam when the beam was required in Hall 2. This location also offered the possibility of parasitic operation in conjunction with experiments in Hall 2

Three-nucleon mechanisms in photoreactions

The $^{12}$C$(\gamma,ppn)$ reaction has been measured for E$_{\gamma}$=150-800 MeV in the first study of this reaction in a target heavier than $^3$He. The experimental data are compared to a microscopic many body calculation. The model, which predicts that the largest contribution to the reaction arises from final state interactions following an initial pion production process, overestimates the measured cross sections and there are strong indications that the overestimate arises in this two-step process. The selection of suitable kinematic conditions strongly suppresses this two-step contribution leaving cross sections in which up to half the yield is predicted to arise from the absorption of the photon on three interacting nucleons and which agree with the model. The results indicate $(\gamma,3N)$ measurements on nuclei may be a valuable tool for obtaining information on the nuclear three-body interaction.Comment: 5 pages, 3 figure

Excitation of spin-isospin giant resonance states in 12c(gamma,pi+)12b using tagged photons

The angular distribution of positive pions from the reaction, 12C(gamma,pi+)12B* were measured at five angles (theta= 35°, 55°, 90°, 125°, 145°) using the tagged photons in the range 176 < Egamma < 182 MeV. The pi+ spectra were measured with a system of DeltaE-E plastic scintillator telescopes. The separation of pi+ from other particles was achieved by measuring the DeltaE vs. E scatter plot and also by observing the delayed decay muons from the stopped pi+ in the E-detector

A new state in 6He following the 7Li(γ,p)6He reaction

A broad excited state was observed in 6He with energy Ex=5+-1 MeV and width Gamma=3+-1 MeV, following the reaction 7Li(γ,p)6He. The state is consistent with a number of broad resonances predicted by recent cluster model calculations. The well-established reaction mechanism, combined with a simple and transparent analysis procedure confers considerable validity to this observation