The (γ, n) reaction on 16O between giant resonance and pion threshold

A new TOF facility has been built for measurements of differential (γ, n) cross sections to discrete final states of light nuclei in the photon energy range between giant resonance and pion threshold. The observed neutron angle θ_n can continuously be varied between 0° and 150°, and additionally measurements at 175° and 180° are possible. Differential cross sections for the reaction ¹⁶O(γ, n₀)¹⁵O are presented for E_γ = 60 MeV (40° ? θ_n ? 149°) and for θ_n = 90° (60 MeV ? E_γ ? 160 MeV). The results, combined with the corresponding (γ, p₀) cross sections, indicate an absorption mechanism of high energy photons by neutron-proton pairs.     

Two-body photodisintegration of 3He and 4He in the Δ(1236) region

Differential cross sections for the reactions ³He(γ, p)d and ⁴He(γ, p)t were measured at proton c.m. angles of 60° and 90° for photon energies ranging from 150–450 MeV with an average resolution of 8 MeV. Bremsstrahlung was used as the photon source; deuterons/tritons were analyzed in a magnetic spectrometer whereas coincident protons were detected in a plastic scintillator telescope. The experimental method includes a calibration by means of ¹H(γ, π⁰)p differential cross section measurement at 90° c.m. in the same photon energy range. The ³He and ⁴He two-body photodisintegration differential cross sections show a monotonically decreasing variation with photon energy. In addition, partial data on the differential cross section of the reaction ⁴He(γ, n)τ at 90° and 120° neutron c.m. angle are given.     

The 12C(γ,p) reaction at Eγ = 60 and 80 MeV

The tagged photon technique has been used to measure the ¹²C photoproton cross section over the angular range θ_p = 30°–135° for E_γ = 60 and 80 MeV. Comparison of the results with quasifree knockout and quasideuteron calculations shows that photon interaction with two-nucleon pairs is dominant at least for high excitation energies in the residual nucleus.     

Differential cross sections for photoemission of deuterons from the nitrogen nucleus

Momentum distributions for inclusive photoemission of deuterons from nitrogen have been measured at laboratory angles of 43° and 50°, and for maximum energies of the bremsstrahlung photon beam of 725, 775 and 825 MeV. The double differential cross sections of deuterons emitted from nitrogen are obtained for photon energies of 750 and 800 MeV and for laboratory angles of 43° and 50°. The photon energy is determined by the photon subtraction method. Results are examined by a π⁰ photoproduction process off a quasi-deuteron in nitrogen (γ″d″→π⁰d) and by a two-step process. The two-step process means the following: first, a nucleon is produced by a photo-pion production process from a quasi-free nucleon in the nitrogen nucleus (γ″N″→πN); second, the nucleon produced at the first step picks up another nucleon in the nucleus and then a deuteron is emitted (N+A→d+anything).     

Angular distribution measurement of 14N(γ, π+) 14Cgs at Eγ=320 MeV

Differential cross sections for ¹⁴N(γ, π⁺)¹⁴C_gs at a photon energy of 320 MeV, near the peak of the free nucleon delta resonance, have been measured at six angles ranging from 35° to 120°. Momentum-space calculations based on the distorted wave impulse approximation yield results which are lower than experiment by a factor of three.     

Measurement of the two-body photodisintegration of3He in theΔ(1236) resonance region

The³He(γ, p)d reaction has been measured in the photon energy region between 200 MeV and 450 MeV at proton c.m. angles between 20° and 150°. Protons and deuterons were detected in coincidence with two time-of-flight spectrometers consisting of scintillation counters; both particles were identified and their energies and angles were measured. The angular distributions show a strong forward peak. The differential cross sections fall off with increasing photon energy without showing a significant influence of theΔ resonance.     

Measurement of the polarization of the recoil proton from the photoproduction of neutral pions on hydrogen in the region of the first resonance. I

The polarization of the recoil proton from the reactionγ+p→π ⁰+p has been measured using liquid helium as polarization analyser. The photon energy has been varied between 250 and 360 MeV, the pion angle (CM) between 58° and 100°.     

Differential cross sections of neutral pion photoproduction from hydrogen in the energy region from 390 MeV to 975 MeV

The differential cross sections of the reaction γp→π⁰p have been measured at photon energies between 390 MeV and 975 MeV with an energy step of 20 or 25 MeV. The angular region covers 11 angles between 15° and 130° in the c.m.s. A combination of a magnetic spectrometer and photon detector was used for the measurement in the angular region from 50° to 130°. At forward angles of 15°, 35° and 50°, π⁰ mesons were detected using a π⁰ detector consisting of a pair of photon detectors. The data obtained with two different detection systems overlap at the angle of 50° and coincide with each other within the experimental error. The results are compared with those of phenomenological partial-wave analyses.     

Proton Compton effect in the Δ(1232) energy region

The proton Compton effectγ+p→γ+p in theΔ (1232) energy region has been thoroughly studied. At the Bonn 500 MeV synchrotron differential cross sections were measured covering a large range of the CM scattering angle (50°≦θ_γ≦130°) and an energy range from 240 to 440 MeV (LAB energy of the primary photon). A contribution to the counting rate from the competing process ofπ ⁰ photoproduction was strongly suppressed by detecting both particles in the final state and by a proper choice of the endpoint energy of the gamma ray beam. The remaining contribution was monitored experimentally. The measured data were complemented by cross sections at 0° which were computed from the totalγ p cross sections with an input from a multipole analysis of single pion photoproduction. Preliminary results of this experiment have been published in Lett. Nuovo Cimento. In the meantime the experimental data were reanalyzed. The final data presented here are larger than the preliminary ones by about 15%.     

Angular dependence of the polarization of the recoil proton from the reaction γp→π0 p in the region of the second resonance

The polarization of the recoil proton from the reactionγp→π ⁰ p has been measured for photon energies between 600 and 1,200 MeV and pion c.m. angles between 90° and 150°.     

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 atK _γ=710 MeV. In the region of the third resonance the data show a greater enhancement than predicted by most of the analyses.     

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 E_γ = 130?525 MeV using the large Mainz NaI(T1) spectrometer atΘ _p ^lab =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.     

Measurements of the (γ, n0) reaction on 12C and 16O and the investigation of the reaction mechanism

Differential (γ, n₀) cross sections on ¹²C and ¹⁶O have been measured for photon energies 60 MeV ? E_γ ? 160 MeV. These results combined with the corresponding (γ, p₀) cross sections support an absorption mechanism of the photon by neutron-proton pairs.     

Angular dependence of the polarization of the recoil proton from the reaction γp→π0 p in the region of the third resonance

The polarization of the recoil proton from the reactionγp→π ⁰ p has been measured for photon energies between 900–1,350 MeV and pion c.m. angles between 70° and 150°. There are significant deviations from recent analysis.     

Nonlinear model-Boltzmann equations and exact solutions

The present paper is devoted to the subject of photonuclear reactions of the type (γ, p), (γ, n), (γ, np), (γ, nn), (γ, pp) as well as nucle on capture reactions as (p, γ) at intermediate photon energies. The theoretical part of this paper reviews and extends earlier calculations of the authors up t gamma energies of E_γ ≈ 400 MeV. The paper is not intended to give a review on different theoretical approaches which have been underta ken in the past. Instead, emphasis is put on the dynamical aspects of the photonuclear reactions in order to see which elementary processes govern the reactions at different photon energies. In view of the success of the present treatment up to E_γ ≈ 120 MeV photon energy, we tempt the predictive power of the model up to photon energies of E_γ ≈ 400 MeV for the reactions in ⁴He and ¹⁶O.     

Measurement of the angular distribution of the polarization of the recoil proton from the photoproduction of neutral pions on hydrogen at a photon energy of 360 MeV. II

The angular distribution of the polarization of the recoil protons from the reactionγ+p→π ⁰+p has been measured at a photon energy of 360 MeV and at pion CM angles of 58°, 75°, 96° and 105°. The polarization of the protons has been determined by the left to right ratio in scattering on a helium gas target. The trajectory of each scattered proton has been measured by a system of spark chambers.     

Photoproduction of neutral pions on hydrogen below 500 MeV

At the Bonn 500 MeV electron synchrotron differential cross sections of the reactionγ+p→Π ⁰+p were measured in the kinematical region of photon energies between 340 and 420 MeV and pionCM angles from 10° to 70° by means of detecting the two photons from the pion decay by two Cherenkov counters.     

The photodisintegration of 40Ar

High resolution measurements of the (γ, n), (γ, 2n), (γ, p), (γ, np) and (γ, 2p) cross sections of ⁴⁰Ar over a photon energy range of 10 to 28 MeV are reported. From this data, the total photon absorption cross section integrated to 26 MeV is found to be 434 ± 40 MeV mb. The results of a dynamic collective model (DCM) calculation compare favourably with the photoabsorption cross section, supporting the use of the DCM in this mass region. It is confirmed that isospin plays an important role in the decay of the ⁴⁰Ar giant dipole resonance.     

Investigation of the <Emphasis Type="Italic">n-γ</Emphasis> angular correlation in the <Superscript>9</Superscript>Be(α,nγ)<Superscript>12</Superscript>C reaction by analysis of the 4.44-MeV <Emphasis Type="Italic">γ</Emphasis>-line shape

A method for evaluating the angular correlation function (ACF) of neutrons and γ quanta produced in nuclear reactions has been developed based on the analysis of Doppler-broadened γ-line shapes. The lineshape analysis of the 4.44-MeV_γ line from the ⁹Be(α, nγ)¹²C reaction at E _α = 4.03 MeV and θ_γ = 0°, 15°, 30°, ..., 150°, taking into account the angular n and γ angular distributions, made it possible to unambiguously fit the ACF parameters. In view of the importance of this reaction for γ diagnostics of plasma, the γ-line shape has been calculated for the monoenergetic component of isotropic α-particle distribution, characteristic of d-t experiments.     

On photonuclear reaction mechanisms at intermediate energies

The¹²C(γ,2N) reactions have been studied using tagged photons ofE _γ = 114–600 MeV at the Mainz microtron (MAMI). The cross sections of the¹²C(γ,pn) reaction and the shape of¹²C(γ,pp) spectra compare well to model predictions over a wide range of photon energies and missing energies, suggesting that the dominant mechanisms of photonuclear reactions are understood.