G. I. Crawford

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

Abstract The tagged photon technique has been used to measure the 12C photoproton cross section over the angular range θp = 30°–135° for Eγ = 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.

Near-threshold measurement of the He-4(gamma,n) reaction

A near-threshold He-4(gamma, n) cross-section measurement has been performed at MAX-lab. Tagged photons from 23 < Ey < 42 MeV were directed toward a liquid He-4 target, and neutrons were detected by time-of-flight in two liquid-scintillator arrays. Seven-point angular distributions were measured for eight photon energies. The results are compared to experimental data measured at comparable energies and Recoil-Corrected Continuum Shell Model, Resonating Group Method, and recent Hyperspherical-Harmonic Expansion calculations. The angle-integrated cross-section data are peaked at a photon energy of about 28 MeV, in disagreement with the value recommended by Calarco, Berman, and Donnelly in 1983.

( gamma,2N) reaction in 12C.

The {sup 12}{ital C}({gamma},{ital pn}) and {sup 12}{ital C}({gamma},{ital pp}) reactions have been measured for photon energies between 80 and 157 MeV using a photon tagging spectrometer and plastic scintillator detectors. The overall energy resolution was {similar_to}7 MeV, sufficient to determine the initial shells of the emitted nucleons. Corrections were made for solid angle and threshold effects by means of Monte Carlo simulations. For the ({gamma},{ital pn}) reaction both the missing energy and recoil momentum distributions are largely consistent with a two-nucleon absorption process on {ital p}-shell an {ital sp} nucleon pairs. For the much smaller {sup 12}{ital C}({gamma},{ital pp}) cross section the reaction mechanism is not yet understood but the recoil momentum distributions suggest that final state interactions are not dominant.

The 16O(γ, p) reaction at Eγ = 60 and 72 MeV

Abstract Angular distributions within the range 25°–125° are presented for the 16O(γ, p) reaction populating low-lying states in 15N at Eγ = 60 and 72 MeV. A comparison of the data with self-consistent Hartree-Fock RPA calculations shows the importance of meson-exchange currents in the photon-absorption process.

Large position sensitive plastic scintillation detectors

Abstract Two large plastic scintillators are described, a 1 m square detector with 2 dimensional position sensitivity and a 1.8 × 0.2 m detector with position sensitivity along its length. Both scintillators are 0.1 m thick. Measurements of pulse height, timing and position resolution, performed using cosmic rays, are presented and the experimental pulse height response is compared with the predictions of Monte Carlo calculations.

The 4He(γ,n) reaction: a potential testing ground for the alpha-particle wavefunction

Differential cross sections (σ(Eγ,θn)) for the 4He(γ,n) reaction have been measured at Eγ=50–71 MeV and θn=30–120°. These data are compared with theoretical predictions where a microscopic calculation of the 3He and 4He wavefunctions has been made within the Alt-Grassberger-Sandhas, integral-equation formalism.

A tagged photon spectrometer for the mainz 180 MeV microtron

A photon tagging system has been installed on the 180 MeV electron microtron at the Institut fuer Kernphysik in Mainz for use in experiments on photonuclear reactions at intermediate energies. The system enables Bremsstrahlung produced photons in the energy range 80-174 MeV to be tagged at rates up to 5x107s-l with an overall efficiency of around 60-65%.

Measurement of theHe4(γ,n)reaction from23<Eγ<70MeV

A comprehensive set of {sup 4}He({gamma},n) absolute cross-section measurements was performed at MAX-lab in Lund, Sweden. Tagged photons from 23<E{sub {gamma}}<70 MeV were directed toward a liquid {sup 4}He target, and neutrons were identified using pulse-shape discrimination and the time-of-flight technique in two liquid-scintillator detector arrays. Seven-point angular distributions have been measured for 14 photon energies. The results have been subjected to complementary transition-coefficient and Legendre-coefficient analyses. The results are also compared to experimental data measured at comparable photon energies as well as recoil-corrected continuum shell-model, resonating group method, and effective interaction hyperspherical-harmonic expansion calculations. For photon energies below 29 MeV, the angle-integrated data are significantly larger than the values recommended by Calarco, Berman, and Donnelly in 1983.

Measurement of the {sup 4}He({gamma},n) reaction from 23<E{sub {gamma}}<70 MeV

A comprehensive set of 4He(,n) absolute cross-section measurements was performed at MAX-lab in Lund, Sweden. Tagged photons from 23<E<70 MeV were directed toward a liquid 4He target, and neutrons were identified using pulse-shape discrimination and the time-of-flight technique in two liquid-scintillator detector arrays. Seven-point angular distributions have been measured for 14 photon energies. The results have been subjected to complementary transition-coefficient and Legendre-coefficient analyses. The results are also compared to experimental data measured at comparable photon energies as well as recoil-corrected continuum shell-model, resonating group method, and effective interaction hyperspherical-harmonic expansion calculations. For photon energies below 29 MeV, the angle-integrated data are significantly larger than the values recommended by Calarco, Berman, and Donnelly in 1983. (Less)

(γ,2N) reaction in 12 C

The {sup 12}{ital C}({gamma},{ital pn}) and {sup 12}{ital C}({gamma},{ital pp}) reactions have been measured for photon energies between 80 and 157 MeV using a photon tagging spectrometer and plastic scintillator detectors. The overall energy resolution was {similar_to}7 MeV, sufficient to determine the initial shells of the emitted nucleons. Corrections were made for solid angle and threshold effects by means of Monte Carlo simulations. For the ({gamma},{ital pn}) reaction both the missing energy and recoil momentum distributions are largely consistent with a two-nucleon absorption process on {ital p}-shell an {ital sp} nucleon pairs. For the much smaller {sup 12}{ital C}({gamma},{ital pp}) cross section the reaction mechanism is not yet understood but the recoil momentum distributions suggest that final state interactions are not dominant.