S.J. Hall

A tagged photon spectrometer for use with the mainz 180 MeV microtron

This paper describes a photon tagging system which has been installed on the 180 MeV electron microtron at the Institut fur 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 5×107 s−1.

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.

The contribution of 2N photon absorption in 12C(γ,2N) reactions for Eγ = 150–400 MeV

Abstract The 12 C(γ,pn) and 12 C(γ,pp) reactions have been studied using tagged photons of energy E γ = 150–400 MeV. Recoil momentum distributions are compared to the results of Monte Carlo calculations based on a two-nucleon photon absorption model and two different phase space models. The 12 C(γ,pn) data at low missing energy are consistent with absorption on 1 p 2 and 1 s 1 p nucleon pairs.

Reduction of afterpulsing in a photomultiplier

Abstract Large pulses in photomultiplier tubes are frequently found to generate smaller pulses approximately 0.5 μs later. Where such large pulses represent some form of background, they can be suppressed in a manner which reduces drastically the number of afterpulses generated. A technique is described for application to 58 AVP/XP1040 photomultipliers, and the effects discussed.

( 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.

A facility for fast neutron time of flight experiments

Abstract Nuclear reaction experiments involving neutrons as primary or secondary particles have been performed on a multi-angle time of flight spectrometer with an electron linear accelerator source. Essential features in the design of a flexible arrangement with good resolution are discussed, and examples are given to illustrate the quality of the experimental results.

Spin-isospin excitation in 12C(γ, π+)12B∗ using tagged photons

The energy spectra of positive pions from the reaction 12C(γ, π+)12B∗ were measured at five angles, using quasi-monochromatic, tagged photons in the energy range from 176 to 182 MeV. Strong transitions to the isovector nuclear states in 12B at 0–1, 4.5, and 7.5 MeV were observed. The differential cross sections to these states have been obtained for the momentum transfers 0.5 < q < 1.2 fm−1. DWIA calculations have been carried out in momentum space for each observed state or group of states. Comparisons are made between the present data, previous data from bremsstrahlung experiments, and various DWIA calculations.

Differential cross section for the reaction 2H(γ, p)n from 133 to 158 MeV

The cross section for the reaction 2H(γ, p)n has been measured at laboratory photon energies Eγ = 133−158 MeV and c.m. angles between 30° and 150°. The reaction was induced by a tagged bremsstrahlung photon beam incident on a liquid deuterium target. The uncertainty in the absolute cross sections is ⩽ 5%. There is now reasonable agreement between recent measurements in this energy region and the overall data set now defines the cross section sufficiently well to provide a test of current models of the reaction.

The angular distribution of ground-state photoneutrons from 40Ca from 18 MeV to 30MeV

Abstract Legendre angular distribution coefficients, a1, a2, a3 and a4 for the reaction 40Ca(γ, n0)39Ca have been measured. The data are compared with an RPA calculation based on a Skyrme III force. Also presented is a measurement of the Legendre angular distribution coefficients for the reaction 2H(γ, n)p.

Opto-coupled system for photomultiplier blanking

Abstract Technical details are given for the circuitry required to perform rapid photomultiplier switching via cathode potential change.