P. Di Giacomo

Inclusive photoproton spectra from 12C at intermediate energies

Abstract Inclusive photoproton spectra have been measured at five laboratory angles and two photon energies: 159 and 198 MeV. A quasi-monochromatic photon beam, obtained by positron annihilation in-flight on a liquid hydrogen target, was used and the photon spectrum was measured on-line by a pair spectrometer. The experimental proton spectra are well described by assuming a dominant photon absorption mechanism by np quasi-deuteron pairs in the nucleus, provided that rescattering effects are taken into account.

The leale photon beam facility at Frascati, obtained by positron annihilation on a liquid hydrogen target

Abstract The Frascati positron beam is used to produce a quasi-monochromatic annihilation photon beam with a continuously variable energy in the 100–300 MeV range. The positron production, beam transport and monitor system are discussed. The photon facility, with liquid hydrogen target, on-line pair spectrometer and monitor systems, is described. The performance of the facility is shown by the first measurements of bismuth photofission and deuteron photodisintegration.

Monte Carlo calculation of energy spectrum and spatial distribution of photons from positron annihilation

Abstract A Monte Carlo computing program has been used to calculate the photon spectrum and emittance from positron annihilation and bremsstrahlung. The positron energy spread, the energy loss and the multiple scattering in the annihilation target have been taken into account. Moreover, the positron emittance, the positron incidence angle and the finite angular acceptance of the photon collimation channel have been explicitly considered. Experimental results, obtained with a pair spectrometer and a beam profile monitor are in good agreement with calculations. In particular, it is shown that the positron emittance has a crucial influence on the shape of the photon spectrum and on the absolute value of the photon flux.

Fission of Bi induced by a quasi-monochromatic photon beam at energies from 100 MeV to 280 MeV

SummaryThe photofission yields of Bi induced by the quasi-monochromatic photon beam from in-flight annihilation of positrons of the Frascati linac were measured. The experiment was performed at sixteen different positron energies, from 120 MeV up to 280 MeV, by collecting the annihilation photons at angles 0.5° < θ < 1°. Fission fragments were detected with glass sandwiches. Photofission cross-section values were deduced by means of an appropriate unfolding method.

On fission experiments by means of a quasi-monochromatic photon beam at the frascati linac

Abstract The best experimental conditions to perform photofission measurements in the energy range 100–300 MeV by using the Frascati annihilation photon beam are deduced. For nuclei with very low fission threshold energies it is shown to be advantageous to collect the photon beam an angle ϑγ > 1°. For nuclei with high fission threshold energies, the annihilation photon contribution to the yield already pre-dominates for 0.5° ⩽ ϑγ ⩽ 1°.

A multiwire chamber for positron and photon beam scanning

Abstract A sensitive profile and positron monitor for low intensity positron (≈ 109 A) and photon (≈ 108γ/s) beams is described. consists of a multiwire chamber with two sensitive planes and additional electronic circuits. The display of the beam profiles, which appears on a standard oscilloscope, is live and quantitative; the effect of adjustments of the beam transport is immediately and continuously visible.

Q2 evolution of the Drell-Hearn-Gerasimov sum rule in the constituent-quark model

SummaryBy means of the constituent-quark model we study theQ2 evolution of the Drell-Hearn-Gerasimov sum rule. The model indicates that high-energy excitations should provide for a natural link between resonance and high-Q2 regions. We obtain contributions to the sum rule integral that could be detected in forthcoming experiments at CEBAF.

Pair currents and constituent quark models

SummaryWe derive explicit expressions for the meson exchange pair currents in the framework of the constituent quark model, taking fully into account the antisymmetrization of the six-quark states, which form the correlated nucleon pairs, and evaluating the vertex functions in terms of the internal quark momentum distribution. We discuss the mapping of the pair current operators from the quark space to the nucleon one, defining effective electromagnetic operators which can be used in practice, in particular for the evaluation of the form factors of the few-nucleon systems.