Interpretation of the High-Energy ProcessesO16(γ,

Abstract

Calculations based on a two-nucleon (n-p) absorption mechanism show that the form factor of the residual nucleus plays an important role in determining the shape of the photoproduction cross section in a wide energy range 60 MeV ^E y ^ 300 MeV.In recent papers 1 ' 2 the <y,P 0 ) reaction has been considered a good source of information of the high-momentum components of nucleon wave functions. Assuming a single-step knockout mechanism, momentum distributions are extracted from the data. 1 With this kind of model, however, it is not possible to describe (K,^0) cr oss sections which are of the same order of magnitude as the fyyPo) reactions. 3 * 4 A promising approach, which gives a good description of the data below the meson threshold, is a model with a one-pion-exchange force. 5 In this model the photon is absorbed mainly by a correlated neutron-proton pair; it is similar to the quasideuteron model used for the description of (y ,np) processes or other processes, e.g., 4 He(y,j!<j). 6 In this Letter it is demonstrated by using the quasideuteron model, that the form factor of the residual nucleus plays an essential role in the interpretation of the measured data in a wide energy range. Figure 1(a) shows the 16 O(y,£ 0 ) 15 N cross sections in a wide range of photon energies for proton angles of 45°, 90°, and 135°. 2 A common feature of the cross sections is the exponential decrease at low photon energies and a change in the slope in the logarithmic plot approximately at 180, 120, and 90 MeV for 45°, 90°, and 135°, respectively.Momentum conservation in the quasideuteron model yields for the cross section 7

Investigation of the40Ca(γ,N) reaction at 60 MeV

“…Indeed, it is clear from theoretical calculations that the inclusion of MEC-effects does lead to an appreciable enhancement in the cross section, without seriously affecting the slope of the energy dependence [11,13]. Related to this is also the fact that the excitation of lh states in the Modified Quasideuteron model [14] involves a coherent sum over all neutrons, while the excitation of 2h-lp states requires the promotion of a neutron from a specific single-particle level to another level, completely destroying the coherence. This then leads to the very different slopes observed in this experiment.…”

Interference effects in relativistic inclusive deuteron electrodisintegration

“…In this section we wish to investigate another situation where one can get sizable interference effects. Consider the quasi-deuteron model [33,34,35,36,37] where the scattering process can be thought as occurring off a NN quasiparticle embedded in a complex nucleus. In such a situation the spatial extent of the quasiparticle can be much smaller than a free deuteron, resulting in a broader momentum distribution, which in turn results in an E/D ratio that tends to drop less rapidly with momentum transfer.…”

Meson exchange currents in a relativistic model for electromagnetic one nucleon emission

“…On the other hand, the fact that the transitions with neutron emission are of the same order of magnitude as those with proton emission addressed to a reaction mechanism where the transferred momentum is shared between two nucleons. Indeed the quasi-deuteron model [3,4,5] was applied with some success to photoreactions at low and medium energy. Various corrections were included in the DKO model [6,7], but were unable to give a consistent description of (γ, p) and (γ, n) data.…”