Surface direct nuclear photoeffect in heavy deformed nuclei

A mechanism of photon-induced direct knockout of a nucleon from the nuclear surface without formation of intermediate excited nuclear state is described. The effect plays an important role at least for the $(\gamma ,p)$ reaction on heavy nuclei in the energy region centered at about 30 MeV where the probability of formation of the giant dipole resonance is already small but quasideuteron photoabsorption still does not prevail. A compact model of the surface direct nuclear photoeffect (SDNP) in heavy deformed nuclei that can be used to calculate the differential $d\sigma (E_{\gamma },\vartheta )/d\mathrm{\Omega }$ and total $\sigma \left(E_{\gamma }\right)$ cross sections of the SDNP under adiabatic approximation is formulated. The model is applied to calculation of the $(\gamma ,p)$ reaction cross sections on ${}^{160}\mathrm{Gd}$ and ${}^{184,186}\mathrm{W}$. The importance of the SDNP effect for these nuclei at $E_{\gamma }\sim 30$ MeV is demonstrated.

Figure 1

Schematic illustration of the proton surface direct photoeffect.

Figure 2

Inset: calculated SDNP cross section of the $(\gamma ,p)$ reaction on ${}^{160}\mathrm{Gd}$ without smearing; black circles with statistical errors: experimental cross section of the (${\gamma ,p)}^{*}$ reaction [35]; blue and green thick lines: cross sections of, respectively, the ($\gamma ,p)$ and ($\gamma ,np)$ reactions proceeding through decay of composite state calculated using the combined model [37]; red line: smeared SDNP cross section of $(\gamma ,p)$; black line: resulting theoretical estimate of the (${\gamma ,p)}^{*}$ cross section. Thin blue solid, dotted, dashed, dash-dotted curves show separate components of the decay cross section of $(\gamma ,p)$: respectively, the $T_{<}$ component of the GDR, the $T_{>}$ component, the isovector quadrupole resonance, the GDR overtone [37].

Figure 3

Comparison of the experimental [34] and calculated cross sections of the $(\gamma ,p)$ reaction on ${}^{184}\mathrm{W}$. Same notation as in Fig. 2.

Figure 4

Comparison of the experimental [34] and calculated cross sections of the $(\gamma ,p)$ reaction on ${}^{186}\mathrm{W}$. Same notation as in Fig. 2.