Analysis of $^4$He($\gamma,p)$T and $^4$He($\gamma,n)$$^3$He Reactions with Linearly Polarized Photons in the Energy Range up to 100\,MeV

TL;DR

This study investigates the $^4$He($ extgamma$,p)T and $^4$He($ extgamma$,n)$^3$He reactions with linearly polarized photons up to 100 MeV, revealing energy-independent cross section ratios and asymmetries, supporting specific nuclear transition models.

Contribution

It provides new experimental data on reaction cross sections and asymmetries with polarized photons, clarifying the role of spin states and challenging previous assumptions.

Findings

Cross section ratios are energy-independent within experimental error.

Cross section asymmetries are also energy-independent, supporting specific nuclear transition models.

Data do not agree with meson exchange current contributions involving spin-flip.

Abstract

In a number of investigations, one can find the data on the $^4$He($\gamma$,p)T and $^4$He($\gamma$,n)$^3$He reaction cross sections in the collinear geometry, which are due to spin {\it S}=1 transitions of the final-state particles. The ratio of the differential cross section in the collinear geometry to the differential reaction cross section at the nucleon emission angle $\theta_N$=90$^\circ$, and specified by the {\it S}=0 electric dipole transition at photon energies in the range 20$\le E_{\gamma}\le$100\,MeV, is independent of the photon energy, within the experimental error. In the meantime, experiments were made to measure the asymmetry of the cross section $\Sigma(\theta_N)$, for the mentioned reactions with linearly polarized photons. It has been found that in the energy range between 20 and 90\,MeV, the $\Sigma(\theta_N)$ value is also independent of the photon energy, within the experimental error. These data are in agreement with the assumption that transitions with spin {\it S}=1 can be due to the contribution of $^3P_0$ states of the $^4$He nucleus, and are inconsistent with the assumption that the spin-flip of the particle system occurred during the reaction as a result of the meson exchange current contribution. The available measured data on the collinear geometry reaction cross sections and the ones on the cross-section asymmetry of the reaction with linearly polarized photons do not agree between themselves. The above mentioned reactions seem to be more convenient for measuring the degree of photon beams linear polarization than the deuteron photodisintegration reactions.