Analog of the giant dipole resonance in ${}^4\mathrm{He}$

We studied the giant dipole resonance (GDR) in ${}^4\mathrm{He}$ by observing its analog via the ${}^4\mathrm{He}$(${}^7\mathrm{Li}$,${}^7\mathrm{Be}$) reaction at an incident energy of 455 MeV and at forward scattering angles. The spin-nonflip ($\Delta S=0$) spectrum was deduced by measuring the 0.43-MeV ${}^7\mathrm{Be}$ γ-ray in coincidence with the scattered ${}^7\mathrm{Be}$. The total cross section of photodisintegration to the GDR in ${}^4\mathrm{He}$ was derived from the $\Delta S=0$ and $\Delta L=1$ spectrum. The result agrees well with the previous total photodisintegration data in which the GDR has a pronounced peak at $E_{\gamma }~27$ MeV.

Figure 1

Singles (a) and coincidence (b) spectra for the ${}^4\mathrm{He}$ and Aramid targets in the (${}^7\mathrm{Li}$,${}^7\mathrm{Be}$) reaction at $E_L=455$ MeV and at ${\theta }_L=0^{°}$. The abscissa is the excitation energy $E_x$ in ${}^4\mathrm{He}$.

Figure 2

Angular distribution of the differential cross sections of the singles spectrum for the ${}^4\mathrm{He}$(${}^7\mathrm{Li}$,${}^7\mathrm{Be}$) reaction. The dashed, dotted, dotted-dashed, and two-dotted-dashed lines denote the cross sections given in the DWBA calculations for particle-hole states with 1${}^{+}$, and 2${}^{+}$, 1${}^{-}$, and 2${}^{-}$, respectively. The solid line is obtained by adding two DWBA cross sections for the 1${}^{-}$ and 2${}^{-}$ states.

Figure 3

Singles spectrum in the ${}^4\mathrm{He}$(${}^7\mathrm{Li}$,${}^7\mathrm{Be}$) reaction at $E_L=455$ MeV and at ${\theta }_L=0^{°}$ decomposed into the $\Delta S=0$ and $\Delta S=1$ spectra for ${}^4\mathrm{He}$.

Figure 4

$E1$ photodisintegration cross sections (•) evaluated from the $\Delta S=0$ cross section for ${}^4\mathrm{He}$ by using Eq. (2) in the text. The hatched area is the sum of ($\gamma ,n$) and ($\gamma ,p$) data recommended by Calarco et al. [3]. Two recent data are also shown: total photodisintegration cross sections (⊗) measured by Shima et al. [5] and ($\gamma ,n$) (⊖) measured by Nilsson et al. [6]. The ($\gamma ,n$) data obtained by Nilsson et al. are simply multiplied by a factor of two. The dashed curve is drawn to guide the eye for data obtained by Shima et al. The solid curve represents the cross sections calculated by using the LIT method with the $\mathit{NN}+3\mathrm{NF}$ potentials [8].