Photodisintegration cross section of ${}^4\mathrm{He}$ in the giant dipole resonance energy region

We simultaneously measured the ${}^4\mathrm{He}(\gamma , n){}^3\mathrm{He}$ and ${}^4\mathrm{He}(\gamma , p){}^3\mathrm{H}$ reactions in the energy range around the giant dipole resonance. A quasi-monoenergetic photon beam produced via the laser Compton scattering technique was irradiated on the active-target time-projection chamber filled with helium gas, and trajectories of charged decay particles emitted from ${}^4\mathrm{He}$ were measured. Our data suggest that the ${}^4\mathrm{He}(\gamma , n){}^3\mathrm{He}$ and ${}^4\mathrm{He}(\gamma , p){}^3\mathrm{H}$ cross sections peak around 26 MeV. This result contradicts the previous experimental data reported by Shima et al. [Phys. Rev. C 72, 044004 (2005)] but is consistent with other experimental results.

Figure 1

Top view of the BL01 at the NewSUBARU synchrotron radiation facility. Infrared photon beams generated with the Nd:${\mathrm{YVO}}_4$ laser modules were guided into the electron storage ring after two reflections. The back-scattered photons resulting from the head-on collision with relativistic electrons coming from the left were guided straight to the GACKO beam hutch. The MAIKo active target and the NaI(Tl) beam monitor were installed in the GACKO beam hutch. On the way to the hutch, photons were sieved with the two collimators to be quasi-monoenergetic photon beams.

Figure 2

Schematic picture of the MAIKo active target. This figure displays a typical ${}^4\mathrm{He}(\gamma , p){}^3\mathrm{H}$ event, in which a ${}^4\mathrm{He}$ nucleus absorbs a photon (thin green arrow emanating from behind the MAIKo active target) to break up into a ${}^3\mathrm{H}$ nucleus (the thickest red line) and a proton (thicker blue line). Electrons generated along the trajectories of the decay particles were drifted with the uniform electric field to the $\mu$-PIC at the bottom. For good visibility, field wires on the front side and the lower half of the GEM are not drawn.

Figure 3

Typical track images measured with the MAIKo active target. (a) and (b) present a ${}^4\mathrm{He}(\gamma ,n){}^3\mathrm{He}$ event, and (c) and (d) indicate a ${}^4\mathrm{He}(\gamma ,p){}^3\mathrm{H}$ event.

Figure 4

(a) Energy spectrum of the LCS photons estimated by the Monte Carlo simulation when the maximum LCS-photon energy is 30.0 MeV. (b) Simulated energy spectrum measured by the NaI(Tl) beam monitor compared with the experimental spectrum. The red solid line shows the simulated spectrum whereas the blue solid circles show the experimental spectrum.

Figure 5

Typical procedure for estimating the LCS-photon number. (a) Energy spectrum measured with the NaI(Tl) beam monitor. (b) Response functions of the NaI(Tl) beam monitor for various photon multiplicities. (c) Best fit result (thick red solid line) to the measured spectrum (blue solid line) by the response functions multiplied by the weight factors (green dashed line). The quenching effect of the signals induced on the NaI(Tl) scintillator was taken into account (see text).

Figure 6

Experimental cross sections of the (a) ${}^4\mathrm{He}(\gamma ,n){}^3\mathrm{He}$ and (b) ${}^4\mathrm{He}(\gamma ,p){}^3\mathrm{H}$ reactions. The present result (upward triangles) is compared with those from previous studies ([32]: circles, [33]: downward triangles, [36] and [37]: squares). Systematic uncertainties of the cross sections are indicated with hatched regions. The theoretical predictions taken from Ref. [39] are shown by the solid lines.