Simultaneous measurement of the photodisintegration ofHe4in the giant dipole resonance region

Abstract

We have performed for the first time the simultaneous measurement of the two-body and threebody photodisintegration cross-sections of 4 He in the energy range from 21.8 to 29.8 MeV using monoenergetic pulsed photons and a 4π time projection chamber containing 4 He gas as an active target in an event-by-event mode. The photon beam was produced via the Compton backscattering of laser photons with high-energy electrons. The 4 He(γ,p) 3 H and 4 He(γ,n) 3 He cross sections were found to increase monotonically with energy up to 29.8 MeV, in contrast to the result of a recent theoretical calculation based on the Lorentz integral transform method which predicted a pronounced peak at around 26−27 MeV. The energy dependence of the obtained 4 He(γ,n) 3 He cross section up to 26.5 MeV is marginally consistent with a Faddeev-type calculation predicting a flat pattern of the excitation function. The cross-section ratio of 4 He(γ,p) 3 H to 4 He(γ,n) 3 He is found to be consistent with the expected value for charge symmetry of the strong interaction within the experimental uncertainty in the measured energy range. The present results for the total and two-body crosssections of the photodisintegration of 4 He are compared to previous experimental data and recent theoretical calculations.

The 4He total photo-absorption cross section with two- plus three-nucleon interactions from chiral effective field theory

“…In both cases, however, the suppression is not sufficient to reach the lowlying data, and in particular those of Ref. [15]. The latter calculations represent a substantial step forward in the study of the 4 He photo-disintegration.…”

“…[13,14,15]). The latest examples date back to the past two years [15,16]. Of particular controversy is the height of the cross section at the peak, alternatively found to be either pronounced or suppressed with differences up to a factor of 2 between different experimental data.…”

“…We compare to experimental data in the region ω < 40 MeV, where corrections to the unretarded dipole approximation are expected to be largely negligible and the relative uncertainty of our calculations is minimal. The data sets from Nilsson et al [16] and Shima et al [15] are chosen here as the latest examples of controversial experiments characterizing the 4 He photo-effect since the 50's (see reviews of available data in Refs. [13,14] and [15]).…”

“…The data sets from Nilsson et al [16] and Shima et al [15] are chosen here as the latest examples of controversial experiments characterizing the 4 He photo-effect since the 50's (see reviews of available data in Refs. [13,14] and [15]). Note that in the upper panel of Fig.…”

“…At higher energies it represents a lower experimental bound for the total cross section, as in the energy range considered here the data of Nillsson et al do not contain the contributions of the 4 He(γ, np)d and four-body break-up channels. Shima et al [15] provide total photo-disintegration data obtained by simultaneous measurements of all the open channels. Finally, we show also an indirect determination of the photo-absorption cross section deduced from elastic photon-scattering on 4 He by Wells et al [40].…”