Compton scattering from ${}^4\mathrm{He}$ at the TUNL $\mathrm{HI}\gamma \mathrm{S}$ facility

Differential cross sections for elastic Compton scattering from ${}^4\mathrm{He}$ have been measured with high statistical precision at the High Intensity $\gamma$-ray Source at laboratory scattering angles of ${55}^{\circ }$, ${90}^{\circ }$, and ${125}^{\circ }$ using a quasi-monoenergetic photon beam with a weighted mean energy value of 81.3 MeV. The results are compared to previous measurements and similar fore-aft asymmetry in the angular distribution of the differential cross sections is observed. This experimental work is expected to strongly motivate the development of effective-field-theory calculations of Compton scattering from ${}^4\mathrm{He}$ to fully interpret the data.

Figure 1

Schematic of the experimental apparatus showing the layout of the cryogenic target and the array of NaI(Tl) detectors. The photon beam is incident from the lower left side of the figure. The target cell is contained inside the aluminum vacuum can.

Figure 2

Simplified diagram of the data acquisition system.

Figure 3

2D spectrum showing energy deposition in the shield detector versus energy deposition in the core detector. An apparent gap around the dashed line is observed between the cosmic-ray events (above the dashed line) and the Compton-scattering events (below the dashed line). The shield energy cut is placed at the dashed line.

Figure 4

Time-of-flight spectrum showing prompt and random regions with shield-energy cut applied. The bin width is 0.4 ns.

Figure 5

Representative energy spectra for full (closed dot) and empty (open dot) targets at $\theta ={55}^{\circ }$, ${90}^{\circ }$, and ${125}^{\circ }$. The bin width is 0.8 MeV. The empty-target spectra have been normalized to the number of incident photons for the full-target spectra.

Figure 6

Representative final energy spectra at $\theta ={55}^{\circ }$, ${90}^{\circ }$, and ${125}^{\circ }$ with empty-target events removed. The bin width is 0.8 MeV. The fit to the data (dashed curve) consists of the electromagnetic background (dot-dashed curve) and the geant4 simulated detector response function (solid curve). At $\theta ={55}^{\circ }$ and ${90}^{\circ }$, the backgrounds are the sum of an exponential low-energy contribution accounting for the atomic scattering and a constant background resulting from the electron-beam-induced bremsstrahlung photons. At $\theta ={125}^{\circ }$, the background is free from the exponential low-energy component and therefore includes the constant contribution only. The vertical dashed lines indicate the ROI used to obtain the yield at each angle.

Figure 7

The reconstructed effective beam energy profile on the target for a $\gamma$-ray beam produced by an electron beam energy of 975 MeV in the storage ring and a laser beam of about 192 nm in the FEL optical cavity. This energy distribution is peaked at 85.1 MeV with a FWHM of 5.5 MeV (6.5%). The weighted mean value is 81.3 MeV.

Figure 8

Compton scattering cross sections of ${}^4\mathrm{He}$ reported in the current work (circles) compared to the results from Lund (squares, $E_{\gamma }=87\mathrm{MeV})$ [12] and the previous measurement at $\mathrm{HI}\gamma \mathrm{S}$ (triangles, $E_{\gamma }=61\mathrm{MeV})$ [11]. The error bars shown are the statistical and systematic uncertainties added in quadrature.