(Feldman)

As a commissioning run for the Compton-scattering setup with the "eggbeater" at HIGS, we have measured Compton scattering on oxygen at and below 40 MeV. The differential cross section of 2 mb/sr (~200 times larger than the 10 nb/sr cross section for deuterium) was more appropriate for the currently available flux of 10⁶ Hz. The study of oxygen as a target nucleus was physically motivated by a search for evidence of the isovector giant quadrupole resonance (IVGQR) in the energy dependence of the polarized-photon-beam asymmetry. The only previous study of the IVGQR with polarized photons was for the case of Pb [Dal92], in which a significant interference structure in the energy dependence of S(120° ) was observed. This structure arises from the interference of the high-energy tail of the dominant giant dipole resonance (E1) with the IVGQR (E2) in this energy region. The parameters of the IVGQR (energy E₀, width G and strength S) were determined by a fit to the energy-dependent asymmetry data.

We investigated the ¹⁶O(g,g)¹⁶O reaction at three energies (25, 31, and 40 MeV) and four backward angles (90° , 120° , 135° , and 150° ) at HIGS. For pure E1 radiation, the energy dependence of the asymmetry is flat and takes on values that depend on the scattering angle. The preliminary analysis of the energy dependence of the ¹⁶O(g,g)¹⁶O data is shown in the bottom panel of Fig. 3 and shows no obvious deviation from the constant value expected for pure E1 radiation (dotted lines). An angular distribution (for the measured back angles) is shown in the top panel for 40 MeV and is generally consistent with the dotted curve shown for pure E1 radiation, which is symmetric about q = 90° . The distribution of E2 strength above the GDR in oxygen is expected to be rather broad and/or fragmented, so the results shown below are not entirely surprising. More data in smaller increments over a range of energies will have to be obtained to get a better sense of the overall energy dependence.

These results for the ¹⁶O(g,g)¹⁶O reaction are only preliminary, and the analysis is still ongoing. However, as a "shakedown" experiment, the experience has provided valuable insights into the issues that will affect future Compton scattering experiments at HIGS. The measured asymmetries demonstrate the accuracy that can be achieved. In this commissioning experiment, we have shown that the Compton-scattering setup is functional and can facilitate careful and systematic measurements of scattered photons using the linearly polarized beam at HIGS.

Figure 3. Top panel: polarized-photon-beam asymmetry for the ¹⁶O(g,g)¹⁶O reaction at 40 MeV. The dotted curve indicates the asymmetry expected for pure E1 radiation. Bottom panel: energy dependence of the asymmetry at q = 135° and 120° . The dotted lines are the pure E1 predictions at these photon-scattering angles.

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Last modified: 03/31/05