(Feldman, O'Rielly, Berman, Briscoe)

The two-body photodisintegration reactions ³H(g,d)n and ³He(g,d)p have been measured simultaneously using tagged photons of energy E_g = 17-52 MeV at SAL. Two identical gas cells with thin windows were located along the photon beam axis; the outgoing deuterons were detected in four solid-state detector DE-E telescopes mounted at 90° in each gas cell. These data constitute the first cross sections for A=3 nuclei obtained in this low-energy region since 1981, and these are the first simultaneous measurements in the A=3 systems. They are also the only photonuclear data for A=3 in this energy region measured with tagged photons. One of the primary motivations for this work is the investigation of charge symmetry in few-nucleon systems, which can be examined by comparing the results on mirror reaction channels using the ³He and ³H targets. Furthermore, with the currently available state-of-the-art calculations from the Bochum group [Ski03a], these high-precision data will serve as a stringent test of the theory.

The analysis of the two-body data set on ³H and ³He is now complete. Using particle identification from the solid-state telescopes, deuterons have been easily separated from protons. Timing (from the tagger) and kinematics (from the reconstructed photon energy) cuts permit a very clean yield of reaction deuterons to be obtained. A non-trivial correction factor due to stolen tagger coincidences had to be applied to the yields, due to the fact that the measurement was performed at very high photon rates (~4 MHz per tagger channel). This factor depends on the instantaneous rate in each tagger channel and the position of the prompt peak in the tagger tdc for that channel – from rate and timing information in the data stream, the factor can be determined well. Using GEANT, we are able to model the detector acceptance and finite-geometry effects of our extended target, which is necessary for determining an absolute cross section. We are also able to account for the effect of energy loss and straggling of outgoing reaction deuterons through the target gas and the exit foils.

Using the extracted yields and the geometrical acceptance factors, final absolute cross sections have been obtained. These results are in reasonable agreement with the results of calculations [Sch01, Ski03a]. The experimental cross sections are shown in Fig. 1 in comparison with these theoretical formulations as well as previous data. The experimental results have been presented in preliminary form [Fel00], including the LOWq Workshop in Halifax [Ori01a] and the EMI2001 Conference in Osaka [Ori01b]. The final results for the ³H and ³He cross sections were presented at a Gordon Conference on Photonuclear Reactions [Ori02].

Since the data for both gas cells were collected simultaneously under the same beam conditions, factors due to photon flux and detector geometry cancel out in the ratio of cross sections for ³H and ³He. Our experimental ³H/³He ratio is given roughly by a constant value of 1.05±0.04 which is in excellent agreement with the Schadow et al. [Sch01] calculations over the entire energy range of the present experiment. These calculations are fully charge symmetric; thus there is no evidence in the current data for any obvious violation of charge symmetry.

Figure 1. Differential cross sections measured at 90° in the lab for the ³H(g,d)n and ³He(g,d)p reactions measured at SAL (top and bottom panels, respectively). The current data are shown as the red points, and previous data are also shown (see legend). Theoretical calculations are also displayed as the solid curves (see legend). These are from [Sch01] using the Bonn, Paris, and Nijmegen potentials and from [Ski03a] using an NN interaction only (AV18) and including a three-body force (Urbana IX).

Before publishing the above results for the absolute cross section, we need to complete our analysis of several internal-consistency checks that were built into the experiment. The most important of these cross-checks was the simultaneous measurement of the photodisintegration of deuterium, which has a very well known cross section in this energy region, and will allow us to confirm our target-length and detector-acceptance factors, as well as our analysis methods for the extraction of yields. We have also obtained data with ⁴He in both gas cells in order to check the sensitivity to target position, gas pressure and photon tagging rate. In addition, an independent check is being performed [Pyw03] using a Monte-Carlo simulation of the SAL tagger system. These analyses are in progress and are nearly complete. A preliminary draft of the paper, already prepared, is only awaiting confirmation from these internal-consistency checks.

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Last modified: 03/31/05