A Comparison of Photon Sources, both Existing and Proposed

Although there is some overlap among the real photon sources, most of the programs are complementary. Different characteristics are required for various classes of experiments; Table provides examples from four general categories of photon-induced reactions. Studies of N excitation and few-body reactions have been combined since their beam requirements are quite similar. The expected event rates for a 1%radiation-length (RL) target of liquid hydrogen are listed for each source, where applicable. (1% RL for either LH2, LD2, or LHe is 8.5 cm.) For Bates-SHR, a luminosity of 10 is assumed.

For low energy QCD tests involving comparisons to PT, the potential of the proposed TUNL/DFELL source looks quite promising. For studies of the -resonance region and of few-body reactions, significant progress could be expected from three of the sources. This is a healthy situation since the number of different experiments and required hours of beam in this category is very large. Only the tagged photon sources in Hall B at CEBAF will be able to address the highest energy issues.

Large solid angle detectors will be needed to control systematics. The IGLOO, CLAS, and SASY spectrometers are presently under construction, and the detector designs for the TUNL/DFELL and Bates-SHR programs are under study. For many of the key problems facing this field, polarized targets will be essential. The TUNL/DFELL group already has a polarized solid He target suitable for neutron Compton scattering measurements. There is expertise at MIT in polarized gas targets suitable for the SHR. There are various options open to the CLAS collaboration involving polarized ammonia or butanol. Finally, the promising new polarized HD target, SPHICE, is already under construction at LEGS. With adequate support, the spectrum of real photon sources in North America can be expected to provide a vigorous attack on the key physics issues facing this field in the next five to ten years.