TUNL/DFELL Compton-Backscattering Facility

A proposal is being developed to use the new free electron laser facility at Duke University to produce an extremely intense source of polarized gamma rays in the 75 to 185 MeV energy range. At the Duke Free Electron Laser Lab (DFELL), coherent undulator radiation from a single bunch of 1.3 GeV electrons produces polarized laser light in the deep ultra-violet. By filling an appropriately timed 2 bunch of the storage ring, backscattered polarized -rays can be produced with total intensities of /sec. This collision process is self-aligning and the emittance of the storage ring is so small that a collimator can be used to pass only the highest energies to the nuclear target. The resulting beam would then consist solely of photons/sec, all within a 0.5 MeV bin and with a linear polarization of 100%.

The very high flux of this source would permit measurements of quite small cross sections. For example, with a 1%radiation-length target of liquid hydrogen, even 1 nb/sr cross sections would result in 150 events/hr into 4. These source intensities, coupled with high resolution and 100%polarization, are ideal for detailed measurements of LET's and nucleon polarizabilities. Of particular interest is the apparent discrepancy between recent indirect determinations of the neutron's magnetic polarizability and calculations using PT. The most model-independent neutron measurements would require detecting Compton scattering at energies below threshold. Two problems have so far precluded such experiments. First, since the photon can couple only to the neutron's magnetic moment, the elastic cross sections are very small (10 nb, total). Second, the leading terms in the Chiral expansion of the cross section for the scattering of unpolarized photons are symmetric under the interchange of the electric and magnetic polarizabilities. Thus, even if two components could be extracted from unpolarized measurements, the ambiguity between the electric and magnetic components would remain. Only the proposed TUNL/DFELL backscattering source has sufficient flux to overcome these small cross sections, and linear polarization which can be used to completely remove the electric/magnetic ambiguity.

The -ray flux of the proposed source is so high that the lifetime of the 2 electron backscattering bunch would only be 4 minutes. Although injection times are only a few seconds, ramping from the present 200 MeV injection energy to 1.3 GeV is much longer because the DFELL magnets are not laminated. The solution to this problem would be to upgrade the injector to 1.3 GeV and top-fill the storage ring. The projected costs for this are unusually low due to a large surplus of spare parts on hand at the DFELL.