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Photoreactions on 12C and Pb (Berman, Ilieva, Niculescu) The g3 group of experiments was approved for 25 days of beam time—13 days for 3He and 6 days each for 4He and 12C. The 3He part was run in December 1999, and was so successful that we were able to use the last part of this scheduled time (g3a) to perform the 4He part of the experiment as well. Thus, at the cost of only about half of the approved beam time, about ¾ of the experimental data have been obtained. There remains the 12C part (g3b) to be completed, and this is what we currently propose to do. However, because we found in our earlier run that we were able to take data at a rate greater than that originally anticipated, we plan to add a foil of NatPb to the carbon (graphite) foils in our target and obtain data on this heavy nuclear target simultaneously with those on our 12C target. We compute that by apportioning our counting rates in the ratio of roughly 12C:Pb = 2:1, we can obtain the required data on both, wholly within the originally approved beam time of 6 days. We defended this as a "jeopardy proposal" last summer (PAC 22), but approval was deferred pending the analysis of more of the g3a data (on 3He and 4He). We believe that we are rapidly approaching completion of the analysis for many reaction channels on these nuclei (see Section B), and we intend to propose this experiment at PAC 25 this coming winter. The binding energy per nucleon, mean radius, and density for 2H, 3He, 4He, 12C, and 208Pb are given in the following table:
It is clear from these values that if we wish to study the density dependence of an interaction, we should compare the data for 2H, 3He, and 4He, whose sizes are not very different but whose densities vary by a factor of about four. This we are already doing, using the data from g2 and g3a. But if we wish to study the size dependence of an interaction, we should compare data for 4He, 12C, and Pb, whose densities (or binding energies/nucleon) are about the same, but whose radii vary by a factor of about four. An example where density dependence is our prime concern is the issue of three-body forces, whose range is much shorter than two-body forces, and whose study therefore requires that the incoming short-wavelength photon interact with the three nucleons when they are close together. We can carry out such studies with our existing data. But for those interactions the study of which depends critically on the mean free path of a particle in nuclear matter, such as the interaction of photoproduced kaons, etas, or Deltas with nucleons, we require data obtained with larger target nuclei. There is a large amount of recent theoretical support for this work. Examples are the work of Bennhold and colleagues at GW on strangeness [Ben98, Lee01] and h [Lee96] photoproduction, that of Wright [Wri98] and Fix [Fix99] on D knockout, Thomas [Tho98], Wambach [Rap99], and Oset [Ose01] on medium modification of the r , and Mosel [Eff00] on meson photoproduction in general. |
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