From or@virginia.edu Tue Nov 25 15:02:39 2003 Date: Tue, 25 Nov 2003 14:22:29 -0500 From: Oscar A. Rondon To: Xiaochao Zheng Subject: Re: proposal, presentation etc.. Hi Xiaochao, Xiaochao Zheng wrote: > (a) I added more discussion about the two-photon exchange based on my > recent discussion with people at Argonne. Basically we need some strategy > to emphasize the physics impact of this work, while without > (inadvertently) criticizing other experiments/proposals. Since both of you > are more or less working on other topics/proposals, your comments will > help to neutralize the tone of this proposal; I agree. I suggest we say that although the two-photon exchange expts. are challenging they should be pursued in parallel because the large size of the observed discrepancy is likely due to more than one cause. By the way, I should mention that the use of transverse polarized targets is quite promising, since vertical field targets (superconducting and conventional) exist already and have been used at BNL, Berkeley, Protvino and other labs, so I would not say that a major facility development is needed for these targets (end of first parag., p. 4). Finally, I would not say that it "may take decades" (p. 4, parag. 4, line 4) to understand two photon exchange. It will take a while, but surely less than 10 years. > (c) I am still a little confused by the calculation for target dilution > and packing factor from 12c and 4He runs. At the moment we need to make > sure there is no obvious flaw in the proposal. Certainly, for the long run > I need to straighten this out before defending it at the PAC. The last equation of the technical note on NH3 dilution factor http://galileo.phys.virginia.edu/~or/nh3nucl.pdf shows how the packing fraction enters in the calculation of the dilution factor. The p.f. is just how much of the target cells we were able to fill up with NH3 (or ND3, LiD, etc.). There are many possible ways of finding how much it is: one could, for example, just weight the material in the cell, etc. However, since ammonia loses its polarization centers if it is heated above 80 K (NH3), it must be kept in liquid nitrogen-LN2, which makes the weighting very tricky, although not impossible: I have measured the densities of NH3 and ND3 at LN2 temperature (77 K) and the numbers everybody uses in spin structure are mostly based on my measurement. However, the density measurement was done using lab apparatus which kept the ammonia in LN2 that evaporates continuously, while the packing fraction would need to be done using the actual target cells before inserting them. The 1.5 m long insert sticks are way too heavy (> 1 Kg) so the addition of about 10 g of ammonia is a 1% effect, and we would need at least a 5% precision on that 1% or 5 parts in 10,000, which is very hard to achieve while at the same time tracking the changing total weight of insert + ammonia + boiling LN2. The weighing could be done more easily if the 25 cm long target cell holder that weighs less than half a pound could be detached from the rest of the insert, but Don and Donal are concerned about the electrical connections to the NMR coils, microwave horns, temperature sensors, etc. that are in the cells. Peter Bosted and Steve Rock came up with the idea of just comparing rates from a target of known thickness to the measured rates from the target as a way of finding the packing fraction. A similar approach, based on transmission of X-rays was devised by Ingo Sick. At the moment, the method we are using is to calibrate the Montecarlo to reproduce the absolute rates of the data runs with C disks of known thickness. This results in a single normalization constant of near unity value (the preliminary constant for RSS is about 0.9). Then, we run the normalized Montecarlo for the ammonia target, which includes NH3, LHe, Al, etc. for different fractions of NH3 and LHe inside the cell (i.e. different packing fractions). The actual p.f. can be found by plotting p.f. as function of the MC rates, and reading the p.f. that corresponds to the observed data rate. This has to be done for each target cell, since they have different loads, depending on the material size, loading operator skill, luck, etc. Each cell has its own p.f. Fortunately, the range of variation is not too large (0.55 +/- 0.05) and the d.f. is not very sensitive to these variations anyway. The He runs are used for further validation of the C normalization and of the cross section models: in RSS and in other experiments that have a broad W range, the p.f. has to be a constant number, independent of W. This can be achieved only if the MC model is good enough so that the ratio of MC/data rates is independent of W. We are still working on improving this for RSS for its eight different configurations (A_parallel and A_perp, NH3 and ND3, HMS at low p_central and at high p_central). Once we are happy with a flat MC/data ratio for NH3, most of the job of getting the d.f. is done too: we just need to take the ratio of our model rates for H to the model rates for the full target with the appropriate p.f. The dilution factor for our elastic scattering proposal involves just a narrow range of W, which means that we only need to concentrate on having a good model for the quasi elastic and elastic regions. Since the only contributions to the rate below the elastic peak come from N, He (and a little Al), taking data on He and subtracting the He rate from the full target data in this region of W will leave pure N rate. Then we can adjust our N cross section model to give the flattest MC/data ratio for this region, which can then be used to extrapolate reliably into the adjacent region under the elastic peak, to get the d.f. My claim of getting the error in the d.f. to be 2.5% or better is based on doing this job right, which I think it is not overly difficult, if we have (as we should) an excellent HMS model including treatment of large beam raster and tracking in the target field, and model cross sections based on our own N data, validated by good C model. > (3) Oscar will take care of the PAC submission forms (please double check > the beam time for the lowest Q2 point, I have just changed it today); The numbers are OK. But the row showing N_tot is a bit unclear: one has to multiply it by f=0.5 to get the N_el number that is related to the error in A_el. It may be better to just show the N_el number. Otherwise, some readers may divide N_tot by total rate and get double the hours you show. Cheers, Oscar