[Gdh_lowq2] comments on your ND3 analysis notes

[kuhn]

> On Feb 8, 2017, at 6:07 PM, Jian-Ping Chen <jpchen at jlab.org> wrote:
> 
> Hi, Xiaochao and Sebastian,
> 
> A comment on 15ND3: The additional neutron in N15 makes dilution worse, but it does reduce the polarized background, since the extra neutron in 14N is now paired, leaving only one proton could be polarized.
> 
> Best regards.
> 
> Jian-ping

Well, it’s a really tricky issue. As far as I know, the original reason that 15N was used in all SLAC experiments and the first round of experiments at JLab has more to do with the fact that it produces a narrower NMR signal, leading to a better way to measure its polarization and to not get confused by background under the NMR signal for d and p. The polarized background is actually LARGER, since the effective polarization of the single proton in 15N and its spin structure functions are bigger than that of the effective bound deuteron in 14N. In the case of a proton (NH3) target, that is not a big deal since in some sense the extra proton in 15N even IMPROVES the dilution factor a little bit; however, as we have come to appreciate, for deuteron (ND3) targets, the background from that unpaired proton is much harder to account for (since its SSFs are NOT proportional to the ones we want to actually measure, namely those of the deuteron), and constitutes a significant background. In that case, the (very slight) polarized background from the effective bound deuteron in 14N acts more like an improvement of the dilution factor. But the devil is in the details…

- Sebastian
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