Brief Look at Helium Bag Window

Aidan M. Kelleher

In the spectrum of events in BigBite, there is a ``bump'' that occurs too early to have come from the poarlized gas in the cell. There has been speculation that this bump comes from the aluminum windows at the end of the helium bag before the target cell. This can be seen in Fig 1.

Figure 1: All Scattered Events along Z. All events in BigBite with no cuts on energy or location along Z

It is clearer if one selects on electrons (energy in the shower greater than 500MeV), as in Fig. 2

Figure 2: Electron Events along Z. Cutting only on energy to select electron events.

No such bump appears if we look at pions (Fig. 3)

Figure 3: Pion Events along Z. Cutting only on energy to select pion events.

We can zoom into the region of the bump (Fig. 4, and combine this with our knowledge of the target geometry. The exit window for the helium bag is at -0.26m. We see nothing of interest at that location.

For comparison sake, the pion spectrum is Fig. 5

Figure 4: Electron Events along Z (zoomed). Zooming into the region of interest.

Figure 5: Pion Events along Z (zoomed). Zooming into the region of interest.

I suspect that this means that the collimators are working, and the ``bump'' is a rise that is collimated out. Between the end window of the helium bag and the end window of the target is a distance of roughly 50mm, this gap is filled with air, which should result in scattering.