Comparing photon background to collimator thickness:

Of VT 10.25 collimator. In some cases a second random seed was used to verify data.

• Original: 21.66 cm
• 30 rad lengths - 16.2 cm (.54)
• 25 rad lenths - 13.5 cm (.54)
• 20 rad lengths - 11.23 cm (.56)
• 1/2 original: 10.83 cm

Table 1: Elastic and Inelastic Backgrounds with and without a lintel

	Elastic Photons	Elastic Photons from coll. region
VT 10.25	0.209581% +/- 0.005073%	0.034230% +/- 0.001653%
	0.208813% +/- 0.004972%	0.042243% +/- 0.001924%
16.2 cm	0.218164% +/- 0.004965%	0.044184% +/- 0.001854%
	0.227141% +/- 0.005137%	0.042107% +/- 0.001797%
13.5 cm	0.211068% +/- 0.004925%	0.041293% +/- 0.001843%
	0.212257% +/- 0.004946%	0.040347% +/- 0.001769%
11.23 cm	0.227675% +/- 0.005110%	0.045156% +/- 0.001756%
	0.252604% +/- 0.005413%	0.053901% +/- 0.002030%
10.83 cm	0.233799% +/- 0.005138%	0.050179% +/- 0.001944%

Plots of these values:

Conclusion: From this 13.5 cm of lead is enough (25 rad lengths via .54, slightly less via .56)