OCA_ACR

Next, the absolute acrylic optical extinction is determined for a subset of tiles and the relative values derived in the previous step are then used to infer the absolute extinctions of all tiles. The acrylic tile extinctions at lower wavelengths are a particularly critical set of parameters which need to be accurately determined. Measurements based on laserball positions within the AV alone are difficult to use for this without dependending on a very accurate knowledge of geometry (i.e. the exact location of the laserball, the angle of light passing through the acrylic and the precise distance to each PMT). Such a dependence may not be robust to systematic uncertainties. Accordingly, a different approach is adopted which involves comparisons between intensities observed when the laserball is located within 200cm of the AV center (again, so photons are not too far from normal incidence on AV and PSUP surfaces), and when it is located in the light water region between the AV and PSUP. For each set of 2 such laserball locations, PMTS are selected which lie within $\sim30$deg of the direction specified by a line passing from inner to outer laserball positions. This eliminates ``extreme'' PMT/collector angles or light paths which approach glancing angles to the AV. As the acrylic is essentially tranparent at wavelengths above 400nm, prompt-peak intensities observed above this wavelength are used to normalize out all geometric uncertainties such as relative laserball positions, PMT/reflector solid angle, laserball angular distribution and overall laser intensity. Hence, the ratio of prompt-peak values for inner to outer laserball positions is computed. Differences in this ratio at lower wavelengths can then be directly related to optical extinction in the acrylic tile though which the photons pass. As mentioned earlier, the relative tile extinctions are then used to infer the absolute acrylic transmissions for all tiles. The absolute acrylic tile transmissions are subsequently assigned to the variables OCA_Acryl(I,IW), where $NTile$ is the tile number and $IW$ is the laserball wavelength index.