July 26th- Day 20

Comparisons.

We set up the PET(002) with a Bragg angle of 11.81 degrees and an IFD of 121mm with a Cobalt source. The optic on the rotational stage was plugged into axis 6. We hooked up the camera with the gain and gamma settings all the way to the right. We took an image and aligned the optic. Afterward, we placed the detector back in position and hooked back up the z axis of the detector in axis 6. We placed the orange film in front of the detector and took another image. The spot was not in the center of the detector, so we moved the detector and tried again. This time the spot was closer to the center. We took the film off the detector and ran scans to find the Co peak. Scans were taken at 25kV, 30kV and 50kV and the number of counts was recorded. The solid angle of each optic was calculated using the equation below:

solid angle = area*sin(Bragg angle)

                                                                   IFD^2
Since the PET optic is comprised of several pieces, the solid angle was only an approximation. The solid angle for the Ge optic was also an approximation, but would be closer to the actual number due to the single piece. The solid angle for Ge(111) was calculated to be 0.0031 and for PET(002) is was 0.024. For each optic, the number of counts at 25kV was divided by the solid angle. This allows the optics to be compared 'apples to apples' by taking into account the differing solid angles. Dividing takes out the solid angle factor. By dividing, the flux is calculated for each of the optics. For Ge(111) it was 3.4E6 and for PET(002) is was 3.8E6. The higher the flux, the higher the efficiency making the PET optic more efficient. The flux for PET might even be higher because the solid angle could be an over approximation making it even more efficient. The optics were scanned using a different source to compare efficiencies.The new source is a Moly source. We calibrated the detector to pick up a certain peak. With this source, the peak moves as a result of varying Bragg angles. For finding the correct peak, specific ROI of 218 and 251 were set. The peak needed to be centered in this range. We tried taking an image using the film, but the detector needed to be realigned. Even after moving the detector the spot was still fairly large. A spectrum was taken but the results were not what we were looking for. Alex messed around with a few things and then took spectra once again at 25kV, 30kV and 50kV. Using the counts at 25kV the flux was calculated to be 2.2E6. Since this number is less than previously calculated, the efficiency of the Moly source was calculated to be less than the Cobalt source.
Once these calculations were complete, spot size measurements were to be taken. Unfortunately, scans were going haywire and were not producing the desired results. The detector had somehow become extremely out of place. After fixing the setup, the background ratios were lower than normal. The spot size was expected to be much larger than that of the Si(111) optic. We tried taking an image of the optic using the film, but could not find the spot. Tomorrow we will attempt to fix the problem.