Superceded by ACIS memo #182
Lab Camera with gamma ray source| CCD: | w459c1 |
| 5 Energies: | C-K (277 eV), O-K (525 eV), Fe-L (705 eV), |
| Al-K (1480 eV), Mn-Ka (5890 eV) | |
| Temperature: | -120 C |
| Node 2: | irradiated warm by soft protons |
| CTI @ 5.9 keV = 14.88 x 10-5 | |
| CTI @ 1.5 keV = 23.70 x 10-5 | |
| Node 0: | undamaged |
| CTI @ 5.9 keV < 2.2 x 10-7 | |
| CTI @ 1.5 keV < 8.7 x 10-7 |
Note: The Fe-L and Al-K sources appear to be offset by 45 pixels in the y-direction, i.e. a histogram of counts vs. ccdy for node 2 resembles node 0 with ccdy(node0)-45. I have offset the Node 0 Fe-L and Al-K data so that the vertical structure in each quadrant is similar. In all cases I am assuming the source illumination is flat along the column direction so that any residual vertical structure can be remove by comparison between node 0 and node 2.
I am investigating the affect of CTI on the quantum efficiency of the ACIS CCDs as a function of incident photon energy and position on the detector. Shown above is the relative QE of node 2 (radiation damaged) to node 0 (undamaged) calculated in two ways. In both cases only G02346 events with a threshold of 38 and a split of 13 are considered. Points marked with a plus (+) were calculated using all the counts in the entire spectrum (`full spectrum' method). This includes photons not in the main spectral peak, i.e. pileup peaks, K-beta peaks, cosmic rays. The points marked with an asterix (*) were calculated using all the counts within ± 3 sigma of the centroid of a Gaussian fit to the spectral line (`3-sigma' method used for the original ground QE calibration). The spectral resolution in node 2 is poor enough that the first pileup peak and, for Mn-Ka, the Kb peak, are within 3 sigma, yielding a higher, but incorrect relative QE. At the lower energies, both techniques do seem to agree that QE at the C-K line is substantially degraded by the CTI, while the O-K, Fe-L and Al-K lines are relatively unaffected. This is because for C-K at high row numbers the event threshold of 38 is of order the pulseheight of the spectral line. At Mn-Ka, the 3-sigma technique is wrongly including the Mn-Kb peak. The full spectrum technique gives a better measurement of the degraded QE. In this case the loss of QE is from grade morphing of valid X-ray events into `bad' grades, primarily G7.
Above is shown the relative QE of Node 2 to Node 0 as a function of row number using the full spectrum technique. The O-K, Fe-L, and Al-K lines show little row-dependent structure, thus the effect of CTI on QE at these energies must be minimal. The C-K and Mn-Ka lines both show a strong degradation of QE at higher row numbers.