6 November 2007Cal-Ops Meeting Mallorca Recent Results from EPIC MOS Life Test Facility Tony Abbey and Craig Brown University of Leicester Space Research Centre Cal Ops Meeting, Mallorca 6 Nov 2007

6 November 2007Cal-Ops Meeting Mallorca The XMM-Newton Life Test: What is it? Launched 10/12/99, nominal mission of 2 years, extended to 31/03/10 Solid state focal plane - CCD22 developed by E2V for UoL for EPIC Over the course of the mission, an unexpected degradation of the EPIC camera has occurred Central CCD, matching the centre of the PSF, shows significant degradation compared to other parts of the EPIC CCDs Early spectra taken show less energy redistribution than later spectra of the same object in the sky Damage must be from X-rays, so not bulk damage Aims of the test are to: Re-create this type of damage in the lab Establish a mechanism for the damage Try and reverse the damage

6 November 2007Cal-Ops Meeting Mallorca In Orbit X-ray Dose – Andy Read _ The flux on the central CCD of MOS1 – for imaging modes only - from SSC files. This is a lower limit, and doesn’t include timing mode – perhaps 30% more. Similar numbers for MOS2. The energy redistribution was being noticed in the focal “patch” after a few 100 revs – ie at 3 or 4 E11 eV..

6 November 2007Cal-Ops Meeting Mallorca Improving and calibrating the test CCD: Uneven dark current at -100°C and removed after annealing at +100°C. Now barely visible.

6 November 2007Cal-Ops Meeting Mallorca XMM-Newton Life Test: What we are doing The XMM-Newton Life Test: What we are doing 2.5cm

6 November 2007Cal-Ops Meeting Mallorca The damage starts It was decided to keep to as close to the in-orbit characteristics as possible Hence the CCD was kept cooled to -100°C and read out continually while the beam was on (remember we reduced to -130°C later) We could therefore see that the whole of the field of the CCD was saturated with charge After 28 hours of irradiation at 5kV 4mA – which was supposed to be equivalent to around 3% of the total, we decided to take a spectrum at low energy. We were not expecting to see any change yet……however… This irradiation was miscalulated, giving each pixel of the central focussed spot 3% of the combined dose of all the in-orbit x-rays on a MOS camera. This mistake gave us plenty of opportunity for annealing experiments.

6 November 2007Cal-Ops Meeting Mallorca Cu-L X-rays at -100°C after 3% of the run In addition to the dark area below the main focussed damage, there are large groups of columns where the x-rays are smeared out, indicating severe CTE problems

6 November 2007Cal-Ops Meeting Mallorca Dark Frame at -50°C after 3% of the run Left image shows image on warming up to -50 after the irradiation, and the right hand image shows cooling down to -50 after a 13 hour room temperature anneal

6 November 2007Cal-Ops Meeting Mallorca The XMM-Newton Life Test: Desperate Measures Annealing for 3.5 days at +100°C and 11.5 days at +130°C (during the last Palermo meeting) 1 day of UV annealing with an EPROM eraser Compensate for a flat-band shifts by tweaks to clock and substrate voltage and increase row transfer time from 1 to 10us per clock phase …CCD performance was greatly recovered and, in addition, the low energy response was improved beyond pre- irradiation levels! Craig developed an analytical technique for showing the ratio of the gaussian fitted peaks in different damage areas Peak Height Ratios Nitrogen line with some C-k contamination

6 November 2007Cal-Ops Meeting Mallorca The XMM-Newton Life Test: Ghost Image Surprisingly, the intensity of the x-ray ghost image was largely unaffected by the annealing, but the dark current on the RHS was reduced. The second image shows the developing “hole” after UV annealing due to flat band shifts, and the improvement to this of voltage/clock width change is evident on the third image.

6 November 2007Cal-Ops Meeting Mallorca The XMM-Newton Life Test: Copper-L Image After the UV annealing the smeared columns (poor CTE) had concentrated down the centre of the CCD. The dramatic improvement of the change from 1 to 10us row transfer clock can be seen here.

6 November 2007Cal-Ops Meeting Mallorca The XMM-Newton Life Test: Carbon - k Images After the UV annealing and the improvement of the change from 1 to 10us row transfer clock, we could now see Carbon X-rays. The left image shows single pixel events with some residual damage in the central area where the beam was strongest. However by using 1-4 pixel events, the dark columns are not evident, showing that there is just a little charge trailing in these columns. In both images the uneven response is due to the x-ray beam, not the CCD.

6 November 2007Cal-Ops Meeting Mallorca On the way – examine the store section response at 700eV, and try to damage it No significant change in spectra or dark current could be observed for 80% of the full XMM dose at the silicon with 1.2KV anode voltage – probably needs a higher energy to cause damage – TBD.

6 November 2007Cal-Ops Meeting Mallorca The XMM-Newton Life Test: Irradiation of image section in the undamaged upper left corner ? This has proceeded slowly over the last 2 months, using an X-ray source voltage of 1.5kV which seems to be too low to cause damage

6 November 2007Cal-Ops Meeting Mallorca Conclusions so far… The enhanced dark current ghost image caused by an overdose of focussed x- rays has not been fully annealed out Thermal and UV annealing combined with clock level and timing changes has restored CTE over the whole of the CCD Energy redistribution effects are subtle The store section has an almost perfect single gaussian energy response, but significant QE only for energy at 700eV and above. This response is undamaged by the full XMM dose of approx 1keV x-rays at the silicon – need higher energy data to show that the electrode structure prevents redistribution effects. A significant fraction of the XMM dose in the image section of 1.2keV x-rays also produces little damage, and no dark current effects. We are comparing the results so far obtained with those described by Janesick et al for back illuminated devices – indeed it appears that the open implant region behaves exactly like the early back illuminated devices studied by Janesick.

6 November 2007Cal-Ops Meeting Mallorca Future work Increase the energy of the x-rays to be more compatible with the EPIC spectrum Repeat the corner irradiation Perform the same on a second CCD22 Perform the same on a CCD66 having the newly developed thin electrode structure