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Quality Control: Radiation-Induced Charge Collection

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As reported on 04/06/2007 in the HAWKI Test report on the Detector and Acquisition System (VLT-TRE-ESO-14800-4062) detector with DET.CHIP.NO = 2 ( DET.CHIP.ID = ESO-Hawaii2RG-chip78 ) is subject of radioactive events. Here we report on the number and the character of the radiation-induced charge collections.

The following data have been analysed:

  • 10 consecutive raw dark frames with DIT=2.0 sec, NDIT=1, DET.NCORRS.NAME=NonDest, acquired on 2008-06-17, 2008-10-20 and 2009-02-20
  • 5 consecutive raw dark frames with DIT=60.0 sec, NDIT=1, DET.NCORRS.NAME=NonDest, acquired on 2008-06-17, 2008-10-20 and 2009-02-20
  • 3 consecutive raw dark frames with DIT=300.0 sec, NDIT=1, DET.NCORRS.NAME=NonDest, acquired on 2008-06-17, 2008-10-20 and 2009-02-20


Fig.1 Difference of two DIT=300sec raw dark frames taken on 2009-02-20.

Fig.2: Shape of the strongest event with 730 ADU peak counts.

|   4    |   3    |
| chip88 | chip79 |
|   1    |   2    |
| chip66 | chip78 |

Fig.3: HAWKI detector head layout and naming scheme (chip number: 1 to 4, and chip id: 66, 79, 79 and 88)

Fig. 4: Details of a difference between two consecutive dark frames (DIT=300, NDIT=1) showing several weak events.
The right head of the event amounts to more than 8000 ADU.


From the events total flux, three frequently occuring types of events can be distinguished:
  • weak events with peak = 10 ADU. This is by far the most frequent type of event.
  • intermediate events with peak 100 ADU.
  • strong events with peak 500-700 ADU (2 found in total in the sample).

Events can show up in any kind of shape from circle-like, ellipsoide, comet-like up to 400 pixel long narrow trails. There is no prefered orientation of the trails (derived by visual inspection).

The number of events is linearly increasing with DIT (exposure time) , The event rate is is about 13 events per minute. The event rate is constant within the last 8 months.

The large number of diffuse events with highly elongated shape which are so far homogeneously distributed over chip #2 with uncorrelated trajectory orientations gives rise to the suggestion that the radioactive source itself is close to the detector plan, and that the source is extended.

The few particle trajectories derived from the two neighboring chips #1 and #3 do not point to chip #2. This is interpreted in the way that the radiation registered on chip #1 is not emitted from chip #2, but emitted in the same way from a source layer in chip #1 with a much smaller event rate of < 1 event per minute. The same holds for chip #3. Chip #4 does not show any stochastic events of this kind.

For a DIT=300 NDIT=1 dark or science exposure, chip #2 registers about 65 events. No detailed distribution of the event characteristics like flux, shape, position is availabale. The ESO HAWKI data reduction pipeline does not correct for these events.

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