The X-ray irradiation chamber (XRIC)

The XRIC allows to carry out the Charge Transfer Efficiency (CTE) characterization and provides with an alternative method to the photon transfer method for the amplifier gain measurement. This task is accomplished exposing the CCD to an x-ray source. The x-ray photons deposit in a single pixel event a known charge, for example a Fe55 source yields 5.9 keV photons which deposit 1620 electrons.
CTE and amplifier gain are obtained from the analysis of the signal distribution following an exposure to the x-ray source.
Figure 1 shows in the upper panel the XRIC while in the lower panel can be seen during the mounting operation with an Infrared-Lab cryostat hosting a CCD E2V 4280. The Fe55 x-ray source is accommodated inside an aluminum holder. As can be seen from figure 1 the cover of the holder acts as a mechanical shutter, this is activated by mean of a pump actuator placed on the bottom place of the camera. The shutter can be operated also manually. This kind of camera can be directly interfaced to various cryostats ( i.e. Infrared-lab or Oxford-Instruments) and allows to operate the CCD and the x-ray source under vacuum conditions (obtained by evacuating from the cryostat valve) , thus assuring high transmittance to x-ray photons.

Figure 1. The XRIC camera. On the lower left panel the XRIC being mounted to an Infrared-Lab cryostat hosting a CCD E2V 4280.

An example of a E2V-4280 CCD obtained by a 10 seconds exposure time to the x-ray source is shown in figure 2 while the gain and charge transfer efficiency (CTE) measurements are shown in Figure 3. The main line (Kalpha) emitted by the source has an energy of 5.9 KeV and thus each photon can generate a charge of 1620 e- on a pixel. The measurement of the position of the line on the plot of Figure 2 (left panel) allows us to determine directly the system gain with a precision better than 1 %. The CTE is measured by stacking together all the columns and by plotting the signal versus the number of pixel transfer (column number). The CTE is given by the formula on the lower part of the right panel (where Charge_loss is the amount of charge lost in Nt transfers).

Figure 2. A 10 seconds of x-ray exposure time for a E2V 4280 CCD. In the rigth panel is shown the selected area in the left

Figure 3. Example of gain (left panel) and CTE (right panel) measurements.