telemetry from the temperature sensors over the Gaia focal plane wasn’t available at the time the project was started and the initial model was being developed.

In the refined CtiPixel implementation an epoch-dependent CCD temperature was used for each camera, derived from the spacecraft telemetry, allowing the direct fit of the characteristic energy level Et of different species.

In the model, the trap species parameters τe (or Et), α and β are set to be independent of time, as they are considered intrinsic characteristics of a defect type, while the traps number densities ρi are allowed to vary to track the increase of defects in the CCD caused by radiation.

The capture parameter α can be converted into a characteristic capture cross section σ of units cm2 using the physical properties of the Gaia CCD pixels: their geometric volume Vg, the serial full well capacity FW of 475K electrons, and the thermal velocity of the electrons (vth ≈1.21x107cm/s):
σ = 2αVg vthFW0.5
(7) The surface area of a readout register pixel is equal to 30 µm in the horizontal (H) direction, to match the width of the pixels in the image area, and 15 µm in the vertical (V) direction, larger compared to CCD image pixels (for a detailed description of the Gaia CCDs architecture see Short et al. 2005).

This design allows serial pixels to handle a minimum of 2.5 times the full-well capacity of an image pixel.

The depth (d) of the readout pixels was estimated at approximately 1 µm, based on pre-launch modelling and measurements by DPAC (private communication).

The readout in the serial direction uses a twophase scheme, therefore the signal packet cloud occupies one half of the pixel width at any one time, resulting in a total geometric volume Vg = H×V×d = 30
2 ×15×1 µm3 = 2.25x10−10cm3, In the initial attempts at fitting the serial calibration data sets using CtiPixel, the number of trap species in the model was set to two, based on prior knowledge that both the Oxygen and the multi-vacancy defects could affect the serial readout in Gaia devices.

The first fits results showed that this choice was not sufficient; on the contrary, it was found that a nonphysical high number of trap species was required to achieve a satisfactory fit to the images.

This was interpreted as a shortcoming of the model, and was tackled by introducing an additional process to the readout sequence: the possibility of re-capturing the electrons just released by previously filled traps within the same pixel.

Charge re-capturing can be activated by setting a flag in CtiPixel .

When the flag is set, the released charge is added to the signal flux of a pixel before the capture probability is computed.

Charge re-capturing showed to improve the best fit results, as will be presented in detail in Section 5.

The code in CtiPixel has been optimised and a set of numerical approximations was introduced to reduce the processing time of its computationally intense pixel-based implementation.

The most significant optimisation is relative to the treatment of the pixels with a common input flux and the same traps state of the neighbouring pixels.

These cases are recognised and flagged in the code so that the results of the model computations can be reused, avoiding unnecessary repetitions.