Amplitude-tuneable, octal LED pulser for emulation of scintillation light in multi-channel photon detectors

Photon detectors like the ALICE Calorimeters are vulnerable to both optical and electrical crosstalk between detector channels belonging to the same particle event. In order to quantify the crosstalk generated by a single channel on its neighbours, the standard method used so far are calibrated electrical pulses in order to quantify the crosstalk between individual frontend preamplifier channels. However due to grounding imperfections, cross-couplings via bias voltage supplies and imperfect electromagnetic shielding, this method introduces biases and is not precise. A new approach is the use of emulated scintillation light over optical fibre bundles in order to effectively generate scintillation light in clustered channels. This method is based on generation of quasi time-coherent light with similar time profiles as scintillation light. Theoretically, and based on linear superposition, pulsing a single channel with calibrated light would be sufficient to measure the crosstalk in the surrounding channels. However, non-linear effects in real detector frontends with energy measurements over cluster sizes like 3x3 can add up to significant errors. Non-linearity is for example due to preamplifier supply voltage drop in the distribution and GND return lines, in particular if power is provided over long cables. This can generate locally reduced or increased signal amplitudes depending on cumulative signal intensity. The presented octal light ALED~(Analogue LED) pulser prototype emulates scintillation light with individually tunable light amplitudes in 8 channels. It can be used for pulsing single channels, or for clustered channels with a predefined amplitude profile. Unbiased single channel crosstalk and non-linear side-effects can be determined for single and multichannel situations.