Micromegas calibration for ACTAR TPC
Résumé
Active targets are gas-filled detection systems where the gas used as the detection medium serves also as a target for
nuclear reactions. They have been used for a wide variety of nuclear physics experiments since the eighties.
Improvements in MPGD (Micro Pattern Gaseous Detectors) and in micro electronics achieved in the last decade have
lead to the development of a new generation of active targets with higher granularity pad planes that allow spatial and
time information to be determined with unprecedented accuracy. A novel active target and time projection chamber
(ACTAR TPC with 16k channels), that will be used to study reactions and decays of exotic nuclei, is presently under
development and will be based on Micromegas technology. A demonstrator version of the ACTAR TPC detection
system with 2048 channels has already been constructed and is presently being tested using the GET (General
Electronics for TPCs) system.
The energy resolution is of primary importance for these experiments to identify the reaction products and reconstruct
precisely the level scheme of the nuclei. Energy measurements are based on the charge collected in the pixels of the
pad plane. One major contribution to the energy dispersion is the non homogeneity of the Micromegas gap. A gap
variation of only 1% can result in more than 2% variation in signal amplitude, which is of the same order as the
resolution obtained with an energy deposition of 5 MeV. One method to calibrate the pad plane homogeneity is the use
of a 2D translational scanning table equipped a radioactive source. The system we are developing will be used to
quantify the gain variation for moderate size detectors up to 25x50 cm2 using different types of gas and pressure
adapted to the performed experiments.
In this poster, the ACTAR TPC detector will be presented. The calibration method will be explained and recent results
obtained with the scanning table will be presented. Results will be compared to simulations and to other methods of
calibration such as cosmic rays or pulser measurements.