Speaker
Description
Ion-Backflow (IBF) of gas-filled detectors for ionising radiation is a limiting factor especially for large Time Projection Chambers (TPC) used in high energy physics experiments such as the ALICE experiment at LHC [1]. ALICE TPC readout chambers consist of a stack of Gas Electron Multipliers (GEM).
IBF leads to space charge in the drift volume which can cause spatial distortions to the drift of electrons within the TPC volume.
A systematic study of the IBF contribution of single GEM foils with respect to the hole sizes and geometries is presented. The GEM foils used in this study were produced using single mask technique by TECHTRA SP. Z O.O., Poland. The GEM foils with a size of 10 x 10 cm$^2$ are divided in four segments with different hole dimensions. The inner hole diameter range is from approximately 40 to 60 $\mu$m.
The exact hole sizes are retrieved using a high-resolution optical scanning system [2,3]. As a result, detailed maps of hole sizes of GEM foils are obtained. These can be used to predict not only the gain behaviour of GEM foils but also the IBF. Non-uniformity in the hole properties will lead to non-uniform gain behaviour. This will lead to variation in IBF, which is the subject of this study.
The effective gain and the IBF of single GEM foils in a standard gas enclosure with a 2D strip readout plane were measured, using two different gas mixtures, Ar-CO$_2$ (70-30) and Ne-CO$_2$-N$_2$ (90-10-5). The strips were combined to 4 sectors, readout in current mode by a set of picoammeters. The X-ray exposure using $^{55}$Fe source was performed separately for each foil segment, allowing us to compare the performance differences of the different hole geometries.
After studying the IBF dependency on the individual GEM hole sizes, shapes and applied electric fields, the GEM foils were then studied on top of two different underlying detectors. We present that choosing the right foils for a readout chamber can effectively limit the IBF.
References
[1] J. Adolfsson et al. [ALICE TPC collaboration], JINST 16 (2021) no.03, P03022.
[2] T. Hildén et al., Nucl. Instrum. Meth. A 770 (2015) 113.
[3] E. Brücken et al., Nucl. Instrum. Meth. A 1002 (2021), 165271.
| Name of the speaker | Hussam Badran |
|---|---|
| Eligible for the Georges Charpak Young Scientist Award. | no |