Demonstrator Detection System for the Active Target and Time Projection Chamber (ACTAR TPC) project

Abstract

21The design, realization and operation of a prototype or "demonstrator" version of an active target and time projection chamber (ACTAR TPC) for experiments in nuclear physics is presented in detail. The heart of the detection system features a micromegas gas amplifier coupled to a high-density pixelated pad plane with square pad sizes of 2×2 mm 2 . The detector has been thoroughly tested with several different gas mixtures over a wide range of pressures and using a variety of sources of ionizing radiation including laser light, an α-particle source and heavy-ion beams of 24 Mg and 58 Ni accelerated to energies of 4.0 MeV/u. Results from these tests and characterization of the detector response over a wide range of operating conditions will be described. These developments have served as the basis for the design of a larger detection system that is presently under construction.1

Commissioning of the ACtive TARget and Time Projection Chamber (ACTAR TPC)

“…The ACTAR TPC detector, shown schematically in Figure 1, follows the construction of two 8-times smaller demonstrator [16,17] using different technologies for the pad wiring. The internal skeleton of ACTAR TPC chamber measures 606 mm × 606 mm × 335 mm.…”

“…As explained in Ref. [16], the use of two field degradation planes ensures a good homogeneity of the drift field by preventing the electric field between the drift cage and the auxiliary detectors to leak into the drift region.…”

“…Based on the demonstrator detector design [16], the pad plane for ACTAR TPC was equipped with a 220 µm gap bulked micromegas amplification system [18,19] made by the CERN PCB Workshop. Usually, the amplification gap in micromegas is about 100 µm, which is the optimum gap for atmospheric pressure operation.…”

“…With the micromegas mesh voltage set to -450 V, the gain was previously measured to be 5000 in Ref. [16]. Hence, all proton tracks can be recorded with a transverse multiplicity of three pads.…”

“…As shown in Ref. [16], a single dynamic range of the GET preamplifiers is not sufficient to record the track of the beam ions and the track of the protons at the same time without satu- rating the electronics. In order to extend the effective dynamic range, the pads located below the beam path were polarized to -100 V, locally reducing the gain of the micromegas from about 5000 to about 250.…”