The MINOS micromegas-TPC Vertex Tracker for ion-beam spectroscopy of very exotic nuclei

Abstract

CEA-Irfu developed a new device, called MINOS (MagIc Numbers Off Stability), to improve both the luminosity and the sensitivity of proton-induced knockout reactions experiments performing in-beam spectroscopy of very exotic nuclei produced at fragmentation facilities (such as RIBF at RIKEN in Japan and in the future at FAIR in Germany). Its innovative approach is that it uses a compact annular 300 mm long Time Projection Chamber (TPC) surrounding a thick cylindrical liquid hydrogen target. The TPC reconstructs the recoiled proton tracks in order to locate the interaction vertex inside the target, allowing a proper Doppler correction to emitted gamma rays measured in a dedicated detector. A bulk-micromegas, segmented in 4608 × 2 mm2 pads, is used to readout the TPC which is filled with an Ar-iC4H10(3%)-CF4(15%) gas mixture. The electronics is composed of 20 analog front-end cards. Each of these is read out by a newly designed digital board, called the Feminos, which is compatible with the AFTER and AGET chips. The internal and external TPC field cages are made of 2 mm thick Rohacell cylinders on which are glued 50 microns thick Kapton foils printed with 381 copper strips. The MINOS device is described, with a focus on the design choices and performances of the TPC characterized in laboratory with cosmic rays. We report on a recent in-beam validation of the TPC at HIMAC (Japan) where a better than 6 mm FWHM vertex resolution was obtained with a 180 and 350 MeV/u 20Ne ion-beam impinging on two 0.5 mm thick CH2 targets.