GEM-MSTPC: An active-target type detector in low-pressure He/CO₂mixed gas

Abstract

We developed an active-target type gas counter operating with low pressure He/CO 2 (10%) detector gas for application in studying low-energy nuclear reactions using radioactive nuclear beams (RNBs). A 400-µm-thick gas electron multiplier (THGEM) was used as the proportional counter for high injection rate capability. We examined the gas gain stability and the influence of ion feedback on particle tracks at high beam injection rates of up to 10 5 particles per second (pps) using a low-energy 12 C beam. By means of modifications to the THGEM, the shift in the relative pulse height of the output signal normalized to that measured at the 12 C injection rate of several hundred pps improved to within 2% at the rate of 10 5 pps. The position distortion induced by ion feedback was suppressed to within 1.5 mm because of the relatively high drift field value of 1 kV/cm•atm and the use of the double GEM configuration. The THGEM was found to be applicable for our active target at high injection rates of up to 10 5 pps.

Active targets for the study of nuclei far from stability

“…By measuring the reaction Q -value leading to 9 Li that has a known mass, the mass of 11 Li could be determined with a precision of ±22 keV [45] by energy-energy and angle-angle kinematics reconstruction. The derived 11 Li two-neutron separation energy is S 2n = 363 (22) keV. This value is in good agreement with values from the MISTRAL transmission spectrometer [46] and the TITAN penning trap measurement [47] of S 2n = 378(5) keV and S 2n = 369.15(65) keV respectively.…”

“…Here the simulation is shown for 0.25 atm of D 2 gas and for a magnetic field of 2 T, in an active target device such as the AT-TPC (see Section 2.7). The figure shows that a very good [19,20] GEM-MSTPC (KEK) [21,22]. resolution can be obtained, about 200 keV (FWHM) in the invariant mass system.…”

“…It has two type of anode pads with a backgammon style geometry, low-gain pads in the central beam region and high-gain pads in the outer regions for detection of light charged-particles [19,20]. The other GEM-MSTPC was built at KEK and has an active-volume dimension of 10 cm × 10 cm × 10cm [21,22]. Various GEM foils of different thickness (50 µm, 400 µm) in single-, double-, and triple-foil configurations were tested for use in this GEM-MSTPC.…”

“…A model of electric field distortion due to positive ion feedback has been made as presented Ishiyama et al [22], which is based on Orthen et al [65]. The distortions in the electric field due to the backflow of positive ions were modeled as a static pillar of positive charge, which represents the average amount of positive charge for a given beam rate, amplification gain, and backflow percentage.…”

Properties of thick GEM in low-pressure deuterium

“…A GEM-TPCbased (Gas Electron Multiplier and Time Projection Chamber) active target system is reported in refs. [1,2] where a He+CO 2 gas was used. We are developing a GEM-TPC-based gaseous active target with a pure deuterium gas, called the Center for Nuclear Study Active Target (CAT), for the purpose of measuring deuteron inelastic scattering off unstable nuclei.…”