Performance evaluation of Mini TPC in CAT-M

Systematic measurements of the incompressibility of the stable isotopes and radioisotopes are necessary to clarify the equation of state of nuclei. To measure the incompressibility of the unstable nuclei, inverse kinematics measurement of isoscalar giant monopole resonance is conducted.

A gaseous active target CAT-M is a TPC developed to realize such measurements by irradiating heavy-ion beams at an intensity up to 1 Mcps. Nuclei of interest enter and react with deuteron which is the nucleus of detector gas. The four momenta of recoil particles are simultaneously measured by CAT-M for missing mass spectroscopy. Our measurements have been done for Kr, Xe, and Sn isotopes at an incident energy of 100 MeV/u.

Such a heavy-ion injection produces a number of delta-rays which causes many noises in an active target. A static magnetic field is formed around the beam axis to remove such noises, so the beam trajectory can’t be detected directly. In addition, the beam itself is also deflected, so it’s necessary to estimate the beam trajectory near the reaction point with a resolution less than 1 mm.

For that purpose, the development of a small TPC called Mini TPC to be placed near the entrance and exit of the field cage is ongoing. Two layers of thick GEMs with a hole diameter of 200 μm and a hole pitch of 500 μm are used as the amplification part of Mini TPC.

In this talk, the structure of Mini TPC and its basic performance such as a bias dependence of gain and charge resolution will be presented.