Development of the collection apparatus for recoil products for study of the deexcitation process of $^{\mathrm{235m}}$U

$^{\mathrm{235m}}$U has very low excitation energy (76.8 eV) and decays predominantly by the internal conversion process. Because the deexcitation of $^{\mathrm{235m}}$U is caused by the interaction between the nucleus and outer-shell electrons, the variation of the decay constant depending on its chemical environment was reported. We are aiming to clarify the deexcitation process of $^{\mathrm{235m}}$U by measuring the decay constants and the energy spectra of the internal-conversion electrons for $^{\mathrm{235m}}$U with various chemical forms. In this work, we developed an apparatus for collecting $^{\mathrm{235m}}$U recoiling out of $^{239}$Pu. We evaluated the performance of the apparatus by using $^{224}$Ra recoiling out of $^{228}$Th. The collection yields of $^{224}$Ra were determined in various applied voltages, air pressures, and $^{228}$Th source shapes. Based on these results, we determined suitable experimental conditions for the collection of $^{\mathrm{235m}}$U from $^{239}$Pu. In addition, the detection apparatus for the low-energy internal-conversion electrons are under development.