Measurement of the energy of heavy ions at around 100 MeV/nucleon with allene-doped liquid argon ionization chamber

Abstract

An allene doped liquid argon ionization chamber with 48 mm x 48 mm x 40 mm sensitive volume has been constructed for precise measurement of heavy ion energy at around 100 MeV/nucleon. The energy resolution of 0.6 percent (FWHM (Full Width at Half Maximum)) was achieved for Ca-40 ions at 94 MeV/nucleon. Allene concentration in Liquid Argon (LAr) was 10, 20, and 80 ppm. Energy resolution of gridded ionization chamber was investigated with oxygen ion and neon ion at around 100 MeV/nucleon. The maximum value of the fraction of collected charge, Q/Q(sub 0), was 40 percent which was obtained under E = 4 kV/cm for 16O ions at 133 MeV/nucleon in 20 ppm allene doped LAr. The best energy resolution is 1.1 percent (FWHM) under E = 1 kV/cm for Ne-20 at 133 MeV/nucleon in 10 ppm allene doped LAr. The energy resolution improved as E was increased from 0.5 to 1 kV/cm, and was degrading as E was increased from 1.5 to 4 kV/cm. The energy resolution started degrading as the beam intensity increases. The degradation of the energy resolution may be due to the variation of the Q/Q(sub 0) made by positive ions left in the sensitive volume of the chamber. Based on a simple model, the additional electric field is estimated to be an order of 1 kV/cm for a beam intensity of 100 cps.

48mm×48mm×40mmの有感体積の標記電離箱を精密エネルギー測定用に構築した。エネルギー分解能0.6%(FWHM(半値全幅))を94MeV/核子のCa-40イオンで達成した。液体アルゴン(LAr)中のアレン濃度は10、20、および80ppmである。グリッド電離箱のエネルギー分解能を100MeV/核子付近で酸素イオンとネオンイオンにより調べた。収集電荷の割合Q/Q(sub 0)の最大値は40%で、20ppmアレンドープLArでの133MeV/核子でE=4kV/cmの下で160イオンに対して得られた。最高エネルギー分解能は1.1%(FWHM)で、10ppmアレンドープLArでの133MeV/核子でE=1kV/cmの下でNe-20に対して得られた。エネルギー分解能はEが0.5から1kV/cmへ増加すると改善され、1.5から4kV/cmへ増加すると低下した。エネルギー分解能はビーム強度の増加とともに劣化し始めた。エネルギー分解能の劣化は電離箱の有感体積中に残された正イオンによるQ/Q(sub 0)の変化に起因する。簡単な模型に基づき、付加電場は100cpsのビーム強度で1kV/cm級であると推定した。