With the development of the heavy ion therapy technique at the Institute Modern Physics, Chinese Academy Sciences, a mini imaging detector system is also developed. The detector system is designed to coincidently measure the two 0. 511 MeVγ-rays oppositely emitted from the annihilation of the positron and electron. The positron is from the decay of the radioactive nuclei With the development of the heavy ion therapy technique at the Institute Modern Physics, Chinese Academy Sciences, a mini imaging detector system is also developed. The detector system is designed to coincidently measure the two 0. 511 MeVγ-rays oppositely emitted from the annihilation of the positron and electron. The positron is from the decay of the radioactive nuclei