Journal of the Korean Physical Society, Vol. 47, No. 2, August 2005, pp. 202∼206 Performance of a Large Volume Liquid Scintillation Detector for the Measurement of Fast Neutrons Jing-Jun Zhu, * Ke-Jun Kang and Yuan-Jing Li Department of Engineering Physics, Tsinghua University, Beijing 100084, China J. W. Kwak, H. Bhang, S. Y. Kim, S. C. Kim, S. K. Kim, † H. S. Lee, J. Lee, S. S. Myung and M. J. Lee Dark Matter Research Center and School of Physics, Seoul National University, Seoul 151-742 J. I. Lee and Y. D. Kim Department of Physics, Sejong University, Seoul 143-747 M. J. Hwang and Y. J. Kwon Physics Department, Yonsei University, Seoul 120-749 I. S. Hahn Department of Science Education, Ewha Woman’s University, Seoul 120-750 H. J. Kim Physics Department, Kyungpook National University, Daegu 702-701 Li Jin Institute of High Energy Physics, Chinese Academy of Science, Beijing 10039, China, and Department of Engineering Physics, Tsinghua University, Beijing 100084, China (Received 25 April 2005) The KIMS collaboration is an experimental group searching for the Weakly Interacting Massive Particle (WIMP) which is one of the strongest candidates of dark matter. In a WIMP search experiment, the neutron is an important background because the nuclear recoil signal of a neutron is indistinguishable from that of a WIMP. We have constructed a 20-liter liquid scintillation detector to measure the neutron background. We present the performance of the neutron detector, as well as its structure and electronics system. We have achieved a good neutron/gamma separation and a good position measurement. A position dependent energy calibration has also been performed to correct the nonlinear response of the detector for different particle incident positions. PACS numbers: 29, 95 Keywords: Dark matter, WIMP, Neutron, BC501A, Liquid scintillator I. INTRODUCTION Dark matter is known to be the main component of the universe [1,2], and the Weakly Interacting Massive Particle (WIMP) is considered as one of the strongest candidates for particle dark matter. There are sev- eral experimental groups, such as DAMA, ZEPLIN, GE- NIUS, EDELWEISS, CDMS [3–7], trying to search for the WIMP. DAMA published a very exciting result by * E-mail: zhujingjun00@mails.tsinghua.edu.cn; † E-mail: skkim@hep1.snu.ac.kr using the annual modulation effect of the WIMP. On the other hand, CDMS and EDELWEISS excluded the signal region claimed by DAMA. Because of the differ- ent experimental techniques used in those experiments, there are some disputes over the discrepancy between the two results. The KIMS (Korea Invisible Mass Search group) group has carried out an experiment search for the WIMP [8, 9]. The experimental program is a collaboration be- tween China and Korea. The KIMS collaboration uses CsI(Tl) crystals as the detection material while other groups mainly use NaI(Tl) crystals or low-temperature -202-