The article describes a gamma ray spectrometer protected by a lead shield (Model 747E Canberra lead shield) and an active shield made of an 80 cm × 80 cm × 3 cm plastic scintillator plate in anticoincidence on top of the lead shield. The detector used as low background gamma-spectrometer is a high purity Germanium crystal of model GC2018 Canberra. | Communications in Physics, Vol. 26, No. 1 (2016), pp. 93-97 DOI: A LOW BACKGROUND GAMMA RAY SPECTROMETER WITH ANTICOSMIC SHIELDING NGUYEN QUOC HUNG† , VO HONG HAI, TRAN KIM TUYET, AND HO LAI TUAN University of Science, HCMC-Vietnam National University, 227 Nguyen Van Cu, Ward 4, District 5, Ho Chi Minh City, Vietnam † E-mail: nqhung@ Received 27 November 2015 Accepted for publication 14 April 2016 Abstract. The article describes a gamma ray spectrometer protected by a lead shield (Model 747E Canberra lead shield) and an active shield made of an 80 cm × 80 cm × 3 cm plastic scintillator plate in anticoincidence on top of the lead shield. The detector used as low background gamma-spectrometer is a high purity Germanium crystal of model GC2018 Canberra. The background count rate currently achieved (30-2400 keV) is cps without anticoincidence. The level of background suppression obtained from the active protection is overall and about for the 511 keV gamma line. The gamma ray spectrometer is installed and operated in the Nuclear Laboratory, Department of Nuclear Physics, University of Science, HCMC-Vietnam National University. Keywords: gamma ray spectrometer, cosmic rays, plastic scintillator, high-purity Germanium detector. Classification numbers: , . I. INTRODUCTION The background of a Germanium detector is due to natural and artificial radioactive elements distributed in the surroundings of the detector, radon and cosmic rays induced radioactivity [1]. The major sources of activity from the crystal and the construction materials are primordial emitters 238 U, 232 Th and their daughters, and 40 K isotope in natural potassium. This contribution to the background can be reduced by means of suitable shielding (very low activity lead or “old” lead) and by selecting pure radioactivity materials surrounding the crystal [2]. The radon component can be reduced by flushing the cavity around a detector
