Background signals from BGO scintillators: Origin of radioisotopic impurity of Bi-207

Abstract

A lead concentrate (PbS), which contains from 0.30 to 0.35 percent of bismuth, has been measured for gamma emitting radionuclides by a Ge detector. The Ge detector was surrounded by the 9.1 kg concentrate, and they were placed in a heavy lead cave. The gamma-ray spectra showed the presence of weak radioactivity of Bi-207 (T(sub 1/2) = 32.2y) at a level of 2.0 mBq/Bi-g. Besides Bi-207, the fission product Cs-137 (T(sub 1/2) = 30.0y) was detected in the concentrate. The Bi-207 activity may be ascribed to the nuclear explosion tests which terminated by early 1960's. The concentrate was prepared through the flotation process at a concentrating mill. One of the origin of the contamination could be from water used in the flotation process.

ガンマ線を放出する放射性核種を求めて0.30-0.35%のビスマスを含む鉛濃縮物(PbS)をGe検出器で測定した。Ge検出器は9.1kgの濃縮物で囲み、重い鉛の穴におさめた。ガンマ線スペクトルはBi-207(T(1/2)=32.2y)の弱い放射能を2.0mBq/Bi-gのレベルで示した。Bi-207の他に、核分裂生成物Cs-137(T(1/2)=30.0y)が濃縮物中に検出された。Bi-207の放射能は、1960年代初期に終止した核爆発試験によるものであろう。この濃縮物は濃縮粉砕機での浮遊選鉱を通して調整された。汚染源の1つは浮遊選鉱に用いられた水の可能性がある。