我们现在的氦源，是地下天然气田中放射元素衰变的副产品，经过数十亿年的累积，逐渐形成的。
Our modern supply of helium has gradually built up over billions of years in subterranean pockets of natural gas as a byproduct of decaying radioactive elements.
为先驱者号们提供舱内热动力源的钚238会有放射性衰变，其发射量会随着时间的推移而越来越小。
As the plutonium-238 that served as the Pioneers' onboard heat source radioactively decayed, it would have emitted less heat over time.
分析了子核具有三个激发态的典型核衰变情况下的级联校正情况，具体给出了测量滤材源的情况下具有代表性的符合校正因子计算的解析表达式。
The true coincidence correction for a specific three excited levels de-excitation case has been analyzed, and the typical analytical expressions of true coincidence correction factors have been given.
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