DarkSide-20k实验的地面宇宙成因活化研究
Study of cosmogenic activation above ground for the DarkSide-20k experiment
AI总结：
针对暗物质直接探测实验DarkSide-20k的地面宇宙射线活化本底问题，估算相关长寿命放射性同位素活度，验证铜、不锈钢活化及UAr中39Ar等同位素活度均在可接受范围内，为实验筹备提供依据。
摘要：
材料因暴露于宇宙射线而产生的活化，可能成为研究稀有事件现象的实验的重要本底来源。DarkSide-20k是目前正在格兰萨索国家实验室（Laboratori Nazionali del Gran Sasso）建造的银河系暗物质粒子直接探测实验，它采用两相液氩时间投影室（TPC），内部填充49.7吨（有效质量）贫化了39Ar的地下氩（Underground Argon, UAr）。尽管氩TPC具备出色的伽马/贝塔本底甄别能力，但这类本底仍需纳入考量——因为它会导致死时间或产生模拟暗物质信号的偶然符合，且与低阈值电子计数测量相关。本文估算了实验中产生的相关长寿命放射性同位素的宇宙成因活度，旨在为实验筹备阶段设定要求与流程，并验证该本底不会超过原生放射性的占比；研究特别关注了DarkSide-20k所用120吨UAr的活化情况。研究详细考量了预期的地面暴露时长，以及实测或计算得到的同位素产生率。根据对宇宙成因同位素在探测器中引发的计数率的模拟结果，可得出结论：铜和不锈钢中的活化不会造成问题。UAr在地表的提取、纯化和运输过程中诱导产生的39Ar活度，经评估为采用同一地下气源的DarkSide-50实验测得的UAr中39Ar活度的2.8%，因此被认为是可接受的。UAr中的其他同位素（如37Ar和3H）因半衰期较短且采用了预设的纯化方法，被证实不具有相关性。
英文摘要：
The activation of materials due to exposure to cosmic rays may become an important background source for experiments investigating rare event phenomena. DarkSide-20k, currently under construction at the Laboratori Nazionali del Gran Sasso, is a direct detection experiment for galactic dark matter particles, using a two-phase liquid-argon Time Projection Chamber (TPC) filled with 49.7 tonnes (active mass) of Underground Argon (UAr) depleted in 39Ar. Despite the outstanding capability of discriminating gamma/beta background in argon TPCs, this background must be considered because of induced dead time or accidental coincidences mimicking dark-matter signals and it is relevant for low-threshold electron-counting measurements. Here, the cosmogenic activity of relevant long-lived radioisotopes induced in the experiment has been estimated to set requirements and procedures during preparation of the experiment and to check that it is not dominant over primordial radioactivity; particular attention has been paid to the activation of the 120 t of UAr used in DarkSide-20k. Expected exposures above ground and production rates, either measured or calculated, have been considered in detail. From the simulated counting rates in the detector due to cosmogenic isotopes, it is concluded that activation in copper and stainless steel is not problematic. The activity of 39Ar induced during extraction, purification and transport on surface is evaluated to be 2.8% of the activity measured in UAr by DarkSide-50 experiment, which used the same underground source, and thus considered acceptable. Other isotopes in the UAr such as 37Ar and 3H are shown not to be relevant due to short half-life and assumed purification methods.
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