--- title: "晶体怎样接住原子核发光时的反冲" description: "把原子核放进固体，为什么能重新看见极窄的伽马共振？" canonical_url: "https://sandboxphysics.com/zh/stories/quantum-evidence/mossbauer-recoil-free-resonance/" language: "zh-CN" page_type: "story" alternate_languages: en: "https://sandboxphysics.com/stories/quantum-evidence/mossbauer-recoil-free-resonance/" last_modified: "2026-10-24" --- [量子物理：读懂证据](https://sandboxphysics.com/zh/stories/quantum-evidence/) 诺奖 A · 证据与操控 # 晶体怎样接住原子核发光时的反冲 Sandbox Physics · 2026-10-15 把原子核放进固体，为什么能重新看见极窄的伽马共振？ ## 发光的原子核也要向后退 一支枪把子弹向前射出，枪就向后退。自由的激发态原子核放出一个伽马光子，也要承担反冲：光子带走动量，核朝反方向运动，分走一小部分能量。反过来，另一个自由核吸收光子，也需要得到反冲动能。 核的共振线可以极窄。发射时光子少带了一点能量，吸收时又要额外付出一点，两端就可能错开许多个线宽。哪怕源和吸收体是同一种同位素，发出的光也可能很难被同样的核共振吸收。 证据阅读：B. Fultz, Mössbauer Spectrometry — technical chapter hosted by the Advanced Photon Source ## 核留在晶体里，情况变了 1958 年，Mössbauer 研究束缚在固体中的铱核，改变源和吸收体的温度，发现低温下共振吸收明显增强。束缚方式改变了发射和吸收的机会。这里最初用的是铱，后来常见的铁谱学属于后续发展。 晶体由许多彼此连接的原子构成。一部分核跃迁发生时，没有增加或减少晶格的振动激发，动量由整个晶体接收。把一个原子核换成重得多的整体，反冲动能便小到相对这条窄线可以忽略。这部分事件称为无反冲通道。 晶体仍接收了动量，也仍有原子振动。关键在于内部振动状态没有在这一次事件中改变。能走这个通道的比例由材料、温度和跃迁共同决定，降温通常有利，却不保证比例达到百分之百。 证据阅读：R. L. Mössbauer (1958), Kernresonanzfluoreszenz von Gammastrahlung in Ir191, Z. Phys. 151, 124–143 — publisher abstract B. Fultz, Mössbauer Spectrometry — technical chapter hosted by the Advanced Photon Source ## 为什么由谁来接住动量这么重要 同样的动量交给重物，动能比交给轻物小。自由核的近似反冲能量见下式：光子能量记为 E 下标伽马，M 是核质量，c 是光速。光子能量越大，反冲能量增加得越快；承受反冲的质量越大，付出的动能越小。这个关系适用于原本静止、反冲速度远小于光速的自由核。 在内部振动不变的事件里，晶体巨大的整体质量把反冲能量压到很小。发射与吸收都落进同一个狭窄窗口，共振就能重新出现。 $$ E_{\mathrm R}\simeq\frac{E_\gamma^2}{2Mc^2} $$ 证据阅读：B. Fultz, Mössbauer Spectrometry — technical chapter hosted by the Advanced Photon Source ## 让源缓缓移动，就能扫描窄得惊人的窗口 有了窄共振，下一步是寻找它。让伽马源朝吸收体缓缓移动，光子得到很小的多普勒增能；让它退去，能量略降。速度远小于光速时，相对能量变化的大小约为沿光路速度除以光速。记录透过去的光子数随速度怎样变化，就得到一个精细的吸收谱。 核周围的电子、电场和磁场会推动谱线移动，或把它分开。分辨这些结构时，还要考虑源速标定、吸收体厚度、非共振背景和无反冲比例。选择合适的参考与能级解释后，这种极窄的核共振便能用来研究材料里原子核周围的环境。 证据阅读：B. Fultz, Mössbauer Spectrometry — technical chapter hosted by the Advanced Photon Source ## 接着操作模型 下面这些教学 Lab 产生的是模拟数据，和上文讨论的历史观测不是一回事。 - [移动模拟伽马源，测穿过吸收体的光子数](https://sandboxphysics.com/zh/labs/mossbauer-doppler-resonance/) ## 来源与阅读说明 1958 年论文的德文摘要可在出版方网站阅读。诺奖 PDF 无法访问，只列作延伸阅读，我们没有读到全文。无反冲比例和多普勒扫描参照 Advanced Photon Source 网站上 Fultz 的技术文献。本文不复制任何历史谱。 1. [R. L. Mössbauer (1958), Kernresonanzfluoreszenz von Gammastrahlung in Ir191, Z. Phys. 151, 124–143 — publisher abstract](https://link.springer.com/article/10.1007/BF01344210) 2. [B. 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