📚 IB Physics: Mechanisms of Matter-Radiation Interaction | 物质与辐射相互作用机制
Matter and radiation continuously exchange energy. When a photon or a particle passes through a medium, it may be absorbed, scattered, or converted into new particles. These mechanisms are not only the core of IB Physics Topic 12 (Quantum and Nuclear Physics) but also the physical basis of medical imaging, radiotherapy, and radiation protection. This article systematically examines each interaction channel, its probability, and its macroscopic consequences.
物质与辐射无时无刻不在交换能量。当一个光子或粒子穿过介质时，它可能被吸收、被散射，或转化为新的粒子。这些机制不仅是 IB 物理 Topic 12（量子与核物理）的核心内容，也是医学成像、放射治疗与辐射防护的物理基础。本文将对各种相互作用通道、其发生概率及宏观效应进行系统梳理。
1. Nature and Classification of Radiation | 辐射的本质与分类
Radiation is usually divided into directly ionizing radiation (charged particles such as α, β and protons) and indirectly ionizing radiation (photons and neutrons). Photons are quanta of electromagnetic energy; their interaction with matter is probabilistic and described by cross-sections. The dominant mechanism changes with photon energy and the atomic number of the absorber.
辐射通常分为直接电离辐射（α、β、质子等带电粒子）和间接电离辐射（光子与中子）。光子是电磁能量的量子，它与物质的相互作用具有概率性，需要用截面来描述。随光子能量和吸收体原子序数的不同，占主导地位的机制也会发生改变。
2. The Photoelectric Effect | 光电效应
A photon can transfer all of its energy to a bound electron, ejecting it from the atom. The condition is hf ≥ φ, where φ is the work function. The maximum kinetic energy of the photoelectron is
Eₖ,ₘₐₓ = hf − φ
This process dominates for photon energies in the low keV range and for absorbers of high atomic number, and it fully absorbs the photon.
光子可将其全部能量交给一个束缚电子，使其脱离原子。条件是 hf ≥ φ，其中 φ 为逸出功。光电子的最大动能为
Eₖ,ₘₐₓ = hf − φ
这一过程在光子能量处于低 keV 量级、吸收体原子序数较高时占主导，而且光子被完全吸收。
3. Compton Scattering | 康普顿散射
For photon energies around 0.1–10 MeV, a photon may scatter inelastically from a loosely bound electron, losing part of its energy. The wavelength shift is
Δλ = (h / mₑc)(1 − cos θ)
The scattering angle θ ranges from 0° to 180°; at 180° the photon loses maximum energy. The scattered photon continues to travel with reduced energy.
当光子能量约为 0.1–10 MeV 时，光子可能与束缚较弱的电子发生非弹性散射，损失部分能量。波长改变量为
Δλ = (h / mₑc)(1 − cos θ)
散射角 θ 在 0° 到 180° 之间；在 180° 时光子损失的能量最大。散射后的光子以较低能量继续传播。
4. Pair Production | 电子对产生
When hf exceeds 2mₑc² (1.022 MeV), a photon in the Coulomb field of a nucleus can be converted into an electron–positron pair. The positron later annihilates with an electron, typically producing two 511 keV photons travelling back-to-back. Pair production dominates above about 5 MeV, especially in high-Z materials.
当 hf 超过 2mₑc²（1.022 MeV）时，在原子核库仑场中的光子可以转化为一个电子–正电子对。正电子随后与电子湮灭，通常产生两个反向飞行的 511 keV 光子。在约 5 MeV 以上，尤其是在高原子序数材料中，电子对产生占主导。
5. Relative Importance of the Three Photon Mechanisms | 三种光子机制的相对重要性
The three principal photon interactions depend strongly on photon energy and absorber atomic number. The following table and the related trends are commonly tested in IB questions.
三种主要的光子相互作用强烈依赖于光子能量和吸收体原子序数。下表及相关规律在 IB 考题中经常出现。
| Mechanism / 机制 | Dominant Energy Range / 主要能量范围 | Outcome / 结果 |
|---|---|---|
| Photoelectric / 光电效应 | Low (keV) / 低能（keV） | Photon absorbed, electron ejected / 光子被吸收，电子逸出 |
| Compton / 康普顿散射 | 0.1–10 MeV | Reduced-energy photon + electron / 低能光子 + 电子 |
| Pair production / 电子对产生 | Above 5 MeV / 5 MeV 以上 | e⁻ + e⁺ pair / 电子–正电子对 |
For low-energy photons and high-Z absorbers, the photoelectric effect dominates; Compton scattering dominates around 1 MeV; pair production dominates at very high energies.
低能光子与高原子序数吸收体中光电效应占优；约 1 MeV 附近以康普顿散射为主；极高能量下以电子对产生为主。
6. Exponential Attenuation Law | 指数衰减规律
When a narrow beam of photons passes through a homogeneous slab, the intensity decreases exponentially:
I = I₀ e^(−μx)
The linear attenuation coefficient μ combines all interaction mechanisms. The half-value layer (HVL) is given by
x₁/₂ = ln 2 / μ
The value of μ depends on photon energy and on the density and atomic number of the absorber.
当一束窄光子束穿过均匀介质时，强度按指数衰减：
I = I₀ e^(−μx)
线衰减系数 μ 综合了所有相互作用机制。半价层为
x₁/₂ = ln 2 / μ
μ 的数值与光子能量、吸收体密度及原子序数有关。
7. Interaction of Charged Particles with Matter | 带电粒子与物质的相互作用
Alpha particles have high charge and low speed; they lose energy mainly by Coulomb excitation and ionization of atoms, so their tracks are straight, dense, and short (a few centimetres in air). Beta particles are lighter and can undergo large-angle elastic scattering; in addition, when accelerated near a nucleus they emit bremsstrahlung. The range of beta particles is larger, and the absorption curve does not show a sharp cutoff.
α 粒子电荷高、速度较慢，主要通过库仑激发和电离原子来损失能量，因此径迹笔直、电离密度大、射程短（空气中仅几厘米）。β 粒子质量轻，会发生大角度弹性散射；此外，在原子核附近被加速时还会发射韧致辐射。β 粒子的射程更大，吸收曲线没有陡峭的截止点。
8. Interaction of Neutrons | 中子与物质的相互作用
Neutrons carry no charge, so they do not ionize directly. They interact with nuclei via elastic scattering (mainly with hydrogen nuclei, the basis of water-moderated reactors) and nuclear reactions. Inelastic scattering and capture can leave the residual nucleus excited or radioactive. This explains why neutron shielding prefers hydrogen-rich materials.
中子不带电荷，因此不能直接产生电离。它通过与原子核的弹性散射（主要是与氢核，水堆减速正是利用这一点）和核反应发生作用。非弹性散射和俘获会使剩余核处于激发态或产生放射性。这也解释了为什么中子屏蔽应优先选用富氢材料。
9. Radiation Quantities and Dose | 辐射量与剂量
Activity is measured in becquerels (Bq); absorbed dose D = E/m is measured in gray (Gy). For radiation protection, the equivalent dose is
H = D × Q
expressed in sieverts (Sv), where the radiation weighting
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