Cross section (physics)

In physics, the cross section is a measure of the probability that a specific process will take place in a collision of two particles. For example, the Rutherford cross-section is a measure of probability that an alpha particle will be deflected by a given angle during an interaction with an atomic nucleus. Cross section is typically denoted Template:Math (sigma) and is expressed in units of area, more specifically in barns. In a way, it can be thought of as the size of the object that the excitation must hit in order for the process to occur, but more exactly, it is a parameter of a stochastic process.

When two discrete particles interact in classical physics, their mutual cross section is the area transverse to their relative motion within which they must meet in order to scatter from each other. If the particles are hard inelastic spheres that interact only upon contact, their scattering cross section is related to their geometric size. If the particles interact through some action-at-a-distance force, such as electromagnetism or gravity, their scattering cross section is generally larger than their geometric size.

When a cross section is specified as the differential limit of a function of some final-state variable, such as particle angle or energy, it is called a differential cross section (see detailed discussion below). When a cross section is integrated over all scattering angles (and possibly other variables), it is called a total cross section or integrated total cross section. For example, in Rayleigh scattering, the intensity scattered at the forward and backward angles is greater than the intensity scattered sideways, so the forward differential scattering cross section is greater than the perpendicular differential cross section, and by adding all of the infinitesimal cross sections over the whole range of angles with integral calculus, we can find the total cross section.

Scattering cross sections may be defined in nuclear, atomic, and particle physics for collisions of accelerated beams of one type of particle with targets (either stationary or moving) of a second type of particle. The probability for any given reaction to occur is in proportion to its cross section. Thus, specifying the cross section for a given reaction is a proxy for stating the probability that a given scattering process will occur.

The measured reaction rate of a given process depends strongly on experimental variables such as the density of the target material, the intensity of the beam, the detection efficiency of the apparatus, or the angle setting of the detection apparatus. However, these quantities can be factored away, allowing measurement of the underlying two-particle collisional cross section.

Differential and total scattering cross sections are among the most important measurable quantities in nuclear, atomic, and particle physics.

With light scattering off of a particle, the cross section specifies the amount of optical power scattered from light of a given irradiance (power per area). Although the cross section has the same units as area, the cross section may not necessarily correspond to the actual physical size of the target given by other forms of measurement. It is not uncommon for the actual cross-sectional area of a scattering object to be much larger or smaller than the cross section relative to some physical process. For example, plasmonic nanoparticles can have light scattering cross sections for particular frequencies that are much larger than their actual cross-sectional areas.

Collision among gas particles

File:CrossSectionFig1.svg

Figure 1. In a gas of particles of individual diameter Template:Math, the cross section Template:Math, for collisions is related to the particle number density Template:Math, and mean free path between collisions Template:Math.

In a gas of finite-sized particles there are collisions among particles that depend on their cross-sectional size. The average distance that a particle travels between collisions depends on the density of gas particles. These quantities are related by

<math>\sigma = \frac{1}{n \lambda},</math>

where

Template:Math is the cross section of a two-particle collision (SI unit: m²),

Template:Math is the mean free path between collisions (SI unit: m),

Template:Math is the number density of the target particles (SI unit: m⁻³).

If the particles in the gas can be treated as hard spheres of radius Template:Math that interact by direct contact, as illustrated in Figure 1, then the effective cross section for the collision of a pair is

<math>\sigma = \pi \left(2r\right)^2</math>

If the particles in the gas interact by a force with a larger range than their physical size, then the cross section is a larger effective area that may depend on a variety of variables such as the energy of the particles.

Cross sections can be computed for atomic collisions but also are used in the subatomic realm. For example, in nuclear physics a "gas" of low-energy neutrons collides with nuclei in a reactor or other nuclear device, with a cross section that is energy-dependent and hence also with well-defined mean free path between collisions.

Attenuation of a beam of particles

Template:See also If a beam of particles enters a thin layer of material of thickness Template:Math, the flux Template:Math of the beam will decrease by Template:Math according to

<math>\frac{\mathrm d \Phi}{\mathrm d z} = -n \sigma \Phi,</math>

where Template:Math is the total cross section of all events, including scattering, absorption, or transformation to another species. The volumetric number density of scattering centers is designated by Template:Math. Solving this equation exhibits the exponential attenuation of the beam intensity:

<math>\Phi = \Phi_0 e^{-n \sigma z},</math>

where Template:Math is the initial flux, and Template:Math is the total thickness of the material. For light, this is called the Beer–Lambert law.

Differential cross section

Consider a classical measurement where a single particle is scattered off a single stationary target particle. Conventionally, a spherical coordinate system is used, with the target placed at the origin and the Template:Math axis of this coordinate system aligned with the incident beam. The angle Template:Math is the scattering angle, measured between the incident beam and the scattered beam, and the Template:Math is the azimuthal angle.

File:Differential cross section.svg

The impact parameter Template:Math is the perpendicular offset of the trajectory of the incoming particle, and the outgoing particle emerges at an angle Template:Math. For a given interaction (coulombic, magnetic, gravitational, contact, etc.), the impact parameter and the scattering angle have a definite one-to-one functional dependence on each other. Generally the impact parameter can neither be controlled nor measured from event to event and is assumed to take all possible values when averaging over many scattering events. The differential size of the cross section is the area element in the plane of the impact parameter, i.e. Template:Math. The differential angular range of the scattered particle at angle Template:Math is the solid angle element Template:Math. The differential cross section is the quotient of these quantities, Template:Math.

It is a function of the scattering angle (and therefore also the impact parameter), plus other observables such as the momentum of the incoming particle. The differential cross section is always taken to be positive, even though larger impact parameters generally produce less deflection. In cylindrically symmetric situations (about the beam axis), the azimuthal angle Template:Math is not changed by the scattering process, and the differential cross section can be written as

<math> \frac{\mathrm{d} \sigma}{\mathrm{d}(\cos \theta)} =\int_0^{2\pi} \frac{\mathrm{d} \sigma}{\mathrm{d} \Omega} \,\mathrm{d}\varphi </math>.

In situations where the scattering process is not azimuthally symmetric, such as when the beam or target particles possess magnetic moments oriented perpendicular to the beam axis, the differential cross section must also be expressed as a function of the azimuthal angle.

For scattering of particles of incident flux Template:Math off a stationary target consisting of many particles, the differential cross section Template:Math at an angle Template:Math is related to the flux of scattered particle detection Template:Math in particles per unit time by

<math>\frac{\mathrm d \sigma}{\mathrm d \Omega}(\theta,\varphi) = \frac{1}{n t \Delta\Omega} \frac{F_\text{out}(\theta,\varphi)}{F_\text{inc}}.</math>

Here Template:Math is the finite angular size of the detector (SI unit: sr), Template:Math is the number density of the target particles (SI unit: m⁻³), and Template:Math is the thickness of the stationary target (SI unit: m). This formula assumes that the target is thin enough that each beam particle will interact with at most one target particle.

The total cross section Template:Math may be recovered by integrating the differential cross section Template:Math over the full solid angle (Template:Math steradians):

<math>\sigma = \oint_{4\pi} \frac{\mathrm d \sigma}{\mathrm d \Omega} \, \mathrm d \Omega = \int_0^{2\pi} \int_0^\pi \frac{\mathrm d \sigma}{\mathrm d \Omega} \sin \theta \, \mathrm d \theta \, \mathrm d \varphi.</math>

It is common to omit the "differential" qualifier when the type of cross section can be inferred from context. In this case, Template:Math may be referred to as the integral cross section or total cross section. The latter term may be confusing in contexts where multiple events are involved, since "total" can also refer to the sum of cross sections over all events.

The differential cross section is extremely useful quantity in many fields of physics, as measuring it can reveal a great amount of information about the internal structure of the target particles. For example, the differential cross section of Rutherford scattering provided strong evidence for the existence of the atomic nucleus.

Instead of the solid angle, the momentum transfer may be used as the independent variable of differential cross sections.

Differential cross sections in inelastic scattering contain resonance peaks that indicate the creation of metastable states and contain information about their energy and lifetime.

Quantum scattering

In the time-independent formalism of quantum scattering, the initial wave function (before scattering) is taken to be a plane wave with definite momentum Template:Math:

<math>\phi_-(\mathbf r) \;\stackrel{r \to \infty}{\longrightarrow}\; e^{i k z},</math>

where Template:Math and Template:Math are the relative coordinates between the projectile and the target. The arrow indicates that this only describes the asymptotic behavior of the wave function when the projectile and target are too far apart for the interaction to have any effect.

After scattering takes place it is expected that the wave function takes on the following asymptotic form:

<math>\phi_+(\mathbf r) \;\stackrel{r \to \infty}{\longrightarrow}\; f(\theta,\phi) \frac{e^{i k r}}{r},</math>

where Template:Math is some function of the angular coordinates known as the scattering amplitude. This general form is valid for any short-ranged, energy-conserving interaction. It is not true for long-ranged interactions, so there are additional complications when dealing with electromagnetic interactions.

The full wave function of the system behaves asymptotically as the sum

<math>\phi(\mathbf r) \;\stackrel{r \to \infty}{\longrightarrow}\; \phi_-(\mathbf r) + \phi_+(\mathbf r).</math>

The differential cross section is related to the scattering amplitude:

<math>\frac{\mathrm d \sigma}{\mathrm d \Omega}(\theta, \phi) = \bigl|f(\theta, \phi)\bigr|^2.</math>

This has the simple interpretation as the probability density for finding the scattered projectile at a given angle.

A cross section is therefore a measure of the effective surface area seen by the impinging particles, and as such is expressed in units of area. The cross section of two particles (i.e. observed when the two particles are colliding with each other) is a measure of the interaction event between the two particles. The cross section is proportional to the probability that an interaction will occur; for example in a simple scattering experiment the number of particles scattered per unit of time (current of scattered particles Template:Math) depends only on the number of incident particles per unit of time (current of incident particles Template:Math), the characteristics of target (for example the number of particles per unit of surface Template:Math), and the type of interaction. For Template:Math we have

<math>\begin{align}

I_\text{r} &= I_\text{i}N\sigma, \\ \sigma &= \frac{I_\text{r}}{I_\text{i}} \frac{1}{N} \\ &= \text{probability of interaction} \times \frac{1}{N}. \end{align}</math>

Relation to the S-matrix

If the reduced masses and momenta of the colliding system are Template:Math, Template:Math and Template:Math, Template:Math before and after the collision respectively, the differential cross section is given byTemplate:Clarify

<math>\frac{\mathrm d\sigma}{\mathrm d\Omega} = \left(2\pi\right)^4 m_\text{i} m_\text{f} \frac{p_\text{f}}{p_\text{i}} \bigl|T_{\text{f}\text{i}}\bigr|^2,</math>

where the on-shell Template:Math matrix is defined by

<math>S_{\text{f}\text{i}} = \delta_{\text{f}\text{i}} - 2\pi i \delta\left(E_\text{f} - E_\text{i}\right) \delta\left(\mathbf{p}_\text{i} - \mathbf{p}_\text{f}\right) T_{\text{f}\text{i}}</math>

in terms of the S-matrix. Here Template:Math is the Dirac delta function. The computation of the S-matrix is the main goal of the scattering theory.

Units

Although the SI unit of total cross sections is m², a smaller unit is usually used in practice.

In nuclear and particle physics, the conventional unit is the barn b, where 1 b = 10⁻²⁸ m² = 100 fm².<ref>Template:SIbrochure8th</ref> Smaller prefixed units such as mb and μb are also widely used. Correspondingly, the differential cross section can be measured in units such as mb/sr.

When the scattered radiation is visible light, it is conventional to measure the path length in centimetres. To avoid the need for conversion factors, the scattering cross section is expressed in cm², and the number concentration in cm⁻³. The measurement of the scattering of visible light is known as nephelometry, and is effective for particles of 2–50 μm in diameter: as such, it is widely used in meteorology and in the measurement of atmospheric pollution.

The scattering of X-rays can also be described in terms of scattering cross sections, in which case the square ångström is a convenient unit: 1 Å² = 10⁻²⁰ m² = Template:Val = 10⁸ b. The sum of the scattering, photoelectric, and pair-production cross-sections (in barns) is charted as the "atomic attenuation coefficient" (narrow-beam), in barns.<ref>Nondestructive Testing Handbook Volume 4 Radiographic Testing, ASNT, 2002, chapter 22.</ref>

Scattering of light

For light, as in other settings, the scattering cross section for particles is generally different from the geometrical cross section of the particle, and it depends upon the wavelength of light and the permittivity, shape, and size of the particle. The total amount of scattering in a sparse medium is proportional to the product of the scattering cross section and the number of particles present.

In the interaction of light with particles, many processes occur, each with their own cross sections, including absorption, scattering, and photoluminescence. The sum of the absorption and scattering cross sections is sometimes referred to as the attenuation or extinction cross section.

<math>\sigma = \sigma_\text{abs} + \sigma_\text{sc} + \sigma_\text{lum}.</math>

The total extinction cross section is related to the attenuation of the light intensity through the Beer–Lambert law, which says that attenuation is proportional to particle concentration:

<math>A_\lambda = C l \sigma,</math>

where Template:Math is the attenuation at a given wavelength Template:Math, Template:Math is the particle concentration as a number density, and Template:Math is the path length. The absorbance of the radiation is the logarithm (decadic or, more usually, natural) of the reciprocal of the transmittance Template:Mathcal:<ref name="Bajpai">Template:Cite book</ref>

<math>A_\lambda = -\log \mathcal{T}.</math>

Combining the scattering and absorption cross sections in this manner is often necessitated by the inability to distinguish them experimentally, and much research effort has been put into developing models that allow them to be distinguished, the Kubelka-Munk theory being one of the most important in this area.

Cross section and Mie theory

Cross sections commonly calculated using Mie theory include efficiency coefficients for extinction <math display="inline">Q_\text{ext}</math>, scattering <math display="inline">Q_\text{sc}</math>, and Absorption <math display="inline">Q_\text{abs}</math> cross sections. These are normalized by the geometrical cross sections of the particle <math display="inline">\sigma_\text{geom} = \pi a^2</math> as <math display="block"> Q_\alpha = \frac{\sigma_\alpha}{\sigma_\text{geom}}, \qquad \alpha = \text{ext}, \text{sc}, \text{abs}. </math> The cross section is defined by

<math>

\sigma_\alpha = \frac{W_\alpha}{I_{\text{inc}}} </math> where <math>\left[W_\alpha \right] = \left[ \text{W} \right]</math> is the energy flow through the surrounding surface, and <math> \left[I_{\text{inc}}\right] = \left[ \frac{\text{W}}{\text{m}^2} \right]</math> is the intensity of the incident wave. For a plane wave the intensity is going to be <math>I_{\text{inc}} = |\mathbf{E}|^2 / (2 \eta)</math>, where <math>\eta = \sqrt{\mu \mu_0 / (\varepsilon \varepsilon_0)}</math> is the impedance of the host medium.

The main approach is based on the following. Firstly, we construct an imaginary sphere of radius <math>r</math> (surface <math>A</math>) around the particle (the scatterer). The net rate of electromagnetic energy crosses the surface <math>A</math> is

<math>

W_\text{a} = - \oint_A \mathbf{\Pi} \cdot \hat{\mathbf{r}} dA </math> where <math display="inline">\mathbf{\Pi} = \frac{1}{2} \operatorname{Re} \left[ \mathbf{E}^* \times \mathbf{H} \right]</math> is the time averaged Poynting vector. If <math>W_\text{a} > 0</math> energy is absorbed within the sphere, otherwise energy is being created within the sphere. We will not consider this case here. If the host medium is non-absorbing, the energy must be absorbed by the particle. We decompose the total field into incident and scattered parts <math>\mathbf{E} = \mathbf{E}_\text{i} + \mathbf{E}_\text{s}</math>, and the same for the magnetic field <math>\mathbf{H}</math>. Thus, we can decompose <math>W_a</math> into the three terms <math> W_\text{a} = W_\text{i} - W_\text{s} + W_{\text{ext}} </math>, where

<math>

W_\text{i} = - \oint_A \mathbf{\Pi}_\text{i} \cdot \hat{\mathbf{r}} dA \equiv 0, \qquad W_\text{s} = \oint_A \mathbf{\Pi}_\text{s} \cdot \hat{\mathbf{r}} dA, \qquad W_{\text{ext}} = \oint_A \mathbf{\Pi}_{\text{ext}} \cdot \hat{\mathbf{r}} dA. </math> where <math>\mathbf{\Pi}_\text{i} = \frac{1}{2} \operatorname{Re} \left[ \mathbf{E}_\text{i}^* \times \mathbf{H}_\text{i} \right] </math>, <math>\mathbf{\Pi}_\text{s} = \frac{1}{2} \operatorname{Re} \left[ \mathbf{E}_\text{s}^* \times \mathbf{H}_\text{s} \right] </math>, and <math>\mathbf{\Pi}_{\text{ext}} = \frac{1}{2} \operatorname{Re} \left[ \mathbf{E}_s^* \times \mathbf{H}_i + \mathbf{E}_i^* \times \mathbf{H}_s \right] </math>.

All the field can be decomposed into the series of vector spherical harmonics (VSH). After that, all the integrals can be taken. In the case of a uniform sphere of radius <math>a</math>, permittivity <math>\varepsilon</math>, and permeability <math>\mu</math>, the problem has a precise solution.<ref>Bohren, Craig F., and Donald R. Huffman. Absorption and scattering of light by small particles. John Wiley & Sons, 2008.</ref> The scattering and extinction coefficients are <math display="block"> Q_\text{sc} = \frac{2}{k^2a^2}\sum_{n=1}^\infty (2n+1)(|a_{n}|^2+|b_{n}|^2) </math> <math display="block"> Q_\text{ext} = \frac{2}{k^2a^2}\sum_{n=1}^\infty (2n+1)\Re(a_{n}+b_{n}) </math> Where <math display="inline">k = n_\text{host} k_0</math>. These are connected as <math display="block"> \sigma_\text{ext} = \sigma_\text{sc} + \sigma_\text{abs} \qquad \text{or} \qquad Q_\text{ext} = Q_\text{sc} + Q_\text{abs} </math>

Dipole approximation for the scattering cross section

Let us assume that a particle supports only electric and magnetic dipole modes with polarizabilities <math display="inline">\mathbf{p} = \alpha^e \mathbf{E}</math> and <math display="inline">\mathbf{m} = (\mu \mu_0)^{-1}\alpha^m \mathbf{H}</math> (here we use the notation of magnetic polarizability in the manner of Bekshaev et al.<ref name="Bekshaev2013">Template:Cite journal</ref><ref name="Bliokh2014">Template:Cite journal</ref> rather than the notation of Nieto-Vesperinas et al.<ref name="Nieto-Vesperinas2010">Template:Cite journal</ref>) expressed through the Mie coefficients as <math display="block"> \alpha^e = 4 \pi \varepsilon_0 \cdot i \frac{3 \varepsilon}{2 k^3} a_1, \qquad \alpha^m = 4 \pi \mu_0 \cdot i \frac{3 \mu}{2 k^3} b_1. </math> Then the cross sections are given by <math display="block"> \sigma_{\text{ext}} = \sigma_{\text{ext}}^{\text{(e)}} + \sigma_{\text{ext}}^{\text{(m)}} = \frac{1}{4\pi \varepsilon \varepsilon_0} \cdot 4\pi k \Im(\alpha^e) + \frac{1}{4\pi \mu \mu_0} \cdot 4\pi k \Im(\alpha^m) </math> <math display="block"> \sigma_{\text{sc}} = \sigma_{\text{sc}}^{\text{(e)}} + \sigma_{\text{sc}}^{\text{(m)}} = \frac{1}{(4\pi \varepsilon \varepsilon_0)^2} \cdot \frac{8\pi}{3} k^4 |\alpha^e|^2 + \frac{1}{(4\pi \mu \mu_0)^2} \cdot \frac{8\pi}{3} k^4 |\alpha^m|^2 </math> and, finally, the electric and magnetic absorption cross sections <math display="inline">\sigma_{\text{abs}} = \sigma_{\text{abs}}^{\text{(e)}} + \sigma_{\text{abs}}^{\text{(m)}}</math> are <math display="block"> \sigma_{\text{abs}}^{\text{(e)}} = \frac{1}{4 \pi \varepsilon \varepsilon_0} \cdot 4\pi k \left[ \Im(\alpha^e) - \frac{k^3}{6 \pi \varepsilon \varepsilon_0} |\alpha^e|^2\right] </math> and <math display="block"> \sigma_{\text{abs}}^{\text{(m)}} = \frac{1}{4 \pi \mu \mu_0} \cdot 4\pi k \left[ \Im(\alpha^m) - \frac{k^3}{6 \pi \mu \mu_0} |\alpha^m|^2\right] </math>

For the case of a no-inside-gain particle, i.e. no energy is emitted by the particle internally (<math display="inline">\sigma_{\text{abs}} > 0</math>), we have a particular case of the Optical theorem <math display="block"> \frac{1}{4\pi \varepsilon \varepsilon_0} \Im(\alpha^e) + \frac{1}{4\pi \mu \mu_0} \Im(\alpha^m) \geq \frac{2 k^3}{3} \left[ \frac{|\alpha^e|^2}{(4\pi \varepsilon \varepsilon_0)^2} + \frac{|\alpha^m|^2}{(4\pi \mu \mu_0)^2} \right] </math> Equality occurs for non-absorbing particles, i.e. for <math display="inline">\Im(\varepsilon) = \Im(\mu) = 0</math>.

Scattering of light on extended bodies

In the context of scattering light on extended bodies, the scattering cross section, Template:Math, describes the likelihood of light being scattered by a macroscopic particle. In general, the scattering cross section is different from the geometrical cross section of a particle, as it depends upon the wavelength of light and the permittivity in addition to the shape and size of the particle. The total amount of scattering in a sparse medium is determined by the product of the scattering cross section and the number of particles present. In terms of area, the total cross section (Template:Math) is the sum of the cross sections due to absorption, scattering, and luminescence:

<math>\sigma = \sigma_\text{abs} + \sigma_\text{sc} + \sigma_\text{lum}.</math>

The total cross section is related to the absorbance of the light intensity through the Beer–Lambert law, which says that absorbance is proportional to concentration: Template:Math, where Template:Math is the absorbance at a given wavelength Template:Math, Template:Math is the concentration as a number density, and Template:Math is the path length. The extinction or absorbance of the radiation is the logarithm (decadic or, more usually, natural) of the reciprocal of the transmittance Template:Mathcal:<ref name="Bajpai" />

<math>A_\lambda = - \log \mathcal{T}.</math>

Relation to physical size

There is no simple relationship between the scattering cross section and the physical size of the particles, as the scattering cross section depends on the wavelength of radiation used. This can be seen when looking at a halo surrounding the Moon on a decently foggy evening: Red light photons experience a larger cross sectional area of water droplets than photons of higher energy. The halo around the Moon thus has a perimeter of red light due to lower energy photons being scattering further from the center of the Moon. Photons from the rest of the visible spectrum are left within the center of the halo and perceived as white light.

Meteorological range

The scattering cross section is related to the meteorological range Template:Math:

<math>L_\text{V} = \frac{3.9}{C \sigma_\text{scat}}.</math>

The quantity Template:Math is sometimes denoted Template:Math, the scattering coefficient per unit length.<ref>Template:GoldBookRef</ref>

Examples

Elastic collision of two hard spheres

The following equations apply to two hard spheres that undergo a perfectly elastic collision.Template:Sfn Let Template:Math and Template:Math denote the radii of the scattering center and scattered sphere, respectively. The differential cross section is

<math>

\frac{d\sigma}{d\Omega} = \frac{R^2}{4}, </math> and the total cross section is

<math>

\sigma_\text{tot} = \pi \left(r + R\right)^2. </math> In other words, the total scattering cross section is equal to the area of the circle (with radius Template:Math) within which the center of mass of the incoming sphere has to arrive for it to be deflected.

Rutherford scattering

In Rutherford scattering, an incident particle with charge Template:Math and energy Template:Math scatters off a fixed particle with charge Template:Math. The differential cross section is

<math>

\frac{d \sigma}{d \Omega} = \left(\frac{q \, Q}{16\pi\varepsilon_0 E \sin^2(\theta/2)} \right)^2 </math> where <math>\varepsilon_0</math> is the vacuum permittivity.Template:Sfn The total cross section is infinite unless a cutoff for small scattering angles <math>\theta</math> is applied.Template:Sfn This is due to the long range of the <math>1/r</math> Coulomb potential.

Scattering from a 2D circular mirror

The following example deals with a beam of light scattering off a circle with radius Template:Math and a perfectly reflecting boundary. The beam consists of a uniform density of parallel rays, and the beam-circle interaction is modeled within the framework of geometric optics. Because the problem is genuinely two-dimensional, the cross section has unit of length (e.g., metre). Let Template:Math be the angle between the light ray and the radius joining the reflection point of the ray with the center point of the mirror. Then the increase of the length element perpendicular to the beam is

<math>\mathrm dx = r \cos \alpha \,\mathrm d \alpha.</math>

The reflection angle of this ray with respect to the incoming ray is Template:Math, and the scattering angle is

<math>\theta = \pi - 2 \alpha.</math>

The differential relationship between incident and reflected intensity Template:Math is

<math>I \,\mathrm d \sigma = I \,\mathrm dx(x) = I r \cos \alpha \,\mathrm d \alpha = I \frac{r}{2} \sin \left(\frac{\theta}{2}\right) \,\mathrm d \theta = I \frac{\mathrm d \sigma}{\mathrm d \theta} \,\mathrm d \theta.</math>

The differential cross section is therefore (Template:Math)

<math>\frac{\mathrm d \sigma}{\mathrm d \theta} = \frac{r}{2} \sin \left(\frac{\theta}{2}\right).</math>

Its maximum at Template:Math corresponds to backward scattering, and its minimum at Template:Math corresponds to scattering from the edge of the circle directly forward. This expression confirms the intuitive expectations that the mirror circle acts like a diverging lens. The total cross section is equal to the diameter of the circle:

<math>\sigma = \int_0^{2 \pi} \frac{\mathrm d \sigma}{\mathrm d \theta} \,\mathrm d \theta = \int_0^{2 \pi} \frac{r}{2} \sin \left(\frac{\theta}{2}\right) \,\mathrm d \theta = 2 r.</math>

Scattering from a 3D spherical mirror

The result from the previous example can be used to solve the analogous problem in three dimensions, i.e., scattering from a perfectly reflecting sphere of radius Template:Math.

The plane perpendicular to the incoming light beam can be parameterized by cylindrical coordinates Template:Math and Template:Math. In any plane of the incoming and the reflected ray we can write (from the previous example):

<math>\begin{align}

r &= a \sin \alpha,\\ \mathrm dr &= a \cos \alpha \,\mathrm d \alpha, \end{align}</math> while the impact area element is

<math> \mathrm d \sigma = \mathrm d r(r) \times r \,\mathrm d \varphi = \frac{a^2}{2} \sin \left(\frac{\theta}{2}\right) \cos \left(\frac{\theta}{2}\right) \,\mathrm d \theta \,\mathrm d \varphi.</math>

In spherical coordinates,

<math>\mathrm d\Omega = \sin \theta \,\mathrm d \theta \,\mathrm d \varphi.</math>

Together with the trigonometric identity

<math>\sin \theta = 2 \sin \left(\frac{\theta}{2}\right) \cos \left(\frac{\theta}{2}\right),</math>

we obtain

<math>\frac{\mathrm d \sigma}{\mathrm d \Omega} = \frac{a^2}{4}.</math>

The total cross section is

<math>\sigma = \oint_{4 \pi} \frac{\mathrm d \sigma}{\mathrm d \Omega} \,\mathrm d \Omega = \pi a^2.</math>

See also

• Cross section (geometry)
• Flow velocity
• Luminosity (scattering theory)
• Linear attenuation coefficient
• Mass attenuation coefficient
• Neutron cross section
• Nuclear cross section
• Gamma ray cross section
• Partial wave analysis
• Particle detector
• Radar cross-section
• Rutherford scattering
• Scattering amplitude

References

Bibliography

• Template:Cite book
• Template:Cite book
• J. D. Bjorken, S. D. Drell, Relativistic Quantum Mechanics, 1964
• P. Roman, Introduction to Quantum Theory, 1969
• W. Greiner, J. Reinhardt, Quantum Electrodynamics, 1994
• R. G. Newton. Scattering Theory of Waves and Particles. McGraw Hill, 1966.
• Template:Cite book

External links

• Nuclear Cross Section
• Scattering Cross Section
• IAEA – Nuclear Data Services
• BNL – National Nuclear Data Center
• Particle Data Group – The Review of Particle Physics
• IUPAC Goldbook – Definition: Reaction Cross Section
• IUPAC Goldbook – Definition: Collision Cross Section
• ShimPlotWell cross section plotter for nuclear data