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 (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). It is important to note that 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.
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
\sigma=
1 | |
nλ |
,
is the cross section of a two-particle collision (SI unit: m2),
is the mean free path between collisions (SI unit: m),
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 that interact by direct contact, as illustrated in Figure 1, then the effective cross section for the collision of a pair is
\sigma=\pi\left(2r\right)2
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.
See also: Attenuation. If a beam of particles enters a thin layer of material of thickness, the flux of the beam will decrease by according to
d\Phi | |
dz |
=-n\sigma\Phi,
\Phi=\Phi0e-n,
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 axis of this coordinate system aligned with the incident beam. The angle is the scattering angle, measured between the incident beam and the scattered beam, and the is the azimuthal angle.
The impact parameter is the perpendicular offset of the trajectory of the incoming particle, and the outgoing particle emerges at an angle . 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. . The differential angular range of the scattered particle at angle is the solid angle element . The differential cross section is the quotient of these quantities, .
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 is not changed by the scattering process, and the differential cross section can be written as
d\sigma | |
d(\cos\theta) |
2\pi | |
=\int | |
0 |
d\sigma | |
d\Omega |
d\varphi
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 off a stationary target consisting of many particles, the differential cross section at an angle is related to the flux of scattered particle detection in particles per unit time by
d\sigma | |
d\Omega |
(\theta,\varphi)=
1 | |
nt\Delta\Omega |
Fout(\theta,\varphi) | |
Finc |
.
Here is the finite angular size of the detector (SI unit: sr), is the number density of the target particles (SI unit: m−3), and 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 may be recovered by integrating the differential cross section over the full solid angle (steradians):
\sigma=\oint4\pi
d\sigma | |
d\Omega |
d\Omega=
2\pi | |
\int | |
0 |
\pi | |
\int | |
0 |
d\sigma | |
d\Omega |
\sin\thetad\thetad\varphi.
It is common to omit the "differential" qualifier when the type of cross section can be inferred from context. In this case, 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.
In the time-independent formalism of quantum scattering, the initial wave function (before scattering) is taken to be a plane wave with definite momentum :
\phi-(r) \stackrel{r\toinfty}{\longrightarrow} ei,
After scattering takes place it is expected that the wave function takes on the following asymptotic form:
\phi+(r) \stackrel{r\toinfty}{\longrightarrow} f(\theta,\phi)
ei | |
r |
,
The full wave function of the system behaves asymptotically as the sum
\phi(r) \stackrel{r\toinfty}{\longrightarrow} \phi-(r)+\phi+(r).
The differential cross section is related to the scattering amplitude:
d\sigma | |
d\Omega |
(\theta,\phi)=l|f(\theta,\phi)r|2.
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) depends only on the number of incident particles per unit of time (current of incident particles), the characteristics of target (for example the number of particles per unit of surface), and the type of interaction. For we have
\begin{align} Ir&=IiN\sigma,\\ \sigma&=
Ir | |
Ii |
1 | |
N |
\\ &=probabilityofinteraction x
1 | |
N |
. \end{align}
If the reduced masses and momenta of the colliding system are, and, before and after the collision respectively, the differential cross section is given by
d\sigma | |
d\Omega |
=\left(2\pi\right)4mimf
pf | |
pi |
l|Tfir|2,
Sfi=\deltafi-2\pii\delta\left(Ef-Ei\right)\delta\left(pi-pf\right)Tfi
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. 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 = = 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.[1]
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.
\sigma=\sigmaabs+\sigmasc+\sigmalum.
Aλ=Cl\sigma,
Aλ=-logl{T}.
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 sections commonly calculated using Mie theory include efficiency coefficients for extinction , scattering , and Absorption cross sections. These are normalized by the geometrical cross sections of the particle as The cross section is defined by
\sigma\alpha=
W\alpha | |
Iinc |
\left[W\alpha\right]=\left[W\right]
\left[Iinc\right]=\left[
W | |
m2 |
\right]
Iinc=|E|2/(2η)
η=\sqrt{\mu\mu0/(\varepsilon\varepsilon0)}
The main approach is based on the following. Firstly, we construct an imaginary sphere of radius
r
A
A
Wa=-\ointA\Pi ⋅ \hat{r
Wa>0
E=Ei+Es
H
Wa
Wa=Wi-Ws+Wext
Wi=-\ointA\Pii ⋅ \hat{r
\Pii=
1 | |
2 |
\operatorname{Re}\left[
* | |
E | |
i |
x Hi\right]
\Pis=
1 | |
2 |
\operatorname{Re}\left[
* | |
E | |
s |
x Hs\right]
\Piext=
1 | |
2 |
\operatorname{Re}\left[
* | |
E | |
s |
x Hi+
* | |
E | |
i |
x Hs\right]
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
a
\varepsilon
\mu
Let us assume that a particle supports only electric and magnetic dipole modes with polarizabilities and (here we use the notation of magnetic polarizability in the manner of Bekshaev et al.[4] [5] rather than the notation of Nieto-Vesperinas et al.[6]) expressed through the Mie coefficients as Then the cross sections are given by and, finally, the electric and magnetic absorption cross sections are and
For the case of a no-inside-gain particle, i.e. no energy is emitted by the particle internally (), we have a particular case of the Optical theorem Equality occurs for non-absorbing particles, i.e. for .
In the context of scattering light on extended bodies, the scattering cross section,, 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 is the sum of the cross sections due to absorption, scattering, and luminescence:
\sigma=\sigmaabs+\sigmasc+\sigmalum.
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:, where is the absorbance at a given wavelength, is the concentration as a number density, and 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 :
Aλ=-logl{T}.
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.
The scattering cross section is related to the meteorological range :
LV=
3.9 | |
C\sigmascat |
.
The quantity is sometimes denoted, the scattering coefficient per unit length.
The following equations apply to two hard spheres that undergo a perfectly elastic collision. Let and denote the radii of the scattering center and scattered sphere, respectively. The differential cross section is
d\sigma | |
d\Omega |
=
R2 | |
4 |
,
\sigmatot=\pi\left(r+R\right)2.
In Rutherford scattering, an incident particle with charge and energy scatters off a fixed particle with charge . The differential cross section is
d\sigma | |
d\Omega |
=\left(
16\pi\varepsilon0E\sin2(\theta/2) |
\right)2
\varepsilon0
\theta
1/r
The following example deals with a beam of light scattering off a circle with radius 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 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
dx=r\cos\alphad\alpha.
\theta=\pi-2\alpha.
Id\sigma=Idx(x)=Ir\cos\alphad\alpha=I
r | |
2 |
\sin\left(
\theta | |
2 |
\right)d\theta=I
d\sigma | |
d\theta |
d\theta.
d\sigma | |
d\theta |
=
r | |
2 |
\sin\left(
\theta | |
2 |
\right).
\sigma=
2\pi | |
\int | |
0 |
d\sigma | |
d\theta |
d\theta=
2\pi | |
\int | |
0 |
r | |
2 |
\sin\left(
\theta | |
2 |
\right)d\theta=2r.
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 .
The plane perpendicular to the incoming light beam can be parameterized by cylindrical coordinates and . In any plane of the incoming and the reflected ray we can write (from the previous example):
\begin{align} r&=a\sin\alpha,\\ dr&=a\cos\alphad\alpha, \end{align}
d\sigma=dr(r) x rd\varphi=
a2 | |
2 |
\sin\left(
\theta | |
2 |
\right)\cos\left(
\theta | |
2 |
\right)d\thetad\varphi.
d\Omega=\sin\thetad\thetad\varphi.
\sin\theta=2\sin\left(
\theta | |
2 |
\right)\cos\left(
\theta | |
2 |
\right),
d\sigma | |
d\Omega |
=
a2 | |
4 |
.
\sigma=\oint4
d\sigma | |
d\Omega |
d\Omega=\pia2.