Nikiya Anton Bettey 2021: 08-14-2021-1848

Sunday, August 15, 2021

08-14-2021-1848 - Bremsstrahlung

Bremsstrahlung /ˈbrɛmʃtrɑːləŋ/^[1] (German pronunciation: [ˈbʁɛms.ʃtʁaːlʊŋ] (listen)), from bremsen "to brake" and Strahlung "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into radiation (i.e., photons), thus satisfying the law of conservation of energy. The term is also used to refer to the process of producing the radiation. Bremsstrahlunghas a continuous spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the energy of the decelerated particles increases.

Broadly speaking, bremsstrahlung or braking radiation is any radiation produced due to the deceleration (negative acceleration) of a charged particle, which includes synchrotron radiation (i.e. photon emission by a relativistic particle), cyclotron radiation (i.e. photon emission by a non-relativistic particle), and the emission of electrons and positrons during beta decay. However, the term is frequently used in the more narrow sense of radiation from electrons (from whatever source) slowing in matter.

Bremsstrahlung emitted from plasma is sometimes referred to as free–free radiation. This refers to the fact that the radiation in this case is created by electrons that are free (i.e., not in an atomic or molecular bound state) before, and remain free after, the emission of a photon. In the same parlance, bound–bound radiation refers to discrete spectral lines (an electron "jumps" between two bound states), while free–bound one—to the radiative combination process, in which a free electron recombines with an ion.

In plasma[edit]

NOTE: this section currently gives formulas that apply in the Rayleigh-Jeans limit $\hbar \omega \ll k_{\rm {B}}T_{e}$ , and does not use a quantized (Planck) treatment of radiation. Thus a usual factor like $\exp(-\hbar \omega /k_{\rm {B}}T_{e})$ does not appear. The appearance of $\hbar \omega /k_{\rm {B}}T_{e}$ in $y$ below is due to the quantum-mechanical treatment of collisions.

In a plasma, the free electrons continually collide with the ions, producing bremsstrahlung. A complete analysis requires accounting for both binary Coulomb collisions as well as collective (dielectric) behavior. A detailed treatment is given by Bekefi,^[5] while a simplified one is given by Ichimaru.^[6] In this section we follow Bekefi's dielectric treatment, with collisions included approximately via the cutoff wavenumber, $k_{\rm {max}}$ .

Consider a uniform plasma, with thermal electrons distributed according to the Maxwell–Boltzmann distribution with the temperature $T_{e}$ . Following Bekefi, the power spectral density (power per angular frequency interval per volume, integrated over the whole $4\pi$ sr of solid angle, and in both polarizations) of the bremsstrahlung radiated, is calculated to be

{dP_{\mathrm {Br} } \over d\omega }={8{\sqrt {2}} \over 3{\sqrt {\pi }}}\left[{e^{2} \over 4\pi \varepsilon _{0}}\right]^{3}{1 \over (m_{e}c^{2})^{3/2}}\left[1-{\omega _{\rm {p}}^{2} \over \omega ^{2}}\right]^{1/2}{Z_{i}^{2}n_{i}n_{e} \over (k_{\rm {B}}T_{e})^{1/2}}E_{1}(y),

where $\omega _{p}\equiv (n_{e}e^{2}/\varepsilon _{0}m_{e})^{1/2}$ is the electron plasma frequency, $\omega$ is the photon frequency, $n_{e},n_{i}$ is the number density of electrons and ions, and other symbols are physical constants. The second bracketed factor is the index of refraction of a light wave in a plasma, and shows that emission is greatly suppressed for $\omega <\omega _{\rm {p}}$ (this is the cutoff condition for a light wave in a plasma; in this case the light wave is evanescent). This formula thus only applies for $\omega >\omega _{\rm {p}}$ . This formula should be summed over ion species in a multi-species plasma.

The special function $E_{1}$ is defined in the exponential integral article, and the unitless quantity $y$ is

y={1 \over 2}{\omega ^{2}m_{e} \over k_{\rm {max}}^{2}k_{\rm {B}}T_{e}}

$k_{\rm {max}}$ is a maximum or cutoff wavenumber, arising due to binary collisions, and can vary with ion species. Roughly, $k_{\rm {max}}=1/\lambda _{\rm {B}}$ when $k_{\rm {B}}T_{\rm {e}}>Z_{i}^{2}E_{\rm {h}}$ (typical in plasmas that are not too cold), where $E_{\rm {h}}\approx 27.2$ eV is the Hartree energy, and $\lambda _{\rm {B}}=\hbar /(m_{\rm {e}}k_{\rm {B}}T_{\rm {e}})^{1/2}$ ^{[clarification needed]} is the electron thermal de Broglie wavelength. Otherwise, $k_{\rm {max}}\propto 1/l_{\rm {C}}$ where $l_{\rm {C}}$ is the classical Coulomb distance of closest approach.

For the usual case $k_{m}=1/\lambda _{B}$ , we find

y={1 \over 2}\left[{\frac {\hbar \omega }{k_{\rm {B}}T_{e}}}\right]^{2}.

The formula for $dP_{\mathrm {Br} }/d\omega$ is approximate, in that it neglects enhanced emission occurring for $\omega$ slightly above $\omega _{\rm {p}}$ .

In the limit $y\ll 1$ , we can approximate $E_{1}$ as $E_{1}(y)\approx -\ln[ye^{\gamma }]+O(y)$ where $\gamma \approx 0.577$ is the Euler–Mascheroni constant. The leading, logarithmic term is frequently used, and resembles the Coulomb logarithm that occurs in other collisional plasma calculations. For $y>e^{-\gamma }$ the log term is negative, and the approximation is clearly inadequate. Bekefi gives corrected expressions for the logarithmic term that match detailed binary-collision calculations.

The total emission power density, integrated over all frequencies, is

{\begin{aligned}P_{\mathrm {Br} }&=\int _{\omega _{\rm {p}}}^{\infty }d\omega {dP_{\mathrm {Br} } \over d\omega }={16 \over 3}\left[{e^{2} \over 4\pi \varepsilon _{0}}\right]^{3}{1 \over m_{e}^{2}c^{3}}Z_{i}^{2}n_{i}n_{e}k_{\rm {max}}G(y_{\rm {p}})\\G(y_{p})&={1 \over 2{\sqrt {\pi }}}\int _{y_{\rm {p}}}^{\infty }dy\,y^{-{\frac {1}{2}}}\left[1-{y_{\rm {p}} \over y}\right]^{\frac {1}{2}}E_{1}(y)\\y_{\rm {p}}&=y(\omega =\omega _{\rm {p}})\end{aligned}}

G(y_{\rm {p}}=0)=1

and decreases with

y_{\rm {p}}

; it is always positive. For

k_{\rm {max}}=1/\lambda _{\rm {B}}

, we find

P_{\mathrm {Br} }={16 \over 3}{\left({\frac {e^{2}}{4\pi \varepsilon _{0}}}\right)^{3} \over (m_{e}c^{2})^{\frac {3}{2}}\hbar }Z_{i}^{2}n_{i}n_{e}(k_{\rm {B}}T_{e})^{\frac {1}{2}}G(y_{\rm {p}})

Note the appearance of $\hbar$ due to the quantum nature of $\lambda _{\rm {B}}$ . In practical units, a commonly used version of this formula for $G=1$ is ^[7]

P_{\mathrm {Br} }[{\textrm {W}}/{\textrm {m}}^{3}]={Z_{i}^{2}n_{i}n_{e} \over \left[7.69\times 10^{18}{\textrm {m}}^{-3}\right]^{2}}T_{e}[{\textrm {eV}}]^{\frac {1}{2}}.

This formula is 1.59 times the one given above, with the difference due to details of binary collisions. Such ambiguity is often expressed by introducing Gaunt factor $g_{\rm {B}}$ , e.g. in ^[8] one finds

\varepsilon _{\mathrm {ff} }=1.4\times 10^{-27}T^{\frac {1}{2}}n_{e}n_{i}Z^{2}g_{\rm {B}},\,

where everything is expressed in the CGS units.

Relativistic corrections[edit]

Relativistic corrections to the emission of a 30-keV photon by an electron impacting on a proton.

For very high temperatures there are relativistic corrections to this formula, that is, additional terms of the order of $k_{\rm {B}}T_{e}/m_{e}c^{2}\,.$ ^[9]

Bremsstrahlung cooling[edit]

If the plasma is optically thin, the bremsstrahlung radiation leaves the plasma, carrying part of the internal plasma energy. This effect is known as the bremsstrahlung cooling. It is a type of radiative cooling. The energy carried away by bremsstrahlung is called bremsstrahlung losses and represents a type of radiative losses. One generally uses the term bremsstrahlung losses in the context when the plasma cooling is undesired, as e.g. in fusion plasmas.

Polarizational bremsstrahlung[edit]

Polarizational bremsstrahlung (sometimes referred to as "atomic bremsstrahlung") is the radiation emitted by the target's atomic electrons as the target atom is polarized by the Coulomb field of the incident charged particle.^[10]^[11] Polarizational bremsstrahlung contributions to the total bremsstrahlung spectrum have been observed in experiments involving relatively massive incident particles,^[12] resonance processes,^[13] and free atoms.^[14] However, there is still some debate as to whether or not there are significant polarizational bremsstrahlung contributions in experiments involving fast electrons incident on solid targets.^[15]^[16]

It is worth noting that the term "polarizational" is not meant to imply that the emitted bremsstrahlung is polarized. Also, the angular distribution of polarizational bremsstrahlung is theoretically quite different than ordinary bremsstrahlung.^[17]

Sources[edit]

X-ray tube[edit]

Spectrum of the X-rays emitted by an X-ray tube with a rhodium target, operated at 60 kV. The continuous curve is due to bremsstrahlung, and the spikes are characteristic K lines for rhodium. The curve goes to zero at 21 pm in agreement with the Duane–Hunt law, as described in the text.

In an X-ray tube, electrons are accelerated in a vacuum by an electric field towards a piece of metal called the "target". X-rays are emitted as the electrons slow down (decelerate) in the metal. The output spectrum consists of a continuous spectrum of X-rays, with additional sharp peaks at certain energies. The continuous spectrum is due to bremsstrahlung, while the sharp peaks are characteristic X-rays associated with the atoms in the target. For this reason, bremsstrahlung in this context is also called continuous X-rays.^[18]

The shape of this continuum spectrum is approximately described by Kramers' law.

The formula for Kramers' law is usually given as the distribution of intensity (photon count) $I$ against the wavelength $\lambda$ of the emitted radiation:^[19]

I(\lambda )\,d\lambda =K\left({\frac {\lambda }{\lambda _{\min }}}-1\right){\frac {1}{\lambda ^{2}}}\,d\lambda

The constant K is proportional to the atomic number of the target element, and $\lambda_{\min}$ is the minimum wavelength given by the Duane–Hunt law.

The spectrum has a sharp cutoff at $\lambda_{\min}$ , which is due to the limited energy of the incoming electrons. For example, if an electron in the tube is accelerated through 60 kV, then it will acquire a kinetic energy of 60 keV, and when it strikes the target it can create X-rays with energy of at most 60 keV, by conservation of energy. (This upper limit corresponds to the electron coming to a stop by emitting just one X-ray photon. Usually the electron emits many photons, and each has an energy less than 60 keV.) A photon with energy of at most 60 keV has wavelength of at least 21 pm, so the continuous X-ray spectrum has exactly that cutoff, as seen in the graph. More generally the formula for the low-wavelength cutoff, the Duane-Hunt law, is:^[20]

\lambda _{\min }={\frac {hc}{eV}}\approx {\frac {1239.8}{V}}{\text{ pm/kV}}

where h is Planck's constant, c is the speed of light, V is the voltage that the electrons are accelerated through, e is the elementary charge, and pm is picometres.

Beta decay[edit]

Beta particle-emitting substances sometimes exhibit a weak radiation with continuous spectrum that is due to bremsstrahlung (see the "outer bremsstrahlung" below). In this context, bremsstrahlung is a type of "secondary radiation", in that it is produced as a result of stopping (or slowing) the primary radiation (beta particles). It is very similar to X-rays produced by bombarding metal targets with electrons in X-ray generators (as above) except that it is produced by high-speed electrons from beta radiation.

Inner and outer bremsstrahlung[edit]

The "inner" bremsstrahlung (also known as "internal bremsstrahlung") arises from the creation of the electron and its loss of energy (due to the strong electric field in the region of the nucleus undergoing decay) as it leaves the nucleus. Such radiation is a feature of beta decay in nuclei, but it is occasionally (less commonly) seen in the beta decay of free neutrons to protons, where it is created as the beta electron leaves the proton.

In electron and positron emission by beta decay the photon's energy comes from the electron-nucleon pair, with the spectrum of the bremsstrahlung decreasing continuously with increasing energy of the beta particle. In electron capture, the energy comes at the expense of the neutrino, and the spectrum is greatest at about one third of the normal neutrino energy, decreasing to zero electromagnetic energy at normal neutrino energy. Note that in the case of electron capture, bremsstrahlung is emitted even though no charged particle is emitted. Instead, the bremsstrahlung radiation may be thought of as being created as the captured electron is accelerated toward being absorbed. Such radiation may be at frequencies that are the same as soft gamma radiation, but it exhibits none of the sharp spectral lines of gamma decay, and thus is not technically gamma radiation.

The internal process is to be contrasted with the "outer" bremsstrahlung due to the impingement on the nucleus of electrons coming from the outside (i.e., emitted by another nucleus), as discussed above.^[21]

Radiation safety[edit]

In some cases, e.g. 32
P, the bremsstrahlung produced by shielding the beta radiation with the normally used dense materials (e.g. lead) is itself dangerous; in such cases, shielding must be accomplished with low density materials, e.g. Plexiglas (Lucite), plastic, wood, or water;^[22] as the atomic number is lower for these materials, the intensity of bremsstrahlung is significantly reduced, but a larger thickness of shielding is required to stop the electrons (beta radiation).

In astrophysics[edit]

The dominant luminous component in a cluster of galaxies is the 10⁷ to 10⁸ kelvin intracluster medium. The emission from the intracluster medium is characterized by thermal bremsstrahlung. This radiation is in the energy range of X-rays and can be easily observed with space-based telescopes such as Chandra X-ray Observatory, XMM-Newton, ROSAT, ASCA, EXOSAT, Suzaku, RHESSI and future missions like IXO [1] and Astro-H [2].

Bremsstrahlung is also the dominant emission mechanism for H II regions at radio wavelengths.

In electric discharges[edit]

In electric discharges, for example as laboratory discharges between two electrodes or as lightning discharges between cloud and ground or within clouds, electrons produce Bremsstrahlung photons while scattering off air molecules. These photons become manifest in terrestrial gamma-ray flashesand are the source for beams of electrons, positrons, neutrons and protons.^[23] The appearance of Bremsstrahlung photons also influences the propagation and morphology of discharges in nitrogen-oxygen mixtures with low percentages of oxygen.^[24]

In electric discharges[edit]

In electric discharges, for example as laboratory discharges between two electrodes or as lightning discharges between cloud and ground or within clouds, electrons produce Bremsstrahlung photons while scattering off air molecules. These photons become manifest in terrestrial gamma-ray flashesand are the source for beams of electrons, positrons, neutrons and protons.^[23] The appearance of Bremsstrahlung photons also influences the propagation and morphology of discharges in nitrogen-oxygen mixtures with low percentages of oxygen.^[24]

Electron–electron bremsstrahlung[edit]

One mechanism, considered important for small atomic numbers $Z$ , is the scattering of a free electron at the shell electrons of an atom or molecule.^[29]Since electron–electron bremsstrahlung is a function of $Z$ and the usual electron-nucleus bremsstrahlung is a function of $Z^{2}$ , electron–electron bremsstrahlung is negligible for metals. For air, however, it plays an important role in the production of terrestrial gamma-ray flashes.^[30]

https://en.wikipedia.org/wiki/Bremsstrahlung

Nikiya Anton Bettey 2021

Blog Archive

Sunday, August 15, 2021

08-14-2021-1848 - Bremsstrahlung

In plasma[edit]

Relativistic corrections[edit]

Bremsstrahlung cooling[edit]

Polarizational bremsstrahlung[edit]

Sources[edit]

X-ray tube[edit]

Beta decay[edit]

Inner and outer bremsstrahlung[edit]

Radiation safety[edit]

In astrophysics[edit]

In electric discharges[edit]

In electric discharges[edit]

Electron–electron bremsstrahlung[edit]

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