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Quantum mechanical scattering of photon and nucleus : ウィキペディア英語版
Quantum mechanical scattering of photon and nucleus

In pair production, a photon creates an electron positron pair. In the process of photons scattering in air (e.g. in lightning discharges), the most important interaction is the scattering of photons at the nuclei of atoms or molecules. The full quantum mechanical process of pair production can be described by the quadruply differential cross section give here:〔Bethe, H.A., Heitler, W., 1934. On the stopping of fast particles and on the creation of positive electrons. Proc. Phys. Soc. Lond. 146, 83–112〕

\begin
d^4\sigma &=
\frac|\mathbf_+||\mathbf_-|
\frac\frac\times \\
&\times\left( \\
&+2\hbar^2\omega^2\frac_-^2\sin^2\Theta_-}_-|\cos\Theta_-)} \\
&+2\left.\frac_-|\sin\Theta_+\sin\Theta_-\cos\Phi}_-|\cos\Theta_-)}\left(2E_+^2+2E_-^2-c^2\mathbf^2\right)\right
). \\
\end

with

\begin
d\Omega_+&=\sin\Theta_+\ d\Theta_+,\\
d\Omega_-&=\sin\Theta_-\ d\Theta_-.
\end

This expression can be derived by using a quantum mechanical symmetry between pair production and Bremsstrahlung.
Z is the atomic number, \alpha_\approx 1/137 the fine structure constant, \hbar the reduced Planck's constant and c the speed of light. The kinetic energies E_ of the positron and electron relate to their total energies E_ and momenta \mathbf_ via

E_=E_+m_e c^2=\sqrt^2 c^2}.

Conservation of energy yields

\hbar\omega=E_+E_.

The momentum \mathbf of the virtual photon between incident photon and nucleus is:

\begin
-\mathbf^2&=-|\mathbf_+|^2-|\mathbf_-|^2-\left(\frac\omega\right)^2+2|\mathbf_+|\frac
\omega\cos\Theta_+ +2|\mathbf_-|\frac \omega\cos\Theta_- \\
&-2|\mathbf_+||\mathbf_-|(\cos\Theta_+\cos\Theta_-+\sin\Theta_+\sin\Theta_-\cos\Phi),
\end

where the directions are given via:

\begin
\Theta_+&=\sphericalangle(\mathbf_+,\mathbf),\\
\Theta_-&=\sphericalangle(\mathbf_-,\mathbf),\\
\Phi&=\text (\mathbf_+,\mathbf) \text (\mathbf_-,\mathbf),
\end

where \mathbf is the momentum of the incident photon.
In order to analyse the relation between the photon energy E_+ and the emission angle \Theta_+ between photon and positron, Köhn and Ebert integrated 〔Koehn, C., Ebert, U., Angular distribution of Bremsstrahlung photons and of positrons for calculations of terrestrial gamma-ray flashes and positron beams, Atmos. Res. (2014), vol. 135-136, pp. 432-465〕 the quadruply differential cross section over \Theta_- and \Phi . The double differential cross section is:

\begin
\frac =
\sum\limits_^ I_j
\end

with

\begin
I_1&=\frac} \\
&\times
\ln\left(\frac_2)^2+4p_+^2p_-^2\sin^2
\Theta_+}(\Delta^_1+\Delta^_2)+\Delta^_1\Delta^_2}_2)^2+4p_+^2p_-^2\sin^2 \Theta_+}(\Delta^_1-\Delta^_2)+\Delta^_1\Delta^_2
}\right) \\
&\times\left(), \\
I_2&=\frac\ln\left(
\frac\right), \\
I_3&=\frac_1p_-c)^2+4m^2c^4p_+^2p_-^2\sin^2\Theta_+
}} \\
&\times\ln\Bigg(\Big((E_-+p_-c)(4p_+^2p_-^2\sin^2\Theta_+(E_--p_-c)+(\Delta^_1+\Delta^_2)
((\Delta^_2E_-+\Delta^_1p_-c) \\
&-\sqrt_1p_-c)^2+4m^2c^4p_+^2p_-^2\sin^2\Theta_+}))\Big)\Big((E_--p_-c)
(4p_+^2p_-^2\sin^2\Theta_+(-E_--p_-c) \\
&+(\Delta^_1-\Delta^_2)
((\Delta^_2E_-+\Delta^_1p_-c)-\sqrt_1p_-c)^2+4m^2c^4p_+^2p_-^2\sin^2\Theta_+}))\Big)^\Bigg) \\
&\times\left^ \\
&+\Big(\\
&+2\hbar^2\omega^2p_- m^2c^3(\Delta^_2E_-+\Delta^_1p_-c)\Big
)
\Big()^ \\
&+\left.\frac_1p_-c)^2-4m^2c^4p_+^2p_-^2\sin^2\Theta_+)(\Delta^_1E_-+\Delta^_2p_-c)}_1p_-c)^2+4m^2c^4p_+^2p_-^2\sin^2\Theta_+)^2}\right], \\
I_4&=\frac_1p_-c)}_1p_-c)^2+4m^2c^4p_+^2p_-^2\sin^2\Theta_+}+\frac_1p_-c)^2}_1p_-c)^2+4m^2c^4p_+^2p_-^2\sin^2\Theta_+)^2}, \\
I_5&=\frac_1)^2-4p_+^2p_-^2\sin^2\Theta_+)
((\Delta^_2E_-+\Delta^_1p_-c)^2+4m^2c^4p_+^2p_-^2\sin^2\Theta_+)} \\
&\times\left\Big]\Big()^\\
&+ \frac_1\Delta^_2
p_-c+2(\Delta^_2)^2E_-+8p_+^2p_-^2\sin^2\Theta_+ E_-)}\\
&-\Big\Big()^\\
&-\left.\frac_1
E_-)}\right], \\
I_6&=-\frac_1)^2-4p_+^2p_-^2\sin^2\Theta_+)}
\end

and

\begin
A&=\frac\frac_-|},\\
\Delta^_1&:=-|\mathbf_+|^2-|\mathbf_-|^2-\left(\frac\omega\right)
+ 2\frac\omega|\mathbf_+|\cos\Theta_+,\\
\Delta^_2&:=2\frac\omega|\mathbf_i|-2|\mathbf_+||\mathbf_-|
\cos\Theta_+ + 2.
\end

This cross section can be applied in Monte Carlo simulations. An analysis of this expression shows that positrons are mainly emitted in the direction of the incident photon.
==References==


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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