Photon absorption, Compton recoil and pair creation
Photoelectric absorption transfers a photon to a bound electron with ion recoil/relaxation; an approximate emitted-electron kinetic energy is hν−B when recoil is negligible.
Key knowledge
Photoelectric absorption transfers a photon to a bound electron with ion recoil/relaxation; an approximate emitted-electron kinetic energy is hν−B when recoil is negligible. For Compton scattering from a free electron initially at rest, E′=E/[1+E(1−cosθ)/(m_ec²)]. Electron–positron pair creation requires recoil as well as rest energy: for an initially stationary bare nucleus of mass M that remains intact, E_thr=2m_ec²(1+m_e/M), tending to 2m_ec² for a very heavy nucleus.
Values and equations
E′=E/[1+E(1−cosθ)/(m_ec²)]; E_thr=2m_ec²(1+m_e/M); T_photo≈hν−B
- E,E′,E_thr,T_photo,B
- Initial/final photon energy, pair threshold, photoelectron kinetic energy and positive binding energy J
- m_e,M
- Electron and recoil-nucleus masses kg
- c
- Vacuum light speed m s⁻¹
- h
- Planck constant J s
- ν
- Photon frequency Hz
- θ
- Scattered photon angle rad
Conditions and scope
- Compton formula assumes a single free resting electron and relativistic photon/electron energy–momentum conservation; atomic binding, initial motion and many-body response change that model.
- The pair threshold follows the invariant-mass minimum M+2m_e for the photon+nucleus channel, without electronic binding/excitation. A lone photon cannot create a pair in empty vacuum while conserving four-momentum; two-photon channels are different. Process strengths depend on energy and material, with no universal low/middle/high-energy dominance rule.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- Introduction to Applied Nuclear Physics: chapter 8 (2012)Paola Cappellaro / MIT OpenCourseWareCourse notes · PDF
- University Physics 3: 6-3-the-compton-effectSamuel J. Ling, Jeff Sanny and William Moebs / OpenStax, Rice UniversityTextbook chapter
- PDG: Kinematics (2025 edition)D. Miller, D. R. Tovey and J. D. Jackson / Particle Data GroupReference guide · PDF
Evidence and review
Introduction to Applied Nuclear Physics: chapter 8 (2012): Section 8.1.4, printed pages 117–119: distinct photoelectric, Compton and field-assisted pair processes. The coarse energy-range table is not treated as universal thresholds or dominance. University Physics 3: 6-3-the-compton-effect: Equations 6.21–6.30: free initially resting electron energy/momentum balance and wavelength shift; the energy expression is algebraically equivalent. Example-specific energy fractions are not generalized. PDG: Kinematics (2025 edition): Equation 49.3, printed page 1: fixed-target invariant energy. Set projectile mass zero and minimum final mass M+2m_e to derive the finite-recoil pair threshold with c restored.
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