Narrow-beam photon attenuation and interaction coefficients
For a narrow monoenergetic photon beam through a homogeneous medium with constant removal coefficient μ, the uncollided intensity obeys I=I₀exp(−μℓ), with mean free path 1/μ.
Key knowledge
For a narrow monoenergetic photon beam through a homogeneous medium with constant removal coefficient μ, the uncollided intensity obeys I=I₀exp(−μℓ), with mean free path 1/μ. For number density n of one target species, μ=nσ_tot. The total specified interaction cross section sums distinct absorption/scattering/pair channels; mass attenuation μ/ρ is distinct from absorbed-energy or dose coefficients.
Values and equations
I/I₀=e^(−μℓ); μ=nσ_tot; ℓ_mean=1/μ; μ_mass=μ/ρ
- I,I₀
- Uncollided photon intensities in a common chosen unit
- μ
- Linear removal coefficient m⁻¹
- ℓ,ℓ_mean
- Path thickness and interaction mean free path m
- n
- Target number density m⁻³
- σ_tot
- Specified total per-target cross section m²
- ρ
- Material mass density kg m⁻³
- μ_mass
- Mass attenuation coefficient m² kg⁻¹
Conditions and scope
- Independent interactions and fixed photon energy in the uncollided beam, μ>0 and path length ℓ≥0. The stated exponential excludes detected photons scattered back into a broad-beam acceptance; mixtures require the appropriate composition model.
- The full physical σ_tot includes relevant channels. The cited NIST compilation explicitly omits photonuclear and certain smaller channels and edge fine structure, so its tabulated coefficient must not be presented as a universal exact total or used here as a dose/risk prediction.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
X-Ray Mass Attenuation Coefficients: section 2: Equations 1–3: narrow-beam exponential, mass thickness and atom cross-section relation; channel list and explicit photonuclear/other omissions; equation 6 homogeneous mixture weights. Mean free path follows integration of the same constant-hazard exponential, and μ=nσ follows number-density conversion. Historical numerical atomic-mass constant is not reused.
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Entry details
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