Competing decay channels and fed daughter populations
Independent constant decay channels have total rate λ=Σ_iλ_i and branching fractions b_i=λ_i/λ.
Key knowledge
Independent constant decay channels have total rate λ=Σ_iλ_i and branching fractions b_i=λ_i/λ. For a closed parent–daughter chain with branch b, Ṅ₁=−λ₁N₁ and Ṅ₂=bλ₁N₁−λ₂N₂. Distinct rates give N₂(t)=N₂₀e^(−λ₂t)+bλ₁N₁₀(e^(−λ₁t)−e^(−λ₂t))/(λ₂−λ₁); equal rates give e^(−λt)(N₂₀+bλN₁₀t). Daughter activity is λ₂N₂, rather than the parent decay rate by default.
Values and equations
λ=Σ_iλ_i; b_i=λ_i/λ; Ṅ₂=bλ₁N₁−λ₂N₂; N₂=N₂₀e^(−λ₂t)+bλ₁N₁₀(e^(−λ₁t)−e^(−λ₂t))/(λ₂−λ₁)
- λ,λ_i,λ₁,λ₂
- Total/channel/parent/daughter decay rates s⁻¹
- b,b_i
- Branching fractions
- N₁,N₂,N₁₀,N₂₀
- Expected populations and initial values
- t
- Elapsed time s
- Ṅ₂
- Expected population rate s⁻¹
For λ₁=λ₂=λ: N₂=e^(−λt)(N₂₀+bλN₁₀t); activity₂=λ₂N₂
- λ,λ₂
- Decay rates s⁻¹
- t
- Elapsed time s
- N₂,N₂₀,N₁₀
- Expected daughter and initial populations
- b
- Parent-to-daughter branch
- activity₂
- Daughter activity Bq
Conditions and scope
- λ₁>0, λ₂≥0, 0≤b≤1, fixed rates and specified initial expected populations; no external production, transport, chemical removal or feedback on decay rates. The equal-rate case is the continuous limit.
- A partial rate λ_i has units s⁻¹ and partial half-life ln2/λ_i where λ_i>0; it is not itself a half-life. Longer networks use the corresponding linear feeding equations and actual branching data.
Sources · 2
References
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Evidence and review
Introduction to Applied Nuclear Physics: chapter 1 (2012): Section 1.3.5, printed page 15: additive decay constants and branching; the incorrectly named partial-half-life quantity bλ is treated as a rate. Radiotracer Generators for Industrial Applications (2013): Section 2.1, printed pages 6–7, equations 1–7: parent decay, daughter feeding and Bateman solution with initially zero daughter. The nonzero-initial daughter, branch multiplier and equal-rate limit are derived by linearity/integration from these displayed differential equations, not quoted as an experimental generator result.
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Entry details
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