Struct CountsBackgroundConfig 

pub struct CountsBackgroundConfig {
    pub alpha_1_init: f64,
    pub alpha_2_init: f64,
    pub fit_alpha_1: bool,
    pub fit_alpha_2: bool,
    pub c: f64,
}

Background model for the counts fitting engine.

In the counts domain, the forward model is: Y(E) = α₁ · [Φ(E) · exp(-Σ nᵢσᵢ(E))] + α₂ · B(E)

where Φ(E) is the incident flux and B(E) is detector/gamma background. The reference Φ(E) / B(E) spectra are supplied by the caller or by spatial pre-processing; this config only controls the fitted scale factors.

Important distinction:

• This is a detector-space counts background model B(E).
• It is NOT the same as the transmission-lift background used by BackgroundConfig, which models additive uplift of the apparent transmission curve (for example gamma-tail structure that pushes transmission upward).

For VENUS MCP/TPX event detectors, the current working assumption is:

• raw/open-beam is the correct normalization baseline
• dark-current / CCD-style electronic offset is not modeled
• rare ghost counts may exist at the hardware level, but are currently treated as negligible unless a detector-background reference spectrum is explicitly provided

This is structurally different from the transmission background model (BackgroundConfig) because:

• Φ and B are reference spectra, not fitted per pixel
• α₁ and α₂ are optional per-pixel scale corrections
• All terms are non-negative (required for valid Poisson NLL)

Fields

alpha_1_init: f64

Research-only. Initial α₁ flux-scale value used by crate::pipeline::evaluate_jacobian_and_fisher (Fisher-info research helper, Epic #394). Not honoured by the production fit path; SolverConfig::PoissonKL profiles the flux via λ̂ and rejects alpha fitting.

alpha_2_init: f64

Research-only. Initial α₂ detector-bg-scale value, same provisions as alpha_1_init.

fit_alpha_1: bool

Research-only. Fit α₁ flag — only consumed by evaluate_jacobian_and_fisher. Passing true through SolverConfig::PoissonKL on counts input yields an error.

fit_alpha_2: bool

Research-only. Fit α₂ flag — see fit_alpha_1.

c: f64

Proton-charge ratio c = Q_s / Q_ob for the counts-KL solver (memo 35 §P1.3 — “make c a first-class API parameter”).

Default 1.0, correct only when the caller has already PC- normalized the open-beam counts so that flux = c · O. For the counts-KL dispatch (SolverConfig::PoissonKL on counts input), set this to the actual Q_sample / Q_open_beam ratio and pass raw open-beam counts — the joint-Poisson solver will profile λ̂ itself. Ignored by LM paths.

Trait Implementations

impl Clone for CountsBackgroundConfig

fn clone(&self) -> CountsBackgroundConfig

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for CountsBackgroundConfig

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for CountsBackgroundConfig

fn default() -> Self

Returns the “default value” for a type.