Initial commit

Base modular structure (phase space, Fermi function)
Initial .gitignore

beta_spectrum/components/fermi.py

@@ -0,0 +1,52 @@
+# components/fermi.py
+import numpy as np
+from scipy.special import loggamma
+
+from ..base import SpectrumComponent
+from ..constants import ALPHA
+from ..utils import nuclear_radius
+
+
+class FermiFunction(SpectrumComponent):
+    def __init__(self, Z: int, A: int):
+        self.Z = Z
+        self.A = A
+        self.alphaZ = ALPHA * Z
+        self.gamma = np.sqrt(1.0 - self.alphaZ**2)
+        self.R = nuclear_radius(A)
+
+    def __call__(self, W: np.ndarray) -> np.ndarray:
+        p = np.sqrt(W**2 - 1.0)
+
+        # Avoid division by zero
+        p = np.where(p == 0, 1e-30, p)
+
+        eta = self.alphaZ * W / p
+
+        # --- log components ---
+
+        # log prefactor
+        log_prefactor = np.log(2 * (1 + self.gamma))
+
+        # log power term
+        log_power = -2 * (1 - self.gamma) * np.log(2 * p * self.R)
+
+        # log gamma ratio
+        z = self.gamma + 1j * eta
+        log_gamma_complex = loggamma(z)
+        log_gamma_abs_sq = 2 * np.real(log_gamma_complex)
+
+        log_gamma_denom = 2 * loggamma(2 * self.gamma + 1).real
+
+        # log exponential
+        log_exp = np.pi * eta
+
+        logF = (
+            log_prefactor
+            + log_power
+            + log_gamma_abs_sq
+            - log_gamma_denom
+            + log_exp
+        )
+
+        return np.exp(logF)