Add isotope lookup dictionary, so isotopes can be loaded from string in config.

src/pg_rad/isotopes/__init__.py

@@ -2,9 +2,8 @@
 __ignore__ = ["logger"]
 
 from pg_rad.isotopes import isotope
-from pg_rad.isotopes import presets
 
-from pg_rad.isotopes.isotope import (Isotope,)
-from pg_rad.isotopes.presets import (CS137,)
+from pg_rad.isotopes.isotope import (CS137, Isotope, get_isotope,
+                                     preset_isotopes,)
 
-__all__ = ['CS137', 'Isotope', 'isotope', 'presets']
+__all__ = ['CS137', 'Isotope', 'get_isotope', 'isotope', 'preset_isotopes']

src/pg_rad/isotopes/isotope.py

@@ -1,3 +1,5 @@
+from typing import Dict, Type
+
 from pg_rad.physics.attenuation import get_mass_attenuation_coeff
 
 
@@ -25,3 +27,24 @@ class Isotope:
         self.E = E
         self.b = b
         self.mu_mass_air = get_mass_attenuation_coeff(E / 1000)
+
+
+class CS137(Isotope):
+    def __init__(self):
+        super().__init__(
+            name="Cs-137",
+            E=661.66,
+            b=0.851
+        )
+
+
+preset_isotopes: Dict[str, Type[Isotope]] = {
+    "CS137": CS137
+}
+
+
+def get_isotope(isotope_str: str) -> Isotope:
+    """Lazy factory function to create isotope objects."""
+    if isotope_str not in preset_isotopes:
+        raise ValueError(f"Unknown isotope: {isotope_str}")
+    return preset_isotopes[isotope_str]()

src/pg_rad/isotopes/presets.py

@@ -1,10 +0,0 @@
-from .isotope import Isotope
-
-
-class CS137(Isotope):
-    def __init__(self):
-        super().__init__(
-            name="Cs-137",
-            E=661.66,
-            b=0.851
-        )