diff --git a/src/pg_rad/isotopes/presets.py b/src/pg_rad/isotopes/presets.py
index 5b8224b..9d07a65 100644
--- a/src/pg_rad/isotopes/presets.py
+++ b/src/pg_rad/isotopes/presets.py
@@ -1,4 +1,10 @@
 from .isotope import Isotope
 
 
-CS137 = Isotope("Cs-137", E=661.66, b=0.851)
+class CS137(Isotope):
+    def __init__(self):
+        super.__init__(
+            name="Cs-137",
+            E=661.66,
+            b=0.851
+        )