4.3. Stopping Power Troubleshooting

Stack problems usually appear as energies that don’t match expectations. The three failure modes below cover most cases; for all of them, the first diagnostic is the same — print(st.stack) and check the areal_density column against what you think is in the beam.

Foil energies are far from expectation

Symptom: mu_E values much too low (or the beam exhausts halfway down the stack) even though the foils are thin; or energies that barely change through what should be a substantial degrader.

Cause: almost always units. The stack input mixes several, and each must be right:

• thickness ('t') is in mm — a 25 um foil is 0.025, and typing 25 makes it a 2.5 cm slab.

• areal_density ('ad') is in mg/cm2.

• mass is in g and area in cm2.

• density is in g/cm3.

Fix: print st.stack and inspect the computed areal_density of each foil — a 25 um Ti foil should be ~11 mg/cm2, a 0.5 mm Al foil ~135 mg/cm2. If a foil is off by a factor of 10–1000, one of its input units is wrong.

“WARNING: Beam stopped in foil N”

Symptom: the warning above, foils downstream of foil N with mu_E near zero, and nonsense fluxes from get_flux for those foils.

Cause: the beam physically ranges out — the summed thickness of the stack exceeds the particle range at E0. (N is the row index shown by print(st.stack), counting from 0, so the offending foil is st.stack.iloc[N].) Every downstream energy and flux is meaningless (Curie transports energy loss only; a stopped beam has no energy left to assign). This is sometimes intentional — a beam dump or catcher at the end — and then harmless, as long as the foils you analyze sit upstream of the stopping point.

Fix: if the stack should pass the beam easily, suspect a thickness-unit error first (previous section) before concluding it truly ranged out. Then compare the range against the summed foil thicknesses — in matching units:

>>> print(10*ci.Element('Al').range(30.0))   # x10: range in mm, like 't'
4.359...

and either raise E0, thin the stack, or — if the stop is intentional — simply ignore the foils at and beyond the stopping depth.

A foil is missing from the results

Symptom: a foil you defined does not appear in st.summarize(), st.plot() or st.fluxes; or get_flux('Ti-01') returns empty arrays.

Cause: foils without a 'name' are deliberately untallied — they degrade the beam but are treated as passive (degraders, spacers, catchers). Only named foils are stored in the results. And get_flux matches the name exactly: 'Ti01' and 'Ti-01' are different strings.

Fix: give every foil you care about a 'name' in the stack definition, and read the exact spellings back from print(st.stack) (the name column) before calling get_flux. Unnamed foils still affect the transport — their absence from the results does not mean they were skipped.