fix(gpuraytrace): add multi-component scintillation genstep collection (#272)

GPU genstep collection was only using SCINTILLATIONTIMECONSTANT1 (fast
component, τ=7ns). LAr has a 25% slow component
(SCINTILLATIONTIMECONSTANT2, τ=1400ns) that was never collected. This
caused GPU to produce zero hits with arrival time >100ns while Geant4
showed 20% of hits in the 100-11,000ns range.

Scintillation fix (src/GPURaytrace.h):

- Read up to 3 scintillation components (SCINTILLATIONTIMECONSTANT1/2/3,
SCINTILLATIONYIELD1/2/3) from material properties
- Split photon count proportionally across components based on yield
fractions
- Create separate gensteps for each component with the correct time
constant
- Last component receives rounding remainder to preserve total photon
count exactly

Shall be merged after rest of the wavelength shifting PRs

---------

Co-authored-by: Dmitri Smirnov <dmixsmi@gmail.com>

Author: ggalgoczi

src/GPURaytrace.h

@@ -531,10 +531,51 @@ struct SteppingAction : G4UserSteppingAction
                                    << G4endl;
                             return;
                         }
-                        G4double SCINTILLATIONTIMECONSTANT1 = MPT->GetConstProperty(kSCINTILLATIONTIMECONSTANT1);
+                        // G4 11.x supports up to 3 scintillation components
+                        const G4int tcKeys[3] = {kSCINTILLATIONTIMECONSTANT1, kSCINTILLATIONTIMECONSTANT2,
+                                                 kSCINTILLATIONTIMECONSTANT3};
+                        const G4int yieldKeys[3] = {kSCINTILLATIONYIELD1, kSCINTILLATIONYIELD2, kSCINTILLATIONYIELD3};
 
-                        U4::CollectGenstep_DsG4Scintillation_r4695(aTrack, aStep, fNumPhotons, 1,
-                                                                   SCINTILLATIONTIMECONSTANT1);
+                        G4double tc[3] = {0, 0, 0};
+                        G4double yield[3] = {0, 0, 0};
+                        G4double yieldSum = 0;
+                        G4int    nComp = 0;
+
+                        for (G4int c = 0; c < 3; c++)
+                        {
+                            if (MPT->ConstPropertyExists(tcKeys[c]))
+                            {
+                                tc[c] = MPT->GetConstProperty(tcKeys[c]);
+                                yield[c] = MPT->ConstPropertyExists(yieldKeys[c]) ? MPT->GetConstProperty(yieldKeys[c])
+                                                                                  : (c == 0 ? 1.0 : 0.0);
+                                yieldSum += yield[c];
+                                nComp = c + 1;
+                            }
+                        }
+
+                        if (yieldSum <= 0.0)
+                        {
+                            G4cout << "WARNING: scintillation yields sum to <= 0 for material '"
+                                   << aMaterial->GetName() << "'; falling back to single component." << G4endl;
+                            yield[0] = 1.0;
+                            yieldSum = 1.0;
+                            nComp = 1;
+                        }
+
+                        G4AutoLock lock(&genstep_mutex);
+                        G4int      nRemaining = fNumPhotons;
+                        for (G4int c = 0; c < nComp; c++)
+                        {
+                            G4int nPhotComp;
+                            if (c == nComp - 1)
+                                nPhotComp = nRemaining; // last component gets remainder
+                            else
+                                nPhotComp = static_cast<G4int>(fNumPhotons * yield[c] / yieldSum);
+                            nRemaining -= nPhotComp;
+
+                            if (nPhotComp > 0)
+                                U4::CollectGenstep_DsG4Scintillation_r4695(aTrack, aStep, nPhotComp, c + 1, tc[c]);
+                        }
                     }
                 }
             }

tests/geom/opticks_raindrop_with_scintillation.gdml

@@ -19,6 +19,7 @@
     <matrix coldim="1" name="FAST_TIME_CONST" values="6.0"/>
     <matrix coldim="1" name="SLOW_TIME_CONST" values="35.0"/>
     <matrix coldim="1" name="YIELD_RATIO" values="0.8"/>
+    <matrix coldim="1" name="YIELD_RATIO_SLOW" values="0.2"/>
     <matrix coldim="2" name="REEMISSION_PROB" values="1.0*eV 0.0 4.0*eV 0.0"/>
   </define>
 
@@ -58,6 +59,7 @@
       <property name="SCINTILLATIONTIMECONSTANT1" ref="FAST_TIME_CONST"/>
       <property name="SCINTILLATIONTIMECONSTANT2" ref="SLOW_TIME_CONST"/>
       <property name="SCINTILLATIONYIELD1" ref="YIELD_RATIO"/>
+      <property name="SCINTILLATIONYIELD2" ref="YIELD_RATIO_SLOW"/>
       <!-- Old Geant4 10.x / Opticks property names -->
       <property name="FASTCOMPONENT" ref="SCINT_SPECTRUM"/>
       <property name="SLOWCOMPONENT" ref="SCINT_SPECTRUM"/>