Physics-based educational reconstruction · model scope in About and the model ledger.
01 · Two different physical mechanisms
Cloud chamber: alcohol from a room-temperature felt meets a dry-ice-cooled floor, and a thin supersaturated layer condenses onto ions; the white traces are droplets, not luminous particles. Bubble chamber: lowering the pressure superheats liquid hydrogen, deposited energy nucleates vapour bubbles, and recompression removes them. There is no alcohol, dry ice or gaseous sensitive layer in that mode.
02 · Dimensional and mechanical scope
The cloud mode is a specified 220 × 220 × 140 mm diffusion instrument. The bubble mode follows the public BEBC envelope (3.7 m diameter, 4 m high, 35 m³, nominal 3.5 T, five cameras) as a 1.717 m-radius × 3.78 m sensitive cylinder. Piston stroke, coil and cryostat detail, optics and pressure timing are reconstruction parameters, and the cutaway liquid tint is an annotation: hydrogen is colourless.
03 · Relativistic transport
Propagation uses an analytic local helix with adaptive arc-length steps, boundary clipping and mean energy loss: p²c² = T(T + 2mc²), r [m] = p⊥ [GeV/c] / (0.299792458 |q/e| B [T]). The field is axial and uniform inside the liquid; fringe fields are not solved, the cloud chamber has no magnet, and no magnetic work is added to kinetic energy.
04 · Interaction approximations
Heavy charged particles use Bethe mean stopping with a density-effect correction; electrons use the PDG collision expression plus a mean radiative-loss term. Alpha ranges follow R = 0.318 T^1.5 cm in reference air, scattering is a Gaussian approximation, and nuclear interactions, showers, delta rays and pion or muon decays are not generated.
05 · Thermodynamics and detector response
Hydrogen saturation uses the NIST Antoine fit, valid from 21.01 to 32.27 K; bubbles form only if P < Psat at the instant of passage, so raising the pressure later cannot create a missed track. The cloud uses a fitted exponential boundary layer and a linear vapour profile, not a coupled heat-transfer solution, and controls select equilibrated states.
06 · What the white points represent
Track centers come from the transport calculation; displayed sites are weighted samples of deposited energy, not ion-pair counts, detection efficiency or source activity. Droplet and bubble radii follow R² growth approximations. In apparatus view unresolved scatterers get a minimum optical footprint; the microscope uses model diameters without it.
07 · Event truth versus measurement
The reference mixture is a curated alpha, electron and grazing-muon stream, not a cosmic-ray flux or decay-chain simulation. K⁰S → π⁺π⁻ is sampled in the rest frame, boosted, and given an exponential decay length with cτ = 26.844 mm; the invariant mass uses generated daughter momenta, and the circle fit uses transport samples rather than photographs.
08 · Time, interaction and repeatability
Particle flight takes nanoseconds and is integrated at physical speed. At 1× cloud detector time runs in real seconds, the 120 ms bubble cycle is shown 100× slower, and microscope inspection pauses detector time. Settings affect only new tracks, the seed is fixed on reset, and gaps longer than five seconds do not advance the model.
