Bidirectional Absorber

The standard story of radiation detection runs forward only: a nucleus emits, a quantum propagates, a detector absorbs. In the Wheeler-Feynman / transactional frame, emission and absorption are co-equal poles of a single transaction.

Every detector broadcasts an advanced confirmation (blue, converging) that meets the source's retarded offer (red, expanding) at the emission event. A detection occurs where and when the handshake completes.

Action Bits (yellow flashes) are discrete symmetry-breaking events: quanta of action exchanged between source and absorber. Each flash is one completed transaction.

The detector array's geometry, material, and energy response shape the confirmation field. A sparse array around a source operates as a configured absorber landscape, selecting which transactions can complete. Optimizing the array means optimizing the confirmation-field structure.

Veto detectors (magenta) subtract from the confirmation field, claiming transactions before they can register as signal. Anti-coincidence shields, muon vetoes, and Compton suppression all intervene on the absorber landscape this way.

Coincidence techniques encode the dual structure implicitly. PET imaging recovers source locations by demanding two 511 keV detections within a tight window across opposing detectors. The transactional reading: the annihilation transaction commits only when both absorber boundaries fire in handshake.

For distributed networks (citizen-science cosmic ray monitors, post-fire watershed sensors), detection capacity becomes a governed resource. Coverage gaps are unallocated transaction-completion capacity: holes in the sensing commons.