We have measured the particle velocity of aluminum foils placed between ultrafine TATB and lithium fluoride, where the TATB thicknesses are varied between 0 and 8 mm. Resolution is about +/-2 ns. The reaction zones are determined from the data, with 0.3 mm being the average. We suggest a 1-D analytical approach based on the idea that the initiating flyer must put in some minimum amount of energy over one reaction zone length. The energy is calculated three ways: flyer input, reaction zone to be filled and the P(2)tau initiation energy. Code calculations show the reaction zones rapidly growing in the region dominated by the fiver followed by a slow increase as the detonation takes hold.