I appreciate the comments of Mike Gruntman (Sept., page 80) concerning the history of using correlated pairs of particles(photons) to determine absolute detector quantum efficiencies. Unfortunately there is an overall misunderstanding of the technique, although Gruntman is in good company. Gruntman refers to section in a book by the eminent Sir Ernest Rutherford et al. In that section there is a description of the 1924 paper of Geiger and Werner, where two microscopes view a single scintillation flash on a single scintillation screen. The claim is made that this allows the quantum efficiency of the human observers to be determined. That is true only as long as the single scintillation event produces enough photons so that each microscope can be guaranteed to collect some of the light. This is fundamentally different from an event that must produce exactly two particles -- that is the basis of the metrology discussed in my article.It appears that the earliest observation of a pair of particles produced by a single event is the 1910 work of Geiger and Marsden . There they observed coincident scintillations on two separate screens carefully shielded so that each observer could see only one screen. These coincident scintillations were produced by pairs of alpha particles emitted by a single nucleus. This fundamental guarantee of two particles created at a time is the basis of the **free lunch** method allowing the quantum efficiency of single particle (photon) detector to be determined without external measurement standards.
Citation: Physics Today
Pub Type: Journals
absolute quantum efficiency, alpha particle, correlated photons, measurement conincidence, phase matching