Question: What American physicist discovered the positron which explained Chinese physicist Chung-Yao Chao's earlier experiments with gamma rays?

Context paragraphs:
Biodistribution: Biodistribution is a method of tracking where compounds of interest travel in an experimental animal or human subject.  For example, in the development of new compounds for PET (positron emission tomography) scanning, a radioactive isotope is chemically joined with a peptide (subunit of a protein).  This particular class of isotopes emits positrons (which are antimatter particles, equal in mass to the electron, but with a positive charge).  When ejected from the nucleus, positrons encounter an electron, and undergo annihilation which produces two gamma rays travelling in opposite directions.  These gamma rays can be measured, and when compared to a standard, quantified.

Ultra-high-energy gamma ray: Ultra-high-energy gamma rays are gamma rays with photon energies higher than 100 TeV (0.1 PeV).  They have a frequency higher than 2.42 × 10 Hz and a wavelength shorter than 1.24 × 10 m. s of 2014 , they are theoretical only and have not been detected.  The highest energy astronomical sourced gamma rays detected are very-high-energy gamma rays.

Carl David Anderson: Carl David Anderson (September 3, 1905 – January 11, 1991) was an American physicist.  He is best known for his discovery of the positron in 1932, an achievement for which he received the 1936 Nobel Prize in Physics, and of the muon in 1936.

Gamma-ray laser: A gamma-ray laser, or graser, would produce coherent gamma rays, just as an ordinary laser produces coherent rays of visible light.  It would be powered by nuclear transitions from a nuclear isomer.  To construct a gamma ray laser, one must identify a suitable isomer, purify it, create a crystal from the purified material, and assemble a configuration that leads to the emission of a coherent gamma-ray beam.  Because the wave length of gamma rays are shorter than that of x-rays, such a device, which has yet to be realized, would potentially be very useful in applications such as high-resolution imaging, surgery, and communications, as well as high-intensity applications.

Meitner–Hupfeld effect: The Meitner–Hupfeld effect is an anomalously large scattering of Gamma rays by heavy elements.  Later on, the Meitner–Hupfeld effect was explained by a broad theory from which evolved the Standard Model, a theory for explaining the structure of the atomic nucleus.  The anomalous gamma-ray behavior was eventually ascribed to electron–positron pair production and annihilation.

Energetic Gamma Ray Experiment Telescope: The Energetic Gamma Ray Experiment Telescope (EGRET) was one of four instruments outfitted on NASA’s Compton Gamma Ray Observatory satellite.  Since lower energy gamma rays cannot be accurately detected on Earth’s surface, EGRET was built to detect gamma rays while in space.  EGRET was created for the purpose of detecting and collecting data on gamma rays ranging in energy level from 30 MeV to 30 GeV.

Magnetar: A magnetar is a type of neutron star with an extremely powerful magnetic field.  The magnetic field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.  The theory regarding these objects was proposed by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays thought to have been from a magnetar had been detected on March 5, 1979.  During the following decade, the magnetar hypothesis became widely accepted as a likely explanation for soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs).

Chung-Yao Chao: Chung-Yao Chao (; 27 June 1902 – 28 May 1998) was a Chinese physicist.  Chung-Yao Chao studied the scattering of gamma rays in lead by pair production in 1930, without knowing that positrons were involved in the anomalously high scattering cross-section.  When the positron was discovered by Carl David Anderson in 1932, confirming the existence of Paul Dirac's "antimatter", it became clear that positrons could explain Chung-Yao Chao's earlier experiments, with the gamma rays being emitted from electron-positron annihilation.

Gamma ray: Gamma rays (also called gamma radiation), denoted by the lower-case Greek letter gamma (γ or formula_1 ), are penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei.  It consists of photons in the highest observed range of photon energy.  Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium.  In 1903, Ernest Rutherford named this radiation "gamma rays".  Rutherford had previously discovered two other types of radioactive decay, which he named alpha and beta rays.

Annihilation radiation: Annihilation radiation is a term used in Gamma spectroscopy for the gamma radiation produced when a particle and its antiparticle collide and annihilate.  Most commonly, this refers to 511-keV gamma rays produced by a normal (negative) electron colliding with a positron.

Supporting paragraphs: Chung-Yao Chao Carl David Anderson
Answer: Carl David Anderson