Please explain the following two phenomena associated with positron emission process: positron range, and noncollinearity of annihilation gamma rays.

Electron–positron annihilation occurs when an electron (

e−

) and a positron (

e+

, the electron's antiparticle) collide. At low energies, the result of the collision is the annihilation of the electron and positron, and the creation of energetic photons:

Naturally occurring electron-positron annihilation as a result of beta plus decay

e−

+

e+

→

γ

+

γ

At high energies, other particles, such as B mesons or the W and Z bosons, can be created. All processes must satisfy a number of conservation laws, including:

Conservation of electric charge. The net charge before and after is zero.

Conservation of linear momentum and total energy. This forbids the creation of a single photon. However, in quantum field theory this process is allowed; see examples of annihilation.

Conservation of angular momentum.

Conservation of total (i.e. net) lepton number, which is the number of leptons (such as the electron) minus the number of antileptons (such as the positron); this can be described as a conservation of (net) matter law.

As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general by elastic scattering.

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.