Question

3. Besides the major types of radioactive decay, there are two others: positron emission and electron capture.

Answer 1

Answer 1: The electron is a subatomic particle that has a negative charge. Electron cannot be sub-divided further. An electron has a mass that ia ~ 1/1836 that of the proton.
The positron is also refereed as an antielectron. The positron has the charge of +1e, a spin of 1⁄2. The similarity between positron and electron is that they have same mass. 

Answer 2: Positron decay occurs in nuclei having heavy mass. Positron emission decreases proton number relative to the neutron number. Positron also exhibits nuclear transmutation thereby producing an atom of a  different element, as compared to parent element,. Them atomic number of daughter element is less as compared to parent element by 1. 

Example: 23 12Mg   → 23 11Na          +           e+         +   energy

Answer 3: Electron capture is also referred as K-electron capture. It involves absorption of  energy, thereby resulting in loss of electron usually from K or L shell. In this process,following thing occurs simultaneously
i) a nuclear proton present in nuclei changes to a neutron, after reacting with an electron which falls into the nucleus from one of its orbitals (preferentially from K or L)
2) Emission of an electron neutrino.
Following is an example of K-electron capture.
7 4Be      +     0 -1e      →      7 3Li      +   energy

Answer 4: Following are the distinct features of transmutations caused by positron emissions and electron capture.
i) Positron: A proton get converted into neutron due to radio-active process. This process results in the emission of positron and neutrino. The process results  in decrease in atomic number by one unit, and however atomic mass number remains unchanged.
ii) Electron capture: A electron from the low energy level (K-shell or L-shell) falls into the nucleus.  Due to this proton is converted into neutron. During this process, neutrino is emitted from the nucleus. As seen in positron emission, the atomic number goes down by one unit, but atomic mass number remains unchanged.