In what circumstances do scientists use observational research and opinion based research?

Observational research is used in early stages of scientific research because it gives us a rough estimate and a general idea at what variables should be look at when we're trying to discover something new.

Opinion based research works through the same principle, someone who is an expert in a field can tell us what does he think could be researched next. Both of these pieces of information are only used as a general idea and not as something definitive.

No. an alpha particle is a pair of protons and a pair of neutrons, i.e. a helium