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How did Rutherford bombard the atom with alpha particles when discovering the nucleus in the Gold Foil Experiment?

I’m confused of how Rutherford 1. isolated an alpha particle, and 2. How did he even know what an alpha particle was or even measure it?? If they hadn’t discovered the nucleus yet how was it possible for him to know to shoot 2 protons and 2 neutrons at the atom? I’d really appreciate if someone could explain this as I haven’t found anywhere that explains these details. Thanks for your help!!
gpt
Rutherford's early work was on different types of radiation. Radioactivity had already been discovered several years earlier, and what Rutherford and others did is categorize it. They found that radioactive substances gave off three distinct rays that they called alpha, beta, and gamma. They understood that alpha rays were positively charged (because they could be deflected in a magnetic field), and that the rays had considerable mass relative to other forms of radiation. Because it seemed to have about four times the mass and twice charge of a hydrogen atom, he figured it was a helium ion, and later that was shown to be the case. Later he and his students and collaborators developed a way to measure how many alphas you had by relying on the fact that they would ionize a gas in a tube, and you could measure that electrical current (a Geiger counter). 

The alpha particles they were using for the gold foil experiment were provided by a radium source. If you want a stream of alpha particles, you build a little metal box, put the radium source in it, and put a tiny hole in the side of it. Only alphas traveling in the direction of the hole will be released, so it acts like a little gun for alphas. 

The experiment consisted of shooting alphas at a very thin gold foil. Most of the alphas went right though. But some were reflected backward at a very sharp angle. By measuring this angle and the number of alphas, he was able to conclude that these were basically bouncing off something very hard and very small. From this he then extrapolated that most of the atom's mass was in a hard nucleus, and could show (through the math) how small the nucleus had to be if it was reflecting the particles at the rate and direction that it was. It is a very elegant experiment and interpretation.

(This experiment was performed by Hans Geiger and his student Ernest Marsden, but Rutherford interpreted the results and used them to show that the nuclear model of the atom was most likely. Rutherford was in contact with Geiger and helped him plan the experiment, which was one of a series of experiments Geiger was doing on alpha particles.)

Lastly, he didn't know about neutrons yet. Those would come later. There were still many unknowns. He didn't even really know about protons — he just assumed, after finding evidence for the nucleus and understanding that different ions had different charges, that they must have positive subatomic particles in them. Rutherford's idea of the atom was one step along the way to our modern understanding of the atom; it wasn't the final step.