Right rear strut looks broken, just not sure what part to get

I've been hearing a loud boom every time I go over a bump in the road from the right rear wheel area for the past few days until I finally decided to jack the car up and look to see what was the problem. It's pretty obvious that there's a giant split in what looks to be the strut. I would like to get the right part just not sure what it would be. Would ordering new struts work? Or is that the whole strut assembly that is broken? I'm also interested in lowering the car so maybe new struts would help anyways. Thanks