I have this old-school copper pipe riser with a sprinkler head on top. However, last night I noticed the sprinkler head had gone missing and water was basically just shooting up into the air. I looked around to see if I could possibly find the missing part, but I seem to be unable to locate it. I suppose the water pressure theoretically could have launched it a ways away. Anyway...I don't know what parts I need to fix this. Looks like the width of the pipe is 1/2" in the thread area.

and here's a picture from the side.

Thanks in advance!