Yes, yes, yes get a helmet, immediately. Both of my worst snowboard accidents were head injuries and because I had no helmet. The first one was not even (totally) my fault. I fell pretty lightly on my toe edge, landed nice and soft on my stomach/forearms, which was only unfortunate when the guy behind me rammed the top of my head with his snowboard as he tried to stop and not hit me (this was on the side of the trail btw, not the middle). Almost blacked out on that one and had to take a sled ride. No concussion, thankfully.
The second time was again my toe edge, trying to stop and caught the heel and got whipped backward and slammed my head on some nice hard pack. Again, no concussion but definitely ruined my night. After that one I went straight down to the ski shop right there at the mountain, despite being overpriced. I will never ride without one again.