Head Shot = Instant kill for FPS enemies

My "Alien Zombies" have nice big fat heads and I'm wondering if it's possible to make it so that if they were shot in the head it would be an instant kill? As it is right now they have 100 hitpoints and my shotgun does 50 damage so it takes two shots to kill them which is fine. But I was thinking it would be more interresting or a bit more difficult if maybe I gave them some more hit points but somehow made it so that a head shot was an instant kill. Don't know if that's possible or not. They are animated characters made in Max 8 so they have a biped if that helps information wise any ;)

1 If your head is individual part of the zombie model, you just tag it to be "Head". When collision happens, if bullet is colliding with head, you give it 100 damage.

2 If your zombie model is all together. You could create a sphere collider with mesh rending disabled and place it at zombie head and make tag to "Head". Rest is same.

Add: That depends on how you calculate damage. If you use a collision detect way with bullet. You could add below script to your bullet:

function OnCollisionEnter(collision : Collision) {
if(collision.gameObject.tag=="Head")
   Player.DoDamage(100);
   Destory(this.gameOjbect);
}