[quote]I tried to get the thorn hand cannon but after I completed the strike (casseme of screams ) I was not rewarded with the thorn[/quote] Because you didn't fulfill the quest's requirements. You're supposed to kill the Arbiters that spawn during the final phase of the encounter instead of killing the boss as quickly as possible.

Did you kill the 2 yellow bar knights in the boss room? They spawn in once the boss is down to half health.