The word Σατανα is always translated as Satan in the New Testament, at least in the ESV version that I have looked up. The following verses use this word to indicate Satan in the gospels:
Then Jesus said to him, “Be gone, Satan! For it is written, “ ‘You shall worship the Lord your God and him only shall you serve.’ ”
But he turned and said to Peter, “Get behind me, Satan! You are a hindrance to me. For you are not setting your mind on the things of God, but on the things of man.”
And he was in the wilderness forty days, being tempted by Satan. And he was with the wild animals, and the angels were ministering to him.
But turning and seeing his disciples, he rebuked Peter and said, “Get behind me, Satan! For you are not setting your mind on the things of God, but on the things of man.”
Apart from the gospels it is also used as Satan as found in:
Acts 26:18, 1 Cor 5:5, 2 Cor 2:11, 2 Cor 12:7, 2 Thess 2:9, 1 Tim 1:20, 1 Tim 5:15, Rev 2:9, Rev 2:13, Rev 2:24, Rev 3:9
I personally do not see any reason why the severity of the rebuke should be of any alarm. ‘Get thee behind me’ is Jesus putting the suggestions of Satan, who had whispered this idea to Peter, out of the way. It is the same as saying ‘Be gone, Satan!’ In Matt 4:10. The only difference is that when Jesus said 'Be gone' the Devil was tempting Jesus to do something wrong. In this instance Satan is trying to get Jesus 'not to do something he must', therefore the Devil was standing directly in front of Christ's straight path. He must get behind him, if Jesus was to continue his mission.
Peter was considering only the outward part of his sufferings, with the shame and scandal that it would cause and did not think of it according to faith. Ironically, he had just made his confession of faith which shows how easy a spiritual view can be turned into a carnal one, with the Devil seizing upon the weakness of our ignorance to insert his own suggestion through even godly men.
Clearly this was one of the most evil suggestions ever offered to Jesus by any man, thus it was from the Devil. Peter was opposing the very purpose for which Jesus came into the world with reasons that would be appealing to Jesus as a temptation. This was the Devil putting forward this idea so why not look past Peter and label it for what it was.