Mathematica Taking only the positive sqrt in Mathematica

[Sagekilla]

As my title says, I need to take only the positive square root of some evaluation. For my calculation, the negative square root is extraneous.

What I do is a normalization of a wave function:

J = Integrate[Psi[x]^2, {x, 0, a}]
sol = Solve[J == 1, A]
A /. sol

My only problem is those above expression will give me A = {-1/sqrt(2), 1/sqrt(2)}.

How can I force Mathematica to only give me positive square root?

 

[FunkyDwarf]

DeleteCases[A,x_/;x<0]

This will ofc only work with expressions mathematica can evaluate numerically or knows to be positive or negative.