[pstricks] size of circle nodes in psmatrix

[Manjusha Joshi]

Hi,
I am trying  following code in which I am using \psmatrix environment.
I set mnode=circle. And Inside node I am writing small letters like a, b, c, 
d, e, f etc. As soon as f (or j, b,d, l etc.) comes the circle around it 
becomes larges than other letters. I want same size for all letters. I tried 
radius option also. But it works only when node has noting inside it.
Is there any way to get same size of nodes i.e I want to set radius for all 
circles to be same. I adjust it with \; and ~ but all the time doing it 
manually is time consuming.
-Thanks
Manjusha 

\documentclass[a4paper,12pt]{article}
\usepackage{pstricks,pst-node,pst-tree,pstricks-add}
%opening
\begin{document}
\SpecialCoor
\pagestyle{empty}
$
\psmatrix[colsep=.5cm,rowsep=.5cm,mnode=circle,radius=5pt,nodesep=0pt,ArrowInside=->,arrowscale=2]
a&b&c&f
\endpsmatrix
$
\end{document}