[Zope] Usersession - how to get one sessionid per open browser?

Hi all, When I open a different browser from a link in one of my pages (<a href="newPage" target="_blank" . . .) I get the same session Id for the new browser window. I would like to treat each open browser as a different session - how do I do this? Surely if I invalidate the session manually on opening the new browser, I will detroy the session data for the other browser (which I don't want to do). Thanks Etienne