Hi Maya,
You forgot to make text_2D invisible in the beginning.
Code:
text_2D = viz.addText('radiation source area', viz.SCREEN )
text_2D.setPosition(0,0,20)
text_2D.color(viz.RED)
text_2D.visible(viz.OFF)
Furthermore: note that your indentation of your EnterProximity and ExitProximity functions is not a tab, but two spaces. Python does not mind it that much, but it would be more consistent to use one tab as indentation level throughout your code.