Question
I am measuring in myQA Accept and get a steep drop in the penumbra and the tail seems to be up-side down (mirrored at the x=0 axis) as in the image. What can I do?
Answer
There are two possibilities that can cause this effect.
1. The background was not measured correctly - therefore, the background subtraction leads to a negative relative dose - as negative relative dose values are not logical myQA Accept takes the modulus |x| of the values - hence it seems as if the values are mirrored at x=0. The steep drop/ peak appears.
2. The CCU is placed to close to the accelerator (primary beam) - hence the scatter radiation leads to a current in the electrometer which is "seen" as negative current/ dose. As this is not logical myQA Accept takes the modulus |x| of the values - hence it seems as if the values are mirrored at x=0. The steep drop/ peak appears.
In order to get rid of the steep drop of please re-measure the background and make sure that the beam is off. Furthermore, put the CCU as far as possible away from the primary beam.