As shown in the figure, the detector is tilted by a certain angle which is about 45 degree. 
The tilted angle is defined as angle B.
We measured angle A using the vertical laser line (red) and papers with a straight line edge. So we can marked
the line denoted by the laser line when the paper is put on the holder.
In the beginning, the angle A is measured as 43.3±0.5 degree. So angle B= 46.7±0.5 degree
After the adjustment of the detector (to make it in the center of the chamber), the angle A is measured as
43.1±0.5 degree with another paper.  So angle B= 46.9±0.5 degree.