The updates 3D layout will show the following

The material type was set to Absorb to make the detector opaque rather than transparent, evident from the layout.

Since we referenced the Detector #7 to the Rectangular Volume and set the Z position as zero, the Detector is located at the front surface of the lightpipe. We want to place this detector 10mm to the right (+Z) of the lightpipe, hence the Z position value should be 2010 mm (thickness of Rectangular Volume + 10). If  we change the thickness of the Rectangular Volume to a different value, the Z position of the detector #7 should also be changed. For convenience, instead of typing the value 2010 mm in the editor, we will place a “Pickup solve” for the Z position of the detector. Then the Z position value in the editor will automatically be calculated to be 10 plus whatever the thickness of object 6 is.

Double or right click on the Z position parameter of object #7 in the editor to open the solve window.

Type the following parameters.

The Parameter #0 in the Non-Sequential Component Editor corresponds to the “Material” column, so for the Rectangular Volume object, parameter #3 corresponds to “Z Length”.

After pressing OK, a letter “P” will appears next to the parameter in the editor indicating the presence of Pickup solve.