Hi Peng,

Thanks for your post here!

There is a Front Only parameter for Detector Rectangle objects, if it is set to 0, then rays may strike the detector from both ront and back sides. You will need to set the Front Only parameter to 11 to ignore rays coming from the back.

You might also want to set the detector material to ABSORB, so that rays are absorbed once they strike the detector.

You may find more info about the detector settings in the Help system at:

The Setup Tab > Editors Group (Setup Tab) > Non-sequential Component Editor > Non-sequential Detectors > Detector Rectangle Object

If these doesn't solve your problem, could you please share more details of your system or share the file so that we can take a closer look?

If you don't want to share the design here on the forums, you can also open a case with us.

Best,

Csilla