Why do I need to move the detector in a Goniometer to 2-omega?

[JanSpintronics]

Hola,

I have a question to a Goniometer or more specific of a Bragg-Bretano- measurement, where you need to move your detector in die double angular speed then you move your Röntgen source. Why is it so? In my thoughts, it makes sense to move them in the same speed of the röntgen source and you will get always the intensity. Because if you have an angle of $\theta$ which is the angle of your primary beam, your reflected beam is just taking the same angle. Why you need to move it just the double angle here?

 

[BvU]

Hi Jan,

you need to move your detector in die double angular speed then you move your Röntgen source

Says who ? How is your detector angle defined ?
Picture on sheet 21 here agrees with you

 

[Charles Link]

The link that @BvU provided (sheet 21) answers this completely: If the sample is rotated (rather than the source), then the angular speed is $2 \omega$. If the source is rotated, then the angular speed is $\omega$.