Impedance matching for broad bandwidth voltage amplifier
You should read the manual for your detector:

This detector is designed to drive a 50-ohm load. Since your amplifier has a 50-ohm input, there is no matching network required. You can just connect the detector to the amplifier through a 50-ohm cable.
In comments you added,

I plan to measure from about 5 MHz to 100 MHz

You should probably include a 100-150 MHz low pass filter in your system to eliminate noise that your 1.2 GHz detector and 1.4 GHz amplifier will be producing outside your signal band.