Remember that RXTE is a satellite orbiting the Earth. The detectors collect X-rays from celestial objects found in the direction they are pointed.

When an X-ray hits the PCA it strikes one of the five mylar-foil covered boxes (PCUs). The X-ray then passes through the foil to the gas inside.

Inside the foil-covered box is Xenon gas. The interaction of the X-ray photon with the Xenon gas is the basis of the X-ray detector.