Proportional Counters

The Assembly of RXTE's Proportional Counter Array
The Proprotional Counter Array from NASA’s RXTE satellite during assembly

One of the early “workhorses” of X-ray astronomy was the proportional counter. A proportional counter is essentially a sealed container with a window where X-rays enter. Inside the container is a gas and electrodes. An X-ray entering the container will interact with the gas, causing a cascade of electrons at the electrodes.

When an X-ray enters through the window, it runs into the gas and ionizes the gas, producing a number of electrons and positively charged atoms of the gas. This is where the electrodes come in – the electrodes set up a strong electric field, and the electrons zoom across the gas chamber toward the positively-charged electrodes (also known as anodes). If they’re moving fast enough, the electrons will ionize other atoms of the gas on their quick trip across the detector, freeing more electrons. Those newly-freed electrons will also zoom across the detector, ionizing more gas atoms on the way, freeing more electrons which ionize more gas atoms which….you get the idea.

In the end, a huge number of electrons will reach the anodes – the number can be in the thousands, depending on the gas used and the energy of the incoming X-ray photon. This process turns the tiny interaction of the X-ray with the gas into a relatively large signal at the anode.

The number of electrons that reach the anode is proportional to the energy of the incoming X-ray. (Recall that energy is another way of measuring the wavelength or frequency of light.) This means that using a proportional counter, scientists can find the spectrum of X-rays from an object in space – that is, they can find out how many photons at each energy (or wavelength) are coming from that object.