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A proportional counter is a type of electromagnetic radiation/particle sensor that senses the energy of each particle (photon or otherwise) that is suitable for detecting and measuring X-ray photons. They have been used as sensors for some X-ray telescopes.
Proportional counters operate similarly to Geiger counters or ionization chambers, but are different in detail in order to carry out the measurement. A proportional counter includes a chamber of gas that is ionized by the incoming particle such that multiple gas atoms are ionized until the incoming particle's energy is spent. Electrodes with a voltage between them maintain an electric field throughout the chamber, and the created ions allow current to flow, the ions reaching the negative electrode and returning to neutral. The amount of current allowed is amplified by an avalanche effect, i.e., ions and electrons drawn toward the electrodes producing more ion/electron pairs. External electronics measures and records the resulting current, which is proportional to the photon energy. This results in detection and measurement of each incoming photon as long as the rate of incoming photons is sufficiently low to detect them individually. My impression is proportional counters can be built that can count hundreds or thousands of photons per second individually.