The 9821B is a high-performance photomultiplier tube designed for precision applications requiring fast timing and high sensitivity. With a diameter of 78 mm (3 inches), this end-window photomultiplier is equipped with a blue-green sensitive bialkali photocathode. The photocathode is mounted on a plano-concave window, which facilitates efficient light collection. The device features 12 beryllium copper (BeCu) dynodes arranged in a linear-focused design, ensuring excellent linearity and timing characteristics. This makes the 9821B an ideal choice for applications in high-energy physics studies and scintillation spectroscopy.

The photomultiplier is engineered to deliver superior single electron response (SER) and high pulsed linearity. It operates effectively within a spectral range of 285 to 630 nm, with a peak quantum efficiency of 30%. The luminous sensitivity of the 9821B reaches up to 75 µA/lm, making it highly sensitive to low light levels. The device is designed to perform optimally even in challenging conditions, maintaining a low dark current at 20 ºC and offering reliable performance across its operational range.

With its robust construction and advanced design, the 9821B photomultiplier is built to withstand rigorous use. It boasts a nominal anode sensitivity of 500 A/lm, which can be increased to a maximum rated anode sensitivity of 2000 A/lm. The photomultiplier's timing capabilities are highlighted by a single electron rise time of 2.1 ns and a full width at half maximum (FWHM) of 3.2 ns, ensuring rapid and accurate detection of fast events. Additionally, the 9821B is designed to minimize the effects of external magnetic fields and temperature variations, making it a reliable component in precision measurement systems.