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Cesium Telluride Photodiodes
|Cesium Telluride (CsTe) Windowed Photodiodes
Cesium telluride (CsTe) photodetectors with magnesium fluoride (MgF2) windows are issued by NIST as transfer standard detectors in the spectral region extending from 116 nm to 254 nm. The photocathode is a semitransparent film of cesium telluride which emits photoelectrons when exposed to ultraviolet radiation. The cesium telluride film is deposited onto a MgF2 window and then mounted on a ceramic body tube and sealed with an indium eutectic alloy. The entire fabrication process is carried out under ultra high vacuum, and the interior of the ceramic tube remains under vacuum for the lifetime of the detector. The ceramic body has three electrical contact rings on it: one for the photocurrent from the CsTe film, one for guarding against leakage currents from the anode to the cathode, and one for the anode connection.
The sensitivity of a windowed photodetector is typically 0.05 to 0.20 electrons / photon throughout the calibration region from 116 nm to 254 nm. Calibration uncertainties (2-sigma) are 2% over most the calibration region, but reach 9% at the shortest wavelengths. These detectors have significantly reduced response to visible radiation (two or more orders of magnitude lower than the ultraviolet response), and are insensitive to radiation with wavelengths shorter than the MgF2 absorption edge around 113 nm. They offer extremely low dark current and noise characteristics. However, the device efficiency may be degraded if the window becomes contaminated in the user's vacuum system or darkens from exposure to radiation in the MgF2 absorption region (wavelengths less than 113 nm).
A fee schedule and ordering information are available.
In addition to these CsTe detectors, the Vacuum UV Transfer Standard Program issues other types of transfer standard detectors:
Detector Construction and Circuit Schematic
Radiation incident on the photocathode through the MgF2 window generates photoelectrons in the cathode. An anode biased 150 V above ground generates an electric field that enhances the emission of electrons from the film.
Typical Efficiency of a NIST CsTe Transfer Standard
Detector efficiency is expressed in terms of electrons per incident photon, and is a function of the incident radiation's wavelength.