Model 422 and 423 (UHV) Rotatable Photodiode Detector
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Model 423 UHV Rotatable Photodiode Detector is a ultra high vacuum, bakeable, windowless, and rotatable photodiode. With a standard metal photocathode, it is capable of detecting radiation of wavelengths shorter than 1500 A. Typical quantum efficiencies are 10% at 584 A and 6% at 1216 and 304 A. However, the use of coatings such as MgF2 can produce yields approaching 40% at 584 A and 304 A. The useful photocathode area is 18 mm x 15 mm.
The photocathode can be rotated in and out of the radiation beam to allow for in situ calibration of the radiation. The rotatable support can accommodate four different types of cathodes, if desired, or can be used for mounting filters.
The diode is mounted in a stainless steel, double sided, metal sealed flange compatible with standard 2-3/4" Varian Conflat flanges. The ceramic-to-metal electrical feedthroughs ( MHV bayonet type) are mounted in mini-Conflat flanges. The entire assembly can be baked to 450' C. The rotary mechanism is an ultra high vacuum bellows sealed unit. The photocathode is demountable to allow for cleaning or for over-coating with any desired material.
Advantages Si photodiodes have over tube type detectors are:
- Small size
- Ease in experimental implementation
- Insensitivity to minute surface contaminants and magnetic fields
- Ultrahigh vacuum and cryogenic compatibility
- A large dynamic range
Single and multi-element Si diodes with no window or coating have successfully been used in a variety of applications. The multi-element array, for instance, found use at the Ring Accelerator Experiment (RACE) at Lawrence Livermore National Laboratory to obtain radiated power vs. length and radius profiles of the plasma.
The National Institute of Standards and Technology (NIST) has a working program for calibration of similar detectors. They have posted some typical response and parameter data on line.
References:
"Photocathodes: Their Efficiency and Stability" by James A. R. Samson - Nuclear Instruments and Methods in Physics Research 222 (1984) 215-220
"Metal Photocathodes as Secondary Standards for Absolute Intensity Measurements in the Vacuum Ultraviolet" by R. B. Cairns and J. A. R. Samson - Jurnal of the Optical Society of America, Vol 56, No 11, 1568-1573 (1966)