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VACUUM PHOTODIODES Content • Definition •Historic of Vacuum Photodiodes •Principles and Operation mode of vacuum photodiodes • Detector properties •Advantages and disadvantages •Limitation of vacuum photodiodes •Applications

Definition • Vacuum photodiode is a photo detector that utilize the external photoelectric effect.

• External photoelectric effect: Generation of free electrons when photons strike the surface of a metal. Electrons are emitted from the surface.

• Photo detector is a device which absorbs light and converts the optical energy to measurable electric current.

The circuit shematic of vacuum photodiode is shown below:

Vacuum photodiode

Historic of Vacuum Photodiodes •Historically, vacuum photodiodes were extremely important during the 1970's and 1980's when conventional silicon x-ray diodes had a dead layer that prevented them from detecting UV radiation. They were used for temperature studies of fast laserproduced plasmas, for impurity measurements on tokamaks that did not emit significantly in the soft x-ray regime and for the study of edge-plasma phenomena such as plasma-wall interactions and H-mode phenomena. In the early 1990's, International Radiation Detectors manufactured silicon photodiodes without a dead layer, and since then silicon diodes have been adopted as the standard for soft x-ray work.

However, vacuum photodiodes are still in use and are being considered for fusion-grade tokamaks such as ITER and JET where the radiation levels are too strong for non-metallic components.

Principles and Operation mode of vacuum photodiodes • Principles Vacuum photodiodes work on the principle of conversion of photons to electrons. Electrons are freed from the surface of metal by the energy absorbed from an incident stream of photons. However, it may absorb photons only if the energy of incident photons is above a certain minimum threshold.

Principles and Operation mode of vacuum photodiodes • Operation mode Vacuum photodiodes operate on the photoelectric effect: sufficiently energetic photons strike a metal plate, called the cathode, and eject electrons that are then collected at the anode as shown in the figure below. The resulting current is measured and gives an estimate of the incident photon flux.

A vacuum photodiode

Figure1: A vacuum photodiode consists of a cathode, in this case a metallic disk, that photo-emits electrons when irradiated by EUV photons, depicted here by the blue incident column. The emitted electrons are then collected at the anode, a metallic cylinder coaxial with the cathode. The photocurrent from cathode to anode is measured and gives an estimate of the photon flux

Principles and Operation mode of vacuum photodiodes • In fact once electrons are liberated from a photoemissive surface (a photocathode), they can be accelerated to an electrode positively charged with respect to the cathode–the anode–and generate a signal current. If the acceleration of photoelectrons is directly from cathode to anode through a vacuum, the device is a vacuum photodiode. Because the electrons in such a device take a very direct path from anode to cathode and can be accelerated by high voltages– up to several kilovolts in a small device–vacuum photodiodes have the fastest response of all photo-emissive detectors. Risetimes of 100 ps or less can be achieved.

Detector properties • Responsivity - Ratio of output current to input optic power - ρ = i/P of units ampere/watt • Spectral Response - Curve of the detector responsivity as a function of wavelength - Because of the rapid change in responsivity with wavelength, different detectors must be used in the windows of optic spectrum where fiber losses are low • Rise Time - The time for the detector output current to change from 10 to 90% of its final value

Advantages and disadvantages Vacuum photodiodes have an excellent time response due to the nearinstantaneous nature of the photoelectric effect, allowing for subnanosecond time resolution. Their simplicity and compact size allow many detectors to be fielded on an experiment.

They are also: -Good detectors for the very low light levels - Risetimes of 100 ps or less can be achieved - They are relatively bulky and fragile and easily damaged by excess light or voltage - Limited dynamic range - Relatively low quantum efficiency - Require a high voltage supply - Compact, robust, monolithic - Very sensitive to temperature and excess bias - Problem in SPC (single photon counting) mode-phenomenon of after pulsing is a byproduct in this device

Limitation of vacuum photodiodes • External connections and electronics are generally the limiting factors in obtaining short risetimes from vacuum photodiodes . However, they are not very sensitive, since at most one electron can be obtained for each photon absorbed at the photocathode. In principle, of course, the limiting sensitivity of the device is set by its quantum efficiency.

Practical quantum efficiencies for photo- emissive materials range up to about 0.4. If the space between photocathode and anode is filled with a noble gas, photoelectrons will collide with gas atoms and ionize them, yielding secondary electrons. Thus, an electron multiplication effect occurs.

However, because the mobility of the electrons moving from cathode to anode through the gas is slow, these devices may have a long response time, typically about 1 ms.

Applications • Instruments for the measurements of Power and Energy • Used for investigations into plasma density perturbation • Used in detection of vacuum uv • Used for applications such as broadband measurements, calibration,dosimetry, spectroradiometry