Photocathode
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A photocathode is a negatively charged electrode in a light detection device such as the input screen in an image intensifier (II) that is coated with a photosensitive compound. When this is struck by light photons, the absorbed energy causes electron emission due to the photoelectric (PE) effect.
Photocathode in an image intensifierComposition
In an II, the photocathode is a thin layer of an alloy of antimony &and caesium (SbCs3)
. The photocathode converts the light photons emitted by the inputentrance phosphor to photoelectrons via the PE effect.:
Electrons
-
electrons are produced in direct proportion to the brightness of the input screen
This process has a - 10-20% conversion efficiency, i.e. approx. 400 electrons are released from the photocathode for each 60 keV x-ray photon absorbed by the phosphor
.Other II considerations
In order to preserve the image faithfully, the input phosphor & photo cathodeand photocathode must be in intimate contact with each other, separated only by a thin transparent wafer which prevents chemical interaction between the 2two layers.
The light spectrum of the input phosphor should also match the sensitivity profile of the photocathode, to maximise the conversion efficiency from light photon to photoelectron.
-<p>A <strong>photocathode </strong>is a negatively charged electrode in a light detection device such as the input screen in an <a href="/articles/image-intensifier">image intensifier</a> (II) that is coated with a photosensitive compound. When this is struck by light photons, the absorbed energy causes electron emission due to the <a href="/articles/photoelectric-effect">photoelectric (PE) effect.</a></p><h4>Photocathode in an image intensifier</h4><p>In an II the photocathode is a thin layer of an alloy of antimony & caesium (SbCs3)</p><p>The photocathode converts the light photons emitted by the input phosphor to photoelectrons via the PE effect.</p><p>Electrons are produced in direct proportion to the brightness of the input screen</p><p>This process has a 10-20% conversion efficiency, i.e. approx. 400 electrons are released from the photocathode for each 60 keV x-ray photon absorbed by the phosphor.</p><h6>Other II considerations</h6><ul>-<li>In order to preserve the image faithfully the input phosphor & photo cathode must be in intimate contact with each other, separated only by a thin transparent wafer which prevents chemical interaction between the 2 layers.</li>-<li>The light spectrum of the input phosphor should also match the sensitivity profile of the photocathode, to maximise the conversion efficiency from light photon to photoelectron</li>-</ul>- +<p>A <strong>photocathode </strong>is a negatively charged electrode in a light detection device such as the input screen in an <a href="/articles/image-intensifier">image intensifier</a> (II) that is coated with a photosensitive compound. When this is struck by light photons, the absorbed energy causes electron emission due to the <a href="/articles/photoelectric-effect">photoelectric (PE) effect.</a></p><h4>Composition</h4><p>In an II, the photocathode is a thin layer of an alloy of antimony and caesium (SbCs3). The photocathode converts the light photons emitted by the <a title="Input phosphor" href="/articles/entrance-phosphor">entrance phosphor</a> to photoelectrons via the PE effect:</p><ul>
- +<li>electrons are produced in direct proportion to the brightness of the input screen</li>
- +<li>10-20% conversion efficiency, i.e. approx. 400 electrons are released from the photocathode for each 60 keV x-ray photon absorbed by the phosphor</li>
- +</ul><p>In order to preserve the image faithfully, the input phosphor and photocathode must be in intimate contact with each other, separated only by a thin transparent wafer which prevents chemical interaction between the two layers.</p><p>The light spectrum of the input phosphor should also match the sensitivity profile of the photocathode, to maximise the conversion efficiency from light photon to photoelectron.</p>
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