Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-06-26
2004-01-06
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C313S536000
Reexamination Certificate
active
06674063
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to photosensors, and more particularly to a photosensor that incorporates high quantum energy photocathodes operated in reflection mode.
2. Description of the Background Art
Research in the area of high-energy physics and high-energy astrophysics is often based on the detection of Cherenkov or fluorescence photons emitted by charged particles in a wide variety of transparent media. Efficient photon detection is a key element that is common to this work.
At present, conventional PhotoMultiplier Tubes (PMTs) are used in most of these applications because they combine affordability with good timing properties. However, PMTs tend to suffer from poor single-photon resolution and low quantum efficiency. Furthermore, PMTs based on reflective photocathodes have reached a very limited application, mainly because of their large dead area and non-uniform angular acceptance.
Hybrid Photon Detectors (HPDs) provide excellent single-photon resolution, but by) fail to offer higher quantum efficiency because they tend to be based on the same photocathode materials as PMTs. On the other hand, HPDs housing GaAs and GaAsP photocathodes in transmission mode have been recently developed that offer peak quantum efficiency of up to approximately 50%. However, due to the complicated Molecular Beam Epitaxy (MBE) process used in manufacturing, these HPDs have been very expensive. In addition, they suffer from a large dead area and difficulties in the formation of compound hexagonally packed imaging cameras. An imaging HPD based on a reflective photocathode formed in a bend structure has been developed as well, but has been relatively difficult to manufacture and suffers from narrow angular acceptance and poor resolution in response time.
In a typical transmission-type photocathode, photons enter the photocathode layer from one side, while the photoelectrons emerge into vacuum on the opposite side of the layer. This concept is far from being optimal since the electrons have to be emitted at a surface that is far from the place of their most abundant creation, i.e., the surface on the opposite side of the layer. Electrons have a relatively low chance to diffuse from the region close to the photon entry surface to the surface on the opposite side. The design of a transmission photocathode has therefore always been a compromise between two conflicting requirements: (a) efficient photon conversion and (b) successful electron diffusion to the surface.
The situation is fundamentally different in reflection-type photocathodes since the electrons are emitted through the same surface the photons have entered. The majority of electron-hole pairs are created very close to the photon entrance surface (due to the Lambert-Beer exponential law) and therefore have a high chance of reaching the same surface and escaping though it into vacuum. As a result, reflection cathodes offer quantum efficiency nearly twice that of transmission photocathodes. The sensitivity to ultraviolet (UV) light is enhanced even more, since the short wavelength photons are absorbed closer to the surface.
Apart from a considerable increase in quantum efficiency and an important widening of the spectral response into the short wavelength range, reflection photocathodes offer other very important advantages. The most important advantage is the significant simplification of the photocathode manufacturing process, and a consequent price reduction. This general feature in particular concerns the most efficient but extremely expensive III-V semiconductor photocathodes (e.g., GaAs, GaAsP, InGaAs, etc.) processed by MBE expitaxial growth in an ultra high vacuum. A typical production process for transmission-type III-V photocathodes consists of approximately ten different steps, starting from expitaxial growth of a thin photocathode layer on top of a crystal substrate with matched lattices constant, fusion of the grown structure to the photube entrance window with the help of previously MBE-deposited additional interface layers, and finally removal of the growth substrate from the opposite side of the photocathode layer.
In contrast, the production of a reflection-type III-V photocathode is much simpler since there is no need to fuse the grown photocathode structure with the glass window and to remove the growth substrate. This leads to a very significant cost reduction that is likely to bring the III-V photocathodes into an affordable price range, with unprecedented high quantum efficiency (e.g., for GaAsP, approximately two to three times higher than that of transmission bialkalai photocathodes). In addition, while for a typical transmission-type photocathode a thick conductive sub-layer has to be deposited between the glass window and the photocathode, in a reflection-type photocathode the thickness and the optical properties of this conductive sub-layer are not critical since photons do not need to pass through. Although only around twenty atomic layers thin when used with transmission photocathodes, this sub-layer (e.g., SnO or indium-tin-oxide) absorbs about 25% of the incoming light, which presents a significant loss even before the light has reached the transmission photocathode. In contrast, reflection photocathodes may even benefit from the conductive layer underneath, since it may serve as a mirror that reflects transmitted light back through the photocathode layer, thus providing the photon with another conversion opportunity.
In spite of these striking advantages, reflection-type photocathodes have never had a wide application in photosensor devices due to the lack of a phototube design that would simultaneously host a photocathode in a reflection configuration, and provide the following important features: (a) negligible dead area; (b) flat angular acceptance and sharp angular cutoff for detected light, (c) fast and position-independent time response, and (d) the possibility of close packing of individual units into large-area multi-pixel honeycomb imaging cameras.
Accordingly, there has been a strong need for a new kind of photosensor that would be able to incorporate photocathodes of the highest quantum efficiency at a relatively low cost. Such photosensors may replace conventional PMTs in many applications (e.g., physics, astronomy, industry, medicine) where high quantum efficiency, single-photon resolution, negligible dead area and other important properties are required. Among the different photocathode types, epitaxially grown III-V photocathodes (such as GaAsP, GaAs, and InGaAs) provide the highest possible quantum efficiency. As stated previously, for example, the quantum efficiency of GaAsP photocathodes may reach almost fifty percent.
Single-photon sensitivity, single-photon resolution, excellent time resolution, low noise, and, most important, high quantum efficiency (e.g., >50%), are key features of an ideal, but so far nonexistent photosensor. A photosensor comprising all of these qualities at a low price would open a new range of sensitivity in different frontiers, such as photon decay, neutrino oscillations, neutrino astronomy, gamma-ray astronomy, and the like. The present invention satisfies those requirements, as well as others, and overcomes deficiencies found in conventional photosensors.
BRIEF SUMMARY OF THE INVENTION
The present invention generally comprises a photocathode operating in the so-called “reflection mode” instead of the traditional transmission mode. By way of example, and not of limitation, a photosensor according to the present invention combines reflective mode photocathode technology with compound parabolic light concentrator (CPC) technology wherein the same vacuum tube components act both as an incoming light concentrator and as a focusing electron lens. The interior of the CPC is electrically conductive and split into two electrodes by a non-conductive interval, such that photoelectrons emitted by the photocathode are electrostatically focused by the same CPC-shape onto a small light collection surface at entr
Allen Stephone B.
O'Banion John P.
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