- Email: [email protected]
A novel photosensor concept
Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
A novel photosensor concept Daniel Ferenc Physics Department, University of California Davis, One Shields Avenue, Davis, CA 95616-8677, USA
Abstract Reflection photocathodes offer quantum efficiency nearly twice that of transmission photocathodes, but have so far not been widely applied due to the lack of an appropriate photosensor configuration. This paper introduces an elegant and very powerful solution: reflection photocathode photosensor, named ReFerence. r 2001 Elsevier Science B.V. All rights reserved. PACS: 07.30.@t; 07.90.+c; 81.05.Ea Keywords: Photosensor; Photomultiplier tube; Vacuum; Photocathode; High-energy physics; Astrophysics; Medical imaging
1. Introduction Numerous present and future experiments in the fields of high energy physics and high energy astrophysics are based on the detection of Cherenkov or fluorescense photons emitted by charged particles in a widest variety of transparent media. The key element common to all these experiments is efficient photon detection. Singlephoton sensitivity, single-photon resolution, excellent time resolution, low noise and, most important, high quantum efficiency (>50%), are the key features of an ideal, needed, but so far nonexistent photosensor. A photosensor comprising all these qualities at a low price would certainly open a new range of sensitivity in different physics frontiers, like proton decay, neutrino oscillations, neutrino astronomy, gamma-ray astronomy, etc. The new photosensor concept introduced in this paper presents a very promising candidate. The presented photosensor may replace traditional Photomultiplier Tubes (PMTs) also in many other E-mail address: [email protected] (D. Ferenc).
applications, including industry, medicine, astronomy, etc. The classical PMTs, the present choice in most projects, combine affordable (though high) cost, with good timing properties, but still with two important drawbacks: poor single-photon resolution and low quantum efficiency. Hybrid Photon Detectors (HPDs) comprise excellent singlephoton resolution, but fail to offer higher quantum efficiency, since they have mostly been based on the same photocathode materials as PMTs. The two exceptions are the small-size HPDs by Intevac [1] and Hamamatsu [2] based on GaAsP photocathodes grown in Molecular Beam Epitaxy process (MBE). These photosensors offer peak quantum efficiency of almost 50%, but at an extremely high cost. The novel photosensor concept introduced in this paper offers a possibility to use a photocathode in the so-called reflection mode, instead of the traditional transmission mode. Reflection photocathodes offer much higher quantum efficiency with the same photocathode material, as elaborated in more detail below. But in spite of
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D. Ferenc / Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
that, they have never been widely used due to the lack of a photosensor configuration that would be able to host the reflection photocathode without drastic sacrifices in effective sensitive area, photon angular acceptance and time resolution. The presented photosensor configuration overcomes those problems and facilitates the long awaited significant increase in quantum efficiency, even providing further important improvements in some other relevant features.
2. Transmission and reflection photocathodes In the photocathode, optical power decays exponentially according to the Lambert–Beer law PðzÞ ¼ Pð0Þ expð@azÞ, where aðlÞ is absorption coefficient specific of the photocathode material and the wavelength of the light l [3] and z is the penetration depth in the material. The distribution of absorbed photons in the photocathode layer, i.e. the loss of optical power 2dPðzÞ, has the same shape as PðzÞ, so the distribution of created electron–hole pairs in the photocathode also follows the same exponential shape. In order to be emitted from the photocathode, the photogenerated electrons in the conduction band of the material must reach by diffusion to the photocathode surface. In the usual transmission-type photocathodes, 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 the two conflicting requirements: efficient photon conversion and 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 (thanks to the Lambert–Beer exponential law) and therefore have a high chance of reaching the same surface and escaping through it into vacuum. As a consequence, reflection photocathodes offer quantum efficiency nearly twice that of transmission photocathodes [3]. The sensitivity to UV light is enhanced even much 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 also other very important advantages. The most important one is in 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 (like GaAs, GaAsP, InGaAs and others), produced by MBE epitaxial growth technique in ultra high vacuum. A traditional production process of transmission-type III–V photocathodes consists of around 10 different steps, starting from epitaxial growth of a thin photocathode layer on top of a crystal substrate with matched lattice constant, fusion of the grown structure to the phototube 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 (for GaAsP about 2–3 times higher than of transmission bialkali photocathodes). In addition, while for a typical transmissiontype photocathode a thin conductive sublayer has to be deposited between the glass window and the photocathode, in case of a reflection-type photocathode the thickness and the optical properties of this conductive sublayer are not critical since
D. Ferenc / Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
photons do not need to pass through it. Although only around 20 atomic layers thin when used with transmission photocathodes, this sublayer (made of SnO or indium–tin–oxide) absorbs already about 25% of the incoming light [3], which presents a significant loss that sets in even before the light had reached the transmission photocathode. In contrast, reflection photocathodes may even benefit from the conductive layer underneath, since it may serve as a mirror reflecting transmitted light back through the photocathode layer, providing thus the photon with another conversion chance.
3. ReFerence photosensor In spite of these striking advantages, reflectiontype photocathodes have never had a wide application in photosensor devices. Essentially all PMTs presently on the market are of transmissiontype. So far the main problem in implementation of reflection-type photocathodes has been in the lack of conceptual phototube design that would simultaneously host a photocathode in reflection configuration, and provide the following important features: (i) negligible dead area, (ii) flat angular acceptance and sharp angular cutoff for detected light, (iii) fast and position-independent time response, (iv) the possibility of close packing of individual units into large-area multi-pixel honeycomb imaging cameras. With a recently proposed imaging HPD with reflection-type photocathode, applied on blends hanging from the entrance window of a phototube [4,5], surprisingly good imaging properties have been achieved, but narrow angular acceptance and non-uniform time response turned out to be the unavoidable drawbacks. The invention presented in this paper solves this problem in completely different, very simple and elegant way. In this special configuration, the same vacuum tube component acts both as a perfect incoming light concentrator and as a perfect focusing electron lens. The former assures that essentially all the incoming photons reach the photocathode, and the later enables all the emitted
231
photoelectons to hit the small semiconductor sensor. This phototube was named according to the performance expectations ‘‘ReFerence’’ (later it became apparent that it would fit also an acronym: REflective photocathode tube by FERENC). The ReFerence phototube is based on the recent discovery that a Winston cone, which provides the most efficient non-imaging light concentration [6,7], may simultaneously act as an ideal focusing electron lens. In most general terms, the ReFerence phototube, shown in Fig. 1, is a Compound Parabolic light Concentrator (CPC) or Winston cone [6,7] which concentrates all the light entering the cone from the left side through the entrance aperture (2), at an incidence angle smaller than a given design cutoff angle (301 in Fig. 1), to the light collection surface (3) covered by a reflection photocathode (6), and simultaneously focuses and accelerates photoelectrons (7) emerging from the photocathode in the opposite direction, onto a point-like electron sensor (8) placed in the middle of the entrance aperture. This electron sensor may be a P–i–N diode, an Avalanche Photo Diode (APD), or any other suitable sensor. It is enclosed in a positive-ion feedback protection electrode (10) [8–10]. While the light concentration is provided by the tube shape that assumes a standard Winston cone parabolic mirror (4,5), electron focusing is facilitated by the electron lens formed from the electrodes that follow the same Winston cone shape, but with the insertion of a single narrow non-conductive gap (9) that divides the cone into two electrodes (4) and (5). The existence and the position of this gap are of crucial importance for the functionality of the ReFerence photosensor. Electrodes (4) and (5) have to be kept at different electric potentials: U4 and U5. From electron optics simulations it follows that a correct electron focusing may be achieved with a continuous multitude of combinations of U4 and U5. This unusual feature offers the important freedom to tune the potential gradient in front of the photocathode surface, and thus allows the optimization of the quantum efficiency and the responsetime properties of the device.
232
D. Ferenc / Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
Fig. 1. Schematic view of the ReFerence photosensor. See text for explanation.
For correct operation, the electrodes (4), (5), (6), (8) and (10) must be kept at potentials rising in the following order: U6oU4oU5oU8oU10. The optimal values for these potentials, and the precise position of the non-conductive interval (9) may be determined empirically in extensive electron–optics simulations. The example presented in Fig. 1 is optimized for a cutoff incidence angle of 301. Equivalent results may be achieved also with other acceptance angles, ranging approximately between 101 and 501. Further, the example in Fig. 1 provides a theoretical maximum light concentration factor (factor 2 in diameter and 4 in area) for the accepted light with incidence angle smaller than 301 and the angular spread at the collection surface being maximal, i.e. 901 [6,7]. The ReFerence tube concept is not limited to CPCs with maximum concentration and may be practiced equivalently with CPCs of a lower concentration factor and a narrower angular spread of light at the collection surface [11]. That may be of certain advantage in
avoiding light reflections from the photocathode surface, but inevitably leads to the increase of the photocathode surface. The entrance section of the phototube may be formed in a hexagonal shape to facilitate hexagonal packing of individual units into a honeycomb structure, without sizeable influence on the electron focusing [11]. Very effective magnetic field protection shielding in the form of a cylinder or cone may be applied [11]. Since the electron velocities are already high in the vicinity of the entrance window, electrons are less sensitive to magnetic deflection, and the shielding does not need to extend beyond the front face of the ReFerence photosensor.
4. Summary In summary, the presented novel ReFerence photosensor concept based on reflection photo-
D. Ferenc / Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
cathode comprises the following important features: *
*
*
*
*
*
*
* *
*
*
*
Manifest conceptual and constructional simplicity. The highest possible quantum efficiency at a low manufacturing cost. Spectral sensitivity extended towards short wavelengths. Optimal usage of photocathode surface, thanks to the highest possible light concentration provided by the Winston cone phototube shape. Fast and position-independent time response [11], in spite of the flat photocathode surface (if needed, could be further improved with slight bending). Flat photocathode surface, required for epitaxially grown photocathodes. Single-photon resolution, thanks to the HPD concept. Negligible dead area. Flat angular acceptance and sharp angular cutoff for the incoming light. Possibility to modify lateral shape of the entrance section of the phototube from circular to hexagonal [11], without significant degradation of the electron focusing performance, and hexagonal honeycomb close packing into largearea imaging cameras. No need for additional light concentrators or stray light protection in applications. Efficient magnetic field screening without acceptance shadowing [11].
233
Acknowledgements Soon after the Stockholm Imaging 2000 conference we lost Tom Ypsilantis, a most exceptional man and physicist. The development of the presented idea has benefited a lot from Tom’s support and encouragement. This paper is devoted to the dearest memory of Tom Ypsilantis.
References [1] Intevac, Hybrid photomultiplier tube with high sensitivity, US Patent No. US5374826. [2] R. Mirzoyan, D. Ferenc, E. Lorenz, Nucl. Instr. and Meth. A 442 (2000) 140. [3] S. Donati, Photodetectors, Prentice-Hall PTR, Englewood Cliffs, NJ, 2000. [4] D. Ferenc, Nucl. Instr. and Meth. A 442 (2000) 150–153. [5] D. Ferenc, New developments in hybrid photon detectors, in: C. Williams, T. Ypsilantis (Eds.), Presented at New Detectors, Erice, Trapani, Sicily, Italy, November 1–7, 1997, World Scientific, Singapore, 1999, pp. 131–140. [6] W.T. Welford, R. Winston, The Optics of Nonimaging Concentrators, Academic Press, New York, 1978. [7] R. Winston, J. Opt. Soc. Amer. 60 (1970) 245. [8] D. Ferenc, D. Hrupec, E. Lorenz, Nucl. Instr. and Meth. A 427 (1999) 518. [9] D. Ferenc, E. Lorenz, R. Mirzoyan, Nucl. Instr. and Meth. A 442 (2000) 124. [10] D. Ferenc, Nucl. Inst. and Meth. A 431 (1999) 460. [11] D. Ferenc, ReFerence Photosensor, Nucl. Instr. and Meth. A, to be submitted for publication.
A novel photosensor concept Daniel Ferenc Physics Department, University of California Davis, One Shields Avenue, Davis, CA 95616-8677, USA
Abstract Reflection photocathodes offer quantum efficiency nearly twice that of transmission photocathodes, but have so far not been widely applied due to the lack of an appropriate photosensor configuration. This paper introduces an elegant and very powerful solution: reflection photocathode photosensor, named ReFerence. r 2001 Elsevier Science B.V. All rights reserved. PACS: 07.30.@t; 07.90.+c; 81.05.Ea Keywords: Photosensor; Photomultiplier tube; Vacuum; Photocathode; High-energy physics; Astrophysics; Medical imaging
1. Introduction Numerous present and future experiments in the fields of high energy physics and high energy astrophysics are based on the detection of Cherenkov or fluorescense photons emitted by charged particles in a widest variety of transparent media. The key element common to all these experiments is efficient photon detection. Singlephoton sensitivity, single-photon resolution, excellent time resolution, low noise and, most important, high quantum efficiency (>50%), are the key features of an ideal, needed, but so far nonexistent photosensor. A photosensor comprising all these qualities at a low price would certainly open a new range of sensitivity in different physics frontiers, like proton decay, neutrino oscillations, neutrino astronomy, gamma-ray astronomy, etc. The new photosensor concept introduced in this paper presents a very promising candidate. The presented photosensor may replace traditional Photomultiplier Tubes (PMTs) also in many other E-mail address: [email protected] (D. Ferenc).
applications, including industry, medicine, astronomy, etc. The classical PMTs, the present choice in most projects, combine affordable (though high) cost, with good timing properties, but still with two important drawbacks: poor single-photon resolution and low quantum efficiency. Hybrid Photon Detectors (HPDs) comprise excellent singlephoton resolution, but fail to offer higher quantum efficiency, since they have mostly been based on the same photocathode materials as PMTs. The two exceptions are the small-size HPDs by Intevac [1] and Hamamatsu [2] based on GaAsP photocathodes grown in Molecular Beam Epitaxy process (MBE). These photosensors offer peak quantum efficiency of almost 50%, but at an extremely high cost. The novel photosensor concept introduced in this paper offers a possibility to use a photocathode in the so-called reflection mode, instead of the traditional transmission mode. Reflection photocathodes offer much higher quantum efficiency with the same photocathode material, as elaborated in more detail below. But in spite of
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 9 7 7 - 9
230
D. Ferenc / Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
that, they have never been widely used due to the lack of a photosensor configuration that would be able to host the reflection photocathode without drastic sacrifices in effective sensitive area, photon angular acceptance and time resolution. The presented photosensor configuration overcomes those problems and facilitates the long awaited significant increase in quantum efficiency, even providing further important improvements in some other relevant features.
2. Transmission and reflection photocathodes In the photocathode, optical power decays exponentially according to the Lambert–Beer law PðzÞ ¼ Pð0Þ expð@azÞ, where aðlÞ is absorption coefficient specific of the photocathode material and the wavelength of the light l [3] and z is the penetration depth in the material. The distribution of absorbed photons in the photocathode layer, i.e. the loss of optical power 2dPðzÞ, has the same shape as PðzÞ, so the distribution of created electron–hole pairs in the photocathode also follows the same exponential shape. In order to be emitted from the photocathode, the photogenerated electrons in the conduction band of the material must reach by diffusion to the photocathode surface. In the usual transmission-type photocathodes, 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 the two conflicting requirements: efficient photon conversion and 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 (thanks to the Lambert–Beer exponential law) and therefore have a high chance of reaching the same surface and escaping through it into vacuum. As a consequence, reflection photocathodes offer quantum efficiency nearly twice that of transmission photocathodes [3]. The sensitivity to UV light is enhanced even much 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 also other very important advantages. The most important one is in 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 (like GaAs, GaAsP, InGaAs and others), produced by MBE epitaxial growth technique in ultra high vacuum. A traditional production process of transmission-type III–V photocathodes consists of around 10 different steps, starting from epitaxial growth of a thin photocathode layer on top of a crystal substrate with matched lattice constant, fusion of the grown structure to the phototube 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 (for GaAsP about 2–3 times higher than of transmission bialkali photocathodes). In addition, while for a typical transmissiontype photocathode a thin conductive sublayer has to be deposited between the glass window and the photocathode, in case of a reflection-type photocathode the thickness and the optical properties of this conductive sublayer are not critical since
D. Ferenc / Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
photons do not need to pass through it. Although only around 20 atomic layers thin when used with transmission photocathodes, this sublayer (made of SnO or indium–tin–oxide) absorbs already about 25% of the incoming light [3], which presents a significant loss that sets in even before the light had reached the transmission photocathode. In contrast, reflection photocathodes may even benefit from the conductive layer underneath, since it may serve as a mirror reflecting transmitted light back through the photocathode layer, providing thus the photon with another conversion chance.
3. ReFerence photosensor In spite of these striking advantages, reflectiontype photocathodes have never had a wide application in photosensor devices. Essentially all PMTs presently on the market are of transmissiontype. So far the main problem in implementation of reflection-type photocathodes has been in the lack of conceptual phototube design that would simultaneously host a photocathode in reflection configuration, and provide the following important features: (i) negligible dead area, (ii) flat angular acceptance and sharp angular cutoff for detected light, (iii) fast and position-independent time response, (iv) the possibility of close packing of individual units into large-area multi-pixel honeycomb imaging cameras. With a recently proposed imaging HPD with reflection-type photocathode, applied on blends hanging from the entrance window of a phototube [4,5], surprisingly good imaging properties have been achieved, but narrow angular acceptance and non-uniform time response turned out to be the unavoidable drawbacks. The invention presented in this paper solves this problem in completely different, very simple and elegant way. In this special configuration, the same vacuum tube component acts both as a perfect incoming light concentrator and as a perfect focusing electron lens. The former assures that essentially all the incoming photons reach the photocathode, and the later enables all the emitted
231
photoelectons to hit the small semiconductor sensor. This phototube was named according to the performance expectations ‘‘ReFerence’’ (later it became apparent that it would fit also an acronym: REflective photocathode tube by FERENC). The ReFerence phototube is based on the recent discovery that a Winston cone, which provides the most efficient non-imaging light concentration [6,7], may simultaneously act as an ideal focusing electron lens. In most general terms, the ReFerence phototube, shown in Fig. 1, is a Compound Parabolic light Concentrator (CPC) or Winston cone [6,7] which concentrates all the light entering the cone from the left side through the entrance aperture (2), at an incidence angle smaller than a given design cutoff angle (301 in Fig. 1), to the light collection surface (3) covered by a reflection photocathode (6), and simultaneously focuses and accelerates photoelectrons (7) emerging from the photocathode in the opposite direction, onto a point-like electron sensor (8) placed in the middle of the entrance aperture. This electron sensor may be a P–i–N diode, an Avalanche Photo Diode (APD), or any other suitable sensor. It is enclosed in a positive-ion feedback protection electrode (10) [8–10]. While the light concentration is provided by the tube shape that assumes a standard Winston cone parabolic mirror (4,5), electron focusing is facilitated by the electron lens formed from the electrodes that follow the same Winston cone shape, but with the insertion of a single narrow non-conductive gap (9) that divides the cone into two electrodes (4) and (5). The existence and the position of this gap are of crucial importance for the functionality of the ReFerence photosensor. Electrodes (4) and (5) have to be kept at different electric potentials: U4 and U5. From electron optics simulations it follows that a correct electron focusing may be achieved with a continuous multitude of combinations of U4 and U5. This unusual feature offers the important freedom to tune the potential gradient in front of the photocathode surface, and thus allows the optimization of the quantum efficiency and the responsetime properties of the device.
232
D. Ferenc / Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
Fig. 1. Schematic view of the ReFerence photosensor. See text for explanation.
For correct operation, the electrodes (4), (5), (6), (8) and (10) must be kept at potentials rising in the following order: U6oU4oU5oU8oU10. The optimal values for these potentials, and the precise position of the non-conductive interval (9) may be determined empirically in extensive electron–optics simulations. The example presented in Fig. 1 is optimized for a cutoff incidence angle of 301. Equivalent results may be achieved also with other acceptance angles, ranging approximately between 101 and 501. Further, the example in Fig. 1 provides a theoretical maximum light concentration factor (factor 2 in diameter and 4 in area) for the accepted light with incidence angle smaller than 301 and the angular spread at the collection surface being maximal, i.e. 901 [6,7]. The ReFerence tube concept is not limited to CPCs with maximum concentration and may be practiced equivalently with CPCs of a lower concentration factor and a narrower angular spread of light at the collection surface [11]. That may be of certain advantage in
avoiding light reflections from the photocathode surface, but inevitably leads to the increase of the photocathode surface. The entrance section of the phototube may be formed in a hexagonal shape to facilitate hexagonal packing of individual units into a honeycomb structure, without sizeable influence on the electron focusing [11]. Very effective magnetic field protection shielding in the form of a cylinder or cone may be applied [11]. Since the electron velocities are already high in the vicinity of the entrance window, electrons are less sensitive to magnetic deflection, and the shielding does not need to extend beyond the front face of the ReFerence photosensor.
4. Summary In summary, the presented novel ReFerence photosensor concept based on reflection photo-
D. Ferenc / Nuclear Instruments and Methods in Physics Research A 471 (2001) 229–233
cathode comprises the following important features: *
*
*
*
*
*
*
* *
*
*
*
Manifest conceptual and constructional simplicity. The highest possible quantum efficiency at a low manufacturing cost. Spectral sensitivity extended towards short wavelengths. Optimal usage of photocathode surface, thanks to the highest possible light concentration provided by the Winston cone phototube shape. Fast and position-independent time response [11], in spite of the flat photocathode surface (if needed, could be further improved with slight bending). Flat photocathode surface, required for epitaxially grown photocathodes. Single-photon resolution, thanks to the HPD concept. Negligible dead area. Flat angular acceptance and sharp angular cutoff for the incoming light. Possibility to modify lateral shape of the entrance section of the phototube from circular to hexagonal [11], without significant degradation of the electron focusing performance, and hexagonal honeycomb close packing into largearea imaging cameras. No need for additional light concentrators or stray light protection in applications. Efficient magnetic field screening without acceptance shadowing [11].
233
Acknowledgements Soon after the Stockholm Imaging 2000 conference we lost Tom Ypsilantis, a most exceptional man and physicist. The development of the presented idea has benefited a lot from Tom’s support and encouragement. This paper is devoted to the dearest memory of Tom Ypsilantis.
References [1] Intevac, Hybrid photomultiplier tube with high sensitivity, US Patent No. US5374826. [2] R. Mirzoyan, D. Ferenc, E. Lorenz, Nucl. Instr. and Meth. A 442 (2000) 140. [3] S. Donati, Photodetectors, Prentice-Hall PTR, Englewood Cliffs, NJ, 2000. [4] D. Ferenc, Nucl. Instr. and Meth. A 442 (2000) 150–153. [5] D. Ferenc, New developments in hybrid photon detectors, in: C. Williams, T. Ypsilantis (Eds.), Presented at New Detectors, Erice, Trapani, Sicily, Italy, November 1–7, 1997, World Scientific, Singapore, 1999, pp. 131–140. [6] W.T. Welford, R. Winston, The Optics of Nonimaging Concentrators, Academic Press, New York, 1978. [7] R. Winston, J. Opt. Soc. Amer. 60 (1970) 245. [8] D. Ferenc, D. Hrupec, E. Lorenz, Nucl. Instr. and Meth. A 427 (1999) 518. [9] D. Ferenc, E. Lorenz, R. Mirzoyan, Nucl. Instr. and Meth. A 442 (2000) 124. [10] D. Ferenc, Nucl. Inst. and Meth. A 431 (1999) 460. [11] D. Ferenc, ReFerence Photosensor, Nucl. Instr. and Meth. A, to be submitted for publication.