- Email: [email protected]
Development of novel photon detectors at UC Davis
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 553 (2005) 165–171 www.elsevier.com/locate/nima
Development of novel photon detectors at UC Davis D. Ferenc, D. Kranich1, A. Laille, E. Lorenz Physics Department, University of California Davis, 1 Shields Av., Davis, 95616 CA, USA Available online 22 August 2005
Abstract Photosensors are indispensable in many areas of fundamental physics research, particularly in the emerging field of particle astrophysics. They also find widespread use in medical imaging, nuclear radiation monitoring, and defense. To make significant progress in future, virtually all these areas require a new, inexpensive, high-quality industrial massproduction photosensor technology. Our group at UC Davis has been working on innovations along that line. r 2005 Elsevier B.V. All rights reserved. Keywords: Photon; Particle astrophysics; Neutrino; Proton decay; Nuclear proliferation; Medical imaging
1. Introduction Many future experiments within the broad field of particle astrophysics will study very energetic or extremely rare phenomena.2 These elusive phenomena require special means of observation, with detectors whose sizes should greatly exceed the dimensions of the largest current experiments. In the construction of detectors on such a large scale, no other option remains than to use natural media—the atmosphere, deep packs of ice, and Corresponding author.
E-mail address: [email protected] (D. Ferenc). Feodor Lynen Fellow. 2 Proton decay, neutrinoless double beta decay, neutrino oscillations, supernova explosions, cosmic neutrinos, cosmic gamma-rays, ultra-high energy cosmic rays, gamma-ray bursts, active galactic nuclei, dark matter, etc.
water. In these transparent media, charged particles that originate in impacts or decays of primary particles radiate Cherenkov or fluorescence light. Photosensors are the only well-proven active detector component3 to convert photons into electrical signals; and consequently, these sensors are the single most important detector element in this field. Photosensors play an equally important role in other similar application areas, particularly in medical imaging, nuclear radiation monitoring, certain classes of analytical instruments, and nuclear proliferation control. Unlike physics, these areas
1
3
There are ongoing activities to explore radio-wave detection or acoustic detection of energetic showers, but these technologies have not yet reached maturity.
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.08.011
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present a real (i.e. large and steady) potential market for new, inexpensive, high-quality, industrially mass-produced photosensors. For various reasons, neither the current vacuum photosensor technology nor the modern semiconductor photosensor technologies may be suitable for such large-area mass applications. In contrast to the semiconductor photosensor technologies that have rapidly evolved during the last few decades (but towards small-pixel devices that are unsuitable for large-area applications), the vacuum photomultiplier tube (PMT) technology has not made any significant progress since the late 1960s. The complex and bulky construction and the labor-intensive manufacture (‘batch processing’) are inherent to the PMT concept—mass production is therefore virtually inconceivable. Also intrinsic to the PMT concept are the important drawbacks in its performance: low photoelectron collection efficiency (at most 70% in large-area PMTs); low quantum efficiency (20–25%) limited only to a narrow spectral region; complicated and expensive installation methods; fragility (as dramatically experienced in the Super Kamiokande disaster); high sensitivity to magnetic fields; and almost a complete lack of single-photon resolution (i.e., the ability to resolve the number of photons in a photosensor pixel). The recent phenomenal success of hi-tech vacuum flat-panel TV technologies, which truly exemplify modern industrial mass production, indicates that an equivalent breakthrough might be possible also in the field of vacuum photosensors. The revolution in the TV industry came as a consequence of a profound conceptual shift— from the bulky cathode ray tube (similar to the PMTs) to the lightweight and attractive flat-panel configuration that perfectly suits continuous production lines. Our search for an equivalent flatpanel vacuum photosensor has resulted in the novel ReFerence photosensor concept [1–4]. The proposed new technology essentially combines three fully established and well-understood technologies: the flat-panel plasma and fieldemission TV-screen assembly, the large-area photocathode deposition, and the semiconductor particle sensor production. In simple terms, the ‘vacuum part’ of the device transforms the photons into electrons and compresses the signal
to such a small area that a small semiconductor sensor may be used for the final readout. ReFerence panels comprise a significantly lower constructional complexity than flat-panel TV screens. In particular, pixels are very large (few cm in diameter), and the tiniest feature within a pixel still looks comfortably large from the manufacturing point of view (the photoelectron sensor of a 1 mm diameter). Based on these and other differences in constructional complexity, we have derived a very rough cost estimate for ReFerence panels—less than one-third of the cost of a plasma TV screen, i.e., currently less than $1000 per square meter. This should be contrasted with $30,000, the cost per square meter of popular 8 in. PMTs, or yet with much higher figures for PMTs of comparable (pixel) sizes, like the ones currently used in the imaging atmospheric Cherenkov gamma-ray telescopes. The ReFerence concept is briefly reviewed in Section 2, where we also report on the current status of the prototype development. In Section 3, we introduce the light amplifier concept—an inexpensive and robust readout alternative for special hybrid photon detectors (HPDs) with a strong photoelectron focusing (ReFerence panels, or some large hemispherical HPDs). Recent developments of new Geiger-mode avalanche photodiodes (G-APDs) are discussed in Section 4. The G-APDs present an inexpensive and robust solution for the readout of various detectors, in particular of a light amplifier version discussed here.
2. Development of ReFerence flat-panel photosensors The ReFerence photosensor is based on a fortunate coincidence: the very same mirror/ electrode shape may act simultaneously as an optical concentrator and as an electron lens. The compact parabolic light concentrator, see Fig. 1a, concentrates light from the entrance aperture (on the left) to the photocathode (on the right) and focuses the photoelectrons to a very small photoelectron sensor (a p-on-n PIN diode, a p-on-n avalanche diode, a fiber-coupled scintillator in the
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Fig. 1. ReFerence photosensor. (a) The functionality of an elementary ReFerence cell, see text for explanation. (b) Result of a prototype test. The bright spot in the middle of the image presents the electron focal point as seen on an inserted phosphor screen. The prototype was hexagonally shaped. (c) Schematic front view of a ReFerence panel. Each hexagonal pixel presents a ReFerence cell. Note the low fraction of dead area. (d) Schematic side-view of a light amplifier panel. The APD array is symbolically located behind the panel and serves as readout for the scintillators coupled to the APDs by means of optical fibers.
light amplifier, or other) placed in the middle of the entrance aperture. The concentration factor is nearly optimal in both directions, which leads to an optimal overall signal concentration. Thus, a very small electron sensor may be used in the focus to detect virtually all the photoelectrons (100% collection efficiency). Note that this small sensor effectively replaces the entire dynode column—the most expensive, bulky, hand-made, rear-end component in a PMT. As demonstrated in a series of generic 3 in. prototype tests carried out at UC Davis, the pointlike photoelectron focusing persists also in a
hexagonally formed ReFerence pixel, see Fig. 1b. This feature is essential, because it proves the feasibility of the honeycomb multi-pixel flatpanel configuration. In these experiments, a 2 mm diameter back-illuminated photocathode was used in order to strictly localize the position of the photoelectron source. This spot-photocathode was then precisely positioned in small increments across the photocathode plane, from the center to the periphery, and the electron focusing was checked by imaging of the electron spot on a phosphor screen in the focal plane. For a correct potential setting, both the position of
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the focal spot and its size were found to be invariant of the photoelectron source position in the photocathode plane. From the area of the spot size, we derived a concentration factor of 1500, which is extremely high for such a simple electron–lens configuration. In addition to its suitability for mass production, as discussed in Section 1, the ReFerence flatpanel concept offers other important advantages: a nearly 100% electron collection efficiency (in contrast to typical 70% efficiency in PMTs); high photon detection efficiency, extending over a much wider spectral range thanks to the reflection-mode photocathode; small dead area; efficient magnetic shielding; and a very low level of thermionic noise, if the photocathode would be cooled in vacuum. In addition, the TransReFerence configuration [4] should provide non-destructive color sensitivity and a very wide spectral sensitivity, but this feature has not been tested yet. After verifying the basic functionality of a ReFerence pixel in a series of single-pixel vacuum-unsealed prototype tests, in 2003 we started with the development of fully functional vacuumsealed flat-panel ReFerence prototypes. The principal goal of that effort was to demonstrate the feasibility of an industrial mass production technology. The photocathode deposition, and the vacuum sealing will be performed at UC Davis, in a new, uniquely equipped laboratory that comprises a special UHV transfer and vacuum-sealing facility; a photocathode evaporation facility (a modified Litton second-generation image intensifier exhaust station); a complete UHV surface science analysis system for photocathode and general surface studies (including Auger spectroscopy, X-ray and ion-induced electron spectroscopy, and other techniques); and two industrial IR power laser sealing facilities (purchased from a leading field-emission TV screen factory, not yet in use). The first flat-panel prototype comprises seven pixels hosted by a 5 in. diameter vacuum enclosure, see Fig. 2. The prototype diameter is limited by the throughput of the transfer system. The enclosure of the first panel prototype is made of glass, while for machining reasons the honeycomb
Fig. 2. Components of the first flat-panel ReFerence photosensor prototype; seven pixels in a honeycomb aluminum structure, 5 in. in diameter. More advanced prototypes will be entirely made of glass.
plates were made of aluminum. We developed a new glass-to-glass sealing technique based on a vacuum-evaporated multi-layer structure consisting of chromium, gold, and indium. Specifically, the indium surface is protected from oxidation with a thin Au–In2 intermetallic compound layer. We have recently started with the development of a series of gradually improving all-glass ReFerence panel prototypes, fabricated in a way that closely approaches the ultimate industrial processing. We have engaged specialized industries to design the tools and produce the prototype components. This process will repeat in several iterations; in the last development phase, we will switch from indium sealing to glass frit sealing, using our IR power laser facilities.
3. Development of light amplifiers A light amplifier (Fig. 1d) receives light on its entire front area (Fig. 1c), and subsequently, both amplifies and concentrates (1500 times) the signal into the scintillator–optical fiber combination. The entire panel area, therefore, effectively shrinks to a very small active readout area (1500
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4. Geiger-mode avalanche photodiodes
Fig. 3. Light amplifier in a spherical tube configuration. The electric field in the tube focuses photoelectrons from the photocathode to a centrally positioned thin scintillator layer. The G-APD array reads out the light emitted by the scintillator upon electron impacts. This configuration provides position sensitivity.
times smaller than the panel area),4 placed outside the vacuum. The other end of the optical fiber may be coupled to a Geiger-mode avalanche photodiode (G-APD), respectively, to a G-APD matrix in case of a large-area device. These secondary photosensors are placed outside the vacuum enclosure. This results in a particularly simple and robust construction, without any electronic components enclosed in the vacuum panel, which dramatically simplifies thermal processing and keeps away from cross-contamination between the photocathode and the semiconductor sensor. It also allows significant readout upgrades and modifications of the actual panel resolution at any time. The light amplifier concept is very general, and it may be applied to any hybrid photon detector with a strong photoelectron concentration. Another example is a large hemispherical HPD, like the Quasar phototube, or the equivalent SmartPMT (Fig. 3) from Philips. In this particular case, one should be able to achieve even a limited position-sensitive performance. Our first experiments with a Quasar tube are just under way. 4 Just to put this number into perspective, let us assume that the light-receiving area in a large next-generation radiation detector is 1500 m2; the 1500 fold concentration would lead to an integral photoelectron readout area of only 1 m2!
In the original version of the Smart PMT/Quasar tube, the secondary light sensor was just a small PMT. This required an additional high-voltage power supply, and did not allow for position measurements of the light spot in the scintillator chip. This position may be correlated with the photon impact position on the photocathode— potentially a very useful feature. The novel Geigermode APDs (G-APDs) provide an elegant and simple solution for an inexpensive and efficient replacement of the secondary PMT. The G-APD concept goes back to developments of small avalanche photodiodes in the 1970s and 1980s. Miniature avalanche diodes could be operated above the breakdown voltage, when connected to a bias voltage by a high ohmic resistor acting as a quencher of an avalanche discharge, very much like the old Geiger tubes. Single-cell devices were produced by RCA-Canada (McIntyre); also the Milano group around Cova carried out many developments, an overview can be found in a recent talk by Cova [5]. In the 1980s and 1990s, physicists in Russia found an elegant solution to overcome the small size [6–8]. They combined many small cells (100–1000 per mm2) on a silicon chip, integrated the quenching resistors onto the chip, and connected all cells together to form macroscopic pixels; for examples see Refs. [6–8]. The operation voltage can be quite low, around 25–100 V, and the gain can range, depending on the construction, between 104 and a few times 106. The output signals are nearly standardized, and in case of a small number of photoelectrons, their number can be simply counted (this allows easy calibration). Signals are extremely fast, and time resolution may range between a few tens of ps to ns. A key quantity is the so-called photon detection efficiency (PDE), i.e., the product of quantum efficiency and the probability to initiate an avalanche breakdown in a cell. Nowadays, one achieves PDE values of around 20% for the best designs, while new developments aim at 40%. Both p-on-n (blue sensitive) and n-on-p (red sensitive) structures are possible. Production methods are relatively simple, and in a long-term the costs are expected to be low.
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Nevertheless, there are some disadvantages, such as a high noise rate (few 100 kHz–MHz/ mm2 at room temperature), and optical cross-talk between cells.5 Also, after firing, the cells need some time to recover (typically 50–1000 ns). This limits applications to the detection of light fluxes to the corresponding number of photoelectrons/ cell/recovery time well below one. In summary, the G-APD concept offers the following advantages:
high PDE, very fast and standardized output signals per photoelectron, high intrinsic gain, very high level of compactness, low operation voltage, insensitivity to magnetic fields, low production costs, high immunity against electromagnetic interference, easy matrix production, and low excess noise factor (1.1–1.5, dominated by optical cross-talk).
Currently, G-APDs of 1–10 mm2 area are produced on the prototype level. Larger-area detectors can be assembled from these elements by using simple, fast adders, which decouple the parasitic parallel capacitances of the smaller units, thus conserving the fast rise-times of the pulses. Since G-APDs have a very high application potential in many areas in research, nuclear medicine [9], analytical instrumentation, etc., we expect that commercial production will start soon. We are currently testing the G-APD readout for the small scintillator in the ReFerence light amplifier panel, and for the large-area light amplifier with modest imaging. The latter device, following the Smart PMT/Quasar concept, but now coupled to a matrix of G-APDs, allows us to ‘mirror’ certain cathode areas to discrete readout cells, thus significantly reducing the unwanted background. For a Quasar-type arrangement, we calculated that on 5 In an avalanche, a few secondary photons are generated (2–3/gain of 105). These photons can travel through the semiconductor and sometimes initiate new avalanches in neighboring cells.
average, one initial photoelectron would produce E15 photoelectrons in a G-APD. Such a signal would be quite large, in fact well above the noise level of the G-APD. We estimated that, by using a matrix of 1 mm2 G-APDs, one could restrict the origin of the initial photoelectron to about 10% of the cathode area. For optimized electron optics in large-area designs, we estimate a photon impact uncertainty on the photocathode of around a few percent of the total surface. This would allow a major improvement in deep-sea neutrino detectors— the ability to discriminate weak optical light flashes from the ubiquitous background from 40K decays, and the strong intrinsic PMT noise.
5. Summary and outlook In the search for rare and/or very energetic phenomena, most of the future experiments in the broad field of high-energy particle astrophysics critically rely on the use of large-area (pixelized) photon detectors. Improved and more cost-effective photon detectors will open prospects for new experiments with higher sensitivity and better performance. Many other areas of research, radiation monitoring, and industrial applications would also profit from such a new photosensor technology. A high-quality industrial mass production technology is needed; we presented the development at UC Davis along certain promising new directions.
Acknowledgements Our project has been supported by the Advanced Detector Research Award of the US Department of Energy and by the US National Nuclear Security Administration. We would like to thank Amos Breskin and Glenn Knoll for useful discussions and help. References [1] D. Ferenc, US Patent No. 6,674,063. [2] D. Ferenc, Nucl. Instr. and Meth. A 471 (2001) 229.
ARTICLE IN PRESS D. Ferenc et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 165–171 [3] D. Ferenc, A. Laille, C. Bratton, Nucl. Instr. and Meth. A 504 (2003) 359. [4] D. Ferenc, Nucl. Instr. and Meth. A 497 (2003) 198. [5] S. Cova, et al., Evaluation and prospects of single photon avalanche diodes and quenching circuits. Contribution to the NIST03 ARDA Workshop, 2003.
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[6] D. Bisello, et al., Nucl. Instr. and Meth. A 367 (1995) 212. [7] V. Golovin, et al., Nucl. Instr. and Meth. A 518 (2004) 560. [8] B. Dolgoschein, et al., Nucl. Instr. and Meth. A 504 (2003) 48. [9] N. Otte, et al., Nucl. Instr. and Meth. A 545 (2005) 705.
Nuclear Instruments and Methods in Physics Research A 553 (2005) 165–171 www.elsevier.com/locate/nima
Development of novel photon detectors at UC Davis D. Ferenc, D. Kranich1, A. Laille, E. Lorenz Physics Department, University of California Davis, 1 Shields Av., Davis, 95616 CA, USA Available online 22 August 2005
Abstract Photosensors are indispensable in many areas of fundamental physics research, particularly in the emerging field of particle astrophysics. They also find widespread use in medical imaging, nuclear radiation monitoring, and defense. To make significant progress in future, virtually all these areas require a new, inexpensive, high-quality industrial massproduction photosensor technology. Our group at UC Davis has been working on innovations along that line. r 2005 Elsevier B.V. All rights reserved. Keywords: Photon; Particle astrophysics; Neutrino; Proton decay; Nuclear proliferation; Medical imaging
1. Introduction Many future experiments within the broad field of particle astrophysics will study very energetic or extremely rare phenomena.2 These elusive phenomena require special means of observation, with detectors whose sizes should greatly exceed the dimensions of the largest current experiments. In the construction of detectors on such a large scale, no other option remains than to use natural media—the atmosphere, deep packs of ice, and Corresponding author.
E-mail address: [email protected] (D. Ferenc). Feodor Lynen Fellow. 2 Proton decay, neutrinoless double beta decay, neutrino oscillations, supernova explosions, cosmic neutrinos, cosmic gamma-rays, ultra-high energy cosmic rays, gamma-ray bursts, active galactic nuclei, dark matter, etc.
water. In these transparent media, charged particles that originate in impacts or decays of primary particles radiate Cherenkov or fluorescence light. Photosensors are the only well-proven active detector component3 to convert photons into electrical signals; and consequently, these sensors are the single most important detector element in this field. Photosensors play an equally important role in other similar application areas, particularly in medical imaging, nuclear radiation monitoring, certain classes of analytical instruments, and nuclear proliferation control. Unlike physics, these areas
1
3
There are ongoing activities to explore radio-wave detection or acoustic detection of energetic showers, but these technologies have not yet reached maturity.
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.08.011
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present a real (i.e. large and steady) potential market for new, inexpensive, high-quality, industrially mass-produced photosensors. For various reasons, neither the current vacuum photosensor technology nor the modern semiconductor photosensor technologies may be suitable for such large-area mass applications. In contrast to the semiconductor photosensor technologies that have rapidly evolved during the last few decades (but towards small-pixel devices that are unsuitable for large-area applications), the vacuum photomultiplier tube (PMT) technology has not made any significant progress since the late 1960s. The complex and bulky construction and the labor-intensive manufacture (‘batch processing’) are inherent to the PMT concept—mass production is therefore virtually inconceivable. Also intrinsic to the PMT concept are the important drawbacks in its performance: low photoelectron collection efficiency (at most 70% in large-area PMTs); low quantum efficiency (20–25%) limited only to a narrow spectral region; complicated and expensive installation methods; fragility (as dramatically experienced in the Super Kamiokande disaster); high sensitivity to magnetic fields; and almost a complete lack of single-photon resolution (i.e., the ability to resolve the number of photons in a photosensor pixel). The recent phenomenal success of hi-tech vacuum flat-panel TV technologies, which truly exemplify modern industrial mass production, indicates that an equivalent breakthrough might be possible also in the field of vacuum photosensors. The revolution in the TV industry came as a consequence of a profound conceptual shift— from the bulky cathode ray tube (similar to the PMTs) to the lightweight and attractive flat-panel configuration that perfectly suits continuous production lines. Our search for an equivalent flatpanel vacuum photosensor has resulted in the novel ReFerence photosensor concept [1–4]. The proposed new technology essentially combines three fully established and well-understood technologies: the flat-panel plasma and fieldemission TV-screen assembly, the large-area photocathode deposition, and the semiconductor particle sensor production. In simple terms, the ‘vacuum part’ of the device transforms the photons into electrons and compresses the signal
to such a small area that a small semiconductor sensor may be used for the final readout. ReFerence panels comprise a significantly lower constructional complexity than flat-panel TV screens. In particular, pixels are very large (few cm in diameter), and the tiniest feature within a pixel still looks comfortably large from the manufacturing point of view (the photoelectron sensor of a 1 mm diameter). Based on these and other differences in constructional complexity, we have derived a very rough cost estimate for ReFerence panels—less than one-third of the cost of a plasma TV screen, i.e., currently less than $1000 per square meter. This should be contrasted with $30,000, the cost per square meter of popular 8 in. PMTs, or yet with much higher figures for PMTs of comparable (pixel) sizes, like the ones currently used in the imaging atmospheric Cherenkov gamma-ray telescopes. The ReFerence concept is briefly reviewed in Section 2, where we also report on the current status of the prototype development. In Section 3, we introduce the light amplifier concept—an inexpensive and robust readout alternative for special hybrid photon detectors (HPDs) with a strong photoelectron focusing (ReFerence panels, or some large hemispherical HPDs). Recent developments of new Geiger-mode avalanche photodiodes (G-APDs) are discussed in Section 4. The G-APDs present an inexpensive and robust solution for the readout of various detectors, in particular of a light amplifier version discussed here.
2. Development of ReFerence flat-panel photosensors The ReFerence photosensor is based on a fortunate coincidence: the very same mirror/ electrode shape may act simultaneously as an optical concentrator and as an electron lens. The compact parabolic light concentrator, see Fig. 1a, concentrates light from the entrance aperture (on the left) to the photocathode (on the right) and focuses the photoelectrons to a very small photoelectron sensor (a p-on-n PIN diode, a p-on-n avalanche diode, a fiber-coupled scintillator in the
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Fig. 1. ReFerence photosensor. (a) The functionality of an elementary ReFerence cell, see text for explanation. (b) Result of a prototype test. The bright spot in the middle of the image presents the electron focal point as seen on an inserted phosphor screen. The prototype was hexagonally shaped. (c) Schematic front view of a ReFerence panel. Each hexagonal pixel presents a ReFerence cell. Note the low fraction of dead area. (d) Schematic side-view of a light amplifier panel. The APD array is symbolically located behind the panel and serves as readout for the scintillators coupled to the APDs by means of optical fibers.
light amplifier, or other) placed in the middle of the entrance aperture. The concentration factor is nearly optimal in both directions, which leads to an optimal overall signal concentration. Thus, a very small electron sensor may be used in the focus to detect virtually all the photoelectrons (100% collection efficiency). Note that this small sensor effectively replaces the entire dynode column—the most expensive, bulky, hand-made, rear-end component in a PMT. As demonstrated in a series of generic 3 in. prototype tests carried out at UC Davis, the pointlike photoelectron focusing persists also in a
hexagonally formed ReFerence pixel, see Fig. 1b. This feature is essential, because it proves the feasibility of the honeycomb multi-pixel flatpanel configuration. In these experiments, a 2 mm diameter back-illuminated photocathode was used in order to strictly localize the position of the photoelectron source. This spot-photocathode was then precisely positioned in small increments across the photocathode plane, from the center to the periphery, and the electron focusing was checked by imaging of the electron spot on a phosphor screen in the focal plane. For a correct potential setting, both the position of
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the focal spot and its size were found to be invariant of the photoelectron source position in the photocathode plane. From the area of the spot size, we derived a concentration factor of 1500, which is extremely high for such a simple electron–lens configuration. In addition to its suitability for mass production, as discussed in Section 1, the ReFerence flatpanel concept offers other important advantages: a nearly 100% electron collection efficiency (in contrast to typical 70% efficiency in PMTs); high photon detection efficiency, extending over a much wider spectral range thanks to the reflection-mode photocathode; small dead area; efficient magnetic shielding; and a very low level of thermionic noise, if the photocathode would be cooled in vacuum. In addition, the TransReFerence configuration [4] should provide non-destructive color sensitivity and a very wide spectral sensitivity, but this feature has not been tested yet. After verifying the basic functionality of a ReFerence pixel in a series of single-pixel vacuum-unsealed prototype tests, in 2003 we started with the development of fully functional vacuumsealed flat-panel ReFerence prototypes. The principal goal of that effort was to demonstrate the feasibility of an industrial mass production technology. The photocathode deposition, and the vacuum sealing will be performed at UC Davis, in a new, uniquely equipped laboratory that comprises a special UHV transfer and vacuum-sealing facility; a photocathode evaporation facility (a modified Litton second-generation image intensifier exhaust station); a complete UHV surface science analysis system for photocathode and general surface studies (including Auger spectroscopy, X-ray and ion-induced electron spectroscopy, and other techniques); and two industrial IR power laser sealing facilities (purchased from a leading field-emission TV screen factory, not yet in use). The first flat-panel prototype comprises seven pixels hosted by a 5 in. diameter vacuum enclosure, see Fig. 2. The prototype diameter is limited by the throughput of the transfer system. The enclosure of the first panel prototype is made of glass, while for machining reasons the honeycomb
Fig. 2. Components of the first flat-panel ReFerence photosensor prototype; seven pixels in a honeycomb aluminum structure, 5 in. in diameter. More advanced prototypes will be entirely made of glass.
plates were made of aluminum. We developed a new glass-to-glass sealing technique based on a vacuum-evaporated multi-layer structure consisting of chromium, gold, and indium. Specifically, the indium surface is protected from oxidation with a thin Au–In2 intermetallic compound layer. We have recently started with the development of a series of gradually improving all-glass ReFerence panel prototypes, fabricated in a way that closely approaches the ultimate industrial processing. We have engaged specialized industries to design the tools and produce the prototype components. This process will repeat in several iterations; in the last development phase, we will switch from indium sealing to glass frit sealing, using our IR power laser facilities.
3. Development of light amplifiers A light amplifier (Fig. 1d) receives light on its entire front area (Fig. 1c), and subsequently, both amplifies and concentrates (1500 times) the signal into the scintillator–optical fiber combination. The entire panel area, therefore, effectively shrinks to a very small active readout area (1500
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4. Geiger-mode avalanche photodiodes
Fig. 3. Light amplifier in a spherical tube configuration. The electric field in the tube focuses photoelectrons from the photocathode to a centrally positioned thin scintillator layer. The G-APD array reads out the light emitted by the scintillator upon electron impacts. This configuration provides position sensitivity.
times smaller than the panel area),4 placed outside the vacuum. The other end of the optical fiber may be coupled to a Geiger-mode avalanche photodiode (G-APD), respectively, to a G-APD matrix in case of a large-area device. These secondary photosensors are placed outside the vacuum enclosure. This results in a particularly simple and robust construction, without any electronic components enclosed in the vacuum panel, which dramatically simplifies thermal processing and keeps away from cross-contamination between the photocathode and the semiconductor sensor. It also allows significant readout upgrades and modifications of the actual panel resolution at any time. The light amplifier concept is very general, and it may be applied to any hybrid photon detector with a strong photoelectron concentration. Another example is a large hemispherical HPD, like the Quasar phototube, or the equivalent SmartPMT (Fig. 3) from Philips. In this particular case, one should be able to achieve even a limited position-sensitive performance. Our first experiments with a Quasar tube are just under way. 4 Just to put this number into perspective, let us assume that the light-receiving area in a large next-generation radiation detector is 1500 m2; the 1500 fold concentration would lead to an integral photoelectron readout area of only 1 m2!
In the original version of the Smart PMT/Quasar tube, the secondary light sensor was just a small PMT. This required an additional high-voltage power supply, and did not allow for position measurements of the light spot in the scintillator chip. This position may be correlated with the photon impact position on the photocathode— potentially a very useful feature. The novel Geigermode APDs (G-APDs) provide an elegant and simple solution for an inexpensive and efficient replacement of the secondary PMT. The G-APD concept goes back to developments of small avalanche photodiodes in the 1970s and 1980s. Miniature avalanche diodes could be operated above the breakdown voltage, when connected to a bias voltage by a high ohmic resistor acting as a quencher of an avalanche discharge, very much like the old Geiger tubes. Single-cell devices were produced by RCA-Canada (McIntyre); also the Milano group around Cova carried out many developments, an overview can be found in a recent talk by Cova [5]. In the 1980s and 1990s, physicists in Russia found an elegant solution to overcome the small size [6–8]. They combined many small cells (100–1000 per mm2) on a silicon chip, integrated the quenching resistors onto the chip, and connected all cells together to form macroscopic pixels; for examples see Refs. [6–8]. The operation voltage can be quite low, around 25–100 V, and the gain can range, depending on the construction, between 104 and a few times 106. The output signals are nearly standardized, and in case of a small number of photoelectrons, their number can be simply counted (this allows easy calibration). Signals are extremely fast, and time resolution may range between a few tens of ps to ns. A key quantity is the so-called photon detection efficiency (PDE), i.e., the product of quantum efficiency and the probability to initiate an avalanche breakdown in a cell. Nowadays, one achieves PDE values of around 20% for the best designs, while new developments aim at 40%. Both p-on-n (blue sensitive) and n-on-p (red sensitive) structures are possible. Production methods are relatively simple, and in a long-term the costs are expected to be low.
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Nevertheless, there are some disadvantages, such as a high noise rate (few 100 kHz–MHz/ mm2 at room temperature), and optical cross-talk between cells.5 Also, after firing, the cells need some time to recover (typically 50–1000 ns). This limits applications to the detection of light fluxes to the corresponding number of photoelectrons/ cell/recovery time well below one. In summary, the G-APD concept offers the following advantages:
high PDE, very fast and standardized output signals per photoelectron, high intrinsic gain, very high level of compactness, low operation voltage, insensitivity to magnetic fields, low production costs, high immunity against electromagnetic interference, easy matrix production, and low excess noise factor (1.1–1.5, dominated by optical cross-talk).
Currently, G-APDs of 1–10 mm2 area are produced on the prototype level. Larger-area detectors can be assembled from these elements by using simple, fast adders, which decouple the parasitic parallel capacitances of the smaller units, thus conserving the fast rise-times of the pulses. Since G-APDs have a very high application potential in many areas in research, nuclear medicine [9], analytical instrumentation, etc., we expect that commercial production will start soon. We are currently testing the G-APD readout for the small scintillator in the ReFerence light amplifier panel, and for the large-area light amplifier with modest imaging. The latter device, following the Smart PMT/Quasar concept, but now coupled to a matrix of G-APDs, allows us to ‘mirror’ certain cathode areas to discrete readout cells, thus significantly reducing the unwanted background. For a Quasar-type arrangement, we calculated that on 5 In an avalanche, a few secondary photons are generated (2–3/gain of 105). These photons can travel through the semiconductor and sometimes initiate new avalanches in neighboring cells.
average, one initial photoelectron would produce E15 photoelectrons in a G-APD. Such a signal would be quite large, in fact well above the noise level of the G-APD. We estimated that, by using a matrix of 1 mm2 G-APDs, one could restrict the origin of the initial photoelectron to about 10% of the cathode area. For optimized electron optics in large-area designs, we estimate a photon impact uncertainty on the photocathode of around a few percent of the total surface. This would allow a major improvement in deep-sea neutrino detectors— the ability to discriminate weak optical light flashes from the ubiquitous background from 40K decays, and the strong intrinsic PMT noise.
5. Summary and outlook In the search for rare and/or very energetic phenomena, most of the future experiments in the broad field of high-energy particle astrophysics critically rely on the use of large-area (pixelized) photon detectors. Improved and more cost-effective photon detectors will open prospects for new experiments with higher sensitivity and better performance. Many other areas of research, radiation monitoring, and industrial applications would also profit from such a new photosensor technology. A high-quality industrial mass production technology is needed; we presented the development at UC Davis along certain promising new directions.
Acknowledgements Our project has been supported by the Advanced Detector Research Award of the US Department of Energy and by the US National Nuclear Security Administration. We would like to thank Amos Breskin and Glenn Knoll for useful discussions and help. References [1] D. Ferenc, US Patent No. 6,674,063. [2] D. Ferenc, Nucl. Instr. and Meth. A 471 (2001) 229.
ARTICLE IN PRESS D. Ferenc et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) 165–171 [3] D. Ferenc, A. Laille, C. Bratton, Nucl. Instr. and Meth. A 504 (2003) 359. [4] D. Ferenc, Nucl. Instr. and Meth. A 497 (2003) 198. [5] S. Cova, et al., Evaluation and prospects of single photon avalanche diodes and quenching circuits. Contribution to the NIST03 ARDA Workshop, 2003.
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