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technospheramag
technospheramag
ТЕХНОСФЕРА_РИЦ
Issue #2/2024
S. K. Kulov, T. D. Alkatseva, G. V. Fedotova, E. I. Sentsova
MCP-PMT – Photon Counting UV-VIS Detectors
MCP-PMT – Photon Counting UV-VIS Detectors
DOI: 10.22184/1993-7296.FRos.2024.18.2.160.165
The development of MCP-PMT – photon counting UV–VIS detectors for optical-physical measurements is reported. MCP-PMTs design and technology features are described, which allow achieving high peak-to-valley ratio, low dark count rate and longer lifetime. We present different versions of PMT constructions (in the form of vacuum units and with a voltage divider integrated into a single housing with a vacuum unit).
The development of MCP-PMT – photon counting UV–VIS detectors for optical-physical measurements is reported. MCP-PMTs design and technology features are described, which allow achieving high peak-to-valley ratio, low dark count rate and longer lifetime. We present different versions of PMT constructions (in the form of vacuum units and with a voltage divider integrated into a single housing with a vacuum unit).
Теги: dark count rate ion feedback peak-to-valley ratio photon counting detectors pmt with integrated voltage divider pulse height distribution амплитудное распределение импульсов ионная обратная связь отношение пик/долина скорость счета темновых импульсов счетчики фотонов фэу со встроенным делителем напряжения
MCP-PMT – Photon Counting UV–VIS Detectors
S. K. Kulov, T. D. Alkatseva, G. V. Fedotova, E. I. Sentsova
VTC “Baspik” LLC, Vladikavkaz
The development of MCP-PMT – photon counting UV–VIS detectors for optical-physical measurements is reported. MCP-PMTs design and technology features are described, which allow achieving high peak-to-valley ratio, low dark count rate and longer lifetime. We present different versions of PMT constructions (in the form of vacuum units and with a voltage divider integrated into a single housing with a vacuum unit).
Keywords: photon counting detectors, pulse height distribution, dark count rate, ion feedback, peak-to-valley ratio, PMT with integrated voltage divider
Article received:11.01.2024
Article accepted:05.02.2024
When detecting extremely weak photon signals, the photon counting method is used to extract the maximum information. Photon counting detectors are one of the main types of detectors for high-energy and nuclear physics. Discrete dynodes PMTs, MCP-PMTs, avalanche dynodes, and hybrid PMTs are usually used as photon counting detectors. Among these detector types, MCP-PMTs are favorably characterized with high amplification and fast response, increased surface area, low power consumption, and stable operation under the exposure to magnetic fields. Solid-state optical detectors concede to MCP-PMTs in terms of noise characteristics, time resolution, and active area.
Low noise characteristic is necessary for photon counting. Detection efficiency depends to a great extent on the threshold level set for the counting (discrimination) mode. The pulse height distribution is a critical feature for photon counting mode. Single-electron peak presence in the single photon counting mode (Fig.1, 2) allows cutting off a large number of small amplitude noise pulses without significant losses of registration efficiency. Since, typically the lower threshold of the pulse amplitude is set (discriminated) at the valley position; the high peak-to-valley ratio in the pulse height distribution creates conditions for more efficient registration of very weak luminescence objects, allowing increasing the signal-to-noise ratio.
On the other hand, PMTs with conventional MCPs (unlike ALD-coated ones) are characterized by such disadvantage as short lifetime. According to historical data [1–3] it is indicated that a significant degradation of the PMT performance is observed when the total charge passing through the MCP exceeds tenths and sometimes even hundredths of Cl/cm2. Short lifetime is determined by ion bombardment of the PMT photocathode. Ions are generated in MCP channels as a result of decomposition and desorption of foreign particles on the channel walls when bombarded by electrons and as a result of ionization of desorbed compounds and residual gases by electron fluxes. The generated ions are accelerated by the electric field towards the photocathode, where they are either absorbed or destroy the molecular structure. As a consequence, the quantum efficiency of the photocathode is reduced. Ion feedback also negatively affects amplitude resolution, peak-to-valley ratio, and dark count rate.
To increase the lifetime of MCP-PMTs, some engineers deposit a protective aluminum film on the first and sometimes on the second MCP (in order not to reduce the physical transparency of the MCP chevron stack input). The film allows electrons to pass through, but is opaque to ions. This technique is used by Hamamatsu in the R 3809U series of photon counters.
However, the aluminum film deposited on the MCP surface absorbs 30–50% of the bombarding electrons, i. e. it reduces the effective signal, worsening the signal-to-noise ratio.
The PMT design and the method of MCP electron degassing developed in VTC “Baspik” LLC allow to practically eliminating the ion feedback. This makes possible minimizing dark count rate density, improve the single-electron pulse height distribution and peak-to-valley ratio to uniquely high values, increase the permissible input load and expand PMTs dynamic range, significantly enlarge lifetime up to Cl/cm2.
We developed 2 modifications of PMTs characterized by the type of photocathode: “Sapphire‑2A” with a tellurium-cesium photocathode (for the UV region of the spectrum) and “Topaz” with a bi-alkali photocathode (for the visible range of the spectrum). Typical spectral characteristics of photocathodes are shown in Fig. 3–4.
The developed PMTs constructively consist of metal-glass vacuum units with end optical input (bi-alkali antimony-potassium-sodium photocathode on glass substrate or tellurium-cesium photocathode on magnesium fluoride substrate), assembled in chevron stack MCPs with active area diameter of 18 mm and channel diameter of 8 µm and metal collector. PMT “Sapphire‑2A” photocathode active area diameter is 15 mm, which of the PMT “Topaz” is 17 mm. The PMTs have an electron immersion lens at the input – a system of electrodes that collect electrons from the photocathode to the input of the first MCP. The design contains a heated gas absorber PC‑1M located in the anode cone part of the cathode chamber. The external view of the PMT is presented in Fig. 5.
Use of immersion lens at the MCP-PMT input contributes to the increase in the rate of permissible load (due to the longer distance between the electrodes, reducing the probability of breakdown at a sharp increase in the input signal). The larger volume of the product than in proximity-focused designs results in the fact that the same amount of gas emitted by electron bombardment during of the PMT operation causes a smaller change in the residual pressure, reducing the number of ions produced.
The developed PMTs can operate for a long time (hundreds of hours in practice) at count rates of 1–3 MHz. The peak-to-valley ratio in the single-electron pulse height distribution of the developed PMTs can reach 20 and more (Fig. 2), whereas in dynode PMTs for similar applications, as well as in MCP-PMTs by other manufacturers (Fig. 1), the typical value is close to 2. Such a high peak-to-valley ratio according to the available information [5] has not been recorded in any MCP-PMT. The authors [5] report that their experiments with PMTs with a stack of 2 MCPs showed the best peak-to-value ratio – 6, amplitude resolution – 86% and they note that, according to their data, these values are the best known for PMTs with two MCPs. Hamamatsu [6] estimates as a great success the peak-to valley ratio of 5.6 and amplitude resolution of 104% in the IIT photon counting tube with a stack of 3 MCPs, while in the “Sapphire‑2A” and “Topaz” PMTs with two MCPs the typical value of these parameters is 10 and 90%, respectively. The counting characteristic of the PMT has a plateau region with duration of almost 300 V (Fig. 6).
Developed PMTs have already been applied in nuclear physics as detectors of Cherenkov radiation.
We can supply PMTs both as vacuum units and with a voltage divider integrated into a single housing with a vacuum unit. In this case, a resistive voltage divider is mounted on the vacuum unit, which provides the necessary potentials of the PMT electrodes. The external view of the MCP-PMT with integrated voltage divider is shown in Fig.7. The entire device is housed and sealed with a compound, attached to the output end of the envelope is a 50-Ohm SMA output connector for output signal acquisition and a high-voltage SHV connector for the power supply.
AUTHORS
Kulov Soslan Kubadievich, Dr. of Sc. (Engin), CEO, LLC VTC “Baspik”, Vladikavkaz, Russia.
Alkatseva Tatyana Danilovna, Cand. of Sc. (Engin), dir. for quality, LLC VTC “Baspik”, Vladikavkaz, Russia.
Fedotova Galina Vasilievna, head lab. MCP detectors, VTC “Baspik”, Vladikavkaz, Russia.
Sentsova Elena Igorevna, senior engineer, lab. MCP detectors, VTC “Baspik”, Vladikavkaz, Russia.
CONFLICT OF INTERESTS
The article has been prepared based on the work of all composite authors. All authors took part in preparation of the article and supplemented the manuscript in terms of their scope of work. The authors declare that they have no conflict of interests.
CONTRIBUTION BY THE MEMBERS
OF THE TEAM OF AUTHORS
The article was prepared based on the work of all members of the team of authors. All authors participated in the writing of the article and contributed to the manuscript in part of their work. The authors declare that there is no conflict between them.
S. K. Kulov, T. D. Alkatseva, G. V. Fedotova, E. I. Sentsova
VTC “Baspik” LLC, Vladikavkaz
The development of MCP-PMT – photon counting UV–VIS detectors for optical-physical measurements is reported. MCP-PMTs design and technology features are described, which allow achieving high peak-to-valley ratio, low dark count rate and longer lifetime. We present different versions of PMT constructions (in the form of vacuum units and with a voltage divider integrated into a single housing with a vacuum unit).
Keywords: photon counting detectors, pulse height distribution, dark count rate, ion feedback, peak-to-valley ratio, PMT with integrated voltage divider
Article received:11.01.2024
Article accepted:05.02.2024
When detecting extremely weak photon signals, the photon counting method is used to extract the maximum information. Photon counting detectors are one of the main types of detectors for high-energy and nuclear physics. Discrete dynodes PMTs, MCP-PMTs, avalanche dynodes, and hybrid PMTs are usually used as photon counting detectors. Among these detector types, MCP-PMTs are favorably characterized with high amplification and fast response, increased surface area, low power consumption, and stable operation under the exposure to magnetic fields. Solid-state optical detectors concede to MCP-PMTs in terms of noise characteristics, time resolution, and active area.
Low noise characteristic is necessary for photon counting. Detection efficiency depends to a great extent on the threshold level set for the counting (discrimination) mode. The pulse height distribution is a critical feature for photon counting mode. Single-electron peak presence in the single photon counting mode (Fig.1, 2) allows cutting off a large number of small amplitude noise pulses without significant losses of registration efficiency. Since, typically the lower threshold of the pulse amplitude is set (discriminated) at the valley position; the high peak-to-valley ratio in the pulse height distribution creates conditions for more efficient registration of very weak luminescence objects, allowing increasing the signal-to-noise ratio.
On the other hand, PMTs with conventional MCPs (unlike ALD-coated ones) are characterized by such disadvantage as short lifetime. According to historical data [1–3] it is indicated that a significant degradation of the PMT performance is observed when the total charge passing through the MCP exceeds tenths and sometimes even hundredths of Cl/cm2. Short lifetime is determined by ion bombardment of the PMT photocathode. Ions are generated in MCP channels as a result of decomposition and desorption of foreign particles on the channel walls when bombarded by electrons and as a result of ionization of desorbed compounds and residual gases by electron fluxes. The generated ions are accelerated by the electric field towards the photocathode, where they are either absorbed or destroy the molecular structure. As a consequence, the quantum efficiency of the photocathode is reduced. Ion feedback also negatively affects amplitude resolution, peak-to-valley ratio, and dark count rate.
To increase the lifetime of MCP-PMTs, some engineers deposit a protective aluminum film on the first and sometimes on the second MCP (in order not to reduce the physical transparency of the MCP chevron stack input). The film allows electrons to pass through, but is opaque to ions. This technique is used by Hamamatsu in the R 3809U series of photon counters.
However, the aluminum film deposited on the MCP surface absorbs 30–50% of the bombarding electrons, i. e. it reduces the effective signal, worsening the signal-to-noise ratio.
The PMT design and the method of MCP electron degassing developed in VTC “Baspik” LLC allow to practically eliminating the ion feedback. This makes possible minimizing dark count rate density, improve the single-electron pulse height distribution and peak-to-valley ratio to uniquely high values, increase the permissible input load and expand PMTs dynamic range, significantly enlarge lifetime up to Cl/cm2.
We developed 2 modifications of PMTs characterized by the type of photocathode: “Sapphire‑2A” with a tellurium-cesium photocathode (for the UV region of the spectrum) and “Topaz” with a bi-alkali photocathode (for the visible range of the spectrum). Typical spectral characteristics of photocathodes are shown in Fig. 3–4.
The developed PMTs constructively consist of metal-glass vacuum units with end optical input (bi-alkali antimony-potassium-sodium photocathode on glass substrate or tellurium-cesium photocathode on magnesium fluoride substrate), assembled in chevron stack MCPs with active area diameter of 18 mm and channel diameter of 8 µm and metal collector. PMT “Sapphire‑2A” photocathode active area diameter is 15 mm, which of the PMT “Topaz” is 17 mm. The PMTs have an electron immersion lens at the input – a system of electrodes that collect electrons from the photocathode to the input of the first MCP. The design contains a heated gas absorber PC‑1M located in the anode cone part of the cathode chamber. The external view of the PMT is presented in Fig. 5.
Use of immersion lens at the MCP-PMT input contributes to the increase in the rate of permissible load (due to the longer distance between the electrodes, reducing the probability of breakdown at a sharp increase in the input signal). The larger volume of the product than in proximity-focused designs results in the fact that the same amount of gas emitted by electron bombardment during of the PMT operation causes a smaller change in the residual pressure, reducing the number of ions produced.
The developed PMTs can operate for a long time (hundreds of hours in practice) at count rates of 1–3 MHz. The peak-to-valley ratio in the single-electron pulse height distribution of the developed PMTs can reach 20 and more (Fig. 2), whereas in dynode PMTs for similar applications, as well as in MCP-PMTs by other manufacturers (Fig. 1), the typical value is close to 2. Such a high peak-to-valley ratio according to the available information [5] has not been recorded in any MCP-PMT. The authors [5] report that their experiments with PMTs with a stack of 2 MCPs showed the best peak-to-value ratio – 6, amplitude resolution – 86% and they note that, according to their data, these values are the best known for PMTs with two MCPs. Hamamatsu [6] estimates as a great success the peak-to valley ratio of 5.6 and amplitude resolution of 104% in the IIT photon counting tube with a stack of 3 MCPs, while in the “Sapphire‑2A” and “Topaz” PMTs with two MCPs the typical value of these parameters is 10 and 90%, respectively. The counting characteristic of the PMT has a plateau region with duration of almost 300 V (Fig. 6).
Developed PMTs have already been applied in nuclear physics as detectors of Cherenkov radiation.
We can supply PMTs both as vacuum units and with a voltage divider integrated into a single housing with a vacuum unit. In this case, a resistive voltage divider is mounted on the vacuum unit, which provides the necessary potentials of the PMT electrodes. The external view of the MCP-PMT with integrated voltage divider is shown in Fig.7. The entire device is housed and sealed with a compound, attached to the output end of the envelope is a 50-Ohm SMA output connector for output signal acquisition and a high-voltage SHV connector for the power supply.
AUTHORS
Kulov Soslan Kubadievich, Dr. of Sc. (Engin), CEO, LLC VTC “Baspik”, Vladikavkaz, Russia.
Alkatseva Tatyana Danilovna, Cand. of Sc. (Engin), dir. for quality, LLC VTC “Baspik”, Vladikavkaz, Russia.
Fedotova Galina Vasilievna, head lab. MCP detectors, VTC “Baspik”, Vladikavkaz, Russia.
Sentsova Elena Igorevna, senior engineer, lab. MCP detectors, VTC “Baspik”, Vladikavkaz, Russia.
CONFLICT OF INTERESTS
The article has been prepared based on the work of all composite authors. All authors took part in preparation of the article and supplemented the manuscript in terms of their scope of work. The authors declare that they have no conflict of interests.
CONTRIBUTION BY THE MEMBERS
OF THE TEAM OF AUTHORS
The article was prepared based on the work of all members of the team of authors. All authors participated in the writing of the article and contributed to the manuscript in part of their work. The authors declare that there is no conflict between them.
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