The photomultiplier tube is a vacuum tube composed of a light incident window, a photocathode, a multiplication stage, and an anode. The photons are irradiated onto the photocathode through the light window, and the photocathode generates photoelectrons, which are then accelerated and aggregated into the multiplication system, where the dynode electrons are multiplied by secondary emission, and the secondary emission is repeated on each dynode. The electrons that are received by the anode multiply by 106 to 107 times, or even more.
Figure 1 (a): PMT (end window type) sectional view
Figure 1 (b): Side window type PMT=
Side window type photomultiplier tubes generally have relatively high gain and are widely used in spectrophotometers and general photometric systems.
Figure 1 (c): End window type PMT
The end window type photomultiplier tube directly forms a photocathode on the inner surface of the light incident window, and is often used for radiation measurement since the scintillator is relatively easy to couple to the light incident window.
Photocathode
Different photocathode materials can make the PMT sensing range have different spectral response characteristics. If the incident window material is matched, the overall sensing range of the PMT can be appropriately modulated. For the photocathode material, since the physical properties of the alkali metal are the most active, almost all of the alkali metal elements are contained. How are these photocathodes discovered, approved and used? Next, let's take a closer look at the PMT "photocathode technology".
1, alkaline photocathode
Compared to other photodetectors, photomultipliers have excellent characteristics in terms of signal-to-noise ratio due to the low noise electron multiplier. In order to further improve the signal-to-noise ratio and obtain higher sensitivity, the quantum efficiency of the photocathode is further improved. Figure 2 shows the relationship between quantum efficiency and the wavelength of a typical photocathode currently in use.
In 1951, American Sommer (Fig. 3 left) invented the photocathode treatment process to produce a multi-alkali photocathode by reacting a layer of Sb with Na, K, and Cs. This photocathode has high sensitivity over a wide spectral range from UV to 850 nm and is used in spectrophotometers and in fluorescence measurements in biological and genetic related fields.
The double-base cathode is made by reacting Sb with K and Cs and has high sensitivity around 400 nm. PMT using this double-base photocathode is widely used for scintillation counting of radiation measurements because this spectral response characteristic matches the emission wavelength of the NaI scintillator. Incidentally, this double-alkali photocathode was also invented by Sommer in 1963. Following Sommer's invention of this photocathode, later professional workers have further improved these two photocathodes in practice, making them the most widely used PMT photocathodes today. The working principle of the photocathode can be described by the energy band model. According to the energy band theory, new semiconductor photocathodes and high-sensitivity double-alkali photocathodes have been developed, which also open the way to enhance the photocathode sensitivity and the extended spectral response range. .
Figure 2: Quantum efficiency versus wavelength for different photocathodes
Figure 3: Dr. Alfred H. Sommer (left) visited Hamamatsu on October 25, 1984. On the right is the former chairman of Hamamatsu Corporation, Ma Mafu.
Quantum efficiency (abbreviated as QE) is the number of photoelectrons emitted by a photocathor divided by the number of incident photons, usually expressed as a percentage.
2. Photocathode band model
Because the photocathode is a kind of semiconductor, its operation can be described by the energy band theory. The band theory has an energy band gap (E g ), an electron affinity (E a ), a Fermi level (E f ), Work function Terms such as φ ). Figure 4 shows an alkali cathode energy band model. When a photon strikes the photocathode, the electrons in the valence band absorb photon energy (hv), which is excited to the conduction band and diffuses toward the photocathode surface. If the energy of these electrons exceeds the vacuum barrier, they are launched into the vacuum. This electron emission process is expressed by WE Spicer as follows.
R : reflection coefficient
a : photon total optical absorption coefficient
aPE: absorption coefficient when electrons are excited to an energy level higher than the vacuum level
L: length of electron diffusion
Ps: Probability of electron escape into vacuum
v: optical frequency
This is called Spicer's three-step model, which explains the photoelectron emission process in three steps: the light absorption process, the electron diffusion process, and the escape process. Applying this expression, the crystal properties of the photocathode can be enhanced by increasing the diffusion length L, and the quantum affinity can be improved by increasing the Ps to reduce the electron affinity.
Figure 4: Alkali metal cathode energy band model
3. High sensitivity alkali metal photocathode
In 2007, Hamamatsu successfully enhanced the quantum efficiency of the alkali cathode by improving the activation process of the photocathode. In the photocathode, when the peak wavelength of the photocathode reaches 350 nm, the quantum efficiency can reach an average of 43%, and is named as ultra bialkali (UBA). In addition to the top dibasic base, we have developed another moderately sensitive photocathode called "Super Double Base", or SBA for short, which has an average quantum efficiency of 35% at a wavelength of 350 nm. . Figure 5 shows typical spectral response characteristics of UBA, SBA, and common double-base photocathodes.
Figure 5: QE curves for UBA, SBA and standard double-alkali cathodes
Photocatalyst is super double base of Hamamatsu high quantum efficiency type PMT
4. Development of semiconductor photocathode
In addition to the enhancement of the alkali bismuth photocathode, researchers are also very keen on the development of semiconductor photocathodes such as GaAs. This study found that the formation of an electric double-layer cerium oxide on the surface of a semiconductor crystal treated by Cs-O activation causes the surface energy band curve to bend downward, so that the electron affinity has a negative value. This photocathode is called a NEA (negative electron affinity) photocathode. Figure 6 shows an energy band model of a single crystal GaAS activated by Cs-O. Since NEA allows electrons to escape at the bottom of the conduction band, its sensitivity can be extended to 900 nm corresponding to the electron band gap.
Figure 6: Energy band model of GaAs photocathode
Photocathode material is GaAs's Hamamatsu side window type PMT
It is inferred from the energy band model that a semiconductor with a higher energy band gap will have a larger NEA. Therefore, with the study of GaAs photocathodes, research on GaAsP photocathodes has also begun. Figure 7 shows the energy band model of GaAsP. At present, the peak quantum efficiency of GaAsP photocathode can exceed 50% in practical applications.
Figure 7: Energy band model of GaAsP photocathode
The photocathode material is GaAsP Hamamatsu MCP-PMT
5, near infrared photocathode
In order to obtain sufficient sensitivity in the wavelength band exceeding 1.1 um, Hamamatsu developed the InP/InGaAs photocathode. This photocathode forms a Schottky junction by evaporating a metal film (such as silver) (about 50 angstroms thick) on the surface of the semiconductor. A bias voltage is applied to the back side of the Schottky electrode and the semiconductor crystal such that an electric field is formed in the photocathode, which greatly reduces the surface barrier, accelerates the photoelectrons, and emits photoelectrons into the vacuum.
Figures 8(a) and (b) show the photoelectron emission band model of a heterojunction field combined with a photocathode. When no bias voltage is applied, the photoelectrons excited by the absorption layer cannot reach the emission surface due to the presence of the conduction band barrier ΔEc between the InGaAs photon absorption layer and the InP electron emission layer, as shown in Fig. 8(a). However, when a certain bias voltage is applied, a depletion layer is formed inside the silver Schottky electrode and the photocathode, and the depletion layer finally reaches the interface between the InGaAs light absorbing layer and the InP electron emission layer, and thus is excited in the absorption layer. The electrons can reach the InP electron emission layer across the barrier. In addition, photoelectrons are accelerated within the InP electron-emitting layer such that they are picked up from the bottom of the conduction band to a higher energy level band L and are emitted from the emission surface into the vacuum while maintaining a high energy level.
Figure 8 (a): Energy band model of InP/InGaAs photocathode
Figure 8 (b): InP/InGaAs photocathode band model with bias voltage applied
Photocathode material is InGaAs's Hamamatsu side window type PMT
6. Photocathode for operation at low temperature
"Dark matter" is a hot topic in the study of astrophysics. It has been suggested to use photomultiplier tubes to capture weak ultraviolet photons to detect dark matter. These photons are emitted by accidental dark matter and scintillator atoms colliding. Liquid helium (-108 ° C) or liquid argon (-186 ° C) was used as the scintillator. At such a low temperature, the surface resistance of the photocathode becomes large, resulting in a limited photocathode current. The linearity of the output due to the increased sheet resistance is degraded, which is very deadly for many tests. The photocathode developed by Hamamatsu for low temperature operation can solve this problem.
A conventional photocathode operating at low temperatures has a layer of aluminum at the bottom of the cathode. Figure 9 shows typical spectral response characteristics of a conventional aluminum-attached photocathode and a novel low-temperature photocathode. The quantum efficiency of the new photocathode at 420 nm is about 28%, although it is slightly lower than the SBA photocathode, but 1.5 times higher than the conventional photocathode. Figure 10 shows a comparison of the linearity of a conventional aluminum-attached photocathode and a novel low-temperature photocathode. When operating at -100 ° C, the output linearity of a conventional photocathode begins to drop rapidly at approximately 0.5 nA, while the low temperature photocathode maintains linearity at 1 uA, where linearity is defined as the output current deviates from the initial value - Current at 5%.
Figure 9: Spectral response characteristics of a novel low temperature double-base photocathode
Figure 10: Comparison of linearity at -100 °C
7. Photocathode for high temperature operation
In the process of oil well exploration and recording, in order to locate the location of oil or natural gas storage, the detector needs to enter the borehole to a depth of 2,000 meters (70 ° C) to 3,000 meters (105 ° C). This requires the development of detectors that can withstand higher temperatures. The study of new alkali source technologies is not only difficult because of the replacement of new detectors during drilling, but also because of the increasing depth of the wells and the high temperature requirements of PMT. It is getting higher and higher. The photocathode of the PMT will gradually dissolve at the high temperature of the oil well exploration, however, the photocathode manufactured by the Sb-K-Na hybrid can withstand such high temperatures. Hamamatsu has also developed a photocathode that can operate at 200 ° C for more than 1000 hours. The photocathode also has a very low dark current at room temperature, making it ideal for low light detection and other applications requiring low noise.
Figure 11 compares the output life characteristics of a conventional high temperature photocathode and a new high temperature photocathode. It can be seen that the new photocathode has a working life at high temperature of about 8 times that of a conventional photocathode.
Figure 11: Output life characteristics at 200 ° C high temperature
The photocathode material is a high temperature double base Hamamatsu end window type PMT
8, ultraviolet cathode
Using GaN semiconductors, our company successfully produced the world's first transmissive UV cathode. GaN is usually formed by epitaxial growth on a sapphire substrate. Later, the development of GaN growth technology using silicon substrates made it possible to grow high-quality epitaxial films on silicon substrates.
Using this technology, a breakthrough in the formation of gallium nitride by buffer layer epitaxial growth on a silicon substrate has been achieved. This technique allows epitaxial growth of GaN crystals to adhere to the glazing, which is then treated to leave only a gallium nitride film for use. We also use an optical cleaning method that uses light to clean the crystal surface. This technique achieves a satisfactory quantum efficiency of 21.5% at a wavelength of 280 nm. Figure 12 shows typical spectral response characteristics of a GaN photocathode and a conventional Cs-Te photocathode.
GaN photocathodes are currently used in UV image intensifiers for low-light detection and fast multi-channel (two-dimensional) measurements including semiconductor wafer inspection, Lehman spectrometers, and high-voltage transmission line discharge detection.
Figure 12: GaN photocathode (quantum efficiency at 380 nm is 21.5%)
Comparison of spectral response characteristics with Cs-Te photocathode
Now, although part of the work of PMT has been replaced by semiconductor detectors, with the development of innovative PMT photocathode technology, PMT has more complex functions and more application possibilities. In the future, PMT will be widely used in low-light detection, medical equipment, biotechnology-related equipment, oil well detection equipment, and astronomical observation equipment for high-energy physics experiments. These applications require higher quantum efficiency, a wider spectral response range (extending into the infrared region), and higher sensitivity in the UV region. Hamamatsu will continue to develop a wider spectral response range and higher sensitivity PMT (QE=100%) to meet these special application needs.
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