Pmt handbook complete hamamatsu

Haneman: Phys. Solids, 11, Gobeli and F. Allen: Phys. Fisher, R. Enstrom, J. Escher, H. Devices, Vol ED, No. Sanford and N. Macdonald: J. Fisher and G. Olsen: J. Bradshaw, W. Choyke and R. Devaty: J. Iams and B. Salzberg: Proc. IRE, 23, 55 Zworykin, G. Morton, and L. Malter: Proc. IRE, 24, Zworykin and J. Rajchman: Proc. IRE, 27, Morton: RCA Rev. Morton: IRE Trans. Hinteregger: Rev. Sternglass: Rev. Young: J. Dormont and P. Saget: J. Radium Physique Appliquee , 20, 23A Goodrich and W.

Wiley: Rev. Figure shows the schematic construction of a photomultiplier tube. This secondary emission is repeated at each of the successive dynodes. This chapter describes the principles of photoelectron emission, electron tra- jectory, and the design and function of electron multipliers. The electron multi- pliers used for photomultiplier tubes are classified into two types: normal dis- crete dynodes consisting of multiple stages and continuous dynodes such as mi- crochannel plates.

Since both types of dynodes differ considerably in operating principle, photomultiplier tubes using microchannel plates MCP-PMTs are separately described in Chapter Furthermore, electron multipliers for vari- ous particle beams and ion detectors are discussed in Chapter The photocathode has the former effect and the latter are represented by the photoconductive or photovoltaic effect.

Since a photocathode is a semiconductor, it can be described using band models as shown in Figure 1 alkali photocathode and 2 III-V compound semiconductor photocathode. When photons strike a photocathode, electrons in the valence band absorb photon energy hv and become excited, diffusing toward the photocathode surface. If the diffused electrons have enough energy to overcome the vacuum level barrier, they are emitted into the vacuum as photoelectrons.

L becomes longer by use of a better crystal and Ps greatly depends on electron affinity EA. Figure 2 shows the band model of a photocathode using III-V compound semiconductors. This bending can make the electron affinity nega- tive. This state is called NEA negative electron affinity. The NEA effect increases the probability Ps that the electrons reaching the photocathode surface may be emitted into the vacuum. In particular, it enhances the quantum efficiency at long wavelengths with lower excitation energy.

In addition, it lengthens the mean es- cape distance L of excited electrons due to the depletion layer. Photocathodes can be classified by photoelectron emission process into a reflection mode and a transmis- sion mode.

The reflection mode photocathode is usually formed on a metal plate, and photoelectrons are emitted in the opposite direction of the incident light. The transmission mode photocathode is usually depos- ited as a thin film on a glass plate which is optically transparent. Photoelectrons are emitted in the same direction as that of the incident light. Refer to Figures , and The reflection mode photocathode is mainly used for the side-on photomultiplier tubes which receive light through the side of the glass bulb, while the transmission mode photocathode is used for the head-on photomultiplier tubes which detect the input light through the end of a cylindrical bulb.

The wavelength of maximum response and long-wavelength cutoff are determined by the combination of alkali metals used for the photocathode and its fabrication process. This "S" number indicates the combination of a photocathode and window material and at present, numbers from S-1 through S have been registered.

However, other than S-1, S, S and S these numbers are scarcely used. Refer to Chapter 4 for the spectral response characteristics of various photocathodes and window materials. Numerical analysis of the electron trajectory using high-speed, large-capacity computers have come into use. This method divides the area to be analyzed into a grid-like pattern to give boundary conditions, and obtains an approximation by repeating computations until the error converges to a certain level.

By solving the equation for motion based on the potential distribution obtained using this method, the electron trajectory can be predicted. When designing a photomultiplier tube, the electron trajectory from the photocathode to the first dynode must be carefully designed in consideration of the photocathode shape planar or spherical window , the shape and arrangement of the focusing electrode and the supply voltage, so that the photoelectrons emitted from the photocathode are efficiently focused onto the first dynode.

The collection efficiency of the first dynode is the ratio of the number of electrons landing on the effective area of the first dynode to the number of emitted photoelectrons. This is usually better than 60 to 90 percent. In some applications where the electron transit time needs to be minimized, the electrode should be designed not only for optimum configuration but also for higher electric fields than usual.

The dynode section is usually constructed from several to more than ten stages of secondary-emissive electrodes dynodes having a curved surface.

To enhance the collection efficiency of each dynode and mini- mize the electron transit time spread, the optimum configuration and arrangement should be determined from an analysis of the electron trajectory. The arrangement of the dynodes must be designed in order to prevent ion or light feedback from the latter stages. In addition, various characteristics of a photomultiplier tube can also be calculated by computer simula- tion. For example, the collection efficiency, uniformity, and electron transit time can be calculated using a Monte Carlo simulation by setting the initial conditions of photoelectrons and secondary electrons.

This allows collective evaluation of photomultiplier tubes. Figures , and are cross sections of photomul- tiplier tubes having a circular-cage, box-and-grid, and linear-focused dynode structures, respectively, show- ing their typical electron trajectories.

Photoelectrons emitted from the photocathode are multiplied by the first dyn- ode through the last dynode up to 19 dynodes , with current amplification ranging from 10 to as much as times, and are finally sent to the anode. Major secondary emissive materials17 used for dynodes are alkali antimonide, beryllium oxide BeO , magnesium oxide MgO , gallium phosphide GaP and gallium phosphide GaAsP.

These materials are coated onto a substrate electrode made of nickel, stainless steel, or copper-beryllium alloy. Figure shows a model of the secondary emission multiplication of a dynode.

Refer to section 4. Because a variety of dynode structures are available and their gain, time response and linearity differ depending on the number of dynode stages and other factors, the optimum dynode type must be selected according to your application.

These characteristics are described in Chapter 4, section 4. Anodes are carefully designed to have a structure optimized for the electron trajectories discussed previ- ously. Generally, an anode is fabricated in the form of a rod, plate or mesh electrode. One of the most impor- tant factors in designing an anode is that an adequate potential difference can be established between the last dynode and the anode in order to prevent space charge effects and obtain a large output current.

Poultney: Advances in Electronics and Electron Physics 31, 39 Seib and L. Ankerman: Advances in Electronics and Electron Physics, 34, 95 Boutot, et al. Hayashi: Bunkou Kenkyuu, 22, Sonnenberg: Appl. Spicer, et al. Pacific, 84, Hagino, et al. Anode output uniformity is thought to be the product of the photocathode uniformity and the electron multiplier dynode section uniformity. Figure shows anode uniformity data measured at wavelengths of nanometers and nanom- eters.

This data is obtained with a light spot of 1 mm diameter scanned over the photocathode surface. This is because the cathode sensitivity near the long-wavelength limit greatly depends on the surface conditions of the photocathode and thus fluctuations increase. Moreover, if the supply voltage is too low, the electron collection efficiency be- tween dynodes may degrade and adversely affect uniformity.

Head-on photomultiplier tubes provide better uniformity in comparison with side-on types. In such appli- cations as gamma cameras used for medical diagnosis where good position detecting ability is demanded, uniformity is an important parameter in determining equipment performance.

Therefore, the photomultiplier tubes used in this field are specially designed and selected for better uniformity. Figure shows typical uniformity data for a side-on tube. The same measurement procedure as for head-on tubes is used. Uniformity is also affected by the dynode structure. As can be seen from Table , the box-and-grid type, venetian blind type and mesh type offer better uniformity.

The following sections explain their measurement procedures and typical characteristics. Figure shows a schematic diagram for the spatial uniformity measurement. The direction of the X-axis or Y-axis is determined with respect to the orientation of the first dynode as shown in Figure The degree of loss of electrons in the dynode section significantly depends on the position of the first dynode on which the photoelectrons strike.

Refer to Figure for specific uniformity data. In some cases, spatial uniformity is measured by dividing the photocath- ode into a grid pattern, so that sensitivity distribution is displayed in two or three dimensions. The spatial uniformity of anode output ranges from 20 to 40 percent for head-on tubes, and may exceed those values for side-on tubes.

The adverse effects of the spatial uniformity can be minimized by placing a diffuser in front of the input window of a photomultiplier tube or by using a photomultiplier tube with a frosted glass window. This dependence on the incident angle is called the angular response. A schematic diagram for the angular response measurement is shown in Figure and specific data is plotted in Figure As the rotary table is rotated, the projected area of the photocathode is reduced. This means that the output current of a photomultiplier tube is plotted as a cosine curve of the incident angle even if the output has no dependence on the incident angle.

Com- monly, the photocathode sensitivity improves at larger angles of incidence and thus the output current is plotted along a curve showing higher sensitivity than the cosine cos curve. This is because the incident light transmits across a longer distance at large angles of incidence.

In addition, this increase in sensitivity usually becomes larger at longer wavelengths. On the other hand, the performance deterioration resulting from the stress imposed by the supply voltage, current, and ambient temperature is called "fatigue". Since the cathode sensitivity of a photomultiplier tube exhibits good stability even after long periods of operating time, the drift and life characteristics primarily depend on variations in the secondary emission ratio.

In other words, these characteristics indicate the extent of gain variation with operating time. Drift per unit time generally improves with longer operating time and this tendency continues even if the photomultiplier tube is left unused for a short time after operation.

Aging or applying the power supply voltage to the photomultiplier tube prior to use ensures more stable operation. Since drift and life characteristics greatly depend on the magnitude of signal output current, keeping the average output current within a few microamperes is usually recommended. At Hamamatsu Photonics, drift is usually measured in the DC mode by illuminating a photomultiplier tube with a continuous light and recording the output current with the operating time. Figure shows specific drift data for typical Hamamatsu photomultiplier tubes.

In most cases, the drift of a photomulti- plier tube tends to vary largely during initial operation and stabilizes as operating time elapses. In addition, there are other methods for evaluating the drift and life characteristics, which are chiefly used for photomultiplier tubes designed for scintillation counting. For more details refer to Chapter 7, "Scintillation counting". Through this aging, drift can be effectively stabilized.

In addition, if the photomultiplier tube is warmed up just before actual use, the drift will be further stabilized. The warm-up period should be longer at the initial phase of photomulti- plier tube operation, particularly in intermittent operation. After a long period of operation warm-up can be shortened. At a higher anode current the warm-up period can be shortened and at a lower anode current the warm-up should be longer. In most cases, a warm-up is performed for several ten minutes at a supply voltage near the actual operating voltage and an anode current of several microamperes.

However, in low current operation average output current from less than one hundred up to several hundred nanoamperes , a warm-up is done by just applying a voltage to the photomultiplier tube for about one hour in the dark state. This phenomenon is known as "hysteresis". Hysteresis is further classified into "light hysteresis" and "voltage hysteresis" depending on the measurement conditions. Some photomultiplier tubes have been designed to suppress hysteresis by coating the insulator surface of the electrode supports with a conductive material so as to minimize the electrostatic charge on the electrode supports without impairing their insulating properties.

This variation is called light hyster- esis. Figure shows the Hamamatsu test method for light hysteresis and typical hysteresis waveforms. I max. Ii I min. The photomultiplier tube is warmed up for five minutes or more at a light level producing an anode current of approximately 1 microampere. Then the incident light is shut off for one minute and then input again for one minute.

This procedure is repeated twice to confirm the reproducibility. Table shows typical hysteresis data for major Hamamatsu photomultiplier tubes. Since most photo- multiplier tubes have been designed to minimize hysteresis, they usually only display a slight hysteresis within 1 percent.

It should be noted that light hysteresis behaves in different patterns or values, depending on the magnitude of the output current. In this case, the photomultiplier tube output may overshoot or undershoot immediately after the supply voltage is changed. This phenomenon is called voltage hysteresis and should be suppressed to the minimum possible value. Generally, this voltage hysteresis is larger than light hysteresis and even tubes with small light hysteresis may possibly exhibit large voltage hysteresis.

Refer to Table below for typical hysteresis data. A photomultiplier tube is operated at a voltage V, which is volts lower than the voltage used to measure the anode luminous sensitivity. The tube is warmed up for five minutes or more at a light level producing an anode current of approximately 0. Then the light level and supply voltage are returned to the original conditions.

This procedure is repeated to confirm the reproducibility. By measuring the variations in the anode outputs, the extent of voltage hysteresis is expressed in percent, as shown in Eq.

In general, the higher the change in the supply voltage, the larger the voltage hysteresis will be. Other characteristics are the same as those for light hysteresis.

Voltage hysteresis may be improved by use of HA coating. Refer to section 8. This output current is called the dark current 1 23 25 33 and ideally it should be kept as small as possible because photomultiplier tubes are used for detecting minute amounts of light and current.

Figure shows a typical dark current vs. Region a is dominated by the leakage current, region b by the thermionic emission, and region c by the field emission and glass or electrode support scintillation. In general, region b provides the best signal-to-noise ratio, so operating the photomultiplier tube in this region would prove ideal. Ion feedback 34 and noise 34 35 36 originating from cosmic rays and radioisotopes will sometimes be a problem in pulse operation. When a photocathode is exposed to room illumination, the dark current will return to the original level by storing the photomultiplier tube in a dark state for one to two hours.

However, if exposed to sunlight or extremely intense light 10, lux or higher , this may cause unrecoverable damage and must therefore be avoided. It is recommended to store the photomultiplier tube in a dark state before use. The dark current data furnished with Hamamatsu photomultiplier tubes is measured after the tube has been stored in a dark state for 30 minutes.

This "minute storage in a dark state" condition allows most photomultiplier tubes to approach the average dark current level attained after being stored for a long period in a dark state. This is also selected in consideration of the work efficiency associated with measur- ing the dark current. If the tube is stored for a greater length of time in a dark state, the dark current will decrease further. The following sections explain each of the six causes of dark current listed above.

This effect has been studied by W. Richardson, and is stated by the following equation. Thus the magnitude of the work function as well as the photocath- ode material govern the amount of thermionic emission. When the photocathode work function is low, the spectral response extends to the light with lower energy or longer wavelengths, but with an in- crease in the thermionic emission.

Among generally used photocathodes composed of alkali metals, the Ag-O-Cs photocathode with a spectral response in the longest wavelength range see Figure exhibits the highest dark current. In contrast, the photocathodes for the ultraviolet range Cs-Te, Cs-I exhibit the shortest wavelength upper limit and provide the lowest dark current. Therefore, as shown in Figure , cooling a photomultiplier tube is an effective technique for reducing the dark current. Although thermionic emission occurs both from the photocathode and the dynodes, the thermionic emission from the photocathode has a much larger effect on the dark current.

This is because the photocathode is larger than each dynode in size and also because the dynodes, especially at the latter stages, contribute less to the output current.

Consequently, the dark current caused by the thermionic emission vs. Figure describes temperature characteristics for dark pulses measured in the photon counting method. In this case as well, the number of dark pulses is decreased by cooling the photocathode. Therefore, the quality of the insulating materials used in the tubes is very important.

For instance, if the insulation resistance is around 10 12 ohms, the leakage current may reach the nanoampere level. The relationship between the leakage current from the insulating materials and the supply voltage is determined by Ohm's law, i. On the other hand, the dark current resulting from thermionic emission varies exponentially with the supply voltage.

Thus, as mentioned in the previous section, the leakage current has relatively more effect on the dark current as the supply voltage is lowered. A leakage current may be generated between the anode and the last dynode inside a tube. It may also be caused by imperfect insulation of the glass stem and base, and between the socket anode pin and other pins. Since contamination from dirt and moisture on the surface of the glass stem, base, or socket increases the leakage current, care should be taken to keep these parts clean and at low humid- ity.

If contaminated, they can be cleaned with alcohol in most cases. This is effective in reducing the leakage current. If these stray electrons impinge on the glass envelope, scintillations may occur and result in dark pulses. In general, a photomultiplier tube is operated with a negative high voltage applied to the photocathode and is housed in a metal case at ground potential.

This arrangement tends to cause stray electrons to impinge on the glass envelope. However, this problem can be minimized by using a technique called "HA coating". Subsequently the dark current increases abruptly. This phenom- enon occurs in region c in Figure and shortens the life of the photomultiplier tube considerably.

Therefore, the maximum supply voltage is specified for each tube type and must be observed. As long as a photomultiplier tube is operated within this maximum rating there will be no problem. But for safety, operating the photomultiplier tube at a voltage 20 to 30 percent lower than the maximum rating is recommended. Even so, there exist residual gases that cannot be ignored. The molecules of these residual gases may be ionized by collisions with electrons.

The positive ions that strike the front stage dynodes or the photocathode produce many secondary electrons, resulting in a large noise pulse. During high current operation, this noise pulse is usually identified as an output pulse appearing slightly after the main photocurrent. This noise pulse is therefore called an afterpulse 38 39 40 and may cause a measurement error during pulsed operation.

Among them, muons can be a major source of photomultiplier tube noise. When muons pass through the glass envelope, Cherenkov radia- tion may occur, releasing a large number of photons.

In addition, most glasses contain potassium oxide K 2O which also contains a minute amount of the radioactive element 40 K. Furthermore, environmental gamma rays emitted from radioisotopes con- tained in buildings may be another noise source. However, because these dark noises occur much less frequently, they are negligible except for applications such as liquid scintillation counting where the number of signal counts is exceptionally small.

There are various methods and terms used to express dark current. The following introduces some of them. The dark current may be measured at a voltage at which a particular value of anode sensitivity is obtained. In this case, the dark current is expressed in terms of equivalent dark current or EADCI equivalent anode dark current input. The equivalent dark current is simply the dark current measured at the voltage producing a specific anode luminous sensitivity, and is a convenient param- eter when the tube is operated with the anode sensitivity maintained at a constant value.

Figure illustrates an example of EADCI data along with the anode dark current and anode luminous sensitivity. A better signal-to-noise ratio can be obtained when the tube is operated in the supply voltage region with a small EADCI.

The lower limit of light detection is determined rather by the fluctuating components or noise. In this case, the noise is commonly expressed in terms of ENI equivalent noise input. The ENI is the value of inci- dent light flux required to produce an output current equal to the noise current, i.

When the ENI is expressed in units of watts W at the peak wavelength or at a specific wavelength, it is also referred to as the NEP noise equivalent power. Normally, these noise components are governed by the dark current generated by the photocathode thermionic emission and the shot noise resulting from the signal current. Both of these noise sources are discussed here. The signal-to-noise ratio referred to in the following description is expressed in r.

The noise component produced in the multiplication process is commonly expressed in terms of the NF noise figure From this equation and Eq. To obtain a better signal-to-noise ratio, the shot noise should be minimized and the following points ob- served: 1 Use a photomultiplier tube that has as high a quantum efficiency as possible in the wavelength range to be measured. These prove that the relation in Eq. Taking into account the contribution of the cathode equiva- lent dark current Id and the noise current NA of the amplifier circuit, Eq.

Pi where Sp is the anode radiant sensitivity and Pi is the incident light power. Detection limits at different bandwidths are plotted in Figure When compared to ENI obtained from Eq. The detection limit can be approximated as ENI when the frequency bandwidth B of the circuit is low up to about a few kilohertz , but it is dominated by the shot noise component originating from signal light at higher bandwidths.

Refer to Chapter 6, "Photon Counting". Since these pulses appear after the signal output pulse, they are called afterpulses. Afterpulses often disturb accurate measurement of low level signals following a large amplitude pulse, degrade energy resolution in scintillation counting See Chapter 7. Types of afterpulses There are two types of afterpulses: one is output with a very short delay several nanoseconds to several tens of nanoseconds after the signal pulse and the other appears with a longer delay ranging up to several microseconds, each being generated by different mechanisms.

In general, the latter pulses appearing with a long delay are commonly referred to as afterpulses. Most afterpulses with a short delay are caused by elastic scattering electrons on the first dynode. The probability that these electrons are produced can be reduced to about one-tenth in some types of photomulti- plier tubes by placing a special electrode near the first dynode. Usually, the time delay of this type of afterpulse is small and hidden by the time constant of the subsequent signal processing circuit, so that it does not create significant problems in most cases.

However, this should be eliminated in time-correlated photon counting for measuring very short fluorescence lifetime, laser radar LIDAR , and fluorescence or particle measurement using an auto correlation technique. In contrast, afterpulses with a longer delay are caused by the positive ions which are generated by the ionization of residual gases in the photomultiplier tube.

These positive ions return to the photocathode ion feedback and produce many photoelectrons which result in afterpulses.

The amplitude of this type of afterpulse depends on the type of ions and the position where they are generated. The time delay with respect to the signal output pulse ranges from several hundred nanoseconds to over a few microseconds, and depends on the supply voltage for the photomultiplier tube. Helium gas is known to produce afterpulses because it easily penetrates through a silica bulb, so use caution with operating environments.

Afterpulses can be reduced temporarily by aging See 4. In actual measurements, the frequency of afterpulses and the amount of charge may sometimes be a prob- lem. The amount of output charge tends to increase when the photomultiplier tube is operated at a higher supply voltage, to obtain a high gain, even though the number of generated ions is the same. In pulse counting applications such as photon counting, the frequency of afterpulses with an amplitude higher than a certain threshold level will be a problem.

As explained, afterpulses appear just after the signal pulse. Depending on the electrode structure, another spurious pulse prepulse may be observed just before the signal pulse output. But, this pulse is very close to the signal pulse and has a low amplitude, causing no problems.

Also it should be noted that light may be polarized at such optical devices as monochromators. When polarized light enters the photocathode of a photomultiplier tube, the photocathode reflectance varies with the angle of incidence. This effect is also greatly dependent on the polarization component as shown in Figure In this figure, Rp is the polarization component parallel to the photocathode surface P component and Rs is the polarization component perpendicular to the photo- cathode surface S component.

It is clear that the photocathode reflectance varies with the angle of incidence. Because this figure shows the calculated examples with the assumption that the absorption coefficient at the photocathode is zero, the actual data will be slightly more complicated.

The polarized light is then focused onto the photomultiplier tube through L2 condenser lens. The dependence on the polarized light is measured by recording the photo- multiplier tube output in accordance with the rotating angle of the polarizer. In this case, the polarization component of the light source must be removed. This is done by interposing a diffuser plate such as frosted glass or by compensating for the photomultiplier tube output values measured when the tube is at 0 degree and is then rotated to 90 degrees with respect to the light axis.

Figure illustrates the polarized-light dependence of a side-on photomultiplier tube with a reflection type photocathode.

In principle, this dependence exists when the light enters slantways with respect to the photocathode surface. In actual operation, the polarization factor P is almost zero when the light enters per- pendicular to the transmission type photocathode surface.

Figure indicates the relative output of a reflection-type photocathode photomultiplier tube as a function of the angle of incident light. It can be seen that the polarization factor P becomes smaller as the direction of the incident light nears the perpendicular of the photocathode surface.

The reflection-type photocathode photomultiplier tubes usually exhibit a polarization factor of about 10 percent or less, but tubes specially designed to minimize the polarization-light dependence offer three percent or less. A single crystal photocathode such as gallium arsenide GaAs has high reflectance and show a polarization factor of around 20 percent, which is higher than that of alkali antimonide photocathodes. In contrast, the polarization that gives the minimum sensitivity is the component parallel to the tube axis S component , independent of the type of tube and wavelength of incident light.

As can be seen from Figure , this is probably due to a change in the photocathode transmittance. The S component increases in reflectance as the angle of incidence becomes larger, whereas the P component decreases.

More- over, as the wavelength shifts to the longer side, the reflectance generally decreases and the polarization factor P becomes smaller accordingly, as shown in Figure In applications where the polarized-light dependence of a photomultiplier tube cannot be ignored, it will prove effective to place a diffuser such as frosted glass or tracing paper in front of the input window of the photomultiplier tube or to use a photomultiplier tube with a frosted window.

Hirohata and Y. Mizushima: Japanese Journal of Applied Physics. Hirohata, T. Ihara, M. Miyazaki, T. Suzuki and Y. Parkhurst, S. Dallek and B. Larrick: J. Soc, , Dallek, W. Parkhurst and B. Cook: Phys. A25, ; 26, Kimble and L. Mandel: Phys. A30, Miyao, T. Wada, T. Nitta and M. Hagino: Appl. A7, A5, MacDonald: J. Domke, T. Mandle, C. Laubschat, M. Prietsch and G. Kaindl: Surf. Niigaki, T. Suzuki, H. Kan and T. Hiruma: Appl. Nakamura, H. Rodway: Surf. Bamford: Phys. Glasses, 3, Kume, K. Koyama, K.

Nakatsugawa, S. Suzuki and D. Fatlowitz: Appl. Opt, 27, Chiba and L. Mmandel: J. B,5, Jones: Appl. Miller, et al. NS-3, 91 Young: Appl. Oxford Morton et al. NS No. Staubert et al. Hall et al. Robber: Appl. Hoenig and A.

Cutler E: Appl. Hora: Phys. Soli Vol a , MEMO This chapter explains how to use the basic circuits and accessories necessary to operate a photomultiplier tube properly. This voltage gradient can be set up using independent multiple power supplies as shown in Figure , but this method is not practical.

Sometimes Zener diodes are used with voltage-dividing resistors as shown in Figure 2. These circuits are known as volt- age-divider circuits. In this case, Ib is obtained by using Eq. This noise becomes significant when the current flowing through the Zener diodes is insufficient. Thus care is required at this point, as this noise can affect the signal-to-noise ratio of the photomultiplier tube output.

This scheme eliminates the potential voltage difference between the external circuit and the anode, facilitating the connection of circuits such as ammeters and current-to- voltage conversion operational amplifiers to the photomultiplier tube. In this anode grounding scheme, how- ever, bringing a grounded metal holder, housing or magnetic shield case near the bulb of the photomultiplier tube, or allowing it to make contact with the bulb can cause electrons in the photomultiplier tube to strike the inner bulb wall.

This may possibly produce glass scintillation, resulting in a significant increase in noise. Also, for head-on photomultiplier tubes, if the faceplate or bulb near the photocathode is grounded, the slight conductivity of the glass material causes a small current to flow between the photocathode and ground. This may cause electric damage to the photocathode, possibly leading to considerable deterioration.

For this reason, extreme care must be taken when designing the housing for a photomultiplier tube and when using an electromagnetic shield case. In addition, when wrapping the bulb of a photomultiplier tube with foam rubber or similar shock-absorbing materials before mounting the tube within its electromagnetic shield case at ground potential, it is very important to ensure that the materials have sufficiently good insulation properties.

The above problems concerning the anode grounding scheme can be solved by coating the bulb surface with black conductive paint and connecting it to the cathode potential. This technique is called "HA coating", and the conductive bulb surface is protected by a insulating cover for safety. In scintillation counting, how- ever, because the grounded scintillator is usually coupled directly to the faceplate of a photomultiplier tube, the cathode is grounded with a high positive voltage applied to the anode, as shown in Figure In actual scintillation counting using this voltage-divider circuit, a problem concerning base-line shift may occur if the counting efficiency increases too much, or noise may be generated if a leakage current is present in the coupling capacitor.

Thus care should be taken regarding these points. Likewise, for other voltage-divider resis- tors, the actual current is the difference between the divider current Ib and the dynode current IDy flowing in the opposite direction through the voltage-divider resistor. The anode current and dynode current flow act to reduce the divider current and the accompanying loss of the interstage voltage becomes more signifi- cant in the latter dynode stages which handle larger dynode currents. However, when the incident light level is increased and the resultant anode and dynode currents are increased, the voltage distribution for each dynode varies considerably as shown in Figure Because the overall cathode-to- 5.

Therefore, the shift in the voltage distribution to the earlier stages results in a collective increase in current amplification, as shown at region B in Figure If the incident light level is increased further so that the anode current becomes quite large, the second- ary-electron collection efficiency of the anode degrades as the voltage between the last dynode and the anode decreases.

This leads to the saturation phenomenon like that shown at region C in Figure To increase the maximum linear output, there are two techniques: one is to use a Zener diode between the last dynode and the anode as shown in Figure 2 and, if necessary, between the next to last or second to last stage as well, and the other is to lower the voltage-divider resistor values to increase the divider current. However, with the former technique, if the divider current is insufficient, noise will be generated from the Zener diode, possibly resulting in detrimental effects of the output.

Because of this, it is essential to increase the divider current to an adequate level and connect a ceramic capacitor having good frequency response in parallel with the Zener diode for absorbing the possible noise.

It is also neces- sary to narrow the subsequent circuit bandwidth as much as possible, insofar as the response speed will permit. With the latter technique, if the voltage-divider resistors are located very close to the photomulti- plier tube, the heat emanating from their resistance may raise the photomultiplier tube temperature, lead- ing to an increase in the dark current and possible fluctuation in the output.

Furthermore, since this tech- nique requires a high-voltage power supply with a large capacity, it is advisable to increase the divider current more than necessary. To solve the above problems in applications where a high linear output is required, individual power supply boosters may be used in place of the voltage-divider resistors at the last few stages.

To prevent this problem, decoupling capacitors can be connected to the last few stages, as shown in Figures 1 and 2. These capacitors supply the photomultiplier tube with an electric charge during the forming of signal pulse and restrain the voltage drop between the last dynode and the anode, resulting in a significant improvement in pulse linearity.

If the pulse width is sufficiently short so that the duty cycle is small, this method makes it possible to derive an output current up to the saturation level which is caused by the space charge effects in the photomultiplier tube dynodes discussed in Chapter 4.

Consequently, a high peak output current, more than several thousand times as large as the divider current can be attained. There are two methods of using the decoupling capacitors: a serial connection method and a parallel connection method as illustrated in Figure below.

The serial connection is more commonly used be- cause the parallel connection requires capacitors which can withstand a high voltage. The following explains the procedure for calculating the capacitor values, using the circuit shown in Figure 1 as an example. It is therefore sug- gested that the voltage-divider circuit be designed with a safety margin in the capacitance value, of about 10 times larger than the calculated values.

If the output current increases further, additional decoupling capacitors should be connected as necessary to the earlier stages, as well as increasing the capacitance values of C1 to C3. Particular care is required when operating at high counting rates even if the output peak current is low.

This is caused by an increase in the electron density between the electrodes, causing space charge effects which disturb the electron current. The intensity of an AC input light varies with time. A pulsed input light arrives as discrete packets of photons. For AC and pulsed light, the capacitance of a photodetector affects the rise time, time jitter, and detection bandwidth.

Dynamic range and linearity should also be taken into account in pulsed and AC light. The intensity of the incident light can be a function of time. It is a DC light if the intensity is constant, AC light if the intensity varies with time, and pulsed light if it arrives as discrete packets of photons.

A DC light poses no additional restrictions on the photodetector but the other two do. For AC and pulsed light, capacitances — junction, parasitic, or terminal — matter: their values affect the output signal rise time, time jitter, and detection bandwidth. Increasing the load resistance increases the output voltage but reduces the bandwidth. Because the quantity of noise in the output increases with bandwidth, a well-designed system has just enough bandwidth for the purpose of the observation.

A detector's rise time t r and its bandwidth B are related by a well-known approximate equation. The measured value of t r depends on the operating conditions of a detector. Detection electronics a load resistor, transimpedance amplifier, or charge amplifier also affect t r and, thus B.

The higher the values of R L and C t , the smaller the bandwidth. Dynamic range and a related concept, linearity, of a photodetector should also be examined in pulsed and AC detection. Is there enough dynamic range to detect the full variation of the incident intensity?

Are the input and output signals linearly related — if not, what is the deviation and is it acceptable? Several factors can influence the dynamic range of a photodetector. A few among them are an intrinsic noise floor, biasing level, output load, and, in the case of a SiPM, number of cells. Hamamatsu provides information on dynamic range and linearity most commonly in the form of graphs, for example, linearity versus the power of incident light or linearity versus the load resistance. Which detector should the experimenter select?

The crucial comparison is the bandwidth. The PD, which is easier to operate and less expensive than the PMT, has enough bandwidth for the required detection and would be an appropriate selection. It is important in the experimental setup that the output has the highest possible signal fidelity. Which of the two photodetectors is a better choice in this case? Given that the output signal fidelity is the primary requirement, the PMT is a better choice because of its larger bandwidth.

If, however, the incoming light is diffuse, focusing optics will not increase the incident power diffuse light cannot be focused ; the only other option is to use a detector with a larger active area. The tradeoff is a higher dark current in the photodetector, which increases noise and, therefore, NEP. If the selection process based on WITS did not yet produce a unique and outstanding choice unlikely but possible , the price may be able to break the tie.

The prices can vary greatly among the different models of a photodetector in a given family; however, when the typical representatives of the families are compared, the highest to lowest prices are for a PMT, SiPM, APD, and PD. This is a price for a stand-alone photodetector. If the potential user needs to design the detection setup from the ground up, the cost of auxiliary equipment such as power supplies, amplifiers, etc.

All of the detectors described in this article are also offered as part of a module. Although in most cases the selection process would end here, there are applications and situations where additional criteria need to be considered.

Examples are ruggedness, immunity to an external magnetic field, ease of use, physical size, and ease of customization. If the device may be customized, how much does it cost? How long is the lead time? The selection process can be taxing, confusing, and potentially costly. Establishing a partnership with the device manufacturer can make the process much quicker and results in the optimal selection.

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Interactive tools. Brochures Research papers. Resources Top. Figure 1. Light from the system under investigation may contain information about the system in the form of spectral composition, intensity light power transferred per unit area , temporal characteristics constant intensity, pulses, exponential decay, etc.

An overview of photodetector types The following four sections describe the four photodetectors, emphasizing their typical physical, optoelectronic, and operational characteristics. Photomultiplier tubes PMT A PMT, depicted in Figure 2, is a vacuum tube that has the following components: a light-sensitive cathode photocathode where the conversion of light signal to electrical signal occurs through the process of extrinsic photoelectric effect; a sequence of dynodes electron multipliers where the electrical signal is amplified through the process of secondary electron emission; and an anode where the signal is received and transferred to outside detection electronics.

Figure 2. Structure of a photomultiplier tube head-on type. Photodiodes PD A photodiode, illustrated in Figure 3, is a solid-state device with a p-n junction. Figure 3. Structure of a photodiode. Avalanche photodiodes APD An APD, portrayed in Figure 4, is a type of a PD that has an internal gain due to impact ionization of lattice atoms by charge carriers in the high-field section of the depletion region.

Figure 4. Structure of an avalanche photodiode APD. Figure 5.