Photomultiplier Anode

K. Hadener, S. Bergamasco and G. Calzaferri, Conversion and amplification of photomultiplier anode current in multiple-frequency phase fluorometry, Rev. Sci. Instrum. 59, 1924-1927 (1988). 

In ProgrRept No 10 (Ref 6) luminosity time tests employing a wide band Tektronix 517 oscillograph and frequency compensated photomultiplier anode circuitry are briefly described. Fig 2 of the rept (reproduced here as Fig 5) shows radiation luminosity waveform from a stepped rod of Tetryl. 

The successive dynodes of a photomultiplier operated at an overall gain of 106 could then amplify this quantity of photoelectrons so that 3.3 x 108 electrons would be collected at the photomultiplier anode, or a charge of 5 x 10-11 C. 

Secondary Emission - Electrons striking the surface of a cathode could cause the release of some electrons and, hence, a net amplification in the number of electrons. This principle is used in the construction of photomultipliers where light photons strike a photoemitting cathode releasing photoelectrons. These electrons are subsequently amplified striking a number of electrodes (called dynodes) before they are finally collected by the anode. 

Fig. 18 Section through an RCA photomultiplier, schematic, dynodes, 10 anode.

Fig. 20 Cross section through a head on photomultiplier [51, 52]. — 1—10 dynodes, 11 anode.

Photoemission is the property of some materials to emit electrons when light falls on them. These materials are used as a cathode and an anode collects the electrons. One application is in the photomultiplier unit that is used in counters. 

Based on the photoelectric effect, electrons in evacuated tubes (photoelectrons) are released from a metal surface if it is irradiated with photons of sufficient quantum energy. These are simple photocells. Photomultipliers are more sophisticated and used in modem spectrophotometers where, via high voltage, the photoelectrons are accelerated to another electrode (dynode) where one electron releases several electrons more, and by repetition up to more than ten times a signal amplification on the order of 10 can be obtained. This means that one photon finally achieves the release of 10 electrons from the anode, which easily can be measured as an electric current. The sensitivity of such a photomultiplier resembles the sensitivity of the human eye adapted to darkness. The devices described are mainly used in laboratory-bound spectrophotometers. 

The ellipsometer used in this study is described elsewhere(3). It consists of a Xenon light source, a monochromator, a polarizer, a sample holder, a rotating analyzer and a photomultiplier detector (Figure 1). An electrochemical cell with two windows is mounted at the center. The windows, being 120° apart, provide a 60° angle of incidence for the ellipsometer. A copper substrate and a platinum electrode function as anode and cathode respectively. Both are connected to a DC power supply. The system is automated with a personal computer to collect all experimental data during the deposition. Data analysis is carried out by a Fortran program run on a personal computer. 

For detection of ultraviolet photons the preferred device is the photomultiplier tube. This device has an electron emissive anode (called a dynode) that photons strike and eject electrons. However, the electrons are attracted toward a second electron emissive surface where each electron generated at the first dynode will eject several electrons toward a third dynode. Approximate 10-12 dynodes are arranged as shown in Fig. 5.12. Each dynode is biased so that it is 90 V more positive than the previous dynode so that the ejected electrons are attracted from one dynode to the next. 

For UV and visible radiation, the simplest detector is a photomultiplier tube. The cathode of the tube is coated with a photosensitive material (such as Cs3Sb, K CsSb, or Na2KSb, etc.) which ejects a photoelectron when struck by a photon. This photoelectron is then accelerated towards a series of anodes of successively greater positive potential (called dynodes). At each dynode, the electron impact causes secondary electron emission, which amplifies the original photoelectron by a factor of 106 or 107. The result is a pulse of electricity of duration around 5 ns, giving a current of around 1 mA. This small current is fed into the external electronics and further amplified by an operational amplifier, which produces an output voltage pulse whose height is proportional to the photomultiplier current. 

A much better time resolution, together with space resolution, can be obtained by new imaging detectors consisting of a microchannel plate photomultiplier (MCP) in which the disk anode is replaced by a coded anode (Kemnitz, 2001). Using a Ti-sapphire laser as excitation source and the single-photon timing method of detection, the time resolution is <10 ps. The space resolution is 100 pm (250 x 250 channels). 

Figure 2.26 Photoemissive tubes. Light enters a simple phototube (a) and causes the release of electrons from the photoemissive alloy of the cathode. Owing to the potential difference between the anode and the cathode, the electrons are captured by the anode and the resulting current can be amplified and measured. Photomultiplier tubes (b) are a development of simple phototubes and result in internal amplification of the current initially developed at the photocathode.

S. Charbonneau, L. B. Allard, J. F. Young, G. Dyck, and B. J. Kyle, Two-dimensional time resolved imaging with 100-ps resolution using a resistive anode photomultiplier tube, Rev. Sci. Instrum. 63(11), 5315-5319 (1992). 

A schematic drawing of a photomultiplier is shown in Figure 3.17. A photomultiplier consists of a photocathode, a chain of dynodes, and a collector (anode). The light to be detected illuminates the photocathode (the active area of the photomultiplier), which generates electrons due to the absorption of incident photons. These electrons are accelerated and amplified by the dynodes and, finally, they arrive at the anode, where are monitored as an induced current. 

Taking a typical value of 5 = 5 and considering 10 dynodes. Equation (3.8) gives a gain of G = (which is of the order of 10 ). The particular value of 5 depends, of course, on the dynode material and on the voltage applied between dynodes. Similarly to the case of the photocathode, the responsivity of a photomultiplier, R , is defined as the current induced in the anode divided by the power of the light reaching the photocathode. Thus, it is very simple to show that... 

Once the electrons have been accelerated and multiplied, they reach the anode. The electrons arriving at the anode produce an electrical current. This current can be measured directly, or indirectly by monitoring the voltage increment induced in a given load resistor, Rl. This load resistor is critical, as it determines the time constant of the photomultiplier. A typical time constant for a photomultiplier is 2 ns, although an adequate choice of the load resistor and anode material could lead to time constants as low as 0.5 ns. 

When time-dependent signals are to be measured by a photomultiplier, the time sensitivity is usually limited by the inhomogeneous transit time. The transit time is the time taken by electrons generated in the cathode to arrive at the anode. If all of the emitted electrons had the same transit time, then the current induced in the anode would display the same time dependence as the incoming light, but delayed in time. However, not all of the electrons have the same transit time. This produces some uncertainty in the time taken by electrons to arrive at the anode. There are two main causes of this dispersion ... 

Figure 3.20 shows the effect of the transit time dispersion on the measurement of an ideal light pulse. Since photoelectrons spend some time traveling from the photocathode to the anode (transit time), the photomultiplier signal is delayed in time with respect to the incident pulse. Furthermore, due to the transit time dispersion, the... 

The current from the anode of the photomultiplier is fed to an RC circuit which effectively acts as an integrator, summing the current pulses and tending to average out the noise. RC times of 1 sec are usually satisfactory for most studies. However, for very weak signals, RC of longer times can be used. 

A second photomultiplier tube looking through a slit scans the oscilloscope trace and converts the time-density function into an intensity versus time plot. This second photomultiplier tube has an RC integration circuit in its anode to improve the statistical sampling by averaging the information. Zarowin states that, if the pulse rate is 100 per sec, and if the RC time is 1 sec, one obtains an effective increase in intensity of 100. This apparatus is shown in detail in Fig. 15. 

Photomultipliers are vacuum tube photocells with a sealed-in set of dynodes. Each successive dynode is kept at a potential difference of 100V o that photoelectrons emitted from the cathode surface are accelerated M each step. The secondary electrons ejected from the last dynode are Collected by the anode and are multiplied so that a 10° — 107 — fold arnpli-t tfion of electron flux is achieved. This allows simple devices such as l croammeters to measure weak light intensities. Background thermal mission can be minimised by cooling the photomultiplier. The schematic... 

At the electron multiplier detector, each arriving ion starts a cascade of electrons, just as a photon starts a cascade of electrons in a photomultiplier tube (Figure 20-12). A series of dynodes multiplies the number of electrons by 105 before they reach the anode where current is measured. The mass spectrum shows detector current as a function of mlz selected by the magnetic field. 

The most important light detector in photochemistry is the photomultiplier (PM) tube. It is based on the photoelectric effect (section 2.1), but the primary electrons released by light are accelerated over a number of dynodes to produce an avalanche of secondary electrons (Figure 7.24). A single photon can produce a pulse of some 106 electrons at the anode. Each of these pulses lasts about 5 ns, so that when the light intensity is rather high these single pulses combine to form a steady electric current. This current is amplified and displayed on a chart recorder or computer. 

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