Phototubes

Photo thermography Phototubes Photovoltaic Photovoltaic cells... 

Analytical Applications. Chemiluminescence and bioluminescence are useful in analysis for several reasons. (/) Modem low noise phototubes when properly instmmented can detect light fluxes as weak as 100 photons/s (1.7 x 10 eins/s). Thus luminescent reactions in which intensity depends on the concentration of a reactant of analytical interest can be used to determine attomole—2eptomole amounts (10 to 10 mol). This is especially useful for biochemical, trace metal, and pollution control analyses (93,260—266) (see Trace and residue analysis). (2) Light measurement is easily automated for routine measurements as, for example, in clinical analysis. 

The sensitivity of a photo-emissive cell (phototube) may be considerably increased by means of the so-called photomultiplier tube. The latter consists of an electrode covered with a photo-emissive material and a series of positively charged plates, each charged at a successively higher potential. The plates are covered with a material which emits several (2-5) electrons for each electron collected on its surface. When the electrons hit the first plate, secondary electrons are emitted in greater number than initially struck the plate, with the net result of a large amplification (up to 106) in the current output of the cell. The output of a photomultiplier tube is limited to several milliamperes, and for this reason only low incident radiant energy intensities can be employed. It can measure intensities about 200 times weaker than those measurable with an ordinary photoelectric cell and amplifier. 

Phototube detectors are normally sensitive either to radiation of wavelength 200 nm to 600-650 nm, or of wavelength 600-1000 nm. To scan a complete spectral range an instrument must therefore contain two photocells a red sensitive cell (600-800 nm) and a blue cell (200-600 nm). 

Lee, J., and Seliger, H. H. (1965). Absolute spectral sensitivity of phototubes and the application to the measurement of the absolute quantum yields of chemiluminescence and bioluminescence. Photochem. Photobiol. 4 1015-1048. 

It is difficult to overestimate the importance of the multiplier phototube, first made available by the Radio Corporation of America, in the detection of x-rays, 7-rays, and nuclear particles. The device is sensitive to x-rays directly, but better results are obtained if the x-rays are first converted to visible light b r a phosphor. A picture of a Du Mont No. 6291 multiplier phototube is shown in Figure 2-5b. 

Fig. 2-5a. Schematic diagram of No. 931-A multiplier phototube. 0 = photocathode 1-9 = dynodes 10 = anode. (Radio Corporation of America.)...

Fig. 2-5b. Photograph of Du Mont No. 0291 multiplier phototube. (Allen B. Du Mont Laboratories, Inc.)...

The multiplication process is repeated in each succeeding stage, the electrons from the (specially shaped) dynode 9 being collected by the anode 10. A multiplier phototube of this type is normally operated at 75 to 100 volts per stage, and an over-all gain of a million can be realized. 

The response time of the multiplier phototube ( 10 9 second) is small enough for all practical counting rates and generally lower by several powers of ten than that of the phosphor used to convert the x-rays to visible light, or of the scaling circuits. 

Fig. 2-6. Schematic diagram illustrating the essential components of the phototimer circuit. When the total x-rays reaching the multiplier phototube, P, generate sufficient current to charge capacitor, C, to a predetermined potential the thyratron, T, fires and turns off the x-rays by means of the relay. (After Morgan, Am. J. Roentgenol. Radium Therapy, 48, 220.)...

In the phosphor-photoelectric detector used as just described, the x-ray quanta strike the phosphor at a rate so great that the quanta of visible light are never resolved they are integrated into a beam of visible light the intensity of which is measured by the multiplier phototube. In the scintillation counters usual in analytical chemistry, on the other hand, individual x-ray quanta can be absorbed by a single crystal highly transparent to light (for example, an alkali halide crystal with thallium as activator), and the resultant visible scintillations can produce an output pulse of electrons from the multiplier phototube. The pulses can be counted as were the pulses-from the proportional counter. 

Scintillation counters also have a characteristic plateau. In the scintillation counter the phototube acts as the primary amplifier with a gain as high as 106. A low-gain linear amplifier may be used in conjunction with a scintillation counter, and, again, the range of amplification in the plateau will be about 103 or 104. 

The top and the bottom x-ray detector each contain a multiplier phototube coated with phosphor. This tube compares the intensity of the x-ray beam entering the detector with that of the light from the reference standard, a discharge lamp. The reference beam is part of a circuit that maintains the x-ray source at constant intensity. The deviation wedge comes to rest when the intensities of the transmitted x-ray beams stand in a predetermined ratio. At this point, the unbalance in the servo system has been compensated, and the position of the deviation wedge consequently indicates the thickness of the strip. In 1955, this application was made fully automatic that is, the unbalance (or error signal) just mentioned was used to readjust tandem cold reduction mills of the United States Steel Corporation. Automatic control proved significantly more effective than manual control. 

The photometer is adequately described in Figure 3-2. In the phosphor-photoelectric detector (2.10), the x-ray beam strikes a silver-activated zinc sulfide phosphor to produce blue-violet light that is changed by the multiplier phototube (Type 931-A) into an electric current that is amplified and read on a suitable micro- or milliammeter. A stable power supply for both x-ray tube and detector circuit are essential, as is clear from the circuit diagrams.10... 

Fig. 3-2. A, Phosphor-photoelectric detector B, sample cell C, sample D, CA-5 x-ray tube and housing E, milliammeter F, amplifier and rectifier vacuum tubes G, regulated power supply for amplifier tubes and multiplier phototube H, control panel.

In the Sunbury x-ray photometer, described by Cranston, Matthews, and Evans,25 commutation between standard and unknown (25 times a second) is achieved in a somewhat different way. This instrument uses two x-ray beams from a single source—one through a standard hydrocarbon, the other through an unknown hydrocarbon on which sulfur is to be determined. The beams strike fluorescent screens in a light-tight box. A chopping disk interposed between the screens and a single multiplier phototube is used to accomplish commutation betw fen standard and unknown. 

Table 8-2 contains unpublished results fron the authors laboratory that illustrate the effectiveness of pulse-height selection in the determination of light elements. In the case of silicon, the background was due mainly to scattered x-rays. In the case of sulfur, multiplier phototube noise was also present. The counting interval was 10 seconds for Nt (total count) and for Nb (background). The excellent results for sulfur could not have been obtained had there not been careful and fortunate selection of the multiplier phototube. 

Nitrocellulose foils as standards of thickness, 297-300 Nobel Prize awards, 2 Noise, from amplifier, 59, 60 from multiplier phototube, pulse-height selection for removal, 219 Nomenclature of x-ray analytical instruments, 124, 125... 

Phototubes, multiplier, 56-59, 222 Placement error, 285-287 Planck s constant, 7, 8 Plastics, characterization by absorptiometry, 78, 79 Plateau, characteristic, 60 Platinum, determination by x-ray emission spectrography, 161, 328 L peaks, measured by Bragg, 25, 26, 35 L spectra, 35... 

Resolution, analysis complicated by insufficient, 201 of x-rays, 61, 113-115 Response time of multiplier phototube, 57 Rhenium, determination by x-ray emission spectrography, 328 Rhodium, determination by x-ray emission spectrography, 328 Risk, consumer, 215 producer, 215... 

In dc amplification, the signal pulses from the phototube anode are converted into photocurrent and the voltage drop produced is read and the output displayed on a recorder. 

In the pulse counting method, each photoelectron pulse arriving at the phototube anode is processed. The pulses are amplified and then used to trigger a pulse generator. The output pulses from the generator are integrated and displayed on a recorder. 

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