Phototubes, light-sensitive

The PM tube has a light-sensitive electrode called the photocathode. It emit electrons when photons strike it. The electrons are then accelerated from the photocathode to the anode of PM tube by the application of approximately 1000 V in steps of approximately 100 V by a series of electrodes called the dynodes. In the PM tube, secondary electrons are produced, resulting in pulses of 10 to 10 electrons. Typically a phototube with 10 dynodes delivers approximately 4 ° electrons. This gain or amplification is dependent on the dynode voltages. 

Light-sensitive phototubes also can be used to determine relative spectral line intensities. Two approaches are used for this purpose. The large, direct reading spectrometers use a battery of phototubes, one for each spectral line desired, located at the individual focal points. Usually the output of the phototube is collected over a specified time interval and stored in a capacitor. After exposure the capacitor is discharged into some type of read-out device. This method integrates the total energy over a time interval to provide a measure of spectral energy. 

A multiplier phototube consists of a light-sensitive photocathode followed by a series of dynodes arranged to multiply the electron emission from the photocathode. The electrical arrangement and basic circuitry used are described in Chapter 9. 

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. 

Slight improvements in sensitivity can be achieved by cooling the phototubes used to detect the emitted light or by increasing the ethylene flow rate. Chemiluminescence produced by the reaction of ozone with ethylene has been designated by the epa as the reference method for monitoring ozone. Several different commercially produced instru< ments are available. 

QUANTUM EFFICIENCY. A measure of the efficiency of conversion or utilization of light or other energy, being in general the ratio of the number of distinct events produced in a radiation sensitized process to the number of quanta absorbed (the intensity-distribution of the radiation in frequency or wavelength should be specified). In the photoelectric and photoconductive effects, the quantum efficiency is the number of electronic charges released for each photon absorbed. For a phototube, the quantum... 

Because of differences among photosensitive cathodes, certain phototubes are more sensitive in certain regions of the light spectrum. Others are more sensitive when used in combination with preliminary filters that screen out specific regions of the spectrum. Accordingly, certain spectrophotometers require an additional filter or a special red-sensitive phototube when they operate in the red or near-IR ranges of the light spectrum. 

Raman spectroscopy. — Routine Raman spectra were obtained on a Cary 83 Raman Spectrometer (488 nm). For higher resolution work, and for compounds sensitive to blue light, a Spex 1401 double monochromator, and a detection system that utilized photon counting techniques was used in conjunction with a variety of laser lines (principally 488, 514.5 and 647.1 nm). The spectrometer was coupled to an on-line computer which allowed the data to be collected, stored, corrected for phototube sensitivity, normalized and plotted. Powdered samples were loaded into 1 mm o. d. quartz capillaries in the Drilab, sealed temporarily with a plug of Kel-F grease, and the tube drawn down and sealed in a small flame outside the drybox. 

The technique of thermoparticulate analysis (TPA) consists of the detection of evolved particulate material in the evolved gases as a function of temperature. In the presence of supersaturated water vapor, these particles provide condensation sites for water, and hence can be detected by light-scattering techniques. Water droplets grow very rapidly on the particulate matter (condensation nuclei) until they are of a sufficient size to scatter light. The scattered light, as detected by a phototube in a dark-field optical system, is proportional to the number of condensation nuclei initially present. It is an extremely sensitive measurement, with the capability of detecting one part of material in 1015 parts of air. The technique was first employed by Doyle (90) and has been reviewed by Murphy (91. 92). 

One of the most satisfactory types of apparatus yet developed is that of Zimm (1948 b). In his arrangement, the cell containing the liquid under study is a small thin-walled glass bulb, immersed in an outer cell containing a liquid of approximately the same refractive index as that in the inner cell. The scattered light is received on a photomultiplier tube, while a portion of the incident beam falls on a phototube of much lower sensitivity. The currents produced in the two tubes are balanced in a precision potentiometer at balance, the setting of the potentiometer shows the ratio of the currents. Details of the electronic circuit are given... 

The optimum light collection geometry for chemiluminescent solutions uses an ellipsoidal reflector with the light source at one focus and a cooled phototube photocathode at the other. Individual colored glass filters are placed near the face of the phototube and the emission spectrum is determined by the differences in filter transmissions. With this idealized system the authors report a sensitivity such that their noise equivalent signal at 500 nm and a bandwidth of 30 nm is (Inaba et at., 1979)... 

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