Sensitive Phototubes

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. 

Different types of photosensitive detectors have been used for spectral intensity measurements, including barrier layer photocells, vacuum and gas photodiodes, and multiplier phototubes. By far the most commonly used device is the multiplier phototube because of its extremely high sensitivity and precision when powered by a voltage-regulated power supply. A variety of multiplier phototubes are available that have maximum response in different wavelength regions. 

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Apparatus. Absorbance measurements shall be made using a spectrophotometer with ultra-violet accessories, matched 1 cm silica cells and a blue sensitive phototube in position. A Beckman Model DU Spectrophotometer has been found to be satisfactory... 

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. 

Block the laser beam or spectrometer entrance slit and adjust the spectrometer to an anti-Stokes shift of 1000 cm Caution Exposure of the sensitive phototube to the intense Rayleigh scattering line can seriously damage the detector. Scan the anti-Stokes spectrum from 1000 to 150 cm in the parallel polarization configuration and, using appropriate sensitivity expansion, j measure the ratio of anti-Stokes to Stokes peak heights for each band. 

Any radiation not absorbed by the sample falls on the detector, where the intensity is converted to an electrical signal that is amplified and read on a meter. The measuring phototube for the visible region has maximum response at 400 nm, with only 5% of this response at 625/nm. Measurements above 625 nm are best made by substituting a red-sensitive phototube (RCA 6953) along with a red filter to remove second-order diffraction from the grating (it passes the desired red radiation but not undesired higher orders). 

FAofomefer—Preferably a spectrophotometer having an effective band width of about 50 nm, and equipped with a blue-sensitive phototube for use at 450 nm, or alternatively a filter photometer equipped with a color filter having a maximum transmission at approximately 450 nm. 

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. 

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... 

Raman spectra were obtained using a Spex lAOl double monochromator and a detection system which utilized photon counting, in combination with a 6A7.1 nm laser exciting line from a krypton laser. 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 X-ray capillaries in the Drilab, sealed temporarily with a plug of Kel-F grease, and the tube drawn down in a small flame outside the drybox. 

S(Av v) is the phototube sensitivity at the wavelength of the (v v) band, is the frequency of the same band and is a proportionality constant that includes parameters such as geometrical factors and the electronic transition moment (assumed to be constant). The term in brackets in eqn (2) represents the absorption process, while the summation represents the subsequent fluorescence pathways (assumed to occur only to the ground state) terminating in the various vibrational levels v. 

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. 

However, oceanic bioluminescence in some regions is not due to dinoflagellates but to a number of zooplankton groups (28). The color of the bioluminescence from other organisms varies with taxonomic group (29-32). For an accurate calibration, both the phototube spectral sensitivity and the types of organisms being stimulated must be known. 

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