Photomultiplier spectral responses

The spectral response of photoemissive tubes depends upon the composition of the cathode and the use of various mixtures of elements permits the production of a wide range of tubes of varying responsiveness (Figure 2.27). Photomultiplier tubes can be used to detect low intensity radiation and even in the absence of any light will still generate a small current due to various emissions from the material of the tube, etc. This dark current has to be compensated for in any measurements that are made. 

The first measurement we make when starting a fluorescence study is not usually a fluorescence measurement at all but the determination of the sample s absorption spectrum. Dual-beam differential spectrophotometers which can record up to 3 absorbance units with a spectral range of 200-1100 nm are now readily available at low cost in comparison to fluorimeters. The wide spectral response of silicon photodiode detectors has made them preeminent over photomultipliers in this area with scan speeds of a few tens of seconds over the whole spectral range being achieved, even without the use of diode array detection. 

Streak cameras and multianode microchannel plate photomultipliers (MCP-PMs) interfaced to a polychromator also permit multiwavelength fluorescence decay measurements, the spectral response of both being determined by the photocathode composition. 

The major characteristics of the photomultiplier with which the user is generally most concerned include (1) sensitivity, spectral response, and thermal emission of photocathodes (2) amplification factor and (3) noise characteristics and the signal-to-noise ratio. 

Note These data were obtained using Westinghouse HCLs and a single experimental setup. No correction has been made for the spectral response of the monochromator/photomultiplier tube system. These data were taken from Reference 2. 

General Electric 4 watt germicidal lamp and a Corion Corporation 2537 interference filter (15% transmission at 2537). The Fluoromonitor s emission detection unit consisted of a RCA 931B (S-4 spectral response) photomultiplier tube and a Corning 7-51 glass filter that transmits light of wavelengths between 310 and 410 nm. 

Spectral responsivity The spectral output quantity of a system such as a photomultiplier, diode array, photoimaging device, or biological unit divided by the spectral irradiance s(X) = dy(X)/di (X), simphfied expression s(X) = Yx/Ex, where Yx is the magnitude of the output signal for irradiation at wavelength X and Ex is the spectral irradiance of parallel and perpendicular incident beam at the same wavelength. 

The amplification and gain per dynode of an ampler rely on the operating voltage. Thus, photomultipliers need a very stable high-voltage power supply. The spectral response depends on the cathode type and... 

Conventional scintillation counters such as the Microbeta (Wallac/Perkin Elmer, Turku, Finland) or the TopCount (Packard, Meriden, USA) use photomultiplier detection systems that count 8 or 12 wells at a time, resulting in a readout time of 40 minutes per 384-well microplate. Bialkali photocathodes (Sb-Rb-Cs or Sb-K-Cs) used in standard photomultipliers have a maximum spectral response at about 420 nm, with a quantum efficiency for detection of up to 30%. Thus, the aforementioned instruments are ideally suited for filtration assays and SPA assays with the blue-emitting YSi and PVT beads. 

Light emitted by the samples is registered by a general purpose side-on photomultiplier tube (Hamamatsu 1P28, the spectral response from 185 to 700nm, the peak sensitivity at 450 nm),and recorded independently for each of eight cells by a multichannel recorder (Hewlett Packard 7418A)... 

Figure 63 Emission spectra of Nal(Tl), CsI(Tl), CsI(Na), and anthracene, compared to the spectral response of two photocathode materials. PMT, photomultiplier tube (from Harshaw Research Laboratory Report, Harshaw Chemical Company, 1978).

A very important parameter of every photomultiplier tube is the spectral sensitivity of its photocathode. For best results, the spectrum of the scintillator should match the sensitivity of the photocathode. The Cs-Sb surface has a maximum sensitivity at 440 nm, which agrees well with the spectral response of... 

In choosing photodetectors for optical sensors, a number of factors must be considered. These include sensitivity, detectivity, noise, spectral response, and response time. Photomultipliers and semiconductor quantum photodetectors, such as photoconductors and photodiodes, are all suitable. The choice, however, is somewhat dependent on the wavelength region of interest. Generally, both types give adequate... 

The spectral response of a photomultiplier tube varies with the coating materials used on the photocathode. Spectral responses of various photomultiplier tubes are given in Table 6-3. Chapter 6 also includes a general discussion of photomultiplier phototubes. The 1P28 tube (S-5 response) is sensitive from 2000 to 6500 A and is frequently used for atomic absorption spectroscopy. The Hamamatsu R106 also has an S-5 response but uses a silica window to lower the usable short wavelength response to about 1700 A. The S-20 response of the RCA 4459 permits measurements to 8500 A and is very useful for most of the alkali metals. 

Figure 10.3 (a) Emission spectra from common scintillators, (b) Spectral response (sensitivity) of common photomultipliers... 

Figure 10.7 (a) The spectral response of a photodiode compared to typical photomultiplier tubes, (b) The emission spectrum of the CsI(Tl) scintillator... 

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