Photomultiplier noise

As was mentioned before, noise is a term used to describe any random output signal that has no relationship with the incoming signal (the incoming light). In photomultipliers, noise can be classified, depending on its origin, into three types dark current, shot noise, and Johnson noise. The differences between these three classes are explained next ... 

The overall energy of the photons from a single p emission is often quite small. The photomultiplier tube used to detect such low-energy events must therefore be very sensitive, but this sensitivity also causes it to detect photomultiplier noise, or spurious counts unrelated to actual p particle emissions. This noise is greatly reduced by... 

Thermoelectrical cooling of the photomultiplier tube at about — 30°C reduces the dark noise current to a very low level. However, as the quantum efficiency of the S-20 type decreases as rapidly as the dark current in the red region, cooling brings only modest increases in the signal-to-noise ratio 23). 

Ideally, any procedure for signal enhancement should be preceded by a characterization of the noise and the deterministic part of the signal. Spectrum (a) in Fig. 40.18 is the power spectrum of white noise which contains all frequencies with approximately the same power. Examples of white noise are shot noise in photomultiplier tubes and thermal noise occurring in resistors. In spectrum (b), the power (and thus the magnitude of the Fourier coefficients) is inversely proportional to the frequency (amplitude 1/v). This type of noise is often called 1//... 

Our first chapter in this set [4] was an overview the next six examined the effects of noise when the noise was due to constant detector noise, and the last one on the list is the first of the chapters dealing with the effects of noise when the noise is due to detectors, such as photomultipliers, that are shot-noise-limited, so that the detector noise is Poisson-distributed and therefore the standard deviation of the noise equals the square root of the signal level. We continue along this line in the same manner we did previously by finding the proper expression to describe the relative error of the absorbance, which by virtue of Beer s law also describes the relative error of the concentration as determined by the spectrometric readings, and from that determine the... 

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

From the above definition it is quite evident that the sensitivity takes no cognizance of the noise-level of the base-line, therefore, it is more or less of no use as a definite guide to the least quantity of an element which may be estimated. However, the sensitivity of a 1% absorption-is a pure theoretical number only that would undergo a change solely depending on the efficiency of the lamp (hollow-cathode-lamp), atomizer, flame-system employed, monochromator (prism, grating used), and finally the photomultiplier used. 

Birch et al. have used the Philips XP2254B, an S20 version of the XP2020Q, to study the fluorescence lifetimes of a series of aminotetraphenylporphyrins in a multiplexed fluorometer. 83 The extended red response (S20R) version of this device, the XP2257B, has been used with IR spark source excitation to study the fluorescence lifetimes of carbocyanine dyes up to 930 nm emission in isotropic and anisotropic media. 55,561 % 84) An improved voltage divider network has been developed for linear focused photomultipliers which reduces thermionic noise from the photocathode by an order of magnitude by restricting the collection of photoelectrons to the center of the photocathode. 84 ... 

The operating principle of an MCP-PM is based on electron multiplication using a continuous dynode structure of ca. 10 um diameter holes, giving a more compact and hence faster time response when compared with conventional photomultipliers. Rise-times of 150 psec and transit-time jitter (i.e., impulse response) of ca. 25 psec FWHM at 200 counts/sec noise at room temperature have been recorded with the 6 fun channel Hamamatsu R3809 MCP-PM.(87)... 

In the particular case of a photocathode, this fluctuation affects both the dark current it) as well as the illumination induced current (/lum)- In the absence of illumination, the only current generated in the photocathode is the dark current, and so the shot noise associated with it is Aif If the light-induced current, /lum. is smaller than the shot noise associated with the dark signal Ai,), then it will be not possible to distinguish any light-induced current. In these conditions, the incident light cannot be detected by the photomultiplier, as it is not possible to separate the noise and the signal. As a consequence, the shot noise associated with the dark current determines the minimum intensity that can be detected by a particular photomultiplier (or by a particular photocathode). This is clearly shown in the next example. 

This noise is due to the thermal motion of the carriers (electrons) in the different resistors used in the photomultiplier. In general, the signal uncertainty caused by this source of noise is much lower than those generated by both dark noise and shot noise. 

---