Noise in Photomultipliers

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  

Even in the absence of illumination (darkness) some electrons, excited by thermal energy, are emitted from the photocathode. Since photocathodes are materials with low working functions, the thermal energy can be high enough to induce the emission of electrons. These emitted electrons give rise to what is known as the dark current or, sometimes, the thermo-ionic current. The dark current varies randomly with time, so that it is considered as noise. It has been experimentally determined that the thermo-ionic current, U, due to photoelectrons emitted by a photocathode in the absence of illumination is given by 

EXAMPLE 3.1 (a) Calculate the dark current intensity at room temperature 

This noise source is associated with the discrete nature of the electric current. When a certain current i is induced or generated in the photocathode, there is some uncertainty in the current, which arises from the quantum properties of electrons. It has previously been demonstrated that the fluctuations in any electrical current with a frequency between / and / + Af are given by 

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. 

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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//... 

Dark noise in photomultipliers is caused by (1) leakage current across insulating supports (2) field emission from electrodes (3) thermal emission from the photocathode and dynodes (4) positive ion feedback to the photocathode and (5) fluorescence from dynodes and insulator supports. Careful design can eliminate all but item (3). Associated with the photocurrent from the photocathode is shot noise. There is also shot noise from secondary emission in the multiplier structure. 

Thermal Noise in Photomultiplier Tubes The energies of the (3 particles from most P emitters are very low. This, of course, leads to low energy photons emitted from the fluors and relatively low energy electrical pulses in the PMT. In addition, photomultiplier tubes produce thermal background noise with 25 to 30% of the energy associated with the fluorescence-emitted photons. This difficulty cannot be completely eliminated, but its effect can be lessened by placing the samples and the PMT in a freezer at -5 to -8°C in order to decrease thermal noise. 

A second way to help resolve the thermal noise problem is to use two photomultiplier tubes for detection of scintillations. Each flash of light that is detected by the photomultiplier tubes is fed into a coincidence circuit A coincidence circuit counts only the flashes that arrive simultaneously at the two photodetectors. Electrical pulses that are the result of simultaneous random emission (thermal noise) in the individual tubes are very unlikely. A schematic diagram of a typical scintillation counter with coincidence circuitry is shown in Figure 6.2. 

DC amplifiers are the simplest and least expensive of the electronic measurement systems. They are most commonly found in commercially available fluorimeters. Ideally, the amplifier stage of a circuit contributes little or no noise to the system the photomultiplier should produce only the noise associated with perfect performance of the photocathode and the electron multiplication process described above. However, the anode dark current of the photomultiplier adds to the noise in the signal, and the amplifier makes its own contribution to the total noise. It is therefore imperative to select the proper photomultiplier tube with low dark current so as to have a higher signal-to-noise ratio. 

Apart from the data acquisition (DAQ) system taking data since November 2003 the collaboration decided to build a Flash-ADC (FADC) based DAQ system for the Grande array. This system will sample the full pulse shape created by the photomultiplier tubes. Having the complete pulse shape recorded a correction for noise in order to improve the data quality is possible and new shower observables to be used in the analysis can be derived. In particular an intrinsic electron to muon separation at individual detector stations will be possible. Since the data will be transmitted optically, it will be resistant against pickup noise. 

In modern absorptiometric instruments, with digital reading of absorbance, the precision depends on the noise of photomultiplier used as a detector. The signal is subjected to electronic processing. The respective curve of precision error has a broad minimum at A equal to about 0.9. Such spectrophotometers can record absorbance with good precision up to values of about 2. 

In analog detection methods (See Section 4.B.2) the photomultiplier output is treated as a continuous variable. At low light-scattering levels, the signal-to-noise ratio in these methods may become small because of various sources of ftostdetection noise in the system (thermal or Johnson noise, PM dark current, etc.). Under these conditions it becomes advantageous to use the digital or photocount autocorrelation method. 

Liquid scintillation counters use two photomultiplier tubes with a coincidence circuit that prevents counting of events seen by only one of the tubes. In this way, false counts due to chemiluminescence and noise in the phototube are greatly reduced. Quenching is a problem in all liquid scintillation counters. Quenching is any process which reduces the efficiency of the scintillation counting process, where efficiency is defined as... 

For low-level light detection, the question of noise mechanisms in photomultipliers is of fundamental importance [4.129]. There are three main sources of noise ... 

Fluctuations of the photomultiplier gain G, which contribute to the noise in analog measurements, see (4.144), are not significant here, since each photoelectron induces the same normalized pulse from the discriminator as long as the anode pulse exceeds the discriminator threshold. 

The lasers used in these experiments produce typical powers greater than 2mW. The detectors often are of nearly unity quantum efficiency. Thus, in one second, A< shot < 10 rad. Even with the decreased efficiency of photomultiplier tubes, and less light intensity in experiments which use many absorption lengths of vapor, one finds A< shot < 10 rad in an observation time (A ) = 1 sec. In practice, the noise in the measured angles often turns out to be larger than the shot noise limit, owing to a combination of mechanical and optical instabilities, and detector noise in some cases. In the most recent experiments, 10" rad can be resolved in less than 1 min of averaging time. 

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