Pulse amplitude distribution

The amplifier gain can be reduced by using a PMT load resistance or amplifier input impedance of more than 50 The increased load resistor in conjunction with the cable capacitance results in a pulse broadening without loss in amplitude. Load resistors of the order of 1 kf2 can be used. To avoid baseline drift in the amplifier, AC coupling is used. The coupling constant is selected as to minimise the baseline walk at higher pulse rates. Nevertheless, pile-up in the amplifier limits the useful pulse rate to typically a few 10 kHz. The general measurement setup is shown in Fig. 6.22. 

Single-board TCSPC modules usually do not give the user direct access to the input of their internal MCA. If an input to the MCA is provided or can be made available, the pulse height distribution can be recorded as shown in Fig. 6.23. 

Because of the high speed of the biased amplifier and the ADC in a TCSPC board, the photon pulses delivered to the ADC need not be broader than 50 to 100 ns. Therefore an extremely high preamplifier gain is not required, and AC coupling can be avoided. The setup can therefore be used up to a count rate of several 10 pulses per second. Examples for pulse height distributions recorded this way are shown in Fig. 6.13, page 227. 

Of eourse, the setup shown in Fig. 6.23 works only with a TCSPC module that uses the TAC/ADC prineiple. Modules with direet time-to digital eonversion do not have an internal MCA and are therefore unable to reeord a pulse height distribution iu the way shown above. 

Another possibility to reeord a pulse height distribution in a TCSPC system is to run a CFD threshold sean. Starting from the lowest possible value, the CFD threshold is gradually inereased and the number of photons reeorded in a given time interval is reeorded versus the CFD threshold. An example for an H5773-20 photosensor module is shown in Fig. 6.24. 

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If the discriminator threshold is set low enough an extremely high peak appears at very low amplitudes. This peak does not originate in the PMT but is caused by electronic noise of the preamplifier and noise pickup from the environment. Some typical pulse amplitude distributions are shown in Fig. 6.13. 

Fig. 6.14 Pulse amplitude distribution of a H7422P-40 for different gain. At high gain the distribution develops a substructure, probably due to the discrete numbers of secondary electrons emitted at the first dynode...

Fig. 6.15 Pulse amplitude distribution for dark pulses and photon pulses of an R5600P-01 PMT...

It is often believed that the width of the pulse amplitude distribution depends on the cathode material. Certainly there are differences in the peak ratios of the main... 

The R3809U tubes have a relatively good SER pulse amplitude distribution which seems to be independent of the eathode type. This is possibly a result of the manufacturing process. The tube elements, spaeers, mierochannel plates and cathode window are enclosed in a vaeuum eontainer. Then the eathode is formed on the input window, the mierochannel plates and the tube elements are outgassed, and the parts are assembled and brazed under vaeuum. Thus the eathode material is not involved in the secondary emission as in eonventional PMTs. 

The pulse amplitude distribution consists of three major eomponents. There are the regular photon pulses, i.e. the pulses originating from eleetron emission at the photocathode, which form a wide peak at relatively high amplitudes. Thermal emission, photoelectron emission, and reflection of primary electrons at the dyn-odes forms a secondary peak at lower amplitudes. At very low amplitudes electronic noise, either from the preamplifier or from the environment, causes a third peak of extremely high eount rate. 

Figure 2.21 Pulse amplitude distributions measured at various diffraction angles from a sample of distilled water using a chromium anode x-ray tube at 50 kV and 10 mA, a scintillation detector, and a LiF(200) crystal.

Another important property of PMTs is the pulse height distribution. The amplification of individual photoelectrons by the PMT is a stochastic process that causes variations in the gain of individual photoelectrons. As a result significant jitter in the amplitude of the output pulses is observed, see Fig. 3.6. These pulse height variations can be more than a factor of 10. The lowest pulse heights mainly consist of (thermal) noise, indicated by the dashed line in Fig. 3.6. The pulse height distribution exhibits a peak corresponding to detected photons. The threshold level of the... 

The theoretical approach is based on the solution to the mixed type linear/nonlinear generalized Schrodinger equation for spatiotemporal envelope of electrical field with account of transverse spatial derivatives and the transverse profile of refractive index. In the quasi-static approximation, this equation is reduced to the linear/nonlinear Schrodinger equation for spatiotemporal pulse envelope with temporal coordinate given as a parameter. Then the excitation problem can be formulated for a set of stationary light beams with initial amplitude distribution corresponding to temporal envelope of the initial pulse. 

Sonntag WE, Xu X, Ingram RL, D Costa A. Moderate caloric restriction alters the subcellular distribution of somatostatin mRNA and increases growth hormone pulse amplitude in aged animals. Neuroendocrinol. 1995 61 601-608. 

Method of Data Processing. The amplitude of the signal pulses depends on the size of the phytoplankters, their trajectory, and their position in the measured volume because of the nonuniformity of the irradiated intensity and the characteristics of the collecting optics (Figure 6). This section presents our method to obtain size information of the particles from the raw data of the pulse-height distribution. 

Figure 3.1. Fourier relationship between an rf pulse of duration Tp and the amplitude distribution A(v) of the frequency components present.

When the same surface coil is used for excitation and for reception, B xy R) enters into the detected signal from the transmitter side as well as from the receiver side. As a result the sensitive volume changes in size compared to excitation or reception only. For a homogeneous sample excited by a single pulse, the distribution of signal amplitude is described by... 

SPREAD saturation pulses with reduced amplitude distribution SPRITE single-point ramped imaging with Ti enhancement SQUID superconducting quantum-interference device... 

Due to the random nature of the detector gain, the amplitude of the single-photon pulses of PMTs and MCP-PMTs varies from pulse to pulse. The pulse height distribution ean be very broad, up to 1 5 to 1 10. Figure 6.11 shows the SER pulses of an R5600 PMT recorded by a 1-GHz oscilloscope. 

The secondary emission coefficient at a particular dynode depends on the dynode material and energy of the primary electrons. For typical interdynode voltages used in PMTs, the secondary emission coefficient, n, is between 4 and 10. Because the secondary emission is a random process the number of the generated secondary electrons varies from electron to electron. The width of the distribution can be expected at least of the size of the standard deviation, n, of a poissonian distribution of the secondary emission coefficient, n. Therefore the single-photon pulses obtained from a PMT have a considerable amplitude jitter. For TCSPC applications it is important that the pulse amplitudes of the majority of the pulses are well above the unavoidable noise background. 

Fig. 6.13 Distribution of the single-photon pulse amplitude of different PMTs and different gain at a count rate of 10 /s. Upper row R5600P-01, H5773P-03. Lower row Two specimens of the R7400-02...

Figure 6.15 shows that the shape of the main peak of the amplitude distribution is the same for the photon pulses and the dark pulses. This is not surprising because the secondary emission process makes no difference between electrons emitted at the cathode thermally or optically. However, the secondary peak at low amplitudes is more pronounced for the dark pulses, probably because some of the thermal electrons are emitted at the dynodes. 

The CFD threshold determines the minimum amplitude of the input pulses that trigger the CFD. The threshold of the CFD in the deteetor channel has a considerable influence on the efficieney of a TCSPC system. As described under Sect. 6.2, page 222, the single-photon pulses of a PMT have a strong amplitude jitter. The general shape of the amplitude distribution is shown in Fig. 7.60, left. 

Fig. 7.60 Amplitude distribution of the PMT pulses (left) and count rate vs. CFD threshold...

Figure 8.23 Amplitude or pulse height and time record of signals from the detector is on the left. Transformation of the data into a pulse height distribution is shown on the right. The FWHM measurement is shown for the higher peak. (From Helsen and Kuczumow, used with permission.)...

An additional consequence of the widened spectral windows at high spectrometer field Is the requirement for enhanced pulse power. In order to assure uniform excitation across the full spectral width the rf pulses have to be sufficiently short. This Is particularly critical to achieve with hlgh-Q multlnuclear probes, and a fully satisfactory solution to this problem has at this time not been fomd. In Figure 5 the transmitter rf amplitude distribution for a... 

Figure 5. amplitude distribution across a 250-ppm NMR spectral window for a 25-ys excitation pulse at three different magnetic field strengths. Percentage numbers indicate total variation of amplitude. 

Acoustic emission Number and intensity of acoustic pulses, amplitude and energy distribution, spectral envelope Accumulation of damage, fracture, adhesion, load bearing capacity... 

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