Detector count rates

The strength of the signal drops dramatically in the apertured case at about 23% porogen load. In the unshielded case, only a gradual small change is detectable. When the porogen load increases above 50% positronium also encounters the interface to the silicon substrate and annihilates more efficiently within the sample. 

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Figure 12. Plot of unaveraged detector count-rate data (top) and airflow temperature (bottom) versus time for the August 25, 1989, flight. Data are observed during the balloon float period at an altitude of 37 km. The 20-s period modulation in the count-rate data is due to tuning the laser on- and off-resonance with the OH absorption. The alternating high and low OH signal is due to measurements with and without NO injection.

The resultant ions (both primary and produced) are mass-selected using a quadruple mass analyzer and measured as count rates by an electron multiplier detector. Count rates of the MH+ species are subsequently converted to ionic densities and then to mixing ratios of constituent M after consideration of instrumental transmission coefficients, temperature, and DT pressure. Instrumental accuracy, which is largely determined by the uncertainties associated with the reported proton transfer reaction rate coefficients (k), is estimated to be better than 30% (Hayward et al, 2002 Lindinger, Hansel and Jordan, 1998). 

Relationship between Tracer Position and Detector Count Rate... 

The various effects that influence the detector count rate were described in the previous sections. If all of the correlation equations are considered, the detector count rate can be expressed as a function of tracer position. In the following section, this formulation is presented and its prediction is compared with measured data. 

The measurement accuracy was experimentally determined by positioning the tracer in known locations inside the bed (Moslemian, 1987). The apparent tracer positions were calculated from the linear regression formulation Eq. 9.13, based on the measured detector count rates and the predetermined calibrations. Sufficient data were taken at each location to allow for statistical determination of mean and standard deviations of the tracer position and velocity. In general, the axial errors were often greater than the radial errors because of the longer axial distance of the bed that the detectors had to monitor. For an empty bed, the mean axial error in determining the tracer position was 4.7 mm and the mean radial error was 3.9 mm The corresponding standard deviations were 1.6 mm in the axial direction and 1.2 mm in the radial direction. These deviations were due to the statistical nature of the radiation detection and are the minimum deviations obtainable for the tracer position. The measured mean axial and radial velocities were approximately zero (<1 cm/s) at all locations inside the bed. However, the standard deviations of the velocities were 7.6 cm/s in the... 

Compared with classic systems, the dead time of advanced TCSPC systems has been considerably reduced. It is however, still on the order of 100 to 150 ns. The fraction of photons lost in the dead time - the counting loss - becomes noticeable at detector count rates higher than 10% of the reciprocal dead time (see Sect. 7.9.2, page 338). The counting loss can be compensated for by a dead-time-compensated acquisition time. Therefore, often a relatively high loss can be tolerated. The practical limit is the maximum useful" count rate, which is defined as the recorded rate at which 50% of the photons are lost. For currently available TCSPC modules, the maximum useful count rate ranges from 3 to 5 MHz, corresponding to a detector count rate from 6 to 10 MHz. 

Pile-up is caused by the detection of a second photon within one signal period [104, 105, 238, 389, 549]. Because a second photon is more likely to be detected in the later part of a signal period, pile-up causes a distortion of the signal shape (see Sect. 7.9.1, page 332). The pile-up distortion is smaller than commonly believed (see Fig. 7.78, page 336), and reasonable results can be obtained up to a detector count rate of 10 to 20% of the signal repetition rate. Nevertheless, the pile-up sets a limit to the applicable count rate. 

The TCSPC chaimels of this system have 100 ns dead time. The maximum useful (reeorded) count rate of each individual channel is 5 10 s The system ean be used at a total recorded count rate up to 20T0 s or a total detector count rate of 40T0 s A typical result is shown in Fig. 5.84. 

The efficiency versus the count rate of a single TCSPC channel and a four-module TCSPC system is shown in Fig. 5.94. The efficiency of the single-channel system remains better than 0.9 and the figure of merit better than 1.05 for count rates up to 1 MHz detector count rate. This is better than for any other lifetime imaging technique. For a detector count rate of 10 MHz, the values are 0.5 and 1.4, respectively. Higher count rates not only result in a substantial loss in efficiency but also increase lifetime errors by pile-up-effect (see Sect. 7.9.1, page 332). For detector count rates above 10 MHz the solution is multimodule systems see Sect. 5.7.5, page 146. 

A system with four parallel TCSPC channels can be used up to 40 MHz detector count rate. When this count rate is compared to the count rates of other time-resolved detection techniques, the high efficiency of TCSPC must be taken into account. Consider a gated image intensifier that is operated at a gate width of 100 to 200 ps, i.e. at a time resolution equivalent to a mediocre TCSPC system. The short gate width then results in an efficiency of 0.05 to 0.1. A four-channel TCSPC system operated at 40 MHz has an efficiency of 50%. The 40 MHz detector count rate of the TCSPC system therefore corresponds to an input count rate of 200 to 400 MHz in the image intensifier. 

Fig. 5.117 Effect of the count rate on the height of the afterpulsing peak. H74222 0 detector, count rate 10 kHz (left) and 100 kHz (right)...

Fig. 5.144 Fluorescence signal recorded in an TCSPC oscilloscope setup. One channel of a Becker Hickl SPC-144, detector count rate 6 MHz, recorded count rate 4 MHz, 1,024 time bins per curve, acquisition time 100 ms per curve. To reproduce the visual impression, two successive traces were overlaid...

Fig. 6.57 Diode laser pulses recorded with an id 100-20 SPAD at 785 nm and an SPC-144 TCSPC module. Left Wavelength 785 nm, pulse width 24 ps, detector count rates of 61 kHz, 2.7 MHz, and 8.1 MHz. Right Wavelength 468 nm, laser pulse width 50 ps, detector count rate 1.4 MHz...

The pile-up distortion of the signal shape is predictable if the detector count rate and the signal repetition rate are known [103, 104, 105, 238, 389, 549]. Suppose the number of counts in a time channel, i, is iV, and the total number of excitation cycles is E. A photon in channel i cannot be detected if a photon in a previous channel, j < i, is detected. The effective number of excitation cycles for channel i is then... 

The influenee of the pile-up on the obtained lifetimes is surprisingly small. A 5% ehange in the intensity-weighted mean lifetime is often tolerable. The corresponding P is about 0.2, i.e. the detector count rate is 20% of the pulse repetition rate. With pile-up eorreetion, even higher count rates appear possible. Pile-up is therefore not a severe problem at pulse repetition rates between 50 to 90 MHz, typical for titanium-sapphire lasers and diode lasers. 

Detector Count Rate, in units of reciprocal dead time... 

Data sheets of TCSPC devices sometimes specify a maximum count rate" that is simply the reciprocal signal processing time. This definition is misleading because for random input pulses it can be reached only for an infinite detector count rate. The term saturated count rate" should better be used instead of maximum count rate". 

The relation between the detector count rate (classic pile-up not included) is... 

Figure 1 Formulation and Filtering of the Earth Occultation Transform, in terms Radon transform, step trans- form and Butterworth high-pass fil-ter. The raw detector counting rate J is also filtered by the same filter to match the filtered step function. A real occultation step after the filter-i ing looks similar to the bottom fig-j ure except the transition width from the maximum to the minimum is fi- nite ( seconds), and thus the recon-4 structed image of a discrete source in 1 the sky will have a finite angular size I This is the angular res-...

Figure 2 Simulation of a point source on TJD 9575. Only one orbital data, consisting of one rising and one setting step, are used in the simulation. Left the simulated point source and the input data to the reconstruction code. The data are the high pass-filtered raw detector counting rates. The data points are not shown when the Earth s limb was not in the FOV. Right the reconstructed image and the residual data. The source is at the right location with approximately the input strength. The residual data are consistent with the input Gaussian noise.

Fig. 26 Detector count rates as a function of time after mixing oppositely labeled n-alkyl-PE05 polymeric micelles squares n = 30 dots n = 24 triangles n = 18 solid line final state. [103]. Reproduced by permission of The Royal Society of Chemistry...

The reactor operator observes neutron population as source range detector count rate. As larger and larger subcritical values of K are reached, the operator must wait longer times for the subcritical count rate to stabilize. If the reactor is almost critical, the time can be longer than ten minutes. 

Assuming a homogeneous sample, our detector count rate can be separated into terms describing the number of electrons emitted per unit volume and the volume of the material analyzed. Some of the terms are sample dependent, and some are instrument dependent. 

A sample of reactor coolant analyzed a few minutes after the loop B pumps were shut off indicated a low boron concentration. This finding, coupled with apparently increasing neutron levels, increased the operators fears of a reactor restart. As explained earlier, the source range neutron detector count rate was increasing because steam bubbles in the downcomer allowed more neutrons to reach the detector. There was no actual danger of recriticality. It is now believed the sample was diluted by condensed steam, causing the indication of low boron concentration. 

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