Detector pulse shape

Figure Bl.10.2. Schematic diagram of a counting experiment. The detector intercepts signals from the source. The output of the detector is amplified by a preamplifier and then shaped and amplified friitlier by an amplifier. The discriminator has variable lower and upper level tliresholds. If a signal from the amplifier exceeds tlie lower tlireshold while remaming below the upper tlireshold, a pulse is produced that can be registered by a preprogrammed counter. The contents of the counter can be periodically transferred to an online storage device for fiirther processing and analysis. The pulse shapes produced by each of the devices are shown schematically above tlieni.

Figure 1. Pulse shapes in the square pulse method (a) and in the conventional pulse method (b). Solid lines give the pulse shape after passage through the catalyst. Trigger level refers to the point at which the pulse detector is triggered by the leading edge of the pulse.

Figure 18.3 Schematic diagram of tin ion chamber that drifts the ionization perpendicular to the particle s path is shown. In this case the anode is segmented and the relative rate of ionization along the path can be determined. The device also contains a Frisch grid between the anode and chamber to improve the pulse-shape response of the device, (c) The schematic version of a detector that drifts the ionization along the particle s path, called a Bragg counter, is shown. The time distribution of the output signal will contain information on the relative rate of ionization all along the particle s path.

The femtosecond laser pulses shaped by the AOPDF are amplified by the CPA up to 0.5mJ/pulse. Ethanol vapor is continuously flow into the vacuum chamber through a micro-syringe (70 pm) with stagnation pressure of 7 Torr at room temperature. The laser pulses are focused on a skimmed molecular beam of the ethanol vapor with an achromatic lens (/ = 145 mm). The focal spot size of the laser beam is 20 pm(j>. The peak intensity of the transform-limited laser pulse is calculated to 4 x 1015 W/cm2. The fragment ions are mass-separated with Wiley-McLaren type time-of-flight (TOF) mass spectrometer, and are detected with a microchannel plate (MCP) detector. 

The pulse shape depends strongly on the detector parameters like the anode diameter and the applied voltage. 

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. 

A disadvantage of the coincidence scintillation cameras is that they have low sensitivity due to low detection efficiency of Nal(Tl) crystal for 511-keV photons, which results in a longer acquisition time. To improve the sensitivity, thicker detectors of sizes 1.6-2.5 cm have been used in some cameras, but even then, coincidence photopeak efficiency is only 3-4%. This increase in crystal thickness, however, compromises the spatial resolution of the system in SPECT mode. Fast electronics and pulse shaping are implemented in modern systems to improve the sensitivity. Also, there is a significant camera dead time and pulse pileups due to relatively increased single count rates in the absence of a collimator in PET mode. Low coincidence count rates due to low... 

As a result of incomplete energy deposition and the statistical nature of the events that take place in the detector, the shape of the pulse-height distribution is different from that of the source energy spectrum. In other words, two spectra are involved in every measurement ... 

The pulse produced at the output of a radiation detector has to be modified or shaped for better performance of the counting system. There are three reasons that necessitate pulse shaping ... 

For special pulse manipulation. The detector pulse may, in certain applications, need special pulse shaping to satisfy the needs of certain units of the counting system. As an example, the signal at the output of the amplifier needs to be stretched before it is recorded in the memory of a multichannel analyzer (see Sec. 10.12). 

Pulse-shape discrimination (PSD) is the name given to a process that differentiates pulses produced by different types of particles in the same detector. Although PSD has found many applications, its most common use is to discriminate between pulses generated by neutrons and gammas in organic scintillators (see also Chap. 14), and it is this type of PSD that will be discussed. 

The purpose of an amplifier to anqilify a voltage pulse in a linear fashion and to shape the pulse so that the event can be analyzed easily and correctly in a short time. A linear amplifier accepts tail pulses as input, usually of either polarity, and produces a shaped and amplified pulse with standard polarity and amplitude span (NIM standard is positive polarity and 0 - 10 V amplitude). On most commercial linear amplifiers, the time constants for the various pulse shaping circuits are adjustable to fit various detector and count-rate requirements. 

Single-Electron Response. Output pulse delivered by a detector for a single photoelectron generated at its input. In most detectors the SER is identical with the pulse shape for a single detected photon. 

The effective resolution of a TCSPC experiment is characterised by its instrument response function (IRF). The IRF contains the pulse shape of the light source used, the temporal dispersion in the optical system, the transit time spread in the detector, and the timing jitter in the recording electronics. With ultrashort laser pulses, the IRF width at half-maximum for TCSPC is typically 25 to 60 ps for microchannel-plate (MCP) PMTs [4, 211, 547], and 150 to 250 ps for conventional short-time PMTs. The IRF width of inexpensive standard PMTs is normally... 

A number of typical detectors are described under Sect. 6.4, page 242. The main selection criteria are the transit-time spread and the spectral sensitivity. Together with the laser pulse shape, the transit-time spread determines the instrument response function (IRF). As a rule of thumb, lifetimes down to the FWHM of the IRF can be measured without noticeable loss in accuracy. For shorter lifetimes the accuracy degrades. However, single-exponential lifetimes down to 10% of the IRF width are well detectable. Medium speed detectors, such as the R5600 and R7400 miniature PMTs, yield an IRF width of 150 to 200 ps. The same speed is achieved by the photosensor modules bases on these PMTs (see Fig. 6.40, page 250). 

Cables are available for Z= 50, 60, 75 and 100 Q. For measurement equipment and other wide-band systems only Z = 50 Q is used. The CFD inputs of TCSPC modules, amplifiers, or routers have internal matching resistors of 50 Q. However, the input impedance of amplifiers or of the pulse shaping network used in CFDs is often far from being ideally resistive. Moreover, PMTs and photodiodes are current sources. Matching at the detector side is avoided because it would decrease the signal amplitude. The resulting reflections at the input cables of a TCSPC device can normally be tolerated, especially if some precaution is taken in adjusting the CFD thresholds. 

The zero cross level adjustment minimises the timing jitter induced by amplitude jitter of the detector pulses. The zero cross level is therefore often called walk adjust". In early TCSPC systems the walk adjust had an enormous influenee on the shape of the instrument response function (IRF). In newer, more advaneed systems the influence is smaller. The reason is probably that detectors with shorter single electron response are used and the discriminators in the newer CFDs are faster. Therefore, the effective slope of the zero cross transition is steeper, with a correspondingly smaller influence of the zero eross level. Figure 7.63 shows the IRF for an XP2020UR linear-focused PMT and an H5773-20 photosensor module for different zero cross levels. 

Typical pulse shapes for LEDs driven by pulses of 3.6 ns FWHM from a Hewlett Packard HPlllOA pulse generator are shown in Fig. 7.88, left. Pulses from a 5 mW, 650 nm laser diode driven by 1 ns pulses from a HP8131A pulse generator are shown right. The detector was a Hamamatsu H5783P photosensor module. 

Some other scintillation materials, such as cesium iodide and bismuth ger-manate, have characteristics that are less favorable than Nal(Tl) for general use, but recommend them for some special measurements. For example, Csl and Nal(Tl) can be combined for coincidence or anticoincidence counting by distinguishing between output from the two detectors by their pulse shapes. 

Use of the LS counter for alpha-particle spectral analysis is discussed in Section 8.3.2. Source preparation is simpler, but energy resolution is worse than with the solid-state detector. Special source preparation and electronic pulse-shape selection can improve resolution. 

Since the signal separation from the prime and rear detector primarily depends on the pulse shape discrimination and the expected event rate in the shield detector is much larger both due to large area and the higher detection efficiency, an ultra-fast RTD system is essential for a very low dead-time. We have developed... 

To evaluate the performance of the detector, 15000 events were collected in the CsI(Tl)-PD detector while it was simultaneously illuminated with Am and Na sources giving lines at 60, 511 and 1275 keV. At the same time a reference spectrum was collected with a standard Nal(Tl)-photomultiplier system. The pulse-shape spectrum for the raw events is shown in Figure 2. Two clear peaks are seen, easily distinguishing the fast events from the silicon and the slower scintillation events. The rise-time spectrum for the Si events is distorted as the readout system was too slow to accurately determine the rise-times of the fastest events. 

Fig. 2. Raw spectra from the hybrid detector showing (left) pulse shape and (right) energy...

Using this pulse-shape spectrum, the cross-over point between the two detectors was set at a rise-time of 3/ s, shown by the dotted line in the figure. Events with faster rise-times were analysed as coming from the photodiode and were essentially all due to 60 keV events all others were assumed to come from the caesium iodide scintillator. The two types of event were analysed individually in order to obtain raw energy spectra from each detector (also shown in Figure 2). The peaks in the energy spectra were then used to calibrate the energy scales for the two systems. 

We have succesfully demonstrated the application of pulse-shape analysis to create a hybrid CsI(Tl)-silicon detector. This technique promises to lower the low energy threshold of such detectors significantly. Areas in which we aim to improve the system include ... 

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