Scintillation detector shape

For simple gamma counting, thallium-activated sodium iodide [Nal(TI)] scintillation detectors, which became commercially available in the early 1950s, continue to render excellent sen/ices. In spite of the introduction of many other scintillation materials, they remained preeminent. Nal(TI) detectors can be manufactured in various sizes and shapes... 

In time-correlated single photon counting Loss of a additional photons detected after the first photon within one same signal period. Pile-up causes distortion of the signal shape and loss in the number of detected events. In high-energy particle detection Detection of several particles within the response of a scintillator, detector and subsequent amplifier. Pile-up causes distortion in the measured energy distribution and loss in the number of detected particles. 

The sample, which is considerably smaller than the bore of the beam collimator ( 10mm dia.), is mounted on a thin mylar foil on top of a cup-shaped scintillation detector (veto counter). The mylar is essentially transparent to the incoming muons. Thus,... 

It has been assumed above that there are no statistical effects at later stages of pulse formation that could change the shape of the photoelectric peak. However, such effects do exist. Consider the scintillation detectors, for instance, which waste much of the potential of energy resolution hidden in the primeval shape of the photoelectric peak. 

The most widely used modern scintillation detector consists of a transparent crystal of sodium iodide that has been activated by the introduction of 0.2% thallium iodide. Often, the crystal is shaped as a cylinder that is 3 to 4 in. in each dimension one of the plane surfaces then faces the cathode of a photomultiplier tube. As the incoming radiation passes through the crystal, its energy is first lost to the scintillator, this energy is subsequently released in the form of photons of fluorescence radiation. Several thousand photons with a wavelength of about 400 nm are produced by each primary particle or photon over a period of about 0.25 ps, which is the dead time. The dead time of a scintillation counter is thus significantly smaller than the dead time of a gas-filled detector. 

The standard shape for a scintillation detector is a simple cylinder with its height equal to its diameter. The ease with which such detectors can be made with precisely reproducible dimensions and properties made the concept of a standard detector a reality. For most routine purposes, the detector of choice is the Nal(TI). Typical off-the-shelf sizes are (in inches) 1 x 1,2 x 2 and 3x3, often still quoted in archaic units. End well detectors of similar sizes are also easily available. 

To reduce interference from Compton scattering, an anticoincidence shield, 76 cm. X 76 cm., was constructed as shown in Figures 2 and 3. The shield consists of two independent type NE-102 plastic phosphor annuli. A 10-cm. bore through the top annulus accommodates the Ge(Li) detector chamber and the cryogenic assembly. The bottom annulus (i.d. diameter 25 cm.) houses a 20-cm. diameter by 15-cm. thick Nal(Tl) scintillator. Normally, the plastic phosphor is used in conjunction with the Nal(Tl) to form a well -shaped anticoincidence shield. Altema-... 

Stop-Flow Mode The LC-ARC StopFlow controller performs two major functions. First, the controller maintains the back pressure during stop-flow data acquisition. This function is crucial for maintaining the shape and resolution of a peak. The second function is to deliver a liquid scintillation cocktail necessary for peak detection by the radiochemical detector. 

As shown in Figure 3, our detectors came equipped with a cylindrical plastic cell about the size and shape of a scintillation counting vial into which a U-shaped tube had been drilled. When this cell has been packed with scintillation grade anthracene a flow cell quite satisfactory for aqueous systems is obtained. 

SEs and BSEs are typically detected by an Everhart-Thornley (ET) scintillator-photomultiplier secondary electron detector. The SEM image is shaped on a cathode ray tube screen, whose electron beam is scanned synchronously with the high-energy electron beam, so that an image of the surface of the specimen is formed [52], The quality of this SEM image is directly related to the intensity of the secondary and/or BSE emission detected at each x- and y-point throughout the scanning of the electron beam across the surface of the material [8],... 

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... 

Liquid scintillators are very useful for measurements where a detector with large volume is needed to increase efficiency. Examples are counting of low-activity )3-emitters ( H and in particular), detection of cosmic rays, and measurement of the energy spectrum of neutrons in the MeV range (see Chap. 14) using the scintillator NE 213. The liquid scintillators are well suited for such measurements because they can be obtained and used in large quantities (kiloliters) and can form a detector of desirable size and shape by utilizing a proper container. 

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. 

Other Types of Detection Systems Scintillation-type transducers consist of a crystalline phosphor dispersed on a thin aluminum sheet that is mounted on the window of a photomultiplier tube. When ions (or electrons produced when the ions strike a cathode) impinge on the phosphor, they produce flashes of light that are detected by the photomultiplier. A specialized version of this type of device is the Daly detector, which consists of an aluminized cathode in the shape of a knoh (the Daly knob) held at a very large negatKc voltage opposite a. scintillation transducer,. Analyte ions collide with the cathode producing secondary electrons that are then attracted to the sur-... 

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. 

Another case which is of increasing importance is where the detector is divided into a large number of well defined pixels of finite size e.g. CCD pixels, discrete scintillator elements, or Germanium detectors). If we assume that all relative positions of the boundaries of the detector pixels and the mask element shadows are equally likely to occur, then a typical value for the sensitivity can be found by considering the coding power of the mask convolved with the detector pixel shape e,g, a square or circular patch). 

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. 

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. 

Bertrand GHV, Hamel M, Normand S, Sguerra F (2015) Pulse shape discrimination between (fast or thermal) neutrons and gamma rays with plastic scintillators state of the art. Nucl Instmm Methods Phys Res Sect A Accelerators Spectrometers Detectors Assoc Equip 776 114-128... 

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