Scintillation detection system

The radiochemical method quantifies gross a activity utilizing either a gas flow proportional counter or a scintillation detection system following chemical separation. In the EPA radiochemical method, the uranium is co-precipitated with ferric hydroxide, purified through anion exchange chromatography, and converted to a nitrate salt. The residue is transferred to a stainless steel planchet, dried, flamed, and counted for a particle activity (Krieger and Whittaker 1980). 

Block Diagram of the Radiometric LC Scintillation Detecting System Courtesy of APackard Inc. 

The Berthold radioactivity monitor (produced by Laboratorium Prof. Dr. Berthold, D-7547, Wildbad 1, Calmbacher Strasse 22) is the only on-line radiometric scintillation detection system suitable for HPLC that is commercially available. The monitor is offered with cells for heterogeneous scintillation counting or as pert of a homogeneous scintillation detector system. Flow cells filled with glass... 

Many radionuclides decay with the emission of gamma rays (photons). These photons can be detected and identified by their characteristic energies by using a germanium detector, a lithimn-drifted germanium detector, or a scintillation detection system. 

Recent papers from the Philips Laboratories37 40 contain thorough discussions of the Geiger counter, the proportional counter, and the scintillation counter, and significant performance data for all three, the emphasis being placed throughout upon x-ray applications. The detection system employed by Parrish and Kohler was particularly noteworthy in that it could conveniently accommodate any one of four detectors. ... 

Some substances, known as fluors or scintillants, respond to the ionizing effects of alpha and beta particles by emitting flashes of light (or scintillations). While they do not respond directly to gamma rays, they do respond to the secondary ionization effects that gamma rays produce and, as a result, provide a valuable detection system for all emissions. 

Of the three commonly used X-ray detectors—(1) Geiger counter, (2) scintillation counter, and (3) proportional counter—the latter is used most frequently for electron-probe microanalysis. In the wavelengths from 1 to 10 A, sealed proportional counters may be used. For longer-waveleiigtli analysis—in the range from 10 to 93 A—the thinnest possible detector window is required to limit spectral attenuation. Nitrocellulose windows have proved successful. Nondispersive detection systems using cooled Li-dnfted Si are also applicable. 

The detection system employed for monitoring antibody interactions with antigens will depend on the type of IA performed. The majority are batch systems and are performed off-line. Quantification is achieved by either measuring the absorbance with spectrophotometer, excitation or emission with flourometer, radioactivity decay with a scintillation counter or visualization of direct precipitation. Typical formats used include microtiter plates or membranes or dipsticks or gels. The assay time can range from less than a minute for precipitation/agglutination assays to minutes for the dipsticks onfield assays and hours for microtiter plate-based laboratory assays (48). 

The most important parameter of the detection system is its response function. We have studied this extensively in Monte Carlo and other calculations. The calculated time-spectrum response to monoenergetic neutrons is composed of a Gaussian timing curve (2.97-ns FWHM), a trapezoidal contribution from detector thickness and non-axial paths, and an exponential tail, calculated by Monte Carlo, from multiple scattering in the neutron scintillator. (Spectrum distortion due to neutrons multiply scattered by structural and other parts of the apparatus and arriving at the neutron... 

For measuring the RTD of a very fast-moving phase, the sensitivity and the response time of the tracer-concentration recording equipment may be a problem. A radioactive tracer offers an advantage in that the scintillation detection counter can be interfaced with very rapid recording systems or multichannel analyzers. 

The nature of the tracer dictates the detection system. For the liquid phase, quite often the tracers (e.g., NaCl, H2S04, etc.) are such that the detection probe is directly inserted into the reactor and continuous monitoring of the concentration at any fixed position is obtained by means of an electrical conductivity cell and a recorder. In this case, no external sampling of liquid is necessary. If the tracer concentration measurement requires an analytical procedure such as titration, etc., sampling of the liquid is required. For the solid phase, a magnetic tracer is sometimes used. The concentration of a solid-phase tracer can also be measured by a capacitance probe if the tracer material has a different dielectric constant than the solid phase. In general, however, for solid and sometimes gas phases, some suitable radioactive tracer is convenient to use. The detection systems for a radioactive tracer (which include scintillation counters, a recorder, etc.) can be expensive. Some of the tracers for the gas, liquid, and solid phases reported in the literature are summarized in Table 3-1. 

The relatively rapid acceptance of the general validity of the radiocarbon method by most researchers is indicated by the rapid establishment of other laboratories to perform analyses. Many who attempted to duplicate the procedures required to obtain acceptable values using the solid carbon technique experienced moderate-to-severe difficulties. The result was the substitution of gas or liquid scintillation counting methods. Because of the greater efficiency of such detection systems, these developments permitted the maximum age range to be extended from about... 

Conventional scintillation counters such as the Microbeta (Wallac/Perkin Elmer, Turku, Finland) or the TopCount (Packard, Meriden, USA) use photomultiplier detection systems that count 8 or 12 wells at a time, resulting in a readout time of 40 minutes per 384-well microplate. Bialkali photocathodes (Sb-Rb-Cs or Sb-K-Cs) used in standard photomultipliers have a maximum spectral response at about 420 nm, with a quantum efficiency for detection of up to 30%. Thus, the aforementioned instruments are ideally suited for filtration assays and SPA assays with the blue-emitting YSi and PVT beads. 

The total time required to complete the above steps is defined as the dead time (r) and is related to the signal integration time that depends on the electronics and the scintillation decay time. During this time the detection system is unable to process a second event, which will be lost. This loss (called the dead-time loss) is a serious problem at high count rates and varies with different PET systems. It is obvious that the dead-time loss can be reduced by using detectors with shorter scintillation decay time and faster electronics in the PET scanners. 

Two models of dead-time behavior have been commonly used the paralysable model and the nonparalysable model (Knoll, 1979). Experimental data suggested that the paralysable model is suitable to describe the current detection system (Sun, 1985). For this model, the statistical relationship of the recorded count rate m to the true scintillation rate n is expressed as... 

The amount of light produced in the scintillator is very small. It must be amplified before it can be recorded as a pulse or in any other way. The amplification or multiplication of the scintillator s light is achieved with a device known as the photomultiplier tube (or phototube). Its name denotes its function it accepts a small amount of light, amplifies it many times, and delivers a strong pulse at its output. Amplifications of the order of 10 are common for many commercial photomultiplier tubes. Apart from the phototube, a detection system that uses a scintillator is no different from any other (Fig. 6.1). 

If the dead time of a detection system using a scintillator is 1 pts, what is the gross counting rate that will result in a loss of 2 percent of the counts ... 

The homogeneous detection system consists of a monitor, detector cell, tubing, pump for delivering scintillator solution and magnetic mixer for mixing the scintillator solution and eluent. The volume of the flow cell is 200 pi, which corresponds to an effective volume of. e.g., 20 ul if the scintillator solution and... 

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-... 

From the discussion of the factors that enter into the counting efficiency it is obvious that the preparation of the counting sample must be done with care and must be r roducible if several samples are to be compared. Counting of a- and jS-emitters in solution is best achieved by means of liquid scintillation counting. Because in this technique the emitters are included in the detection system itself the efficiency is very high and reproducible. 

Alternatively, the transfer of the radionuclides energy to these bound electrons raises them to an excited state in the atom or molecule. When the excited species returns to its ground state energy level, the excited atom or molecule may emit electromagnetic energy in the ultraviolet to visible (UV/Vis) region. This light can be detected by a photomultiplier tube (PMT) in a scintillation counter system. 

Bremsstrahlung is of interest in radioanalytical chemistry because some of the energy of electrons stopped in detector-shielding material is converted to X rays that can penetrate the shield. Cherenkov radiation permits scintillation counting of radionuclides in plain water samples if the electron energy is sufficiently high and the detection system is sufflciently sensitive. 

In a liquid scintillation (LS) system, the sample is mixed with a cocktail that consists of an organic scintillator dissolved in an organic solvent. The cocktail and the usual aqueous sample form an emulsion. The radiation emitted by the intimately mixed radionuclide deposits its energy in the solvent, which transfers it to the scintillator. The scintillations are then detected by the PMT. The LS counter is useful for detecting alpha particles and low-energy beta particles from samples that... 

The distilled water is counted in an LS system at a selected water-to-cocktail ratio (usually 1 1). Samples may be counted without purification if other radionuclides are known to be absent or can be differentiated clearly from tritium by pulse-height discrimination in the detection system, and if chemicals that cause excessive quenching or fluorescence are known to be absent. Section 15.4.3 illustrates an extension of this measurement technique, where is measured in flowing water with a scintillation counter. 

The radiation detection systems employed in radioanalytical chemistry laboratories have changed considerably over the past sixty years, with significant improvement realized since the early 1980s. Advancements in the areas of material science, electronics, and computer technology have contributed to the development of more sensitive, reliable, and user-friendly laboratory instruments. The four primary radiation measurement systems considered to be necessary for the modern radionuclide measurement laboratory are gas-flow proportional counters, liquid scintillation (LS) counters. Si alpha-particle spectrometer systems, and Ge gamma-ray spectrometer systems. These four systems are the tools used to identify and measure most forms of nuclear radiation. 

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