Scintillation process

Helf White (Ref 2) interpret the above behavior of the nitrocompds in inhibiting the scintillation process as one of simple light absorption rather than as a true chemical quenching (ae-excitation process). To substantiate this, the UV and near-visible spectrum of each of the light compds in toluene—PPO soln was measured using the 50% extinction concn for each nitrocompd (as determined from Fig 1). 

Fig 1 Quenching of the Scintillation Process by Organic Nitrocompounds in Toluene-14C-PPO... 

The addition of a secondary solute or wavelength shifter can serve to offset much if not all of the action of tagged nitrocompds in reducing counting efficiency. For expl nitrocompds, a shift of the emission spectrum considerably into the visible region where absorption effects are not so pronounced is clearly indicated. The secondary solute POPOP has been found to be most efficient for this purpose (Ref 2). This enhanced effect on the scintillation process is illustrated in Fig 2 for p-Nitrotoluene... 

The scintillation process, in detail, begins with the collision of emitted j8 particles with solvent molecules, S (Equation 6.10). 

Chemical quenching occurs when chemical substances in the scintillation solution interact with excited solvent and fluor molecules and decrease the efficiency of the scintillation process. To avoid this type of quenching the sample can be purified or the fluors can be increased in concentration. Modern scintillation counters have computer programs to correct for color and chemical quenching. 

The internal standard ratio method for quench correction is tedious and time-consuming and it destroys the sample, so it is not an ideal method. Scintillation counters are equipped with a standard radiation source inside the instrument but outside the scintillation solution. The radiation source, usually a gamma emitter, is mechanically moved into a position next to the vial containing the sample, and the combined system of standard and sample is counted. Gamma rays from the standard excite solvent molecules in the sample, and the scintillation process occurs as previously described. However, the instrument is adjusted to register only scintillations due to y particle collisions with solvent molecules. This method for quench correction, called the external standard method, is fast and precise. 

In a scintillation detector, a fraction of the energy deposited by the primary radiation in the detector is converted to light that, in turn, is converted into an electrical signal. Conceptually, the process can be divided into the scintillation process itself (energy —> light), the collection and conversion of the light into electrons... 

The details of the scintillation process are complicated and depend very much on the molecular structure of the scintillator. In organic crystals, the molecules of the organic solid are excited from their ground states to their electronic excited states (see Fig. 18.18). The decay of these states by the emission of photons occurs in about 10-8 s (fluorescence). Some of the initial energy absorbed by the molecule is dissipated as lattice vibrations before or after the decay by photon emission. As a result, the crystal will generally transmit its own fluorescent radiation without absorption. 

The scintillation process in inorganic scintillators differs from that in organic scintillators. Consider the structure of an ionic crystal, as shown in Figure 18.19. When an energetic electron passes through the crystal, it may raise valence electrons from the valence band to the conduction band. The electron vacancy in the valence... 

Figure 1. Scintillation processes and their probabilities in a binary system...

An important result of our experimental investigation is the fact that within experimental error the same results were obtained for the three series of sample mentioned above as well for the fluorescence quantum spectra and transmittance distributions as for the measurements of fluorescence intensity under -irradiation. From this it follows that in the samples prepared by a solvent the remaining solvent in the scintillator does not influence the scintillation processes. 

Measurement of radioactive decay can also be affected by various components present in, or added to, the scintillation cocktail. These components can cause quenching that is, they can decrease the efficiency of the scintillation process. Scintillation counting provides data in counts per minute (cpm). Quenching dictates that the counts per minute detected is less than the actual decay rate, or disintegrations per minute (dpm). Almost every sample encountered experimentally is quenched to some degree for example, 02 picked up by the scintillation fluid from contact with air serves as a quencher. Therefore, researchers frequently count an additional sample containing a standard of known de-... 

In both scintillator and gas detectors, the absorption of radiation causes excitation and ionization however with the scintillation process, the absorbed energy produces a flash of light, rather than a pulse of current. The principal types of scintillation detectors found in the clinical chemistry laboratory are the sodium iodide crystal scintillation detector and the organic liquid scintillation detector. Because of the crystal detector s relative ease of operation and economy of sample preparation, most clinical laboratory procedures have been developed to measure nucfides, such as which can be counted efficiently in a crystal detector. A liquid scintillation detector is used to measure pure (3-emitters, such as tritium or C. 

The response of an organic scintillator to monoenergetic neutrons depends on effects similar to those discussed in the previous section for proportional counters, with the exception of electric field distortions. The most important cause of a response different from the ideal rectangular distribution shown in Fig. 14.9 is the nonlinear relation between the energy of the proton and the amount of light produced by the scintillation process. For organic scintillators. 

Ngen IS the number of photons generated by the scintillation process,... 

The scintillation process in an organic liquid scintillator (From Kessler 1989 used by permission of Perkin Elmer)... 

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