Scintillator solgel

Methods of incorporating inorganic radiochemicals into liquid and solgel scintillators that exhibit high scintillation yield are described. Liquid-scintillation measurements have been made at the National Bureau of Standards on 31 different radionuclides for a variety of application in radionuclide metrology. Sample preparation techniques are described for a number of radionuclides that differ markedly in their chemical behavior as well as in their nuclear-decay characteristics. Particular emphasis is given to radionuclides such as Fe and... 

Commercial solgel (emulsion) scintillators used were Aquasol II and Biofluor (New England Nuclear), Beckman GP (Beckman Instr.), and PCS (Amersham Corp.). Solvents used were p-xylene (ICN spectral grade) and toluene (Packard "Puresolv") and the scintillators were p-bis-Co-methylstyryl)-benzene (bis-MSB from Eastman Kodak) and 2-(4 -t-butylphenyl)-5-(4"-biphenylyl)-1,3,4-oxadiazole (butyl-PBD from ICN). The chelating agent was di-2-ethylhexyl phosphoric acid (HDEHP) obtained from K K Laboratories and used without further purification. Radionuclide solutions of known radioactivity concentration were supplied by other workers in the NBS Radioactivity Group (Mann, 1979). 

In a typical experiment, carriers and chelating agent are added to a flask containing 100 mL of solgel scintillator. If necessary, up to 5% water by volume may be added to insure dissolution of the carrier salts. This "buffered scintillator" is then pipetted into the special cells, in the case of systems (i) and (ii), or for the commercial counter (iii), into glass vials, which meet the dimensional requirements of the International Electrotechnical Commission standard (lEC, 1977). The aqueous sample, typically about 30 mg, is transferred gravimetrically to the cell and mixed with the scintillator (BIPM, 1975). It should be noted that these 30-mg aqueous samples contain only a few yg of carrier. Thus, the final emulsion will be carrier free unless additional carrier is added. 

It appears that any of the commercial solgel scintillators can be modified for use with these radionuclides. Several workers for example, have used Insta-Gel (Packard Instr.) ... 

The highest counting efficiency observed for Fe for the 10-roL vial was 45%. In this experiment the sample consisted of 17 ppm Fe in 0.5 N HCl and the final solgel scintillator contained only 3 yg Fe per 10 mL of emulsion. At such low carrier levels it may be necessary to coat the walls with a silicone compound to avoid plateout (BIPM, 1975). 

The results for the lanthanides shown in Table II demonstrate that the solgel scintillators can be used for carrier-free samples, such as the Pm, as well as for solutions containing high concentrations of carrier, such as the Ho. 

No problems were observed for uranium or americium radionuclides, but HDEHP was needed for the Am- Np equilibrium mixture. Other solgel scintillators have been used for the actinides. Horrocks (1974) has described the use of Readysolv VI (Beclonan Instr.) for normal uranium and Miglio (1978) has used Aquasol II for americium, curium and californium radionuclides. 

For measurements such as those described here, which involve very small aqueous samples, the general-purpose solgel scintillators do have a drawback in that they contain excess emulsifiers. The manufactures should be able to develop products that exhibit >50% counting efficiency for 20 mg of... 

This results in an error not correctable by external standardization or by internal standardization unless the internal standard has a distribution coefficient identical with the labeled molecule in the sample. Further, since phase contact in some solgel scintillators varies with the aqueous sample concentration, the accuracy of the quench correction will vary with sample size. In short, phase contact in solgel scintillators is at present an undefined variable which Ccin result in large and unsuspected errors in experimental results in the radioassay of tritium-labeled compounds. 

A plot of these data, if desired, give the familiar water sample quenching curve for the solgel scintillator. The phase contact ratio for each concentration is then calculated by dividing the relative counting efficiency for tritiated water at each concentration by the relative counting efficiency for the tritium-labeled toluene sample at the Scime concentration ... 

FIGURE 1. Variation in Phase Contact Ratio For Various Solgel Scintillators... 

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