DE2452540C2 - Use of a dispersion of plastic particles with a content of scintillator substances in a homogeneous aqueous phase - Google Patents

Description

Emulsifier. Emulsifiers with longer oxyethyl sequences have proven to be less suitable. Suitable emulsifiers are sodium dodecyl sulfate and other suifisrte alkanes with up to 20 carbon atoms, optionally sulfated reaction products of substituted phenols with not more than 15 mol ethylene oxide or the salts of fatty acids with up to 20 carbon atoms called The amount of emulsifier can be in the range of 0.2 to 15%, based on the weight of the water phase. Since generally the lowest possible Particle size is sought after, large amounts of emulsifier; H. in the range of 1 to 8%, usually cheap.

| e should be lower ass particle size, the lower the Feststoffanteii also has the dispersion can be selected. At high fast substance concentrations, particles of very small size result in highly viscous dispersions which are hardly suitable for use according to the invention. A solids content of 50% by weight is likely to be an upper limit for useful scintillating plastic dispersions, but the particle size must be at the upper limit of the permissible range. For the preferred dispersions with a particle size of 20 to 100 nm, a solids content of 30% by weight should not be exceeded. In order to produce extremely small particles, it can be useful to reduce the solids content to a few percent. A solids content of 1% in the final measurement solution may represent the lower limit of the range in which measurements are still possible. With a solids content of 3%, reproducible values are obtained with sensitive measuring instruments. In general, the dispersions used according to the invention contain 10 to 30% by weight of plastic.

The dispersed plastic particles contain, in a known manner, the actual scintillation substance, which converts nuclear rays, generally short-wave, ultraviolet light. As an example of these additives, also referred to as primary solutes, which contain an amount between about 0.05 -> 5% in the plastic, are, in addition to the "classic" scinfillation substances mentioned at the beginning, such as p-terphenyl , for example 2-phenyl-5- {4-biphenylyl) -13 ₁ 4-oxadiazole (abbreviation PBD) and 2,5-diphenyloxazoI, 2- {4 't-butylphenyl) -5- {4 "-biphenylyI) -i , 3,4-oxadiazoL 2 £ -diphenyl-1,3,4-oxadiazoI, 2,5-di-4- (biphenylyl) -1, 3,4-oxadiazole, 1,1'-dinaphthyL 2 (1-naphthyl ) -5-phenyloxazole, 2 ^ -diphenylfuran; p-quaterphenyl; 2- (2-naphthyl) indole. In addition, the scintillator particles contain fluorescence in an amount between about 0.001 and 0.3% by weight Dye, which is also known as the "wavelength shifter" or secondary solute. This substance converts the short-wave UV light of around 350 nm emitted by the primary solute into low-wave UV light or visible light. For this purpose, 2,2'- p-PhenyIenbis (5-phenyloxazole) (Abbrev upg POPOP), 1,4-di- (4-methyl-5-phenyIoxazo! yI) -benzoL 4,4'-diphenylstilbene, 2 ^ '- p-phenylenebis (5 ^ naphthyl) -oxazole); 1,4-Di- [25 (p-tolyOxazoIyl)] - benzoI, 2 £ -Di- {4-biphenyIyl) -oxazoI or 1,4-bis (p-dimethoxystyryl) -2 £ -dimethoxybenzene are well suited for the preparation of the According to the invention to be used dispersions, the primary and the secondary solute are used together with the monomer phase. Water-soluble or water-dispersible active substances can be dissolved or distributed directly in the dispersion. In many cases the active substance is in the form of an aqueous solution, which is then mixed with the aqueous synthetic material dispersion. The mixing ratio naturally depends on the activity to be expected and the plastic content of the dispersion. For the first indicative measurement of a substance of unknown activity in aqueous solution, this can be done, for example, with the same volume of a 30% ig :. π scintillator dispersion are mixed. The lower the activity of the solution to be investigated, the more should be used and the more plastic should be used for the measurement. The most favorable compromise must be sought between the plastic concentration, which should be as high as possible, and the particle size, which should be as small as possible.

To determine the Me0bedingup.gen, i.e. plastic content in the final measurement solution, the size of the article and the amount or fill level of the measurement solution, one should look at the chain of events from the decay of a radioactive isotope, such as CI-36, C-I4, J-131, P -32, S-35, Ca-45, K-40, Ca-137, Am-241, Ni-63, Tc-99 and Sr-90, until the flash of light hits the photocell of the secondary electron multiplier Keep eyes. It is well known that the various types of radiation differ in their range. A Λ decay can only be registered if a plastic particle is reached on the very short path of the particle. The probability of hitting a plastic particle increases with its concentration. The flash of light excited in the struck plastic particle can only be registered, however, if it is not absorbed or scattered on the way to the photocell. Since the radiation intensity also decreases with the square of the distance, flashes of light from plastic particles from outside a certain distance to the photocell contribute practically nothing to the measurement result. This results in a limit value for the fill level in the measuring cuvette, beyond which there is no Zjr.sh ~ .c the Sckundarstrahiungskisiung. No icaUusiciicn isu This limit value is to be determined in each individual case depending on the type of radiation and is often in the order of 3 to 10 cm. If a fill level below this limit value is used, the fill level is included as a parameter in the measured variable. If the limit value is exceeded, the measurement result is independent of the level.

The range of the / radiation is considerably greater than that of the «radiation. The likelihood that the eo / -Particle reached in a plastic particle, is therefore much larger than with a «-particle. One can as a result, measure with a lower plastic concentration. While in Λ-decay the place of the decaying Isotope almost coincides with the starting point of the secondary radiation, can in the case of the primary / Radiation is the secondary radiation emanating from the plastic particle struck from another location go out. If the particle hit is on the way between the place of / decay and the photocell, the resulting flash of light is correspondingly less due to scattering or absorption weakened A limit value of the fill level, beyond which the counting efficiency or the response probability no longer increases, of course also results from / -decomposition. He hangs among other things. also from the energy of the primary

radiation. Here, too, the most accurate measurements are made at fill levels above the limit value mentioned obtain.

The / «radiation has the greatest range of the measurable types of radiation. She is - in terms of here in Considerable orders of magnitude - practically unlimited Whether a measurable secondary radiation occurs depends only on whether a scintillator molecule is excited in the area of the measurement solution. The starting point the secondary radiation is almost independent of the location of the decay. The limit value of the filling level results in this case solely from the range of the secondary radiation.

For all types of radiation, the limit value of the fill level, beyond which no increase in secondary radiation is registered, the greater the smaller the plastic particles, the fewer particles per unit volume and the harder the primary radiation.The accuracy of the measurement increases according to the rules the statistics with the square root of the counted pulses, i.e. also with the duration of the measurement. Otherwise, the working technique of activity measurement when using dispersions according to the invention is largely similar to working with liquid scintillators; so for example it is convenient to use Quarzküvettcn with mirrored sides.

example
A. Preparation of the dispersion

In a Wittschen Τ ~ ορί {7 ι) mil reflux condenser, stirrer and feed vessel, 4.0 g of sodium lauryl sulfate are dissolved in 207 g of distilled water ^{at 8O 1- C 0.6 g of ammonium peroxydisulfate} .4 hours, an emulsion consisting of 274 g vinyl toluene, 206 g methyl acrylate, 1.44 g ammonium peroxydisulfate, 5.0 g sodium lauryl sulfate, 12.0 g butyl PBD, 0.24 g POPOP and 913 g distilled water 80 ⁰ C was added dropwise upon complete addition of the emulsion, the mixture is kept for 2 hours at 80 ⁰ C, then cooled to room temperature (ca.25 ° C).

A dispersion is obtained with an average particle radius of approx. 25-30 nm.

B. Activity determination

5 ml of an aqueous be measured activity solution containing the pure / -? Radiator χ ³⁶ C with a half time of 3 10 ^s a and contains a maximum energy of 0.71 MeV in the form of sodium chloride are added with the same volume of the above dispersion and in a measuring cell with shaking The mixed solution to be measured activity showing a specific activity of 1 χ 10- ² μΟ / ΐη1. After 1: 1 dilution of the test solution has a polymer content of 15.8 ⁰ Io. The measuring cell is inserted into a conventional liquid scintillation measuring head, which contains a secondary electron multiplier, and the integral counting rate is measured with the aid of a recording unit.

a) measured activity rate:

b) repetition:

Measurement after 64 hours waiting time:

34 679 pulses / min probability

31.2% answer

33 886 pulses / min = 30.5% response probability

34 528 pulses / min = 31.1% response probability

It is not necessary to purge the measurement solution with nitrogen. Furthermore, there are no phosphorescence effects. The use of mirrored quartz measuring cells increases the counting yield.

Claims (1)

Claim: Use of a dispersion of plastic particles of less than 1 μΐη diameter with a Content of scintillator substances in a homogeneous aqueous phase as a scintillator for liquid scintillation measurement of radioactivity. ίο For the measurement of charged particles, ^ -quants and fast neutrons one uses scintillators, which convert this radiation into luminescent light, which is then done by means of secondary electron multipliers can be measured. Such scintillators are e.g. B. solutions of anthracene, stilbene, or diphenyloxazole p-Terphenyl in toluene, xylene or similar aromatic solvents. The activity of radiating substances which are not soluble in the solvents mentioned can only be measured to a limited extent with such systems will. In the case of water-soluble, radiation-active substances, solubilizers are used that allow one of the aqueous solution of the active material and the aromatic solution of the scintillation substance to form homogeneous phase. The accuracy of the activity determinations obtained with such systems suffers from dependence on a large number of external parameters. Another class of scintillators contains the scintillation substance, such as p-terphenyl, etc., in a solid Plastic matrix, such as B. polyvinyl toluene. These solid scintillators are in the form of solid plastic blocks for determining the activity of water-soluble substances only in the form of containers, flow cells, threads or Stripes suitable Only the phase boundary layer is used for low-energy radiators or those with a short range recorded A better surface / volume ratio can be achieved if these solid Scintillators in the form of a bead polymer suspended in an aqueous solution of the active substance But even in this case of activity determination in a heterogeneous system, the measurement result is one of many Depending on the parameters, for example the counting yield increases with the particle size of the scintillator beads. Solid scintillaries of the last-mentioned type are known from US Pat. No. 3,457,180. They contain them Scintillator substances in a plastic matrix, which consists of a copolymer of vinyl toluene and methyl methacrylate consists In US-PS 35 13 102 the production of fluorescent or scintillating coatings from a Dispersion of plastic particles, of less than 1 μm in diameter, the scintillator substances in a polymer matrix contained, described in a homogeneous aqueous phase. The dispersion is applied to the surface of a solid body applied and dried to form a layer of the scintillating polymer Although the dispersion is likely to fluoresce even when exposed to radioactive radiation, their use for the ticvis measurement has not been disclosed or suggested in this publication. The present invention follows a new way of determining the activity of water-soluble or in Water dispersible active substances. The invention relates to the use of a dispersion of Plastic particles of less than 1 μm diameter with a content of scintillator substances in a homogeneous aqueous phase as a scintillator for liquid scintillation measurement of radi * activity The use of such dispersions has compared to that of organic solutions of the scintillators Advantage that the former in any ratio without additives such. B. solubilizer, miscible with water are. This is particularly advantageous for measuring biological systems. Compared to the pearl polymers they have the advantage that they are statistically more evenly distributed in the system to be measured and not for Tend to self-sedimentation. Dispersions with very small particle diameters of 20 to 100 nm are particularly favorable. In such dispersions System is losing little light through scattering. The lower limit of this range is in the order of magnitude of the diameter of the tangle of a single macromolecule and can therefore in principle no longer be exceeded
The same plastics that are used are suitable as the plastic matrix for the dispersed scintillator particles has already been used for the production of plastic scintillators in the form of blocks or beads. here are primarily polyvinyltoluene and polystyrene as well as copolymers containing at least 30% vinyltoluene or contain styrene. Other aromatic vinyl monomers, like styrene or vinyltoiuol or possibly in addition to these, am Construction of the plastics may be involved, but technically less accessible or harder to use polymerize are vinylxylenes, p-vinyl-diphenyl, 1-vinylnaphthalene, 4-vinyl-p-terphenyl or 2-vinyl-9.10-diphenylanthracene. In general, the aromatic vinyl monomers mentioned form at least one 50% by weight of the plastic Non-aromatic monomers that can also be involved in the synthesis of plastics are primarily the esters of acrylic or methacrylic acid. Of these, the methyl and ethyl esters are particularly suitable. The propyl and butyl esters can also be used, the esters with a larger carbon number in the alkyl radical are less suitable. In principle, it is expedient to select the non-aromatic comonomers so that the resulting copolymer does not form a film at room temperature, ie has a softening temperature above room temperature. Methyl methacrylate is the most favorable among the non-aromatic comonomers. A composition of 57.2% by weight vinyltol and 42.8% by weight methyl methacrylate has the particular advantage that this monomer mixture polymerizes "azeotropically", ie that the copolymer formed consists entirely of macromolecules of this composition. The dispersions used according to the invention can be prepared by the customary methods of emulsion polymerization getting produced. For this purpose, non-ionic, cationic or, better, anionic ones are used