BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to organic scintillators , and more particularly, to organic scintillators that are made elastic for more convenient use.

2. Description of the Prior Art

Organic scintillators ^' are commonly used as relatively inexpensive materials to detect ionizing radiation. In them particles of ionizing radiation produce flashes of photons, or scintillations, that are then registered with suitable photodetectors, such as photomultiplier tubes.

There are two widely spread types of organic scintillators: liquids and plastics. Typically, they comprise an aromatic base material into which small amount of fluorescing compounds called fluors are dissolved. Among liquid scintillators the base material is an aromatic solvent like benzene, toluene, xylene or pseudocu ene, although less hazardous solvents like alkylated benzenes and naphtalenes have recently gained popularity. In plastic scintillators the base material is an aromatic polymer, most often polystyrene or polyvinyltoluene, resulting in a rigid material.

Scintillation mechanisms are essentially the same for both types of scintillators. The radiation excites the base material whereafter the excited states rapidly migrate to fluor molecules. Their subsequent de-excitation leads to emission of several photons, i.e. a scintillation. To obtain best sensitivity for the photomultiplier tube two different fluors are often employed. The near ultraviolet photons from

the primary fluor are absorbed by the secondary fluor that re-emits them in visible region through a fluorescence process. Examples of common primary fluors are diphenyloxazole (PPO) and phenylbiphenylyloxadiazole (PBD) , whereas bis (phenyloxazolyl)benzene (POPOP) and bis (methylstyryl)benzene (bis-MSB) are typical secondary fluors.

Organic scintillators are employed in many kinds of radiation detection systems. A widespread application is their use in containers or tanks of various sizes to detect external radiation. However, some problems are faced when designing these containers. Liquids can leak off from joints or be corrosive to surrounding container materials. Moreover, many of them liberate harmful vapors. With plastics, mechanical stresses may develop that can damage the container-photodetector assembly, especially if considerable changes occur in surrounding temperature.

The object of the present invention is to produce a versatile and easily usable scintillator material that does not have the problems described above.

SUMMARY OF THE INVENTION

The object of the present invention is to produce a new type of scintillator that is elastic and, consequently, easy to use in various applications.

The inventors have found that it is possible to dissolve liquid silicone into appropriate aromatic solvents with at least one fluor and then vulcanize the silicone to yield an elastic composite that acts as a scintillator. The resulting composite is not fluid and thus cannot leak. Moreover, because of elasticity it can tolerate considerable stress without damage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is accomplished by dissolving liquid silicone into appropriate aromatic solvents together with at least one fluor and then vulcanizing the silicone to yield elastic composite that acts as a scintillator. The vulcanized silicone presumably forms a matrix that retains the solvent and the fluors therein. Analogous silicone matrices are known in somewhat different field for containment and release of volatile organic liquids, such as fragrances; see e.g. the U.S. Patent 4,725,575.

According to the present invention conventional aromatic solvents, such as benzene, toluene, xylene, or pseudocumene can be used, but they may be harmful because of the fumes they develop. Often it is more preferable to employ less volatile solvents of which alkylbenzenes with the alkyl group having at least four carbon atoms are very convenient. Good solvents in this class are commercially obtainable from Shell under the trade name Dobane. Among them Dobane 102 is well applicable comprising a mixture of alkylbenzenes with the number of carbon atoms in alkyl groups being around twelve.

The preferable silicones in the present invention belong to a class known as room temperature vulcanizable (RTV) silicones. They are supplied as pourable liquids that can be vulcanized, or cured, at easily attainable temperatures to yield stiff rubber-like material. At 25°C the cure time is typically at least one day, but by using elevated temperatures, e.g. about 100°C, the cure time can be shortened to less than some hours. For practical work, two-part RTV silicones are most convenient. In these systems the curing of the main silicone (part A) is initiated by adding a special curer solution (part B) .

There are many types of RTV silicones with different

chemical side groups and physical appearance. The requirements for the silicones usable in the present invention are that they should be clear and colourless to allow maximum transmission of scintillation light and that they should be compatible (soluble and curable) with the used solvents. A silicone type fulfilling these requirements is obtainable e.g. from General Electric with the code RTV 615.

A wide variety of fluors exist and practically all of them can be utilized in the present invention provided that they are soluble in the solvent used. PPO as the primary fluor and bis-MSB as the secondary fluor are easily obtainable and well employable.

EXAMPLE

A versatile scintillator can be prepared in the following way. First, 500 mg of PPO and 25 mg of bis-MSB were dissolved into 100 ml of Dobane 102. Into 6 parts of this solution 4 parts of RTV 615A (the main silicone) and 0.4 parts of RTV 615B (the curer) were added followed by vigorous mixing. The mixture was then poured into the desired container and cured in 90°C for at least one hour. The resulting composite is the required elastic scintillator. Naturally, the relative amounts of components can be varied for optimization purposes.