Technique for fabricating radiation sensitive dosimeters

The finely-divided particles of a selected phosphor material are combined with an aqueous solution of a suitable binder material to form an emulsion which is spread uniformly on the smooth surface of a support member. The wet coating is then heated to remove the water and the resulting film is sintered to produce a strong, continuous film. The film is then peeled from the support member resulting in a uniformly thick sheet of phosphor and binder from which dosimeters can be cut or stamped to any desired size.

BACKGROUND OF THE INVENTION 
The ever increasing demands for energy and the continuing development and 
significance of nuclear energy has resulted in countless techniques for 
the exploration of uranium. One characteristic which is now being 
considered to identify the location of uranium deposits involves the 
monitoring of radon emanating from the earth. A particularly suitable 
technique for monitoring radon as an indication of potential uranium 
deposits includes the positioning of radon responsive dosimeters a few 
feet beneath the surface of the earth and the subsequent reading of the 
dosimeters to determine the level of radon emanating from a particular 
site. The application of radon responsive dosimeters, such as calcium 
sulfate activated by dysprosium (CaSO.sub.4 :Dy) for uranium detection is 
described and illustrated in U.S. Pat. No. 4,053,772, issued Oct. 11, 
1977, assigned to the assignee of the present invention and incorporated 
herein by reference. The mechanism for determining the presence of radon 
is the alpha-sensitivity of the dosimeter. When alpha-particles from radon 
strike the dosimeter electrons in the dosimeter material are raised to a 
higher energy level. These electrons are trapped, as described in the 
above-referenced U.S. patent, until the dosimeter material is heated, 
whereupon the electrons move to the conduction band and then return to 
their original valence band. During this movement, light energy is emitted 
and the measurement of the light energy emitted is measured as an 
indication of the presence of uranium. 
While commercially available dosimeters, or phosphor compositions as 
identified above, are suitable for alpha storage, non-uniformity in the 
dimensions of the dosimeter can result in a misleading interpretation of 
the radon concentration. Furthermore, the techniques for fabricating 
commercial dosimeters, which typically consist of slicing bar stock, limit 
the practical effective diameter of the dosimeters. Commercial dosimeters 
presently available are nominally 0.6 centimeters in diameter and 0.02 
millimeters in thickness. Efforts to slice dosimeter wafers from bars of 
larger diameters has proven unsatisfactory. The problems encountered in 
the larger diameter bar stock are due in part to the phosphor powder 
itself. Efforts to increase the concentration of phosphor material as 
occurs when larger diameter bars are developed, increases the 
susceptibility of the bar to flaking or chipping. Poor uniformity in 
thickness plus dimensional irregularities can result if the phosphor 
content is too high. While increase phosphor content improves dosimeter 
sensitivity, efforts to commercially produce dosimeter wafers of an 
effective diameter larger than 0.6 centimeters at phosphor concentrations 
of greater than 33% have not proven commercially practical. Thus, a 
requirement for a larger diameter dosimeter necessitates an undesirable 
reduction in the phosphor content. 
SUMMARY OF THE INVENTION 
An emulsion of a suitable substrate material, i.e., Teflon, and a selected 
phosphor, i.e., CaSO.sub.4 :Dy is made by mixing together finely-divided 
phosphor particles, i.e., finer than 200 mesh, with an aqueous 
polytetrafluoroethylene dispersion, commercially available from E. I. 
Depont deNemours and Company under the trade name Teflon 30 TFE. The 
Teflon serves as a binder material for the phosphor. A non-ionic wetting 
agent, surfactant, contained in the commercially available TFE dispersion, 
aids in the mixing of this dispersion. The surfactant in this instance is 
Triton X100. The surfactant serves to maintain the Teflon particles in 
suspension in the aqueous polytetrafluoroethylene dispersion and prevents 
clumping of the Teflon particles. 
The phosphor particles constitute at least 34% by weight of the solids 
content of the emulsion. The maximum ratio of phosphor to Teflon is 
governed by the practical mechanical strength of the final film. 
Experimentation indicated that this concentration can be increased to as 
much as 75%. The ability to increase the percent by weight content of the 
phosphor above the 34% level of commercially available dosimeters permits 
significant increase in the sensitivity of the resulting dosimeter. A 
technique for forming dosimeter wafers is disclosed in U.S. Pat. No. 
3,471,699. This technique employs dry ingredients which are compressed and 
heated and cooled under pressure. The dosimeter composition is limited to 
a phosphor to binder weight ratio of 1:2, wherein the phosphor by weight 
content is approximately 33%. 
Once the emulsion of the subject invention is thoroughly mixed, it is 
applied onto a smooth surface support member and spread evenly over the 
surface to form a layer having a thickness which, following removal of the 
water, will produce a dosimeter film of desired thickness. Following the 
spreading of the emulsion, the emulsion is heated to remove the water and 
the resulting film is sintered to produce a strong continuous film. The 
film is then peeled from the support member. Dosimeters of desired size 
and shape are then cut or stamped from the film with the result that the 
dosimeters may have effective diameters substantially greater than 0.6 
centimeters.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 2 there is disclosed a novel technique for 
fabricating uniformly thick radiation dosimeter sheet material having 
surface areas substantially greater than that achievable from conventional 
dosimeter fabrication methods. 
The most significant departure from the prior art techniques is the concept 
of forming an aqueous emulsion which permits the uniform spreading of a 
relatively thin layer L of a phosphor particle P and binder B composition 
on the smooth surface S of a support member M for the fabrication of large 
surface area, uniformly thick dosimeter sheet material. 
For the purposes of discussion, the phosphor material selected is calcium 
sulfate activated by dysprosium (CaSO.sub.4 :Dy) and the aqueous binder 
solution is an aqueous polytetrafluoroethylene dispersion containing resin 
particles of between 0.05 and 0.5 microns. The calcium sulfate-dysprosium 
phosphor is selected for the purposes of discussion inasmuch as it 
exhibits the desired alpha-sensitivity which renders it particularly 
suitable for uranium detection operations as described in the 
above-referenced U.S. Patent. It is apparent that other phosphor materials 
can be substituted in accordance with the operational requirements of the 
dosimeter. Various calcium sulfate phosphor mixes are described in U.S. 
Pat. No. 3,883,748. While Teflon, as described above as the aqueous binder 
solution, is particularly suitable for dosimeter fabrication, other 
materials such as silicon rubber, when thinned with a solvent such as 
trichloroethylene also works effectively as a binder for the phosphor 
powder. Similarly, polyimide has been successfully employed as a binder. 
Referring now to the process steps of FIG. 2, the novel dosimeter 
fabrication technique consists of the following steps: 
1. Mixing radiation sensitive phosphor particles of approximately 200 mesh 
size or finer and a binder dispersion in water to form an aqueous 
emulsion; 
2. Spreading the emulsion on a smooth surface of a support member to form a 
uniform emulsion coating; 
3. Heating the emulsion coating to remove the water and form a dry coating; 
4. Sintering the dry coating to form a continuous dosimeter film; and 
5. Removing the dosimeter film from the smooth surface of the support 
member. 
In the above process, it has been determined experimentally, that a heating 
temperature of approximately 120.degree. C. will effectively remove the 
water and form a dry coating, while a temperature of approximately 
370.degree. C. will effect the desired sintering which yields a strong 
continuous dosimeter film. The sintering step will also remove much or all 
of the wetting agent or surfactant, of the binder dispersion. 
Experimentally it has been determined that sintering should occur for at 
least three hours to achieve the desired results. Experimentation has 
further indicated that an aqueous coating of approximately 4.5 mills 
spread over a smooth surface of a support member will yield a dosimeter 
film of approximately 2.0 mills. 
The size of the phosphor particles and the viscosity of the aqueous 
emulsion are critical considerations. If the phosphor particle size is 
much larger than 200 mesh, there is a tendency for the phosphor particles 
to settle out of the emulsion and not remain completely encapsulated by 
the binder. 
It has been determined experimentally that the amount of water added to the 
phosphor particles and binder dispersion should be such as to form an 
aqueous emulsion with a viscosity greater than 30 centipose, i.e., 
approximately 60 centipose. The upper limit of the viscosity would be that 
at which "caking" would occur.