GB2067060A

GB2067060A - Ultraviolet radiation dosimeter

Info

Publication number: GB2067060A
Application number: GB8039570A
Authority: GB
Original assignee: SECR DEFENCE; UK Secretary of State for Defence
Current assignee: SECR DEFENCE; UK Secretary of State for Defence
Priority date: 1979-12-19
Filing date: 1980-12-10
Publication date: 1981-07-15
Also published as: GB2067060B

Abstract

A photochemical UV dosimeter has a photosensitive element which has an action spectrum (response vs wavelength) very close to that of human skin in the erythemal (skin reddening) range of the ultra-violet (ie wavelengths below about 320 nm). The preferred photosensitive element in the dosimeter comprises a photosensitive material, especially a polysulphone, particularly dimethyl polysulphone, having an effective thickness of below about 10 microns, preferably below about 1 micron. The photosensitive element may be dispersed in an inert matrix or, preferably, mounted on a non-photoactive support. In either case the preferred material is polyvinylalcohol or regenerated cellulose strip (cellophane Trade Mark). The photochemical response, as read at 302 nm is independent of dose rate, at least from 0.5 to 2.5 J/cm<2>, and of temperature, from -4 DEG to 50 DEG C, giving A302 of 0.2 for 1J/cm2 of radiation at 280 +/- 0.5 nm. The action spectrum is independent of dose and closely approximates to that of human skin.

Description

SPECIFICATION Ultraviolet radiation dosimeter This invention relates to a dosimeter for monitoring erythemal (skin-reddening) ultra violet radiation.

Adverse reactions of humans to ultraviolet radiation have been known for many years, the most common form being the sunburn reaction which is characterised clinically be a delayed erythema response. More recently such adverse reactions have become more commonplace as for example an increasing number of photosensitising substances have been released into the atmosphere and the number of industrial occupations associated with the generation of ultraviolet radiation, for example arc welding, have also increased.

Various studies are now being undertaken to determine, for example, the exposure of particular social groups, employees etc to such environmental and industrial sources of ultraviolet radiation had the effect of such exposure upon them. In addition, the recent rapid expansion in the photochemical treatment of certain diseases, e.g. psoriasis, wherein an oral or topical administration of a photoactive drug to a patient is followed by exposure of the subject to ultraviolet radiation, and led to similar studies being performed in the medical field.

Such studies involve numerous measurements of doses of erythemal ultraviolet radiation and hence there is a need for radiation dosimeters which are cheap and easy to use but provide an effective measurement of integrated exposure to erythemal ultraviolet radiation below about 320 nm. These requirements could be met by a chemical dosimeter. However such dosimeters should be insensitive to wavelengths above about 320 nm and have a response to shorter wavelengths which is independent of dose rate; the efficiency of conversion (ie sensitivity) should be high, but the photoproducts should be substantially transparent to the erythemal U V radiation. The most important requirement, however is that the action spectrum (plot of change in optical density for a constant dose at varying wavelength) of the dosimeter should be closely matched to that of human skin.

Previous ultraviolet chemical dosimeters, for example as disclosed by Zweig and Henderson in Photochemistry and Photobiology, 24, 543, (1976), have been based on the irreversible photochemical conversion of colourless chemical compounds to coloured photoproducts. Generally however the sensitivity of these dosimeters to erythemal ultraviolet radiation has not closely matched the corresponding sensitivity of human skin and their use has been generally of a qualitive nature, giving users an estimate only of the erytnemal ultraviolet dose to which they have been exposed. However if a more accurate measurement of received erythemal ultraviolet dose is required such dosimeters would be of little use.Davis, Deane and Diffey (Nature, 261, 169, (1 976)) disclcsed the possible use of a dimethyl polysulphone in an ultraviolet dosimeter.

However the sample employed by these authors, which was a 40 micron thick layer, did not have a wavelength response tO erythemal UV radiation that closely matched the corresponding wavelength response of human skin. The half-height sensitivity of this polysulphone sample was at 325 nm. whereas the half-height sensitivity of the erythemal action spectrum of human skin is at 300 nm.

Further, the above polysulphone film is sensitive to UV radiation up to about 330 nm. whilst the biological effectiveness of UV radiation is negligible at wavelength above about 320 nm.

This small difference in spectral response can lead to significant errors in estimates of the biologically effective UV radiation (wavelengths between about 280 and 320 nm), when the film is used to monitor natural UV radiation, due to the rapid increase in intensity of the solar spectrum between 320 and 330 nm.

It has now been found that the thickness of the photosensitive element in the dosimeter may affect the spectra! response, and that by varying said thickness of said element a closer agreement between the erythemal action spectrum and the spectral response of the photosensitive element may be obtained.

According to the present invention, therefore, a photosensitive element for use in a dosimeter for monitoring erythemal ultraviolet radiation of wavelength below 320 nm comprises a shaped article prepared from a photosensitive material and having a spectral response to erythemal ultraviolet radiation of wavelength below 320 nm that is substantially the same as the spectral response of human skin to said radiation.

The shaped article will in many cases be a thin film of a photosensitive material and such an article will preferably be mounted on or between at least one supporting layer which is preferably transparent, at least, to erythemal ultraviolet radiation below about 320 nm.

However, this is clearly not essential if only a rear (ie away from the source of radiation) supporting layer is used.

Alternativey the shaped article may comprise a photosensitive material dispersed in a suppor:inrj matrix, said matrix preferably being both transparent and insensitive, at least, to eryhemal ultraviolet radiation below about 320 nm.

Preferably the shaped article is insensitive to radiation of wavelength above about 320 nm., but, if this is not the case, a filter which is substantially opaque to radiation of wavelength above about 320 nm, may be superimposed on said article, in which case, when said article is a film the filter may also form the said at least one supporting layer.

Preferably the photosensitive material comprises a polysulphone, especially a polyaryl ether sulphone, which may be derived from the condensation between a dihydric phenol and a dihalosulphone. A preferred class of polysulphones have the formula:

Formula 1 where R, R' are the same or different, and represent alkyl, particularly lower alkyl, especially methyl groups, and n represents the degree of polymerisation which may vary widely, but is typically between about 50 and 80.

These polymers have good stability to thermal and oxidative degradation and to high energy radiation.

When the shaped article comprises a film of a polysulphone, the film, in order to give the required spectral response, should have a thickness of less than about 10 microns, and preferably less than about 1 micron. Although the sensitivity of such thin layers is less than for the 40 micron layers previously used it is still sufficient to allow doses as low as 100 m J cm-2 of erythemal ultraviolet radiation to be measured accurately.

Whilst such thin layers may sometimes be used as a shaped article without support, it is generally necessary to use a supporting layer. Where a rear supporting layer is used, it may be of any convenient rigid materal such as metals, ceramics or organic polymers such as polyethylene or polypropylene. However such supporting layers can give rise to variations of UV sensitivity due to boundary phenomena between the supporting layer and the shaped article. To minimise such effects the supporting layer should bond well to the shaped article, preferably comprising a thin film of polysulphone, be readily produced with an even surface (generally better than 0.1 micron variation) and have a refractive index similar to that of the shaped article, preferably comprising a thin film of polysulphone.Materials which meet these criteria, and are also substantially transparent to erythemal UV radiation, at least, below 320 nm include polyvinyl alcohol and regenerated cellulose sheet ("Cellophane' '-Trade Mark). Thus these materials are preferred as supporting layers. These materials are also preferred as supporting matrices, when the shaped article comprises a photosensitive material dispersed in a supporting matrix.The shaped article may be prepared by casting a photosensitive material for example, a polysulphone, onto the surface of a preformed supporting layer, from a solution in a suitable solvent, especially a chlorinated hydrocarbon such as chloroform, dichloromethane or dichloroethane, preferably at less than 25g/1 00m1, especially 1 5-20g/1 OOml. Alternatively the shaped article may be prepared by mixing a photosensitive material, such as polysulphone with the supporting matrix and the solvent to form a co-solution or suspension (if the matrix is insoluble in the solvent) and then casting the mixture onto an inert surface from which the shaped article is then removed.When the shaped article comprises a phtosensitive material, especially polysulphone, dispersed in a supporting matrix the required thickness of the shaped article will depend upon the concentration of the photosensitive material in the matrix.

Filtering or inert layers may be superimposed on the shaped article, however made, if required.

The present invention further provides a dosimeter for monitoring erythemal ultraviolet radiation of wavelength below about 320 nm. including a photosensitive element according to the present invention.

The dosimeter may be employed in any situation which requires a cheap and easy to use apparatus to measure erythemal ultraviolet exposure. Such situations would include, for example, the exposure of humans to erythemal ultraviolet radiation at work and during medical treatment.

Preparation of a photosensitive element according to the invention will now be described by way of example with reference to the accompanying drawings in which: Figure I shows in diagrammatic form the apparatus used in the preparation of a photosensitive element, and Figure 2 shows the wavelength sensitivity of a polysulphone film of thickness 0.88 + 0.04 microns.

CASTING THE FILM Referring to Fig. 1, a rigid rectangular wooden base 1 is provided near one end with a spring clip 2 which holds in place a foam pad 3 and plastics (polyethylene) sheet 4. A gantry 5 spans the width of the base and is free to move longitudinally above the base.

A support layer comprising a 250 by 400 mm film of regenerated cellulose ("Cellophane" Trade Mark) was placed on the plastics sheet 4 and secured by the clamp 2. Working in subdued light, the Cellophane surface was cleaned and smoothed using a pad soaked in solvent.

The spreading rod 6 with a diameter of 1.05 cm and a fine wire 7 rapeed around it in a helix to produce 133.3 turns/cm, was also cleaned with solvent and was then clamped into the gantry 5 by clamps 8, 9 to be across the cellulose surface at right angles to the direction of motion of the gantry. The total weight of the rod on the film was of the order of 500 grammes. The gantry and clamps were constructed so that the rod could not roll, but only slide on the polymer surface.

A small amount of dimethyl polysulphone (Formula 1; R = R' = CH3) in solution (see below) was poured onto the film along the length of the rod, and the gantry driven across the film at a rate of about 1 Ocm/sec by means of a variable-speed motor. At the end of the travel, the rod was removed and the film allowed to dry for a few minutes. The film was then placed in a darkened vacuum chamber at room temperature for 24 hours, to dry.

Measurement of film thickness Advantage was taken of the softening effect of water upon the cellulose to part the polysulphone from the cellulose when measuring the film thickness: a square of film of known dimensions was cut from the sheet, and soaked in a water bath. After about one minute the polysulphone film could be floated off, dried under vacuum and then weighed. The thickness could then be computed from the surface area, the weight and the density (1.24 grammes cm - 2).

Effect of the polysulphone solution First, a solution of 1 5g poiysulphone in 100ml of chloroform was used: the mixture was left to stir in the dark for several hours and was found to produce a film of just over 1 micron thickness. However, the surface of the film was not smooth, being composed of waves left by the scraping action of the casting rod. A coarser rod was tried, with a weaker solution of polysulphone, in the hope that the thicker, less viscous film left by the rod would evaporate down to a thin smooth film as the chloroform evaporated. However, the range of casting rods available was such that evaporation took a long time, and air currents over the surface, and imperfections in the flatness of the supporting surface led to variations in thickness of the film.

It was then decided to use a solvent with a slightly higher boiling point than chloroform (B.P.

61 'C). so that the longer drying time would allow the surface tension of the viscous fluid to smooth small-scale irregularities. Dichloroethane, which has a boiling point of 83"C, was found to be a satisfactory solvent for polysulphone, and so a solution was made up with the original concentration, 1 5 grammes of polysulphone in 100ml of dichloroethane, and the fine spreading rod was used. Thus was found to give a film thickness of 0.88 microns + 0.04, while a solution of 20 grammes polysulphone to 100ml dichloroethane gave a thickness of 1.00 + 0.06 microns: both films were smooth and adhered well to the substrate, Cellophane.

When a solution of 25 grammes of polysulphone to 100ml dichloroethane was tried, however, the polysulphone crystallised out and did not adhere. Best results were obtained with a film made with 1 8 grammes of polysulphone to 100ml dichloroethane ('Analar' or spectroscopic' grade).

Optical Properties of the film Before casting polysulphone onto regenerated cellulose sheet (Cellophane) it was ascertained that the Cellophane showed no optical density changes after irradiation with UV or visible radiation. The absorbance of Cellophane within the wavelengths range 295-315 nm was measured and the film exposed to broad band radiation from a medium pressure mercury lamp for an hour. This treatment produced an optical density change within the spectrum 295-315 nm of about .003. Under the same conditions, polysulphone-on-cellulose film was observed to undergo an optical density change of the order 0.7, as was a sample of 1 micron-thick polysulphone floated from its cellulose substrate.

Choice of readout wavelength The readout wavelength was chosen as that at which the optical density change was greatest for a given dose and at which the slope of the curve of optical density change against readout wavelength was flat, so that small variations between badges and fluctuation in readout wavelength should have a minimal effect. Thus the absorbance spectrum was recorded in the range 285-328 nm, before and after irradiation with 290 + 20 nm radiation. A plot of the change in optical density against wavelength showed that the optimum readout wavelength was 302 nm.

Action Spectrum (Plot of change in optical density for a constant dose at varying wavelength).

The action spectrum was found by irradiating polysulphone films of thickness 0.88 + 0.04 microns to a given dose at wavelengths from 265-320 nm, with a bandwidth of 2 nm. The dose given was 0.1 8 J cm - 2, though later investigations showed the action spectrum to be dose independent.

The UV induced changes in the optical density of a film at 302 nm may be expressed by the empirical relationship K(y)D= 10.5a+ 15awl + 125a3 wherein a = the change in the optical density of the film (measured at 302 nm) after irradation of the film with 0.18 J/cm2 of a given wavelength of UV radiation, K(y) = the wavelength response of the film normalised to unity at an irradiation wavelength at 302 nm for a UV dose of 0.1 8 J/cm2 at a given wavelength, D =0.18J/cm2 The values of K(y) determined by irradiating polysulphone films of thickness 0.88 + 0.04 microns to a dose of 0.18 J/cm2 at wavelengths from 265-320 nm are given graphically in Fig. 2.

The values for K(y) obtained are substantially identical in the 295-320 nm region to the ultraviolet exposure standard proposed by the National Institute for Occupational Safety and Health (NIOSH, 1 972, "Occupational Exposure to UV Radiation" HSM 73-11009, USDHEW, Washington D.C.). This standard is based on an envelope action spectrum which combines the action spectrum for eye photokeratitis and Caucasian skin erythema.

The action spectrum obtained (Fig. 2) also follows closely the erythemal action spectrum for monochromatic irradiation as quoted byNakayama et al ("Sunlight and Man": Fitzpatrick Ed; Univ Tokyo Press) Cripps et al; (Br J Derm 82; 584-592; 1970) and Mackenzie et Al (Br J Derm 89; 251-264; 1973).

Dose response To obtain the dose response, polysulphone film of thickness 0.88 + 0.04 micron was irradiated at a wavelength of 280 + 5 nm to doses from 0.01 Jcm-2 up to 10 Jcm-2. By moving the film further from the light source, it was possible to vary the dose rate over the range 0.5 mWcm-2 to 2.5 mWcm-2: over this range the film showed no dose rate dependence. No temperature dependence was found over the range - 4 to + 50"C and response was unaffected by irradiating in an oxygen free nitrogen atmosphere.

The sensitivity of the polysulphone film is good, though less than for 40 micron polysulphone, for a dose of 1 Jcm-2 the change in A302 is about 0.2 which is easily measurable (compare 0.3 for 1 Jcm-2 for 40 micron polysulphone).

Claims

1. A photosensitive element for use in a dosimeter for monitoring erythemal ultraviolet radiation of wavelength below 320 nm comprising a shaped article prepared from a photosensitive material and having a spectral response to erythemal ultraviolet radiation of wavelength below 320 nm thst is substantially the same as the spectral response of human skin to said radiation.

2. A photosensitive element according to Claim 1 wherein the shaped article comprises a thin film of a photosensitive material.

3. A photosensitive element according to Claim 1 or 2 wherein the shaped article is mounted on at least one supporting layer.

4. A photosensitive element according to Claim 3 wherein the supporting layer comprises a metal, a ceramic material or an organic polymer.

5. A photosensitive element according to Claim 4 wherein the organic polymer comprises polyethylene or polypropylene.

6. A photosensitive element according to any preceding Claim wherein the shape article is mounted on or between at least one suporting layer that is transparent to erythemal ultra violet radiation of wavelength br jw 320 nm.

7. A photosensitive elL.nent according to Claim 6 wherein the UV transparent supporting layer comprises polyvinyl alcohol or regenerated cellulose sheet.

8. A photosensitive element according to Claim 1 wherein the shaped article comprises a photosensitive material dispersed in a supporting matrix which is both transparent and insensitive to erythemal ultra-violet radiation of wavelength below 320 nm.

9. A photosensitive element according to Claim 8 wherein the supporting matrix comprises polyvinyl alcohol or regenerated cellulose sheet.

10. A photosensitive element according to any preceding Claim wherein the photosensitive material comprises a polysulphone.

11. A photosensitive element according to Claim 10 wherein the polysulphone is a polyaryl ether sulphone.

1 2. A photosensitive element according to Claim 11 wherein the polyaryl ether sulphone has the formula

wherein R an R' are the same or different and represent alkyl groups, and n is from 50 to 80.

1 3. A photosensitive element according to Claim 1 2 wherein R and R' are both methyl groups.

1 4. A photosensitive element according to any one of Claims 2 to 7 and 10 to 1 3 wherein the thickness of the thin film of photosensitive material is less than 1 0 microns.

1 5. A photosensitive element according to Claim 1 4 wherein the thickness of the thin film of photosensitive material is less than 1 micron.

1 6. A photosensitive element according to any preceding Claim wherein the shaped article is insensitive to radiation of wavelength above 320 nm.

1 7. A photosensitive element according to any preceding Claim wherein a filter, which is substantially opaque to radiation of wavelength above 320 nm, is superimposed on the shaped article.

1 8. A photosensitive element according to any one of Claims 3 to 7 and 8 to 1 6 wherein a filter, which is substantially opaque to radiation of wavelength above 320 nm forms the supporting layer for the shaped article.

1 9. A photosensitive element for use in a dosimeter for monitoring erythemal ultraviolet radiation of wavelength below 320 nm substantially as hereinbefore described with reference to the Example and Fig. 2.

20. A dosimeter for monitoring erythemal ultraviolet radiation of wavelength below 320 nm including a photosensitive element according to any preceding Claim.

21. A process for producing a ph.otosensitive element according to Claim 14 comprising dissolving a photosensitive material in a solvent, casting the solution onto a support and allowing the solvent to evaporate to leave a film of the photosensitive material of thickness less than 10 microns.

22. A process according to Claim 21 wherein the film of photosensitive material is of thickness less than 1 micron.

23. A process according to Claim 21 or 22 wherein the support comprises a supporting layer for the film.

24. A process for producing a photosensitive element according to Claim 8 comprising mixing a photosensitive material with a supporting matrix and a solvent to form a co-solution or suspension. casting the co-solution or suspension onto a support and allowing the solvent to evaporate to leave a photosensitive material dispersed in a supporting matrix.

25. A process according to any one of Claims 21 to 24 wherein the photosensitive material is dissolved or mixed in a chlorinated hydrocarbon.

26. A process according to Claim 25 wherein the chlorinated hydrocarbon comprises chloroform, dichloromethane or dichloroethane.

27. A process according to any one of Claims 21, 22, 23, 25 or 26 wherein the concentration of the photosensitive material in the solvent is less than 25g/ml.

28. A process according to Claim 27 wherein the concentration is from 1 5 to 20g/ml.

29. A process for producing a photosensitive element substantially as hereinbefore described with reference to the Example and Fig. 1.