GB2227633A

GB2227633A - Radiation dosemeter

Info

Publication number: GB2227633A
Application number: GB8901663A
Authority: GB
Inventors: John Hargraves; Timothy Peter Maish; Annette Frances Morris; Andrew Wright
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1989-01-26
Filing date: 1989-01-26
Publication date: 1990-08-01
Also published as: GB8901663D0

Abstract

A thermoluminescent phosphor radiation dosemeter comprises separate phosphors 9 and 10 for measuring the doses received by the eye lens and the skin respectively. These phosphors are located in recesses 4 and 5 on opposite sides of a plate 1 to which are hinged a rear cover 2 and a front cover 3. The skin phosphor 10 overlying the eye phosphor 9 thereby acts as part of the filtration for radiation reaching the eye phosphor as does the base 13 of the recesses. The cover 3 over the skin phosphor 10 includes a hole covered by a thin opaque sheet 12 which provides filtration for radiation reaching the skin phosphor. The plate, covers and integral hinges are formed as a one-piece injection moulding of carbon-filled polypropylene. <IMAGE>

Description

Radiation dosemeter This invention relates to personal radiation dosemeters and has particular application to measuring the dose of nuclear radiation received by the eyes when, eg working with radioactive materials.

The Ionising Radiation Regulations 1985 (IRR85) state that if any dose is greater than one-tenth of the relevant dose limit, systematic assessment of dose is necessary. This means that if the eyes receive greater than 15mSv per annum, dose assessment is required. Dose assessment to the eyes can take the form of systematic assessment from a body dosemeter or measurement of dose by a dosemeter.

The International Commission of Radiological Protection (ICRP) recommends that, in situations where the body extremities receive a larger dose than its trunk, a second dosemeter should be worn if the dose thereto is likely to exceed three-tenths of the relevant limit recommended by the ICRP. In the case of the eyes this means that an eye dosemeter should be worn, if the dose to the eye lens is likely to exceed 15mSv per annum. For this purpose they do not specify a specialised eye dosemeter, but suggest the use of a conventional dosemeter from which one can estimate both the effective dose equivalent at the surface of the body and the effective dose equivalent at a depth of lOmm. With this information, the eye dose can be adequately controlled.

The use of such conventional dosemeters is not ideal because they do not contain the correct filtration for measurement of dose to the lens. To obtain a more representative measurement, a specialised eye dosemeter worn as near to the eye as possible, eg at the front of a cap or headband, or attached to protective spectacles, can be used. The eye lens is located at a depth of between 2.5 and 3.5mm, and specialised eye dosemeters contain a detector at a similar depth to the lens, corresponding to 250-350mgcm 2 of a tissue equivalent material, in order to simulate its location.

Lithium fluoride (LiF) is a near tissue equivalent thermoluminescent (TL) phosphor which can be used for skin, body and eye dose measurement by being located below the appropriate depth of a tissue equivalent filter, and such thermoluminescent dosemeters (TLD's) are known. One such commercially available dosemeter comprises a 0.2mm thick LiF/PTFE disc below a 7mgcm filter for skin dose measurement, designated Hs(0.07) by the ICRP, and a 0.4mm thick LiF/PTFE disc below a 300mgcm 2 for lens dose measurement, designated Hp(3) by the ICRP, mounted side-byside on a binary coded card for identification, which is secured to a cap etc. by a safety pin.However, this design has certain disadvantages in use, eg size and compatibility with some existing readers, and the present invention provides an alternative form.

According to the present invention, in a thermoluminescent phosphor radiation dosemeter comprising separate phosphors for simultaneously measuring eye and skin doses, the skin phosphor overlies the eye phosphor and thereby acts as part of the filtration for radiation reaching the eye phosphor.

Said phosphors may be removably located in recesses on opposite sides of a plate, the plate having an opaque openable cover on each side of the plate for retaining the phosphors in place and the base of the recesses acting as a further part of the filtration for radiation reaching the eye phosphor. The cover over the skin phosphor may include a hole covered by a thin opaque sheet which provides filtration for radiation reaching the skin phosphor.

The covers may be hinged to opposite edges of the plate and the plate, hinges and covers may be formed as an integral assembly from an opaque plastics material.

The present invention will now be described, by way of example, with reference to the accompanying drawings wherein: Fig 1 is a perspective view, partially exploded, of an embodiment of the invention, shown with its hinged covers opened.

Fig 2 is a diagram illustrating the filtration system in the embodiment of Fig 1.

Referring to Fig 1, the dosemeter comprises a centre plate 1, a rear cover 2 and a front cover 3. The centre plate 1 includes a circular recess 4 facing the rear cover, and a similar recess 5 (hidden in Fig 1) facing the front cover. The centre plate 1 has an outwardly extending rib 6 midway between its front and rear faces to which the front and rear covers are hinged by integral plastic hinges 7. The aforesaid assembly is a one-piece injection moulding of carbon-filled polypropylene.

The covers 2 and 3 are a secure fit on the plate 1 when the assembly is closed, the outer surfaces of the covers then being flush with the outer edge of the rib 6. A recess 8 is formed in the edge of each cover (hidden in cover 3) to enable the covers to be easily prised apart.

Loosely located in the recess 4 is a LiF/PTFE rear disc 9 of 0.4mm thickness, and in the recess 5 a similar front disc 10 of 0.2mm thickness. Disc 9 measures the eye lens dose and disc 10 the skin dose. They are held in the recesses when the assembly is closed up and removed for insertion in a conventional TLD reader.

The required filtration is provided by the thickness of the polypropylene and by the discs themselves. Fig 2 shows the mass per unit area of the rear cover 2, the rear disc 9, the common base 13 of recesses 4 and 5, and the front disc 10. To obtain the required filtration, the front cover 3 would have required a moulded-in window of carbon-filled polypropylene less than 0.lmm thick. It was found more convenient to form a round, outwardly bevelled hole 11 in the cover 3 over which is stuck a strong paper label 12 of mass thickness 9mgcm . This label is also used to carry details of the wearer's identity.

As will be seen from Fig 2, the disc 10, as well as measuring skin dose also acts as part of the filter overlying disc 9, so that he total filtration for disc 9 is 2 258m -2 (9 + 48 + 200)mgcm 2 e 258mgcm 2, ie a filtration thickness suitable for measuring the dose received by the lens. The rear cover 2 provides filtration for back-scattered radiation in a known manner. The discs 9 and 10 are obtainable from Vinten Instruments.

Fig 1 is approximately to scale, the assembly in the closed condition measuring approx 18mm square and 7mm thick, and the recesses approx 14mm diameter and lmm deep. The hole 11 is approx 7mm in diameter. The covers 2 and 3 are each 2mm thick and the base 13 is 2mm thick. The total weight is 3gm.

The dosemeter is attached to the wearer by inserting it in a small nylon bag (3mgcm ) (not shown) which is easily attached to eg a hat by a safety pin. The bag also protects the dosemeter from contamination. It is important that the dosemeter is worn with the skin dose disc 10 facing outwards. This is ensured by printing FRONT on the label 12, and stamping BACK on one side of the nylon bag.

Claims

1. A thermoluminescent phosphor radiation dosemeter comprising separate phosphors for simultaneously measuring eye and skin doses, wherein the skin phosphor overlies the eye phosphor and thereby acts as part of the filtration for radiation reaching the eye phosphor.

2. A dosemeter as claimed in Claim 1 wherein said phosphors are removably located in recesses on opposite sides of a plate, the plate having an opaque openable cover on each side of the plate for retaining the phosphors in place and the base of the recesses acting as a further part of the filtration for radiation reaching the eye phosphor.

3. A dosemeter as claimed in claim 1 wherein the cover over the skin phosphor includes a hole covered by a thin opaque sheet which provides filtration for radiation reaching the skin phosphor.

4. A dosemeter as claimed in claim 2 or claim 3 wherein the covers are hinged to opposite edges of the plate.

5. A dosemeter as claimed in claim 4 wherein the plate, hinges and covers are formed as an integral assembly from an opaque plastics material.

6. A thermoluminescent phosphor radiation dosemeter substantially as hereinbefore described with reference to the accompanying drawings.