DOSIMETRIC QUALITY ASSURANCE INTERPRETED FOR ISO 17025 IN PUBLIC HEALTH ENGLAND'S PERSONAL DOSIMETRY SERVICE.

Many individual monitoring services (IMSs) have long experience in delivering high-quality dosimetry, and many follow rigorous quality assurance (QA) procedures. Typically, these procedures have been developed through experience and are highly effective in maintaining high-quality dose measurements. However, it is not always clear how the range of QA procedures normally followed by IMSs maps on to the various requirements of ISO 17025. The Personal Dosimetry Service of Public Health England has interpreted its QA procedures both in operating existing services and in developing a new one.

USE OF A SIMPLE THERMALISED NEUTRON FIELD FOR QUALITY ACCEPTANCE OF WHOLE BODY TLDS.

The individual monitoring service of Public Health England (PHE) uses Harshaw™ whole-body and extremity thermoluminescent dosemeters (TLDs) with high-sensitivity lithium fluoride LiF:Mg,Cu,P, together with Harshaw 8800™ automated readers. The neutron-insensitive, (6)Li-depleted variety of TLD material is used by PHE because the service provides separate neutron and photon dosemeters. The neutron dosemeters are not sensitive to photons and vice versa Since insensitivity to neutrons is a supply requirement for TLDs, there is a need to test every new (annual) consignment for this. Because it is thermal neutrons that produce a response in (6)Li TLDs, a thermal field is needed. To this end, PHE has adopted the simple approach of sandwiching the TLDs between two ISO water-filled slab phantoms. In this arrangement, the fast neutrons from an Am-Be source are effectively thermalised. Details of the method are given, together with the results of supporting MCNP calculations and some typical results.

Monte Carlo modelling of 90Sr/90Y and 85Kr beta fields for Hp(3) measurements.

In support of research aimed at developing a thermoluminescence dosemeter capable of accurately measuring ionising radiation doses to the lens of the eye, Monte Carlo modelling of a standard beta exposure set-up has been performed. It was found that electrons with an energy distribution corresponding to the beta emission spectrum from (85)Kr deposit negligible doses at a depth of 3 mm in tissue, but doses from (90)Sr/(90)Y are significant; free in air and fluence-to-Hp(3,θ°) and -Hp(0.07,θ°) conversion coefficient data were found for this field for exposures at 0°, 30° and 60° angles of incidence, and the response characteristics of the new eye dosemeter were evaluated. It was shown that the results were not affected greatly by the shape of the calibration phantom. However, it was demonstrated that the presence of intermediating air and beam flattening filters hardens the energy distribution of the field at the point of test, relative to a raw (90)Sr/(90)Y source, and this impacts dose depositions.

Type testing of a head band dosemeter for measuring eye lens dose in terms of H(P)(3).

A new head band dosemeter, for the measurement of eye lens dose in terms of Hp(3), has been type tested by Public Health England's Centre for Radiation, Environmental and Chemical Hazards [formerly part of the UK Health Protection Agency (HPA)]. The type tests were based on the International Standard ISO 12794, drawing also upon earlier work at HPA. The results show that, unlike many existing dosemeters, the new head band dosemeter correctly measures Hp(3) for beta radiations as well as photons.

Long-term stability of Harshaw LiF:Mg,Cu,P TLDS.

The sensitivity of Harshaw™ magnesium/copper/phosphorus-doped lithium fluoride is shown to be stable over long time periods, even when, as is often the case in practical operations, the TLDs are read out only three times a year.

Evaluating the performance of the Harshaw™ DXTRAD LiF:Mg,Cu,P finger ring with two different thickness filters.

Amongst the dosemeters offered by the Health Protection Agency (HPA) Personal Dosimetry Service are finger rings, which use the Harshaw™ DXTRAD element. This paper describes restricted-type testing carried out by HPA on the previously untried combination of the LiF:Mg,Cu,P material and a thicker filter (42 mg cm(-2)). The tests were based on ISO 12794 [International Organization for Standardization. ISO 12794:2000 nuclear energy--radiation protection--individual thermoluminescence dosemeters for the extremities and eyes. ISO (2000)] and included energy and angle dependence of response for photons. The conclusion was, for photon dosimetry applications the thicker filter is acceptable for use in the HPA service.

Practical low dose limits for passive personal dosemeters and the implications for uncertainties close to the limit of detection.

Recent years have seen the increasing use of passive dosemeters that have high sensitivities and, in laboratory conditions, detection limits of <10 µSv. However, in real operational use the detection limits will be markedly higher, because a large fraction of the accrued dose will be due to natural background, and this must be subtracted in order to obtain the desired occupational dose. No matter how well known the natural background is, the measurement uncertainty on doses of a few tens of microsieverts will be large. Individual monitoring services need to recognise this and manage the expectations of their clients by providing sufficient information.

Type testing of a new TLD for the UK Health Protection Agency.

The UK Health Protection Agency is currently commissioning a new personal dosimetry system based on the use of Harshaw two-element thermoluminescent dosemeter cards using LiF:Mg,Cu,P. Results of extensive type testing carried out with reference to IEC 61066, "Thermoluminescence Dosimetry Systems for Personal and Environmental Monitoring", have been presented.

Comparison of time effects, decision limit and residual signal in Harshaw LiF:Mg,Ti and LiF:Mg,Cu,P.

The personal dosimetry service of the UK Health Protection Agency--formerly of the National Radiological Protection Board (NRPB)--is currently commissioning a body thermoluminescence dosemeter (TLD) system based on the use of Harshaw 8800 readers and two-element cards. As part of the process, studies have been carried out into the long-term time dependence of response, the limit of detection and the magnitude of the signal remaining after recommended processing. TLD cards containing both conventional lithium fluoride (LiF:Mg,Ti) and the high-sensitivity material LiF:Mg,Cu,P were available, thus allowing a comparison between the two types of material.

Comparison of the effects of exposure to light in Harshaw LiF:Mg,Ti and LiF:Mg,Cu,P.

The response of thermoluminescence dosemeters (TLDs) to light, in various conditions, has been studied. TLD cards containing both conventional lithium fluoride (LiF:Mg,Ti) and the high-sensitivity material LiF:Mg,Cu,P were available, so permitting a comparison between the two types. Also available for the tests were Harshaw extremity EXT-RAD (LiF:Mg,Cu,P) dosemeters. The LiF:Mg,Ti body TLD cards and the EXT-RAD extremity dosemeters both showed some response to fluorescent light, while the LiF:Mg,Cu,P cards showed no significant response. It is therefore concluded that LiF:Mg,Cu,P body cards need no special precautions to protect them from the effects of light. For LiF:Mg,Ti cards and extremity dosemeters, effects are small, but steps to avoid excessive light exposure should be considered.

Experience of radiation passbooks in the UK NRPB dose record keeping service.

The National Radiological Protection Board (NRPB)--now part of the new UK Health Protection Agency (HPA)--has operated an automated Dose Record Keeping (DRK) Service for over 30 y, and currently maintains records for some 10,000 workers. A proportion of these are designated as Outside Workers, and are issued with Radiation Passbooks, under the Ionising Radiations Regulations 1999 (IRR99). The present study reports the operational experience of NRPB's DRK Service in issuing passbooks, providing advice to users and issuing tailored reports. Indication is given on some problems commonly encountered by employers and dosimetry services.

Type testing of an extremity finger stall dosemeter based on Harshaw TLD EXTRAD technology.

A new type of extremity dosemeter, which incorporates the Harshaw TLD EXTRAD dosemeter element into a PVC finger stall, has been developed. The dosemeter uses high-sensitivity lithium fluoride, (7)LiF:Mg,Cu,P (TLD-700H) in a thin 7 mg cm(-2) layer, with alternative coverings of PVC at 10 mg cm(-2) and aluminised polyester at 3.2 mg cm(-2). Results are presented of the type testing of both versions of the finger stall dosemeter against published standards.

Results for NRPB dosimetry services in the 2000 EURADOS trial performance test. National Radiolgical Protection Board. European Radiation Dosimetry Group.

During 2000 a trial performance test for individual monitoring services in Europe was organised by the European Radiation Dosimetry Group (EURADOS), covering whole-body beta/photon, whole-body neutron and extremity beta/photon dosimetry for both monoenergetic and simulated workplace fields. The UK National Radiological Protection Board (NRPB), which supplies routine dosemeters to some 50,000 wearers in the UK and overseas, participated in this trial performance test. This paper presents the results obtained for the NRPB whole-body TLD, neutron (PADC) and extremity dosimetry services and comments on their performance in comparison with the overall results.

Extremity TLD high dose response.

The Extremity TLD service provided by the UK National Radiological Protection Board (NRPB) supplies dosemeters to 3200 wearers in various occupations, primarily in the industrial, research and medical sectors. The dosemeters and associated processing equipment were supplied by Bicron/NE Technology Ltd and the service in this form has been operating successfully for over 15 years. The dosemeter uses LiF powder, which is deposited onto the adhesive face of a carbon-loaded Kapton tape. This paper is a periodical re-evaluation of the system's high dose response. Whereas there are no UK requirements for extremity dosimetry services to hold approval for assessment of absorbed doses >0.5 Gy, it is useful to characterise how the dosemeter behaves at such levels.

The NRPB PADC neutron personal dosimetry service.

The UK National Radiological Protection Board has been operating a successful contract neutron personal dosimetry service based on poly-allyl diglycol carbonate (PADC, also known by the trade name CR-39) since 1986, covering about 1500 workers and serving major nuclear sites in the UK and abroad. In that time approximately 100,000 dosemeters have been issued. Since the service was launched, a number of aspects have undergone evolution and it is therefore worthwhile to give an updated summary of how the service operates and performs today. The description covers the choice of plastic and of etching technique, the design of the dosemeter, the features of the automated image analysis, and the characteristics of the control software. Also described are the approaches to calibration and traceability and to compensation for energy and angle dependence, with particular mention of the ability of the service to estimate the quantity Hp(10) in accordance with the requirements of the European Council Directive 96/29/Euratom.