Chemical Dosimetry
Chemical dosimetry involves determination of dose by the measurement of chemical changes induced in materials by ionising radiation.
As part of the latest NMS programme the Chemical Dosimetry group at NPL is undertaking development and research that will enhance NPL's reference dosimetry measurement capability in the industrial and radiotherapy areas.
A large proportion of worldwide industrial dosimetry, particularly for sterilisation applications, relies on calibrations and chemical dosimetry services provided by NPL. As part of the dissemination of standards to the user community NPL also provides consultancy and advice to other laboratories and major industrial irradiation companies throughout the world.
At NPL, world-class facilities are available to provide supporting infrastructure to the NMS research programme. Within the chemical dosimetry group, three cobalt-60 irradiators, high quality uv-visible spectrophotometers, a class 100,000 chemical clean room and two EPR spectrometers are regularly available. This equipment is used to maintain both standard reference services and support NMS research and development activities.
Dosimeters based on the amino acid alanine have a wide dose range that enables potential usage in both the radiotherapy and industrial radiation areas. NPL currently provides dose measurement ranging from at the lowest ~ 5 Gy (dose determinations that are of particular use for the radiotherapy area) to, at the other extreme, a maximum dose of 70 kGy. It is the intention to extend this range even higher as many industrial sterilisation plants now require precise dose determination up to doses of 100 kGy. The objective is to extend the range of the current alanine reference dosimeter with a minimal impact on the overall quoted uncertainty by investigating parameter settings and measurement set-up.
In the industrial area, doses in the range from 100 kGy up to 600 kGy, may be used for various industrial techniques including, polymer degradation, polymer cross-linking and vulcanisation. Although the required accuracy of dose determination is less at these high doses, there is currently a lack of suitable systems for these applications. This is because at these dose levels most of the existing techniques either fail or have very poor uncertainties. Research work at NPL is ongoing for the investigation of innovative dosimetric materials and techniques that could be used in this dose range. The viability of producing a reference or routine dosimeter will be studied, with the aim of providing new NPL dosimetry services.
The alanine dosimeter once irradiated is relatively stable over timescales of months. However recent studies have indicated that during the first 48 hours after irradiation, small, but significant changes in the response of the alanine dosimeter take place. The increased use of alanine as an 'in-plan' dosimeter often requires measurement within hours, or even minutes, after irradiation. NPL will conduct research in this area, to better quantify and characterise the alanine response over the first 48 hours post irradiation.
The biotechnology industry increasingly has the need to irradiate cooled or frozen samples. Facility development over the next three years includes the commissioning of a cryogenic irradiator sample compartment to enable irradiations between room temperature and 77K. With this new development, more detailed studies of dosimetry systems at low temperatures will be performed, in particular the response of alanine. This will improve both direct irradiation facilities and reference dosimetry to the sterilisation / biotechnology community.
NPL already has expertise in flat ophthalmic applicators and has developed a world first calibration service for curved ophthalmic applicators. The ophthalmic applicator calibration service makes use of specially prepared thin ~ 0.5 mm alanine pellets - originally developed at NPL for use in electron beams - to provide absorbed dose measurements traceable to the primary standard micro-calorimeter.
A project is ongoing to develop Fricke gels with a view to their use for measuring dose distributions, for example, near HDR brachytherapy sources. Improvements will be made to the gel manufacturing process with the aim of improving reproducibility and reducing diffusion.
Alanine dosimetry is also involved in the small field dosimetry project for new and emerging radiotherapy techniques including, Intensity Modulated Radiotherapy (IMRT) and the establishment of a primary standard and traceability chain for proton radiotherapy.
NPL now has a web server specifically dedicated to industrial and chemical dosimetry
For further information on NPL's Chemical Dosimetry standards and research activities, please contact Dr Peter Sharpe