3D relative dose measurement with a μm thin dosimetric layer
Introduction
Radiation processing applications are widely used around the world and address different domains such as sterilization, polymer modification or food decontamination. Process control, either for IQ/OQ/PQ (Installation, Operational and Performance Qualification) or in routine monitoring, uses dosimeters and dosimetry systems (ISO/ASTM, 2020) to demonstrate that the process has been conducted in a controlled manner. Dosimeters are adapted to the radiation modality (gamma, electron beam (e-beam) and X-ray), the target dose range to be covered and the product being irradiated when it comes to PQ especially.
All dosimetric materials currently in use are influenced by either light, irradiation temperature, humidity or atmosphere, radiation type, energy, dose rate and time after irradiation which require – if possible - corrections to their response and often provide specific challenges (ISO/ASTM, 2013b). Moreover, dosimeter shape, whether in the form of film, stripe, sticker or pellet, may cause additional issues, such as electronic equilibrium, dose gradient inside the dosimeter or radiation field disturbance. A major drawback common to all dosimetry systems presently in use is the 1D or 2D (ISO/ASTM, 2013a) form of the dosimeters that are carrying the radiation-sensitive material. This drastically limits the required evaluation of absorbed dose for complex 3D surfaces (e.g. bottles, hip joints, syringes, catheters …). The single currently available technique for such application is to employ multiple dosimeter strips of appropriate lengths, which get attached to the object’s surface as best as possible and are placed individually in the regions of interest. They are required to be collected after exposure and read out externally (Helt-Hansen and Miller, 2004). However, the positioning often does not closely match the objects surface/3D geometry due to their insufficient flexibility, size or the complexity of the object surface. Furthermore, the thickness of the dosimetric material may be larger than the object and in any case it does not provide a very good measure of the surface dose because of its own thickness. Especially for low energy irradiation, the dosimeter thickness requires extrapolation to surface dose.
In order to obtain the desired estimate, which is the surface dose of the object, the dosimetric material would have to follow the object’s surface closely in order to allow dose evaluation for every position of the irradiated three-dimensional object. This requires a dosimetric material of rather high viscosity, like lacquer or paint, but no dosimetric material has ever been incorporated into ink or spray, which would allow easy coating of objects, while potentially being of a thickness not requiring corrections.
This paper is not a full characterization of the response of a novel dosimeter but rather demonstrates the proof of concept of a new approach to measure surface doses of complex three-dimensional objects, based on the radiation properties of a dosimetric active phosphor (NaYF4:Yb3+,Er3+), which can be incorporated into inks and sprays. By employing μm-thin radiation sensitive material and a well-adapted spray coating procedure, the final dosimetric layer thickness obtained is smaller than that of existing dosimeters and, furthermore, fully adheres to the object’s surface to be dose mapped. Thus, a potential way of measuring the Dμ (dose in the first micron of the irradiated object) is available instead of having to extrapolate measured data down to μm level (Helt-Hansen et al., 2010). This applies especially for low energy e-beam treatments, but is also of interest for all irradiation modalities and related energies, as outlined e.g. in ISO 11137–3:2017 (ISO, 2017).
Section snippets
Requirements for 3D dose determination of complex geometries
There, the potential advantages of a spray coating dosimetric material with related readout equipment for application in industrial processes become apparent. With a sprayed-on dosimeter the entire surface of an irradiated object can be dose-mapped, including surfaces and complexly shaped parts where current dosimeters cannot be attached. This applies especially to locations such as holes and irregular surfaces of e.g. implants or plastic bottles - geometries which could generate radiation
Materials and methods
The dose concept employed here is based on the phenomenon of changes in luminescence decay times due to ionizing radiation in upconversion materials (Ban and Hersh, 1972; Hersh and Ban, 1972). The luminescent phosphor used in this study is the rare earth fluorate NaYF4:Yb3+, Er3+ (Lumilux SP Green, Honeywell). For this material, dose dependency of luminescence decay time has been shown for several luminescence emissions under near-infrared excitation (Härtling et al., 2012; Reitzig et al., 2013
Properties of the new dosimetric material and measurement approach on flat substrates
The dosimetric properties of monolayers of NaYF4:Yb3+, Er3+-particles printed onto flat paper substrates were evaluated. Based on the evident similarity of printed (Fig. S1, A + B) and spray-coated (Fig. S1, C + D) phosphor particle monolayers, it appears reasonable to assume that the following properties apply to sprayed monolayer coatings (identical particle size distribution) as well.
Qualification studies were conducted as follows: Before and after ionizing irradiation, the time-dependent
3D reader and irradiated objects
The dosimetric approach described above provides the opportunity to determine dose distributions for complex geometries. They can be considered as surface doses because of the minute thickness (particle diameter d50 = 1.5 μm) of the single layer of dosimetric material. Providing 3D data requires measurements with a scanning device at the requested resolution along at least two axes of an object. Such a dosimetric approach of scanning large areas of an irradiated object is unprecedented, with
Discussion
The measurement system used here was not optimized for monolayers of the phosphor, which leaves room for improvement of signal to noise ratio, as well as repeatability and reproducibility for the dim signals observed from the thin layer. However, the interest in such a new approach here is primarily in establishing dosimetric patterns at this stage, and it can be accepted that the technical potentials are not exhausted in this initial 3D-setup.
The relative dose map for the single-sided
Conclusions
The current study provides the proof of concept and examples of a novel approach to measure surface dose data of complex sample geometries. The concept is intensity-independent and relies on the dependency of the luminescence decay time to ionizing radiation of the inorganic phosphor NaYF4:Yb3+, Er3+. Employing micro particles (d50 = 1.5 μm) of the phosphor material the dosimeter can be spray-coated directly on irregular object surfaces. Considering the special importance of the thickness of a
CRediT authorship contribution statement
Christiane Schuster: Formal analysis, Investigation, Visualization, Funding acquisition, Writing - original draft. Florent Kuntz: Conceptualization, Validation, Formal analysis, Investigation, Funding acquisition, Writing - original draft. Alain Strasser: Supervision, Writing - review & editing. Thomas Härtling: Funding acquisition, Supervision, Writing - review & editing. Kay Dornich: Supervision, Writing - review & editing. Daniel Richter: Investigation, Validation, Project administration,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The results were acquired within the „READ“ project (01QE1625B) in the framework of the European research program Eurostars, funded by the German Federal Ministry of Education and Research (BMBF) and the French Banque Publique d’Investissement. The authors are indebted to the following persons for their assistance: Eric Moszczynski and Sébastien Riegler (both Aerial) for irradiations and dosimeter read-out during qualification; Anne-Katrin Stange and Lenz Fiedler (both Freiberg Instruments) for
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Depth dose curve and surface dose measurement with a μm thin dosimetric layer
2022, Radiation Physics and ChemistryCitation Excerpt :The average film thickness is 1.6 μm (Fig. S1, B). Before and after ionizing irradiation of the phosphor films, its time-resolved luminescence is recorded as described in Schuster et al. (2020). In short, the phosphor is excited by 1 ms pulses from a 980 nm laser diode and the luminescence is recorded by means of a silicon photodiode.