Project description:

Within the project RADICALS (Radiation Dosimetery using in-vivo Calcium L-Band-Spectrometry) a portable device is being developed. This will make the measurement of radiation exposure dosage in a living human possible. Humans exposed to radiation experience the breaking of molecular bonds in oxyapetite in teeth and bones. The loose molecular chain ends result in free oxygen radicals which can be measured in the same way as for carbon dating in archaeology. Electron spin (paramagnetic) resonance can be used for these purposes. Hitherto extraction of a tooth was necessary in order to produce a vague reconstruction or radical determination.

Traditionally, people who are likely to be exposed to radiation during their work carry film dosimeters. These are analysed every 1-2 months providing an indication of radiation exposure during this time. Problems occur with inappropriate application [1] (worn at the wrong position or simply forgotten etc.). In addition, daily radiation exposure (medicinal X-Ray, sunlight etc.) is not of interest and only serves to complicate the situation. An alternative is to measure the exposure dosage through thermo-luminescence-detection [3]. Such analysis systems require very sensitive detectors and radiation estimates grow even more complex when ingestion is involved [4].

Exposure to ionising radiation results in the breaking of a very limited number of molecular bonds in carbonat ions in teeth and bones. The number of free CO₂ radicals produced by the loose molecular chain ends can be measured. Electron spin (paramagnetic) resonance (known as ESR or EPR) can be used for these purposes.

Materials subjected to intense magnetic fields (around 340mT) absorb microwave energy (traditionally in X-Band - around 10 GHz) depending on the amount of free radicals present. The generation of such magnetic field densities requires physically large electromagnets and a sensitive microwave measurement bridge. This has three distinct disadvantages: Firstly, the total weight of such a system is around 1 tonne and is enormously expensive. Secondly, measurements cannot be made in-vivo because the absorption of microwave radiation in water at these frequencies is simply too high. Thirdly, a subject tooth must first be extracted, the enamel removed and powdered before measurements may commence.

Although the resolution is somewhat reduced, an alternative is to use L-Band (1 to 2 GHz) ESR. This allows much smaller magnetic field densities (approximately 50 mT) to be used. A further advantage is the much lower absorption of microwave radiation in water at L-Band frequencies.

Nevertheless, a prerequisite for concentration measurement with ESR is a very high degree of magnetic field homogeneity at the measurement volume. Because wound inductors with high field homogeneity tend to be physically large, the Mechatronics Research Unit at the HS Regensburg have utilised specially manufactured NdFeB rare earth permanent magnets as shown in figures 1 & 2. This yields an overall homogeneity of around 0,3 mT at 40mT Center Field Density within a 1 cm³ volume which is adequate for a human tooth.

The second aspect is the microwave field. This must be coupled into the tooth enamel by means of an antennae or resonator as shown in figure 3. This is not a simple problem owing to the somewhat longer wavelength (at least 15cm) of L-Band compared to that of X-Band (3cm). This means that a more sophisticated design than a simple loop antennae is required.

Figure 3: In-viveo dosimetery schematic

The present prototype measuring head is capable of being placed in the mouth. The additional microwave and data analysis equipment presently being developed will make up the rest of a portable system weighing under 100 kg. Current measurement results have yielded measurements well under 5 Gy without additional data processing. Adaptive filtering and autocorrelation techniques are expected to improve the sensitivity significantly.

Partner Links:
Forschungszentrum für Umwelt und Gesundheit (GSF)
Institut de radioprotection et de sûreté nucléaire (IRSN)