MCP-N (LiF:Mg,Cu,P) TLDs for radon measurements with charcoal canisters

A method of measurement of radon concentration in air was developed, based on high-sensitivity LiF:Mg,Cu,P (MCP-N, TLD Poland) thermoluminescent detectors installed in charcoal canisters. The canisters were exposed typically for 72 h in a calibration chamber with a radon concentration ranging from 100 Bq x m(-3) to 87 kBq x m(-3). It was found that in these conditions the signal registered by the TL detectors was proportional to the 222Rn concentration and the lowest limit of detection (LLD) was at a level of 100 Bq x m(-3). The proposed method can be used in large-scale, multi-site surveys aimed at screening for high levels of indoor radon concentration or for measuring ground radon exhalation rates.

Dosimetry of low-energy protons and light ions

For the vertical beam facility at the 14 MV Munich tandem accelerator, various techniques for dosimetry were tested for radiation fields of low-energy protons and light ions (4He, 12C and 16O). A reference dose was determined from the fluence of particles by counting individual particles. A parallel-plate Markus chamber with a small sensitive air volume was used for beam dosimetry applying the ICRU protocol. The doses measured with the ionization chamber were compared with doses evaluated from the fluence measurements. Alternative dose measurements were performed using MTS-N LiF:Mg, Ti thermoluminescence detectors (TLDs) and a photometrically evaluated Fricke chemical dosimeter. An uncertainty of 8% was found in the determination of the dose relative to the reference method. Effects of an inhomogeneous energy loss and a finite track length of the projectiles in the sensitive detector volume of the dosimeters had to be taken into account.

Calculation of relative biological effectiveness for proton beams using biological weighting functions

PURPOSE

The microdosimetric weighting function approach is used widely for beam comparison studies. The suitability of this model to predict the relative biological effectiveness (RBE) of therapeutic proton beams was studied. The RBE(alpha) (i.e., linear approximation) dependence on the type of biological end point, initial proton energy, energy spread of the input proton beam, and depth of beam penetration was investigated.

METHODS AND MATERIALS

Proton transport calculations for a proton energy range from 70 to 250 MeV were performed to obtain proton energy spectra at a given depth. The corresponding microdosimetric distributions of lineal energy were calculated. To these distributions the biological response function approach was applied to calculate RBE(alpha) the biological effectiveness based on a linear dose-response relationship. The early intestinal tolerance assessed by crypt regeneration in mice and the inactivation of V79 cells were taken as biological end points.

RESULTS

The RBE(alpha) values approach about 1 in the plateau region and gradually increase with the proton penetration depth. In the center of the Bragg peak, at the maximum dose delivery, the values of RBE(alpha) range from 1.1 (250-MeV beam, early intestinal tolerance in mice) to 1.9 (70-MeV beam, Chinese hamster V79 cells in G1/S phase). Distal to the Bragg peak, where only a small fraction of dose is delivered, the RBE(alpha) was found to be even higher. For modulated proton beams we found an increasing RBE(alpha) with depth in the spread-out Bragg peak (SOBP). Values up to 1.37 at the distal end of the SOBP plateau (155-MeV beam, SOBP between 5.3 and 13.2 cm) were obtained.

CONCLUSION

More experimental work on the determination of microdosimetric weighting functions is needed. The results of the presented calculations indicate that for therapy planning it may be necessary to account for a depth dependence on proton RBE, especially for lower energy.

Microdosimetry of tritium

Microdosimetric aspects of tritium radiotoxicity are discussed. The level of coincidence of tritium locations and of radiation-sensitive sites does not appear to be decisive for relative effectiveness of tritium exposures. The actual target structure and dimensions seem to be of more importance. Particularly efficient germ cell destruction by chronic exposures to tritiated water is unlikely to be explainable on the basis of classical principles of microdosimetry. The phenomenological approach of investigating biological response functions may be a useful alternative for the interpretation of unexpected experimental results, especially when combined with bidimensional microdosimetry. The latter allows for clustering of nanometer sites within a target of cellular dimensions.