arXiv subjects

Gerhard Hilgers

Publications and source records attributed to Gerhard Hilgers.

5 recordsLinked to original sources

arXiv2025

Active Eye Lens Dosimetry With Dosepix: Influence of Measurement Position and Lead Glass Shielding

In this work, the effect of the measurement position on measurements of $H_\text{p}(3)$ of a new active eye lens dosemeter prototype based on the Dosepix detector is examined. A comparison between measuring directly in front of the eye and measuring at the side of the head of an Alderson phantom showed no significant influence on the resulting $H_\text{p}(3)$ for different radiation qualities and angles. In addition, to account for the absorption effect of radiation safety glasses, pieces of lead glass were attached to the front and side of the dosemeter. Corresponding effects and consequences for radiation protection measurements have been investigated by using a human like Alderson head phantom as well as thermoluminescent dosemeters (TLDs) and the active eye lens dosemeter prototype. For specific angles, the radiation bypassed the radiation safety glasses and lead glass pieces, leading to an increase in the measured $H_\text{p}(3)$. Compared to TLDs behind radiation safety glasses, measurements with the lead glass shielded prototype resulted in lower values for $H_\text{p}(3)$. Furthermore, the results did not reproduce previous findings where larger dose values were found for Dosepix behind lead glass pieces than for the TLDs behind radiation safety glasses. A possible reason might be that the dimensions of the lead glass pieces are not representative of the radiation safety glasses in front of the eye but, ultimately, it is not yet clear what the main reason for the deviation is. Therefore, it is advisable to test the same methodology in future investigations with other eyewear models and lead glass pieces to investigate whether similar behaviors occur.

physics.med-ph↗

arXiv2024

Nanodosimetric investigation of the track structure of therapeutic carbon ion radiation. Part 2: Detailed radiation transport and track structure simulation

Previously reported nanodosimetric measurements of therapeutic-energy carbon ions penetrating simulated tissue have produced results that are incompatible with the predicted mean energy of the carbon ions in the nanodosimeter and previous experiments with lower energy monoenergetic beams. The purpose of this study is to explore the origin of these discrepancies. Detailed simulations using the Geant4 toolkit were performed to investigate the radiation field in the nanodosimeter and provide input data for track structure simulations, which were performed with a developed version of the PTra code. The Geant4 simulations show that with the narrow-beam geometry employed in the experiment, only a small fraction of the carbon ions traverse the nanodosimeter and their mean energy is between 12 % and 30 % lower than the targeted values. Only about one-third or less of these carbon ions hit the trigger detector. The track structure simulations indicate that the observed enhanced ionization cluster sizes are mainly due to coincidences with events in which carbon ions miss the trigger detector. In addition, the discrepancies observed for high absorber thicknesses of carbon ions traversing the target volume could be explained by assuming an increase in thickness or interaction cross-sections in the order of 1 %. The results show that even with strong collimation of the radiation field, future nanodosimetric measurements of clinical carbon ion beams will require large trigger detectors to register all events with carbon ions traversing the nanodosimeter. Energy loss calculations of the primary beam in the absorbers are insufficient and should be replaced by detailed simulations when planning such experiments. Uncertainties of the interaction cross-sections in simulation codes may shift the Bragg peak position.

physics.med-ph↗

arXiv2024

Nanodosimetric investigation of the track structure of therapeutic carbon ion radiation. Part 1: Measurement of ionization cluster size distributions

At the Heidelberg Ion-Beam Therapy Center, the track structure of carbon ions of therapeutic energy after penetrating layers of simulated tissue was investigated for the first time. Measurements were conducted with carbon ion beams of different energies and polymethyl methacrylate (PMMA) absorbers of different thicknesses to realize different depths in the phantom along the pristine Bragg peak. Ionization cluster size (ICS) distributions resulting from the mixed radiation field behind the PMMA absorbers were measured using an ion-counting nanodosimeter. Two different measurements were carried out: (i) variation of the PMMA absorber thickness with constant carbon ion beam energy and (ii) combined variation of PMMA absorber thickness and carbon ion beam energy such that the kinetic energy of the carbon ions in the target volume is constant. The data analysis revealed unexpectedly high mean ICS values compared to stopping power calculations and the data measured at lower energies in earlier work. This suggests that in the measurements the carbon ion kinetic energies behind the PMMA absorber may have deviated considerably from the expected values obtained by the calculations. In addition, the results indicate the presence of a marked contribution of nuclear fragments to the measured ICS distributions, especially if the carbon ion does not cross the target volume.

physics.med-ph↗

arXiv2022

Correlated ionisations in two spatially separated nanometric volumes in the track structure of 241Am alpha particles: comparison with Monte Carlo simulations

The production of two double strand breaks in spatially separated locations on the DNA molecule can cause the loss of a whole DNA loop, which can be of substantial length depending on the geometrical position of the two damaged sites and depends on the degree of correlation between ionisation clusters formed in sites of several nanometres in size. In the first part of this paper, nanodosimetric measurements of alpha particle tracks in 1.2 mbar H2O, 1.2 mbar C3H8 and 1.2 mbar C4H8O with the PTB ion counter nanodosimeter were reported. In this second part, the focus is on the geometrical characterisation of the two sites simulated with the nanodosimeter in the three target gases and on the comparison of the measurement results with Monte Carlo simulations. The measurements in 1.2 mbar C3H8 were simulated with a version of the track structure code PTra dedicated to modelling the PTB ion counter nanodosimeter. Further simulations were performed with Geant4-DNA for 241Am alpha particle tracks in liquid water. Simulations of the measurements and the actual measurement results are found to be in good agreement for the investigated irradiation geometries.

physics.ins-det↗

arXiv2021

Characterisation of the PTB ion counter nanodosimeter's target volume and its equivalent size in terms of liquid H2O

For the first time a dedicated investigation of the target size of a nanodosimeter device has been carried out in order to investigate to what extent measured ionisation cluster size distributions can serve as benchmark data for modelling approaches, with particular focus on the target size in terms of liquid H2O. To this end, measurements with alpha particles from a 241Am source were carried out using three different target gases, H2O, C3H8 and C4H8O. For each of the three target gases, three different drift-time windows were applied to realise three different target sizes. A method has been developed to determine the dimensions of the simulated nanometric target volume in liquid H2O for cylindrical and spherical shape, as often used in approaches to model radiation effects to DNA. Simulations with nanometric targets of dimensions determined with this method agree very well with the corresponding measurements. Scaling of the spatial distribution of the extraction efficiency for different target gases and drift-time windows, which corresponds to the nanodosimeter's target volume, in terms of liquid H2O using ($ρλ_{ion}$)-ratios has also been investigated and proved to yield an estimate of the target volume in liquid H2O. Thus, it can be concluded that ionisation cluster size distributions measured with a nanodosimeter device are suited as benchmark data for approaches that model radiation induced damage to DNA in nanometric volumes of liquid H2O in simple geometries such as cylinders or spheres, provided that the nanodosimeter's target volume is characterised accordingly. By proper selection of drift-time window length as well as target gas and density a wide range of target volume dimensions in terms of liquid H2O can be realised with the PTB Ion Counter nanodosimeter according to specific requirements of modelling approaches.

physics.ins-det↗