Search

Purpose: To study the agreement between proton microdosimetric distributions measured with a silicon-based cylindrical microdosimeter and a previously published analytical microdosimetric model based on Geant4-DNA in-water Monte Carlo simulations for low energy proton beams. Methods and material: Distributions for lineal energy (y) are measured for four proton monoenergetic beams with nominal energies from 2.0 MeV to 4.5 MeV, with a tissue equivalent proportional counter (TEPC) and a silicon-based microdosimeter. The actual energy for protons traversing the silicon-based microdosimeter is simulated with SRIM. Monoenergetic beams with these energies are simulated with Geant4-DNA code by simulating a water cylinder site of dimensions equal to those of the microdosimeter. The microdosimeter response is calibrated by using the distribution peaks obtained from the TEPC. Analytical calculations for y‾F and y‾D using our methodology based on spherical sites are also performed choosing the equivalent sphere to be checked against experimental results. Results: Distributions for y at silicon are converted into tissue equivalent and compared to the Geant4-DNA simulated, yielding maximum deviations of 1.03% for y‾F and 1.17% for y‾D. Our analytical method generates maximum deviations of 1.29% and 3.33%, respectively, with respect to experimental results. Conclusion: Simulations in Geant4-DNA with ideal cylindrical sites in liquid water produce similar results to the measurements in an actual silicon-based cylindrical microdosimeter properly calibrated. The found agreement suggests the possibility to experimentally verify the calculated clinical y‾D with our analytical method.

Neutron spectrometry measurements with an extended-range Bonner Sphere Spectrometer (BSS), as well as neutron H*(10) measurements using an extended-range rem meter WENDI-2, a conventional rem meter LB 6411 and a tissue-equivalent proportional counter, were performed inside and around the Fixed-Beam Treatment Room at the proton therapy facility of Essen, in Germany. The WENDI-2 stood out as the easiest detector for making accurate neutron H*(10) measurements, since its direct measurements were equivalent to the H*(10) rates obtained with the BSS. The measurements were also compared to simulation results obtained with MCNPX 2.7.0 using two different selections of physics models for the hadron interactions above 150 MeV: the Bertini & Dresner models and the CEM03 model. For neutron H*(10) rates outside the treatment room, factors of 1.6–1.8 were obtained between the results of the two simulations, the Bertini & Dresner models yielding the largest values in all positions. The comparison of the simulation results with the WENDI-2 and BSS measurements for positions inside the treatment room showed that the Bertini & Dresner models reproduce the global neutron production in the water phantom relatively well, whereas the CEM03 model underestimates it by a factor of ∼1.3. At the most-forward angle, however, the Bertini model (unlike the CEM03 model) seemed to overestimate the production of neutrons with energies above 100 MeV. Outside the shielding, the simulated H*(10) overestimated the WENDI-2 measurements by factors of 2–3 with the Bertini & Dresner models, and 1.1–1.7 with the CEM03 model. Both simulations were thus conservative with respect to the neutron fluxes transmitted through the concrete walls. This conservative behaviour is probably caused by a combination of several uncertainties, including for instance uncertainties on the proton and neutron interaction cross-sections and uncertainties on the concrete composition and density., SCOPUS: ar.j, info:eu-repo/semantics/published

A tissue-equivalent proportional counter (TEPC) simulates micrometric volumes of tissue, if the energy deposited in the counter cavity is the same as that in the tissue volume. Nevertheless, a TEPC measures only the ionizations created in the gas, which are later converted into imparted energy. Therefore, the equivalence of the simulated diameter in two gases should be based on the equality of the mean number of ions pairs in the gas rather than on the imparted energy. Propane-based tissue equivalent gas is the most commonly used gas mixture at present but it has the drawback that its composition may change with time. From this point of view, the use of pure propane offers practical advantages: higher gas gain and longer stability. In this work, microdosimetric measurements performed with pure propane, at site sizes between 0.05 mg/cm2 and 0.3 mg/cm2, demonstrate that the response of a propane-filled detector in gamma and in neutron fields is almost the same if an appropriate gas density is used.

Tissue-equivalent proportional counters (TEPCs) measure distributions of ionizations, produced in the gas cavity by the radiation field which are afterwards converted into distributions of energy imparted by applying a calibration factor. To calibrate the pulse height spectra, firstly, a marker point must be identified in the measured spectrum. Then, an accurate value of lineal energy must be assigned to this marker. A common marker that is often used for calibration is the so-called proton-edge (p-edge). It is a distinctive feature of a proton or neutron spectrum which corresponds to the maximum amount of energy that a proton can deposit in the active volume of the detector. A precise method to identify the marker point was applied to identify the p-edge with an uncertainty below 2%. To evaluate the final uncertainty of the calibration, the uncertainty of the energy value assigned to the p-edge must also be considered. This value can be evaluated using different energy-range tables. This study investigates how the choice of different input databases for calibration purposes influences the calibration. The effect of three different frequently used sets of input data was analysed for pure propane gas and for propane-TE gas mixture.

Real time Radiation Monitoring Device (RRMD-2 and -3) were installed in Space Hub in the S/MM (Shuttle/ Mir Mission), and the radiation data and the environmental data necessary for the estimation of the dose equivalent were transmitted to the NASA Johnson Space Center via Space Hub and analyzed together with the orbit data. The radiation data, the environmental data and the orbit data were also transmitted to the Tsukuba Space Center in Japan through Internet and analyzed similarly almost in real time. Simultaneous measurement with a Tissue Equivalent Proportional Counter (TEPC) which was considered to be a standard of the dosimetry until now was also carried out. Estimated dose values by a plastic track detector (CR-39) and by a Thermoluminescence Dosimeter (TLD) were also compared with RRMD data for overall evaluation of RRMD. The results clearly indicated that the use of the assumption introduced for the real time measurement of TEPC led to erroneous values of effective quality factor, namely, the shape of the LET (Linear Energy Transfer) distribution. In addition, the absorbed dose rate obtained by RRMD-3 showed a large discrepancy by 1.24 times of the average value obtained by TLD., S/MM(シャトル/ミールミッション)において実時間放射線モニタ装置(RRMD-2および-3)をスペースハブ内に取り付け、線量当量の推定に必要な放射線データおよび環境データをスペースハブを経由してNASAジョンソン宇宙センターに送信し、軌道データとともに解析した。また、放射線データ、環境データ、軌道データはいずれもインターネットを通じて日本の筑波宇宙センターにも送信され、同様にほぼ実時間で解析した。これまで線量計測の標準と見なされてきた組織等価比例計数管(TEPC)との同時測定も行った。またプラスチック検出器(CR-39)と熱ルミネセンス線量計(TLD)からの推定線量との比較も行い、RRMDに対し総合的な評価を加えた。その結果、TEPCの実時間計測のために導入された仮定の使用が誤った実効線質係数、すなわちLET(線エネルギー付与)分布の形状を導くことを明確に示した。またRRMD-3で求めた吸収線量率はTLDによる平均の値の1.24倍と大きな食い違いを示していた。

Ionising radiation causes a wide variety of effects in biological targets. In experimental microdosimetry these effects are studied by investigating the statistical distribution of energy deposition events at the microscopic level employing a Tissue-Equivalent Proportional Counter (TEPC). A TEPC is usually designed with a cavity of a few centimetres in diameter containing a central anode wire and operated with a tissue-equivalent wall and counting gas at low pressure to simulate tissue volumes of micrometer dimensions. However, in experimental microdosimetry there are some conditions such as high flux therapy beams and energy deposition on nanometer level (nanodosimetry) in which TEPCs with very small cavities are needed. The construction of a central wire in a small cavity is extremely difficult and expensive. It becomes even more difficult when multi-element cavities are applied to increase the counter sensitivity. Therefore, a new type of TEPC was developed based on a Gas Electron Multiplier (GEM). The design and construction of a TEPC-GEM is described. The successful operation of this counter has been shown by the first microdosimetric measurements of the TEPC-GEM exposed to a 14 MeV monoenergetic neutron beam and a californium (252Cf) source for a cavity diameter of 1.8 mm, simulating a 1.0 μm tissue site size. The measured spectra show an excellent agreement with spectra from the literature. Furthermore, the first microdosimetric responses for low energy X-rays are presented for counter cavities of 1.8 mm, 1.0 mm and 0.5 mm diameter at various pressures, simulating tissue site sizes down to 140 nm. This new design not only simplifies the construction of mini multi-element counters but it also facilitates microdosimetry measurements (e.g. in nanodosimetry, personal dosimetry and space dosimetry) in a relatively easy manner. Because of the relatively simple design and construction, the TEPC-GEM is not only relatively cheap and easy to produce but also its compo, Interfaculty Reactor Institute