Methane-based Tissue-equivalent Gas Research Articles

Measurements and calculations of gas gains in methane-based TEG mixtures

The gas gains in the methane-based tissue equivalent gas (TEG) mixture (64.4% CH4 + 32.5% CO2 + 3.1% N2) were measured in single wire proportional counters and calculated with Magboltz simulation program for various mixture pressures. The calculated gain values were always greater than the measured ones for all pressures. The source of the discrepancies was investigated in terms of dissociation of the molecules and non-equilibrium conditions. The impacts of both energy loss mechanisms on gas gain rise as the mixture pressure decreases.

Dose-average linear energy transfer of electrons released in liquid water and LiF:Mg,Ti by low-energy x-rays, 137Cs and 60Co gamma

For many years, track-average linear energy transfer (LET), has been used to quantify the radiation-induced phenomena in biological and physical systems. However, due to the need for including into the radiotherapy treatment planning system, parameters that are clinical and biologically relevant, a precise knowledge of the dose-average LET, becomes essential. Besides, several dosimetric studies have revealed that is fundamental to describe the dosimeter’s response induced by photons. The most important data sets publicly available for of electron generated by photons are those reported for measurements performed in methane-based tissue-equivalent gas. However, comparing to liquid water, the electron spectra generated by low photon energy might not be similar due to the photoelectric effect. Thus, this work aimed at investigating the of electron spectra generated in liquid water and LiF:Mg,Ti by ten x-ray beams from 20 kV to 300 kV, 137Cs and 60Co gamma. The results suggest that is more sensitive to the surrounding environment than and consequently, it might be a more appropriate parameter to quantify the radiation effect and damage in matter induced by photons. Besides, good agreement (6% to 12% differences versus 10% to 15% uncertainties in the experiments) was observed between the data obtained in this work for liquid water and the experimental values published for methane-based tissue-equivalent gas at energies above 60 keV. Whereas at lowest energies, the minimum difference is around 18% which can be associated to the difference between the two media.

Measurement of the first Townsend ionization coefficient in a methane-based tissue-equivalent gas

Tissue-equivalent gases (TEGs), often made of a hydrocarbon, nitrogen, and carbon dioxide, have been employed in microdosimetry for decades. However, data on the first Townsend ionization coefficient (α) in such mixtures are scarce, regardless of the chosen hydrocarbon. In this context, measurements of α in a methane-based tissue-equivalent gas (CH4 – 64.4%, CO2 – 32.4%, and N2 – 3.2%) were performed in a uniform field configuration for density-normalized electric fields (E/N) up to 290Td. The setup adopted in our previous works was improved for operating at low pressures. The modifications introduced in the apparatus and the experimental technique were validated by comparing our results of the first Townsend ionization coefficient in nitrogen, carbon dioxide, and methane with those from the literature and Magboltz simulations. The behavior of α in the methane-based TEG was consistent with that observed for pure methane. All the experimental results are included in tabular form in the Supplementary material.

Accuracy of the electron transport inmcnp5and its suitability for ionization chamber response simulations: A comparison with theegsnrcandpenelopecodes

In this work, accuracy of the mcnp5 code in the electron transport calculations and its suitability for ionization chamber (IC) response simulations in photon beams are studied in comparison to egsnrc and penelope codes. The electron transport is studied by comparing the depth dose distributions in a water phantom subdivided into thin layers using incident energies (0.05, 0.1, 1, and 10 MeV) for the broad parallel electron beams. The IC response simulations are studied in water phantom in three dosimetric gas materials (air, argon, and methane based tissue equivalent gas) for photon beams ((60)Co source, 6 MV linear medical accelerator, and mono-energetic 2 MeV photon source). Two optional electron transport models of mcnp5 are evaluated: the ITS-based electron energy indexing (mcnp5(ITS)) and the new detailed electron energy-loss straggling logic (mcnp5(new)). The electron substep length (ESTEP parameter) dependency in mcnp5 is investigated as well. For the electron beam studies, large discrepancies (>3%) are observed between the MCNP5 dose distributions and the reference codes at 1 MeV and lower energies. The discrepancy is especially notable for 0.1 and 0.05 MeV electron beams. The boundary crossing artifacts, which are well known for the mcnp5(ITS), are observed for the mcnp5(new) only at 0.1 and 0.05 MeV beam energies. If the excessive boundary crossing is eliminated by using single scoring cells, the mcnp5(ITS) provides dose distributions that agree better with the reference codes than mcnp5(new). The mcnp5 dose estimates for the gas cavity agree within 1% with the reference codes, if the mcnp5(ITS) is applied or electron substep length is set adequately for the gas in the cavity using the mcnp5(new). The mcnp5(new) results are found highly dependent on the chosen electron substep length and might lead up to 15% underestimation of the absorbed dose. Since the mcnp5 electron transport calculations are not accurate at all energies and in every medium by general clinical standards, caution is needed, if mcnp5 is used with the current electron transport models for dosimetric applications.

Simulation of the Charge Produced by Protons inside a Tissue Equivalent Ionization Chamber in a Mixed Neutron/Gamma Field in BNCT

The neutron and gamma dose in boron neutron capture therapy (BNCT) can be determined by using ionization chambers of different materials. However, inexplicable results, such as negative doses, are sometimes obtained. Computer simulations using MCNPX can help one to understand the behavior of ionization chambers. This paper deals with a part of this investigation: the contribution of protons to the total measured charge in a tissue equivalent (TE) ionization chamber that is flushed with methane-based TE gas. The inherent problem is that the Monte Carlo code MCNPX cannot track protons below 1 MeV.A custom-made program, called Proton Produced Ionization Chamber Charge (PPICC), calculates the deposited energy and thus the charge in the TE gas per proton. For this, it uses the stopping powers for protons in TE plastic and gas. MCNPX provides the total number of protons produced by all neutron interactions near the gas. To check this new procedure, measurements and simulations have been performed using a validated mixed beam of neutrons and gammas. The neutron fluence consists of 12% fast neutrons and 87% epithermal neutrons. In one setup the chamber is free-in-air (epithermal/fast neutron field) and in the other is in a cubic polymethylmethacrylate phantom at 25 mm depth (thermal/epithermal neutron field).The total charge is the sum of the charges due to electrons, originating from primary and neutron-induced gammas, and protons from 1H(n,n)1H and 14N(n,p)14C reactions. The total measured and calculated charges in the two setups have acceptable uncertainties and are in good agreement. The charge collected in a TE ionization chamber can be simulated in a mixed field of neutrons and gammas. The charge resulting from proton recoil in the gas is unexpectedly large.

Gas electron multiplier (GEM) operation with tissue-equivalent gases at various pressures

We have studied the operation of two different Gas Electron Multiplier (GEM) structures in both methane and propane based Tissue-Equivalent (TE) gases at different pressures varying from 0.1 to 1 atm. This work was motivated to explore the possibility of using a GEM for a new type of Tissue Equivalent Proportional Counter. In methane based TE gas, a maximum safe GEM gain of 1.5×10 3 has been reached while in propane based TE gas this is 6×10 3. These maxima have been reached at different gas pressures depending on GEM structure and TE gas. Furthermore, we observed a decrease of the GEM gain in time before it becomes stable. Charge up/polarisation effects can explain this.

Photon quality correction factors for ionization chambers in an epithermal neutron beam

Photon quality correction factors (kQγ) for ionization chamber photon dosimetry in an epithermal neutron beam were determined according to a modified absorbed dose to water formalism which was extended to mixed radiation fields. We have studied two commercially available ionization chambers in the epithermal neutron beam optimized for BNCT at the facility at Studsvik, Sweden. One of the chambers is nominally neutron insensitive; a magnesium-walled detector flushed with pure argon gas (denoted by Mg/Ar). The second chamber has approximately the same sensitivity for neutrons and photons; it is considered a ‘tissue equivalent’ detector, with A-150 walls flushed with methane-based tissue-equivalent gas (denoted by TE/TE). The kQγ-factors in epithermal neutron beams have previously been assumed to be equal to unity or estimated from measurements in clinical accelerator produced photon beams. In this work the kQγ-factors have been determined from absorbed dose calculations using cavity theory together with Monte Carlo derived electron fluences obtained with the MCNP4c system for water and PMMA phantoms. The calculated quality correction factors differ substantially from unity, being in the order of 10% for the Mg/Ar detector at shallow phantom depths, and between 2 and 4% for other depths and for the TE/TE chamber.

W values and radial dose distributions for protons in TE-gas and air at energies up to 500 MeV

The dosimetry of charged-particle beams is frequently performed by measuring the ionization yield produced in gas-filled ionization chambers and converting it into absorbed dose using the mean energy W expended in a gas per ion pair formed, or its differential value for the filling gas. Because of the increasing importance of proton therapy and the lack of accurate values of W and at high proton energies in particular, we investigated the ionization yield formation by protons in air and methane-based tissue-equivalent gas in the energy range up to 500 MeV. For this purpose, we analysed the available experimental values of W and and, in a second step, investigated the ionization-yield formation by protons as a function of their initial energy using an analytical method based on the Spencer-Fano equation in the framework of the continuous-slowing-down approximation. To estimate possible ionization losses which can be expected if the ionometric method is applied in the case of an incomplete secondary-electron equilibrium, a new analytical model has been developed which can be used to calculate the radial distribution of the ionization yield produced by secondary electrons around high-energy proton tracks.

Neutron W Values in Methane-Based Tissue Equivalent Gas up to 60 MeV

Neutron W Values in Methane-Based Tissue Equivalent Gas up to 60 MeV

Neutron W Values in Methane-Based Tissue Equivalent Gas up to 60 MeV

Neutron W Values in Methane-Based Tissue Equivalent Gas up to 60 MeV

New Analytical Representation of W Values for Protons in Methane-Based Tissue-Equivalent Gas

New Analytical Representation of W Values for Protons in Methane-Based Tissue-Equivalent Gas

New Analytical Representation of W Values for Protons in Methane-Based Tissue-Equivalent Gas

Abstract The interpretation of ion yields measured with ion chambers or proportional counters in terms of energy deposited depends on a precise knowledge of the W values, i.e. the mean energy expended to form an ion pair. Methane-base tissue-equivalent (TE) gas is widely used in neutron dosimetry. Neutrons ionise indirectly by induced charged particles. In tissue and TE material, recoil or reaction protons account for a major fraction of the energy deposited. This report presents a new analytical representation of proton-W values. New experimental results are included which strongly influence the analytical fit, as they have small measurement uncertainties and include W values for proton energies as low as 1 keV and as high as 3.46 MeV. The differences between the present and older evaluations as found in the literature are discussed. The new analytical fit includes a 'bump' in W values for proton energies at about 300 keV. The covariance matrix of the new model is studied and the influence of the new fit on Wn values and on microdosimetric spectra for neutrons is discussed.

A Comparison of Calculated and Measured W Values in Tissue-Equivalent Gas Mixtures

We calculated the mean energy required to produce an ion pair (W) in methane-, propane- and butane-based tissue-equivalent (TE) gas mixtures from W values in pure constituent gases according to various models for energy partition among gas components. We found an agreement between the experimental and calculated W values in the methane-based TE gas regardless of the model concept. In contrast, only those models which take into account differences in the stopping powers, total ionization cross sections and model constants of gas components give acceptable results for the propane-based TE gas. The calculated W value for high-energy electrons in the isobutane-based TE gas mixture is 25.2 eV for high-energy electrons and 28.0 eV for approximately 5 MeV alpha particles.

W values for low-energy protons in methane-based tissue equivalent gas and its constituents

The mean energy, W, expended per ion pair formed has been determined experimentally for protons completely stopped in methane-based tissue-equivalent gas (TE gas) and its constituents, methane, carbon dioxide and nitrogen, covering the energy range from 1 to 100 keV. Differential omega values were derived from the energy dependence of W. While the W values for TE gas and methane decrease monotonously with increasing energy up to 100 keV, the results for carbon dioxide and nitrogen show a flat minimum around 20 keV. For practical application, analytical functions have been fitted to the experimental data. Using a simple additivity rule of the ionization yield, it is possible to determine the W values for this TE gas mixture from those of the pure gas components within the limits of experimental uncertainty.

W value measurements for 241Am alpha particles in various gases

Measurements of W, the mean energy expended per ion pair formed, have been performed for 241Am alpha particles stopping in a number of the gases commonly used in ionisation chambers. Both absolute and relative measurements were made with nitrogen used as the reference gas for the relative measurements. The absolute method entailed a simultaneous measurement of a quantity of charge and the number of alpha particles which produced that charge, whilst the relative approach simply required a determination of the ratio of ion currents. Results were obtained for methane-based tissue-equivalent gas, for its constituents, i.e. methane, carbon dioxide and nitrogen, for argon, and also for a gas mixture with a composition approximately equivalent to A-150 plastic.

W value measurements for protons in tissue-equivalent gas and its constituent gases

Values of W (the mean energy required to create an ion pair when a charged particle is stopped in a gas) are needed in radiation dosimetry to convert ionisation chamber current readings to absorbed dose. For neutron dosimetry the most important W values are those for protons, and this paper describes absolute measurements for protons with energies between 1.2 and 3.5 MeV for methane-based tissue-equivalent gas and its constituent gases, i.e. methane, carbon dioxide and nitrogen. An ionisation chamber with a parallel-plate arrangement was used to determine the integrated charge created by a measured number of protons, and W values were determined with an estimated uncertainty of about +or-0.5%. The results are presented and compared with previous measurements. In general the present data tend to be somewhat lower than previously reported values. Finally the accuracy of a formula for predicting W values for gas mixtures from the values for the individual constituents is investigated.

Initial recombination of ions in electron tracks: an evaluation of the predictions of Lea's model and a modified track structure model

The formulas estimating initial recombination of ions created by electrons in high pressure parallel plate ionization chambers are modified and developed to comply with a modified model for recombination. Appropriate average values for restricted ionization density related parameters of recombination are derived to describe the situation of irradiation with electron or photon spectra. It is shown that these average values are related to the dose mean of the restricted ionization density (cut-off energy 500 eV) in methane based tissue equivalent gas. An experimental comparison between the predictions of the modified model and Lea's model for electron recombination is presented and the modified model shows a closer agreement with experiment in all situations studied. The modified model predicts the experimentally observed currents to within 0.1% over the photon energies studied (60–1225 keV) and over all high field strengths applied to the chamber, at a constant pressure of 8 atm.

A measurement of WN/e for methane-based tissue-equivalent gas in a d(4)-Be neutron field

Measurements of absorbed dose were carried out in a d(4)-Be neutron field using tissue-equivalent (TE) plastic ionisation chambers and a TE plastic calorimeter in a TE plastic phantom. The gamma -ray dose fraction at the same position was measured using two types of Geiger-Muller (GM) dosemeters. Good agreement was found in the absorbed dose as determined by the use of the two types of ionisation chambers: spherical and thimble-shaped. Bragg-Gray theory was employed, along with measurements using the ionisation chambers and the calorimeter to compute a value for the ratio of absorbed dose to specific charge, which is proportional to the product of the stopping power ratio for secondary charged particles in the chamber wall and gas (Sw,g)N, and the average energy expended in the gas per ion pair formed (WN/e). The value of WN/e for the d(4)-Be field studied was found to be 31.0+or-1.8 JC-1 for methane-based TE gas, assuming the stopping power ratio to be 1.000+or-0.044 for neutrons.

Stopping power and average energy to form an ion pair for 42 MeV oxygen ions

Total ionisation produced by 42 MeV oxygen ions when stopped in nitrogen, carbon dioxide, methane and propane, and methane-based tissue-equivalent (TE) gases was measured using a variable pressure ionisation chamber. From these measurements average energy required to form an ion pair (W) for oxygen ions in the above gases was determined. Ionisation produced by oxygen ions in traversing a given path length in the chamber filled with various gases was also measured and used to determine the stopping power (Linfinity ). W and Linfinity for methane-based TE gas was calculated by adding the contributions from constituent gases. Calculated values of Linfinity obtained (a) using the proton stopping power data of Janni (1966) and Andersen and Ziegler (1977) and (b) using Northcliff and Schilling tables (1970) were compared with measured values for various gases. Calculated values were higher than those measured except for nitrogen based on Andersen and Ziegler (1977) data. W-values determined in the present study for oxygen ions in nitrogen and methane-based TE gas were consistent with published values.

Characteristics of A-150 plastic-equivalent gas in A-150 plastic ionization chambers for p(66)Be(49) neutrons.

The evaluation of a gas mixture having an atomic composition similar to that of A-150 tissue-equivalent (TE) plastic has been extended to a high-energy neutron therapy beam. "A-150" gas, air, and methane-based TE gas were each flowed through A-150 plastic-walled ion chambers of different sizes and irradiated with p(66)Be(49) neutrons. A tentative value for W(A-150) of 27.3 +/- 0.5 JC-1 was derived for this beam. The W value of the A-150 gas mixture is compared to those of methane-based TE gas and of air for the p(66)Be(49) neutron beam as well as to corresponding values found in similar experiments using 14.8-MeV monoenergetic neutrons.