Tissue Equivalent Gas Mixtures 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.

Lower size limit of the simulated biological target by TEG counter

The gas gain in the range of 1.0–2 × 105 was measured for propane-based tissue equivalent gas mixtures in the pressure range of 9–500 hPa. The Zastawny and Mitev et al. models of avalanche growth were used to analyse the obtained data. Both models showed that for low pressures of the working mixture, (i.e., below 80 hPa), there were non–equilibrium states of the development of the electron avalanche. The minimum size of the simulated biological target was determined on the basis of the measurements.

Gas gain in low pressures proportional counters filled with TEG mixtures

Gas gain in the range from 1 to 2 × 105 has been measured for both methane- and propane-based tissue equivalent gas mixtures in the pressure range of 9–60 hPa. The Diethorn and Williams&Sara models of the first Townsend coefficient have been used to determine the basic gas properties. Effective ionisation potentials of the mixtures, mean electron ionisation free paths, increase in the average electron kinetic energy between two successive ionising collisions and the values for the reduced electric field strength at which multiplication starts have been determined as a function of mixtures pressure. It was found that both Diethorn and Williams&Sara plots of the measured gas gain data have to be fitted to a double linear relation, for low and high gas gains, separately. It has also been obtained that the volume of the avalanche multiplication zone for the pressure p∼10 hPa accounts for 10% of the volume of the whole detector so the proportionality of the counter is strongly reduced.

Factors limiting the linearity of response of tissue equivalent proportional counters used in micro- and nano-dosimetry

Proportional counters filled with tissue equivalent gas mixtures are extremely useful instruments and are being used extensively as sensitive detectors for all types of radiations to measure the energy transferred to small tissue volumes. The linearity of their response is of primary importance. So the investigation and clarification of the physical phenomena taking place in the counter and of the limits within which useful results may be obtained would contribute to a more efficient use and a wider application of these counters. The linearity of response in the dose and in the gas gain has been determined. Linearity in the dose is limited by the total count rate effect, while linearity in the gas gain is limited by secondary processes occurring in the electron avalanche and by the self-induced space charge effect.

Operation of gas electron multiplier (GEM) with propane gas at low pressure and comparison with tissue-equivalent gas mixtures

A Tissue-Equivalent Proportional Counter (TEPC), based on a single GEM foil of standard geometry, has been tested with pure propane gas at low pressure, in order to simulate a tissue site of about 1µm equivalent size. In this work, the performance of GEM with propane gas at a pressure of 21 and 28kPa will be presented. The effective gas gain was measured in various conditions using a 244Cm alpha source. The dependence of effective gain on the electric field strength along the GEM channel and in the drift and induction region was investigated. A maximum effective gain of about 5×103 has been reached. Results obtained in pure propane gas are compared with gas gain measurements in gas mixtures commonly employed in microdosimetry, that is propane and methane based Tissue-Equivalent gas mixtures.

Gas gain limitation in low pressure proportional counters filled with TEG mixtures—part II

Proportional counters filled with tissue equivalent gas mixtures are extremely useful instruments and are being used extensively as sensitive detectors for all types of radiations to measure the energy transferred to small tissue volumes. The linearity of their response is of primary importance. So the investigation and clarification of the physical phenomena taking place in the counter and of the limits within which useful results may be obtained would contribute to a more efficient use and a wider application of these counters. The measured gas gain curves have been fitted to the Diethorn and Williams & Sara gas gain models. The models parameters and their dependence on gas pressure have been determined. It was shown that reduced ionization coefficient α/p is not univocal function of the reduced electric field strength EFS/p.

Microdosimetric response of proportional counters filled with different tissue equivalent gas mixtures

In pulse mode operation each detector at the output gives pulse height spectra. These pulse height spectra from proportional counters filled with tissue-equivalent gas are recalculated to microdosimetric quantity for expressing the radiation quality. Energy spectra of 90Sr, 109Cd and 137Cs sources have been measured for different in geometry proportional counters for Methane- and Propane-based tissue equivalent gas (TEG) mixtures. The mixtures pressures were varied in the range from 20 hPa to 200 hPa to have simulated tissue target diameter of the order of a few μm, from 0,3 μm to 7 μm. Differences, up to 20% in the average deposited energy due to different TEG mixtures were observed.

Gas gain limitation in low pressure proportional counters filled with TEG mixtures

Proportional counters filled with tissue equivalent gas mixtures (TEPC) can be used to simulate interactions and energy transferred to small tissue volumes. One criteria which allows to use TEPC as the dose meter is that the particle ranges are larger compared to the gas volume. TEPC achieve this by operating at low gas pressures. Single ionization events dominate the distribution of low-LET radiation at low gas pressure and therefore their detection is of primary importance, a high gas gain is necessary. Therefore gas gain factor has been measured for Methane- and Propane-based tissue equivalent gas mixtures. The highest stable gas gains, second ionization Townsend coefficient and electron avalanche dimensions have been determined.

Monte Carlo tools to supplement experimental microdosimetric spectra

Tissue-equivalent proportional counters (TEPCs) are widely used in experimental microdosimetry for characterising the radiation quality in radiation protection and radiation therapy environments. Generally, TEPCs are filled with tissue-equivalent gas mixtures, at low gas pressure, to simulate tissue site sizes similar to the cell nucleus (1 or 2 µm). The TEPC response using Monte Carlo (MC) codes can be applied to supplement experimental measurements. Most of general-purpose MC codes currently available recourse to the condensed-history approach to model the electron transport and do not transport low-energy electrons (<1 keV), which can lead to systematic errors, especially in thin layers and in gas-condensed medium interfaces. In this work, a comparison between experimental microdosimetric spectra of (60)Co and (137)Cs radiation at different simulated sizes (from 1.0 to 3.0 μm) in pure propane versus simulated spectra obtained with two general-purpose codes FLUKA and PENELOPE, which include a detailed simulation of electron-photon transport in arbitrary materials, including gases, is presented.

Nanodosimetric measurements with an avalanche confinement TEPC.

This paper illustrates a tissue-equivalent proportional counter designed to have high gas gain and good energy resolution at nanometric simulated site sizes. Microdosimetric neutron and gamma spectra were measured in dimethyl ether and in propane-based tissue-equivalent gas mixture down to 35 nm. The comparison of experimental data with the results of Monte Carlo calculations shows a satisfactory agreement.

Ionization coefficient in propane, propane-based tissue equivalent and dimethyl-ether in strong non-uniform electric fields

Gas amplification in a proportional counter filled with propane, propane-based tissue-equivalent gas mixture and dimethyl-ether is measured. By using low pressures (2-50 kPa) relatively strong reduced electric fields are produced, which enabled a study of non-equilibrium effects. The gas gain data are analysed according to the field-gradient model and the reduced ionization coefficient /P is thus obtained. The assumption of electrons being in equilibrium with the electric field fails to be valid in low-pressure gases, where the electric field strengths at the anode surface, Sa, above about 5 × 105 V m-1 kPa-1 are obtained. Under non-equilibrium conditions /P is not a unique function of the reduced electric field strength, but depends on the gas pressure as well. Good agreement with other experimental or theoretical /P obtained mostly under equilibrium conditions is obtained at relatively weak electric field strengths. The presented /P in strong fields may be used for a more accurate modelling of low-pressure counters.

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.

The Fano factor for electrons in gas mixtures

Data on statistical fluctuations of ionization yield, or the Fano factor, for electrons in gas mixtures are systematized. Empirical results indicate that the Fano factor for electrons in a regular mixture, Fmix, can be represented by the Fano factors FA and FB for the pure components: Fmix = (FA − FB) z + FB, where z is an energy partition parameter. Good agreement between Fmix thus calculated and experimental or theoretical Fano factors is obtained for binary mixtures, while the difference between the measured Fano factor and Fmix is about 10% for tissue-equivalent gas mixtures.

The Mean Energy Required to Form an Ion Pair for Low-Energy Photons and Electrons in Polyatomic Gases

The mean energy required to form an ion pair (W) is measured for photons having energy 0.277, 1.49, and 5.89 keV in several polyatomic gases and two tissue-equivalent gas mixtures. An increase of W values with decreasing energy is observed. Generally, a continuous decrease of W with an increasing number of atoms in a series of organic compounds was reported (ICRU Report No. 31), while our measurements show an oscillation of W values in alkanes; the W value for an alkane with an even number of carbon atoms in the molecule (ethane and butane) is lower than that for the next higher alkane with an odd number of C atoms (propane and pentane, respectively). The W value for low-energy electrons was calculated from experimental photon W values. A good agreement between the calculated and the experimental W values in the low-energy region, and the high-energy W values is obtained. Our results fill the gap between the very low-energy region (below 300 eV) and the high-energy (above 10 keV) region.

Statistical fluctuations in the ionisation yield of low-energy photons absorbed in polyatomic gases

The statistical fluctuations of the number of ion pairs created by monoenergetic photons in several polyatomic gases and gas mixtures were derived by iterative deconvolution of measured pulse height spectra obtained by means of the proportional counter. The response of the proportional counter to the unit charge (the single-electron spectrum) has been determined precisely, making possible the measurement of the number of electrons created per absorbed photon. W values (mean energy per ion pair), generalised Fano factors and primary ion pair distribution characteristics have been derived as functions of incident photon energy for ten polyatomic gases, mostly alkanes, and two tissue-equivalent gas mixtures. Monoenergetic photons having an energy equal to 0.277 keV (carbon Kalpha line), 1.49 keV (aluminium Kalpha line) and 5.89 keV (55Fe) have been used throughout the experiment. It is shown that the primary distribution of ion pairs in all investigated gases and gas mixtures is asymmetrical and more peaked than the Gaussian distribution for low-energy incident photons, approaching the Gaussian shape for 6 keV photons.

The Average Energy per Ion Pair, W̄, for Hydrogen and Oxygen Ions in a Tissue Equivalent Gas

Measurements are reported for the value of W (the average energy required to form an ion pair) for hydrogen and oxygen ions in the Rossi-Failla, tissue-equivalent gas mixture. The University of Virginia kevatron linear dc ion accelerator was used to produce the monoenergetic ions. For the six different incident monoenergetic hydrogen ions employed with kinetic energies ranging from 3.72 to 48.24 keV per atom, the values for W ranged from 46.5 to 30.4 eV per ion pair. For the incident monoenergetic oxygen ions employed with kinetic energies of 24.52 and 49.05 keV per atom, values for W of 66.0 and 52.2 eV per ion pair, respectively, were obtained. Thus, a general increase in W with decreased kinetic energy was observed. Comparisons were made with computed W values which are predicted from stopping power theory.

Computed ionization and Kerma values in neutron irradiated gases

As an aid to accurate neutron dosimetry the mean W values, W, have been calculated for a tissue-equivalent gas mixture, ethylene, acetylene and carbon dioxide at twenty-one different neutron energies in the range 0.1-15.5 MeV. Kerma and the energy expended as elastic nuclear scattering are also evaluated at each neutron energy from the differential secondary particle spectra.

Dependence of the energy per ion pair on the photon energy below 6 keV in various gases

An attempt has been made to measure W( E), i.e. the energy required for formation of an ion pair, as a function of photon energy in various gases. The method consists in performing accurate measurements of pulse height spectra of K lines of low Z elements using a proportional counter. The W values obtained in the energy range from 277 to 5890 eV did not depart more than + 3% and − 1% from the value measured at 5890 eV in four investigated gases: Ar + 10% CH 4, C 3H 8 and two tissue equivalent gas mixtures. Below the carbon K line (277 eV) the incident photon beam consisted of a narrow band of soft X-rays obtained by filtration of bremsstrahlung. The mean energy as well as the width of the filtered X-ray spectrum were calculated. The measurement of W( E) in propane revealed a steady increase of W below 200 eV.

Recent Measurement of Energy Distribution in Tissue of Cobalt-60 Gamma Rays: Improvements of Experimental Technic and Calibration Method

An attempt has been made to extend the gamma-ray energy loss distribution measurement in tissue toward very small spherical volumes of tissue, ranging from 0.2 to 1.0 µ, using the experimental technic developed by Rossi and Rosenzweig (1). A more detailed consideration of the factors which determine the performance of the counter and the associated electronics showed that the smallest volume of tissue which can be simulated by a spherical counter of a given diameter is determined by the lowest gas pressure inside the counter at which the gas multiplication required to raise a substantial portion of measured spectrum above the noise is still in the proportional region. A relation could be derived which enables one to predict the lowest attainable tissue size, using a particular experimental arrangement. To determine the energy spectrum for very small tissue volumes it is desirable to use (a) a low density tissue equivalent gas which has a broad proportional region, enabling one to raise the gas multiplication up to 5·104 or more, (b) a well-designed counter, having an optimal ratio of anode-to-grid diameters in order to exploit fully the proportional region of a particular gas, (c) a low noise preamplifier and a pulse-shaping circuit consisting of a two-step differentiation and one-step integration network, and (d) a reliable energy calibration method taking into account that for very small counter diameters and low pressures only the very low energy electrons will expend their entire energy in the counter volume. A substantial lowering of the counter pressure and thus the tissue sphere which it represents has been achieved by redesigning the experimental apparatus according to a, b, and c. A tissue-equivalent gas mixture consisting of 55 per cent propane, 39.6 per cent CO2, and 5.4 per cent N2 has been successfully used down to 3.4 Torr, which corresponds to an 0.2 µ diameter tissue sphere. The high gas multiplication of the order of 104 is readily obtained near the end of the proportional region. This improvement is attributed to the stronger quenching action of propane; TE gas containing methane reaches the breakdown point at a multiplication ∼103 at these low pressures. The counter geometry has been changed in order to obtain a more uniform field near the anode. The 2.5 mil anode wire has been replaced by a 1 mil wire and the helix diameter doubled, from 1/16 to 1/8 of an inch. The preamplifier noise has been reduced from 750 to ∼250 e− RMS by using a FET preamplifier, reducing the stray capacitance of the collector and choosing the optimal RC values of the pulse-shaping networks. Typical values are 0.8–3.2 µ sec. for both differentiation and integration; second differentiation is applied only at very high counting rates to avoid pulse pile-up. The use of a low-energy calibrating source proved to be instrumental in making possible an accurate measurement of energy loss distributions and single electron spectra.

Determination of the quality factor through the utilization of a balanced, tissue equivalent, ionization chamber

Accelerator dosimetry tends toward either specifying the field by way of its components or using a tissue equivalent chamber with a failsafe Quality Factor (QF). This investigation attempts a third approach, the determination of QF directly. A Lucite, three-parallel plate ionization chamber was constructed with a common center electrode and two independent outer electrodes. This configuration allows the position of the center electrode to be manually adjusted to effect first order balance; vernier balance is obtained electronically through adjustment of one of the paired electrometers. The chamber can be pressurized to at least 17 atm with a tissue equivalent gas mixture and, therefore, approximates tissue rad response. This ionization chamber is capable of detecting differences in LET of any radiation by operating on the principle of columnar recombination. The previous investigators use either a single ion chamber or operate their respective systems in the general recombination mode. This technique requires low polarizing voltages and, therefore, is useful only for low dose rates. Both the single chamber and the general recombination system are limited in accelerator dosimetry applications. Very high instantaneous dose rates and extreme variability of sources dictate a balanced ionization chamber system that utilizes columnar recombination. In the present instrument, one of the chambers is operated at low polarizing potential while the other is variable. The difference current between the two chambers is adjusted to zero for background levels and when sensing radiation this difference current is a measure of QF. In operation, the slope, with voltage, of the difference current is proportional to the QF of the radiation. This proportionality is apparently linear at 10 atm over a range of QF from one to at least 7.5. This response has been shown to be dose-rate independent. The directional dependence and pressure dependence of this chamber have been studied and will be reported.