Water Positron Research Articles

M-TAG: A modular teaching-aid for Geant4

Geant4 is a versatile Monte Carlo radiation transport simulation toolkit with a steep learning curve. This work introduces a user-code called M-TAG (Modular Radiation Teaching-Aid for Geant4), built on top of Geant4. M-TAG is designed to help gradually introduce the Geant4 toolkit to new users.The goal of Geant4 is to record quantities from the simulated radiation as it is transported through geometries. M-TAG simplifies the inclusion of new geometric elements and detector components in the simulation by including new classes. M-TAG also provides basic validated examples for some common detector development tasks. Geant4 intercom modules, called messenger classes, manage these classes. To validate M-TAG, simulations were performed to calculate the range of positrons in water. One hundred million decays at the center of a water-filled sphere with a radius of 1 m were allowed for fluorine-18, carbon-11, oxygen-15 and gallium-68. These results were compared to literature values. An inexperienced Geant4 user was tasked with creating a simulation model for a plastic scintillator-based detector and conducting basic tests to assess the effectiveness of M-TAG as a teaching tool. The simulation involved calculating the dose to the detector's sensitive volume using a 2x2 cm planar monoenergetic photon source spanning energies from 20 to 100 keV. One billion particles were simulated twice: once with the actual detector geometry and once with the sensitive volume replaced by water. The validity of M-TAG was also verified by computing dose ratios and comparing them with mass-attenuation ratios obtained from NIST XCOM data sets.The mean positron travel distances were within the distribution of literature values. Simulated positron energy spectra means were within 1.8% of literature means. Simulated dose ratios agreed with literature values within uncertainties.We have developed and verified a modular Geant4 teaching aid called M-TAG. It was used to introduce a new user to Geant4, who used it to perform further validation simulations.

Optical imaging of decayed positrons and muons with different collimators

Although optical imaging of decayed positrons and muons can provide promising methods, it has been attempted only for muons without a collimator, and the beam characteristics with collimators, such as peak position or beam spread in depth and lateral directions, have not yet been evaluated. Therefore, we conducted optical imaging of decayed positrons and muons with different collimators. For the imaging of decayed positrons, Cherenkov-light imaging in fluorescein (FS) water was used, while imaging of a plastic scintillator block was used for the imaging of muons. We conducted these imaging trials during irradiation with 84.5-MeV/c positive muons to an FS water phantom or a plastic scintillator block using a cooled charge-coupled device (CCD) camera with each collimator of a different diameter attached to the beam port. We could measure the Cherenkov-light images of FS water of decayed positrons and optical images of muons using the plastic scintillator block for all collimators. The depth profiles of the Cherenkov-light images were slightly wider for the muons with the collimators of larger diameters, although the estimated peak depths were nearly the same for all collimators. The lateral profiles of the Cherenkov light were wider for the muons when using collimators of larger diameters. Asymmetry in the directions of positron emissions from the muons was observed for all collimators. The depth profiles of the optical image of muons using a plastic scintillator block had nearly the same shape. The estimated lateral widths of the optical images of the plastic scintillator block were the same sizes as the collimator diameters within a 1.1-mm difference at a 10-mm depth of the scintillator block, and the widths were wider at the Bragg peak. With these measured optical images, we conclude that Cherenkov-light imaging of decayed positrons in water and optical imaging of muons using a plastic scintillator block with collimators are useful methods for determining not only peak position but also beam width as well as the asymmetry of the directions of positron emissions from the muons.

Position distribution calculation of annihilation radiations and bremsstrahlung x rays in water during irradiation of positive muons: a Monte Carlo simulation study

Range, momentum and deviation of momentum determinations of muons are important for quality assessment (QA) of beams. Imaging of annihilation radiations emitted from positrons decayed from positive muons and that of bremsstrahlung x-rays emitted from positrons and secondary electrons from positive muons are possible methods of imaging muons. However, the energies and intensities as well as position distributions of these radiations have not been obvious. Thus we calculated the energy spectrum and the distributions of annihilation radiations as well as bremsstrahlung x-rays produced in water during irradiation of positive muons using Monte Carlo simulation. The calculations were conducted for 84.5 MeV /c positive muons, which is the same beam condition used in an experimental facility at the Japan Proton Accelerator Research Complex (J-PARC). We were able to calculate the energy spectrum as well as the position distributions of annihilation radiations and bremsstrahlung x-rays. The energy spectrum showed a broad distribution of bremsstrahlung x-rays, mainly from decayed positrons with an energy range up to 50 MeV with higher intensity in low-energy bremsstrahlung x-rays. The spectrum also showed a sharp peak at 511-keV from annihilation radiations. The position distribution of annihilation radiations was wider than those of the bremsstrahlung x-rays. The position distribution of the bremsstrahlung x-rays were nearly identical to the Cerenkov-light position distribution emitted by the decayed positrons in water. We conclude that imaging of bremsstrahlung x-rays from decayed positrons by using an x-ray camera is a promising method for the QA of positive muons and that higher spatial resolution images of positron distributions will be measured than those measured by annihilation radiations.

Spatial profiles of positrons injected at low energies into water: influence of cross section models

We present a collated set of interaction cross sections for positrons in water, and study how the choice of ionisation energy sharing and of anisotropy in elastic cross sections influences low-energy spatial transport. A Monte Carlo code has been developed to model the transport of a beam of positrons injected at 60 eV into water, in which we compare several ionisation energy sharing models, and also vary the anisotropic scattering behaviour for elastic collisions. While the cross sections are primarily applicable to water vapour, we have investigated the inclusion of coherent elastic scattering which is present in liquid water. We present comparative profiles of positron number density, energy deposition, positronium formation, and secondary electron generation. Anisotropic scattering can increase radial diffusion by a factor of two, and a similar effect occurs as a result of coherent elastic scattering, though only at energies below the positronium formation threshold. The results emphasise the need for detailed knowledge of scattering cross sections that are differential in both scattering angles and energy transfer.

Effect of the magnetic field on positron range using GATE for PET-MR.

Positron range is an important spatial resolution limiting factor in PET. When imaging inside a magnetic field the positron range is non-uniformly affected. A decrease of the positron range is expected in the directions perpendicular to the direction of the magnetic field, whereas no variation is expected in the direction of the magnetic field. Monte Carlo simulations were performed to validate these expectations. GATE simulation package [1] (version 5.0.0) was used to calculate the annihilation distribution of positrons in water, lung and rib bone. A point source placed in the centre of a spherical phantom was simulated. Six different positron emitters were used: 11 C, 13 N, 15 O, 18 F, 68 Ga, 82 Rb. The simulations were performed without and with static magnetic field set in the axial direction for various field strengths. In total 105 annihilation events were simulated per configuration and the annihilation coordinates were obtained from the Geant4 output and analysed with Matlab. Previous investigations indicated an increase of the positron range along the axis of the magnetic field [2,3]. As an attempt to confirm the previous findings, the effect of the magnetic field on the positron emission range was investigated. GATE simulations of the positron annihilation process indicated a general reduction of the mean positron range inside the phantom volume, specifically for the high energy emitters 82 Rb and 68 Ga. The evaluation of the positron annihilation distance across the directions perpendicular to the magnetic field showed a reduction of the mean positron range, as theoretically expected [4,5]. These results are compared and found in accordance with the theoretical expectations [2,3]. Contrary to previously published results [2,3], no increase in the positron range along the direction of the magnetic field is detected. The results of this study can be used to improve positron range correction algorithms for simultaneous PET-MR acquisition.

Positron follow-up in liquid water: I. A new Monte Carlo track-structure code

When biological matter is irradiated by charged particles, a wide variety of interactions occur, which lead to a deep modification of the cellular environment. To understand the fine structure of the microscopic distribution of energy deposits, Monte Carlo event-by-event simulations are particularly suitable. However, the development of these track-structure codes needs accurate interaction cross sections for all the electronic processes: ionization, excitation, positronium formation and even elastic scattering. Under these conditions, we have recently developed a Monte Carlo code for positrons in water, the latter being commonly used to simulate the biological medium. All the processes are studied in detail via theoretical differential and total cross-section calculations performed by using partial wave methods. Comparisons with existing theoretical and experimental data in terms of stopping powers, mean energy transfers and ranges show very good agreements. Moreover, thanks to the theoretical description of positronium formation, we have access, for the first time, to the complete kinematics of the electron capture process. Then, the present Monte Carlo code is able to describe the detailed positronium history, which will provide useful information for medical imaging (like positron emission tomography) where improvements are needed to define with the best accuracy the tumoural volumes.

Moving from organ dose to microdosimetry: contribution of the Monte Carlo simulations

When living cells are irradiated by charged particles, a wide variety of interactions occurs that leads to a deep modification of the biological material. To understand the fine structure of the microscopic distribution of the energy deposits, Monte Carlo event-by-event simulations are particularly suitable. However, the development of these track structure codes needs accurate interaction cross sections for all the electronic processes: ionization, excitation, Positronium formation (for incident positrons) and even elastic scattering. Under these conditions, we have recently developed a Monte Carlo code for electrons and positrons in water, this latter being commonly used to simulate the biological medium. All the processes are studied in detail via theoretical differential and total cross sections calculated by using partial wave methods. Comparisons with existing theoretical and experimental data show very good agreements. Moreover, this kind of detailed description allows one access to a useful microdosimetry, which can be coupled to a geometrical modelling of the target organ and then provide a detailed dose calculation at the nanometric scale.

On the Mechanism of Formation of Intratrack Yields of Water Radiolysis Products upon Irradiation with Fast Electrons and Positrons: 2. Concentration and Time Dependences of Yields

A mathematical model for the formation of main transient and final radiolysis products generated in tracks of fast electrons and positrons in water and aqueous solutions was constructed and described in terms of equations of inhomogeneous chemical kinetics in part 1 of this study. The model takes into account the reactions of a solute with epithermal electrons, thermal, and hydrated electrons; the ambipolar character of diffusion of charged intratrack particles; and new pathways of the formation of hydrogen and positronium due to the appearance of weakly bound states of electrons. In the present paper, the model was quantitatively fitted to experimental data on both time variation in the yields of radiolytic products (H3O+, e aq − , H, OH, OH−, H2O2) in pure water and the yields of hydrogen (H2, H), hydrated electron (e aq − ) and positronium (Ps) in various dilute and concentrated aqueous solutions.

On the mechanism of formation of intratrack yields of water radiolysis products upon irradiation with fast electrons and positrons: 1. Model formulation

A mathematical model is constructed for the formation of main transient ( , H, OH) and final ( H 2 , H 3 O + , OH - , Ps) radiolytic products formed in tracks of fast electrons and positrons in water and aqueous solu- tions. The model takes into account the occurrence of reactions of epithermal electrons, along with thermal and hydrated electrons, with a solute; the ambipolar character of diffusion of charged intratrack particles; and new pathways of the formation of hydrogen and positronium due to the appearance of weakly bound adducts of elec- trons with their scavengers, in addition to the traditional routes.

Comparison of vasodilatory effect of carbon dioxide inhalation and intravenous acetazolamide on brain vasculature using positron emission tomography

Carbon dioxide (C02) and acetazolamide are increasingly being used as vasodilators to detect cerebrovascular reserve capacity in patients of chronic cerebrovascular disease. The functional cerebrovascular reserve or ability of cerebral vessels to lower their resistance in response to decrease in cerebral perfusion pressure is expressed as change in cerebral blood flow from baseline under a vasodilatory stimuli. Theoretically a vasodilator causing maximum vasodilation, and thereby expressing complete reserve capacity would be more suitable for such a purpose. We quantitatively compared the vasodilating effect of 5% CO2 inhalation and 1 g of intravenous acetazolamide by positron emission tomography. Cerebrovascular reserve was quantified in six patients with chronic cerebrovascular disease in the same sitting, using oxygen-15 labeled water (H2 150) positron emission tomography at rest, during 5% CO2 inhalation and after 7 g intravenous acetazolamide. A significant linear correlation in both nonlesion hemisphere (r = 0.7071 p< 0.007) and in lesion hemisphere (r = 0.6261 p< 0.005) was found between CO2 and acetazolamide for cerebrovascular reserve capacity. This correlation improved by considering cerebrovascular reserve per unit change in arterial carbon dioxide (r = 0.7441 P<·0.007 in nonlesion hemisphere and r = 0.7271 P< 0.001 in lesion hemisphere). The quantitative value of global reserve capacity was different by CO2 stimuli (5.2%) and acetazolamide (49.7%). Though a similar vasodilatory response is elicited by both vasodilators, acetazolamide seems to be more potent and therefore should be preferred to detect patients with exhausted cerebrovascular reserve capacity. [Neural Res 1997; 19: 139–144]

Monte Carlo Calculations of Radical Proximity Functions around Thermalising Positrons in Water

Monte Carlo Calculations of Radical Proximity Functions around Thermalising Positrons in Water

Temperature dependence of positron lifetime in water

According to our measurements, the long lifetime τ 2 of positrons in water shows a sudden transition at about 4°C. The transition in the peak-rate of angular correlation of annihilation photons is situated at 0°C.

Positrons annihilation in water and ice with an external magnetic field

The angular distribution of 2γ annihilation of positrons in water and in ice has been measured with a 15 kG magnetic field. Measurements show that the magnetic field enhances the narrow component of the angular distribution in water. The angular distribution in ice seems to be not influenced by the magnetic field.