high-Z Materials Research Articles

A new model for predicting proton SEE cross sections based on heavy ion data

Based on the physical mechanism of the proton single event effects (SEEs), a new analytical model is proposed to predict the proton SEE cross sections of electronic devices from the heavy ion data. The experimental data of five SRAMs from our experiments and the literature are used to test the new model, and the predicted results agree well with them. The existing models only apply to the devices with low LET threshold whose proton SEE cross sections are dominated by the p + Si reactions, and cannot predict the angular effects. Compared to them, our model applies to the devices with from low to high LET threshold whose proton SEE cross sections are dominated by the proton interactions with silicon and/or high-Z materials, and can also predict the angular effects. Moreover, for two SRAMs with low LET threshold, our model performs much better than two classic models, the BGR and microdosimetry models, in predicting their proton SEE cross sections at normal incidence. A new physical concept called the indirect ionization power of protons, which is very helpful for understanding the proton SEE mechanism, is put forward and used in our model. It is expected that the new model will be widely used in studying the proton SEE mechanism, predicting the proton SEE cross sections and on-orbit error rates of electronic devices in space.

Feasibility study of a prototype muon tomography system based on a plastic scintillator and WLS fibers

Muon tomography is a technique for monitoring special nuclear material, such as uranium, inside the shielding material. It is a 3D imaging system capable of identifying atomic number (Z) using muon particles, high energy natural radiation. The following technique shows an advantage in no radiation exposure due to its usage of naturally occurring cosmic ray muons.In this study, a prototype muon tomography system was developed using four muon detectors in top and bottom structures. The four muon detectors based on a plastic scintillator and WLS fibers were implemented in the previous study. Two detectors above the target object and the other two below the target object measure a muon’s incoming and outgoing trajectories, respectively. Four detectors constituting the prototype were connected by coincidence to confirm the muon detection.The purpose of this study was to develop the prototype muon tomography system and to reconstruct the image of the high-Z material, such as lead and uranium. The performance of the prototype was confirmed by imaging under various conditions, such as measurement time, position, and thickness changes, using lead blocks. In addition, the feasibility of the prototype was verified by performing imaging using three materials with different atomic numbers, such as lead, iron, and aluminum.

Functional Materials for Fusion Nuclear Power Cores (A Technical Memorandum)

Nuclear energy science is nothing new, since World War II (WW II) and the successful Manhattan Project started in the deserts of New Mexico around 1940’s with Fermi introducing his experimental fission reactor in basement of University of Chicago in state of Illinois. Following that project and the exploring application of atom energy for peaceful goals using famous Einstein’s theory E=mc^2 has turned a new chapter for nuclear power energy by introducing a new source for electricity production. Fusion nuclear power generation that totally works in opposite direction that of fission nuclear power by splitting atoms nuclei of high Z-materials such as Uranium (U) or Plutonium (Pu), it uses the technology of thermonuclear reaction concepts driven by two isotopes of Hydrogen elements mainly Deuterium (D) and Tritium (T). Recent progress in research towards the development of fusion power driven by means of magnetic confinement approach, using reactor such as Tokamak device and the impressive advances research made in this particular reactor in the past few years, seems very promising to strike break-even of thermonuclear driven fusion reaction taking into consideration approach of Magnetic Confinement Fusion (MCF). Tokamak devices currently under construction will demonstrate the break-even condition or scientific feasibility of fusion power. Exciting and innovative ideas in mirror magnetic confinement are expected to culminate in high-Q devices making open-ended confinement a serious contender for fusion reactors. However, with such an innovative approach comes its own technical challenges associated with the machinery such as Tokamak that internally requires in a harsh temperature that in nature can be seen at the surface of our Sun, thus we are in need of materials that can sustain such high-temperature heat and be able to maintain their integrity and extend the life-cycle of such device while in full commercial operation within period of 24x7x360. Here in this Technical Memorandum (TM), we discuss potential materials that are possibly can deliver such sustainability one need them to operate within high-temperature and maintain their both physical and chemical properties from physics of solid-states point of view. With also today’s research in Nanotechnology and Memory-metals reaching to such a goal seems to be achievable, which is subject of this Technical Memorandum (TM)

Fabrication of Black Body Grids by Thick Film Printing for Quantitative Neutron Imaging.

Neutron imaging offers deep penetration through many high-Z materials while also having high sensitivity to certain low-Z isotopes such as 1H, 6Li, and 10B. This unique combination of properties has made neutron imaging an attractive tool for a wide range of material science and engineering applications. However, measurements made by neutron imaging or tomography are generally qualitative in nature due to the inability of detectors to discriminate between neutrons which have been transmitted through the sample and neutrons which are scattered by the sample or within the detector. Recent works have demonstrated that deploying a grid of small black bodies (BBs) in front of the sample can allow for the scattered neutrons to be measured at the BB locations and subsequently subtracted from the total measured intensity to yield a quantitative transmission measurement. While this method can be very effective, factors such as the scale and composition of the sample, the beam divergence, and the resolution and construction of the detector may require optimization of the grid design to remove all measurement biases within a given experimental setup. Therefore, it is desirable to have a method by which BB grids may be rapidly and inexpensively produced such that they can easily be tailored to specific applications. In this work, we present a method for fabricating BB patterns by thick film printing of Gd2O3 and evaluate the performance with variation in feature size and number of print layers with cold and thermal neutrons.

Development and commissioning of a compact Cosmic Ray Muon imaging prototype

Due to the muon tomography’s capability of imaging high Z materials, some potential applications have been reported on inspecting smuggled nuclear materials in customs. A compact Cosmic Ray Muons (CRM) imaging prototype, Lanzhou University Muon Imaging System (LUMIS), is comprehensively introduced in this paper including the structure design, assembly, data acquisition and analysis, detector performance test, and material imaging commissioning etc. Cast triangular prism plastic scintillators (PS) were coupled with Si-PMs for sensitive detector components in system. LUMIS’s experimental results show that the detection efficiency of an individual detector layer is about 98%, the position resolution for vertical incident muons is 2.5 mm and the angle resolution is 8.73 mrad given a separation distance of 40.5 cm. Moreover, the image reconstruction software was developed based on the Point of Closest Approach (PoCA) to detect lead bricks as our target. The reconstructed images indicate that the profile of the lead bricks in the image is highly consistent with the target. Subsequently, the capability of LUMIS to distinguish different materials, such as Pb, Cu, Fe, and Al, was investigated as well. The lower limit of response time for rapidly alarming high-Z materials is also given and discussed. The successful development and commissioning of the LUMIS prototype have provided a new solution option in technology and craftsmanship for developing compact CRM imaging systems that can be used in many applications.

Developing solid-surface plasma facing components for pilot plants and reactors with replenishable wall claddings and continuous surface conditioning. Part A: concepts and questions

It is estimated that pilot plants and reactors may experience rates of net erosion and deposition of solid plasma facing component (PFC) material of 103–105 kg yr−1. Even if the net erosion (wear) problem can be solved, the redeposition of so much material has the potential for major interference with operation, including disruptions due to so-called ‘unidentified flying objects (UFOs)’ and unsafe dust levels. The potential implications appear to be no less serious than for plasma contact with the divertor target: a dust explosion or a major UFO-disruption could be as damaging for an actively-cooled deuterium-tritium (DT) tokamak as target failure. It will therefore be necessary to manage material deposits to prevent their fouling operation. This situation appears to require a fundamental paradigm shift with regard to meeting the challenge of taming the plasma–material interface: it appears that any acceptable solid PFC material will in effect be flow-through, like liquid–metal PFCs, although at far lower mass flow rates. Solid PFC material will have to be treated as a consumable, like brake pads in cars. ITER will use high-Z (tungsten) armor on the divertor targets and low-Z (beryllium) on the main walls. The ARIES-AT reactor design calls for a similar arrangement, but with SiC cladding on the main walls. Non-metallic low-Z refractory materials such as ceramics (graphite, SiC, etc) used as in situ replenishable, relatively thin—of order mm—claddings on a substrate which is resistant to neutron damage could provide a potential solution for the main walls, while reducing the risk of degrading the confined plasma. Separately, wall conditioning has proven essential for achieving high performance. For DT devices, however, standard methods appear to be unworkable, but recently powder droppers injecting low-Z material ∼continuously into discharges have been quite effective and may be usable in DT devices as well. The resulting massive generation of low-Z debris, however, has the same potential to seriously disrupt operation as noted above. Powder droppers provide a unique opportunity to carry out controlled studies on the management of low-Z slag in all current tokamaks, independent of whether their protection tiles use low-Z or high-Z material.

Determination of double-layer gamma build-up factor using Monte Carlo code, FLUKA: Development of new empirical formula

Designing shield for coupled neutron and gamma radiation field is a complex problem as it involves combination of high-Z and low-Z materials in composite or laminar form with optimized thickness. In the present work we have utilized general purpose open-source Monte Carlo code, FLUKA for estimation of gamma exposure build-up factors (EBF) of single and multilayer shields. Excellent agreement of FLUKA calculated results of EBF for iron, water and poly-methyl methacrylate (PMMA) (for different energy point-isotropic gamma source) with standard codes/database like EGS4 and ANSI, demonstrates the validity and reliability of this code for the present purpose. Subsequently, the code is utilized for determination of double-layer exposure build-up factor (DEBF) of iron/PMMA shield. In this regard, a new empirical formula is constructed for predicting DEBF of any arbitrary combination of layer materials, applicable over wide range of layer thicknesses and gamma energies. The coefficients of thus constructed empirical formula have been associated with radiation interaction parameters of individual components. Further, the proposed formula is shown to predict DEBF of iron/water shield with good accuracy, thereby ascertaining the robustness of the formula. The quantitative accuracy with which the proposed formula is able to predict DEBF for both Iron/PMMA and Iron/Water shield lies within 12% of FLUKA calculated data.

Developing solid-surface plasma facing components for pilot plants and reactors with replenishable wall claddings and continuous surface conditioning. Part B: required research in present tokamaks

The companion part A paper (Stangeby et al 2022) reports a number of independent estimates indicating that high-duty-cycle DT tokamaks starting with pilot plants will likely experience rates of net erosion and deposition of solid PFC, plasma facing component, material in the range of 103 to 104 kg yr−1, regardless of the material used. The subsequent redeposition of such large quantities of material has the potential for major interference with tokamak operation. Similar levels and issues will be involved if ∼continuous low-Z powder dropping is used for surface conditioning of DT tokamaks, independent of the material used for the PFC armor. In Stangeby et al (2022) (part A) it is proposed that for high-duty-cycle DT tokamaks, non-metallic low-Z refractory materials such as ceramics (graphite, SiC, etc) used as in situ replenishable, relatively thin—of order mm—claddings on a substrate which is resistant to neutron damage could provide a potential solution for protecting the main walls, while reducing the risk of degrading the confined plasma. Assessment of whether such an approach is viable will require information, much of which is not available today. Section 6 of part A identifies a partial list of major physics questions that will need to be answered in order to make an informed assessment. This part B report describes R&D needed to be done in present tokamaks in order to answer many of these questions. Most of the required R&D is to establish better understanding of low-Z slag generation and to identify means to safely manage it. Powder droppers provide a unique opportunity to carry out controlled studies on the management of low-Z slag in current tokamaks, independent of whether their protection tiles use low-Z or high-Z material.

Liquid lithium wetting and percolation in a porous tungsten/liquid Li plasma facing component (PFC)

This work explores the concept of a hybrid material system designed to protect the plasma from high-Z material emission while also protecting an underlying high-Z wall material from erosion by integrating a low-Z component in the liquid phase. Plasma-surface interaction properties of a porous tungsten (W)-liquid metal hybrid system, having the favorable bulk thermomechanical properties of W while serving as a scaffold for a liquid metal with self-healing and radiative vapor shielding characteristics, are examined. W-substrates with 70% density of bulk W and 1–5 μm sized pores have been fabricated with 50-nm W powders using spark plasma sintering. Enhanced lithium (Li) wettability, driven by percolation through the porous W architecture, is demonstrated with in-situ liquid Li drop measurements. Results show complete wetting of liquid Li at 250 °C in the porous W, 100 °C lower temperature than what has been observed on traditional W surfaces. In operando ion beam analysis of Li films deposited on porous W substrates provides microscopic evidence of Li percolation into the substrate and a first look at D retention up to a depth of 100 nm after plasma exposure at a flux of 1020 m−2s−1, 250 eV/amu D+ plasma to a fluence of 1.16 × 1023 m−2 at a temperature of 200 °C.

Neutron transmission imaging with a portable D-T neutron generator

A portable fast-neutron imaging system is being developed to provide complementary information to field X-ray imaging. Applications include inspection of vehicles and infrastructure for corrosion, measurement of material levels in containers, and inspection of munitions and suspicious packages. While fast-neuron imaging generally provides lower imaging resolution compared to X-rays, fast-neutron interaction cross-sections have a weak dependence on material Z. This enables imaging of low-Z materials inside high-Z materials. Here, we discuss the limitations and current improvements in fast-neuron imaging. Limitations in portable fast-neutron imaging systems include low D-T neutron generator output, low light production in ZnS(Cu) imaging scintillators, low resolution due to scintillator thickness and D-T spot size, and digital-panel darknoise that varies in time and position and that can be 100× larger than the neutron signal. We have made improvements in these areas through development of a segmented high light yield scintillator, panel noise mitigation techniques, and testing of new high-output, small spot size D-T neutron generators. The segmented high light yield fast-neutron scintillator demonstrated 5× increase in light compared to ZnS(Cu). An additional 2× improvement in signal-to-noise was demonstrated with panel-noise mitigation techniques. Our MCNP calculations also show good agreement with neutron imaging results We have demonstrated improvements in fast-neutron imaging through development of a segmented high light yield neutron scintillator, mitigation of digital panel noise, and preliminary testing with new high-output, small spot size D-T neutron generators. We have also demonstrated good results modeling fast-neutron images and scatter effects using MCNP.

Proceedings to the 7th Annual Conference of the Particle Therapy Cooperative Group North America (PTCOG-NA)

Proceedings to the 7th Annual Conference of the Particle Therapy Cooperative Group North America (PTCOG-NA)

Monte Carlo simulations of energy, time and spatial evolution of primary electrons generated by 511 keV photons in various scintillators

The intermediate phases between the initial interaction of ionizing radiation in scintillators and the production of scintillation light have been extensively studied over the years. Observable features of scintillators such as energy and time resolution have been in part explained through pre-scintillation mechanisms. Whereas the early energy transfer processes following the photoelectric (or Compton) interaction of an energetic photon with an atomic electron in scintillator materials were fully described, their spatio-temporal characteristics have had limited quantification. For 511 keV photon interaction in (TOF-)PET detectors, the production of a primary electron carrying most of the energy leads to non-localized energy deposit distribution having some temporal extent. Here, we characterize the spatial, temporal and energy evolution of the primary electron generated by 511 keV photons in different scintillators using Monte Carlo simulations. We aim to provide a general quantitative description detailing the dynamics of energy transfer, such as the electron ejection angle, track length and tortuosity, time course and remaining energy as a function of the initial interaction point. Simulation results show that the electron track is tortuous with a short extent and a rapid energy transfer within 100 μm in high-Z (e.g., BGO) or high density (e.g., LuAP:Ce) materials while the extent nearly doubles in lighter materials like BaF2 in which the track tortuosity is 10%–50% lower. The time fluctuations of the photoelectron full energy deposit remain below 1 ps FWHM, therefore adding negligible jitter to the time resolution of coincident detectors. These simulations provide a better understanding of the main carrier dynamics from simple characteristics as some detector concepts require low-range, contained energy deposit (e.g., pixelated crystal arrays) whereas others benefit from a greater extent of energy deposition (e.g., structures designed for energy sharing).

Physics of runaway electrons with shattered pellet injection at JET

Runaway electrons (REs) created during tokamak disruptions pose a threat to the reliable operation of future larger machines. Experiments using shattered pellet injection (SPI) have been carried out at the JET tokamak to investigate ways to prevent their generation or suppress them if avoidance is not sufficient. Avoidance is possible if the SPI contains a sufficiently low fraction of high-Z material, or if it is fired early in advance of a disruption prone to runaway generation. These results are consistent with previous similar findings obtained with Massive Gas Injection. Suppression of an already accelerated beam is not efficient using High-Z material, but deuterium leads to harmless terminations without heat loads. This effect is due to the combination of a large magnetohydrodynamic instability scattering REs on a large area and the absence of runaway regeneration during the subsequent current collapse thanks to the flushing of high-Z impurities from the runaway companion plasma. This effect also works in situations where the runaway beam moves upwards and undergoes scraping-off on the wall.

Evaluation of scattering effects for radiation shielding or filter materials by using Monte Carlo simulation

Most radiology departments utilize ordinary concrete and lead for radiation shielding as the primary radiation can be reduced through photon absorption. There are many studies done focusing on the transmitted photons that penetrate the shielding materials for radiation shielding. However, the scattering from the shielding materials would be ignored. When high-energy photons impinge on thick shields, most of the incident energy is absorbed in the shielding materials, but some of it can also be deflected sideways or in a backward direction. This is important as the backscatter radiation can contribute to unnecessary additional radiation dose to healthcare workers. Hence, this study evaluates several shielding materials namely aluminium, iron, copper, lead, ordinary concrete, and heavy concrete particularly for its attenuated and scattered photons for radiation shielding. The shielding materials were evaluated using the Monte Carlo simulation, specifically PHITS code. In the simulation, all shielding materials were modelled as a fixed 30 x 30 cm rectangular shape with a fixed thickness of 10 cm. Mono-energy and pencil beam photon energies ranging from 100 keV until 1 MeV were directed to the shielding materials. As a result, at 100 keV, lead shielding showed the least amount of transmitted dose compared to other shielding materials. However, lead shielding also showed the highest reflected dose at the same incident photon energy. As copper showed the least amount of reflected dose at this incident energy, hence applying a thin layer of this material to lead shielding can tolerate the compromise between low transmitted dose and high reflected dose. Therefore, this can improve the radiation shielding at various irradiation facilities. In conclusion, the reflected dose for all materials studied will increase or higher when the incident photon energy increase, except for lead as well as for low-Z element materials rather than high-Z element materials.

A novel conceptualization in the analysis and design of passive neutron area monitors based on gold foil activation

In the detection and measurement of neutron fields, with energies between 10 and 20 MeV, current passive neutron area monitors based on gold foil sensors usually do not have a perfect fitting of their dose response functions with the neutron fluence-to-ambient dose equivalent conversion function from ICRP74. However, apart from the radiative capture in 197Au, the common channel considered in these monitors, other nuclear reactions in 197Au can be considered to improve the fit between both functions. Therefore, this work aimed to develop a mathematical combination of response functions in passive monitors with gold foils, considering the (n, γ) and (n, 2n) channels in 197Au, to extend their response up to 20 MeV, improving their performance under neutron fields with high energies. The proposed methodology avoids introducing modifications in the original device, such as the insertion of sheets with high-Z materials, and simplifies the design and manufacturing of passive monitors, while reducing costs.

Computational and Experimental Approaches for Evaluating Dose under a Block in TBI Geometry

Total Body Irradiation (TBI) patients are often treated at extended distances of several meters, with blocking made from high-Z materials placed close to the patients’ skin. Evaluating the dose under a block (e.g., for implanted medical device shielding purposes) in such a geometry is challenging. We compare the performance of two commonly used dose calculation algorithms, Anisotropic Analytical Algorithm (AAA) and Acuros XB, with Optically Stimulated Lumine- scence (OSLD) and ion chamber measurements in phantoms. The calculations and phantom measurements are also compared with in-vivo OSLD measure- ments. We find that OSLD and ion chamber measurements in phantom are good predictors of in-vivo measurements, while both AAA and Acuros XB sys- tematically overestimate the block transmission. We found Acuros XB to be accurate enough for a rough upper estimate (dose under block overestimated by 7% - 22%), while for AAA the overestimate was more severe (90% - 110%); the reason is that AAA does not account for the increase in pair production cro- ss-section in high-Z materials.

Quality assessment of cadmium telluride as a detector material for multispectral medical imaging

Cadmium telluride (CdTe) is a high-Z material with excellent photon radiation absorption properties, making it a promising material to include in radiation detection technologies. However, the brittleness of CdTe crystals as well as their varying concentration of defects necessitate a thorough quality assessment before the complex detector processing procedure. We present our quality assessment of CdTe as a detector material for multispectral medical imaging, a research which is conducted as part of the Consortium Project Multispectral Photon-counting for Medical Imaging and Beam characterization (MPMIB). The aim of the project is to develop novel CdTe detectors and obtain spectrum-per-pixel information that make the distinction between different radiation types and tissues possible. To evaluate the defect density inside the crystals — which can deteriorate the detector performance — we employ infrared microscopy (IRM). Posterior data analysis allows us to visualise the defect distributions as 3D defect maps. Additionally, we investigate front and backside differences of the material with current-voltage (IV) measurements to determine the preferred surface for the pixelisation of the crystal, and perform test measurements with the prototypes to provide feedback for further processing. We present the different parts of our quality assessment chain and will close with first experimental results obtained with one of our prototype photon-counting detectors in a small tomographic setup.

Image quality evaluation of multimodal imaging of muon

Both the information about the scattering of muons due to their interaction with material and the information about the material-stopped muons generating secondary induced neutrons effectively are used for multimodal imaging of muon. In order to evaluate the image quality of multimodal imaging of muon, the detection model is established based on Geant4 and the reliability of the detection model is verified. Both the multiple Coulomb scattering module and the muon induced neutron module prove to be reliable. The multimodal imaging simulation program is developed, and the images are reconstructed on the basis of the simulated data. Four imaging models are developed. The first model is a line pair model used to study the spatial resolution of reconstructed images with imaging time ranging from two hours to two weeks. The line pair model is composed of <sup>235</sup>U and the length of each line pair is set to be 100 mm. The cross sections are set to be 4<sup>2</sup>, 4<sup>2</sup>, 6<sup>2</sup>, 6<sup>2</sup>, 10<sup>2</sup>, 10<sup>2</sup>, 20<sup>2</sup>, and 20<sup>2</sup> mm<sup>2</sup>, respectively. The second model is a cube model used to study the material resolution of reconstructed images with imaging time ranging from one hour to twelve hours. The side length of each cube is 100 mm. The third model is the cladding model used to test the reliability of multimodal imaging images in complex shielding situations. The outermost layer is of lead, with the side length being 140 mm and the thickness 40 mm. The middle layer is of iron, with the side length being 100 mm and the thickness 40 mm. The innermost layer of <sup>235</sup>U, with the side length being 60 mm. The last letter model is used to calculate the structural similarity of reconstructed images, with imaging time ranging from half an hour to twelve hours. The letter model is made of <sup>235</sup>U and consists of cubes with side length of 50 mm. The letters “E” and “P” are made up of 16 cubes and 15 cubes respectively. The spatial resolution reaches 4 mm when imaging time is within 12 hours. The <sup>235</sup>U and other common high-z, medium-z, and low-z material can be distinguished when imaging time is on the order of hours. Muon scattering imaging image of the cladding model will cause misjudgment. However, the multimodal imaging image can correctly reflect the existence of <sup>235</sup>U. The structure similarity between the reconstructed image and the reference image in different imaging times proves that multimodal imaging has higher quality than single imaging method. The study indicates that the multimodal imaging of muon has better imaging quality, can adapt to more complex imaging scenes and has more advantages in the detection and recognition of special nuclear material than muon imaging method with single interaction information.

Multi-Energy Gamma-Ray Attenuations for Non-Destructive Detection of Hazardous Materials

We present a non-destructive method (NDM) to identify minute quantities of high atomic number (Z) elements in containers such as passenger baggage, goods carrying transport trucks, and environmental samples. This method relies on the fact that photon attenuation varies with its energy and properties of the absorbing medium. Low-energy gamma-ray intensity loss is sensitive to the atomic number of the absorbing medium, while that of higher-energies vary with the density of the medium. To verify the usefulness of this feature for NDM, we carried out simultaneous measurements of intensities of multiple gamma rays of energies 81 to 1408 keV emitted by sources 133Ba (half-life = 10.55 y) and 152Eu (half-life = 13.52 y). By this arrangement, we could detect minute quantities of lead and copper in a bulk medium from energy dependent gamma-ray attenuations. It seems that this method will offer a reliable, low-cost, low-maintenance alternative to X-ray or accelerator-based techniques for the NDM of high-Z materials such as mercury, lead, uranium, and transuranic elements etc.

Plasma-wall self-organization in magnetic fusion

This paper introduces the concept of plasma-wall self-organization (PWSO) in magnetic fusion. The basic idea is the existence of a time delay in the feedback loop relating radiation and impurity production on divertor plates. Both a zero and a one-dimensional description of PWSO are provided. They lead to an iterative equation whose equilibrium fixed point is unstable above some threshold. This threshold corresponds to a radiative density limit, which can be reached for a ratio of total radiated power to total input power as low as 1/2. When detachment develops and physical sputtering dominates, this limit is progressively pushed to very high values if the radiation of non-plate impurities stays low. Therefore, PWSO comes with two basins for this organization: the usual one with a density limit, and a new one with density freedom, in particular for machines using high-Z materials. Two basins of attraction of PWSO are shown to exist for the tokamak during start-up, with a high density one leading to this freedom. This basin might be reached by a proper tailoring of ECRH assisted ohmic start-up in present middle-size tokamaks, mimicking present stellarator start-up. In view of the impressive tokamak DEMO wall load challenge, it is worth considering and checking this possibility, which comes with that of more margins for ITER and of smaller reactors.