Galactic Cosmic Ray Radiation Research Articles

SPACE RADIATION-INDUCED BYSTANDER EFFECT IN ESTIMATING THE CARCINOGENIC RISK DUE TO GALACTIC COSMIC RAYS

Space radiobiology is an interdisciplinary science that examines the biological effects of ionizing radiation on humans involved in aerospace missions. The knowledge of the risk assessment of the health hazard related to human space exploration is crucial to reducing damages induced to astronauts from Galactic Cosmic Rays (GCRs) and sun-generated radiation. GCRs have been identified as one of the primary sources of radiation exposure in space. In this context, an accurate characterization of the possible risk of carcinogenesis induced by exposure to GCRs particles is mandatory for safe human space exploration, and one of the most crucial open problems is the contribution to carcinogenesis due to the effects on the cells directly and not directly irradiated, indicated as Target Effects (TEs) and Non-Target Effects (NTEs), respectively. It is accepted that the detrimental effects of ionizing radiation are not restricted only to the irradiated cells but also to non-irradiated distant cells manifesting various biological effects. Tumor Prevalence (TP) is often used to investigate the effects of NTEs in predictions of chronic GCR exposure risk. This paper reports the status of the research on this topic at the INFN Roma Sapienza Alpha Magnetic Spectrometer (AMS) research group, where is in progress an extensive study about the risk evaluation of the NTEs that the GCRs radiation will imply when added to the TE. A theoretical framework is presented for TP-induced NTEs modeling, ready to be used with the data collected from the AMS02 detector. Finally, a possible example of the use of the tool is shown for an accurate estimate of the tumor prevalence function of the exposure period for different typical space protons energies.

Immediate effects of acute Mars mission equivalent doses of SEP and GCR radiation on the murine gastrointestinal system-protective effects of curcumin-loaded nanolipoprotein particles (cNLPs).

Missions beyond low Earth orbit (LEO) will expose astronauts to ionizing radiation (IR) in the form of solar energetic particles (SEP) and galactic cosmic rays (GCR) including high atomic number and energy (HZE) nuclei. The gastrointestinal (GI) system is documented to be highly radiosensitive with even relatively low dose IR exposures capable of inducing mucosal lesions and disrupting epithelial barrier function. IR is also an established risk factor for colorectal cancer (CRC) with several studies examining long-term GI effects of SEP/GCR exposure using tumor-prone APC mouse models. Studies of acute short-term effects of modeled space radiation exposures in wildtype mouse models are more limited and necessary to better define charged particle-induced GI pathologies and test novel medical countermeasures (MCMs) to promote astronaut safety. In this study, we performed ground-based studies where male and female C57BL/6J mice were exposed to γ-rays, 50 MeV protons, or 1 GeV/n Fe-56 ions at the NASA Space Radiation Laboratory (NSRL) with histology and immunohistochemistry endpoints measured in the first 24 h post-irradiation to define immediate SEP/GCR-induced GI alterations. Our data show that unlike matched γ-ray controls, acute exposures to protons and iron ions disrupts intestinal function and induces mucosal lesions, vascular congestion, epithelial barrier breakdown, and marked enlargement of mucosa-associated lymphoid tissue. We also measured kinetics of DNA double-strand break (DSB) repair using gamma-H2AX- specific antibodies and apoptosis via TUNEL labeling, noting the induction and disappearance of extranuclear cytoplasmic DNA marked by gamma-H2AX only in the charged particle-irradiated samples. We show that 18 h pre-treatment with curcumin-loaded nanolipoprotein particles (cNLPs) delivered via IV injection reduces DSB-associated foci levels and apoptosis and restore crypt villi lengths. These data improve our understanding of physiological alterations in the GI tract immediately following exposures to modeled space radiations and demonstrates effectiveness of a promising space radiation MCM.

Astrocytes regulate vascular endothelial responses to simulated deep space radiation in a human organ-on-a-chip model.

Central nervous system (CNS) damage by galactic cosmic ray radiation is a major health risk for human deep space exploration. Simulated galactic cosmic rays or their components, especially high Z-high energy particles such as 56Fe ions, cause neurodegeneration and neuroinflammation in rodent models. CNS damage can be partially mediated by the blood-brain barrier, which regulates systemic interactions between CNS and the rest of the body. Astrocytes are major cellular regulators of blood-brain barrier permeability that also modulate neuroinflammation and neuronal health. However, astrocyte roles in regulating CNS and blood-brain barrier responses to space radiation remain little understood, especially in human tissue analogs. In this work, we used a novel high-throughput human organ-on-a-chip system to evaluate blood-brain barrier impairments and astrocyte functions 1-7 days after exposure to 600 MeV/n 56Fe particles and simplified simulated galactic cosmic rays. We show that simulated deep space radiation causes vascular permeability, oxidative stress, inflammation and delayed astrocyte activation in a pattern resembling CNS responses to brain injury. Furthermore, our results indicate that astrocytes have a dual role in regulating radiation responses: they exacerbate blood-brain barrier permeability acutely after irradiation, followed by switching to a more protective phenotype by reducing oxidative stress and pro-inflammatory cytokine and chemokine secretion during the subacute stage.

Evaluating Shielding Materials for High Energy Space Radiation

Space radiation is very hazardously harmful to human health and physically damaging to electronics equipment. The major radiation sources are Solar Particle Events (SPE) and Galactic Cosmic Rays/Radiation (GCR). GCR composed of high energy protons, alpha particles and electrons travelling from outside solar system are especially difficult to shield. Previous study has been shown that aluminum and higher atomic number structural alloys, provide fairly little shielding which may increase secondary radiation. In this work, we evaluate the range of materials for GCR shielding. Materials are aluminum, for its low atomic number, Liquid Hydrogen, Polyethylene,Water, Polybenzoxazine, Lithium hydride, Liquid Methane and Hydrogenated Graphite Nano fiber (HGNF) materials which containing high proportions of hydrogen which effectively slowing secondary neutrons and GCR radiation. Boron nitride storing hydrogen absorbs the secondary neutrons, GCR, and SPE effectively. Liquid Methane performance is also noticeable. The dose equivalent for each shielding materials are calculated using On-line Tool for the Assessment of Radiation In Space (OLTARIS) and High Charge and Energy TRaNsport (HZETRN) and compared for 2010 Solar minimum GCR. The results show that Liquid hydrogen is the good space radiation shielding material for a given thickness, followed by Lithium Hydride, Liquid Methane, Boron nitride storing hydrogen, Polyethylene, Polybenzoxazine, HGNF, and then aluminum. In addition, we plotted dose equivalent for Liquid hydrogen and Polyethylene at different densities.

On the Probable Change of the Status of the Current Unusual Failure of 11-Year Solar Cyclicity from Local to Global

The anomalous increase in the area of the 23rd solar cycle detected by galactic cosmic rays (GCRs) was a precursor of failure of the 11-year cyclicity: in accordance with the author’s hypothesis about the invariant of the 22-year cycle, an increase in the area of the 23rd cycle was followed by a decrease in the area (energy intensity) of the next 24th cycle. A decrease in energy intensity is accompanied by a decrease (relative variation) in the luminosity of the Sun, which in the 24th cycle has decreased to the level of the average value of the three previous cycles, i.e., almost twice. If the invariant of the 22-year cycle is preserved, the restoration of the 11-year cyclicity in the 25th cycle should be expected. If the invariant of the 22-year cycle is violated, the level of the GCR radiation background at the maximum of the forthcoming 25th cycle (from 2024 to 2025) should not be lower than the background radiation level at the maximum of the 24th cycle (2014). In this case, the state of the 23rd–24th–25th cycle unusual failure will change from local to global.

Consideration of particle wave diffraction to enhance spacecraft radiation shielding

Recent advances in atomic microscopy and interferometry have made evident the possibility that the wave-like behavior of Galactic Cosmic Ray (GCR) particle radiation and its secondary emissions can be exploited by diffraction through passive shielding materials to reduce the harmful impact to humans and equipment traveling in deep space. This research examines an additional physical mechanism to the well accepted “stopping power” and “nuclear collision” processes determined by the Bethe-Bloch and Bradt-Peters equations, in order to enhance shielding effectiveness: that the wave-like behavior of GCR radiation and its secondary emissions can be used to reduce the harmful impact of the radiation. Thus, it is shown theoretically possible to passively alter the path of some GCR particle radiation by diffraction, thereby directing them laterally and presenting the possibility that diffraction “safe zones” could be created behind a shielding material that may enhance its effectiveness. Preliminary analysis indicates the best application may be in shielding dangerous neutron secondary emissions.

The response of clouds and aerosols to cosmic ray decreases

AbstractA method is developed to rank Forbush decreases (FDs) in the galactic cosmic ray radiation according to their expected impact on the ionization of the lower atmosphere. Then a Monte Carlo bootstrap‐based statistical test is formulated to estimate the significance of the apparent response in physical and microphysical cloud parameters to FDs. The test is subsequently applied to one ground‐based and three satellite‐based data sets. Responses (>95%) to FDs are found in the following parameters of the analyzed data sets. AERONET: Ångström exponent (cloud condensation nuclei changes), SSM/I: liquid water content, International Satellite Cloud Climate Project (ISCCP): total, high, and middle, IR‐detected clouds over the oceans, Moderate Resolution Imaging Spectroradiometer (MODIS): cloud effective emissivity, cloud optical thickness, liquid water, cloud fraction, liquid water path, and liquid cloud effective radius. Moreover, the responses in MODIS are found to correlate positively with the strength of the FDs, and the signs and magnitudes of the responses agree with model‐based expectations. The effect is mainly seen in liquid clouds. An impact through changes in UV‐driven photo chemistry is shown to be negligible and an impact via UV absorption in the stratosphere is found to have no effect on clouds. The total solar irradiance has a relative decrease in connection with FDs of the order of 10−3, which is too small to have a thermodynamic impact on timescales of a few days. The results demonstrate that there is a real influence of FDs on clouds probably through ions.

Atmospheric radiation modeling of galactic cosmic rays using LRO/CRaTER and the EMMREM model with comparisons to balloon and airline based measurements

AbstractWe provide an analysis of the galactic cosmic ray radiation environment of Earth's atmosphere using measurements from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) aboard the Lunar Reconnaissance Orbiter (LRO) together with the Badhwar‐O'Neil model and dose lookup tables generated by the Earth‐Moon‐Mars Radiation Environment Module (EMMREM). This study demonstrates an updated atmospheric radiation model that uses new dose tables to improve the accuracy of the modeled dose rates. Additionally, a method for computing geomagnetic cutoffs is incorporated into the model in order to account for location‐dependent effects of the magnetosphere. Newly available measurements of atmospheric dose rates from instruments aboard commercial aircraft and high‐altitude balloons enable us to evaluate the accuracy of the model in computing atmospheric dose rates. When compared to the available observations, the model seems to be reasonably accurate in modeling atmospheric radiation levels, overestimating airline dose rates by an average of 20%, which falls within the uncertainty limit recommended by the International Commission on Radiation Units and Measurements (ICRU). Additionally, measurements made aboard high‐altitude balloons during simultaneous launches from New Hampshire and California provide an additional comparison to the model. We also find that the newly incorporated geomagnetic cutoff method enables the model to represent radiation variability as a function of location with sufficient accuracy.

Supply Voltage Decision Methodology to Minimize SRAM Standby Power Under Radiation Environment

In static random access memory, standby power is the summation of scrubbing and leakage powers. In terrestrial environments, the leakage power is more dominant than the scrubbing power. Hence, the conventional methodology to reduce SRAM standby power is to lower ${V_{DD}}$ to possible minimum voltage under process, voltage, and temperature variations. However, under severe radiation environments such as space, high scrubbing rate is indispensable to prevent the accumulation of soft-errors, making the scrubbing power have a substantial portion of total standby power. Since the soft-error rate becomes higher with the ${V_{DD}}$ scaling, the conventional methodology may not be valid under radiation environments. We present a methodology to decide optimal supply voltage with respect to standby power under radiation. We visualize our methodology under solar max/min galactic cosmic ray radiation environment of geosynchronous earth orbit and three error correction code (ECC) scenarios: Hamming code, double-error-correction (DEC) Bose–Chaudhuri–Hocquenghem (BCH) code, and triple-error-correction (TEC) BCH code. In 65 nm CMOS, Hamming code fails to deliver our target decoded bit-error-rate. Under other ECCs, the proposed methodology shows that 0.97 V (for DEC BCH) and 0.8 V (for TEC BCH) are optimal. Here, we can obtain 30% (for DEC BCH) and 60% (for TEC BCH) standby power savings compared to nominal voltage (= 1.2 V), respectively.

MSL-RAD radiation environment measurements.

In this study, results are presented from the on-board radiation assessment detector (RAD) of Mars Science Laboratory (MSL). RAD is designed to measure the energetic particle radiation environment, which consists of galactic cosmic rays (GCRs) and solar energetic particles (SEPs) as well as secondary particles created by nuclear interactions of primary particles in the shielding (during cruise) or Martian soil and atmosphere (surface measurements). During the cruise, RAD collected data on space radiation from inside the craft, thus allowing for a reasonable estimation of what a human crew travelling to/from Mars might be exposed to. On the surface of Mars, RAD is shielded by the atmosphere (from above) and the planet itself (from below). RAD measures the first detailed radiation data from the surface of another planet, and they are highly relevant for planning future crewed missions. The results for radiation dose and dose equivalent (a quantity most directly related to human health risk) are presented during the cruise phase, as well as on the Martian surface. Dose and dose equivalent are dominated by the continuous GCR radiation, but several SEP events were also detected and are discussed here.

Does the worsening galactic cosmic radiation environment observed by CRaTER preclude future manned deep space exploration?

AbstractThe Sun and its solar wind are currently exhibiting extremely low densities and magnetic field strengths, representing states that have never been observed during the space age. The highly abnormal solar activity between cycles 23 and 24 has caused the longest solar minimum in over 80 years and continues into the unusually small solar maximum of cycle 24. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of galactic cosmic rays in the space age and relatively small solar energetic particle events. We use observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter to examine the implications of these highly unusual solar conditions for human space exploration. We show that while these conditions are not a show stopper for long‐duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains a significant and worsening factor that limits mission durations. While solar energetic particle events in cycle 24 present some hazard, the accumulated doses for astronauts behind 10 g/cm2 shielding are well below current dose limits. Galactic cosmic radiation presents a more significant challenge: the time to 3% risk of exposure‐induced death (REID) in interplanetary space was less than 400 days for a 30 year old male and less than 300 days for a 30 year old female in the last cycle 23–24 minimum. The time to 3% REID is estimated to be ∼20% lower in the coming cycle 24–25 minimum. If the heliospheric magnetic field continues to weaken over time, as is likely, then allowable mission durations will decrease correspondingly. Thus, we estimate exposures in extreme solar minimum conditions and the corresponding effects on allowable durations.

An observation‐based GCR model of heavy nuclei: Measurements from CRIS onboard ACE spacecraft

We present an observation‐based elemental Galactic Cosmic Ray (GCR) heavy nuclei spectra model, based on ACE/CRIS measurements. Then we extrapolate the spectra model to the lower energy range of ACE/SIS instrument. In addition, we compare the modeling results with both the CRIS and SIS measurements. The flux of lower energetic particles measured by SIS are despiked since there are Solar Energetic Particle (SEP) events. The good agreement between the modeling and the observation results, especially for the solar minimum, indicates the validity of our model in the energy range 30–500 MeV/nuc. Compared with two GCR radiation environment models, CRÈME96/Nymmik model and Badhwar & O'Neill model, our model can provide an improved fit to the GCRs spectra measured by ACE. Furthermore, our model is a phenomenological one, without consideration of the physical process during GCRs' propagating through the heliosphere. Therefore, it is more straightforward and applicable in practice. Finally, the model can reproduce and predict the GCR spectra in the past and future, respectively, based on the strong correlation between the model parameters and solar activity indicated by sunspot number.

Physical basis of radiation protection in space travel

The health risks of space radiation are arguably the most serious challenge to space exploration, possibly preventing these missions due to safety concerns or increasing their costs to amounts beyond what would be acceptable. Radiation in space is substantially different from Earth: high-energy ($E$) and charge ($Z$) particles (HZE) provide the main contribution to the equivalent dose in deep space, whereas $\ensuremath{\gamma}$ rays and low-energy $\ensuremath{\alpha}$ particles are major contributors on Earth. This difference causes a high uncertainty on the estimated radiation health risk (including cancer and noncancer effects), and makes protection extremely difficult. In fact, shielding is very difficult in space: the very high energy of the cosmic rays and the severe mass constraints in spaceflight represent a serious hindrance to effective shielding. Here the physical basis of space radiation protection is described, including the most recent achievements in space radiation transport codes and shielding approaches. Although deterministic and Monte Carlo transport codes can now describe well the interaction of cosmic rays with matter, more accurate double-differential nuclear cross sections are needed to improve the codes. Energy deposition in biological molecules and related effects should also be developed to achieve accurate risk models for long-term exploratory missions. Passive shielding can be effective for solar particle events; however, it is limited for galactic cosmic rays (GCR). Active shielding would have to overcome challenging technical hurdles to protect against GCR. Thus, improved risk assessment and genetic and biomedical approaches are a more likely solution to GCR radiation protection issues.

Does the space environment affect the ecosphere?

The Sun is now emerging from a deep and protracted solar minimum, when the power, pressure, flux, and magnetic flux of solar wind were at their lowest levels [McComas et al., 2008; Schwadron and McComas, 2008;Connick et al., 2011]. Because of an anomalously weak heliospheric magnetic field and low solar wind pressure, galactic cosmic rays (GCRs)—protons, electrons, and ionized nuclei of elements accelerated to high energies—achieved the highest fluxes observed in the space age (Figure 1) [Mewaldt et al., 2010]. Related observations have shown remarkably rapid changes in the fluxes of energetic neutral atoms (ENAs) used by NASA's Interstellar Boundary Explorer mission to image the global heliosphere surrounding the solar system [McComas et al., 2010]. These changes in ENAs are caused by decreasing solar wind pressure. Does the recent anomalous deep solar minimum hint at larger changes in store? And how do changing GCR fluxes and conditions on the Sun influence Earth's ecosphere? Given the fact that GCR radiation can damage living tissue, causing cellular mutagenesis, the changing state of the Sun may have serious implications for life on the planet.

Galactic cosmic ray radiation hazard in the unusual extended solar minimum between solar cycles 23 and 24

[1] Galactic cosmic rays (GCRs) are extremely difficult to shield against and pose one of the most severe long-term hazards for human exploration of space. The recent solar minimum between solar cycles 23 and 24 shows a prolonged period of reduced solar activity and low interplanetary magnetic field strengths. As a result, the modulation of GCRs is very weak, and the fluxes of GCRs are near their highest levels in the last 25 years in the fall of 2009. Here we explore the dose rates of GCRs in the current prolonged solar minimum and make predictions for the Lunar Reconnaissance Orbiter (LRO) Cosmic Ray Telescope for the Effects of Radiation (CRaTER), which is now measuring GCRs in the lunar environment. Our results confirm the weak modulation of GCRs leading to the largest dose rates seen in the last 25 years over a prolonged period of little solar activity.

On the validity of the aluminum equivalent approximation in space radiation shielding applications

The origin of the aluminum equivalent shield approximation in space radiation analysis can be traced back to its roots in the early years of the NASA space programs (Mercury, Gemini and Apollo) wherein the primary radiobiological concern was the intense sources of ionizing radiation causing short term effects which was thought to jeopardize the safety of the crew and hence the mission. Herein, it is shown that the aluminum equivalent shield approximation, although reasonably well suited for that time period and to the application for which it was developed, is of questionable usefulness to the radiobiological concerns of routine space operations of the 21st century which will include long stays onboard the International Space Station (ISS) and perhaps the moon. This is especially true for a risk based protection system, as appears imminent for deep space exploration where the long-term effects of Galactic Cosmic Ray (GCR) exposure is of primary concern. The present analysis demonstrates that sufficiently large errors in the interior particle environment of a spacecraft result from the use of the aluminum equivalent approximation, and such approximations should be avoided in future astronaut risk estimates. In this study, the aluminum equivalent approximation is evaluated as a means for estimating the particle environment within a spacecraft structure induced by the GCR radiation field. For comparison, the two extremes of the GCR environment, the 1977 solar minimum and the 2001 solar maximum, are considered. These environments are coupled to the Langley Research Center (LaRC) deterministic ionized particle transport code High charge ( Z) and Energy TRaNsport (HZETRN), which propagates the GCR spectra for elements with charges ( Z) in the range 1 ⩽ Z ⩽ 28 (H–Ni) and secondary neutrons through selected target materials. The coupling of the GCR extremes to HZETRN allows for the examination of the induced environment within the interior of an idealized spacecraft as approximated by a spherical shell shield, and the effects of the aluminum equivalent approximation for a good polymeric shield material such as generic polyethylene (PE). The shield thickness is represented by a 25 g/cm 2 spherical shell. Although, one could imagine the progression to greater thickness, the current range will be sufficient to evaluate the qualitative usefulness of the aluminum equivalent approximation. Upon establishing the inaccuracies of the aluminum equivalent approximation through numerical simulations of the GCR radiation field attenuation for PE and aluminum equivalent PE spherical shells, we further present results for a limited set of commercially available, hydrogen rich, multifunctional polymeric constituents to assess the effect of the aluminum equivalent approximation on their radiation attenuation response as compared to the generic PE.

Geant4 Monte Carlo Simulations of the Galactic Cosmic Ray Radiation Environment On-Board the International Space Station/Columbus

A characterization of the Galactic cosmic ray (GCR) induced radiation environment on-board Columbus and the International Space Station (ISS) has been carried out using the Geant4 Monte Carlo particle transport toolkit and detailed geometry models of Columbus and ISS. Dose and dose equivalent rates, as well as penetrating particle spectra are presented. Simulation results indicate that the major part of the dose rates due to GCR protons are associated with secondary particles produced in the hull of ISS. Neutrons contribute about 15% of the GCR proton dose equivalent rate and mesons about 10%. More than 40% of the simulated GCR proton dose and dose equivalent rates are due to protons in the energy range above 10 GeV. Protons in the energy range above 50 GeV contribute only 5% to the dose rates. The total simulated dose and dose equivalent rates at solar maximum are 63 muGy/d and 123 muSv/d, respectively. The dose equivalent rate underestimates measurements made during the 2001 solar maximum. The discrepancy can be attributed to deficiencies in hadronic ion-nuclei interaction models for heavy ions and to the lack of such models above 10 GeV/N in Geant4.

The relative biological effectiveness of densely ionizing heavy-ion radiation for inducing ocular cataracts in wild type versus mice heterozygous for the ATM gene

The accelerated appearance of ocular cataracts at younger ages has been recorded in both astronauts and airline pilots, and is usually attributed to high-energy heavy ions in galactic cosmic ray radiation. We have previously shown that high-LET 1-GeV/nucleon (56)Fe ions are significantly more effective than X-rays in producing cataracts in mice. We have also shown that mice haploinsufficient for ATM develop cataracts earlier than wild-type animals, when exposed to either low-LET X-rays or high-LET (56)Fe ions. In this paper we derive quantitative estimates for the relative biological effectiveness (RBE) of high energy (56)Fe ions compared with X-rays, both for wild type and for mice haploinsufficient for ATM. There is a clear trend toward higher RBE's in haploinsufficient animals, both for low- and high-grade cataracts. Haploinsufficiency for ATM results in an enhanced sensitivity to X-rays compared with the wild type, and this enhancement appears even larger after exposure to high-LET heavy ions.

Ground-based simulations of galactic cosmic ray fragmentation and transport

Since mean free paths for nuclear fragmentation are of the order of the ranges of primary Galactic Cosmic Ray (GCR) nuclei, determination of the radiation field produced by successive fragmentations of nuclei in material and tissue is essential to accurate assessment of GCR radiation risk to humans on long-duration missions outside the geomagnetosphere. We describe some recent measurements made at the Bevalac of heavy ion transport through materials, with representative results and examples of how they may be applied to aspects of the space radiation problem, including efforts to devise analytical tools for predicting biological effects and for designing spacecraft shielding.

Galactic cosmic ray radiation levels in spacecraft on interplanetary missions

Using the Langley Research Center galactic cosmic ray (GCR) transport computer code (HZETRN) and the computerized anatomical man (CAM) model, crew radiation levels inside manned spacecraft on interplanetary missions are estimated. These radiation-level estimates include particle fluxes, LET (linear energy transfer) spectra, absorbed dose, and dose equivalent within various organs of interest in GCR protection studies. Changes in these radiation levels resulting from the use of various different types of shield materials are presented.