NIR Environmental Vegetation Monitoring For Ecosystems and Precision Agriculture

Using Unmanned Aerial Vehicles (UAVs or "Drones") we can do scanning in the visual spectrum of large areas in the surrounding environment without the high comparative cost of using manned vehicles such as helicopters or planes or expensive space-based monitoring infrastructure. UAVs allow for fast, local and energy efficient surveillance for environmental monitoring.

In the case of ecosystem analysis, it is key to have active monitoring of plant health and distribution in order to gauge the health of an ecosystem and the support potential for the various species that depend on healthy, diverse and broad growth of vegetation.

Applications of UAVs in environmental protection, forestry and plant agriculture agriculture, forestry and environmental protection include:

• Local Ecosystem Monitoring
• High Frequency Vegetation Growth Analysis
• Monitoring of Agricultural Impact on Environment
• Empiracle Measurement of Unidentified Vegetation Die-off (UVD)
• Monitoring of Agricultural Impact on Environment
• Monitoring Health of Agricultural Plant Crops
• Detection of Water/Soil Stress on Plants
• Early Detection of Disease and Pest impact on Plants
• Monitoring Pollution and Spill Impacts on Plants

UAV Drones can provide a fast, cheap, efficient (both in energy and in time) and yet very effective way to perform environmental diagnostics and information retrieval without depending on more complex infrastructure such as satellite and manned aircraft.

UAVs can be used in crop monitoring and within what is called "precision agriculture", which works in order to optimize plantation management and assess more accurately the optimum density planting, in addition to making decisions regarding the use of fertilizers, irrigation frequency and other possibilities, such as to predict more accurately the crop production and allowing for sustainable use of the limited resources of water, soil and land available for agriculture.

The UAV can perform scheduled flights to carry out surveys of areas of vegetation (an indicated use, for example, for the of monitoring vulnerable ecosystems), and compare the spectral data taken from sensor cameras with available visual tomography, provided by accurate and updated maps, to detect and deduce root causes of detected instances of stress in the areas of plant growth.

In the case of the visual tomography we can update existing maps with relative ease by simply referencing 2D aerial or satellite survey maps with the measured heights at certain locations in order to predict water flow and effects of soil creeping and leeching. Several map providers and simulation tools exist for modelling water flows in an environment, all of which is a discussion for another time.

However, with the sensor information we need to think a little more abstractly and find key variables which indicate the state of plant health.

Near-Infrared (NIR) and Plant Health

Near-Infrared, NIR is a small portion of the much larger region called infrared (IR), located between the visible and microwave portions of the electromagnetic (EM) spectrum.

NIR makes up the part of IR closest in wavelength to visible light and occupies the wavelengths between about 700 nanometers and 1500 nanometers (0.7 µm – 1.5 µm). NIR is not to be confused with thermal infrared, which is on the extreme other end of the infrared spectrum and measures radiant (emitted) heat.

In most commercially available cameras, most of which based on silicon semiconductor Charged-Coupled Devices (CCD) detectors, absorb visible light from about 390nm until about 1200 nm,

NIR radiation can be blocked by special glass or plastic windows designed to ensure natural colour images are produced without undesired shifts towards the extremes of  the red or blue parts of the EM spectrum, the shift depending on the nature of the semiconductor CCD sensor. So-called "hot mirrors" are used to negate the influence of the NIR on camera images which would otherwise induce a reddening effect.

A replacement of the hot mirror by a neutral glass or plastic filter can therefore be used for infrared photography. As a consequence of this a significant increase of commercial camera modularity can be achieved by this special optical set up and users can decide whether they want to generate natural colour (true colour) or near infrared (NIR) images, depending on the external lens filter sets applied. This allows the camera to detect the infrared light necessary for producing NDVI images.

Using multispectral and in particular near-infrared (NIR) surface reflectance cameras, the various monitoring parameters for vegatation can be gained easily, sometimes in a single flight sweep over an area, to generate quality indicators of plant health.

When studying the vegetation reflectance spectrum in detail, The near-infrared spectrum itself allows a relatively high detailed probe of the health of plants at a cellular level.

In photosynthesis, the chloroplasts in plant cells take absorb light energy sing chlorophyll which aborbes photons and creates and electronic channel to fix carbon and water to form glucose, the basis of carbohydrates and hence food for the plant.

The forms of chlorophyll in plants absorb light at specific frequencies, typically, in the red and blue light. The green portion of light is effectively reflected, this makes the plant was seen in the range of green in the visible spectrum.

This information is also important for developing lighting systems, for indoor agriculture for example, of which light frequencies are the most efficient for growing plants as exploited by so-called "grow lights" for indoor plant growth. This is another topic in precision agriculture which can be explored further.

In any case, Chlorophyll pigments A and B absorbs most energy at about 450 nm (blue) and 650 nm (red) respectively, with significant overlap between the 2 as shown in the diagram above. Other pigments absorb more visible wavelengths, but the most absorption occurs in the red and blue portions of the spectrum. This absorption removes these colors from the amount of light that is transmitted and reflected, causing the predominant visible color that reaches our eyes as green. This is the reason healthy vegetation appears as a dark green.

Unhealthy vegetation, on the other hand, will have less chlorophyll and thus will appear brighter (visibly) since less is absorbed and more is reflected to our eyes. This increase in red reflectance along with the green is what causes a general yellow appearance of unhealthy plants.

Plants reflect strongly in the NIR however not because of chlorophyll but because of a spongy layer of lignin found on the bottom surface of the leaf, but not strongly in the red.

IR reflectance is advantageous to plants, as it is reflects electromagnetic energy that the plant cannot use and moreover would probably damage the plant tissues, especially during high levels of sunshine where the IR radiation would heat the plant tissues and slow down or damage cellular processes.

Plant stress causes an increase in visible light transmission in the green as the chlorophyll decays and the much more obvious effect of an increase in the reflectance of red, this is why the leaves of deciduous trees turn orange and red when they die-off in the autumn.

The near-infrared plateau (NIR, 700 nm - 1100 nm), is a region where biochemical absorptions are limited to the compounds typically found in dry leaves, primarily cellulose, lignin and other structural carbohydrates.

However, and this is critical, NIR reflection in this region is also affected by multiple scattering of photons within the leaf, related to the internal cellular structure, fraction of air spaces in the xylem vessels of the plant, and most important as a general indicator, the air-water interfaces that refract light within leaves. The reflectance and transmittance in the middle-infrared also termed the shortwave-infrared (SWIR, 1100 nm - 2500 nm) is also a region of strong absorption, primarily by water in green leaves. The primary and secondary absorptions of water in leaf reflectance are greatest in spectral bands centered at 1450, 1940, and 2500 nm, with important secondary absorptions at 980 nm, and 1240 nm (Carter, 1991). These are the bands which create the primary NIR reflectance in healthy plants.


The cohesion-adhesion model of water transport in vascular plant tissue describes how hydrogen bonding in water to explain many key components of fluid movement through the plant's xylem and other vessels.

Within a vessel, water molecules hydrogen bond not only to each other, but also to the cellulose chain itself which comprises the wall of plant cells. This creates a capillary tube which allows for capillary action to occur since the vessel is relatively small. This mechanism allows plants to pull water up into their roots. Furthermore,hydrogen bonding can create a long chain of water molecules which can overcome the force of gravity and travel up to the high altitudes of leaves.

Cohesion-adhesion model of water transport in cellulose-based plant vascular tissue 

Therefore the existing interface between the water in plants and the cellulose chains is a key indicator that a plant is #1 not under dehydration and #2 has structural integrity. If any one of these factors are removed, this indicates poor plant health and this is detected by the reflectance in the NIR.

Soil, on the other hand, reflects both NIR and Red . However, when a plant becomes dehydrated or sickly, the spongy layer collapses and the lack of water itself and its interfacing with the plant all contributes to ceasing to reflect as much NIR light. Thus, a combination (approximated as linear) of the NIR reflectivity and red reflectivity should provide excellent contrast between plants and soil and between healthy plants and sick plants.

The Normalized Difference Vegetation Index (NDVI) is a simple graphical indicator that can be used to analyze remote sensing measurements, such as in aerial and space based surveys, and assess whether the target being observed contains live green vegetation or not.

It turns out which combination is not particularly important, but the NDVI index of (NIR-red)/(NIR+red) does happen to be particularly effective at normalizing for different irradiation conditions.

Hence, plants in a given area that are adequately hydrated show high absorbance of NIR light in this absorbance band (and low reflectance), whereas those subject to drying shows greater reflectance in this band.

Specifically, NDVI was developed by a NASA scientist Dr. Compton Tucker in a 1977 paper
entitled, “Red and Photograghic Infrared Linear Combinations for Monitoring Vegetation.”

Tucker examined 18 different combinations of NIR (Landsat MSS 7 800-1100 nm), red (Landsat MSS 5 600-700 nm), and green (Landsat MSS 4 500-600 nm) and compared these results with the density of both wet and dry biomass to in an attempt determine which combination correlated best.

His findings were that

• NIR/red, 
• SQRT(NIR/red), 
• NIR-red,
• (NIR-red)/(NIR+red), 
• SQRT((NIR-red)/(NIR+red)+0.5) 

were all very similar indicators for estimating the density of photosynthetically active biomass.

Using this a threshold level for near IR reflectance from healthy plants can be deduced, allows for a way to label plant health remotely.

Using a near-infrared spectral camera, a drone can easily monitor vegetation for signs of sickness and determine the health of both agricultural crops and the plants at the base of a foodchain in ecosystems to monitor an environment which is in a constant state of change. This is of utmost importance in parts of the world which are suffering from environmental destruction, both natural and increasingly induced by humans. The surveillance of vegetation in endangered areas is of very high importance and new methods need to be introduced to ensure the survival of the most vulnerable biomes on earth, namely the tropical, temperate and boreal forests.

Using the concept of a stereoscopic split-screen camera lens we can combine a Near-IR Pass Filter in conjunction with a clear visible light window to create a basic multi-spectral camera system, as described in the schematic below:

A common NIR Filter is the "Congo Blue" gel filter which passes NIR, which is detected as red by a CCD sensor, and does not allow green to pass.

"Congo Blue" filter for one window of the multi-spectral camera

Such split screen lenses can be included on UAV cameras as well as on high-definition portable camera technologies, i.e. smartphones, which can be light enough (and which are now so ubiquitous) that they can be used as diagnostic instruments for ecosystem monitoring.

Carbon-fiber drone fitted with split-screen camera

Using these techniques, a portable UAV with a camera fitted with a dual near-infrared + true-color lens fitting to fly over such an environment that lies close to a town area to monitor vegetation health.



The experimental data can then be fed back via Wifi feed to a smartphone or a computer, from which the data can be analysed using computer software to retrieve vegetation information from surveillance drone by calculating the vegetation index and overlaying the indices pixel by pixel on the composite image.

Developing this further we could also make a live NIR-camera feed application for use on portable computer, i.e. tablets and smartphone, technology which can allow for fast and easy remote diagnostics of plant health.

The stereoscopic split-screen lens mount is universally adaptable for use in smartphones, so for local use, or for active viewing on a height say, the diagnostic can be done entirely on portable computer infrastructure itself. It is hoped that such apps can become more widespread and applicable for simple to install multi-spectral camera attachments.

All of this could save energy and finance for performing the relatively simple step of diagnosis of plant health and help spur-on the the real work of finding solutions to remedy the problem of plant disease, dehydration and malnutrition in an environment.

Notes:

#1 Interesting still is the fact that Nitrogen (N-H bonds), which form have a first harmonic overtone at 1510 nm and a series of combination bands at 1980, 2060, and 2180 nm (Wessman, 1990). The secondary structure of any protein involves interactions (mainly hydrogen bonds) between neighboring polypeptide backbones which contain Nitrogen-Hydrogen bonded pairs and oxygen atoms.

Since both N and O are strongly electronegative, the hydrogen atoms bonded to nitrogen in one polypeptide backbone can hydrogen bond to the oxygen atoms in another chain and visa-versa. Though they are relatively weak,these bonds offer great stability to secondary protein structure because they repeat a great number of times.

Hence a non-linear vegetation index with increasing factors could be in principle be formulated with the addition of detecting nitrogen deficiency from a distance in soils.

The Nuclear Isomer EMP Weapon Controversy

Introduction to Nuclear Isomers

A nuclear isomer of a particular element is an atom of that element with the same atomic number Z and the same mass number, A, in a state of nuclear excitation, that is excitation of one or more of the particles in the atomic nucleus, i.e. the nucleons (protons or neutrons).

The higher states of nuclear excitation are metastable with respect to the ground state, meaning that they decay more slowly due to the requirement of an change of nuclear angular momentum, I.

Nuclei usually exist in their ground state with the individual nucleons paired up subject to energy constraints. In some nuclides, for example resulting from radioactive decay, one or more nucleons can be excited into one or more higher spin states. These nuclei can revert back to the ground state by the emission of gamma radiation. If this emission is delayed by more than 1 μs, the nucleus is said to be a nuclear isomer and the process of releasing energy is known as isomeric transition.

There are two very different ways that such nuclei can possess a quantum variable known as spin angular momentum or spin. Either the nucleus rotates as a whole, or several nucleons can orbit the nucleus independently in a non-collective rotation. The latter case can result in the nucleons being trapped in high spin states such that they have much higher lifetimes. Nuclides with even-Z and even-N (i.e. with a whole number of He-4 nuclei, which are Bosons, particles with whole integer spin) can also have high excess rotational spin due to alpha particles rotating independently around the nucleus. Examples here are 12C, 16O, 20Ne, and 24Mg.

Therefore, Nuclear isomers include excited states of nuclei that electromagnetically decay slowly enough for energy storage. However, the emitted gamma rays of the isomer decay come in a burst. Therefore, one would think, that a controlled triggering of the isomer decay could allow stored energy to be released on demand, and nuclear isomers represent a potential stand-alone energy source. Barriers to developing a practical energy source are triggering and production.

Nuclear Isomer Triggering Theory

Induced gamma emission can be triggered by means of stimulating nuclei in a long-lived excited energy level in a nucleus is analogous therefore to the process of stimulated emission of a photon from a long-lived excited energy state of an atom.

Hence with induced gamma emission the isomer must be then be an element in which the excited, metastable, state of the element is more stable than the ground state. Polonium-212 is an example where the isomer has a much longer halflife than the ground state. With a spin of 18, the half-life of 45 s is very much longer than the ground state half-life of 300 ns.

The excited isomer can then be considered as two neutrons and two protons (i.e. an alpha particle) being excited to a nuclear higher nuclear energy level, in analogy to the electron energy levels in atomic physics except know we are dealing with the particles around the nucleus rather than the electrons around the atom. The excited alpha particle then orbits in its higher energy level around the "doubly magic" lead-82 nucleus. The high spin state decays by alpha emission which carries off the 18 units of spin.

Other examples are hafnium-178 (spin 16 due to 4 (1-alpha particle) of the 78 nucleons orbiting the nucleus), tungsten-178 (spin 25 due to 8 unpaired nucleons (i.e. 2-alpha particles orbiting the nucleus). The energy stored in these excited nuclear orbitals are can be very large. For example, the excitation of 1-alpha particle to the first metastable state of hafnium 178m2 is 10,000 times as much energy per gram as TNT.

In theory, isomer high-energy density materials (HEDMs) have potential energy yields orders of magnitude greater than existing chemical energetics. While the development of useful propellants, explosives, or energy sources based on this phenomenon is probably decades away, such extraordinary energy density has the potential to revolutionize all aspects of power generation on demand.

Current nuclear batteries in development use small amounts (milligrams and microcuries) of radioisotopes with high energy densities. In one design, radioactive material sits atop a device with adjacent layers of P-type and N-type silicon, so that ionizing radiation directly penetrates the junction and creates electron-hole pairs. Nuclear isomers could replace other isotopes, and with further development it may be possible to turn them on and off as needed. Current candidates for such use include 108Ag, 166Ho, 177Lu, and 241Am. As of 2016 the only isomer which had been proven to be successfully triggered was 180mTa, which incidentally required more photon energy to trigger than was released.

Fission of an isotope such as 177Lu releases gamma rays by decay through a series of internal energy levels within the nucleus, and it is thought that by learning the triggering cross sections with sufficient accuracy, it may be possible to create energy stores that are 10^6 times more concentrated than high explosive or other traditional chemical energy storage. [Ref 1]

Potential aerospace applications range from very high-density energetics for propulsion and potential high-energy and power density primary sources to power spacecraft or satellites, again in the realm of nuclear batteries, and to be controlled and triggered sources of gamma rays for use in particle and nuclear physics research, in particular particle-antiparticle pair production.

Proposed Nuclear Isomer Production - Application, Methods and Feasibility

Famously, it was the goal of DARPA's Stimulated Isomer Energy Release program is to develop a technique to control the release of the energy contained in nuclear isomers. Its mission was to develop a way to make these isomers in gram-size quantities and then demonstrate that as much energy can be released as is used to initiate the reaction (i.e. a breakeven experiment). Program Plans outlined in February 2004 include efforts to determine if the hafnium isomer can be triggered with photons in the x-ray range that will release more than 50 times the energy input of trigger and moreover release the energy in the form of controlled gamma rays. The project intended to identify a hafnium isomer production process that is affordable and cost effective, and to develop a physics approach to a chain reaction for the hafnium isomer.

DARPA supported a group led by Carl Collins at the University of Texas at Dallas. In early 1999 Collins claimed to have demonstrated triggering energy release from a hafnium-178 isomer using a dental X-ray machine (Physical Review Letter 25 Jan, 1999). [Ref 2]

The Collins groups claimed that when they bombarded the metal with soft X-rays, the hafnium-178 released a burst of gamma rays 60 times more powerful than the X-rays.

This would be a very important discovery for an organisation like DARPA or any global security or military intelligence agency for that matter, as a controlled high energy gamma ray source such as this could be key for, among other things, a directed energy weapon system that would also create a significant directed EMP (Electromagnetic Pulse) if such a weapon was fired into the atmosphere. Such a weapon would be considered a Weapon of Mass Destruction, as it would cause significant damage to a nation's infrastructure in a first strike tactic.

This would work by means of the Gamma rays creating Compton Scattering of electrons from Oxygen atoms in the atmosphere. 

In Compton Scattering, an incident gamma ray photon loses some of its energy to a bound electron, which excites the electron which then has enough kinetic energy to escape from the atom and recoils away from the atom. The scattered photon moves away at an opposite and equal angle to the emitted electron.

The recoil electrons would then spiral in line with the Earth's own magnetic field and release high energy Radio and Microwave Synchrotron Radiation in a pulse, which is the EMP itself,  which would fry any piece of electronic equipment attached to an antenna, or anything that acts as a antenna. 

Hence, power lines, telecom towers, mobile communications and most semiconductors would be either badly disrupted or completely destroyed. This is the ultimate non-lethal way to win a war - leaving the buildings and people intact but disabling or destroying most or perhaps all machines and weapons. This can happen with all forms of nuclear weapons when detonated in the atmosphere, but nuclear weapons have additional fallout making them highly lethal weapons of mass destruction.

In 2001 physicists from the Lawrence Livermore National Laboratory, in collaboration with scientists at Los Alamos and Argonne national laboratories, conducted tests that strongly contradicted reports claiming an accelerated emission of gamma rays from the nuclear isomer 31-yr. hafnium-178, and the opportunity for a controlled release of energy. The triggering source in the original experiment was a dental X-ray machine.

Using the Advanced Photon Source at Argonne, which has more than 100,000 times higher X-ray intensity than the dental X-ray machine used in the original experiment, and a sample of isomeric Hf-178 fabricated at Los Alamos, the team of physicists expected to see an enormous signal indicating a controlled release of energy stored in the long lived nuclear excited state. However, the scientists observed no such signal and established an upper limit consistent with nuclear science and orders of magnitude below previous reports. When the team turned the APS X-ray beam onto the sample of 31-yr. Hf-178, no detectable increase of the isomer decay occurred. In other words, the X-ray irradiation did not decrease the time it takes for hafnium to decay; a result that is consistent with nuclear physics.

Anatoli Andreev of Moscow State University wrote in 2007 "Recently, there have been reports in the mass media about plans to build what became known as an “isomeric bomb” based on Hf-178. What all the publications are speaking about is no less than the possibility of building a radically new weapon that does not fall under a single article of the existing nonproliferation treaties. The publications were based on the sensational results on induced decay of the long-lived isomer Hf-178m2 (16+, 2446 keV, 31 yr), obtained in 1999-2004 by a group of researchers headed by Carl B Collins, the Director of the Center for Quantum Electronics, University of Texas at Dallas.

The results show the following. The production of several grams or more of the isomer 178m2-Hf is an extremely difficult task and, so far, no effective process for such production has been described in the literature.

The initial discovery of 178m2Hf was the ridiculously daunting result of irradiating 100 mg of HfO2 for two years in a high neutron flux reactor facility [Ref 3]

, with thermal neutron fluxes > 4 × 10^14 n/cm2/s,  and required an additional three years to decay and process, resulting in an estimated 25 picograms of 178m2Hf. Considerations of large scale processing with reactor irradiation conclude that it is impractical to produce even gram quantities in this manner.

In a  paper by Karamian, et al. [Ref 4] the production cross section for 178m2Hf was measured (along with other isotopes of Hf). From that paper the production of 178m2Hf can be estimated by the expression:

 [Ref 5]

where Φ is the neutron flux, N177 is the amount of 177Hf which serves as the “feed stock” for the production and N178m2 is the amount of 178m2Hf produced. The cross sections (measured in barns, b) reported by Karamian, et al. provide an estimate for the production:

It is instructive to calculate the total quantity of 178m2Hf that Helmer and Reich would have produced. Starting from 100 mg of HfO2, with 177Hf at 18.6% abundance the initial amount of “feed stock” would be roughly 16 mg. Estimating the reactor flux for 2 years of running to be Φ = 6.3 × 10^21 n/cm2 yields roughly 0.075 ng of 178m2Hf.

To obtain gram quantities of 178m2Hf it would require processing 10 metric tonnes of HfO2

Accelerator production might be possible via the reaction 179Hf(n,2n)178m2Hf. The cross section at 18 MeV incident neutron energy is calculated to be 10 mb. The shape of the cross section above 18 MeV is uncertain. The total neutron cross section 179Hf(n,X) is approximately 2.5 b. Each incident neutron incident on the 179Hf target makes 0.004 178m2Hf nuclei, or 250 incident neutrons to make a single 178m2Hf.

Neutrons would be made by accelerating deuterons to high energy and directed onto a Li target to produce neutrons in the appropriate energy range. A thick Li target would yield roughly 1/3 of a neutron out in the energy range of interest. A high intensity machine would accelerate 6×10^18 deuterons/s/Ampere. The neutron yield would be 2×10^18 neutrons/s/A.

Assuming that 120 MeV deuteron accelerator can be designed and built with roughly 100mA beam currents, the neutron yield would be 2 × 10^17 neutrons/s.

The 178m2Hf production for one year of running would be 2 × 10^22 atoms, or roughly 6 g.

Additional issues with accelerator production of 178m2Hf are the enrichment of 179Hf from natural stock, and the processing of the irradiated target to recover the 178m2Hf. There are also considerable technical challenges regarding the accelerator, the Li target and processing the 178m2Hf from the 179Hf target. Finally, not all the cross sections relevant for the estimating production are known. 

Burdensome expenditures from state defense budgets to even produce the necessary quantities may prove completely useless: no energy can be liberated by the method as described in Collins’s articles. The cross sections of the induced decay of the isomer 178m2-Hf measured by that method do not agree with the current ideas about the physics of the nucleus and the physics of electromagnetic nuclear processes.

Summary:

Summarizing the obtained proposals, methods and results, it would be noted the following:

Theoretical calculations and the analysis of the existing experimental data suggest that the hafnium problem, as presented by the works of Collins's group, does not exist. The hullabaloo over the hafnium bomb was due to meaningless experimental data and the incompetence of certain individuals, and their thirst for fat military and black project budgets. rather than to the real possibility of building any radically new technology based on 178-Hf in particular.

Nevertheless, the potential for developing nuclear isomers known to be triggered such as 180mTa and future developments on other isomers, may make on-demand triggered x-ray and gamma ray sources possible for experimentation as well as nuclear and particle physics research and applied technologies in the field of energy, material science and applied nuclear physics in particular. However, due to the difficulty in creating significant quantities of pure nuclear isomers, this makes it largely unfeasible to pursue nuclear isomers as a practical energy storage medium, let alone a practical weapon.

It is also more important to focus this research away from the often low-integrity thinking of the military and instead peruse the more integral issue of understanding the nature of how inverted populations of excited alpha particles in the energy orbitals of nuclei in materials, which is also relevant in the study of Bose-Einstein condensates in general. Since alpha particles are bosons and the inverted populations of can be theoretically generated in a coherent avalanche in an induced series of nuclear reactions. This, in and of itself, has much wider applications in the fields of experimental, theoretical and applied physics and this is most likely what warrants investigation, rather than developing an EMP "super-weapon". 

References:

Ref 1- [Ref- M.S. Litz and G. Merkel (2004-12-00 [sic]). "Controlled extraction of energy from nuclear isomers"]

Ref 2 - https://www.aps.org/publications/apsnews/200706/backpage.cfm

Ref 3 - [Ref- R. Helmer and C. Reich, Decay of an isomeric state in 178Hf with K ≥ 16, Nuclear Physics A, 114 (1968), pp. 649–662.] 

Ref 4 - [Ref - S. Karamian, J. Carroll, J. Adam, E. Kulagin, and E. Shabalin, Production of long-lived hafnium isomers in reactor irradiations, High Energy Density Physics, 2 (2006), pp. 48–56.] 

Ref 5 - [Ref - C. B. Collins, N. C. Zoita, F. Davanloo, S. Emura, Y. Yoda, T. Uruga, B. Patterson, B. Schmitt, J. M. Pouvesle, I. I. Popescu, V. I. Kirischuk, and N. V. Strilchuk, Accelerated Decay of the 31-yr Isomer of Hf-178 Induced by Low-Energy Photons and Electrons, Laser Physics, 14 (2004), pp. 154–165.]

The Spacecraft That Requires no Fuel: Graphene Photoelectric Solar Sails

Graphene is much lauded for its remarkable properties like robustness, high tensile strength while maintaining flexibility, low weight along with high electric and heat conduction in its atomic-scale thickness. However sometimes ignored is the potential of graphene's electrical properties to transform light energy directly into kinetic energy and motion, an interesting visual phenomenon but also one of importance when considering the concept of using the power of the stars to traverse the immense distances of space.

Photo-kinetic Properties of Levitated Graphene

During the process of using lasers of graphene, we have found that under strong magnetic fields we can create propulsion of graphene by light illumination alone.

Interesting still is the amount of propulsion achieved when exposed to focused sunlight.

We can focus solar radiation very effectively using a Fresnel Lens.


Graphene itself has very high resistance to the damage caused by the heat of focused solar radiation. Graphene has a melting point of 4700 Kelvins.






We can see the advantage of graphene-based solar sail technology when compared with aluminium, which has a much lower melting point of 934 Kelvins. This alone is reason to consider constructing solar sails from graphene, particularly if focused solar or laser radiation is being used to propel the craft.

In the experiment, like using the laser, we set up a chessboard grid of ~1.2 Tesla permanent NdFeB magnets. Graphene has excellent photothermal properties, so that the focused solar radiation can heat up the graphene film in one region instantly, which affects its magnetic susceptibility, making it tilt and hence move.

The fact that graphene also dissipates heat rapidly, in the form of quantised lattice vibrations (phonons), allows the process to be instantly reversible, which is what allows the film to move so responsively. Hence, the graphene film does not simply collapse onto the magnet as it loses its magnetic susceptibility, it simply tilts in one region and stays levitating on the region opposite from the laser focus as the heat dissipates before it reaches there.



This effect of propulsion would be substantially more under high vacuum and also in the vacuum of space itself where there is no air resistance or weight of atmosphere added to the craft. However without a magnetic field to create the effect of diamagnetism we will lose the effect of motion, at least in the way demonstrated.

However, that shouldn't stop us from experimenting with such films on Earth. The fact that the graphene film levitates, in effect ignoring the effect of gravity, is a good set-up in testing the effect of light on graphene films. Moreover, the fast heat transfer capability and the speed at which the magnetic dipoles in the graphene film undergo reversal is also a strong indication that the electrons are moving very fast in graphene which is an important fact in exploring new applications and features in solar sail technology.

New applications include concentrating not only on the affect of photon momentum on the sail, but on the possible mechanism in which electrons can be emitted from the sail and create additional thrust to propel the sail much faster than conventionally possible by photon momentum alone.

Photon and Electron Momentum-driven Solar Sails

In quantum mechanics, light is described as being made up of particles called photons, each of which has its own momentum. Each photon has an energy E and travels at the speed of light, c. It also has a momentum p, given by: p = E/c

In the vacuum of space, the inherent momentum of photons from the sun can either be absorbed or reflected off of a solar sail. If absorbed we are simply transferring momentum proportionally into the sheet, causing it to move with the same momentum as the photon. if reflecting off of the sheet, the effect of momentum conservation would impart twice the momentum onto the surface of the sheet.

Either way, we cause a reflective solar sail to move in zero gravity and since we cannot have a perfectly reflecting surface, both effects of absorption and reflection would cause the sail to move in space.

However there may be other effects that would contribute to the motion of the solar sail in space. An interesting fact is that when we expose the graphene sheet to the focused laser or sunlight, as described in the article here, by drawing up the light energy the graphene in the magnetic field creates a deflagration effect in the spin of electrons in the region illuminated which transmits itself as a quantised spin-wave.

In space, with no significant magnetic field, this importation of energy should instead create a net movement of electrons, an electrical current, which could be released from the film by the photoelectric effect. This would, in effect, propel the sheet in space with a defined momentum.

The photoelectric effect is explained in terms of the quantum model where the energy of each photon is the frequency, v, multiplied by Planck's constant h.

In the photoelectric effect, the kinetic energy (Ekin) of the photoelectrons to the energy of the absorbed photons (hv) and the potential energy (ϕ) of the surface:

The work function, W, is defined as the energy necessary to remove an electron from the surface of a metal:  

W=−eϕ−EF

Here, eϕ is the energy of the electron just outside of the surface of the metal and EFis the Fermi energy. 

The work function is the energy difference of the electron between being just outside of the metal and being at the Fermi Fermi energy.

The Fermi energy itself is the maximum energy occupied by an electron at 0K (degrees Kelvin). By the Pauli exclusion principle, we know that the electrons will fill all empty energy levels before they will share energy levels, and the top of that "Fermi sea" of electrons is called the Fermi energy. 

In the metal there are electrons with energies higher than EF and there are electrons with lower energy than EF. We say the electrons with lower Fermi energy are easier to remove and are, in effect, near the energy surface in the "Fermi sea", wheras the electrons with a higher Fermi Energy are "deeper" in the "Fermi sea" and are harder to remove as they are effectively screened by the lower energy electrons at the surface.

This formula works with metals under illumination of light in vacuum. The metal target has a certain work function, where the electrons are localized in a "sea" at the top of the metal's energy surface and are detected as "free electrons" under illumination.

If a reverse field potential is applied across the metal energy surface, by and introduced electric current say, we can reduce the kinetic energy of the electrons by a certain degree, in effect the induced photoelectric current in the material being reduced.

If the reverse potential is large enough, the photocurrent will be stopped, and this stopping potential is determined. The maximum kinetic energy, Kmax, of the photo-electrons is related to the stopping potential by: Emax = e0Vs

In semiconductor materials, the reverse field potential that creates the characteristic stopping potential of the released photo-electrons is the inherent band-gap energy of the material. 

The band-gap (Eb) is essentially the quantised energy levels of the electrons in the material. In a similar way to how electrons are quantised in energy levels in the atomic model, in semicondincdutors materials such as silicon, germanium and of course our graphene the electrons within are quantised in specific energy bands.

In semiconductors therefore, the energy of the photoelectrons must be reduced by the band gap energy (Eb) of the electrons trapped in the material. 

Another way to think about this is that the band gap energy must be delivered in order to allow the electrons to "quantum jump" from their characteristic band.

Band structure of materials can be constructed where we include the concept of the electrons being confined within bands in the "Fermi sea", as described earlier, with the top of the sea being the "Fermi Level"

We also include the definition of the momentum,

k||  of electrons moving throughout the surface of the semiconductor material. This allows us to properly describe the conservation of momentum in the system of the photon, with intrinsic momentum, impacting the confined energy bands and releasing an electron, which now carries the characteristic momentum of the photon K|| , from one of those bands at a displaced angle, θ .

F

The conserved momentum of the released electron (of mass m) is equated as:

In monolayer graphene, the unit cell consists of two carbon atoms - A and B.

The band structure of graphene exhibits two bands intersecting at two inequivalent points K and K' in the reciprocal space (kx,ky).

Near these points, the electronic dispersion resembles that of relativistic Dirac electrons. For this reason, K and K' are commonly referred to as the “Dirac points” at the tips of "Dirac Cones". A simulation of monolayer graphene shows this clearly, using the tight binding model.

The band structure of graphene is as follows:

With excitation energy (γ < 0), obtained within a nearest neighbour tight binding model, with a distance a between nearest neighbour carbon atoms. Here, we have set E = 0 at the K–points, where the valence band and the conduction band touch each other.

From this we get the Graphene energy dispersion (showing valance and conduction bands) from the π-bonding in graphene.

The valence and conduction bands are degenerate at the Dirac points. Put another way, the electrons are confined in one cone and the holes are confined in another and hop over the Dirac points under excitation.

Graphene is, in this sense, a band-gap semiconductor. How the band-gap can be changed is crucial for its application in making devices. There are two ways to lift the degeneracy of the two bands at the Dirac points.

One is to hybridize the electronic states at K and K' which requires breaking of the translational symmetry. The other is to engineer a strain in the graphene lattice so to break the equivalence between the A and B atoms in the lattice, which does not require any translation symmetry breaking.

Introducing more layers to the graphene will also decrease the band-gap energy, as the movement of electrons will behave as though the the multiple lattices are more equivalent to each other as more of them are added, effectively functioning as electron transport in a metal. If designing graphene as a semiconductor we would like a large band-gap energy to work with, hence few to single layer graphene is the best material to make graphene-based semiconductors with.

On average, bilayer graphene is calculated to have an energy band gap of 0.48 eV around the Fermi level, using our model. In experimental practice, a band-gap of anywhere from 0.1eV to 0.5eV is typical of few-layer graphene.

However, when considering use for a photoelectric solar sail, we want a small band-gap energy so multi-layer graphene is ideal. Multi-layer graphene, with its small band-gap energy should therefore release photoelectrons with a relatively high kinetic energy, according to the formula.

Therefore by Newton's third law the net thrust by the release of photoelectrons released from a solar-illuminated graphene sail should also be relatively high and should provide an additional effect of forward propulsion, along with photon absorption and reflection, which would move a solar sail in the vacuum of space.

We might then envision using this information in the approach in creating a light-powered propulsion system, i.e. a solar-sail spacecraft, for future developments in space exploration.

We can perform an experiment on a graphene solar sail prototype using a small vacuum vessel to observe the motion of a sail under concentrated light.

Using the effect of magnetic levitation, we can in effect negate the effect of gravity on our film. However, as stated in a previous article, the effect of the films motion is influenced by the effect of photo-illuminated magnetic spin deflagration changing the magnetic susceptibility of the film. Hence the ideal way to perform the experiment must be in either free-fall under high vacuum (relatively difficult) or in the vacuum of space itself to be a fully proven technology.

How fast, theoretically, could a Photoelectric Graphene Solar Sail Go?

Photons are massless particles, so the energy–momentum relation for photons is simply E = pc. For electrons in a graphene lattice, we know that the wavenumber, measured relative to the energy band minimum, and multiplied by ¯h, is the crystal momentum. More precisely

Theoretically we can attempt to calculate this using the nature of the electrons in graphene. Electrons in the conduction band of graphene may be considered as massless relativistic particles (”massless Dirac fermions”), moving not with the speed of light, but rather with a Fermi velocity.

To find the Fermi velocity we have to get the excitation energy relation, under an excitation quanta gamma, for the graphene electrons at the Fermi level, i.e. near the conduction band minimum:

The derivation is quite messy (willing to share if requested) and is based on the and structure of graphene we have shown earlier. In the end we get a formula that relates the excitation energy in graphene to the velocity of the electrons.

With the Fermi Velocity itself being

The nearest neighbour distance in graphene is a ≃ 1.4 ˚A,


A photon is characterized by either a wavelength, denoted by λ or equivalently an energy, denoted by E. There is an inverse relationship between the energy of a photon (E) and the wavelength of the light (λ) given by the equation:

where h is Planck's constant and c is the speed of light.

h = 6.626 × 10 -34 joule·s

c = 2.998 × 108 m/s

By multiplying to get a single expression, hc = 1.99 × 10-25 joules-m

When dealing with "particles" such as photons or electrons, a commonly used unit of energy is the electron-volt (eV) rather than the joule (J). An electron volt is the energy of a charged particle when place across a potential difference of 1 Volt (i.e. using 2 capacitor plates) at which the charged particle undergoes a quantised change in momentum.The eV unit of energy is universally used in particle physics. A photon with an energy of 1 eV = 1.602 × 10-19 J.

Therefore, we can rewrite the above constant for hc in terms of eV:

hc = (1.99 × 10-25 joules-m) × (1ev/1.602 × 10-19 joules) = 1.24 × 10-6  eV-m

Further, we need to have the units be in µm (the units for λ):

hc = (1.24 × 10-6 eV-m) × (10^6 µm/ m) = 1.24 eV-µm

By expressing the equation for photon energy in terms of eV and µm we arrive at a commonly used expression which relates the energy and wavelength of a photon, as shown in the following equation:

We know that most of the light emitted by our star, the Sun, is in the visible portion of the electromagnetic spectrum. Being a yellow star, most of the radiation emitted is centered around the yellow-green region with significant overlap in the red and infrared, blue, violet and ultraviolet portions. By measuring the light received from the Sun we know that its radiation corresponds to a surface temperature of about 6300 K (units of Kelvin).

From the calculation, we find that the range of photon energies for visible light emitted from the sun, from red to violet, ranges from about 1.63 to 3.26 eV respectively. This is important knowledge to have for designing semiconductor devices, such as photovoltaics, to harvest solar energy efficiently. In other words, it makes sense to design solar photovoltaic devices using semiconductors which have band gaps in between these energies.

Lets say the peak wavelength energy for exciting the graphene is around 405nm (0.405μm) (the same wavelength as our near-UV laser), i.e. 3eV.

so if γ ∼ 3 eV,

the velocity of the electrons is then calculated as:

Which is roughly 3% light speed. Assuming a 100% efficient mass action-reaction this is the fastest the sail could theoretically go by photoelectric propulsion.

Remember this is assuming that there is no mass being towed and that the energy of the incoming photons remains at a level of 3 eV. Any additional inertial mass, m0, will retard the rate of change in momentum.

In traversing space, the given gravity is the sum of the accelerations due to gravity that influences the craft near the the planet the craft is orbiting or flying past, g0, along with the total gravity of the host star, gS.

The performances of a spacecraft's thrust are characterized by the specific impulse, Isp, which measures the thrust, T, in Newtons produced by unit of mass of propellant at the given gravity.

The maximum effective specific impulse, Isp, of a photoelectric solar sail drive is equal to:

Where C0 is the photoelectric collimation factor (the fraction of the amount of radiation that hits the sail, from the sun or laser source, which will actually create the emission of a photoelectron), Vexh is photoelectron velocity, and ∑g is the sum of standard accelerations due to gravity (Note: this factor is not necessary if Isp is measured in N·s/kg or m/s).


Away from the earth, the value of g0, the Earth's gravity, on the total sum of gravitational influence on the craft will diminish and the Sun's gravity will be the major force retarding the specific impulse. As the sail gets farther from the sun, the value of C0 will decrease, however so too will the inverse relationship of the Sun's gravity gS. At some point the two effects will balance and conservation of momentum would allow the craft to maintain its maximum velocity as long as it encounters nothing to slow it down.

The solar sail craft could in fact maintain its top speed until it reaches another star system in which the value of C0 will increase and the sail will in fact decelerate, as it gets closer to the star, in the opposite direction as it enters another star system. This would allow for a much more autonomous form of long-distance space exploration, in which the craft could be in a state of dormancy during traversing interstellar space from system to system and becoming active within the star system being explored.

Moreover, a large solar sail transport system would in and of itself provide a large reflector dish for radio or laser communications across interstellar space.

Single figure percentages of light speed may not sound very impressive, but even 1% light speed is many times faster than the top record holders of any spacecraft built so far. It took numerous gravity assists, in planetary arrangements that only happen once every 250 years or so, that has flung Voyager 2 into the edges of the solar system and now into the first frontiers of interstellar space. It is currently travelling at about 55,000 km/h, which is only about 0.0051% the speed of light.

Therefore there might be good reason to develop spacecraft designs that get us to at least single percentage figures of light speed and solar sail technology is one with much potential.