.comment-link {margin-left:.6em;}


2Physics Quote:
"In atoms, electronic wave functions describe - among other things - charge distributions existing on length-scales that are many orders of magnitude removed from our daily experience. In physics laboratories, experimental observations of charge distributions are usually precluded by the fact that the process of taking a measurement changes a wave function and selects one of its many possible realizations. For this reason, physicists usually know the shape of charge distributions through calculations that are shown in textbooks. But in the last few years, this has started to change. Recent experiments have visualized the nodal structure of electronic states of hydrogen and helium on two-dimensional detectors."
-- Aneta Stodolna, Marc J.J. Vrakking
(Read Full Article: "Atoms Under the Magnifying Glass: Direct Observation of the Nodal Structures of Electronic States" )

Sunday, November 23, 2014

Imaging Spin-Valley-Layer Locking in a Transition-Metal Dichalcogenide

Transition-metal dichalcogenides (TMDs) of the form MCh2, where M is a transition-metal and Ch a chalcogen, have received much attention in recent years. They can be stabilised as single Ch-M-Ch monolayers, which display a host of attractive materials properties including direct band gaps in the visible region and ambipolar conduction, suggesting a range of applications in electronics and optoelectronics [1]. Moreover, they host degenerate band extrema at the corners of the hexagonal Brillouin zone, which gives rise to a so-called valley degree of freedom. A combination of strong spin-orbit interactions with broken inversion symmetry in the monolayer causes this valley pseudospin to become strongly coupled to the real spin [2]. Valley-dependent optical selection rules combined with suppressed inter-valley scattering resulting from their coupled spin-valley texture has opened new possibilities for optical control of spin and valley pseudospins [3-5].

This suggests unique potential to exploit TMDs in novel schemes of electronics exploiting the spin (a.k.a. spintronics) or valley (a.k.a. valleytronics) degrees of freedom, with the ultimate potential for faster, smaller, and more energy-efficient devices. One might naturally expect this potential to be lost for their bulk counterparts, where the most common structure (the 2H polymorph) is formed by stacking single TMD monolayers together with a 180° rotation between neighbouring layers (other stacking sequences host their own interesting properties [6], but we don’t consider these here). The bulk unit cell therefore contains two such monolayers, having a centre of inversion. It is well established that such inversion symmetry, together with time-reversal symmetry, enforces all electronic states in solids to be spin-degenerate.

In our recent work published in Nature Physics [7], performed in a collaboration between my group in St Andrews (UK) and researchers at the Norwegian University of Science and Technology, the universities of Tokyo (Japan), Aarhus (Denmark), and Suranaree (Thailand), the Max-Planck Institute in Stuttgart (Germany), MAX-IV Laboratory (Sweden) and Diamond Light Source (UK), we have instead observed spin-polarised states persisting in centrosymmetric bulk WSe2.

We used angle-resolved photoemission spectroscopy (ARPES) to probe the electronic structure of bulk crystals of 2H-WSe2. Through a process of Mott scattering, we also measured the spin polarisation of the emitted photoelectrons, and discovered that the electronic states around the corners of the Brillouin zone were almost 100% spin polarised. Surely this would seem to contradict the inversion symmetry that this material possesses?

Figure 1: Valence band dispersions of WSe2 measured by angle-resolved photoemission, showing excellent agreement with theoretical calculations of the kz-dependent bulk electronic structure (coloured lines). The spin texture measured at the K and K’ points of the Brillouin zone is shown schematically by coloured arrows.

Through a combination of photon-energy dependent ARPES experiments and first-principles theoretical calculations, we observed how, for the electronic states close to the Brillouin zone corners, their wavefunctions are spatially localised within single monolayers of the bulk crystal structure where locally, inversion symmetry is not present. The combination of this inversion symmetry breaking together with strong spin-orbit coupling drives these states to develop huge spin polarisations, leading to spin-valley locking as for isolated monolayers.
Figure 2: Angle-resolved photoemission measurements of WSe2 throughout the Brillouin zone, schematically showing the intertwined layer- and momentum-dependent spin texture uncovered here.

The 180° rotation between neighbouring monolayers in the bulk crystal structure, however, imposes an additional layer-dependent sign change of the spin polarisation for a given valley, as found in previous theoretical calculation [8], and recently also suggested from polarisation-resolved optical experiments [9]. By exploiting photon energy-dependent interference between photoelectrons emitted from different crystal layers, we could tune the measured photoelectron spin-polarisation nearly to zero, effectively averaging over neighbouring layers, or could selectively probe just the top monolayer of the crystal. These measurements together provide the first direct observation of the entangling of the spin with valley and layer pseudospins in a bulk transition-metal dichalcogenide.

Moreover, our study provides an experimental observation that local, rather than global, inversion symmetry breaking is sufficient to stabilise spin-polarised states in solids [10], contrary to conventional wisdom. This is exciting because it reveals that a whole new class of materials which we previously thought must have only spin-degenerate energy bands can in fact locally host spin-polarised states. Controlling this could bring fantastic new opportunities for spin- and valleytronics, and a whole arsenal of new materials in which we can achieve this.

[1] Qing Hua Wang, Kourosh Kalantar-Zadeh, Andras Kis, Jonathan N. Coleman, Michael S. Strano, "Electronics and optoelectronics of two-dimensional transition metal dichalcogenides". Nature Nanotechnology, 7, 699 (2012). Abstract.
[2] Di Xiao, Gui-Bin Liu, Wanxiang Feng, Xiaodong Xu, Wang Yao, "Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides". Physical Review Letters, 108, 196802 (2012). Abstract.
[3] Hualing Zeng, Junfeng Dai, Wang Yao, Di Xiao, Xiaodong Cui, "Valley polarization in MoS2 monolayers by optical pumping". Nature Nanotechnology, 7, 490 (2012). Abstract.
[4] Kin Fai Mak, Keliang He, Jie Shan, Tony F. Heinz, "Control of valley polarization in monolayer MoS2 by optical helicity". Nature Nanotechnology, 7, 494 (2012). Abstract.
[5] Xiaodong Xu, Wang Yao, Di Xiao, Tony F. Heinz, "Spin and pseudospins in layered transition metal dichalcogenides". Nature Physics, 10, 343 (2014). Abstract.
[6] R. Suzuki, M. Sakano, Y. J. Zhang, R. Akashi, D. Morikawa, A. Harasawa, K. Yaji, K. Kuroda, K. Miyamoto, T. Okuda, K. Ishizaka, R. Arita, Y. Iwasa, "Valley-dependent spin polarization in bulk MoS2 with broken inversion symmetry". Nature Nanotechnology, 9, 611 (2014). Abstract.
[7] J.M. Riley, F. Mazzola, M. Dendzik, M. Michiardi, T. Takayama, L. Bawden, C. Granerød, M. Leandersson, T. Balasubramanian, M. Hoesch, T.K. Kim, H. Takagi, W. Meevasana, Ph. Hofmann, M.S. Bahramy, J.W. Wells, P. D. C. King, "Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor". Nature Physics, 10, 835 (2014). Abstract.
[8] Zhirui Gong, Gui-Bin Liu, Hongyi Yu, Di Xiao, Xiaodong Cui, Xiaodong Xu, Wang Yao, "Magnetoelectric effects and valley-controlled spin quantum gates in transition metal dichalcogenide bilayers". Nature Communications, 4, 2053 (2013). Abstract.
[9] Aaron M. Jones, Hongyi Yu, Jason S. Ross, Philip Klement, Nirmal J. Ghimire, Jiaqiang Yan, David G. Mandrus, Wang Yao, Xiaodong Xu, "Spin–layer locking effects in optical orientation of exciton spin in bilayer WSe2". Nature Physics, 10, 130 (2014). Abstract.
[10] Xiuwen Zhang, Qihang Liu, Jun-Wei Luo, Arthur J. Freeman, Alex Zunger, "Hidden spin polarization in inversion-symmetric bulk crystals". Nature Physics, 10, 387 (2014). Abstract.

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Sunday, November 16, 2014

The Plasmoelectric Effect: A New Strategy for Converting Optical Energy into Electricity

Matthew Sheldon

Author: Matthew Sheldon

Affiliation: Department of Chemistry, Texas A&M University, USA.

Link to Sheldon Research Group >>

A plasmon resonance is a remarkable optical phenomenon that occurs in metallic nanostructures and other nanoscale materials that have high electrical conductivity. As reported in 'Science' on October 30 [1], we have demonstrated a new way to use plasmon resonances to generate electrical potentials during optical excitation. Our work could lead to new ways of converting optical energy into electrical energy, and may guide new opportunities in the very active research areas of plasmonics and nanophotonics.

A plasmon resonance results from the oscillations of electrons (or other electrical carriers) lining up with the oscillating electric field of incident radiation. This resonance causes significant concentration of light energy within the small sub-wavelength volume defined by the nanostructure. Because the resonant frequency can be tailored by controlling the nanoscale geometry, plasmonic materials have been the subject of considerable scientific activity for a host of applications that benefit from the ability to tune and concentrate radiation, such as Raman spectroscopy, cell labeling, sub-wavelength optical communication, or enhanced light trapping in solar cells, to list a few examples [2]. However, much of the confined optical energy is quickly absorbed by the metal and converted to heat. This heating is generally regarded as a limitation for optical applications.

Despite this loss of optical energy as heat, we questioned whether the strong plasmonic concentration of energy could still be utilized to perform electrical work. Electrical work can be understood as the movement of electrons through a circuit load, so it seemed natural to wonder if the plasmon resonance, which fundamentally results from the coupling of light to the motion of electrons, could also move electrons through a circuit. As reported in Science on October 30th [1], we discovered a mechanism by which optical absorption in plasmonic resonances indeed produces an electrical potential, a necessary first step towards performing useful electrical work. We have labeled this phenomenon the ‘plasmoelectric effect’. In conjunction with a thermodynamic model we developed, our analysis shows how a plasmonic resonance can act as a heat engine that uses thermal energy from the absorption of light to move electrons and produce static electric potentials.

Currently, the photovoltaic effect is the primary mechanism used in technology for the production of electrical potentials from the absorption of light, i.e. photo-voltages. The photovoltaic effect is the generation of excess electrical carriers in semiconductors during optical excitation with energy greater than the band gap energy. Our discovery, the plasmoelectric effect, is a fundamentally different mechanism for generating an electrical potential, and instead results from the dependence of the plasmon resonance frequency on electron density in conductors.

Recent works from other researchers studying plasmonic systems [3-5] have demonstrated that it is possible to tune the plasmon resonance frequency of a nanostructure by modulating electron density. Specifically, these researchers applied a static electric potential to inject or remove electrons from resonant structures, and they observed a shift to higher or lower frequency, respectively, of the plasmonic absorption resonance. In essence, the electrical state of the conductor, whether it is charged positively, negatively, or neutral, is coupled with the frequency of the plasmonic absorption. This behavior is analogous to how the resonant pitch of a musical instrument, such as a flute, would change if you modify the density of the air in the acoustic cavity.

Inspired by these experiments, we considered the extent to which the optical absorption, the plasmon resonance frequency, and the charge state are linked in this way, and if the reverse of this behavior would also occur. That is, can optical excitation with off-resonant light cause a change in the electron density of a plasmonic structure that shifts the plasmonic absorption into resonance with the illumination, and thereby induce an electrical potential? Considering the acoustic analogy above, this would be like the chamber of a flute adopting a slightly modified air density in order to become resonant with a loud pitch playing nearby that would otherwise be slightly out of tune.

To probe this possibility experimentally, we monitored the electric potential of a conductive surface coated with plasmonic Au nanoparticles using Kelvin probe force microscopy (KPFM). For KPFM a conductive atomic force microscope (AFM) tip is maintained a few nanometers above a sample surface, and the electrical potential between the tip and sample is measured. During KPFM experiments we also illuminated the nanoparticles with a tunable laser, varying the output from higher frequency to lower frequency through the plasmon resonance. We observed that higher frequency light caused negative surface potentials and that lower frequency light caused positive surface potentials, but there was no potential measured when the incident light was the same frequency as the plasmon resonance. This is the exact behavior expected if the nanoparticles are adjusting charge density so that the plasmon resonance is better matched with the frequency of the optical excitation.

Our report also details a thermodynamic model that anticipates this behavior for plasmonic materials. We show how the condition of minimum free energy, the preferred thermodynamic state of a system, corresponds to a configuration of charge density that modulates the plasmon resonance frequency in order to maximize the amount of heat produced via optical absorption. However, the energy required to electrically charge the structure moderates how much the plasmon resonance can shift. Therefore, for a given optical intensity, single frequency light induces a specific charge state that balances these counteracting effects. In general, during illumination a plasmonic structure will only remain neutral if incident light is the same frequency as the plasmon resonance of the neutral structure.

To show that the behavior is general to plasmonic systems, we also measured the optical response of periodic arrays of nanoscale holes in thin gold films that have strong, tunable plasmonic resonances across the visible spectrum based on the hole pitch. These fabricated hole arrays also displayed electrical potential trends consistent with our description of the plasmoelectric effect, as summarized in Fig. 1.
Figure 1: Plasmoelectric effect (a) Schematic of a metal nanoparticle that becomes electrically charged by illumination. (b) Electron microscopy image of the metal nanocircuit, composed of an array of nanoscale holes in a 20-nm-thin gold film. The scale bar is 500 nanometer. (c) Measured optical absorption spectra for metal nanocircuits with different spacings between the holes (175, 225, 250, and 300 nm). (d) Electrical potential of the nanocircuits in (c) as a function of wavelength of the incident light. The measured potentials range from -100 mV to +100 mV as the wavelength of the incident light is tuned from high frequency blue light to low frequency red light.

We believe our results are exciting for two fundamental reasons: First, we have demonstrated a new way to generate an electrical potential by the absorption of radiation. There is general interest in materials that can convert light to electrical potentials for sensing and for optical power conversion, for example, and our report lays the groundwork for these possible applications. Second, we believe our analysis provides more insight into the basic thermodynamic behavior of plasmonic materials. Given the very active research in this area by scientists from many different disciplines, these insights may open new opportunities in plasmonics research.

[1] Matthew T. Sheldon, Jorik van de Groep, Ana M. Brown, Albert Polman, Harry A. Atwater, "Plasmoelectric potentials in metal nanostructures". Science, 346, 828–831 (2014). Abstract.
[2] Albert Polman, "Plasmonics Applied". Science, 322, 868–869 (2008). Abstract.
[3] Carolina Novo, Alison M. Funston, Ann K. Gooding, Paul Mulvaney, "Electrochemical Charging of Single Gold Nanorods". Journal of the American Chemical Society, 131, 14664–14666 (2009). Abstract.
[4] S. K. Dondapati, M. Ludemann, R. Müller, S. Schwieger, A. Schwemer, B. Händel, D. Kwiatkowski, M. Djiango, E. Runge, T. A. Klar, "Voltage-Induced Adsorbate Damping of Single Gold Nanorod Plasmons in Aqueous Solution". Nano Letters, 12, 1247–1252 (2012). Abstract.
[5] Guillermo Garcia, Raffaella Buonsanti, Evan L. Runnerstrom, Rueben J. Mendelsberg, Anna Llordes, Andre Anders, Thomas J. Richardson, Delia J. Milliron, "Dynamically Modulating the Surface Plasmon Resonance of Doped Semiconductor Nanocrystals". Nano Letters, 11(10), 4415–4420 (2011). Abstract.

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Sunday, November 09, 2014

A Rare Middle-Weight Black Hole in a Nearby Galaxy

Dheeraj R. Pasham

Author: Dheeraj R. Pasham1,2

1Astronomy Department, University of Maryland, College Park, USA 
2Astrophysics Science Division and Joint Space-Science Institute, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

Black holes are among the most exotic and mysterious objects in the Universe, serving as one-way portals for matter, light, or anything else that gets too close. Our modern conception of black holes stems directly from Albert Einstein's theory of gravity, the general theory of relativity, proposed in 1915. However, it was not until the 1970s that new evidence emerged that these objects not only exist, but actually power the brightest objects in the Universe [1,2].

It is now established that there are at least two classes of black holes in the Universe: (1) the so-called "stellar-mass" black holes which weigh anywhere between 3-50 times the mass of our Sun and (2) "super-massive" black holes that are a million to a few billion times more massive than the Sun. Although we understand that the former are produced by spectacular deaths of the heaviest stars, the formation and the growth of super-massive black holes that are responsible for shaping the nature of many galaxies is still a mystery.

Understanding the formation of super-massive black holes holds a key to understanding the growth of galaxies that are the building blocks of our Universe. Current evidence indicates that super-massive black holes might have grown by accumulation of matter onto middle-weight black holes that are a few hundred to a thousand times more massive than the Sun and formed by the collapse the massive, first generation of stars formed when the Universe was only ~ 5% of its current age [3]. However, although candidates for such middle-weight black holes exist, no definitive mass measurements have yet been made. This is primarily because these objects are faint and thus the traditional methods used to weigh stellar-mass and supermassive black holes have not yet yielded any meaningful results [4,5].

In our recent result published in Nature [6], we used a new technique involving the 3:2 frequency ratio, X-ray resonance oscillations arising from close to the black hole in the galaxy M82, to measure its black hole mass to be 428±105 heavier than the Sun.
Figure 1 (click on the figure to view higher resolution version )

The basic idea behind the measurement is as follows. A subset of stellar-mass black holes exhibit the so-called high-frequency quasi-periodic oscillations (QPOs). Often in these systems the high-frequency QPOs occur in pairs of two, with their frequencies in a 3:2 ratio [7,8]. The power spectra of three such systems showing the twin pair QPOs are shown in Figure 1 [7]. The respective timescales of these oscillations (~0.01 seconds: 100-450 Hz) are comparable to the Keplerain orbital periods of test particles near the innermost stable circular orbit (ISCO) of these black holes. For example, for a non-rotating black hole weighing 10 solar masses, the Keplerian frequency of a test particle at ISCO is 220 Hz. In addition, for a given source, these frequencies appear to be stable to within a few percent for changes in the source luminosity. The fact that their frequencies are stable and appear to be originating from close to the ISCO (for a non-spinning black hole, the ISCO radius is 3 times the radius of the event horizon) suggests that they originate from very near to the black hole where strong gravity dominates and hence tied to the black hole's mass [9,10]. Under the assumption that these oscillations originate from a fixed characteristic radius within the accretion disk around the black hole, their frequencies should scale inversely with the black hole mass, and there is observational support that they do for stellar-mass black holes [7].

Some recent studies on X-ray variability of stellar-mass and supermassive black holes suggest that supermassive black holes behave as scaled-up stellar-mass black hole systems. More specifically, the qualitative nature of the variability of both the smaller stellar-mass and the heavier supermassive black holes appears to be the same (they appear to vary the same way) with the respective timescales of supermassive black holes being longer than than those of their stellar-mass counterparts. McHardy et al. (2006) [11] have demonstrated that these timescales scale inversely with the black hole mass after taking into account the rate at which matter fall onto the black hole. Under this black hole unification paradigm [11], if middle-weight black holes exist, some of them should exhibit these 3:2 pairs but at frequencies scaled down (longer timescales) according to their black hole masses.
Figure 2

Combing 6 years of archival X-ray data, we recently discovered such stable, twin-peak (3.3 and 5 Hz, 3:2 frequency ratio) X-ray oscillations from an object named M82 X-1 (see Figure 2) at frequencies roughly 50 times lower (or at timescales 50 times longer) than stellar-mass black holes. Scaling these frequencies to the oscillations of the black holes of known stellar mass implies that M82 X-1's black hole is 428±105 heavier than our Sun.

[1] C.T. Bolton, "Identification of Cygnus X-1 with HDE 226868". Nature, 235, 271 (1972). Abstract.
[2] B. Louise Webster, Paul Murdin, "Cygnus X-1—a Spectroscopic Binary with a Heavy Companion?" Nature, 235, 37 (1972). Abstract.
[3] Piero Madau and Martin J. Rees, "Massive Black Holes as Population III Remnants". Astrophysical Journal Letters, 551, L27 (2001). Abstract.
[4] T.P. Roberts, J.C. Gladstone, A.D. Goulding, A.M. Swinbank, M.J. Ward, M.R. Goad, A.J. Levan, "(No) dynamical constraints on the mass of the black hole in two ULXs". Astronomische Nachrichten, 332, 398 (2011). Abstract.
[5] D. Cseh, F. Grisé, P. Kaaret, S. Corbel, S. Scaringi, P. Groot, H. Falcke, E. Körding, "Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1". Monthly Notices of the Royal Astronomical Society, 435, 2896 (2013). Abstract.
[6] Dheeraj R. Pasham, Tod E. Strohmayer, Richard F. Mushotzky, "A 400 solar mass black hole in galaxy M82". Nature, 513, 74 (2014). Abstract.
[7] Ronald A. Remillard and Jeffrey E. McClintock, "X-Ray Properties of Black-Hole Binaries". Annual Review of Astronomy & Astrophysics, 44, 49-92 (2006). Abstract.
[8] T. M. Belloni, A. Sanna, M. Méndez,"High-frequency quasi-periodic oscillations in black hole binaries". Monthly Notices of the Royal Astronomical Society, 426, 1701 (2012). Abstract.
[9] Marek A. Abramowicz, Włodek Kluźniak, Jeffrey E. McClintock, Ronald A. Remillard, "The Importance of Discovering a 3:2 Twin-Peak Quasi-periodic Oscillation in an Ultraluminous X-Ray Source, or How to Solve the Puzzle of Intermediate-Mass Black Holes". Astrophysical Journal Letters, 609, L63 (2004). Abstract.
[10] Robert V. Wagoner, "Diskoseismology and QPOs Confront Black Hole Spin". Astrophysical Journal Letters, 752, LL18 (2012). Abstract.
[11] I. M. McHardy, E. Koerding, C. Knigge, P. Uttley, R. P. Fender,"Active galactic nuclei as scaled-up Galactic black holes". Nature, 444, 730 (2006). Abstract

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Sunday, November 02, 2014

Antineutrino Monitoring for the Iranian Heavy Water Reactor

[From Left to Right] Eric Christensen, Patrick Huber, Patrick Jaffke, Thomas E. Shea

Authors: Eric Christensen1, Patrick Huber1, Patrick Jaffke1, Thomas E. Shea2

1Center for Neutrino Physics, Virginia Tech, Blacksburg, Virginia, USA
2TomSheaNuclear Consulting Services, Gorgengasse 10/25, 1190 Vienna, Austria

Antineutrino reactor monitoring has been proposed in the 1970s; steady progress in detector technology will make this technology practically feasible within 1-2 years. In this contribution we investigate how this decades-old idea can be applied to the recent situation in Iran, more specifically we discuss the benefits of using antineutrinos to monitor the heavy water reactor at Arak, the IR-40. One of the key findings is that a detector situated outside the reactor building would meet the verification goals identified by the International Atomic Energy Agency for plutonium production or diversion from declared inventories.

All nuclear reactors produce plutonium during operation, however only very specific types of reactors are suitable for the production of weapons-grade plutonium. Historically, all plutonium used in nuclear weapons has been obtained from either heavy water or graphite moderated reactors fueled with natural uranium. At the beginning of their respective weapons programs these reactors typically had a thermal power output in the 20-250 MWth range, which is considerably less than typical commercial power reactors. As shown in previous work [1,2] reactors with a power output in this range represent a particularly suitable target for antineutrino reactor monitoring.

Iran is constructing a 40 MWth heavy water moderated, natural uranium fueled reactor at Arak, the so-called IR-40. Given its design characteristics, this reactor is ideally suited to make weapons-usable plutonium with an annual output of about 10 kg. It is estimated that about 4 kg of plutonium are sufficient to make a simple explosive device with a yield upwards of several kilotons of TNT equivalent [3]. Iran states that the IR-40 will be used exclusively for peaceful purposes, in particular isotope production for medical applications. Nonetheless, the IR-40 is, after the uranium enrichment program, one of the key issues in the ongoing negotiations between Iran and the P5+1 countries (i.e., these are the five permanent members of the UN Security Council: United States, Russia, China, United Kingdom, France plus Germany). A possible solution could be the conversion of the reactor to operate with enriched uranium reducing its plutonium output significantly without limiting its ability to provide isotopes for civilian applications [4].

Any agreement reached about the the future of the IR-40 will present a challenge for effectively monitoring its implementation. The historic example of the first nuclear crisis in the Democratic People's Republic of Korea (DPRK) in the 1990's serves as a stark reminder that providing reliable safeguards and timely warning of possible breaches is very difficult in a host country which is not deterred by international isolation. In the DPRK example the consequences were intermittent, but critically timed, denials of inspector access at the required level and ultimately, this allowed the DPRK to further its nuclear weapons capabilities while negotiating for an end of the plutonium program [5].

Antineutrino monitoring was proposed in the late 1970s by Borovoi and Mikaelyan [6]. The number of antineutrinos produced and their energy spectrum depends in a well-defined manner on the reactor power and on the relative contribution to fission from the various fissile isotopes. Thus a careful measurement of both the number of antineutrinos and their energy distribution allows, in principle, to infer the reactor power and the amount of plutonium in the core. Plutonium production is an inevitable result of reactor operation and thus in itself not indicative of any malfeasance. A quantitative indicator for a diversion of plutonium is a mismatch between the amount of plutonium produced by a reactor core and the amount of plutonium residing in the reactor core. Neither of these two quantities is easily measured directly and thus, suitable proxies are used: the reactor's power history provides a means to compute the amount of plutonium produced whereas a detailed history of reactor fueling allows to track the in-core plutonium content. Both indicators rely on a complete, uninterrupted data record -- should there be any loss of this so-called continuity of knowledge (CoK), the ability to determine any mismatch between produced and in-core plutonium is lost; any recovery is difficult and may be only partial. We argue that antineutrino monitoring could provide a robust and non-intrusive method to recover from a loss of the CoK.

Specifically, we consider a hypothetical IR-40 case which to some extent draws on the experience with the DPRK: There is full safeguards access for N-1 months. The reactor is shut down in the Nth month and at the same time the CoK is lost. The reasons for loss of the CoK can span a wide range from merely technical issues, to a diplomatic standoff, to an attempt at proliferation. The basic question arising in this scenario is: Was the reactor refueled or not? Obviously, finding an answer in a timely fashion would be of prime interest to the international community and in many cases also be in the best of Iran's interests.

In our earlier work [1] we demonstrated quantitatively that a measurement of the energy spectrum of antineutrinos emitted from a reactor makes it possible to determine the burn-up and, thus, the plutonium content with good accuracy and in a timely manner. Here, we make the same assumptions as in Refs.[1,2] about the detection system, we assume that product of efficiency and number of target protons is 4.3 X 1029, which for a realistic detector translates to a detector mass in the 5-15 ton range. This is still light enough to envisage a detector system which fits inside a standard 20 feet intermodal shipping container. Furthermore, we assume sufficient background rejection capabilities to allow for surface operation.

For a long time this was considered not possible for antineutrino detectors, but recently a Japanese group succeeded in the detection of reactor antineutrinos from the back of a van [7]. Nonetheless, we have to point out that at this point in time no detector system with all the required characteristics exists. At the same time, there are about a dozen collaborations world-wide attempting to produce suitable detectors (for a different purpose) and it seems very likely that fully functional prototypes will demonstrate feasibility within 12 months. In our estimate, a detector inside its shipping container can be deployed outside the reactor containment building of the IR-40 at a distance of 19 m from the center of the reactor core.

Figure 1: In the upper panel, data points show the event rate spectrum obtained in a 90 day data taking period for a core of average age of 45 days. The error bars indicate the statistical error in each bin. The blue line indicates the corresponding expected event rate spectrum for a core of average age of 315 days. The lower panel shows the difference in event rates between the 45 day core and the 315 day core and the corresponding statistical error bars. Figure and caption from Ref.[2].

Our analysis of the IR-40 is using standard reactor physics calculations made using commercially available software and the details are given in Ref.[2]. This calculation establishes the relationship between the fission rates and the content of the fissile isotopes in the core. In Fig.1 we show the resulting event rate spectrum for a core of 45 day average age (data points with statistical error bars) and for comparison the expected event rates for a core of 315 days of age (blue line). It can be seen clearly, that the older core emits neutrinos with a lower mean energy corresponding to 7 kg of plutonium, whereas the fresh core has no plutonium. This difference remains statistically significant even in the presence of realistic systematic uncertainties.

Figure 2: Shown is the 1σ accuracy for the determination of the plutonium content of the reactor as a function of time in the reactor cycle. The data-taking period is 90 days each. Dashed error bars indicate the accuracy from a fit to the plutonium fission rate fPu, whereas the solid error bars show the result of a fit constrained by a burn-up model. The blue line indicates operation without refueling and the orange line indicates operation with a refueling after 270 days. Figure and caption from Ref.[2].

The quantitative sensitivity to the plutonium content is shown in Fig.2, where the vertical axis shows the amount of plutonium in the reactor core as a function of time. The blue curve shows the change of plutonium content for the case of no refueling, whereas the orange curve assumes that the irradiated core, containing 8\,kg of plutonium, was replaced with a fresh core after 270 days of irradiation. 270 days is the time at which the isotopic content of plutonium-239 [8] drops to 93% of all plutonium and thus formally ceases to be considered weapons-grade. Within the first 90 days after the IR-40 shutdown (shown as gray vertical band) the two cases can be clearly distinguished by the antineutrino monitoring data. Even partial core refuelings corresponding to as little as 2 kg of removed plutonium could be detected at 90% confidence level. Alternatively, a full core refueling would be detected within about 9 days at 90% confidence level.

To summarize, in this note, based on the results of Ref.[2] we demonstrate that antineutrino monitoring of the IR-40 would provide a high-level tool to assess the amount of in-core plutonium as well as the amount of produced plutonium. Both tasks can be accomplished withing the 90 day period set by the International Atomic Energy Agency (IAEA) and with a quantitative accuracy greatly exceeding 1 significant quantity (8 kg) as required by the IAEA. This technique is non-intrusive and independent from any other safeguards information, in particular the CoK is not required. This combination of features appears to be a considerable and practically valuable characteristic not offered by any other known method. Needless to say, these advantages would not only arise for antineutrino monitoring of the IR-40 but for any reactor with a power output in the 20-250 MWth range, which are the most likely candidates for being an entry point for a plutonium-based nuclear weapons program. Antineutrino reactor monitoring would not replace other techniques but in combination with those techniques can enhance the overall effectiveness and reliability of non-proliferation safeguards. A practical system appears feasible on a timescale of 1-2 years and the next step would be an actual antineutrino reactor monitoring experiment.

This work was supported by the U.S. Department of Energy under contract DE-SC0003915 and by a Global Issues Initiative grant by the Institute for Society, Culture and Environment at Virginia Tech.

[1] Eric Christensen, Patrick Huber, Patrick Jaffke, "Antineutrino reactor safeguards - a case study". arXiv:1312.1959v2 [physics.ins-det] (2013).
[2] Eric Christensen, Patrick Huber, Patrick Jaffke, Thomas E. Shea, "Antineutrino Monitoring for Heavy Water Reactors". Physical Review Letters, 113, 042503 (2014). Abstract.
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posted by Quark @ 6:37 AM      links to this post

Sunday, October 26, 2014

Testing the Strong-Field Dynamics of General Relativity

Tjonnie G. F. Li

[Tjonnie G. F. Li is the recipient of the 2013 Stefano Braccini Thesis Prize administered by the Gravitational Wave International Committee (GWIC) for his PhD thesis “Extracting Physics from Gravitational Waves: Testing the Strong-field Dynamics of General Relativity and Inferring the Large-scale Structure of the Universe” (PDF). His thesis work was carried out at Nikhef - Dutch National Institute for Subatomic Physics, the Netherlands and the Ph.D was awarded by Vrije Universiteit, Amsterdam, the Netherlands.

The Stefano Braccini Thesis Prize was established to honor the memory of a talented gravitational wave physicist whose promising career was cut short. Stefano worked with the French-Italian Virgo project, and contributed to the superattentuator design, to the integration and commissioning of Virgo and to its data analysis efforts. -- 2Physics.com]

Author: Tjonnie G. F. Li

Affiliation: Rubicon Postdoctoral Fellow, LIGO Laboratory, California Institute of Technology, USA.

Motion of celestial objects

Humans have been watching the sky for thousands of years. In early times, humans tracked the motion of the Sun and the Moon to make calendars and to associate it with Earthly events such as tides and seasons. By tracking the motion of celestial objects, the early notion of the orbit of the Sun, the Moon and the planets started to form. Building on earlier work developed by Greek astronomers, Claudius Ptolemy (90–168) introduced an accurate model of the planetary orbits by including the notion of a smaller circular orbit (epicycle) augmenting the primary circular orbit.

During the Renaissance, our knowledge of the sky started to change. Johannes Kepler (1571–1630) introduced three laws that described the planetary orbits as ellipses with the Sun at the focus. Later, Isaac Newton (1642-1727) showed that Kepler’s laws of planetary motion can be derived from a law that not only describes the motion of planets, but also describes how all objects are attracted to each other. Newton’s law of universal gravitation states that all objects “pull” on each other through the gravitational force, and the strength of this force is determined by the masses of the two objects.

Despite the success of Newton’s law of universal gravitation, it could not account for the shift in Mercury’s perihelion, the point in Mercury’s orbit that is closest to the Sun. It was Albert Einstein (1879–1955) who refined Newton’s law of universal gravitation by introducing the general theory of relativity. Einstein’s general theory of relativity states that the curvature of spacetime dictates the way in which matter flows through it, and conversely, matter curves spacetime around it. Einstein’s theory explained the shift in Mercury’s perihelion, and so far seems to be the correct description of the motion of planets, stars and even galaxies.

Gravitational waves: a new window into the Universe

The general theory of relativity does more than just predicting the motion of objects. It also predicts a new type of radiation, known as gravitational radiation or gravitational waves. Gravitational waves are ripples in the curvature of spacetime, which propagate at the speed of light. The effect of gravitational waves is the periodic expansion and contraction of space and time (see Fig. 1).
Figure 1: Example of the distortion of spacetime due to a incident gravitational wave onto a ring of test particles. The top and bottom row represent the effects of the two polarisation states as a function of the phase of the gravitational wave.

The existence of gravitational waves has only been inferred indirectly through the motion of two stars orbiting each other. In particular, in 1974, Russell Hulse and Joseph Taylor found two pulsars (neutron stars that emit highly collimated beams of electromagnetic radiation) in a binary system that appeared to behave exactly as if the system was loosing energy and angular momentum in the form of gravitational waves (see Fig. 2). Today this discovery is regarded as the first indirect evidence of gravitational waves, and earned Hulse and Taylor the 1993 Nobel Prize in Physics [1].

Figure 2: Change in the time of the periastron of the binary pulsar “PSR B1913+16” as a function of time (red dots). These observation are compared to the prediction of general relativity (blue line). This data is considered as the first indirect evidence of gravitational waves.

Quest for strong gravity

So does this mean that general relativity has been fully verified? From a theoretical perspective, we might be inclined to say that general relativity cannot be the final answer, because of the current inability to describe it using a complete quantum theory. Therefore, it is currently not possible to unify gravity with the other forces of nature (electromagnetic, strong and weak force) into a grand unified theory, which some might argue is an indication that general relativity cannot be the final answer. In other words, a theory must exist that, in the low-energy regime, behaves like general relativity.

From an experimental perspective, one can argue that all of the tests of general relativity have so far been done in the regime of weak gravity. A figure of merit which describes the strength of gravity is the quantity ϵ ~ GM∕(Rc2), where G is the gravitational constant, M is the total mass of the system, R is the characteristic length scale of the system and c is the speed of light. Near a black hole the strength is ϵ ~ 10-1, whereas for solar system tests, and for binary pulsar tests, this strength is about ϵ ~ 10-6 [2]. Therefore, there is a whole new regime of gravity to explore experimentally.

Scientists all over the world are working hard on the quest for strong gravity. Amongst many interesting questions, they also hope to uncover empirical insight into the quantisation of gravity, which could refine or guide new theories of gravity. One of the ways in which we could hope to probe the regime of strong gravity is through the direct measurement of gravitational waves. Such measurements could probe gravity close to black holes and other exotic astrophysical objects.

Advanced LIGO and Virgo

Large-scale physics experiments such as the USA-based LIGO (see Fig. 3) [3] and the Italy-based Virgo [4] aim to, for the first time in the history of mankind, detect the influences of gravitational waves directly. These experiments are set up to measure tiny changes in distances of about one thousandth of the diameter of a proton. These tiny perturbations of spacetime could lead us down a new path in our quest for strong gravity.

Figure 3: Aerial view of the LIGO-Hanford detector

The motion of the source closely dictates the characteristics of the gravitational waves emitted. So by mapping out the distortions caused by the incident gravitational wave, one could infer a wealth of information about its origins. In other words, where astronomers needed telescopes to determine the motion of planets, stars and galaxies, measurements of gravitational waves can provide an additional way to map the dynamics of celestial objects.

In particular, a promising class of candidates for the first detection of gravitational wave is the compact binary coalescence [5]. Compact binary coalescence typically refers to (especially in the context of LIGO/Virgo) the mergers of binary black holes or neutron stars (see Fig. 4). The components of such systems spiral toward each other as energy and momentum are radiated away through the emission of gravitational waves. Finally, when the objects are sufficiently close to each other, they merge to form a single black hole which then continues to ring down as it reaches a quiescence state. The dynamics of coalescence of a compact binary can be seen through simulations as in Ref. [6].
Figure 4: Image from a binary black hole simulation.

Testing strong-field gravity with compact binary coalescences

Compact binary coalescences are attractive systems to probe strong gravity, because close to black holes and neutron stars the effects of gravity can be considered strong. Moreover, these systems are relatively easy to understand theoretically, because they mainly involve the application of general relativity. In contrast, mechanisms behind, for example, supernovae, which are also candidates to be measured by Advanced LIGO/Virgo, involve a complicated interplay amongst many branches of physics. The direct measurement of gravitational waves emitted from a compact binary coalescence will therefore give us access to the motion of black holes in orbit around each other’s strong gravitational pull.

However, in order to extract this information, we need specialised algorithms to dig deep into the data. One of such algorithms is called Test Infrastructure for GEneral Relativity (TIGER). This algorithm tries to answer the question “is the signal consistent with general relativity?” through the application of Bayesian hypothesis testing [7]. This framework ensures the optimal use of available information, and allows one to combine information across multiple detections of compact binary coalescences. Using this algorithm in a simulation environment, we have shown that the Advanced LIGO-Virgo network is indeed capable of probing gravity in uncharted territories, to an accuracy never seen before.

Of course, many challenges have to be faced. Detection of gravitational waves is a major challenge by itself on which hundreds of scientist are currently working. Moreover, once the Advanced LIGO-Virgo network is making confident detections, we need to analyse the motion of black holes or neutron stars in the presence of noise that can be orders of magnitude louder than the signal. Nevertheless, we are on the brink of the first direct detection with Advanced LIGO coming online as early as 2015. A hundred years after the introduction of general relativity, Advanced LIGO/Virgo could either put the crown on Einstein’s work, or showcase its limitations.

To be continued…

[1] “The Nobel Prize in Physics 1993”. Link in: Nobelprize.org.
[2] C. M. Will. “The Confrontation between General Relativity and Experiment”. In: Living Reviews in Relativity 17.4 (2014). Link.
[3] http://www.ligo.org .
[4] http://wwwcascina.virgo.infn.it .
[5] B.S. Sathyaprakash and Bernard F. Schutz. “Physics, Astrophysics and Cosmology with Gravitational Waves”. In: Living Reviews in Relativity 12.2 (2009). Link.
[6] Download from: http://numarch.aei.mpg.de/numrel-webpages/movies/bbh08_small.mov .
[7] T. G. F. Li, W. Del Pozzo, S. Vitale, C. Van Den Broeck, M. Agathos, J. Veitch, K. Grover, T. Sidery, R. Sturani, A. Vecchio, “Towards a generic test of the strong field dynamics of general relativity using compact binary coalescence”. Physical Review D, 85, 082003 (2012). Abstract.

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