Paragraph free essay sample

Digital Bangladesh Digital Bangladesh is a wonderful vision that is dreamt by the government and the literate class for the technological development of Bangladesh. Digitalization has become a buzzword in the new era of information technology. We can now learn in an instant what is happening in the furthest corner of the world. The electronic transfer of information via the internet has created an interconnected world of information. Bangladesh is going to observe digital year in 2011 to mark the Journey to digital Bangladesh. The government has taken up an initiative for setting up Union Information Centres (UIC) in 1000 unions in line with the dream to build Digital Bangladesh by 2021. This programme is a welcome development towards reaching information technology facilities to the doorsteps of rural people. The village people will easily get information about agriculture, health, education, marketing and employment from the UlCs. The government has also moved to formulate new laws empowering an authority to set up six hi-tech parks with the aim of establishing oreign investment in the information technology sector. We will write a custom essay sample on Paragraph or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page The Supreme Court (SC) is going to be digitalized soon. All cases related to information of the SC will be available in its website. Dhaka Metropolitan Police is also going to install a hi-tech monitoring system in the central control room to make contact with the on-duty police anywhere within the metropolitan area. Digitalization will largely change banking and financial activities. Worldwide money transfer and transaction of usiness have now become a matter of clicking the mouse of a computer. Some products like software and TV programmes are also amenable to digital transmission. We will be able to buy and sell goods through the electronic screen if we can make a Digital Bangladesh. Undoubtedly, encouraging development towards digitalization of Bangladesh by 2021 can be possible. The ambitious dream for a Digital Bangladesh will remain a dream if the government can not equip the people with the technology needed for establishing the digital era.

Het Behouden Huis essays

Het Behouden Huis essays Ik heb dit boek gekozen nadat ik de achterkant had gelezen, hierop stonden een paar uitspraken over dit boek die mij nieuwsgierig maakten, zoals bijvoorbeeld: het behouden huis heeft zeker niets kinderlijks en dient zeer nadrukkelijk voor volwassenen te worden voorbehouden. Jos. Panhuysen, Het Binnenhof 10-5-52. of Het is in het geheel niet aangenaam om te lezen . Het boek van Anne de Vries zal dat voor de meeste meer zijn. Jos. Panhuysen, De Gelderlander 21-5-52. Ik had nog nooit van deze mensen gehoord maar toch wilde ik graag weten wat ze met deze uitspraken bedoelden. Ook is het een boek dat over de oorlog gaat, wat ik altijd een interessant onderwerp vindt om over te lezen. Ik verwachtte een spannend boek waar veel in gevochten zou worden, maar na het lezen bleek het een vrij saai boek te zijn dat eigenlijk helemaal niet leuk is om te lezen, net als verteld werd in de recensies. Als soldaat in een partizanen leger ergens in Oost Europa bevrijdt de hoofdpersoon een dorp, In het leger zelf heeft hij geen vrienden, hij kan de meeste mensen zelfs niet verstaan. De enige waar hij een beetje contact heeft is een Spanjaard die hij Yesero noemt, ook al is dat niet zijn echte naam. In het dorp krijgt hij van zijn sergeant opdracht te gaan zoeken naar boobytraps, en tijdens zijn tocht door het dorp ziet hij een mooi huis staan. Hij gaat naar binnen en verbaast zich erover dat alles nog zo netjes en onderhouden is, er is zelfs nog warm water. Als hij een tijdje in het huis is en een warm bad genomen heeft besluit hij dat hij hier blijft wonen en zich uitgeeft voor de eigenaar van het huis. Op een gegeven moment staat er een Duitse officier bij hem aan de deur die hem vraagt of er Duitse soldaten in het huis gestationeerd mogen worden. Hij stemt toe om zichzelf niet te ve ...

Management Case Analysis (Boeing Case) Essay Example | Topics and Well Written Essays - 750 words

Management Case Analysis (Boeing Case) - Essay Example Place: As an international player, it has a broader market base to serve which can ensure a perennial demand for its products that a down trend in one economy may be compensated by the demand hike in another. However, being an international player in itself has its own costs that if all the segments of the market are not properly served, chances that when one economy is experiencing a downtrend, the other segment may be favoring its rival’s products. Thus search for newer markets like the developing nations etc. should always be forayed into. Product: Customizing and at the same time convincing officials to save the jobs of its workers like the incident at Los Angeles proves that the company is striving hard to make the both ends meet to gain the confidence of both the workers and customers also. The main point to note is that pricey contracts cannot be struck by anybody’s intention to help. The deals are struck only when the company identifies the needy customers who are ready to pay for the services of the company. The developing markets may throw an opportunity in this regard to the company. Promotion: The Company exhibits confidence in its estimates to grow up to 35% from the current 26% of the market share within 20 years from now which amounts to sale of 3890 Cargo flights in a year in place of current number of 1,950 flights per year. Investments for this size of market have to be arranged at a low cost model failing which; the company may fall short of its estimates. With other players lagging behind in the cargo section, a diligent performance can win accolades to the company in the cargo section. Price: The currently operating commercial flights are aptly priced that the company is able to maintain its second position in the market notwithstanding the evils of delay in the trial run. However, the rival is gaining more on the basis of pricing that its products are delivering higher advantage for the price they demand. If the

Globally relevant Essay Example | Topics and Well Written Essays - 1500 words

Globally relevant - Essay Example This results in no disarmament at all. Also, certain counties have threats from other countries. Such countries have, therefore, a national interest in having weapons. Countries that have weapons are facing a lot of pressure from other countries. The powerful countries of the world are co-operating the least in the movement against weapons. Such countries have the capacity to create weapons of mass destruction that include nuclear and biological weapons. However, poorer countries are also gaining access to such weapons. That is continuously giving chances to terrorists to strengthen themselves and create big disasters. Further, many countries allot a great portion of their budgets to making or acquiring of weapons. The same amount can also be spent on education, environment or other development projects, but it does not happen due to the requirement of a strong military. There is also very limited gun control in many countries which is why the percentage of gun violence related incid ents is very high. According to Stockholm International Peace Research Institute (SIPRI)’s recent trends summary, world military expenditure in 2010 was estimated to have reached $1.63 trillion at 2010 prices. This is almost 2.6 percent of world gross domestic product (GDP) or $236 for each person in the world. Out of the total military expenditure in the whole world, the expenditure by the US accounts for 41% of it. SIPRI also found out that the major portion of the total military expenditure in the world is made by large countries. 15 major countries of the world spend 81.8% of the total military expenditure. An important point is that the world faced a very bad economic depression in 2008 hence resulting in many countries cutting their spending in various sectors. However, the spending in the military sector is continuously on a rise. It does not seem to be justified but it has valid reasons. (GlobalIssues) Just before the time the depression arrived, it was not reasonably foreseeable. It could not have been expected that a crisis of such magnitude was about to hit. Many countries were happy with their economic growths and they had easy access to credit. They had their fixed or pre-planned foreign policy objectives, the knowledge of available resources were satisfactory, the peacekeeping operations were expected to go as normal and the policies were fixed. Countries like China and India saw a boom in the economies and they increased the spending on military. Also, the high prices of minerals and fossil fuels enabled quite a few countries to increase military expenditure. However, after the 2008 financial crisis had hit, military spending still appears to have increased. It has been observed by SIPRI that some nations like China and India did not face a financial crisis but continued to grow. As the financial crisis hit big countries like the US, the governments responded by employing expansionary fiscal policies according to which they increased the government expenditure. Among these expenditures which were made to counter the crisis, many new military projects were started too which is why there has been an increase in the total military spending. Most importantly, many countries have continued to put the strategic and geopolitical concerns above other matters. If these concerns demanded an increase in military expenditures, the governments did not hesitate to increase the spending despite dire economic straits. In contrast,

Cyber War Essay Example | Topics and Well Written Essays - 250 words

Cyber War - Essay Example This is proofing to be a threat in 21st century. However, there is one serious risk of cyber war. This threat is an attack of the computer systems by the malware (Gartzke, 2012). This is because it can cripple a country infrastructure with a coordinated move. This means the primary sectors of the economy will fail to perform, and this can lead to unprecedented loss. For example, if the cyber criminals attack power plants, the whole nation could be plunged into darkness (Gartzke, 2012). This mean most of the countries can remain in a position that was there two hundred years ago. This can paralyze a country before the problem is rectified. Huge amount of resources will need to be mobilized to resolve the threat. According to my thinking, the first country to establish a large-scale cyber offensive will be South Korea. This is because the country is heavily dependent on the internet. In fact, virtually all sectors of the economy depend on the latest technological advancements. North Korea, on the other hand, has little to lose, as it is less dependent on the technology (Gartzke,

The Transmission Electron Microscopy Biology Essay

The Transmission Electron Microscopy Biology Essay The transmission electron microscope operates on the same basic principles as the light microscope but uses electrons instead of light. What you can see with a light microscope is limited by the wavelength of light. TEMs use electrons as light source and their much lower wavelength make it possible to get a resolution a thousand times better than with a light microscope. TEM uses a technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera. TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small de Broglie wavelength of electrons. This enables the instruments user to examine fine detail-even as small as a single column of atoms, which is tens of thousands times smaller than the smallest resolvable object in a light microscope. TEM forms a major analysis method in a range of scientific fields, in both physical and biological sciences. TEMs find application in cancer research, virology, materials science as well as pollution, nanotechnology, and semiconductor research. History of TEMs The first operational electron microscope was presented by Ernst Ruska and Max Knoll in 1932, and 6 years later Ruska had a first version on the market. In 1986 Ruska received a Nobel Prize in physics for his fundamental work in electron optics and for the design of the first electron microscope. The following table gives a basic outline of the history of the electron microscope by decades. Year Specimens Application/development Instrumentation/theory Resolution 1940s Replicas oxide carbon plastics surfaces slip steps extracted particles fractography -50kV, single condenser -little or no theory; a first basic theory of electron microscopy was published in 1949 by Heidenreich. ~10nm 1950s Thin foils: from bulk deposited defects phase transitions -100kV -contrast theory developed. ~0.5-2nm 1960s metals semiconductors ceramics minerals Dynamic in-situ studies substructure of solids radiation damage microdiffraction -high voltage electron microscopes (Toulouse: 1.2 and 3MeV) -scanning electron microscopes -accessories for in-situ studies -controlled experiments 0.3nm (transmission) ~15-20nm (scanning) 1970s catalysts quasicrystals High resolution imaging lattice imaging -Analytical transmission electron microscopy -scanning transmission electron microscopy -energy dispersive x-ray spectra -electron energy loss spectroscopy -commercial high voltage electron microscopy (0.4-1.5MeV) -high resolution imaging theory 0.2nm (transmission) 7nm (standard scanning) 1980s virtually all materials atomic resolution in close-packed solids surface imaging small particles -commercial medium-voltage high-resolution/analytical electron microscopy (300-400kV) -improved analytical capabilities -energy filtering imaging -ultra-high vacuum microscopes 0.15nm (transmission) 5nm (scanning at 1kV) 1990s fast computation for image simulation alloy design nanostructures integrated digital scanning and image processing -surface atomic microscopy -orientation imaging microscopy 0.1nm (transmission) 3nm (scanning at 1kV) 2000s Electron microscopy in the 1960s In 1969 RCA dropped out of the electron microscope business, having decided that they could make more money selling record albums and consumer electronic devices.   General Electric had never become a major power in the electron microscope business. This left the field wide open for companies such as JEOL, Hitachi, and Akashi in Japan, and Philips, Siemens, and Zeiss in Europe. The resolution of the best TEMs was now approximately 0.3 nm (3 Ã…); JEOL claimed a resolution of 0.2 nm (2 Ã…) for its 1968 model JEM-100B. Accelerating voltages were still typically in the 100 kV range, although JEOL marketed a 200 kV instrument in 1967 called the JEM-200. Philips marketed a very popular 100 kV microscope called the EM 300 in 1966. They claimed that this was the first fully-transistorized electron microscope, and that it could attain a point resolution of 0.5 nm (5 Ã…). More than 1,850 units of the EM 300 were sold. Another approach to the study of materials that emerged in the 1960s involved increasing the accelerating voltage of the electron gun to extreme levels up to 3 MeV in an effort to penetrate more deeply into thicker samples. CEMES-LOE/CNRS at Toulouse, France, developed a 3MeV instrument around 1965, followed closely by JEOL, which released a 1 MeV microscope, the JEM-1000, in 1966. (One MeV represents a million electron volts, while one kV is a thousand electron volts. So 1,000 kV= 1 MeV.) These ultrahigh voltage EMs were so large that they typically occupied their own two-story building. The electron gun and its associated high voltage electronics were located near the ceiling of the second story, while the operator sat at the bottom of the microscope column looking at the fluorescent screen. Hitachis 1964 model HU-500 stood 4 meters tall; later, higher MeV versions eventually made this look small. On the left is a photograph of the 1 MeV Atomic Resolution Microscope (ARM) at the Lawrence Berkeley Laboratory. Electron microscopy in the 1970s The 1970s were a time of rapid development on all fronts in the electron microscope industry. Further improvements in TEM came from brighter electron sources (lanthanum hexaboride and field emission guns). The resolution of the TEM was pushed to 0.2 nm (2 Ã…) in the 1970s, with better results reported in some cases for lattice imaging resolutions; Hitachi claimed a 1.4 Ã… lattice resolution for its 1975 model H-500 TEM, and JEOL claimed the same resolution for its 1973 model JEM-100C. Accelerating voltages of 100 kV maximum had become the norm. In contrast to the low cost instruments, Philips 1972 model EM 301 TEM was designed for high performance and versatility for the skilled operator who had the time to coax the best results from his instrument. The EM 400 introduced in 1975 used a LAB6 electron gun, which was ten times as bright as the standard tungsten filament at the time. On the down side, the reactivity of lanthanum hexaboride required an ultra-clean vacuum system of 10-6 Torr. In 1977 Philips introduced accessories for the EM 400, including a secondary electron detector for topographical studies and a field emission gun (FEG) a single crystal tungsten tipped filament that emits electrons from a very localized region of the tip to produce narrow, bright electron beams. FEGs can have100 to 1,000 times the brightness of a LAB6 filament, with electron beam diameters as small as 1 nm. Vacuum requirements for these FEGs are 10-10 Torr. JEOL started with the JEM-100B Analytical model in 1970, which added scanning ability and an EDX x-ray spectrometer to the TEM. This was improved upon by the JEM-100C in 1973, with its 1.4 Ã… resolution, and further upgraded by the JEM-100CX Analytical model in 1976, which added an ultraclean vacuum system and a LAB6 electron gun. In the ultrahigh voltage EM market, The Hitachi 3MeV HU-3000 was installed at Osaka University in 1970. This accelerating voltage was the highest ever for an electron microscope. A resolution of 4.6 Ã… was reported for this instrument. The 1976 model H-1250 had a maximum voltage of 1250 kV, but a superior resolution of 2.04 Ã…. Electron microscopy in the 1980s During the 1980s TEM resolutions were further reduced to 1.0 to 1.5Ã…, making imaging of atoms in lattice planes possible. Microprocessor control of microscopes and computerized analysis of data became common due to the emergence of the personal computer in the early 80s. This microprocessor control brought about such features as an auto-stigmator and auto-focus, freeing the microscope operator from the mundane tasks that had always been involved in using the instrument. Electron energy loss spectroscopy (EELS) detectors were incorporated in STEMs and AEMs, allowing detection of low atomic number elements that could not be seen using x-ray techniques. The demands of the fast-growing integrated circuits industry produced electron microscopes designed for non-destructive testing of semiconductor wafers and for functional testing of ICs. Smaller electron beam sizes made it possible to switch from microprobe to nanoprobe technology. Elemental mapping of a samples surface could now be done on a nanometer level. Development of low cost instruments was not a priority in the 1980s. Some that were developed in the 1970s continued to be sold, but development was focused on high-performance, high-resolution, microprocessor-controlled instruments. JEOL produced 7 new TEM units between 1980 and 1986. These included the JEM-1200 EX (1981), which added microprocessor control to the JEM-100 CX (1976). The same model equipped with an EDS x-ray spectrometer was called the JEM-1200 EX/Analytical microscope. The 1984 model JEM-2000 FX/Analytical had a maximum voltage of 200 kV and a resolution of 2.8 Ã…; this instrument marked the switch from a microprobe beam to a nanoprobe. The JEM-4000 FX/Analytical microscope introduced in 1986 raised the acceleration voltage to 400 kV, which produced a beam probe size only 2 nm in diameter. After years of a standard 100 kV accelerating voltage with a few ultrahigh voltage units thrown in, these medium-voltage microscopes finally became popular. Electron microscopy in the 1990s The 1990s produced several corporate mergers in the electron microscope industry. Carl Zeiss and Leica joined to form LEO Electron Microscopy, Inc. In 1996 Philips bought Electroscan, the developer of the environmental SEM in the 1980s, to form Philips Electroscan. The following year Philips Electron Optics and a company called FEI merged under the name FEI to continue manufacturing electron microscopes. Hitachi and JEOL remained independent entities. The resolution of TEMs had already reached its theoretical limit (the best possible resolution predicted by calculations), so the 1Ã… resolution obtained using field emission gun (FEG) electron sources remained the standard. Medium voltage range instruments up to 300 kV were common, although 100 kV instruments still kept their long lasting popularity. Computers were now a vital part of every electron microscope, with graphical user interfaces (GUIs) being the norm. They were involved in both the control of the instrument and the processing of data, including post-analysis enhancement of micrographs using contrast-enhancing software. JEOL offered TEMs with maximum accelerating voltages of 120, 200, and 300 kV. The 120 kV model JEM1230 had a resolution of 0.2 nm (2Ã…). The JEM-2010 F FasTEM (200 kV) and the JEM-3000 F FasTEM (300 kV) both used FEG sources and achieved resolutions of 0.1 nm (1.0 Ã…). Three meetings of the Electron Microscopy Society of America (1968, 1975, and 1980) The Electron Microscopy Society of America (now known as the Microscopy Society of America) was founded in 1942, when it began holding annual meetings for instrument makers and users to gather and discuss the technology and its applications. The topics of papers given at these meetings present a snapshot of the state of electron microscopy at the time. A brief look at three of these meetings shows the evolution of the technology and its applications over a 12-year period. In the brief twelve-year span of 1968 to 1980, the physical sciences overtook the biological sciences at EMSA meetings, judging solely on number of papers presented. A large part of this development is probably due to the emergence of the scanning electron microscope in 1965, which made examination of the surface of bulk specimens possible for the first time. Since physical scientists could now look at real samples instead of replicas or thin films, activity in microscopy of materials increased dramatically. With no similar dramatic development in biological microscopy, the balance shifted. The Science of TEMs Comparison of Light (LM) and Electron Microscopes. a. Similarities 1) Illumination system: produces required radiation and directs it onto the specimen. Consists of a source, which emits the radiation, and a condenser lens, which focuses the illuminating beam (allowing variations of intensity to be made) on the specimen. 2) Specimen stage: situated between the illumination and imaging systems. 3) Imaging system: Lenses which together produce the final magnified image of the specimen. Consists of i) an objective lens which focuses the beam after it passes through the specimen and forms an intermediate image of the specimen and ii) the projector lens(es) which magnifies a portion of the intermediate image to form the final image. 4) Image recording system: Converts the radiation into a permanent image (typically on a photographic emulsion) that can be viewed. b. Differences 1) Optical lenses are generally made of glass with fixed focal lengths whereas magnetic lenses are constructed with ferromagnetic materials and windings of copper wire producing a focal length which can be changed by varying the current through the coil. 2) Magnification in the LM is generally changed by switching between different power objective lenses mounted on a rotating turret above the specimen. It can also be changed if oculars (eyepieces) of different power are used. In the TEM the magnification (focal length) of the objective remains fixed while the focal length of the projector lens is changed to vary magnification. 3) The LM has a small depth of field, thus different focal levels can be seen in the specimen. The large (relative) depth of field in the TEM means that the entire (thin) specimen is in focus simultaneously. 4) Mechanisms of image formation vary (phase and amplitude contrast). 5) TEMs are generally constructed with the radiation source at the top of the instrument: the source is generally situated at the bottom of LMs. 6) TEM is operated at high vacuum (since the mean free path of electrons in air is very small) so most specimens (biological) must be dehydrated. 7) TEM specimens (biological) are rapidly damaged by the electron beam. 8) TEMs can achieve higher magnification and better resolution than LMs. 9) Price tag!!! (100x more than LM) Figure below shows the cross-sectional view of a standard TEM. Figure shows the transmission electron microscope at The Chinese University of Hong Kong. Figure shows a schematic outline of a TEM. A TEM contains four parts: electron source, electromagnetic lens system, sample holder, and imaging system. A. Electron Source The electron gun produces a beam of electrons whose kinetic energy is high enough to enable them to pass through thin areas of the TEM specimen. The gun consists of an electron source, also known as the cathode because it is at a high negative potential, and an electron-accelerating chamber. There are several types of electron source, operating on different physical principles, which we now discuss. i. Thermionic Emission Figure 3-1 shows a common form of electron gun. The electron source is a V-shaped (hairpin) filament made of tungsten (W) wire, spot-welded to straight-wire leads that are mounted in a ceramic or glass socket, allowing the filament assembly to be exchanged easily when the filament eventually burns out. A direct (dc) current heats the filament to about 2700 K, at which temperature tungsten emits electrons into the surrounding vacuum by the process known as thermionic emission. Figure 3-1. Thermionic electron gun containing a tungsten filament F, Wehnelt electrode W, ceramic high-voltage insulator C, and o-ring seal O to the lower part of the TEM column. An autobias resistor, RB (actually located inside the high-voltage generator, as in Fig. 3-6) is used to generate a potential difference between W and F; thereby controlling the electron-emission current, Ie. Arrows denote the direction of electron flow that gives rise to the emission current. Raising the temperature of the cathode causes the nuclei of its atoms to vibrate with increased amplitude. Because the conduction electrons are in thermodynamic equilibrium with the atoms, they share this thermal energy, and a small proportion of them achieve energies above the vacuum level, enabling them to escape across the metal/vacuum interface. The rate of electron emission can be represented as a current density Je(in A/m2) at the cathode surface, which is given by the Richardson law: Where T is the absolute temperature (in K) of the cathode and A is the Richardson constant (~106Am-2K-2), which depends to some degree on the cathode material but not on its temperature; k is the Boltzmann constant (1.38 x 10-23J/K), and kT is approximately the mean thermal energy of an atom. ii. Schottky emission The thermionic emission of electrons can be increased by applying an electrostatic field to the cathode surface. This field lowers the height of the potential barrier (which keeps electrons inside the cathode) by an amount, the so-called Schottky effect. A Schottky source consists of a pointed crystal of tungsten welded to the end of V-shaped tungsten filament. The tip is coated with zirconium oxide (ZrO) to provide a low work function (~2.8 eV) and needs to be heated to only about 1800 K to provide adequate electron emission. Because the tip is very sharp, electrons are emitted from a very small area, resulting in a relatively high current density ( Je ~ 107A/m2) at the surface. Because the ZrO is easily poisoned by ambient gases, the Schottky source requires a vacuum substantially better than that of a LaB6 source. iii. Field emission If the electrostatic field at a tip of a cathode is increased sufficiently, the width (horizontal in Fig.3-4) of the potential barrier becomes small enough to allow electrons to escape through the surface potential barrier by quantum-mechanical tunneling, a process known as field emission. The probability of electron tunneling becomes high when the barrier width, w is comparable to de Broglie wavelength of the electron. This wavelength is related to the electron momentum p by p=h/ÃŽÂ » where h= 6.63 x 10-34 Js is the Planck constant. Because the barrier width is smallest for electrons at the top of the conduction band, they are the ones most likely to escape. Because thermal excitation is not required, a field-emission tip can operate at room temperature, and the process is sometimes called cold field emission. As there is no evaporation of tungsten during normal operation, the tip can last for many months or even years before replacement. It is heated (flashed) from time to time to remove adsorbed gases, which affect the work function and cause the emission current to be unstable. Even so, cold field emission requires ultra-high vacuum (UHV: pressure ~ 10-8 Pa) to achieve stable operation, requiring an elaborate vacuum system and resulting in substantially greater cost of the instrument. B. Electromagnetic Lens System The TEM may be required to produce a highly magnified (e.g, M = 105) image of a specimen on a fluorescent screen, of diameter typically 15 cm. To ensure that the screen image is not too dim, most of the electrons that pass through the specimen should fall within this diameter, which is equivalent to a diameter of (15 cm)/M = 1.5  µm at the specimen. For viewing larger areas of specimen, however, the final-image magnification might need to be as low as 2000, requiring an illumination diameter of 75  µm at the specimen. In order to achieve the required flexibility, the condenser-lens system must contain at least two electron lenses. The first condenser (C1) lens is a strong magnetic lens, with a focal length f that may be as small as 2 mm. Using the virtual electron source(diameter ds) as its object, C1 produces areal image of diameter d1. Because the lens is located 20 cm or more below the object, the object distance, u ~ 20 cm >> f and so the image distance v ~ f. The second condenser (C2) lens is a weak magnetic lens ( f ~ several centimeters) that provides little or no magnification (M ~ 1) but allows the diameter of illumination (d) at the specimen to be varied continuously over a wide range. The C2 lens also contains the condenser aperture (the hole in the condenser diaphragm) whose diameter D can be changed in order to control the convergence semi-angle of the illumination, the maximum angle by which the incident electrons deviate from the optic axis. Figure shows lens action within the accelerating field of an electron gun, between the electron source and the anode. Curvature of the equipotential surfaces around the hole in the Wehnelt electrode constitutes a converging electrostatic lens (equivalent to a convex lens in light optics), whereas the non-uniform field just above the aperture in the anode creates a diverging lens (the equivalent of a concave lens in light optics). C. Sample Holder To allow observation in different brands or models of microscope, TEM specimens are always made circular with a diameter of 3 mm. Perpendicular to this disk, the specimen must be thin enough (at least in some regions) to allow electrons to be transmitted to form the magnified image. The specimen stage is designed to hold the specimen as stationary as possible, as any drift or vibration would be magnified in the final image, impairing its spatial resolution (especially if the image is recorded by a camera over a period of several seconds). But in order to view all possible regions of the specimen, it is also necessary to move the specimen horizontally over a distance of up to3 mm if necessary. The design of the stage must also allow the specimen to be inserted into the vacuum of the TEM column without introducing air. This is achieved by inserting the specimen through an airlock, a small chamber into which the specimen is placed initially and which can be evacuated before the specimen enters the TEM column. Not surprisingly, the specimen stage and airlock are the most mechanically complex and precision-machined parts of the TEM. There are two basic designs of the specimen stage: side-entry and top-entry. In a side-entry stage, the specimen is clamped (for example, by a threaded ring) close to the end of a rod-shaped specimen holder and is inserted horizontally through the airlock. The airlock-evacuation valve and a high-vacuum valve (at the entrance to the TEM column) are activated by rotation of the specimen holder about its long axis; see figure (a). One advantage of this side-entry design is that it is easy to arrange for precision motion of the specimen. Translation in the horizontal plane (x and y directions) and in the vertical (z) direction is often achieved by applying the appropriate movement to an end-stop that makes contact with the pointed end of the specimen holder. A further advantage of the side-entry stage is that heating of a specimen is easy to arrange, by installing a small heater at the end of the specimen holder, with electrical leads running along the inside of the holder to a power supply located outside the TEM. The ability to change the temperature of a specimen allows structural changes in a material (such as phase transitions)to be studied at the microscopic level. Specimen cooling can also be achieved, by incorporating (inside the side-entry holder) a heat-conducting metal rod whose outer end is immersed in liquid nitrogen (at 77 K). One disadvantage of the side-entry design is that mechanical vibration  picked up from the TEM column or from acoustical vibrations in the external air, is transmitted directly to the specimen. In addition, any thermal expansion of the specimen holder can cause drift of the specimen and of the TEM image. These problems have been largely overcome by careful design, including choice of materials used to construct the specimen holder. As a result, side-entry holders are widely used, even for high-resolution imaging. In a top-entry stage, the specimen is clamped to the bottom end of a cylindrical holder that is equipped with a conical collar; see Figure (b). The holder is loaded into position through an airlock by means of a sliding and tilting arm, which is then detached and retracted. Inside the TEM, the cone of the specimen holder fits snugly into a conical well of the specimen stage, which can be translated in the (x and y) horizontal directions by a precision gear mechanism. The major advantage of a top-entry design is that the loading arm is disengaged after the specimen is loaded, so the specimen holder is less liable to pick up vibrations from the TEM environment. In addition, its axially symmetric design tends to ensure that any thermal expansion occurs radially about the optic axis and therefore becomes small close to the axis. However, in disadvantage views, it is more difficult to provide tilting, heating, or cooling of the specimen. Although such facilities have all been implemented in top-entry stages, they require elaborate precision engineering, making the holder fragile and expensive. Because the specimen is held at the bottom of its holder, it is difficult to collect more than a small fraction of the x-rays that are generated  by the transmitted beam and emitted in the upward direction, making this design less attractive for high-sensitivity elemental analysis. D. Imaging System The sample is placed in front of the objective lens in a form of thin foil, thin section or fine particles transparent for the electron beam. (Figure. 3). The objective lens forms an image of the electron density distribution at the exit surface of the specimen based on the electron optical principles. The diffraction, projection and intermediate lenses below the objective lens are used to focus and magnify either the diffraction pattern or the image onto a fluorescent screen, which converts the electrons into visible light signal. There are three important mechanisms, which produce image contrast in the electron microscope: mass-thickness contrast, phase contrast and diffraction or amplitude contrast. i. Mass-thickness contrast arises from incoherent elastic scattering of electrons. As electrons go through the specimen they are scattered off axis by elastic nuclear interaction also called Rutherford scattering. The cross section for elastic scattering is a function of the atomic number (Z). As the thickness of the specimen increases the elastic scattering also increases since the mean-free path remains fixed. Also specimens consisting of higher Z elements will scatter more electrons than low-Z specimens. This will create differential intensity in an image formed from thicker regions where fewer electrons will be transmitted to the image compared to a thinner or low atomic number region, which will be brighter in the image plane. In TEM, the mass-thickness contrast is affected by the size of the objective aperture and the accelerating voltage. Smaller apertures will increase the difference in the ratio of scattered and transmitted electrons and as a consequence will increase the contrast between regions of different thickness of mass. Lowering the accelerating voltage will lead to similar effect since the scattering angle and the cross section increase which also will cause increase in the relative contrast between higher mass and lower mass regions. ii. Phase contrast. Some of the electrons leaving the specimen are recombined to form the image so that phase differences present at the exit surface of the specimen are converted into intensity differences in the image. Phase contrast is the dominant mechanism for object detail iii. Diffraction contrast. Diffracted electrons leaving the lower surface of a crystalline specimen are intercepted by the objective aperture and prevented from contributing to the image. Alternatively only one diffracted beam forms the image. Diffraction contrast is the dominant mechanism delineating object detail >15 Ã… in crystalline specimens and is important and widely used contrast mechanism for study of crystal defects. Using this approach considerable quantitative information about the defect structure of the specimen may be obtained without operating the microscope at maximum resolution. Vacuum System Electron microscopes cannot operate in air for a number of reasons. The penetration of electrons through air is typically no more than 1 meter, so after coming on meter from the gun, the whole beam would be lost to collisions of the electrons with the air molecules. It is also not possible to generate the high charge difference between the anode and cathode in the gun because air is not a perfect insulator. Finally, the beam on the specimen while in air would trap all sorts of rubbish (air is full of hydrocarbon molecules) on the specimen, crack them (removing hydrogen, oxygen, etc.) and thus leave a thick carbon contamination layer on the specimen. Each electron microscope therefore has a vacuum system. The degree of sophistication of the vacuum system depends on the requirements. Simple imaging of biological thin sections is much less demanding than cryo applications or small-probe analysis in materials science and a thermionic gun can operate under much worse vacuum than a Field E mission Gun (FEG). The most basic vacuum system consists of a vessel connected to a pump that removes the air. The vacuum system of an electron microscope is considerably more complicated, containing a number of vessels, pumps, valves (to separate different vessels) and gauges (to measure vacuum pressures). From the bottom up we can distinguish four vessels in the vacuum system: The buffer tank The projection chamber The column (specimen area) The electron gun area Sometimes a tubomolecular pump (TMP), essentially a high-speed turbine fan, is used in place of (or to supplement) a diffusion pump. Usually an ion pump is used to achieve pressures below 10-4Pa, as required to operate a LaB6, Schottky, or field-emission electron source. By applying a potential difference of several kilovolts between large electrodes, a low-pressure discharge is set up (aided by the presence of a magnetic field) which removes gas molecules by burying them in one of the electrodes. Figure shows cross section through a diffusion pump. The arrows show oil vapor leaving jets within the baffle assembly. Water flowing within a coiled metal tube keeps the walls cool. Frequently, liquid nitrogen is used to help in achieving adequate vacuum inside the TEM, through a process known as cryo

Symptoms and Diagnosis of Meningitis and Encephalitis Essay -- Biology

Symptoms and Diagnosis of Meningitis and Encephalitis Abstract- Meningitis and Encephalitis symptoms are almost identical to those of the flu or common cold. Most symptoms are exceedingly subtle, and immediate diagnosis is crucial, because meningitis and encephalitis can become deadly in a matter of hours. There are many different forms of diagnosis, each equally important. Differentiating between bacterial and viral forms of the disease is important because treatment and severity differs. Meningitis is more prevalent in the elderly, the very young, and those with immune deficiency diseases. After his first trip to Africa, Brad Pitt came down with a mild case of viral meningitis. Luckily, Brad was diagnosed and treated in time by the finest doctors in Los Angeles. However, the meningitis could have turned deadly had the star not sought immediate medical attention. Meningitis can turn deadly in a matter of hours, but so few people recognize the symptoms. Mostly meningitis symptoms are comparable tom the common cold. There are signs, though, and accurate diagnosis procedures to ensure full and healthy lives for everyone. Meningitis is a potentially deadly disease with generally common symptoms. It can be either a viral or bacterial infection of a person?s spinal fluid. It also affects the fluid surrounding the brain. Meningitis usually started with either a viral or bacterial infection generally of the respiratory tract. Meningitis is much more common in the very young, very old, and people with immune systems that have been affected by an STD, such as HIV/AIDS. Symptoms in babies are often extremely difficult to detect as they cannot voice their grievances Meningitis symptoms in infants are almost impossible to visibly sepa... ...Control And Prevention. 12 Oct. 2005. 31 July 2006 http:///www.cdc.gov/ncidod/DBMD/diseaseinfo/meningococcal_g.htm. "Infections Meningitis." Kids Health For Parents. Ed. Elena Pearl Ben- Joseph, MD and Neil Izenberg, MD. Mar. 2004. Nemours Foundation. 31 July 2006 http://www.kidshealth.org/parent/infections/lung/meningitis.html "Meningitis." Wikipedia. 31 July 2006 http://en.wikipedia.org/wiki/ Meningitis. Meningitis. Ed. Mary L. Gavin, MD and Joel Klein, MD. Nov. 2004. Kids Health. 1 Aug. 2006 meningitis.html>. Encephalitis. Ed. Barbara P Homeier, MD and Joel Klein, MD. Jan. 2005. Kids Health. 1 Aug. 2006 bacterial_viral/encephalitis.html>. "Encephalitis." Wikipedia. 30 July 2006. 1 Aug. 2006 en.wikipedia.org/wiki/Encephalitis>. Symptoms and Diagnosis of Meningitis and Encephalitis Essay -- Biology Symptoms and Diagnosis of Meningitis and Encephalitis Abstract- Meningitis and Encephalitis symptoms are almost identical to those of the flu or common cold. Most symptoms are exceedingly subtle, and immediate diagnosis is crucial, because meningitis and encephalitis can become deadly in a matter of hours. There are many different forms of diagnosis, each equally important. Differentiating between bacterial and viral forms of the disease is important because treatment and severity differs. Meningitis is more prevalent in the elderly, the very young, and those with immune deficiency diseases. After his first trip to Africa, Brad Pitt came down with a mild case of viral meningitis. Luckily, Brad was diagnosed and treated in time by the finest doctors in Los Angeles. However, the meningitis could have turned deadly had the star not sought immediate medical attention. Meningitis can turn deadly in a matter of hours, but so few people recognize the symptoms. Mostly meningitis symptoms are comparable tom the common cold. There are signs, though, and accurate diagnosis procedures to ensure full and healthy lives for everyone. Meningitis is a potentially deadly disease with generally common symptoms. It can be either a viral or bacterial infection of a person?s spinal fluid. It also affects the fluid surrounding the brain. Meningitis usually started with either a viral or bacterial infection generally of the respiratory tract. Meningitis is much more common in the very young, very old, and people with immune systems that have been affected by an STD, such as HIV/AIDS. Symptoms in babies are often extremely difficult to detect as they cannot voice their grievances Meningitis symptoms in infants are almost impossible to visibly sepa... ...Control And Prevention. 12 Oct. 2005. 31 July 2006 http:///www.cdc.gov/ncidod/DBMD/diseaseinfo/meningococcal_g.htm. "Infections Meningitis." Kids Health For Parents. Ed. Elena Pearl Ben- Joseph, MD and Neil Izenberg, MD. Mar. 2004. Nemours Foundation. 31 July 2006 http://www.kidshealth.org/parent/infections/lung/meningitis.html "Meningitis." Wikipedia. 31 July 2006 http://en.wikipedia.org/wiki/ Meningitis. Meningitis. Ed. Mary L. Gavin, MD and Joel Klein, MD. Nov. 2004. Kids Health. 1 Aug. 2006 meningitis.html>. Encephalitis. Ed. Barbara P Homeier, MD and Joel Klein, MD. Jan. 2005. Kids Health. 1 Aug. 2006 bacterial_viral/encephalitis.html>. "Encephalitis." Wikipedia. 30 July 2006. 1 Aug. 2006 en.wikipedia.org/wiki/Encephalitis>.