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If you can’t find what you’re looking for, try the newer version.
CLICK HERE TO REDIRECT.
Image intensifier screens (fig below) allow the production of an image changing in real time, which can be advantageous, particularly when used with high contrast material like Ba which shows the passage of barium sulphate through the gut, for example. It does, however, subject the patient to long exposure times, hence high radiation dose. Since the objective is to minimize this, we look elsewhere for more sophisticated techniques.
Faster computing in the 1970’s yielded CAT scanning (Computerised Axial Tomography) Tomos is Greek for ‘slice’ and the technique involves imaging a slice through the body by rotating digital detectors and emitters, before moving on to the next slice, consequently reducing exposure time. It is to X-rays what digital cameras are to photography. A brain scan takes less than a second to image. (see p705) Resolution (remember Rayleigh) has much improved over the last three decades and kidney stones less than 2mm in diameter can be detected.
A disadvantage is that exceptional skill is required to diagnose with this technique.
Positron Emission Tomography. A source of positrons (beta + emitters) is injected. These annihilate with the electrons in the tissues of the patient producing a pair of identical photons traveling in opposite directions, each with an energy of 1.02/2 MeV = 0.511MeV. Detectors find the point of annihilation with a resolution of about 1mm, allowing abnormality at cellular level to be visualized . Best for metabolic studies with the biologically active molecule fludeoxyglucose (FDG), an analogue of glucose. The concentrations of tracer imaged will indicate tissue metabolic activity because of the local glucose uptake. Use of this tracer to explore the possibility of cancer metastasis (i.e., spreading to other sites) is the most common type of PET scan.
MRI Magnetic Resonance Imaging. better resolution, contrasts soft tissues, no patient dose, much more expensive.
Charged particles have a property called ‘spin’, aligning themselves either up or down in an external magnetic field. These two states have different energies. Energising a sample containing H for example with a radio frequency, a ‘spin-up’ proton absorbs resonant photons to a higher energy ‘spin down’ state, subsequently decaying back to spin-up with the emission of a characteristic photon of the same frequency, an event which detectors can record and display. The rate at which these transitions take place is the important part, since this is directly related to tissue type. The location mechanism depends on the application of uniform fields to act as a background to the resonant changes so only one plane has the correct value of B for absorption to occur. I think it unlikely that you will be asked for further detail.
X-rays are produced when charged particles like electrons are accelerated around nuclei. Since they are negatively charged, they are accelerated towards a positive nucleus, emitting as they curve around a ‘braking radiation’ or‘bremsstrahlung’ in German. The degree of ‘bend’ determines the energy of the X rays produced, so the radiation emitted is over a continuum of wavelengths .
Additionally, the electrons may directly promote other electrons in the metal atoms of the target from its lower to higher energy levels- when these decay back they emit characteristic X-photons. Here two characteristic jumps are superposed on the bremsstrahlung background. The relative intensity is a measure of how likely this event is.
We won’t go into details here, except the only real difference between these and gamma rays is that gamma rays are emitted spontaneously from excited nuclei. X rays pass through human tissue, being absorbed by denser material. In 1901, Wilhelm Roentgen was the first person ever to win the Nobel Prize for Physics and his discovery revolutionised the medical world.
Some schools have a small version of one of these.
Electrons are accelerated in a vacuum towards a metal target, W in this case. X rays are produced – notice they are not subject to a ‘law of reflection’ – and pass through and out of a collection window for use. At diagnostic voltages (140KV for a chest X-ray) the mechanism for energy loss in the body is photoelectric. The anode gets very hot and has to be cooled, the target is often rotated otherwise the heat generated would destroy it. The X rays are partially absorbed by the area of interest in the patient and the data collected on photographic film as a negative image where high absorption is seen as a light area and vice versa.
This site is fascinating. It also helps us to answer a few questions about how to do experiments.
This is a fossil skull of Homo Erectus, found in Africa. How old do you think it might be? Dating geological specimens involves an interdisciplinary approach using more than one dating method and cross-validating the results. Absolute dating methods include radiometric, luminescence and incremental dating. Relative dating methods fall under the science of stratigraphy.
Radiometric Dating Radiometric dating is based on the knowledge that certain naturally occurring radioactive isotopes decay, or transform into a different element, at known rates. The half-life, or amount of time it takes for half of the measured isotope to decay can be measured directly if it’s quite short, but long half lives can’t – we have to compare the percentage of the isotope today with its stable daughter or we have to know how much isotope there was before the decay started. In other words, an isotope with a very short half-life can’t be used to date very old fossils. Radiometric dating includes carbon dating, (illustrated) used to date specimens up to about 75,000 years old, and 40K (potassium)-40Ar (argon) dating, which is used to date much older fossils. This involves measuring the percentage of the K isotope which is radioactive (a long half life of 2,4×108y ) with Ar which is stable. The older the sample, the smaller the ratio K-Ar. This is how we know that the Earth is about 4,5 billion years old.
Luminescence Dating Geologists and archeologists use luminescence dating by observing photons, or light, emitted from minerals such as quartz, diamond, feldspar and calcite. As radiation from photons is stored in sediment layers, age since last exposure to sunlight can be calculated from this information.
Incremental Dating Incremental dating incorporates several techniques including dendrochronology, ice cores and varve analysis. Dendrochronology (tree-ring dating) is used to date wood fossils. Shallow ice cores are dated exactly by counting layers; each layer represents a year. Varve analysis is used to date archaeological specimens based on patterns of glacial deposit.
Stratigraphy This method is based on the assumption that geological layers can provide relative ages for the specimens found within them, provided that the deeper rock layers formed earlier than the shallower ones. Sequences of rock layering reveal the general patterns that describe the geochronology of the planet. Because of the way fossils are formed, fossil specimens found in rock layers must be older than the surrounding rock. Support for evolutionary theories is found in part from the observation that rock layers of similar age contain fossils of similar flora and fauna .
A short introduction is attached. This is a vast subject – no pun intended. I have left the wiki links intact so that you can explore further if you like. Click on the link below. Pictured is the impressive M31 in Andromeda.
More Distant Objects
Astronomy is an ancient science and it was only about 5-600 years ago that we began to make sense of what the ancients saw. The bedrock for modern thinking lies with Copernicus, Tycho Brahe – the first real astronomer – Johannes Kepler , Galileo and Newton. It’s an interesting study to trace the emergence of knowledge from these few names.
Imagine a time when the Earth and its planetary siblings were nothing but cosmic dust. Yet astronomers agree that this was the state of affairs some 4.5/4.6 billion years ago. Our sun was a fledgling protostar, continually amassing more matter via gravity and steadily cranking up its internal nuclear fusion. There was no solar system, only a giant, amorphous rotating cloud of particles called the solar nebula.
To figure out how all that leftover gas and dust led to planets, astronomers have largely studied the structure of our own solar system for clues. They’ve also looked to distant, younger solar systems still in varying stages of development.
With the formation of the sun, the remaining gas and dust flattened into a rotating protoplanetary disk. Within this swirling debris, rocky particles from long extinct supernova remnants began to collide, forming larger masses that soon attracted even more particles by gravitational attraction. Their volume contracted under gravity to create planetesimals, which collided with one another to become the solid inner planets. Meanwhile, gases froze into giant balls that would build the outer gas giants.
Why did rocky planets form closer to the sun and the gas giants farther away? One theory involves the solar wind, the steady flow of plasma that emanates from a star. When the sun first came into being, this wind was far stronger than it is today, strong enough to blast lighter elements such as hydrogen and helium away from the inner orbits. When these expelled elements reached the outer orbits, the strength of the solar wind dropped off. The gravity of the outer gas giants quickly drew these elements in, bloating them into their current forms: solid cores of rock and ice covered with gas.
This spreadsheet summarises much of our current understanding of our solar system. Kepler’s Laws do not need to be memorised but they are so fundamental they’re worth more than a look. Study this link also my spreadsheet entitled Solar System Data.
I want you to look for and note any patterns that you think you have found. In particular, use the spreadsheet data to verify the Law of Periods, in other words,
Make sure the units are SI.
Are the rules the same all over the Universe? Maybe not…
Strange Objects in Space