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More than 100 years back, when the world of science were moving on a rapid pace, Wilhelm Conrad Röntgen had discovered what he had coined as X-Rays, also known as Röntgen rays. The nature of the x-rays was unknown at the time of discovery. The main equipment used when the X-rays was found was of simple tools such as the discharge tube. He had "covered the discharge tube with a black cardboard, so that no visible light could escape". Still, a light was observed on a fluorescent substance placed at a distance from the tube when a high voltage was applied and the cathode rays struck the back of the tube. He was certain that the fluorescence was caused by an "invisible radiation which can penetrate cardboard, wood and thin sheet metals". The fluorescent substance used was "crystals of platinum barium cyanide". Very much later it was discovered that these rays were produced when the cathode rays or later known as electrons (electrons have not been discovered at that period) stopped when it struck an object. In addition to fluorescent produced in some salts, x-rays could also have an effect on photographic plate and ionizes gases. The dynamic life of x-rays were exploited early on right after the discovery of x-rays and employed in many fields besides physics, such as medicine, chemistry, biology and even astronomy till now. Many were able to make use of x-rays for their own experiments without any difficulty could be due to the fact that equipment used was easily obtained and also affordable. Many of the employed uses of x-rays were not only useful for research purposes but also of daily life.
According to the Columbia Encyclopedia, x-ray defined as "invisible and highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light". It was deduced from Sommerfeld's calculation that the x-rays have a range of wavelength from 0.01 to 0.1 nm, between the wavelength of gamma rays and UV light. The rays can neither be refracted nor has a reflection like light, yet it is electrically neutral like one. It travels with speed of light in a straight line. It also could be not be influenced by magnetic and electric field. This shows that the ray does not have any charge. The rays were, however, "diffusely scattered by all substances, and were partially absorbed by matters of all kinds". The level of absorption is much higher for elements of high atomic weight compared to low atomic weight. The scattered rays were also polarised in a certain direction, since when a second block is placed in the paths of the rays, it were scattered only in a particular direction relative to the incident beam. This made Röntgen come to a conclusion that the x-rays were actually not an electromagnetic radiation but differed fundamentally from visible or ultraviolet light. He assumed that the x-rays were wavelike in nature and were of longitudinal wave similar to sound waves but of higher frequencies but was not able to demonstrate the "familiar optical phenomena of reflection, refraction, interference and diffraction".
X-rays can be identified as soft x-rays or hard x-rays, depending on the penetrating power. Soft x-rays have lower energy compared to hard x-rays, therefore incidentally the former has a longer wavelength (low frequency) while the latter has shorter wavelength (high frequency). This is accordance to Planck's equation,
E = hf (Eq. 1)
with E represents energy; h represents Planck's constant, 6.63 x 10-34 J/s; and f represents the frequency. The penetration or "hardness" of the x-rays increases with the rise in voltage, while it decreases as the atomic number of the absorbing material increases. When it comes to absorption of x-rays, it seems to be different for different materials. Some absorb strongly, while others do not. To calculate the material absorbance of x-rays, the intensity of the beams with accordance with the path (thickness of the material) can be calculated using the following equation,
I = I0e -Î¼Ï° (Eq.2)
represented by I as the intensity of the rays after passing through a layer of thickness with thickness Ï°, and I0 represent the intensity when Ï° is zero, while Î¼ represent the "absorption coefficient". From Eq. (2), it can be deduced that, there is a fractional decrease in intensity of the beams per unit conduit through the absorbing medium. This can be seen with Eq. (3),
Î¼ = - dI / IdÏ° (Eq.3)
By this, a shield from the intensity of radiation can be designed, by calculating the effective shield in accordance to the thickness of the material. The most common shield used against x-rays is lead because of its high density of about 11, 340 kg/m3.
Soft x-rays and hard x-rays can be applied for medical use. Almost immediately after the discovery of x-rays, x-ray machines were produced to make a diagnosis of bone fracture. X-rays can traverse through materials of low density without interruptions, such as flesh. X-rays can be absorbed or reflected when it hits on high density materials, such as the bones. Therefore, when an x-ray image was taken, the areas of high density appear whitish against the black background, and lower density appears greyish or black, accordingly to the different densities. Soft x-rays are used to images of the bones and internal organs. It generally does not cause any tissue damage unless being used too often. Hard x-rays are used in radiotherapy. It is used to kill cancer cells and also to shrink tumours. Cyclotrons or synchrotrons are usually used to produce hard x-rays using high voltage. Both cyclotrons and synchrotrons are particle accelerator. Both make use to magnetic field to circulate the particles, while the electric field is used to accelerate the particles.
In a search to discover of the structure of deoxyribonucleic acid, or better known as DNA, which carries the genetic information in living cells, Rosalind Franklin had stumble upon the answer to the million dollar question using x-ray diffraction. Figure 1 shows the image of the famous Photograph 51 by Franklin in 1952 of the DNA. X-ray was used because their "wavelength are so short that the x-rays bounce off the atoms, scattered or diffracted into different directions, leaving behind a print on the photographic film as the x-ray exit". The "X-shaped diffraction pattern" shows that DNA is helical and that it consists of two strands coiled around each other in the same axis.
Figure 1: Photograph 51
This photograph is obtained using x-ray diffraction technique. Diffraction can be described as a physical phenomenon where it occurs when a wave encounters an obstacle. The wave will bend around the source of obstacle or spread out as it moves when it encounters a small space. The diffraction is generally considerable when the obstacle encountered is smaller than the wavelength of the wave diffraction, so that the obstacle could not block the wave. If there is a multiple, closely-spaced openings provided by the obstacle, another phenomenon can be observed which the superposition of the waves or better known as interference. There are two types of interference, which are the destructive interference and constructive interference. Destructive interference occurs when two waves of the same frequency and wavelength meet from the opposite direction and they cancel each other out, while constructive interference occurs when the amplitude of two waves adds on. "X-ray diffraction is caused by the interaction of electromagnetic waves with the matter in the crystal, particularly with the electrons". Then the electrons will scatter the waves or the electron itself will become a source of x-ray. The waves which were scattered will interact with each other, either becoming stronger or cancel each other out. This is the interference effect from x-ray diffraction. Therefore, based on x-ray diffraction, x-ray crystallography was born. X-ray crystallography was used to determine the structure of the crystals based on scattering of x-rays by the atoms of the crystal. Crystals have a regular pattern of atom arrangement which is repeated as unit cells throughout the crystal. Crystals act towards x-rays as almost the similar manner as a grating acts towards light. Grating is a process where a beam of light can be dispersed into different its wavelengths to produce a spectrum. Max von Laue had demonstrated and calculated how x-rays can be diffracted by the three-dimensional array of the crystal. His work was then interpreted by William Lawrence Bragg and came about the Bragg's law, Eq. (4),
nÎ» = 2d sin Ï´ (Eq. 4)
where n is an integer, Î» represents the wavelength of the x-ray, d or d-spacing is the space between the atoms in the crystal (measured in Angstrom, Å) and Ï´ is the angle of diffracted beam. This can be clearly seen in Figure 2.
Figure 2: Illustration of Bragg's Law
Bragg's law is used to calculate the distance between 2 planes of atoms, given the frequency, wavelength and angle at which the x-rays enter the crystal. From this information, the structure of a crystal and its unit cell can be determined. However, this only works if the constructive interference occurs, which means the amplitude of the waves add-up, and therefore being in phase. This also means the integer, n, is of suitable value, n = 1, 2, 3, etc. According to the Eq. (4), change in angle, Ï´, can result in the change of the wavelength of the rays reflected by the crystals. X-ray crystallography has been used by scientists for years in studying the structure and function of proteins. The first protein to be identified through x-ray crystallography was myoglobin. Myoglobin is a globular heme-containing protein that can be found in muscle cells which can transport oxygen to the cells.
X-rays microscopy has been used as a tool of high resolution imaging in areas like biology and material science. It can be used for viewing a specimen in higher magnification, especially the interior of it. Some of the advantages of using x-ray microscopy are that there is no need of processing, especially of hydrated thicker specimen. The specimen can be viewed in their natural state. Moreover, "actual location of a specific element within a cell can be visualized when the proper wavelengths of x-rays are used" and the specimen can also be observed in three-dimensional view with a "single exposure to with divided x-ray beams in different direction". This technique generally uses soft x-rays with wavelengths range around 1 to 10 nm. "A shorter wavelength generally gives a much higher resolution than longer wavelengths". This wavelength was chosen in contrast to the visible light wavelength which is about 400 to 700 nm. Sometimes the chemical composition of a sample can also be deduced since x-ray can cause fluorescence in some materials. X-ray microscopy can also be used as an alternative to x-ray crystallography if the sample is too small to be interpreted. This technique been used extensively in biology especially, such as an image of Kupffer cell and red blood cells infected with malaria had been taken.
All the application of x-rays introduced above was just a small part of role played by x-ray in many areas.