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Unit 10: EFFECT OF X-RAYS
X rays are today most used at hospitals in radiology service for imaging purpose.
Key unit Competence
By the end of the unit, I should be able to analyze and evaluate the effects of X-rays.
My goals
• Explain the production of X-rays
• State the properties of X-rays.
• Explain the origin and characteristic features of an x-ray spectrum.
• Outline the applications of X-rays in medicine, industries, and scientific research
• Solve problems involving accelerating potential and minimum wavelength of X-rays.
• Recognize how the intensity and quality of X-rays can be controlled.
• Appreciate the use of X-rays in medicine and industry
Introductory activity
When a person goes to the hospital with pain in her/his chest, or with an internal fracture of the bone, physicians do normally recommend the patient to pass by radiology service. Hence try to answer the following questions:
1. Why do physicians recommend patients to pass by radiology service?2. Radiology means that there are radiations. Discuss different types of radiations that are found in there?
3. Discuss the production of X-ray radiations.
4. What are the positive and negative effects of X-ray radiation on the human body?
10.1 PRODUCTION OF X-RAYS AND THEIR PROPERTIES
Activity 10.1: Investigating the production of X-rays
Read the following text and answer the questions that follow.
Discovery of X-rays: Becquerel’s discovery wasn’t the only important accidental one. In the previous year W.C. Roentgen unexpectedly discovered X-rays while studying the behavior of electrons in a high-voltage vacuum tube. In that instance, a nearby material was made to fluoresce. Roentgen named them X- rays because he didn’t know what they were.
Within twenty years of this discovery, diffraction patterns produced using X-rays on crystal structures had begun to show the finer structure of crystals while, at the same time, giving evidence that X-rays had a wave nature. Since then, X-ray radiation has become an indispensable imaging tool in medical science.
Questions:
1. What do you understand by X-rays?
2. How are X-rays produced?
3. Where are X-rays used?
10.1.1 X-ray production
X-rays are produced when fast moving electrons strike matter (see Fig.10.1). They were first produced in 1895 by Wilhelm Rontgen (1845-1923), using an apparatus similar in principle to the setup shown in Fig.10.1. Electrons are emitted from the heated cathode by thermionic emission and are accelerated toward the anode (the target) by a large potential difference V . The bulb is evacuated (residual pressure 10-7 atm or less), so that the electrons can travel from the cathode to the anode without colliding with air molecules. It was observed that when V is a few thousands volts or more, a very penetrating radiation is emitted from the anode surface.
The above figure is an illustration of the Coolidge tube which is the most widely used device for the production of X-rays. The electrons are produced by thermionic effect from filament, which is the cathode of the tube, heated by an electric current. These electrons are accelerated towards a metal target that is the anode due to the high potential voltage between the cathode and the anode.
The target metals are normally Tungsten or Molybdenum and are chosen because they have high melting point and higher atomic weights. The accelerated electrons interact with both electrons and nuclei of atoms in the target and a mysterious radiation is emitted. This radiation was referred to as X-rays.
About 98% of the energy of the incident electron is converted into heat that is evacuated by the cooling system and the remaining 2% come out as X-rays.
10.1.2 Types of X-rays
Sometimes X-rays are classified according to their penetrating power. Two types are mentioned:
• Hard X-rays: those are X-rays on upper range of frequencies or shorter wavelength. They have greater energy and so they are more penetrating.
• Soft X-rays: they are X-rays on lower range of frequencies or longer wavelength. They have lower energy and they have very low penetrating power. The Fig.10.2 below shows the relative location of the different types of X-rays.
Hard X-rays are produced by high accelerating potential. They have high penetrating power and short wavelength while soft X-rays are produced by lower accelerating potential, have relatively low penetrating power and relatively long wavelength.
10.1.3 Properties of X-rays
Activity 10.2: Understanding the pros and cons of X-rays
Make intensive research on the production and the properties of X-rays, then write a report about your findings.
The following are the main properties of X-rays:
a. X-rays can penetrate through most substances. However, their penetrating power is different.
b. X-ray can produce fluorescence in different substances.
c. X-rays can blacken photographic plate. The degree of blackening depends upon the intensity of x-rays incident upon the plate. Thus, X-ray intensity can be measured with the help of photographic plates.
d. X-rays ionize the gas through which they travel. The ionizing power depends on the intensity of the x-ray beam. Thus, X-ray intensity can also be measured by measuring their ionizing power.
e. X-rays are not deflected by electric or magnetic fields. This proves that unlike cathode rays or positive rays they are not a beam of charged particles.
f. X-rays travels on a straight lines like ordinary light.
g. X-ray are both reflected and refracted.
h. X-rays can be diffracted with the help of crystalline substances. They can also be polarized.
From the above characteristics it can be seen that X-rays have the properties that are common to all electromagnetic radiations.
10.1.4 Checking my progress
1. Describe the process by which X-rays are produced.
2. Discuss and describe the types of X-rays?
3. What is the meaning of the X in X-ray?
4. How are X-rays diffrent from other electromagnetic radiations?
10.2 THE ORIGINS AND CHARACTERISTIC FEATURES OF AN X-RAY SPECTRUM
Activity 10.3 investigating the X-ray spectrum
During the production of X-rays, a high voltage must be applied across the x rays tube to produce enough acceleration of electrons towards the target. Search internet, then discuss and explain the relationship between the applied
10.2.1 Variation of the X-ray intensity with wavelength
Depending on the accelerating voltage and the target element, we may find sharp peaks superimposed on a continuous spectrum as indicated on Fig.10.3. These peaks are at different wavelengths for different elements; they form what is called a characteristic x-ray spectrum for each target element.
X-rays of different wavelengths are emitted from X-ray tube. If the intensity is measured as a function of the wavelength and the variation is plotted graphically then a graph of the nature shown on the figure above is obtained.The graph has the following features:
a. Minimum wavelength
b. Continuous spectrum
c. Characteristic peaks
10.2.2 Origin of the continuous spectrum
It is known that when charged particles such as electrons are accelerated or decelerated they emit electromagnetic radiation of different frequencies. In doing so a part of their kinetic energy is transformed in the energy of the emitted radiation. Electrons inside the x-ray tube decelerate upon hitting the target and as a result they emit electromagnetic radiations with a continuous distribution of wavelength starting from a certain minimum wavelength. This mechanism of producing electromagnetic radiation from an accelerated or decelerated electron is called bremsstrahlung.
For a transition between K and L-shells. Thus the energy of the emitted photon depends on the binding energies in the K and L shells and hence the x-ray spectral lines have definite frequencies and wavelengths which are characteristic of the target atom.
For a given target material more than one spectral lines are observed as transitions may occur between different energy levels.
The X-ray lines originating from the transition between the different electron levels are usually labelled by the symbols α, β, γ, etc.
From L-level to K-level transition produces Kα-line
From M-level to K-level transition produces Kβ –line
From M-level to L-level transition produces Lα –line
From N-level to L- level transition produces Lβ –line
10.2.4 Checking my progress
1. What is the characteristic of X-ray characteristic peak radiation?
2. How is X-ray continuum produced via bremsstrahlung?
3. X-rays are generated when a highly accelerated charged particle such aselectrons collide with target material of an X-ray tube. The resulting X-rays have two characteristics: the continuous X-rays (also called white X-rays) and characteristic X-rays peaks. The wavelength distribution and intensity of continuous X-rays are usually depending upon the applied voltage and a clear limit is recognized on the short wavelength side.
a. Estimate the speed of electron before collision when applied voltage is 30kV and compare it with the speed of light in vacuum.
b. In addition, establish the expression of the shortest wavelength limit λmin of X-rays generated with the applied voltage V. it is obtained when the incident electron loses all its energy in a single collision.
10.3 APPLICATIONS AND DANGERS OF X-RAYS
Activity 10.4: Investigation of X-rays uses and dangers
1. Using the historical background of X-ray discovery, what are the uses of X-rays in real life?
2. Discuss the dangers that X-rays may cause when they are used in a wrong way.
X-rays have many practical applications in medicine and industry. Because X-ray photons are of such high energy, they can penetrate several centimetres of solid matter. Hence they can be used to visualize the interiors of materials that are opaque to ordinary light, such as broken bones or defects in structural steel.
10.3.1 In medicine
X-ray imaging utilizes the ability of high frequency electromagnetic waves to pass through soft parts of the human body largely unimpeded. For medical applications, parts of the human body are exposed to moderated X-rays intensity and images are produced in similar way as light on a photographic plate or digital recorder to produce a radiograph (See Fig.10.7).
By rotating both source and detector around the patient’s body a “slice” image can be produced in what is called computerized tomography (CT). Although CT scans expose the patient to higher doses of ionizing radiation the slice images produced make it possible to see the structures of the body in three dimensions.
In 1895, the Dutch Wilhelm Roentgen (See Fig.10.8) discovered that light energy could be used to take photographs through substances such as paper, cloths and wood. Roentgen also discovered that this invisible form of light energy, called X-rays could be used to take the pictures of structures inside the body as shown in Fig. below. Bone tissue appears clearly on an X-rays.
The object to be visualized is placed between an X-ray source and an electronic detector (like that used in a digital camera) or a piece of photographic film (Fig.10.8 or Fig.10.8B). The darker area in the recorded images by such a detector, the greater the radiation exposure. Bones are much more effective X-ray absorbers than soft tissue, so bones appear as light areas. A crack or air bubble allows greater transmission and shows as a dark area.
A widely used and vastly improved x-ray technique is computed tomography; the corresponding instrument is called a CT scanner. The x-ray source produces a thin, fan-shaped beam that is detected on the opposite side of the subject by an array of several hundred detectors in a line. Each detector measures absorption along a thin line through the subject. The entire apparatus is rotated around the subject in the plane of the beam, and the changing photon-counting rates of the detectors are recorded digitally. A computer processes this information and reconstructs a picture of absorption over an entire cross section of the subject.
In the middle 1970, CT (Computer Tomography) scanning machines were introduced in human medicine.
X-rays are also used in the following: • Killing of cancerous cells • Radiography is also used in industry for examining potentially damaged machinery to ascertain the cause of damage and to verify castings or welded joints • X-rays are used to study the structure of crystals (crystallography). • When a handgun is fired, a cloud of gunshot residue (GSR) is ejected from the barrel. The x-ray emission spectrum of GSR includes characteristic peaks from lead (Pb), antimony (Sb), and barium (Ba). If a sample taken from a suspect’s skin or clothing has an x-ray emission spectrum with these characteristics, it indicates that the suspect recently fired a gun.
10.3.2 Examining luggage cargo and security.
X-rays are being used in airports to examine luggage for weapons or bombs. Note that the metal detector that you walk through in the airport does not X-ray you. It uses magnetic waves to detect metal objects. X-rays are also being used to examine cargo luggage for illegal or dangerous material as in Fig.10.9
10.3.3 In industry
They can be used to detect structural problems and cracks in metals that cannot be seen from the outside. X-rays are used on commercial airplanes, bridges metals and pipe lines, to make sure there are no stress fractures or other dangerous cracks in the material.
10.3.4 In scientific research
• X-ray diffraction provides one of the most important tools for examining the three-dimensional (3D) structure of biological macromolecules and cells.
• They are also used in crystallography, where X-ray diffraction and scattered waves show the arrangement of atoms in the crystal.
The array of spots formed on the film is called a Laue pattern and show the atom structure of the crystal.10.3.5 Dangers of X-rays
• X rays cause damage to living tissues. As X-ray photons are absorbed in tissues, their energy breaks molecular bonds and creates highly reactive free radicals (such as neutral H and OH), which in turn can disturb the molecular structure of proteins and especially genetic material. Young and rapidly growing cells are particularly susceptible, which is why X-rays are useful for selective destruction of cancer cells.
• Because X-rays can kill living cells, they must be used with extreme care. When improperly used they can cause severe burns, cancer, leukemia, and cataracts. They can speed aging, reduce immunity to disease, and bring about disastrous changes in the reproductive cells.
• Lead screens, sheets of lead-impregnated rubber, and leaded glass are used to shield patients and technicians from undesired radiation.
• The effect of X-ray radiations is cumulative. That is, many minor doses over a number of years is equivalent to a large dose at one time.
• Unnecessary exposure to x-rays should be avoided. MRI (Magnetic Resonance Imaging) uses magnets and sound energy to form pictures of the internal organs without exposing patients to harmful X-rays.
• When they are used in hospitals, the sources should be enclosed in lead shields.
• A careful assessment of the balance between risks and benefits of radiation exposure is essential in each individual case.
10.3.6 Safety precaution measures of dangers caused by X-rays
Medical and dental X-rays are of very low intensity, so that the hazard is minimized. However, X-ray technicians who go frequently behind the lead shield while operating X-rays need to be protected because of the frequency of exposure. A person can receive many medical or dental X-rays in a year with very little risk of getting cancer from it. In fact, exposure to natural radiation such as cosmic rays from space poses a greater risk.
The following are some of the precautions:
I. Protective suits and wears such as gloves and eye glasses made of lead are used always when handling these radiations. These shields protect the workers from X-ray exposure.
II. Workers who operate equipment’s that use X-rays must wear special badges which detect the amount of radiation they are exposed to.
III. Food and drinks are not allowed in places where X-radiations are present.
IV. Experiments that involve these radiations (X-rays) substances should be conducted in a room surrounded by thick concrete walls or lead shields.
V. Equipment that use X-rays should be handled using remote-controlled mechanical arms from a safe distance.
10.3.7 Checking my progress
1. How do we create different X-ray images in medicine?
2. What are the dangers that may be caused by using excessive dose of X-rays?
10.4 PROBLEMS INVOLVING ACCELERATING POTENTIAL AND MINIMUM WAVELENGTH.
10.4.1 Accelerating potential and minimum wavelength
Activity 10.5 Calculation of accelerating potential in X-ray tube
An x-rays tube operates at 30 kV and the current through it is 2.0 mA. Calculate:
a. The electrical power output
b. The number of electrons striking the target per second.
c. The speed of the electrons when they hit the target
d. The lower wavelength limit of the X-rays emitted.
When a high voltage with several tenso fk Vis applied between two electrodes, the high-speed electrons with sufficient kinetic energy is drawn out from the cathode and collides with the anode. The electrons rapidly slow down and lose kinetic energy. Since the slowing down patterns(method of losing kinetic energy) varies with electrons, continuous X-rays with various wavelength are generated. When an electron loses all its energy in a single collision, the generated X-ray has the maximum energy(orthe shortest wavelength ).The value oft he shortestwavelengthlimitcanbeestimatedfromtheacceleratingvoltageVbetween electrodes.
Typical X-ray wavelengths are 10-12 m to 10-9m . X-ray wavelength can be measured quite precisely by crystal diffraction techniques. X-ray emission is the inverse of the photoelectric effect. In photoelectric emission there is a transformation of the energy of a photon into the kinetic energy of an electron, in X-ray production there is a transformation of the kinetic energy of an electron into energy of a photon.
In X-ray production we usually neglect the work function of the target and the initial kinetic energy of the boiled off electrons because they are very small in comparison to the other energies.
. X-ray wavelength can be measured quite precisely by crystal diffraction techniques. X-ray emission is the inverse of the photoelectric effect. In photoelectric emission there is a transformation of the energy of a photon into the kinetic energy of an electron, in X-ray production there is a transformation of the kinetic energy of an electron into energy of a photon. In X-ray production we usually neglect the work function of the target and the initial kinetic energy of the boiled off electrons because they are very small in comparison to the other energies.
Example Electrons in an X-ray tube accelerate through a potential difference of 10.0 kV before striking a target. If an electron produces one photon on impact with the target, what is the minimum wavelength of the resulting X- rays? (Answer using both SI units and electron volts.
Answer To produce an x-ray photon with minimum wavelength and hence maximum energy, all of the electron’s kinetic energy must go into producing a single x-ray
Constructive interference occurs only when the path difference between rays scattered from parallel crystal planes would be an integer number of wavelengths of the radiation. When the crystal planes are separated by a distance d, the path length difference would be 2dsin θ.
10.5 END UNIT ASSESSMENT
10.5.1 Multiple choices
1. Choose the letter that best matches the true answer: I. X-rays have
a. Short wavelength b. high frequency c. Both A and B d. Longest wavelength
I. If fast moving electrons rapidly decelerate, then rays produced are
a. Alpha rays c. beta rays b. x-rays d. gamma rays
II. Energy passing through unit area is
a. Intensity of x-ray c. wavelength of x-ray b. Frequency of x-ray d. Amplitude of x-ray
IV. X-rays are filtered out of human body by using
a. Cadmium absorbers c. Copper absorbers b. Carbon absorbers d. Aluminum absorbers
V. Wavelength of x-rays is in range
a. 10-8 m to 10-13 m c. 10-10 m to 10-15 m b. 10-7 m to 10-14 m d. 102 m to 109m
10.5.2 Structured questions
2. A plot of x-ray intensity as a function of wavelength for a particular accelerating voltage and a particular target is shown in Fig.10.5.
Fig.10. 5Characteristics of x rays spectrum
a. There are two main components of this x-ray spectrum: a broad range of x-ray energies and a couple of sharp peaks. Explain how each of these arises.b. What is the origin of the cut-off wavelength min λ of the Fig.10.5 shown below? Why is it an important clue to the photon nature of x-rays?
c. What would happen to the cut-off wavelength if the accelerating voltage was increased? What would happen to the characteristic peaks? Use a sketch to show how this spectrum would look if the accelerating voltage was increased.
d. What would happen to the cut-off wavelength if the target was changed, keep the same accelerating voltage? What would happen to the characteristic peaks? Use a sketch to show how the spectrum would look if some other target material was used, but the accelerating voltage was kept the same
3. Electrons are accelerated from rest through a p.d of 10 kV in an x ray tube. Calculate:
I. The resultant energy of the electrons in eV.
II. The wavelength of the associated electron waves. (1.23x10-11m)
III. The maximum energy and the minimum wavelength of the x ray radiation generated
4. MonochromaticX-rayofwavelength 1.2x 10-10m are incidentonacrystal. The1st order diffraction maximum is observed at when theangle between the incident beam and the atomicplaneis120. Whatistheseparationoftheatomicplanesresponsibleforthediffraction?
5. An x-ray operates at 30 kV and the current through it is 2.0 mA. Calculate:
I. The electrical power output
II. The number of electrons striking the target per second.
III. The speed of the electrons when they hit the target
IV. The lower wavelength limit of the x-rays emitted.
6. An x-ray machine can accelerate electrons of energies 4.8x10-15J . The shortest wavelength of the x- rays produced by the machine is found to be 4.1x10-11 m Use this information to estimate the value of the plank constant.
7. The spacing between Principal planes of Nacl crystal is 2.82A0 . It is found that the first order Bragg diffraction occurs at an angle of 100. What is the wavelength of the x rays?
8. What is the kinetic energy of an electron with a de Broglie wavelength of 0.1 nm. Through what p.d should it be accelerated to achieve this value? Assume e, me, h.
9. You have decided to build your own x-ray machine out of an old television set. The electrons in the TV set are accelerated through a potential difference of 20 kV. What will be the ƛmin for this accelerating potential?
10. A tungsten target (Z = 74) is bombarded by electrons in an x-ray tube. The K, L, and M atomic x-ray energy levels for tungsten are -69.5, -11.3 and -2.30 keV, respectively.
a. Why are the energy levels given as negative values?
b. What is the minimum kinetic energy of the bombarding electrons that will permit the production of the characteristic K α and K β lines of tungsten?
c. What is the minimum value of the accelerating potential that will give electrons this minimum kinetic energy?
d. What are the Kα and Kß wavelengths?
11. Using the following illustration figure Fig.10.6, label each part marked by letter from A to H and explain the function of each part A, B, C, D, E, F and H.
Fig.10. 6 X ray tube illustration