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  • Subject area(s): Science
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  • Published on: 15th October 2019
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Task 1:

a) Draw a labelled diagram to show the structure of an atom.

The dictionary definition for an atom is the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction. Atoms are the 'building blocks' of life. They describe matter and are made from smaller particles called subatomic particles.

At the centre of an atom is a nucleus which contains protons and neutrons. Protons have a relative charge of +1 while neutrons have a relative charge of 0 (neutral). Protons and neutrons are made of quarks. After the invention of the particle accelerator, it was discovered that the electrons and neutrons were made up of quarks (sub-particles) which are held by gluons.  

Protons are found in the centre of the atoms with the neutrons they make up the nucleus. Protons have a charge of +1 and atomic mass of 1 which is approximately equal to 1.66x10-24. The number of protons defines the identity of the element (an atom with a 1 proton is hydrogen and with 2 protons is helium).  As such, protons are stable, their number only changes during radioactive decay.

Electrons have a charge of -1. Protons and neutrons are stationary while the electrons move about in the space outside the nucleus. Electrons are liable, they can be transferred from one atom to another and through this the atoms become charged.

Neutrons are a subatomic particle with the same mass as a proton. They have no electric charge and are in all atomic nuclei expect hydrogen.

Relative Charge Relative Mass Charge/c Mass/Kg

Electrons -1 1/2000 -1.6 9.11x10-31

Protons +1 1 1.6 x 10-19 1.67x10-27

Neutrons 0 1 0 1.67x10-27

b) Explain what it is mean by radioactivity and complete the table attached. For each particle you must describe each of the properties listed.

Radioactivity:

Spontaneous emission of radiation, either directly from unstable atomic nuclei or as a consequence of a nuclear reaction.

Alpha Decay:

This is the decay where the atomic nucleus emits an alpha particle and therefore it decays into an atom with a mass number which is reduced by 4 and atomic number is reduced by 2. The helium nucleus is identical to the helium atom which also has 2 protons and 2 neutrons. The alpha particles are expelled at high speed from the nucleus. It is also has low penetration power and the slowest speed, only 10% the speed of light. Alpha particles are very high in ionising power and have the largest mass and charge of all decays'. It is simply stopped by a thin sheet of paper and only lasts a few cm in air.

Beta (plus) Decay:

This decay is a subtype of beta decay and occurs when the nucleus has too many protons where it can be transformed into neutron, inside the atomic nucleus. This process allows the ratio of protons and neutrons to become optimal. While the neutron is lost a protons appears this is called positron emission.

Beta (minus) Decay:

In this beta minus decay a neutron is lost and a proton appears and the process produces an electron. Here the weak interaction increases the atomic number by 1 while emitting an electron. Beta minus decay is only able to happen if the daughter nucleus has a larger binding energy than the parent nucleus.

Gamma Decay:

 This is high frequency therefore it consists of high energy photons. Gamma decay is a type of radioactivity which has unstable atomic nuclei. In gamma decay the nucleus is changed from a high energy to a low energy through the process of electromagnetic radiation. As the number of protons does not change, the daughter and parent nuclei are the same chemical.

Task 1/P2:

c) Draw a labelled diagram of a diagnostic X ray tube and explain how x rays are produced.

X-rays are used for medical diagnostic procedures or for research purposes. Radiographers can change the current and voltage in the x-ray machine in order to operate the properties of the x-ray beam produced. Different x-ray beam spectra are applied to different body parts.

X-rays are produced when a beam of electrons are produced from the filament. The electrons are liberated from the heat filament and accelerated by a high voltage towards the metal target. The x-rays are produced when the electrons collide with the atoms and nuclei of the metal target.

X-rays pass through the soft tissue of the in the body where the electrons are absorbed. They have low ionising power so they do not harm the inside of the body. Below is a diagram of the x-ray tube structure. The voltage is provided by the 2 ends of the tube. The filament heats up and ejects high moving electrons. They are accelerated towards the anode as they are rejected by the negative pole of the battery.

To summarise the power is sent to the x-ray tube via the cables the milliamperage (mA) is sent to the filament on the cathode side.  This causes the filament to heat up and the electrons are produced.  A positive voltage is applied to the anode. The negative electrons are attracted across the tube to the positive anode. The electrons slow down and finally rest, the electron beam is focused from the cathode to the anode target by the focusing cup.

Cathode: The cathode is functioned to eject the electrons from the electrical circuit and to focus the electrons into a beam aimed at the anode.

Anode: The component which the x - radiation is produced is the anode. It is a large piece of metal which is connected to the positive side of the electrical circuit. The two functions of the anode are to 1.convert electronic energy into x-radiation and 2. To dissolve the heat created in the process.

Glass/metal envelope: Both the anode and cathode are in an airtight enclosure or envelope. The envelope and the contents are known as the tube insert which is part of the tube that has a limited lifetime and can be replaced. The function is to provide support to the electrical insulation of the anode and cathode and to maintain a vacuum in the tube. The gasses in the x-ray tube allow the electricity to flow through freely rather than only in an electron beam.

d) Describe how ultrasound is produced and using ultrasound how images are produced. Explanation must include the interaction of ultrasound with tissue when waves are transmitted into the body. Use diagrams to aid your explanations.  

Ultrasound is produced and detected using an ultrasound transducer. Ultrasounds are produced when the wave is generated with an electric field and is applied to the piezoelectric crystals located in the transducer surface. The electrical stimulation causes the distortion of the crystals resulting in the vibration and production of the sound waves. The ultrasounds are produced in pulses where each pulse consist of 2 or 3 sounds cycles all part of the same frequency.

Attenuation: Attenuation is the decreasing intensity of the sound wave when it has passed or passing through the medium. This is the result of the boundaries of the tissue with the variety of densities.

Reflection: The larger smoother surfaces, e.g. bone the sound wave can be reflected back in a singular direction, without any disturbance to the image. Soft tissue in the body is classified as a diffuse reflector as the adjoining cells create an uneven surface which causes the reflection to return in various directions to the transmitted beam. Yet the sound is able to get back to the transducers in a uniform manner.

Refraction: The reflections that are generated in the ultrasounds are not returned back to the transducer. The angle of the refraction happening is dependent on the angle the soundwave hits on the boundary between tissues and the how fast the signal travels over time.

Scattering: scattering occurs at interfaces involving structures of small dimensions.  This is common with red blood cells (RBC), where the average diameter of an RBC is 7''m, and an ultrasound wavelength may be 300''m (5 MHz).  When the sound wave is greater than the structure it comes in contact with, it creates largeness in all directions with little or no reflection returning to the transducer.

Sound waves need a medium to travel e.g. air particles are needed for waves to travel. They are high frequency waves.

Task 2:

a) 'Radioactive decay is random and unpredictable':  

The decay rates of the radionuclides are known to vary depending on the environment. Radioactive decay involves the spontaneous transformation of one element into another. The only way that this can happen is by changing the number of protons in the nucleus. This means that the reason why radioactive decay is random and unpredictable is because decay rate depends on the environment and the environment cannot be controlled or predicted.

The half-life of a certain type of atom does not describe the exact amount of time that every single atom experiences before decaying. Rather, the half-life describes the average amount of time it takes for a large group of amounts to reach the point where half of the atoms have decayed.

There is no way of telling when the decay will happen, and no way to speed up the process.  The chemical reactions involve the outer shell electrons; radioactive decay involves the nucleus.

The rate of the disintegration of the nuclide is directly proportional to the number of atoms left at the time:

Exponential decay is the change that occurs when an original amount is decreases by a consistent rate over time.

b) State the standard equation for radioactive decay and explain the terms in the equation.

Alpha decay equation:

This decay is when the atomic nucleus emits an alpha particle and therefore decays into a number when the mass is reduced by 4 and the atomic number is reduced by 2. Alpha decay occurs often in larger nuclei which have too much of a large proton and neutron ratio. In alpha decay the atomic number changes meaning that the parent and daughter nuclei will have different chemical properties.

A = atomic mass (number of protons + neutrons)

Z = atomic number (number of protons)

X = chemical symbol (as shown on the Periodic Table)

Equation:

Example:

Radon decays into polonium when it emits an alpha particle. The radon is the parent nuclei and the polonium is the daughter nuclei.

Beta Minus equation:

This is where the neutron changes into proton with the addition of an electron. The proton stays within the nucleus which allows the electron to leave the atom at high energy.  When a beta particle is emitted from the nucleus the nucleus has one more proton and one less neutron. This means that the atomic number increases by 1 and atomic mass number does not change.

Equation:

Example:

Potassium emits into calcium when it emits Beta minus. Potassium is the parent nuclei and calcium is the daughter nuclei.

Beta plus equation:

This occurs when there are too many protons present. Here a proton is converted into a neutron. This is called positron and has the same mass as an electron but the opposite charge (positive).

Example:

Protactinium emits into Thorium when it emits beta plus. Protactinium is the parent nuclei and Thorium is the daughter nuclei.

The factors which affect the x ray quantity are:

The factors which affect the x ray quality:

' Tube current (mA)

' Exposure time (s)

' Tube potential (kVp)

' Waveform

' Distance (FSD)

' Filtration ' Tube Potential

' Filtration

' Wave Form

The 2 types of x-ray spectra are

' Characteristic X-ray

' Bremsstrahlung/Braking X-ray

Characteristic X-ray:

A high energy electron collides with an inner shell electron this causes both to be ejected from the tungsten atom. When the electron leaves a hole is left behind in the inner layer. This is then filled by an outer shell electron.

Bremsstrahlung/Braking X-ray:

When an electron passes near a nucleus it is then slowed and the path is deflected. The energy which has been lost is emitted as a Bremsstrahlung x ray photon.

This spectrum can be manipulated by changing the X-ray tube current or voltage settings, or by adding filters to select out low energy X-rays. In these ways radiographers are able to apply different spectra of X-ray beams to different body parts.

 The x ray tube has the function of creating the photons from the electric energy supplied by the x ray generator. The process is inefficient and only 1% is converted to X ray photons and the remaining is converted into heat. Therefore the x ray tube must withstand be able to dissipate a great deal of heat. The design of the x ray tube determines the characteristics for the X ray beam such as X ray energy spectrum and focal spot size.

Factors which affect the X ray Spectra:

Tube Current: The number of x-ray photons produced depends on the number of electrons which are striking the target and thus should depend on the tube current. It has been found that the intensity is proportional to the tube current. An increase in current will results in an increase in quantity but no change in quality.

Filtration: Adding filtration is called hardening the x-ray beam because of the increase in average energy. Filtration more effectively absorb low-energy x-rays than high energy x-rays. Characteristic spectrum is not affected and the maximum energy of the x-ray emission is not affected. Adding to the useful beam reduced the x-ray beam intensity while increasing the average energy. Filtration in x-rays is a process of removing signals of the undesired frequencies from the input signal.

Peak tube potential: The does have an effect on the x-ray spectra and there will be a change in quantity of protons and a change in quality as the spectrum shifts to a much higher energy, as shown in figure 5.

Voltage: The voltage can be ac or dc (alternating or direct). For the peak generators it is measure in peak voltage applied (kVp). The maximum energy which is to be produced depends on the voltage.

Target material: The greater the efficiency of the X ray production the higher the atomic number will be.

Bibliography:

Task 1:

' Jim Clark. (2010). A SIMPLE VIEW OF ATOMIC STRUCTURE. Available: http://www.chemguide.co.uk/atoms/properties/gcse.html. Last accessed 18th Nov 2015.

' http://chemistry.tutorvista.com/inorganic-chemistry/structure-of-an-atom.html , Last accessed 18th Nov 2015

' https://whatisnuclear.com/articles/radioactivity.html

Task 2:

' http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/radiation/radioactiverev7.shtml

' http://www.wikihow.com/Calculate-Half-Life

' http://www.cyberphysics.co.uk/topics/radioact/Radio/half_life.htm

' http://www.s-cool.co.uk/gcse/physics/radioactivity/revise-it/half-life

' http://www.s-cool.co.uk/gcse/physics/radioactivity/revise-it/types-of-radiation

' http://spacem11.imascientist.org.uk/2011/03/why-is-radioactive-decay-random-and-spontaneous

Task 3:

' By K. Kirk Shung, Michael Smith, Benjamin M.W. Tsui (1992). Principles of Medical Imaging. London: Academic Press, Inc. pages 13-16.

' http://hsc.uwe.ac.uk/radscience/xray_prod/factors_04.pdf

' http://csep10.phys.utk.edu/astr162/lect/light/absorption.html

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