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What is Ionising Radiation?

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1 What is Ionising Radiation?
During this course we are going to look at various aspect of radiation protection. The first topic is ‘What is radiation’ - we are all familiar with the terms - natural radiation, man-made radiation, background radiation and cosmic radiation - but what do they mean? - is man-made radiation different from natural radiation and so on - we will look at this topic in more detail later. Radiation is used in a wide variety of situations - from medical uses - eg in radiotherapy or X-rays - to commercial uses - eg in smoke detectors or food sterilisation - to research and power stations. Radiation is dangerous - but it depends on the type and concentrations involved. What is Ionising Radiation? James Gray University RPA

2 Radioactivity - a natural and spontaneous process by which the
unstable atoms of an element emit or radiate excess energy in the form of particles or waves. After emission the remaining daughter atom can either be a lower energy form of the same element or a completely different element. The emitted particles or waves are called ionising radiation because they have the ability to remove electrons from the atoms of any matter they interact with.

3 Review of Atomic Structure – ‘High School’ Physics
The Bohr Model (1913) – negatively charged electrons orbiting a positively charged nucleus. Electrons only in ‘allowable’ orbits. AX Z Only works for hydrogen atom electrons are not ‘point like’ particles electrons do not ‘orbit’ the nucleus in a traditional sense electrons carry one unit of (-ve) electrical charge

4 The Nucleus: Two particles: protons & neutrons (hadrons) Proton mass = x10-27 kg = amu Neutron mass = x10-27 kg = amu amu = atomic mass unit, defined relative to carbon 12 Charge: protons carry one (+ ve) unit of electrical charge neutrons are chargeless Forces: electrical – protons repel each other – infinite range strong nuclear – short range (~10-15m) attractive force between quarks – is 137x stronger than electrical forces the nucleus is held together by a balance of these forces when the nucleus is in balance it is called stable the key to the balance is the neutron:proton ratio

5 Summary: Size of atom 10-10m, size of nucleus 10-15m Made up from 3 particles – proton, neutron, electron Electrons exist outside of nucleus in discrete allowable orbits Electrons can move between orbits by absorbing/emitting energy Electrons carry one unit of electrical charge (-ve) Protons and neutrons exist within the nucleus They have roughly the same mass Protons carry one unit of electrical charge (+), neutron has no charge Stable nucleus there is a balance between SNF and electrical force When the balance is upset the nucleus is unstable

6 Definition: Atoms with the same number of protons/electrons have the same physical and chemical properties, these are called elements e.g. all oxygen atoms have 8 protons. Elements are arranged in order of increasing proton number and are characterised with the symbol - Periodic Table Elements can have different numbers of neutrons and these are called isotopes Isotopes can be stable or unstable

7 Isotope - atoms of the same element with different numbers of
Slide 2 – Stable and r/a isotopes. It is possible for atoms of the same element (i.e. the same no. of protons) to have nuclei with a different numbers of neutrons. These atoms are called isotopes of the element, the example shown in the slide is for the element hydrogen. Here we have the ‘normal’ hydrogen atom, i.e. containing one proton + 1 electron, the second image shows the nucleus with one proton and one neutron, this is the isotope deuterium and the third image depicts one proton with two neutrons giving the isotope tritium. In the example shown the isotopes hydrogen and deuterium are stable isotopes whereas the isotope tritium is unstable. Stability of the nucleus is related to its ratio of neutrons to protons, for low atomic number elements that ratio is approximately 1:1. for elements of higher atomic number this ratio rises to approximately 1.6:1. Departures from the stable ratio lead to unstable nuclei. Stability is regained by one or more of 5 radioactive decay processes. Isotope - atoms of the same element with different numbers of neutrons. Isotopes of Hydrogen Hydrogen - 1 proton + 1 electron - stable Deuterium - 1 proton + 1 neutron + 1 electron - stable Tritium - 1 proton + 2 neutrons + 1 electron - unstable Stability - related to n:p ratio low atomic number - n:p ~ 1:1 high atomic number - n:p rises to ~ 1.6:1 Stability regained by radioactive decay processes

8 Radioactive decay processes.
1. Beta (minus) decay We start by looking at Beta (minus) decay. This occurs when the neutron to proton ratio is too high. A neutron can ‘transform’ into a proton within the nucleus, charge neutrality is conserved by the ejection, from the nucleus of an electron. Electrons ejected in this way from the nucleus are called ‘beta’ particles. [neutron rich isotopes are generally produced within nuclear reactors] In this example the isotope potassium 40 has 19 protons and 21 neutrons. The excess neutron is transformed into a proton, with the ejection of an electron and a almost massless and chargeless particle called an anti-neutrino, the anti-neutrino is needed to satisfy conservation of energy. In reaching stability there is a release of surplus energy which is shared in varying amounts between the beta particle and the anti-neutrino. In the slide the beta particle for this reaction can have a maximum energy of 1.32 MeV. You can see by the right hand side of the equation that the number of protons in the nucleus has changed, a new atom has been formed with one more proton and one less neutron than the original atom. We call the starting atom the ‘parent’ and the new atom the ‘daughter’, in this case the daughter atom is Calcium 40. General equation for beta minus decay:

9 Radioactive decay processes.
Another type of beta decay is beta plus decay, sometimes known as positron decay. This occurs when the neutron to proton ratio is too low, i.e. there is excess protons (can be achieved with the use of a particle accelerator), stability can be reached by the transformation of a proton into a neutron within the nucleus, in this case charge neutrality is conserved by the ejection of a positively charged electron (called a positron) and a neutrino. Again the energy is shared between the two particles. The positron is the anti-particle of the normally found electron, and when a positron and an electron collide they ‘annihilate’ each other, producing two photons of energy equal to the rest mass energy of the electron. The energy of each photon is given by their rest mass mec2 = MeV. The other particles involved in the two beta decay processes, i.e. the neutrino and anti-neutrino are chargeless and virtually massless. They have a negligible chance of interacting with matter and pass through with complete ease. We are continually being irradiated with a large flux of solar neutrinos due to beta decay processes occurring in the sun. 2. Beta (plus) decay Parent nucleus Neon - 19 Daughter Nucleus Fluorine - 19 + + annihilation radiation General equation for beta plus decay: annihilation radiation = mec2 = MeV (x2)

10 Radioactive decay processes.
3. Electron capture: Excess of protons, stability reached by different process than + Orbital electron is captured by the nucleus, neutrino emitted. If the nucleus has an excess of protons stability can be reached by another process called electron capture. In this case an orbital electron from a shell close to the nucleus ‘falls’ into the nucleus and combines with a proton to produce an neutron. The only particle emitted from the nucleus is an anti-neutrino. If this were the end of the story then the electron capture process would be almost impossible to detect. However, it commonly occurs that after the capture and transformation of the proton the nucleus is left with excess energy - we say it is in an excited state. The nucleus rids itself of this excess energy by emitting a gamma ray photon [10-14s], photons which originate from the nucleus are called gamma ray photons, photons which originate from outwith the nucleus are called variously X-ray, UV, visible etc depending on the mechanisms involved. In all cases involving electron capture there is the emission of an X-ray photon whose energy is characteristic of the daughter atom remaining. This X-ray is produced when an orbital electron from a higher energy state drops down to fill the space left by the captured electron, the energy of the X-ray is the energy difference between the two levels involved. Commonly nucleus is left in an ‘excited’ state and returns to its ground state by emitting a gamma-ray photon from the nucleus In all cases a characteristic X-ray photon is emitted by the atom. The general equation for the electron capture process is:

11 Radioactive decay processes.
4. Gamma decay: Typically, after a decay reaction, the daughter nucleus is often left in an excited state and this excess energy can be released as gamma ray photons. Just as the electrons in orbit around the nucleus occupy discrete energy levels (we say they are quantised) the nucleons in the nucleus also have quantised energy levels, this means that the emitted gamma ray photons from a particular nucleus will have a unique energy spectrum. You will find that most alpha and beta emitters also emit gamma rays as part of their decay process, there is no such thing as a ‘pure’ gamma emitter. 60Co Ni MeV - + 1.17 MeV  MeV  Nucleons have quantised energy levels - emitted -ray photons from a particular nucleus have a unique -ray spectrum. -ray spectrum can be used to identify unknown isotopes and calibrate instruments.

12 Radioactive decay processes.
Elements with atomic number greater than 82 achieve stability by a series of radioactive decay until they are transformed into a stable isotope of lead. One of the decay processes is alpha decay. Alpha decay can be regarded as a spontaneous fission process where one of the fragments is an alpha particle, consisting of 2 protons and 2 neutrons tightly bound together, i.e. a helium nucleus. The energy of the emitted alpha particle is discrete and of the order 5 MeV. An example of a decay chain is the decay of Uranium 238 through a series of alpha and beta (minus) decays until the stable isotope Lead 206 is achieved. The Health Physics notes contains an example of a decay chain. An understanding of the alpha decay process requires a grasp of quantum mechanics which is beyond the scope of this course. 5. Alpha decay: Nuclides with Z > 82  particle = 4He2 (helium nucleus) and are monoenergetic Decay chain: Generally, unstable heavy elements require a series of alpha and beta decays until a lighter more stable element is reached

13 Radioactive decay processes.
6. Neutron emission produced by three methods: Nuclear fission Deuterium bombardment of a tritium target Bombarding beryllium target with alpha particles

14 X-ray Generation: ‘Characteristic’ X-ray emission Bremsstrahlung Man made

15 Penetrating Distances
 < 4cm air, will not penetrate skin.  - several mtrs in air, penetrates skin ~ 0.8 cm, use ~ 6 mm plastic shielding. X - penetrating, speak of half- thickness 1/2, use lead shielding.  - more penetrating than X-rays, use lead or concrete shielding. The different types of radiation produced by the decay process have different penetrating distances. For example, an alpha particle will travel only a few cm in air and can be stopped completely by a single sheet of paper. A beta particle can travel several metres in air and will require about 6 mm of plastic shielding to completely stop it. X-rays, because they are electromagnetic energy are more penetrating than either alpha or beta particles and will not be completely stopped by shielding, instead we say that the X-ray photons are attenuated by the shielding material. Similarly gamma rays are still more penetrating than X-rays. In both cases we describe shielding materials in terms of their half-thickness, this is the thickness of shielding required to attenuate the beam by 1/2. In general lead or concrete is used to shield X and gamma ray emissions. The last particle on the diagram is neutron emission, you should not normally come across this type of emission in the normal laboratory. Neutron emission generally occurs in nuclear reactors

16 Activity and half-life
A radioactive nuclide decays spontaneously at a rate which is proportional to the number of original atoms present. This is expressed mathematically as dN/dt = -N, dN/dt means the rate of change of the quantity N, the minus sign indicates that the quantity is reducing and lamda is the proportionality constant called the decay constant. It is defined as the instantaneous fraction of atoms decaying per unit time and is unique for each nuclide. The solution to this equation takes the form shown, where Nt is the number of nuclides remaining at time t, and N0 is the original number of nuclides. The exponential factor on the right hand side of the equation indicates that the nuclide decays in an exponential manner, as shown on the next slide. A radioactive nuclide decays at a rate proportional to the number of original nucleii present: :where  = decay constant Integrating the above gives the decay equation: e-t term indicates that radioactive atoms decay exponentially

17 Half life (1/2): The time required for amount of radioactive material
The graph is a plot of the decay equation and clearly shows the exponential character of radioactive decay. This area marked in red has a special significance, this is the point where the no. of nuclides has reached half of its original value and it is called the half-life of the nuclide. If we put N = 1/2 N0 into the decay equation we can get an expression for the half-life of any nuclide and from a knowledge of the half-life we can determine the decay constant for any nuclide. Half life (1/2): The time required for amount of radioactive material to decrease by one-half:

18 Units: The disintegration rate of a radioactive nuclide is called its
Activity. The unit of activity is the becquerel named after the discoverer of radioactivity. The disintegration rate is called the activity, the SI unit of activity is called the Becquerel and is defined as 1 disintegration per second. The unit is named after Henri Becquerel who discovered radioactivity when he placed a small quantity of uranium sulphate onto a photographic plate wrapped in black paper. When he developed the plate he found that it contained an image of the crystals. The Becquerel is a small unit and we normally give the activity of an isotope in units of kilobecquerels, megabecquerels etc. 1 Bq = 1 disintegration per second this is a small unit, activity more usually measured in: kilobecquerel (kBq) = 103 Bq Megabecquerel (MBq) = 106 Bq Gigabecquerel (GBq) = 109 Bq Terabecquerel (TBq) = 1012Bq ANTOINE HENRI BECQUEREL

19 Units: Old units still in use:
You may find that some books and manufacturers still use the old unit which was the curie, named after pierre and marie curie. The conversion between the cutie and the becquerel is 1 curie equals 37 GBq. In other words the curie is a large unit Units: Old units still in use: Curie (Ci) = 3.7 x 1010 disintegration per second therefore: 1 Ci = 3.7 x 1010 Bq = 37 GBq 1 mCi = 3.7 x 107 Bq = 37 MBq 1 Ci = 3.7 x 104 Bq = 37 kBq 1 MBq ~ 27 Ci

20 Common Isotopes Used In The Lab:
3H: 1/2 = 12.3 yrs, - emitter (19 keV, ‘soft’) Cannot be detected using Geiger counter Bremsstrahlung radiation may be significant Shielding < 0.1 mm plastic Some common isotopes which you may encounter in the laboratory are: Tritium has a half-life of 12.3 years and emits beta particles which can have a maximum energy of about 19 keV - this is a very low energy for a beta particle and we sometimes call it a soft beta emitter. Because of the low energy tritium cannot be detected using a portable Geiger counter and we have to rely on swabbing and counting in a liquid scintillation counter. This means that you cannot continually monitor experiments using tritium and you have to be especially careful to avoid contamination. Tritium radiation will not penetrate gloves but because of the chemical nature of the isotope it can be easily absorbed thro’ the skin. Carbon 14 has a half-life of 5730 years, it also emits beta particles with a maximum energy of 157 keV - these can be detected using a thin end window Geiger counter. A shielding level of 3 mm of plastic should be adequate although the preferred level is 6 mm plastic An example of a beta emitter with a short half-life is Phosphorous 32, its half-life is 14.3 days and it emits beta particles with a maximum energy of 1.71 megaeV, we usually call P32 a hard beta, this will easily be detected with a Geiger counter. An adequate shielding level for this isotope is 6 mm plastic. An example of an X-ray emitter is iodine this has a half-life of about 60 days and emits X-rays with an energy of 27 keV and 31 keV, it also emits gamma rays. To detect this isotope you have to use a portable scintillation counter. Because iodine 125 emits electromagnetic radiation you need to use a denser shielding material - lead is a good choice and the half-thickness for lead for this energy is 0.02 mm, so a shielding level of 1 mm of lead should be adequate. 14C: 1/2 = 5730 yrs, - emitter (157 keV, ‘soft’) Can be detected using Geiger counter Bremsstrahlung radiation may be significant Shielding ~ 3 mm plastic 32P: 1/2 = 14.3 days, - emitter (1.71 MeV, ‘hard’) Can be detected using Geiger counter Shielding ~ 6.3 mm plastic 125I: 1/2 = 60 days, X-ray emitter Can be detected using a portable scintillation counter Shielding ~ 1 mm lead

21 Interaction with Matter
Moving on to the interaction of radiation with matter. The radiation that we have been discussing can interact with matter in 2 ways. It can ionise an atom or molecule - ionisation is the removal of an electron from an atom leaving an ion. It can excite an atom or molecule - excitation is the addition of energy to the atom leaving the atom in an excited state, which will then loose this excess energy through other mechanisms. If we look at the charged particles first: An alpha particle carries an electrical charge of 2 and travels at about 1/20th of the speed of light, because of its high charge, relatively large mass and slow speed it will interact strongly with matter and will virtually ionise every atom or molecule it encounters. Beta particles are much lighter than alpha’s, carry a single electrical charge and travel at close to the speed of light, because of this they will not interact as strongly as the alpha particle and ionise about 1 in every 1000 atoms or molecules they encounter. After each ionisation the alpha or beta particle will loose some of their initial energy and with a sufficient number of ionisations will eventually be stopped, we say that these particles have a finite range. This explains why the alpha particle has a much shorter range than the beta - it is because of its much stronger interaction with matter. Range is measured in gcm-2, therefore if you know the density of your absorber in gcm-3 you can determine the thickness of material needed to provide adequate shielding.  and X-rays interact with matter in 2 major ways: Ionisation: removal of an electron from an atom leaving an ion. Excitation: addition of energy to the atom, giving an excited state. Charged particles: -particle: 2+, 1/20 c, virtually ionises every molecule encountered. -particle: 1-, ~ c, ionises one in every 1000 molecules. After each ionisation the charged particle will lose energy and will finally be ‘stopped’ - i.e.  radiation has a finite range. Range is measured in gcm-2 R = E / 2 gcm-2 R = E / 1000 gcm-2 &

22 Example of a range calculation
Q. What is the range of a 35S beta particle in perspex? A. The max. energy of the 35S beta particle is MeV  the range of the particle is gcm-2 The density () of perspex = 1.2 gcm-3  the penetration depth in cm (t) is given by t = range /  = / 1.2 = 0.07 cm = 0.7 mm

23 Interaction with Matter
Moving on to X and gamma rays, these interact with matter via the Compton effect the Photoelectric effect and Pair production. Starting with the photoelectric effect. An X or gamma-ray photon is absorbed by an inner orbital electron, if the energy of the incoming photon is high enough then the electron can gain enough energy to escape from the atom. E=h f The photoelectric effect is predominant for photons of low energy, typically this means X-rays. X and -rays: Chargeless, more penetrating than or . Interact via: photoelectric effect, the Compton effect and pair production. X or  -ray photon photoelectron Photoelectric Effect

24 Interaction with Matter
The Compton effect can be thought of as an elastic collision process. A high energy photon ‘collides’ with a valence electron, ejecting the electron from the atom. A photon of lower energy than the original is produced that travels at an angle to the direction of the incident photon, determined by conservation of momentum. The Compton effect generally occurs for high energy photons (>0.1 MeV) in other words for gamma rays X and -rays: Chargeless, more penetrating than or . Interact via: photoelectric effect, the Compton effect and pair production. Compton Effect Photon Compton electron

25 Interaction with Matter
I’ve only included pair-production for completeness, you do not have to remember this as it is not an important effect. Pair production occurs when a photon is absorbed by the nucleus, this leaves the nucleus in an excited state. It recovers by the emission of an electron-positron pair. X and -rays: Chargeless, more penetrating than or . Interact via: photoelectric effect, the Compton effect and pair production. X or  photon Pair Production

26 Interaction with Matter
X and gamma radiation are types of electromagnetic waves and as such are chargeless and virtually massless. The probability of interaction with the orbital electrons of an atom is much smaller than that of alpha and beta radiation. This accounts for the well-known penetrability of X and gamma radiation. Since the probability of interaction with matter is small we find that a given thickness of absorber produces the same fractional reduction in intensity, we say that the incident radiation is attenuated the degree of attenuation is dependent on the absorber material and the energy of the radiation. For all absorber materials this attenuation is exponential in character and unlike the charged particle radiations there is no thickness of material which will completely ‘stop’ X or gamma radiation. The concept of the half-life discussed earlier can be applied to absorber thickness and this is called the half-thickness of absorber material, I.e. the thickness of absorber needed to reduce the intensity of the radiation by one half. X and  rays are types of electromagnetic radiation. They are not ‘stopped’ by matter but are attenuated. Attenuation depends on energy of radiation, thickness and density of absorber material. Given thickness of absorber produces the same fractional reduction in intensity. Analogous to half-life - called half-thickness - thickness of absorber required to reduce intensity by 1/2.

27 If we use three half-thickness' of absorber then this will reduce
the intensity by: 1/2+1/2+1/2 = 1/8

28 Summary: Unstable atoms (excess p or n) can regain stability by emitting radiation Two types – particle and electromagnetic Particle: β minus – electrons (-1 charge) β plus – positrons (+1 charge) α – helium nuclei (+2 charge) neutrons (chargeless) EM: γ – ray – originate from inside nucleus X – ray – originate outside nucleus or man made Shielding: charged particles – low density materials γ/X rays – high density materials Units Becquerel (Bq) old unit Curie (Ci) Excellent physics website:

29 Common laboratory isotopes
32P – pure beta (minus) 33P - pure beta (minus) 14C - pure beta (minus) 3H - pure beta (minus) 35S - pure beta (minus) 125I – electron capture – gamma and X-rays 131I – beta (minus) + gamma

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