radioactivity & atomic structure

NAME: ………………………………………………………..

Physical Sciences 3C Atomic Structure (Criterion 7)

Radioactivity (Criterion 6)

Theory and

Examples

Atomic Structure 2015 Radioactivity

Rosny College - 2 - Physical Sciences 3C

Rosny College Science Department 2015

INDEX:

PAGE • ATOMIC STRUCTURE 3

• ISOTOPES 4

• FORMATION OF IONS 9

• ATOMIC STRUCTURE EXERCISES 10

• RADIO-ISOTOPES 11

• ALPHA RADIATION 12

• BETA RADIATION 13

• GAMMA RADIATION 14

• POSITRON EMMISSION 15

• STABILITY CURVE 16

• PROPERTIES OF ALPHA, BETA & GAMMA 17

• WHAT IS IONISING RADIATION? 18

• EFFECTS OF IONISING RADIATION 18

• ARTIFICIAL NUCLEAR REACTIONS 19

• NUCLEAR FISSION & FUSION 20

• BALANCING NUCLEAR EQUATIONS 22

• RUTHERFORD’S EXPERIMENT 23

• RADIOACTIVE HALF-LIFE 24

• UNITS USED IN RADIOACTIVITY 27

• THE DETECTION OF IONISING RADIATION 28

• HAZARDS OF IONISING RADIATION 30

• SAFETY PRECAUTIONS 31-32

• EVERYDAY USES FOR RADIOACTIVITY ETC 33-38

• RADIOACTIVITY REVISION TEST 39-40

• REVISION TEST ANSWERS 41-43



ATOMIC STRUCTURE A simplified view of an atom:

The atomic model shows that atoms are comprised of 3 main sub-atomic particles: ………………. = positively charged ………………. = negatively charged ………………. = neutral (i.e. no electrical charge) The protons and neutrons are of similar mass (1.67 x 10−27 kg) and are located in a tiny central core of the atom called the nucleus. Surrounding the nucleus at comparatively large distances are the electrons moving in regions called orbitals. Electrons are about 2000 times lighter than protons and neutrons. (i.e. mass of an electron is 9.11 x 10−31 kg) The nucleus contains almost all of the atom’s mass but occupies only a tiny portion of the atom’s volume; i.e. one part in 10 million million.

…………………

…………………



SUMMARY PARTICLE SYMBOL MASS ELEC. CHARGE LOCATION Proton

1 u positive within the nucleus

Electron

(1/1835) u

negative

in orbitals outside the nucleus

Neutron

1 u neutral within the nucleus

(u = atomic mass unit or amu)



ATOMIC NUMBER (Z): Z is the number of protons in the nucleus of an atom. Each element is uniquely identified by its atomic number. Examples 1 Carbon (Z = 6): all carbon atoms have ….... protons in each nucleus. 6C Aluminium (Z = 13): all aluminium atoms have 13 ……………. in each nucleus. 13Al Copper (Z = 29): all …………….. atoms have 29 protons in each nucleus. 29Cu Gold (Z = ……..): all gold atoms possess 79 protons in each nucleus. 79Au Hydrogen (Z = 1): all hydrogen atoms possess 1 proton in each nucleus. …… Note: An electrically neutral atom will have the same number of electrons and protons. MASS NUMBER (A):

A is the number of protons + the number of neutrons in the nucleus.

A = Z + N

[Protons and neutrons are sometimes referred to as NUCLEONS – they are in the nucleus.] We write to represent element X with Z protons and A nucleons. Examples 2

Symbol Element Z A N

carbon 14

38 50



ELEMENTS: The chemical properties of an atom are determined by its atomic number Z. Atoms can have any number of protons from 1 to over 100 ( i.e. Z = 1, Z = 100 etc). An element is a substance which contains atoms all with the same atomic number (i.e. the same number of protons in the nucleus). Some example of elements are: oxygen, sodium, uranium, ………………………….. and ……………….… THE PERIODIC TABLE: The 100 or so elements are listed on the periodic table according to their atomic number (Z). www.webelements.com has lots of information about each element. [The periodic table and more detailed electron structure will be looked at after we have studied the properties of metals and non-metals.] ISOTOPES: Atoms of the same element can have different numbers of neutrons. There are consequently atoms of the same element possessing different mass numbers. These are called isotopes of that element.

Isotopes of an element have the same number of protons but different numbers of neutrons. i.e. the same Z but different A

Since the chemical properties of an atom are determined by the number of protons, different isotopes of an element have the same chemical properties.



Example 3 Magnesium atoms (Z = 12) must possess 12 protons but the number of neutrons in Mg can vary.

Symbol Z N A 24Mg

13

26Mg

Thus, magnesium exists in three different isotopic forms. Example 4 Hydrogen (atomic number 1) also occurs in three isotopic forms; Z N A

“normal” hydrogen

deuterium

tritium

Average Mass Number A naturally occurring element is a mixture of all its isotopes, therefore its measured mass number will be a weighted average of the individual mass numbers. Example 5 Naturally occurring chlorine consists of 75.8% of chlorine-35 and 24.2% of chlorine-37. Find the average mass number for chlorine. FORMATION OF IONS An electrically NEUTRAL atom must have the same number of protons and electrons. All the elements existing in a chemically uncombined form are electrically neutral. e.g. a piece of shiny metallic zinc (Zn(s)) comprises atoms each possessing 30 protons and 30 electrons. Although not normally done, we could indicate the neutral electrical charge by writing the symbol as Zn0

(s).



When atoms lose or gain electrons, they then have an overall electrical charge and are now called ions. A neutral atom GAINING ELECTRONS results in a NEGATIVE ION (called an ANION) e.g. Cl + 1e– → Cl–

↓ ↓

(17 protons & 17 electrons) → (17 protons & 18 electrons)

A neutral atom LOSING ELECTRONS results in a POSITIVE ION (called a CATION) e.g. Al → Al3+ + 3e–

↓ ↓

(13 protons & 13 electrons) → (13 protons & 10 electrons)

Aluminium atom

+ +

+ +

N

N



ATOMIC STRUCTURE EXERCISES

Complete the following table, noting that the mass number of any particular isotope is the number of protons plus neutrons which in total is called the number of nucleons. ELEMENT PROTONS NUCLEONS NEUTRONS ELECTRONS (a) Li

7

(b) U

238

(c)

88

135

88

(d) Al

14

(e)

83

209

83

(f)

56

30

(g) Ra

228

88

(h) Co2+

33

(i) Fe3+

55

(j)

92

143

89

(k) SO4

2–

48

From the above table, which pairs represent isotopes of the same element?



Identifying isotopes As the chemistry of all elements is identical regardless of isotope, how can we tell what and how much of different isotopes are present in a sample? We could try and weigh each single atom, but atoms are very light and conventional scales are nowhere near sensitive enough; also there are a lot of atoms in a typical sample! A method called Mass Spectrometry was developed (almost by accident) that allows scientists to determine how massive particles are. A mass spectrometer works by

• firstly ionising each atom in the sample. • accelerating the ion towards a detector that picks up where the ion hits.

In the diagram below, the electron beam ionises the sample. Positive ions are attracted to the cathode and repelled by the anode, making them accelerate towards the cathode. The cathode has a slit in it allowing some of the sample through. This beam passes through a beam focuser (not shown) which makes sure all the ions are travelling at the same velocity, and then it enters a magnetic field. This field bends or deflects the beam; the amount it is deflected is determined by the mass of the particle, thereby separating the lighter ions from the heavier ones.

http://www.astarmathsandphysics.com/ib-physics-notes/quantum-and-nuclear-physics/ib-physics-notes-the-mass-spectrometer.html



The detector counts how many ions hit at each point along it, and a graph like the one below is generated, giving the scientist an idea of the isotopic and chemical makeup of the sample (by looking at the spectral peaks).

http://en.wikipedia.org/wiki/Mass_spectrometry

A mass spectrometer is used for many things:

- Identifying compounds in a sample. e.g. presence of steroids/drugs in an athlete’s urine sample.

- Identifying which atoms are present in a sample. e.g. determining the chemical makeup (empirical formula) of a new drug.

- Identifying which isotopes are present in a sample e.g. identifying the isotopic makeup of moon rocks to determine its origins. See http://www.universetoday.com/98052/isotopic-evidence-of-the-moons-violent-origins/ for an interesting article on this.

For more information on mass spectrometers, see the Howstuffworks.com page (http://science.howstuffworks.com/mass-spectrometry.htm)



INTRODUCTION TO RADIOACTIVITY Radio-isotopes Some particular combinations of protons and neutrons are much more stable than others: e.g. sodium-22 (22Na)with 11 protons and 11 neutrons has an unstable nucleus. sodium-23 (23Na)with 11 protons and 12 neutrons has a stable nucleus. sodium-24 (24Na)with 11 protons and 13 neutrons has an unstable nucleus. Also

carbon-12 (12C) with ……protons and ……..neutrons has a stable nucleus carbon-13 (13C) with ……protons and ……..neutrons has a stable nucleus carbon-14 (14C) with …… protons and …….neutrons has an unstable nucleus Isotopes with unstable nuclei are called RADIOISOTOPES. They will, at some stage, undergo a process called NUCLEAR DECAY and turn into more stable nuclei, usually of a new element! As radioisotopes these isotopes decay, they emit excess energy in the form of radiation. Note: Even though radioisotopes have unstable nuclei, their chemical properties (due to the electrons) are identical to those of the stable isotopes and will undergo normal chemical reactions. There are 3 main ways by which a nucleus can become more stable: each results in the release of one or more different forms of radiation called alpha, beta or gamma radiation. [There are also several more radiation types.]



1. ALPHA RADIATION (α)

Alpha particles are the very stable helium-4 nuclei ( ). They have a +2 charge. Alpha emitters occur in isotopes of elements with mass numbers above 209. e.g. radium-226 is an alpha emitter.

→ + + energy

N.B. Left and right of the arrow; (i) the totals of mass numbers (the upper numbers) on either side of the arrow is the same. (226 on either side) (ii) the totals of atomic numbers or positive nuclear charge numbers (the lower numbers) on either side of the arrow is the same. (88 on either side)

Alpha particle emission results in the formation of a new element with an atomic number of two less than the original and a mass number four less. → + + energy

Generally, α-particles have very limited penetration through air, traveling only a few centimetres. Alpha particles travel at between 1%-10% the speed of light. The α-particles released from nuclear decay are the origin of all helium (He) gas on Earth, including that used in party balloons!



2. BETA RADIATION (β) Beta particles are high speed electrons that come from within the nucleus as a neutron changes into a proton. A beta particle (β) has a charge of -1. It is often represented symbolically as because, as an electron, it has an atomic number (electrical charge number) of −1 and a (relative) mass number of effectively zero in comparison to a proton or neutron. Beta decay can occur with any element but normally it occurs within isotopes of a given element with the higher mass numbers where there is an excess of neutrons. None of the elements with atomic numbers below 82 undergo alpha emission and yet these lighter isotopes may have unstable nuclei due to an excess of neutrons. For example, carbon-12 (6 neutrons) is stable but carbon-14 (8 neutrons) decays by way of beta emission. The transformation of a neutron into a proton plus electron (beta particle) can be represented by the nuclear equation:

i.e. → + + ν

In addition to the emission of the beta particle, energy is also released in the form of a particle called an ANTINEUTRINO (ν). The antineutrino has no mass or charge and is often ignored in writing the equation. For example, consider the radioactive decay of strontium-90, a beta emitter; i.e. → + + ν + ENERGY

In this case, the new nucleus formed (often referred to as a ‘daughter’ nucleus) as a result of the beta decay is yttrium-90 nucleus. Beta particle emission results in the formation of a new element with an atomic number of one greater than the original.

The average penetration of beta radiation through air is in the order of only 10-20 cm and they are stopped even by a thin sheet of aluminium foil. β-particle speeds range from 5%-99% of the speed of light. → + + ν + ENERGY



3. GAMMA RADIATION (γ) Gamma rays (γ) are a form of high energy electromagnetic radiation (thereby travelling at the speed of light) similar to x-rays but with even greater energy. Gamma rays are often emitted following alpha and beta radiation where the new nucleus formed still has excess energy to emit; the γ emission often occurs many hours or days after the α or β emission.

Gamma emission does not result in any change to atomic number or mass number of the nucleus. Gamma rays have extremely large penetration distances through air.



4. POSITRON (BETA +) RADIATION (β+) All natural beta radiation involves the emission of electrons. β- radiation, of course, results from an excess of neutrons in the nucleus. However it is possible to artificially create an excess of protons in the nucleus (by simply bombarding the nucleus with protons for example). In this case, a different type of beta decay occurs where a proton inside the nucleus turns into a neutron and a positive electron (a positron) along with a neutrino (often ignored).

→ + + ν

Positron emitters are widely used in medical diagnostics. Most of those used in Australia are produced at ANSTO, Lucas Heights near Sydney. Fluorine-18 is a commonly used positron emitter. → Sodium-22 is another.

The Positron is an example of anti-matter. It is the anti-particle of the electron; when a positron meets an electron they both disappear, generating two gamma rays which head off in opposite directions at the speed of light! This property is very useful in medical imaging.



Stability Curve for Isotopes [NOTE: This is not examined.]

For nuclear stability:

• For small nuclei, N = Z approximately, e.g. for oxygen, Z = .…, N = ….. • As the size of the nuclei increase, the number of neutrons increases more rapidly

than the number of protons e.g. for mercury, Z = …….., N = …….

Instability leading to nuclear radiation occurs when: • An excess of neutrons (above the stability region) results in ……….. emission.

• An excess of protons (to the right of the stability region) results in

………………… emission.

• Alpha emission occurs when ……………………….



Summary of Properties of Alpha, Beta and Gamma Radiation. Alpha and Beta particles have a lot of energy due largely to their very high speeds. [Similar to a bullet fired from a gun which has much higher energy than one which is thrown by hand.] This energy is transferred to any matter the particles pass through as they slow down. Once they come to rest, the particles are harmless and undergo ordinary chemical reactions. For example, an alpha particle gains two electrons to become a stable helium atom. Gamma particles, being electromagnetic photons, behave slightly differently. The photon gives up all of its energy and disappears as it interacts with an atom. However the chance of interaction is quite low as the gamma photon is not charged so it may travel a considerable distance before interacting.

Radiation α (alpha) β (beta) γ (gamma) Nature of radiation

Helium nuclei

High energy electrons

electromagnetic

Electric charge +2 -1 neutral Speed relative to light

0.01 – 0.1 0.05 – 0.99 1.0

Distance in air A few cm 20 – 30 cm A few km Stopped by Paper Aluminium

sheet Several cm of lead

Effect of electric or magnetic field

Deflected Deflected in opposite direction to α radiation

No effect

While alpha particles generally have the most energy, they do not travel far as they have very strong interaction with the matter they pass through. They give up this energy very quickly due their greater charge (+2) and relatively lower speeds. α particles therefore have a greater effect but over a shorter distance than β particles and γ rays.



What is Ionising Radiation? Radiation is energy in the form of particles or waves which spread out (“radiate”) from a source. We are bombarded by radiation continuously; but most of it is harmless low-energy radiation. Low-energy (non-ionising) radiation includes visible light, infra-red and radio waves. Ionising radiation is radiation that has sufficient energy to ionise atoms and molecules. Electrons are removed from the atoms by this high-energy radiation. Ionising radiation has sufficient energy to cause damage to living tissue. Most of this damage is caused by ionisation of the atoms and molecules of the DNA in the animal or plant cells. The cells then cease to function correctly; in most cases, the cells simply die but sometimes they reform incorrectly giving rise to cancer cells. If only a small number of cells die, the organism can continue to function and replace the dead cells. A large dose of radiation, however, may kill too many cells for the body to handle and the whole organism dies. Ionising radiation includes alpha and beta particles, gamma rays as well as X-rays, neutrons and some UV light. Effects of Ionising Radiation

While mutation or killing of cells by ionising radiation is of concern, it does have its uses. Ionising radiation can be used to kill cancer cells; kill harmful micro-organisms in food etc, and genetic mutation may be of benefit in improving agricultural output.



Artificial Nuclear Reactions – Nuclear Bombardment Alpha, beta and gamma decay are naturally occurring processes. However, there are many other processes which are ARTIFICIALLY produced. These involve bombarding nuclei with small particles (protons, neutrons or alpha particles for example) which results in a new nuclear type. The bombarding particle often needs to be travelling at a high speed to penetrate the nucleus. The new nucleus is usually unstable, i.e. is radioactive. There are many uses for these artificially produced radio-nuclides; they are widely used in medicine for diagnostics and treatment and as tracers in a wide range of industrial and scientific applications. The most common sources of the bombarding particles are:

• Nuclear reactors which are a rich source of high speed neutrons. Lucas Heights near Sydney is home to Australia’s only nuclear reactors and produces many of our radio-nuclides.

• Particle Accelerators which can accelerate protons to high speeds using electric fields. There are many of these around the country – in hospitals and universities as well as at Lucas Heights. The world’s largest accelerator, to be used for research, has just been constructed by CERN in Switzerland.

Examples i) A process to produce neutrons (as used by the “radio-active boy scout”) is to bombard beryllium with alpha particles.

+ → + ….. ii) Bombardment of boron nucleus by a neutron

+ → + iii) Bombardment of lithium nucleus by a proton.

+ → +



Two other very types of nuclear processes are: NUCLEAR FISSION and NUCLEAR FUSION. NOTE: Nuclear Fusion is not examinable

1. NUCLEAR FISSION: Fission occurs where a heavy nucleus is bombarded with SLOW moving neutrons and then splits into two lighter nuclei of approximately equal mass. e.g. + → + + 3( ) + ENERGY!

Note that many other fission pairs of products may also be formed. The two new nuclei formed will always have atomic numbers that add to give 92. e.g. (atomic no. Ba + atomic no. Kr) = 92 (atomic no. Hf + atomic no. Ca) = 92 (atomic no. Cd + atomic no. Ru) = 92 This fission process is the basis of most nuclear power stations. The fission reaction is controlled by neutron absorbing materials which prevent the process from becoming a "chain reaction". Uncontrolled fission is the basis of the atomic bomb.



2. NUCLEAR FUSION: Fusion occurs where small particles such as neutrons, hydrogen or helium nuclei combine to form a heavier nucleus such as lithium. This process is normally accompanied by the release of enormous amounts of energy due to the transformation of mass differences into energy ( E = m.c2) e.g. 2 ( ) + 2( ) → + (3 x 109 )kJ

+ → + + (3 x 108 )kJ + → + + (2 x 109 )kJ

Nuclear fusion is the basis of the sun's energy output as well as the "hydrogen bomb". As yet, scientists have been unable to control the fusion process so that energy is released at a suitably slow rate for electricity generation on a large scale.



BALANCING NUCLEAR EQUATIONS Complete the following nuclear equations by identifying the missing nuclide indicated by (?). The released energy (E) is shown. This will be in the form of an antineutrino (ν) when β emission occurs. (a) → ? + + E (alpha) (b) → ? + + E (c) → ? + + E (beta particle)

(d) → ? + + E (e) → ? + + E (‘positron’ = a positive electron) (f) → ? + + E (g) ? → + + E (h) ? → + + E (i) ? → + + E (j) + → + ? + E (k) + → ? + + E

(l) + → + ? + 3( ) + E

(m) + → + ? + 3( ) + E

(n) + → ? + + E

(o) + → + ? + E

(p) + → + ? + E

(q) + ? → + + E



Atomic Models over the Years [These are not examinable.] The present day model for atomic structure is based upon nearly 200 years of scientific research and suggests all matter is formed from various combinations of building units called atoms. The word atom is derived from the Greek ‘atomos’ meaning indivisible. Some of the important pioneers in the development of our knowledge of the atom were: Dalton Thompson Rutherford Bohr Schrodinger J.J Thomson discovered the electron and proposed the “plum-pudding” of the atom. However, this was soon replaced by Rutherford’s nuclear model. RUTHERFORD’S GOLD FOIL EXPERIMENT With the discovery of radioactivity by Henri Becquerel in 1896, a number of important scientific experiments were then carried out using α-particles to investigate the structure of atoms. The New Zealand physicist, Ernest Rutherford hoped to use high speed α-particles as ‘bullets’ to fire at atoms so as to break them open and hopefully identify any particles that emerged. Rutherford’s experiment was carried out by his two research students Marsden and Geiger and their results were totally unexpected. They took a very thin layer of gold foil and suspended it in a vacuum adjacent to an α-particle emitter. The foil was so thin that it was only a few hundred atoms thick. They used a fluorescent screen and a microscope to observe where the alpha particles were after hitting the gold foil. To their surprise most α-particles passed straight through the gold foil as if there was nothing there at all! (See diagrams)



Occasionally an α-particle was deflected through a large angle or even rebounded from the gold foil. It was as if this thin foil was like a netting fence with large holes through which nearly all the bullets passed unaffected. Only when an α-particle hit something hard did the deflection or rebound occur. Rutherford’s experiment led his research team to conclude that atoms are mostly empty space with the vast majority of the atom’s mass concentrated in a tiny core which we now know as the atom’s nucleus.

A mathematical treatment of these results led Rutherford to conclude that the nucleus in atoms occupies only about one ten million millionth part of the atomic volume; i.e. The volume of an atom = 1013 x the volume of its nucleus The outer emptiness of the atom is the orbital region occupied by electron clouds.



RADIOACTIVE HALF-LIFE

As the parent nuclei of a radioactive isotope decay, they become the nuclei of another element. The average length of time it takes for half the parent nuclei in a given sample to decay is called the HALF-LIFE of the radioactive isotope. After one "half-life" has elapsed for a given isotope, not only will there be only half the number of that type of nuclei remaining but the activity or rate of emission will have decreased to half its original value (usually measured in becquerel (Bq)) The half-life does not indicate which nucleus will decay but rather it indicates the statistical chances of decay. Half-lives vary from fractions of seconds to billions of years! e.g. RADIOISOTOPE HALF-LIFE bismuth-214 20 minutes bromine-82 36 hours carbon-14 5730 years iodine-131 8.2 days polonium-214 0.00015 seconds radium-226 1600 years uranium-238 4.5 billion years



Example Consider strontium-90, a β emitter with a half-life of 28 years. i.e. → + If we had an initial sample of strontium-90 with a mass of 100 mg having an activity of 3200 Bq, then in 28 years time, the amount of would be only 50 mg and the activity would have dropped to 1600 Bq. In 56 years time (2 half-lives) the amount of strontium-90 would now be only 25 mg and the activity down to 800 Bq. i.e. 100 mg 28 y 50 mg 28 y 25 mg 28 y 12.5 mg 3200 Bq 1600 Bq 800 Bq 400 Bq N.B. The decrease in mass of strontium-90 is accounted for in terms of the new mass of the decay product which is yttrium-90.

Example: An ancient wooden artifact contains 5.6 g of C-14. Given that the artifact is 28 650 years old, how much C-14 did it contain when new?



UNITS USED IN RADIOACTIVITY

ACTIVITY

becquerel (Bq)

This is a derived S.I. unit named after the physicist

who discovered radioactivity.

It measures the activity of a radioactive sample in terms

of number of nuclear transformations per second.

1 Bq = 1

transformation/sec

Formerly activity was measured in curies (Ci) where

1 Ci is the activity of 1 g of

radium.

1 Ci = 3.7 x 1010

Bq

NOTE: The Count Rate (counts per second etc) as measured by a “Geiger counter” is not the same as the Activity (decays per second). The Geiger tube only detects a fraction of the radiation emitted and may also receive radiation from other sources. Different radiation types have different energies and different effects on the body. There are other units such as “gray (Gy)” and “sievert (Sv)” which reflect these differences.



The Detection of Ionising Radiation Ionising radiation is detected and counted for scientific study and is also monitored for personal or environmental safety. Most detectors rely on the ionising properties of the radiation. You should be familiar with the use and operation of the GM tube and the cloud chamber. GEIGER-MUELLER TUBE A GM tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when radiation interacts with the wall or gas in the tube. These pulses are converted to a reading on the instrument meter. If the instrument has a speaker, the pulses also give an audible click. CLOUD CHAMBER There is a temperure gradient between the felt ring soaked in meths and the dry ice. Near the dry ice, the air is saturated with meths vapour. When ionising radiation penetrates this area, it leaves a trail of ionised air molecules; these in turn cause cause the meths to condense, causing a “vapour trail” along the particle’s path. What we see is a trail of doplets of methylated spirits. SCINTILLATION COUNTER If a material gives off light when radiation interacts with it, it is said to scintillate. The amount of light given off is proportional to the amount of incident radiation. This light can be measured and the information used to calculate the amount of radiation exposure. Different types of scintillating material are used to detect different types of radiation. For example, a thin layer of zinc sulfide is generally used to detect alpha radiation; an anthracene crystal is used for beta; and a sodium iodide thallium activated crystal detects gamma. Scintillation detectors can exist in any physical state (i.e., solid, liquid, or gas), but the most common detectors are solid crystalline materials. The basic functioning of the detector is the same no matter what type of scintillation material is used.



Since scintillation materials have a higher density and higher effective atomic number, they present more target atoms for interaction with ionising radiation than gas filled detectors. Scintillation detectors of a similar size as a gas filled detector will produce a greater number of excited electrons making them more sensitive to lower activity levels of the radiation being measured. PERSONAL MONITORING People working in an environment are required to wear a radiation dosimeter (a badge which records their exposure to ionising radiation). This badge is analysed on a regular basis to see if the wearer has received an excessive exposure. The badge DOES NOT prevent the exposure, it simply records it. FILM BADGES The film badge is a beta/gamma sensitive device that measures total whole body dose. The badge itself is a small plastic holder that contains a photographic film packet. Inside the packet are two pieces of photographic film, tightly wrapped in a paper envelope to prevent light from exposing the film. One piece of film is sensitive to low radiation exposure levels and the other is sensitive to high exposure levels. When radiation interacts with the film emulsion, it produces ions that chemically activate silver molecules in the emulsion. When the film is put into a developing solution, the chemically activated silver atoms are changed into elemental silver, which turns black. The degree of this blackness or its density is read on a machine called a densitometer and the reading is an indication of the beta and gamma dose. The film badge holder has an open window that allows beta and gamma radiation to enter. This means that the blackness of the film (after it has been developed) in the area behind the window is a measure of the total beta and gamma dose. Most film badges have different inserts in other parts of the holder to shield out betas and lower energy gammas. For example, one part of the holder might have a plastic insert, another an aluminium insert, and still another a lead insert. After the film is developed, the blackness will vary behind the different inserts, depending on the ability of different energy gammas to penetrate them. This ability to penetrate is a measure of the incident photon’s energy, so the film badge can tell us what energy radiation we have been exposed to as well as how much of each energy radiation. THERMOLUMINESCENT DOSIMETER (TLD) Many stations now use thermoluminescent dosimeters (TLDs) instead of film badges. This is because the TLD is not subject to the interpretation of the densitometer. It is a more modern device and lends itself to automated reading and record keeping. Externally, the TLD looks the same as a film badge but it may be slightly larger or smaller. Inside, it is quite different. Instead of film, the TLD contains a piece of thermoluminescent material. Thermoluminescent material is material that will give off light when heated in proportion to the amount of radiation it has been exposed to.



Hazards of Ionising Radiation In considering the hazards of ionising radiation, we must consider the

• Intensity of the radiation source and • Time near the source • The type of radiation

Different radiation types have different hazards. Alpha radiation is very highly ionising but has low penetration; it cannot penetrate the outer, dead layer of our skin. An alpha-emitting radioisotope is therefore quite safe if it is not ingested (e.g. swallowed); however is highly dangerous if it gets inside the body, if a decay occurs near vital organs such as the lungs the alpha emitted can cause considerable damage . Radon (a daughter of radium and part of the uranium decay chain), an alpha emitter, is a gas and is therefore easily taken into the lungs. Radon is present near many rocks and in concrete and brick buildings as a result of decay of other isotopes. It is in fact a major contributor to our radiation exposure. Beta Radiation is also quite highly ionising but is stopped by around 20 cm of air. Beta emitters are also most dangerous if ingested. Radioactive fallout from nuclear (“fission”) bombs and reactors contains radio-isotopes of many elements vital to human function so can be taken up by our metabolism. Iodine and potassium are examples of beta-emitting radioisotopes. Strontium can replace calcium in the bones and krypton is a gas which can be inhaled. Gamma Radiation is less ionising but is highly penetrating. Most gamma rays which hit the body will pass through but a significant fraction will cause damage. With gamma emitters it is vital that people do not spend too much time near them, that they are stored away from people and that adequate shielding is in place. Remember: Time, Distance, Shielding

USING RADIOACTIVE SOURCES



SAFETY PRECAUTIONS LABORATORY PRECAUTIONS: 1. Ingestion (eating or breathing in) of any radioactive material whatsoever should be avoided. 2. If any radioactive material lodges in the lungs (e.g. through the inhalation of radioactive dust or gases such as radon) there can be very serious carcinogenic (cancer forming) effects because the lung tissue is not protected by a layer of ‘dead’ cells such as those that exist on and protect our skin. 3. In our experimental work where radioactive sources are used, be sure to observe the following safety precautions:

• Don’t eat anything whilst performing the experiments • Avoid breathing the air from within the lead-lined boxes that hold the sources. • Hold any radioactive material well away from your body and do not point the

radioactive sources at anyone. • Keep the radioactive sources inside the lead boxes when not in use. • Reduce the time of contact with the radioactive sources to a minimum.

IONISING RADIATION: The radiation from radioactive substances can have very serious effects on living organisms. They can cause burns and destruction of living cells and may result in initiating serious cellular changes leading to cancer and genetic mutations. Radioactive sources must therefore be treated with extreme caution because the effects of the radiation are to some extent cumulative. A small daily dose of radiation may cause irreparable cellular damage over a prolonged exposure period. The damage caused by the radiation from radioactive sources arises from the ionisation they produce where electrons are removed from molecules resulting in the formation of positive ions. i.e. X + radiation → X+ + e– Alpha radiation (α) produces intense ionisation effects but as their range in air is so small that they wouldn’t pass through the surface layers of skin. There is little danger from α-active materials provided that they are not taken into the body directly via the stomach or lungs. The danger of inhalation of α-active materials is particularly the case with radium sources which emit radon gas which is itself an alpha emitter. i.e. → + + energy

Radium sources should be kept in sealed containers to prevent the dispersion of the α-active radon gas. Beta (β) radiation is less strongly ionising but its penetration is greater than α-radiation. A thin sheet of wood (6 mm) will prevent any penetration of β-radiation. The main danger arising



from exposure to β-radiation is again associated with it entering the body directly via the stomach or lungs. Gamma (γ) rays constitute the chief danger when handling radioactive materials due to their great penetrating power. For example a 10 mm thickness of lead metal only absorbs about 50% of the γ-radiation incident upon it. All γ-radiating materials used in school science laboratories must be low level activity and stored in lead-lined boxes. Their handling requires great care to minimize the exposure level to safe levels. In the treatment of certain types of cancer, γ-radiation is used to specifically target and kill cells that are associated with the tumour development. This branch of medicine is called radiotherapy. GENERAL PRECAUTIONS: 1. Old watches and clocks may have dials which have a radioactive radium paint to make the numerals luminous. These should be avoided although they don’t normally present a high safety risk as the α-particles are generally absorbed by the metallic backs of the watch or glass face of the clock. 2. It is important to limit the amount of radiation you receive from x-rays and U.V. light. Excessive exposure to these forms of radiation increases the risk of cancer significantly. The latest research recommends that people with fair skin should avoid any “sun-bathing” and always use protective sun-screens when out in direct summer sunlight. 3. Keep all the rooms of your house well ventilated with a gentle stream of fresh air. Sealing up a room of your house that makes it virtually ‘air-tight’ will result in radioactive radon gas accumulating in the room. The radon results from decay processes occurring within the slightly radioactive gypsum in the plaster used to line houses. By itself this radon presents minimal threat to home occupants but with sealed rooms and prolonged exposure, risks of cancer are increased. 4. A distinction between naturally occurring radiation and so-called ‘artificial’ radiation cannot be made. An 8 MeV α-particle from any source is equally dangerous. 5. Not all radiation is undesirable. Under carefully controlled conditions, gamma radiation treatment for cancer cells is particularly successful in preventing the spread of tumours. 6. Some natural repair takes place within the body after radiation damage has occurred so that a small rate of exposure for a long time may cause less damage than the same dose given at a high rate over a short period of time. 7. Background radiation levels vary significantly throughout the world. The plaster board used to line modern homes has a measurable level of radioactivity and yet it is regarded as sufficiently low to be harmless. Regions with granite rocks (e.g. Coles Bay) tend to have slightly higher levels of background radiation than others.



Everyday uses for radio-isotopes, radioactivity and ionising radiation. What are the properties of ionising radiation and radio-isotopes that make them so useful? o Radio-isotopes have the same chemical properties as ordinary isotopes of the same

element. A radio-isotope of an element can replace an ordinary isotope in a molecule for example.

o The half-life of given isotope is constant and well-known. o Different isotopes have half-lives ranging from micro-seconds to eons. o Gamma radiation can penetrate many materials, including the human body. o Ionising radiation will ionise material through which it passes. o Ionising radiation can kill cells & whole organisms.

Ionising Radiation and Radio-isotopes can be used for: o Medical Diagnosis (finding problem areas in the body). o Medical Therapeutics (treating cancers etc). o Tracers (finding leaks in pipes, following flows of liquids). o Establishing the age of objects (carbon dating etc). o Sterilising objects or preserving food. o Testing materials for cracks or flaws. o Controlling the thickness of paper etc. Some useful references. http://www.ausetute.com.au/nuclesum.html http://webext.ansto.gov.au/index.htm Follow the links under Nuclear Facts. MEDICAL DIAGNOSTICS



Radioactive Tracers The body is made of, and uses, many varied chemical compounds. Different elements and compounds tend to be used in different parts of the body; iodine in the thyroid and iron in the blood for example. Glucose is also used for energy so it concentrates in areas of high metabolic activity. Radiopharmaceuticals are those compounds which have radio-isotopes attached to them. When they move to the selected part of the body the medical scientist can monitor the radiation emitted to gauge the function of that part of the body. Medical Imaging – PET Scans In Positron Emission Tomography (PET), the patient is injected with a compound which is “tagged” with a positron emitter. This compound is chosen to target specific areas of the body. When the isotope decays, it emits a positron; this positron quickly meets an electron, they annihilate, emitting two gamma ray photons in opposite directions. These are detected by a gamma camera.

PET scans show the metabolic activity of different areas in the body using

radioactively labelled glucose. Areas of high glucose consumption are represented as dark spots, and signify areas of growth. Here you can see a tumour that has developed in the left lung (red arrow). The PET scan can also show whether the cancer has metastasized, or spread to other areas

in the body (blue arrow).In this picture the brain and genitalia show high metabolic activity as well - this is because the brain requires a vast

amount of energy to function, and the genitalia are the site of sperm production (meiosis).



MEDICAL THERAPEUTICS - Radiation Therapy Radiation can cause cancer. Normally, cells that are exposed to radiation die. However, if the radiation does not kill the cell, the cell may be mutated by the radiation, and can start to reproduce. These multiplying mutant cells constitute cancer.

However, although radiation can cause cancer, it can also be used to treat it. Rapidly multiplying cancer cells are particularly prone to radiation. Doctors can administer radiation to kill these cancerous cells. However, some surrounding normal cells are unavoidably killed as well, due to the large doses of radiation required. Thus patients who undergo radiation therapy also experience side effects such as radiation sickness. To try to minimise these side-effects, narrow beams of radiation are used, and the beam continuously rotates around the body, minimising the radiation to normal areas but concentrating it around the cancerous region.

Radiation is also used to sterilise surgical equipment and bandages. By bombarding the object to be sterilised with radiation, bacteria and viruses can be killed. This procedure is very common in hospitals for cleaning reusable surgical equipment between operations and sterilising blood for transfusions. It is also used in the manufacture of disposable medical products such as bandages and syringes where sterilisation is required before sale or use. Radiation-based sterilisation is especially useful in sterilising heat-sensitive or steam-sensitive materials.

SMOKE DETECTORS Inside a radioisotope-based smoke detector, the americium-241 is placed inside an air chamber. The ionising alpha radiation emitted collides with air molecules in the air chamber, ionising them (making the air particles charged). The ionised air molecules are then able to conduct an electrical current between two electrodes on either side of the air chamber.

When smoke enters the air chamber, the ionised air molecules attract the smoke particles, causing a decrease in the current conducted. (The ionised air molecules are now "carrying" extra smoke particles, decreasing the flow of current.) This current decrease is then detected by electrical circuitry, activating an alarm.

The americium-241 used in smoke detectors exists in the form of americium oxide AmO2. This chemical is expensive - the US Atomic Energy Commission sells it for about US$1500 per gram. However, 1 gram of americium-241 is enough for over 5000 household smoke detectors. The americium-241 radioisotope was first discovered 50 years ago as part of the Manhattan project - the US's attempt to build the first nuclear bomb. Americium-241 is formed in nuclear reactors and has a half life of 432 years.

Typical smoke detectors use less than 35 kilobecquerels of americium-241, which is a very small amount. The radiation emitted by a smoke detector containing this amount of americium poses less danger to human health than background radiation in the atmosphere, so smoke detectors being "radioactive" are not a health hazard. Even still, Australian standards require that detectors



be labelled as containing radioactive material, and must not be disposed through general garbage collection, although in some states smoke detectors are compulsory in all homes.

RADIOACTIVE DATING Radioactive carbon-14 atoms exist naturally. They are everywhere around us: in our clothes, in the food we eat, even in the air we breathe. However, there are not many of these - only 1.3 × 10-

12 percent of all carbon atoms are the carbon-14 isotope. This is why they do not pose danger to us - there are so few of them.

The ratio of radioactive carbon-14 atoms to stable carbon-12 atoms in the atmosphere has remained constant over thousands of years. Although carbon-14 naturally decays, it is also continually being formed. Carbon-14 atoms are formed when neutrons from the sun's cosmic radiation collide with nitrogen-14 atoms in the atmosphere. Thus the decay of carbon-14 is reasonably balanced with its production, resulting in a constant ratio of carbon-14 to carbon-12.

Carbon dioxide (CO2) molecules in the air can contain either isotope of carbon. This CO2 is continually used by plants to grow. Because the ratio of carbon-14 to carbon-12 in atmospheric CO2 is constant, the intake of CO2 by a plant results in a constant ratio of the two isotopes in the plant's body while it is alive. However, when the plant dies it will no longer take in CO2. As a result, the carbon-14 decaying in the dead plant will not be replenished by a "fresh supply" of more CO2, resulting in the ratio of carbon-14 to carbon-12 decreasing over time.

Because animals eat plants, the ratio of carbon-14 to carbon-12 in them also decreases once they die, since the carbon-14 cannot be replenished.

This process of dating using carbon-14 is used by palaeontologists. Paleontologists burn a small sample of a fossil to react the carbon in it with oxygen, to form CO2. The CO2 that contains carbon-14 will be radioactive, and the amount can be easily measured using a radiation counter. Burning is done to facilitate measuring the level of carbon-14. The major problem using radiocarbon dating is the chance of getting carbon from the samples mixed up with "fresh" carbon.

Carbon-14 has a half life of about 5730 years. This means that in a given sample of a carbon-containing substance, (without the carbon-14 being replenished) the ratio of carbon-14 to carbon-12 will decrease by half every 5730 years. Suppose for example, some archaeologists uncovered ancient manuscripts and found that the ratio of carbon-14 to carbon-12 in the paper was half of that found in living trees. This would mean that the manuscripts would be about 5730 years old.

The use of radioactive carbon-14 for dating was first done by William Libby, an academic at the University of Chigaco, USA, in 1947.

The relatively short half-life of carbon-14 (5730 years) means that the amount of carbon-14 remaining in materials and objects older than about 80,000 years is too small to be measured with today's equipment. Thus carbon dating is limited to objects which are not older than this. However, the abundance of other atoms with longer half-lives, such as uranium-238 (half-life 4.5 × 109 years) can be measured in place of carbon-14. Geologists measure the amounts of other



radioactive metal isotopes such as uranium-238, rubidium-84 and potassium-40 (see below) found in rocks to determine their age. Measurements show that the oldest rocks on Earth are about 4.6 billion years old - which is a reasonably accurate estimate of the Earth's age. Similarly, analysis of fossilised plants shows that they first occurred on Earth about 3 billion years ago.

FOOD TREATMENT AND PRESERVATION Ionising radiation is used as an alternative to chemicals in the treatment and preservation of foods. A French scientist first discovered that radiation could be used to prolong food shelf life in the 1920s and it became more widely used in World War II. Today, astronauts often eat radiation-preserved food while on space missions.

In meats and other foods of animal origin, irradiation destroys the bacteria that cause spoilage as well as diseases and illnesses such as salmonella poisoning. This allows for a safer food supply, and meats that can be stored for longer before spoilage. Additionally, irradiation also inhibits tubers that cause fruits and vegetables to ripen. The result is fresh fruits and vegetables that can be stored for longer before ripening.

The irradiation technique is particularly important when exporting to countries with tropical climates, where foods can be spoiled easily due to the warm temperatures.

Irradiation of food is carried out using accelerated electrons (beta radiation), and ionising radiation from sources such as the radioisotopes cobalt-60 and cesium-137. X-rays are also sometimes used. None of these sources of radiation used have enough energy to make the exposed foods radioactive.

Inside the food treatment plant there is a conveyor belt or similar system that transports the food to the radiation source, so that workers do not have to move close to the radiation. The source is packaged in a pencil like device, about 1cm in diameter. The room where irradiation takes place is shielded by concrete walls to prevent radiation from escaping into the environment, although the radiation risk is considerably much less than that from a nuclear reactor. Where gamma radiation is used from a radioisotope source, the radioisotope is stored in a pool of water while not in use, to also help prevent radiation from escaping. However, the plant is in many ways similar to any other - refrigeration is still important. No process can make food completely spoil-proof.

Food irradiation is a well-tested process. Scientists have performed numerous decades of research, and it has been shown that irradiation will not cause significant chemical changes in foods that may affect human health, nor will it cause losses that may affect the nutritional content of food. (Chemical residues left behind by irradiation are in concentrations equivilant to about 3 drops in a swimming pool. Chemical-based preservatives and treatments usually leave more residues.) Taste is usually unaffected. The World Health Organisation and food safety authorities in many countries have approved irradiation as a safe method of food treatment and preservation.

Radiation-treated food is still not very widely used today. Despite the scientific evidence and approvals, many activist organisations claim that irradiation is unsafe and exploit the lack of



public awareness and concerns about food safety and nuclear issues. Some even say that irradiation is a way that governments can utilise nuclear wastes left over from weapons testing or power generation. (However, the wastes left cannot be used in food processing because they do not provide the right type of ionising radiation.) Consequently, these scare tactics deter the public and some food producers are reluctant to use irradiation for fear of consumer boycotts. However, a recent survey conducted in mid-1998 by the Food Marketing Institute (a United States organisation) revealed that less than one percent of all those surveyed identified irradiation as a concern. Most said that spoilage and microbial hazards were of great concern - they very problem that irradiation addresses. Another study by an academic revealed that about 99% of consumers were willing to buy irradiated food after they were shown scientific data and irradiated food samples. This compared to 50% before shown this data.

Irradiation poses less of a risk to human health than many chemical treatments that are used today, which include the addition of chemical preservatives. The use of radiation is sometimes favoured to using chemical preservatives, because no allergic side-effect results. It is also better than heat-sterilisation because irradiation does not destroy nutrients and vitamins, whereas heat treatment does.

Gauging Radioisotope gauging is based on the principle that the radiation emitted from a radioisotope will be reduced in intensity by matter located between the radioisotope and a detector. The amount of this reduction can be used to gauge the presence or absence of the material, or even to measure the quantity of material between the source and the detector. The advantage of this form of gauging is that there is no contact with the material being measured. One application is in the manufacturing of plastic film. The film runs at high speed between a radioactive source and a detector and the detector signal strength is used to control the thickness of the plastic film. A similar technique is used to measure the height of coal in hoppers feeding power station furnaces.

[from: http://www.ansto.gov.au/__data/assets/pdf_file/0018/3564/Isotopes.pdf]



NAME ................................................................................ LINE............................. PHYSICAL SCIENCES 3C ATOMIC STRUCTURE AND RADIOACTIVITY REVISION TEST Q1. Write balanced nuclear equations for the following nuclear processes. a) neodymium-147 undergoes beta-particle and gamma radiation emission. b) thorium-232 undergoes alpha-particle and gamma radiation emission. c) a neutron forms a proton plus a beta particle (6 marks) Q2. The radioactive isotope cerium-144 ( ) has a half-life of 284 days. It decays by way of beta-particle and gamma radiation emission. a) Write the nuclear equation for this decay process. b) A sample of cerium-144 has a mass of 160µg. What will be the amount of cerium-144 remaining after 1136 days? (c) what has happened to the "missing" mass? (4 marks) -2- Q3. Describe the composition of a 65Zn2+ ion in terms of proton, neutron and electron composition Q4. The radioactive isotope tungsten-185 ( ) has a half-life of 75 days and it decays by way of beta-particle and gamma radiation emission. (a) Write the nuclear equation showing the beta-decay process described above.



(2 marks) (b) A sample of tungsten -185 has an initial radiation count of 6400Bq. How long will it be before this radiation count has dropped to 400Bq? Explain your answer. (2 marks)

(c) Sketch and label a graph showing the change of radiation count for the tungsten-185 sample from its initial value of 6400Bq. Show the decay process for a period of 5 half-lives.

(4 marks)

Q5. After their manufacture, medical syringes are packed into plastic bags and sterilised using radiation from a suitable radioactive source. (i) What types of radiation would be suitable for the sterilising of syringes that are already sealed inside plastic bags? Suggest TWO types. ………………………… or ………………………………. (1 mark) (ii) Explain your choices. (2 marks)

(ii) Is there a risk of the syringe or the plastic bag becoming radioactive because of this sterilising process? Explain your reasoning. (3 marks)



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



Q5. After their manufacture, medical syringes are packed into plastic bags and sterilised using radiation from a suitable radioactive source. (i) What types of radiation would be suitable for the sterilising of syringes that are already sealed inside plastic bags? Suggest TWO types.

gamma or beta (1 mark)

(ii) Explain your choices.

Gamma and beta radiation are both ionising radiation so they

will ionise molecules and disrupt cell functions of living creature.

Sufficient radiation will kill organisms such as bacteria and

viruses on the syringes. Gamma is highly penetrating, so will

penetrate inside the package and the syringe to kill the organisms;

beta is moderately penetrating so will be effective provided the

packaging is thin.

Alpha radiation, while being highly ionising would not penetrate

the plastic bag – alphas are stopped by a sheet of paper for

example. (2 marks)

(ii) Is there a risk of the syringe or the plastic bag becoming radioactive because of this sterilising process? Explain your reasoning.



No. Ionising radiation such as gamma and beta interact with the

electrons of atoms but do not have sufficient energy to penetrate

the nucleus of the atom. Radioactivity is a property of the

nucleus of atoms. Therefore the materials being treated do not

become radioactive as their nuclei are unchanged by the

sterilization.

[However, there is some debate as to whether ionisation may

cause chemical change to foods when ionising radiation is used

for food preservation. Some people fear that such chemical change

may be harmful.] (3 marks)

radioactivity & atomic structure

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