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- He knew from the work of J.J. Thomson that every atom
contains one or more electrons. Thomson had shown that the electron
is a negatively charged particle inside every atom but no-one knew
until Rutherford' theory was confirmed experimentally how the positive charge in the atom was distributed. Thomson thought the atom could be like a 'currant bun' - with electrons dotted in the
atom like currants in a bun. In this model, the positive charge was supposedly spread throughout the atom like the dough of the bun.
contains one or more electrons. Thomson had shown that the electron
is a negatively charged particle inside every atom but no-one knew
until Rutherford' theory was confirmed experimentally how the positive charge in the atom was distributed. Thomson thought the atom could be like a 'currant bun' - with electrons dotted in the
atom like currants in a bun. In this model, the positive charge was supposedly spread throughout the atom like the dough of the bun.
Remember: Rutherford knew that the atoms of certain elements were unstable and emitted radiation. It had been shown that there were tree types of radiation: alpha radiation, beta radiation and gamma radiation.
-what does Rutherford know about alpha decay ?therefore what/how did he use it to test JJ-Thompsons atom model? what result surprised him / showed that JJ - Thompsons plum pudding model was wrong ?
-what does Rutherford know about alpha decay ?therefore what/how did he use it to test JJ-Thompsons atom model? what result surprised him / showed that JJ - Thompsons plum pudding model was wrong ?
Rutherford knew that alpha radiation consisted of fast-moving positively charged particles. He and his co-workers used this type of radiation to probe the atom. He reckoned that a beam of alpha particles directed at a thin metal foil might be scattered slightly by the atoms of the foil if the positive charge was spread throughout each atom. He was astonished
when he discovered that some of the particles bounced back from the foil
when he discovered that some of the particles bounced back from the foil
-Rutherford used a narrow beam of alpha particles, all of the same kinetic energy, in an evacuated container to probe the structure of the atom. Figure 1 shows an outline of the arrangement he used. A thin metal foil was placed in the path of the beam. Alpha particles scattered by the metal foil were detected by a detector which could be moved round at a constant distance from the point of impact of the beam on the metal foil.
-Rutherford used a microscope to observe the pinpoints of light emitted when a particles hit the atoms of a fluorescent screen.He measured the number of a particles reaching the detector per
minute for different angles of deflection from zero to almost 180°
-Rutherford used a microscope to observe the pinpoints of light emitted when a particles hit the atoms of a fluorescent screen.He measured the number of a particles reaching the detector per
minute for different angles of deflection from zero to almost 180°
1 )most alpha particles passed straight through the foil with little or no
deflection; about 1 in 2000 were deflected,
2 )a small percentage of alpha particles (about 1 in 10000) were deflected
through angles of more than 90°
> Rutherford realised the -scattering measurements could be explained by assuming every atom has a 'hard centre' much smaller than the atom.His interpretation of the data was:
-most of the atom's mass is concentrated in a small region, the nucleus, at the centre of the atom,
- the nucleus is positively charged because it repels alpha particles(which carry positive charge) that approach it too closely.
deflection; about 1 in 2000 were deflected,
2 )a small percentage of alpha particles (about 1 in 10000) were deflected
through angles of more than 90°
> Rutherford realised the -scattering measurements could be explained by assuming every atom has a 'hard centre' much smaller than the atom.His interpretation of the data was:
-most of the atom's mass is concentrated in a small region, the nucleus, at the centre of the atom,
- the nucleus is positively charged because it repels alpha particles(which carry positive charge) that approach it too closely.
• alpha particle C collides 'head-on' with the nucleus and rebounds back so its angle of deflection is 180°.
• alpha particles A, B, and D are deflected through different angles. The closer the initial direction of an a particle is to the 'head-on direction,
• the greater its deflection is because the electrostatic force of repulsion between the a particle and the nucleus increases with decreased separation between them,
• the smaller the least distance of approach of the alpha particle to the alpha particle to the nucleus is
• a particle E does not approach the nucleus closely enough to be significantly deflected.
• alpha particles A, B, and D are deflected through different angles. The closer the initial direction of an a particle is to the 'head-on direction,
• the greater its deflection is because the electrostatic force of repulsion between the a particle and the nucleus increases with decreased separation between them,
• the smaller the least distance of approach of the alpha particle to the alpha particle to the nucleus is
• a particle E does not approach the nucleus closely enough to be significantly deflected.
-Using Coulomb's law of force (i.e., the law of force between charged objects) and Newton's laws of motion, Rutherford used his nuclear model to explain the exact pattern of the results.
-By testing foils of different metal elements, he also showed that the magnitude of the charge of a nucleus is +Ze, where e is the charge of the electron and Z is the atomic number of the element.
-By testing foils of different metal elements, he also showed that the magnitude of the charge of a nucleus is +Ze, where e is the charge of the electron and Z is the atomic number of the element.
Rutherford's golden foil experiment :
for such a single scattering by a foil that has n layers of atoms, the probability of an alpha particle being deflected by a given atom is therefore about 1 in 10000n.
-what does this probability depend on ?
-what is the area ratio ( nucleus to atom ) equal to ?
-what is a typical value of n ?
for such a single scattering by a foil that has n layers of atoms, the probability of an alpha particle being deflected by a given atom is therefore about 1 in 10000n.
-what does this probability depend on ?
-what is the area ratio ( nucleus to atom ) equal to ?
-what is a typical value of n ?
-During the golden foil experiment , what happens if the alpha particle hits the nucleus in a head on impact ?
-What is the potential energy of the alpha particle at this point ?
using what equation ?-in a head on impact, the alpha particle stops momentarily at the least distance of approach, d.
-At this point, the potential energy of the alpha particle in the electric field of the nucleus is equal to to the initial kinetic energy Ek of the alpha particle. use the equation :What is Marie Curie do?-Marie Curie established the nature of radioactive materials. She showed how radioactive compounds could be separated and identifiedWhat did Rutherford find out about radiation ?-ionised air, making it conduct electricity. He made a detector which could measure the radiation from its ionising effect.
-was of two types. One type which he called alpha (a) radiation was easily absorbed. The other type which he called beta (B) radiation was more penetrating. A third type of radiation, called gamma (y) Radiation, even more penetrating than ß radiation, was discovered a year later.Rutherford investigations on radioactivity:
- further testes showed that a magnetic field deflects which types of radiation?in which direction?
+ has no effect on which type of radiation?
-from the deflection what was concluded?-Further tests showed that a magnetic field deflects α and ß radiation in
opposite directions and has no effect on y radiation.
-From the deflection direction, it was concluded that alpha radiation consists of positively charged
particles and beta radiation consists of negatively charged particles. y radiation was later shown to consist of high energy photons.Remember: don't confuse the radiation with the source; the source is radioactive not the radiation-The ionising effect of each type of radiation can be investigated using what ?
-explain how ?-The ionising effect of each type of radiation can be investigated using an ionising chamber and a picoammeter .
- The chamber contains air at atmospheric pressure. Ions created in the chamber are attracted to the oppositely charged electrode where they are discharged. Electrons pass through the picoammeter as a result of ionisation in the chamber. The current is proportional to the number
of ions per second created in the chamber.What are the properties of alpha radiation ?alpha radiation causes strong ionisation. However, if the source is moved away from the top of the chamber, ionisation ceases beyond a certain distance. This is because a radiation has a range in air of no more than a few centimetres.What are the properties of beta radiation?Beta radiation has a much weaker ionising effect than a radiation. Its range in air varies up to a metre or more. A Beta particle therefore produces fewer ions per millimetre along its path than
an alpha particle does.What are the properties of gamma radiation ?y radiation has a much weaker ionising effect than either alpha or ßeta radiation. This is because photons carry no charge so they have less effect than alpha or Beta particles do.Explain hoe cloud chambers work / form cloud chamber observations?A cloud chamber contains air saturated with a vapour at a very low temperature. Due to ionisation of the air, an alpha or a ßeta particle passing through the cloud chamber leaves a visible track of minute condensed vapour droplets. This is because the air space is supersaturated. When an ionising particle passes through the supersaturated vapour, the ions produced trigger the formation of droplets.What are the chamber observation of alpha particles?Alpha particles produce straight tracks that radiate from the source and are casily visible. The tracks from a given isotope are all of the same length, indicating that the a particles have the same range.Absorption test
-what is the count rate?
-what must be measured before the source is tested?-The number of counts in a given time is measured and used to work out
the count rate which is the number of counts divided by the time taken.
-Before the source is tested, the count rate due to background radioactivity
must be measured. This is the count rate without the source present.Absorption test:
-what can be used to investigate absorption by different materials?
-how does it work?-A grieved tube and a counter may be used to investigate absorption by different materials.
-Each particle of radiation that enters the tube is registered by the counter as a singleWhat are the chamber observation of beta particles?B particles produce wispy tracks that are easily deflected as a result of collisions with air molecules. The tracks are not as easy to see as alpha particle tracks because ß particles are less ionising than a particlesDescribe the process of the absorption test ?-Before the source is tested, the count rate due to background radioactivity must be measured. This is the count rate without the source present.
-The count rate is then measured with the source at a fixed distance from the tube without any absorber present. The background count rate is then subtracted from the count rate with the source present to give the corrected (i.e., true) count rate from the source.
-The count rate is then measured with the absorber in a fixed position between the source and the tube. The corrected count rates with and without the absorber present can then be compared.
>>By using absorbers of different thickness of the same material, the effect of the absorber thickness can be investigated. Figure 6 shows a typical set of measurements for the absorption of radiation by aluminium. Note that the count rate scale is a logarithmic scaleExplain how the grieved tube worksThe Geiger tube is a sealed metal tube that contains argon gas at low pressure. The thin mica window at the end of the tube allows alpha and Beta particles to enter the tube. y photons can enter the tube through the tube wall as well. A metal rod down the middle of the tube is at a positive potential as shown in Figure 7. The tube wall is connected to the negative terminal of the power supply and is earthed.Absorption summary ?-alpha radiation is absorbed completely by paper and thin metal foil
-beta radiation absorbed completely by 5mm of metal
-gamma radiation is absorbed completely by several centimetres of leadExplain what happens when a particle of ionising radiation enters a tube?When a particle of ionising radiation enters the tube, the particle ionises the gas atoms along its track. The negative ions are attracted to the rod and the positive ions to the wall. The ions accelerate and collide with other gas atoms, producing more ions. These ions produce further ions in the same way so that within a very short time, many ions are created and discharged at the electrodes. A pulse of charge passes round the circuit through resistor R, causing a voltage pulse
across R which is recorded as a single count by the pulse counter.The arrangement in Figure 5 ( just a source in sealed container and Geiger tube ) without the absorbers may be used to investigate the range of each type of radiation in air.
-describe how ?The count rate is measured for different distances between the source and the tube,
starting with the source close to the tube. The background count rate must also be measured in the absence of the source so the corrected count rate can be calculated for each distance.The arrangement in Figure 5 ( just a source in sealed container and Geiger tube ) without the absorbers may be used to investigate the range of each type of radiation in air.
- explain what you will see if a particle with alpha radiation enters the tube-alpha radiation has a range of only a few centimetres in air. The count rate decreases sharply once the tube is beyond the range of the a particles. This can be seen in Figure 4 as the tracks from the source are the same length indicating that the particles from a given source have the same range and therefore the same initial kinetic energy. The range differs from one source to another indicating that the initial kinetic energy differs from one source to another.The arrangement in Figure 5 ( just a source in sealed container and Geiger tube ) without the absorbers may be used to investigate the range of each type of radiation in air.
- explain what you will see if a particle with beta radiation enters the tube ?-B radiation has a range in air of up to about a metre. The count rate gradually decreases with increasing distance until it is the same as the background count rate at a distance of about 1 m. The reason for the gradual decrease of count rate as the distance increases is that the B particles from any given source have a range of initial kinetic energies up to a maximum. Faster B particles travel further in air than slower B particles as they have greater initial kinetic energy.Explain what the dead time of a Geiger tube is ?The dead time of the tube, the time taken to regain its non-conducting state after an ionising particle enters it, is typically of the order of 0.2 ms. Another particle that enters the tube in this time will not cause a voltage pulse. Therefore, the count rate should be no greater than about 5000 s^-1 (= 1/0.2 ms)The arrangement in Figure 5 ( just a source in sealed container and Geiger tube ) without the absorbers may be used to investigate the range of each type of radiation in air.
- explain what you will see if a particle with gamma radiation enters the tube ?y radiation has an unlimited range in air. The count rate gradually decreases with increasing distance because the radiation spreads out in all directions, as shown in Figure 2. The proportion of the y photons from the source entering the tube decreases according the Inverse square law.Describe the nature of alpha radiation ?alpha radiation consists of positively charged particles. Each a
particle is composed of two protons and two neutrons, the same
as the nucleus of a helium atom.Explain how Rutherford proved that the spectrum of light from the tube was the same as from a tube filled with helium gas.-Rutherford devised an experiment in which a particles were collected as a gas in a glass tube fitted with two electrodes. When a voltage was applied to the electrodes, the gas conducted electricity and emitted light. Using a spectrometer, he proved that the spectrum of light from the tube was the same as from a tube filled with helium gas.For Alpha radiation :
-describe the nature of the particle ?
-range in air ?
-direction in a magnetic field?
-absorption?
-ionisation?
-energy of each particle/photon ?>Nature: 2 protons + 2 neutrons
>Range: fixed, depends on energy , can be up to 100mm
>Direction in a magnetic field: deflected
>Absorption: stopped by paper or thin material foil
>Ionisation: produces about 10^4 ions per mm in air at standard pressure
>energy of each particle/photon: constant for a given sourceFor Bata radiation :
-describe the nature of the particle ?
-range in air ?
-direction in a magnetic field. ?
-absorption ?
-ionisation?
-energy of each particle/photon ?>Nature: β−= electron ( β+=positron)
>Range: range up to about 1m
>Direction in a magnetic field: opposite direction to alpha particles, and more easily deflected
>absorption: stopped by approx 5mm of aluminium
>Ionisation: produces about 100 ions per mm in air at standard pressure
>energy of each particle/photon:varies up to a maximum for a given sourceFor gamma radiation :
-describe the nature of the particle ?
-range in air ?
-direction in a magnetic field. ?
-absorption ?
-ionisation?
-energy of each particle/photon ?>Nature:photon of energy of the order of MeV
>Range: follows the inverse square law
>Direction in a magnetic field:not deflected
>absorption: stopped or significantly reduced by several centimetres of lead
>Ionisation:very weak ionising effect
>energy of each particle/photon:constant for a given sourceDescribe and explain the experiment that was devised by Rutherford that allowed him to discover the proton and PREDICT (NOT discover ) the existence of neutrons.
>>the existence of the neutron was established later in 1932 by James Chadwick-Rutherford made the discovery that neutralised a particles are the same as helium some years before his discovery that every atom contains a nucleus. After he established the nuclear model of the atom, it was realised that the nucleus of the hydrogen atom, the lightest known atom, was a single positively charged particle which became known as the proton. Rutherford realised that other nuclei contain protons and he predicted the existence of neutral particles of similar mass, neutrons, in the nucleus. For example, the helium nucleus carries twice the charge of the hydrogen nucleus and therefore contains two protons. However, its mass is four times the mass of the hydrogen nucleus so Rutherford predicted that it contained two neutrons as well as two protons.Describe the nature of beta radiationBeta radiation from naturally occurring radioactive substances consists of fast-moving electrons.Explain how the nature of beta radiation was proved ?This was proved by measuring the deflection of a beam of ß particles using electric and magnetic
fields. The measurements were used to work out the specific charge (which is the charge/mass) of the particles. This was shown to be the same as the specific charge of the electron. An electron is created and emitted from a nucleus with too many neutrons as a result of a neutron suddenly changing into a proton.-describe the stability of a nucleus with too many protons? What does cause the nucleus to do?A nucleus with too many protons is also unstable and emits a positron, the antiparticle of the electron, when a proton changes to a neutron.Unstable nuclei (i.e too many protons) are not present in naturally occurring radioactive substances.
-explain how they are created ?They are created when high-energy protons collide with nuclei.-describe the nature of gamma radiation? how was this was discovered ?y radiation consists of photons with a wavelength of the order of 10^-11 m or less. This discovery was made by using a crystal to diffract a beam of y radiation in a similar way to the diffraction of light by a diffraction grating.Define the intensity of the radiation?The intensity, I , of the radiation is the radiation energy per second passing normally through unit area .-What is the formula for the intensity I of the radiation from the distance, d ?
-using the formula explain what Intensity varies with?>remember:The intensity, I , of the radiation is the radiation energy per second passing normally through unit area .
- for a point source that emits n yphotos per secound ( each of energy hf) the radiation energy per secound from the source = nhf
- At distance r from the source, all the photons emitted from the source pass through a total area of 4 π r^2 ( the surface area pf a sphere of radius r)What is the relationship between the intensity ,I(0),of a gamma source and the distance ,x,from the source ?-therefore at distance 2x the intensity is I(0)/4
-at distance 3x the intensity is I(0)/9How do you verify the the inverse square law for a gamma source ?-use a Geiger counter to measure the count rate C at different measured distance , d, from the tube and the backgrounds count rate, C(0), without the source present. The corrected count rate C-C(0) is proportional to the intensity of the radiation.
-As shown in the figure below the source is at unknown , d(0), inside its sealed container. Using the inverse square law for gamma radiation gives the corrected count rate :What is the equation for alpha emission ?Why is Ionising radiation is hazardous ?
>> such radiation includes X-rays, protons and neutrons as well as α, β, and γ radiation.-Ionising radiation is hazardous because it damages living cells.-what happens during β− decay/ emission ?
-what is the formula for it ?-a neutron in a neutron rich nucleus emits a β− particle.In effect, a neutron in a neutron rich nucleus changes into a proton and an( electron) antineutrino is emitted at the same time as the β− is created-what happened during β+ decay/emission ?
-what is the equation for β+ decay ?-In effect, a proton in a proton rich nucleus changes into a neutron and an electron neutrino is emitted at the same time as the β+ particle is created-explain what happens during electron capture?
-write the formula for electron capture ?-some proton rich nuclides can capture an inner shell electron.This causes a proton in the nucleus to change into a neutron with the emission of the electron neutrino at the same time.The inner shell vacancy is filled by an outer shell electron, as a result causing an X-ray photon to be emitted by the atomExplain why ionising radiation affects living cells ?-it can destroy cell membranes which causes cells to die, or
- it can damage vital molecules such as DNA directly or indirectly by creating 'free radical' ions which react with vital molecules. Normal cell division is affected and nuclei become damaged. Damaged DNA may cause cells to divide and grow uncontrollably, causing a tumour which may be cancerous. Damaged DNA in a sex cell (i.e., an egg or a sperm) may cause a mutation which may be passed on to future generations.-what happens to living cells as a result of exposure to ionising radiation?-As a result of exposure to ionising radiation, living cells die or grow uncontrollably or mutate, affecting the health of the affected person (somatic effects) and possibly affecting future generations (genetic effects).
-Cell mutation and cancerous growth occurs at low doses as well as at high doses. There is no evidence of the existence of a threshold level of ionising radiation below which living cells would not be damaged.
-High doses of ionising radiation kill living cells.Anyone using equipment that produces ionising radiation must wear what ?-Anyone using equipment that produces ionising radiation must wear a film badge to monitor his or her exposure to ionising radiation-What happens during gamma emission/decay ?
-when is a gamma photon emitted ?>no chnage occurs in the number of protons or neutron of a nucleus when it emits a gamma photon
>A gamma photon is emitted if a nucleus has excess energy after it has emitter an alpha or β− particleExplain how film badges work ?The badge contains a strip of photographic film in a light-proof wrapper. Different areas of the film are covered by absorbers of different materials and different thicknesses. When the film is developed, the amount of exposure to each form of ionising radiation can be estimated from the blackening of the film. If the badge is overexposed, the wearer is not allowed to continue working with the equipment.-what does the biological effect of ionising radiation depends?
-explain how ?-The biological effect of ionising radiation depends on the dose received and the type of radiation. -The dose is measured in terms of the energy absorbed per unit mass of matter from the radiation. The same dose of different types of ionising radiation has different effects. For example, a radiation produces far more ions per millimetre than y radiation in the same substance so it is far more damaging. However, a radiation from a source outside the body cannot penetrate the skin's outer layer of dead cells so is much less damaging than if the source were inside the body.Key points to remember about background radiation ?-We are all subject to background radiation which occurs naturally due to cosmic radiation and from radioactive materials in rocks, soil and in the air.
-Background radiation does vary with location due to local geological features. For example, radon gas which is radioactive can accumulate in poorly ventilated areas of buildings in certain locations.
>Figure 3 shows the sources of background radiation in the UK.Remember: ALARA stands for ' as low as reasonably achievable'.Risk are always reduced by increasing the distance from source and shortening the time exposureRemember: because radioactive materials produce ionising radiation, they must be stored and used with care. In addition, disposal of a radioactive substance must be carried out in accordance with specific regulations. Only approved institutions are allowed to use radioactive materials. Approval is subject to regular checks and approved institutions are categorised according to purpose.Explain how radioactive substances should be stored ?-Storage of radioactive materials should be in lead-lined containers. Most radioactive sources produce y radiation as well as α or β radiation so the lead lining of a container must be thick enough to reduce the radiation from the sources in the container to about the background
level. In addition, regulations require that the containers are under lock and key' and a record of the sources is kept.Remember : When using radioactive materials, established rules and regulations must be followed.
> explain what these regulation are ?-No source should be allowed to come into contact with the skin.
-Solid sources should be transferred using handling tools such as
tongs or a glove-box or using robots. The handling tools ensure
the material is as far from the user as practicable so the intensity of
the y radiation from the source at the user is as low as possible and
the user is beyond the range of a or B radiation from the source.
-Liquid and gas sources and solids in powder form should be in
sealed containers. This is to ensure radioactive gas cannot be
breathed in and radioactive liquid cannot be splashed on the
skin or drunk.
-Radioactive sources should not be used for longer than is
necessary. The longer a person is exposed to ionising radiation,
the greater is the dose of radiation received.Explain what the intensity of a γ beam that passes though an absorber decreases exponentially with ?The intensity of a y beam that passes through an absorber decreases exponentially with the
thickness of the absorber. If a certain thickness of a material cuts the intensity of a y beam to half,
twice the thickness will cut the intensity to a quarter, and so on.Explain what happens when a nucleus of a radioactive isotope emits a alpha or beta particle ?>When a nucleus of a radioactive isotope emits an alpha or a beta particle, it becomes a nucleus of a different elements because its proton number changes.The number of nuclei of the initial radioactive isotope therefore decreases.
>The mass of the initial isotope decreases. The figure below shows how the mass decreases over time.The curve is referred to as a decay curve. The mass of the isotope decreases with time at a slower and slower rate.Measurements show that the mass decreases exponentially which means that the mass drops by a constant factor in equal intervals of time.
E.g
If the initial mass of the radioactive isotope is 100g and the mass decreases to a factor of x0.8 every 1000s then :
-after 1000s, the mass remaining = 0.8g (0.8x100)
-after 2000s, the mass remaining = 64g(=0.8x0.8x100)
-after 3000s, the mass remaining = 51g(=0.8x0.8x0.8x100)A convent measure for the rate of decrease is the time taken for a decrease by half , this is the half life of the process .
Define the half life of a radiative isotope ?The half life, T(1/2) of a radioactive isotope is the time taken for the mass of the isotope to decrease to half the initial mass.
-this is the same as the time taken for the number of nuclei of the isotope to decrease to half the initial number.Example of half life calculation :
Consider a sample of a radiative isotope X which initially contains 100g of the isotope.
What is the mass of the isotope after :
-one half life ?
-two half lives?
-three half lives ?
-n half lives ?-After 1 half life , the mass of X remaining= 0.5x100=50g
-After 2 half lives from the start , the mass of X remaining= 0.5x0.5x100=25g
-After 3 half lives from the start , the mass of X reaming = 0.5x0.5x0.5x100=12.5g
- You Carry on until the mass that remain is half the mass of the initial mass, the EXACT time taken for that to happen is the half life ( so the half life might not be a integer i.e 3.8)
>>After n half lives from the start, the mass of X remaining =How do you calculate the count rate after n half lives ?initial count rate / 2^nRemember : although in theory a radiative decay curve never falls to zero, in practice it eventually falls to a level which is indistinguishable from the background radiationExplain why the mass of a radioactive isotope decreases exponentially?The mass of a radioactive isotope decreases exponentially because radioactive decay is a random process and the number of nuclei that decay in certain time is in proportion to the nuclei of X remaining.-Define the activity , A, of a radioactive isotope ?
-what is the unit for A?-The Activity , A , of a radioactive isotope is the number of nuclei of the isotope that disintegrate per second. In other words, it is the rate of change of the number of nuclei of the isotope.
The unit of activity is the becquerel( Bq), where 1 Bq= 1 disintegration per secondExplain what is the activity of a radioactive isotope proportional to ?The activity of a radioactive isotope is proportional to the mass of the
isotope. Because the mass of a radioactive isotope decreases with time
due to radioactive decay, the activity decreases with time.Describe an experiment that shows what the activity of a radioactive isotope proportional to ?>Figure 3 shows an experiment in which the activity of a radioactive isotope of Protactinium 234,91}Pa is measured and recorded using a Geiger tube and a counter. This isotope is a β-emitter produced by the decay of the radioactive isotope of thorium 234,90}Th.
-In this experiment, an organic Solvent in a sealed bottle is used to separate protactinium from thorium to enable the activity of the protactinium to be monitored.Before the experiment is carried out, the backsround count rate is measured without the bottle present. The bottle is then shaken to mix lie aqueous and solvent layers and then placed near the end of the Geiger tube. The lavers are allowed to separate as shown in Figure 3. The protactinium is Collected by the solvent and the thorium by the layer. The Gelger tube detects & particles emitted by the decay of the protactinum nuclei in the solvent layer.
-The counter is used to measure the number of counts every 10s. The our rate is the number of counts in each ten second interval divided by 10s. The background count rate is subtracted to give the corrected count rate: since the activity is proportional to the corrected count rate, a graph of the corrected count rate against time, as in Figure 4, shows how the activity of the protactinium decreases with time.What is the energy transfer per second from a radioactive source equal ?-For a radioactive source of activity A that emits particles (or photons) of the same energy E, the energy per second released by radioactive decay in the source by the radiation is the product of its activity and the energy of each particle. In other words, the power of the source = AE.
The energy transfer per second from a radioactive source = AEIf the source is in a sealed coriander and emits only alpha particles which are all absorbed by the container , what type of energy will the the container gain? Equal to what ?-If the source is in a sealed container and emits only a particles which are all absorbed by the container, the container gains thermal energy from the absorbed radiation equal to the energy transferred from the source.
- For example, for a source that has an activity of 30MBq and emits particles of energy 2. 5 MeV, the energy transfer per second from the source = 30 x10^6 Bq × 2.5 MeV = 7.5 x 10^7 MeVs-1
=1.2 x 10^-5Js^-1-How does an unstable nucleus become stable ?
-what type of event is this ?-an unstable nucleus becomes stable by emitting and alpha and beta particle or a gamma photon .
-this is an unpredictable event-What is the probability for a nucleus of a radioactive isotope undergoing radioactive event? nucleus of a radioactive isotope has an equal probability of undergoing radiative decay in any given time interval.
-Therefore, there is large number of nuclei of a radioactive isotope-Every nucleus of a radioactive isotope has an equal probability of undergoing radiative decay in any given time interval.
-Therefore, there is large number of nuclei of a radioactive isotopeWhat does the number of nuclei that disintegrate in a certain time interval depends ?The number of nuclei that disintegrate in a certain time interval depends only on the total number of nuclei.( since the every nucleus of a radioactive isotope has an equal probability of undergoing radiative decay in any given time interval.)Consider a sample of a radioactive isotope X that initially contains N(0) nuclei of the isotope.Let N represent the number of nuclei of X remaining at time t after the start. Suppose in time Δt,the number of nuclei that disintegrate is ΔN.
-what is ΔN proportional to ?
-Therefore what does this show us ?Because radiative disintegration is a random process,ΔN is proportional to :
1) N , the number of nuclei of x remaining at time t
2) the duration of the time interval
-Therefore :Explain/ Derive what the activity, A ,of N atoms of a radioactive isotope is given by ?-What is the solution to the equation for the rate of disintegration ?
-what is the equation for in n half lives , what is the remaining number of nuclei ?-What does the graph for the N( number of nuclei remaining after decay ) are against T look like ?
-what equation is this graph represented by ?Explain what the equation for the mass decrease during radioactive decay for radioactive isotope is ?Explain what the equation for the activity of a sample of N nuclei of an isotope decay is ?-What is the unit of decay constant is ?
-what does a large value of λ mean regarding decay rate and half life ?-the unit of decay constant is S^-1.
-A large value of λ means fast decay and short decay-what is the corrected count rate, C, due to a sample of a radioactive isotope at a fixed distance from a Geiger tube is proportional to ?
-Therefore , what does this mean ?-The corrected count rate, C, due to a sample of a radioactive isotope at a fixed distance from a Geiger tube is proportional to the activity of the source.
-Therefore,Worked example :
-A sample of a radioactive material initially contains 1.2 x 10^20 atoms of the isotope.The decay constant for the isotope is 3.6 x 10^-3 s^-1 . Calculate :
A) the number of atoms of the isotope remaining after 1000s
B) the active of the sample after 1000sDefine the decay constant ?The decay constant, λ , is the probability of an individual nucleus decaying per second.-In general , what is the change in the number of nuclei ΔN in time Δt given by ?
-then what is the probability of decay ?
-so what is the probability per unit time ?Remember : the half life , T(1/2) , of a radioactive isotope is the time taken for half the initial number of nuclei to decay.
- the longer the half life , how does this effect the size of the decay constant ?why?
-the half life is related to the decay constant according to what equation ?
-how else can that same equation be written ?-the longer the half life, the smaller the decay constant because the probability of decay per second is smaller.Proof that T(1/2)=ln2/λHow do you calculate N at time t , given the values of N(0) , the number of nuclei , and T(1/2) ?Remember : radioactive waste from a nuclear reactor contains a range of unstable isotopes with different half-lives . The waste products must be stored for many years until their activity is no more than backgroundRadioactive isotopes are used for many purpose.
-what does the choice of an isotope for a particular purpose depend on ?> its half life
> the type of radiation it emits
>For some uses, the choice also depends or how the isotope is obtained and on whether or not it produces a stable decay product.
>addition, the toxicity and biochemical suitability of the pharmaceuticals to which it is attached need to be considered in medical and related applicationsWhat are the different types of radioactive dating>carbon dating
>Argon datingExplain how carbon dating works ?-Living plants and trees contain a small percentage of the radioactive isotope of carbon 14,6}C which is formed in the atmosphere as a result of cosmic rays knocking out neutrons from nuclei. These neutrons then collide with nitrogen nuclei to form carbon-14 nuclei.
-Carbon dioxide from the atmosphere is taken up by living plants as a result of photosynthesis. So a small percentage of the carbon content of any plant is carbon-14 nuclei . This isotope has a half-life of 5570 years so there is negligible decay during the lifetime of a plant. Once a tree has died, no further carbon is taken in so the proportion of carbon-14 in the dead tree decreases as the carbon-14 nuclei decay. Because activity is proportional to the number of atoms still to decay, measuring the activity of the dead sample enables its age to be caleulated, provided the activity of the same mass of living wood is known.Carbon dating :
Writing the equation that represents this process ?Worked example :
A certain sample of dead wood is found to have an activity of 0.280 Bq.An equal mass of living wood is found to have an activity of 1.30 Bq.Calculate the age of the sample.Give your answer in years to 3 s.f ?-explain how argon dating works , using equations as well ?Argon Dating:
-What is the effective half life of the decay of 40,20}Ca ?1250 million yearsArgon Dating :
Explain how the age of the rock (i.e the time from when it solidified) can be calculated ?-the age of the rock can be calculated by measuring the proportion of argon-40 to potassium-40 :
1) For every N potassium-40 atoms now present, if there is 1 argon-40 atom present, there must have originally N+9 potassium atoms (i.e , 1 decayed into argon-40+8 that decayed into calcium-40+N remaining) .
2)What is a radioactive tracer used to do ?-A radioactive tracer is used to follow the path of a substance through a system.In general, the radioactive isotope(s) in the tracer should :
> have a half life which is stable enough for the necessary measurements to be made and short enough to decay quickly after use
> emit beta radiation and gamma radiation so it can be detected outside the flow pathWhat are examples of the application of radioactive tracers ?>detecting underground pipe leaks
>modelling oil reservoirs mathematically to improve oil recovery
>investigating the uptake of fertilisers by plants
> monitoring the uptake of iodine by the thyroid glandsExample of radioactive tracers : detecting underground pipe leaks
-Describe the method ?
-what type of tracer is used ? +why ?-radioactive tracer injected into the flow. A detector on the surface above the pipeline is used to detect leakage
-injected fluid contains a β-emitter or a γ-emitter (depending
on factors such as depth, soil density) as α-radiation would be
absorbed by the pipesExample of radioactive tracers:modelling oil reservoirs mathematically to improve oil recovery
-describe the method used ?
-what type of tracer is used ?-water containing a radioactive tracer is injected
into an oil reservoir at high pressure, forcing some
of the oil out. Detectors at the production wells
monitor breakthrough of the radioactive isotope
-'tritiated' water 3,1}H0, a ß-emitter with a half-life of 12 yearsExample of radioactive tracers:investigating the uptake of fertilisers by plants
-describe the method used ?
-what type of tracer is used ?-plant watered with a solution containing a
fertiliser. By measuring the radioactivity of the
leaves, the amount of fertiliser reaching them can be determined
-fertiliser contains phosphorus, 32,15} P, a B-emitter with a half-life of
14 daysExample of radioactive tracers: monitoring the uptake of iodine by the thyroid glands
-describe the method ?
-what type of tracer is used ?-patient is given a solution containing sodium solution of sodium iodide contains iodide which contains a small quantity of
iodine 133,53}I . The activity of the patient's half-life of 8 days thyroid and the activity of an identical sample prepared at the same time is measured 24h later
-solution of sodium iodide contains iodine 131,53}I , a β-emitter with a half life of 14 daysWhat some important example of industrial uses of radioactivity ?> engine wear
> thickness monitoring
> power sources for remote devicesExplain how radioactivity can be used for engine wear ?The rate of wear of a piston ring in an engine can be measured by
fitting a ring that is radioactive. As the ring slides along the piston
compartment, radioactive atoms transfer from the ring to the engine
oil. By measuring the radioactivity of the oil, the mass of radioactive
metal transferred from the ring can be determined and the rate of
wear calculated. A metal ring can be made radioactive by exposing it to neutron radiation in a nuclear reactor. Each nucleus that absorbs a neutron becomes unstable and disintegrates by β− emission.Explain how radioactivity can be used for thickness monitoring ?Metal foil is manufactured by using rollers to squeeze plate metal on a continuous production line. A detector measures the amount of radiation passing through the foil. If the foil is too thick, the detector
reading drops. A signal from the detector is fed back to the control system to make the rollers move closer together and so make the foil thinner. The source used is a ß- emitter with a long half-life. Alpha radiation would be absorbed completely by the foil and gamma radiation
would pass straight through without absorption.Explain how radioactivity can be used as power sources for remote devices ?-Satellites, weather sensors, and other remote devices can be
powered using a radioactive isotope in a thermally insulated sealed
container which absorbs all the radiation emitted by the isotope. A
thermocouple attached to the container produces electricity as a result of the container becoming warm through absorbing radiation.radioactivity can be used as power sources for remote devices :
- explain why the source needs to have a reasonably long half life for this application ?-For a mass m of the isotope, its activity A=λN, where N is the number of radioactive atoms present in mass m. If each disintegration of a nucleus nucleus releases energy E, the energy transfer per second from the source= λNE. The source needs to have a reasonably long half life, so it does not need to be replaced frequently, but a very long half-life may be require too much mass to generate the necessary power .Explain a useful way to survey nuclear stability ?A useful way to survey nuclear stability is to plot a graph of the neutron number N against the proton number Z for all known isotopes, as shown in Figure 1. Bach isotope is plotted on the graph according to its values of N and Z. The graph shows that stable nuclei lie along a belt curving upwards with an increasing neutron-proton ratio from the origin to N = 120, Z = 80 approximately.What is a light isotope ?Z (proton number ) from 0 to no more than 20Describe What does the N-Z graph look like for light isotopes ?For light isotopes (Z from 0 to no more than 20), the stable
nuclei follow the straight line N = Z. Such nuclei have equal
numbers of protons and neutrons.Explain why the N-Z graph looks the way it does as Z of the isotopes increases beyond 20 ?-As Z increases beyond about 20, stable nuclei have more neutrons than protons. The neutron/proton ratio increases. The extra neutrons help to bind the nucleons together without introducing repulsive electrostatic forces as more protons would do.On the N-Z graph , at what point does alpha emitters occur ? + explain why?α emitters occur bevond about (proton number) Z = 60, most of them with more than 80 protons and 120 neutrons. These nuclei have more neutrons than protons but they are too large to be stable. This is because the strong nuclear force between the nucleons is unable to overcome
the electrostatic force of repulsion between the protons.On the N-Z graph, when does β− emitter occur?+explain why ?emitters occur to the left of the stability belt where the isotopes are neutron-rich compared to stable isotopes .neutron-rich isotopes become stable or less unstable by 'converting' a neutron into a proton and emitting a ß- particle (and an electron antineutrino) at the same time.On a N-Z graph , when does a β+ emitter occur ? + explain why ?-β+ emitters occur to the right of the stability belt where the isotopes are proton-rich compared to stable isotopes. As explained previously, proton-rich isotopes become stable or less unstable by 'converting' a proton into a neutron and emitting a β+ particle (and an electron neutrino) at the same time. Electron capture also takes
place in this region.The change that takes place when an unstable nucleus becomes stable or less unstable can be represented on the N-Z graph as shown in general in figure 1 and in detail in figure 2
- Describe/ explain how ?>A nucleus that emits an a particle loses two protons and two neutrons so it moves diagonally downwards to the left across two grid squares.
>A nucleus that emits a β− particle loses a neutron and gains a proton so it moves diagonally downwards to the right across one grid square.
>A nucleus that emits a β+ particle (or captures an electron) loses a proton and gains a neutron so it moves diagonally upwards to the left across one grid square.Many radioactive isotopes decay to form another isotope which might itself be unstable.
- what happens when an unstable nucleus emits an alpha particle regarding its position on the N-Z graph ?-when an unstable nucleus emits an alpha particle, its position on the N-Z plot moves downwards parallel to the N=Z line to a new position with a greater neutron proton ration.Many radioactive isotopes decay to form another isotope ( the 'daughter' nucleus) which might itself be unstable.
-what happens if the 'daughter' nucleus is also unstable ?-If the 'daughter' nucleus is also unstable, it will decay to form a nucleus of a different isotope ( which may itself be stable or unstable) by either:
>a further alpha particle, or :
>a β− particle if its position is to the left of the stability belt, or :
>a β+ particle or by undergoing electron capture if , i both cases, its position is to the right of the stability belt.
> this an unstable nucleus, before it becomes stable, may undergo a series of isotopic changes in which each change involves an emission of an alpha or a beta particle.Naturally occurring radioactive isotopes decay through what ?Naturally occurring radioactive isotopes decreases thought a series of such changes with one or more of the changes having a very long half life-hence the reason why such isotopes have not decayed completely .-How can a β− emitter be manufactured ?
-How can β+ emitter be manufactured? + explain why ?-β− emitters can be manufactured by bombarding stable isotopes with neutrons.
-β+ emitters can only be produced by bombarding stable isotopes with protons.The protons need to have sufficient kinetic energy to overcome coulomb Remus ion from the nucleus-After an unstable e nucleus emits a alpha or a beta particles or undergoes electron capture, what might it emit ?
-what is the effects of this ?
-when does this happen ?-it might emit a gamma photon.
-emission of a gamma photon does not chase the number of protons or the number of neutrons in the nucleus but it does allow the nucleus to lose energy.
-this happen if the 'daughter' nucleus is formed in an excited state after it emits an alpha or beta particle or undergoes electron capture. The excited state is usually sort lived and the nucleus moves to its lowest energy state, its ground state, either directly or via one or more lower energy excited states.What is the technetium generator used for ?The technetium generator is used in hospitals to produce a source which emits gamma radiation only.Explain how technetium generators work?-some radioactive isotopes such as the technetium isotope 99,43}Tc form in an excited state after an alpha emission or a beta emission and stay in the excited sae long enough to be separated from the parent isotope.Such a long lived excited state is said to be a metastable state. Nuclei of the technetium isotope 99,43Tc form in a metastable state( indicated by the symbol 99,43Tc^m) after β− emission form nuclei of the molybdenum isotope 99,42Mo which has a half life of 67h. 99,43}Tc^m has a half life of 6h and decays to the grounds sate by gamma emission.
-Technetium 99,43}Tc in the ground sate is a β− emitter with a half life of 500000 years and it forms a stable product.Therefore, a sample of 99,43}Tc^m with no molybdenum present effectively emits only gamma photons.Samples of technetium 99,43Tc^m are used in medical diagnosis application, explain how ?-the technetium generator cos it's of an ion exchange column containing ammonium molybdenite exposed to neutron radio at ion several days earlier to make a significant number of the molybdenum nuclei unstable.When a solution of the sodium chloride is passes through the column , some of the chlorine ions exchange with pertechnate ions but not with only molybdenate ions so the solution that emerges contains 99,43}Tc^m nuclei.What are examples of uses of 99,43}Tc^m (technetium generator )>monitoring blood flow
>the gamma cameraWhat is a much more accurate method to measure the diameter of different nuclides ?Using high energy electronsExplain how we can much more accurately measure the diameter of the nucleus ?How is 99,43}Tc^m used in monitoring blood flow ?It is used in monitoring blood flow through the rain using external detectors after a small quantity of sodium pertechnate solution is administered intravenouslyHow is 99,43}Tc^m used in the gamma ray camera ?The camera is designed to 'image' internal organs and bones by detecting y radiation from sites in the body where a γ- emitting isotope such as 99,43}Tc^m nuclei is located. For example, bone deposits can be located using a phosphate tracer labelled with 99,43}Tc^m. The γ camera itself consists of detectors called
photomultiplier tubes in a lead shield behind a lead collimator grid which ensures each tube only detects γ photons emitted from nuclei located at a well defined spot directly in front of the tube.What is is a much more accurate method used to measure the diameter of different nuclides compared to just estimating it using the two other methods ?
-explain how ?-A much more accurate method used to measure the diameter of different nuclides is using high electron electrons.
-When a beam of high energy electrons is directed at a thin solid sample of an element, the incident electrons are diffracted by the nuclei of the atoms in the toil. The beam is produced by accelerating electrons through a potential difference of the order of a hundred million volts. The electrons are diffracted by the nuclei because the de Broglie wavelength of such high-energy electrons is of the order of 10^-15 m which is about the same as the diameter of the nucleus. A detector is used to measure the number of electrons per second diffracted through different angles.From the high electron diffraction experiment what did it show us ?The measurements show that as the angle θ of the detector to the 'zero order' beam is increased, the number of electrons per second
(i.e., the intensity of the beam) diffracted into the detector decreases
then increases slightly then decreases again.High energy electron diffraction experiment :
-what does the scattering of the beam electron by the nuclei occur due to ?
-what does this effect cause ?Scattering of the beam electrons by the nuclei occurs due to
their charge. This is the same as a-scattering by nuclei except the
electrons are attracted not repelled by the nuclei. This effect causes
the intensity to decrease as angle θ increases.High energy electron diffraction experiment:
-what does the diffraction of the beam electrons cause ? This happens provided what ?
-what are the superimposed intensity variations similar to ?Diffraction of the beam electrons by each nucleus causes intensity maxima and minima to be superimposed on the effect above. This happens provided the de Broglie wavelength of the electrons in the beam is no greater than the dimensions of the nucleus. These
superimposed intensity variations are, on a much smaller scale, similar to the concentric bright and dark fringes seen when a parallel beam of monochromatic light is directed at a circular gap or obstacle. The angle of the first minimum from the centre, θmin is measured and used to calculate the diameter of the nucleus, provided the wavelength of the incident electrons is known.High energy electron diffraction experiment :
-how do you calculate the wavelength of high energy electrons is calculated ?High energy electron diffraction experiment:
Remember: in practice, electrons need to be accelerated through pads greater than 100 million Volts to be diffracted significantly by the nucleusHigh energy electron diffraction experiment :
-what does the angle θmin depend on ?-The angle θmin depends on the radius R of the nucleus in accordance with the equation R sinθ(min) =0.61λ , λ is the de Broglie wavelength of the electrons.1Mev = how many J ?1.6 x 10^-13 JUsing samples of different element, the radius R of different nuclides can be measured.By plotting suitable graphs ( different x and y axis depending on what you want to show ) it can be shown that R depends on what ?Using samples of different element, the radius R of different nuclides can be measured.By plotting suitable graphs ( different x and y axis depending on what you want to show ) it can be shown that R depends on R=r(0)A^1/3
-what graph is used to confirm that the power of A =1/3Using samples of different element, the radius R of different nuclides can be measured.By plotting suitable graphs ( different x and y axis depending on what you want to show ) it can be shown that R depends on R=r(0)A^1/3
-what graph is used to give a accurate value for r(0)-what is the equation for the volume of a nucleus ?
-what assumption do we make for this ?
-what does this tell us ?-This means that the nuclear volume V is proportional to the mass of the nucleus. In other words, the density of the nucleus is constant,independent of its radius, and is the same throughout a nucleus. From this, we can conclude that nucleons are separated by the same distance regardless of the size of the nucleus and are therefore evenly separated inside the nucleus.-what can we use to calculate the density of the nucleus ?
-what is the density of the nucleus , use calculation/ show working out ?Remember:
-A cubic millimetre of nuclear matter would have a mass of about
340 million kilograms, about the same as the total body mass of
about 4 million adults.
-A neutron star is almost as dense as the nucleus of an atom for
Example, a neutron star of diameter 25 km and a mass of about 4 x 10^30 kg (about 2 solar masses ) has a density of 6 x10^16 kgm^-3
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