1 origin and nature of nuclear radiation

1 Origin and nature of nuclear radiation particles particles rays

Nature helium nuclei Fast-movingelectrons

EM wavessimilar to X-ray

Charge +2 1 no charge

Speed up to 10% ofspeed of light

up to 90% ofspeed of light

speed of light

Very weak (0.01% of )

Weak

(10% of )StrongIonizing

power

No deflectionLarge deflection

Very small deflection

Effect of Fields

01/18504Mass (nucleon unit)

Properties of , and radiations (2)

Properties of , and radiations (3)

Never fully absorbed : reduced to half by 25 mm of lead

Stopped by

5 mm of aluminium

Stopped by a sheet of paper

Penetrating power

~500 m~5 m~5 cmRange in air

Properties of , and radiations (4)

No transmutation

Radioactive transmutation

Photographic

filmCloud chamberGM tube

Photographic

filmCloud

chamberGM tube

Photographic

filmIonization

chamberCloud chamberSpark counterThin window

GM tube

Detectors

HeYX AZ

AZ

42

42

eYX AZ

AZ

011

5 Deflection in electric & magnetic fielda In electric field

(+)

(–)

electric field

radioactive source

+

mass of >> mass of deflection: <<

5 Deflection in electric & magnetic field

b In magnetic field

B-field (into paper)

radioactive source

F

B

I

Flow of particles particles = Flow of positive chargespositive charges = Direction of CurrentCurrent

Flow of particles particles = Flow of negative chargesnegative charges = OppositeOpposite direction of currentcurrent

b In magnetic field

radiation tracks in a very strong B-field (Photo credit: Lord Blackett’s Estate)

Radiation Detectors

Photographic Film To detect , and radiations

Spark counter To detect -particles

Ionization Chamber To detect -particles

Cloud Chamber To detect and particles

Geiger-Müller Tube To detect , and radiations

Photographic Film

The photographic film has been blackened by radioactivity except in the shadow of the key.

Spark Counter

The spark counter consists of positively charged wire mounted under an earthed metal grid.

It produces sparks in the presence of ionized particles.

It can only be used to detect α-radiation.

Earthed grid

To the positive terminalof the EHT supply

When radiation enters the metal can, the gas inside is ionized.

Under the influence o the electric field, electrons move towards the anode while the positive ions move towards the cathode.

As a result, a small ionization current is produced and is recorded by an electrometer.

2 kV d.c(+)

Radioactive source

electrometer

source

Metal cylinder

Brass rod

Note 1. When the applied voltage (V)

is increased, the ionization current is larger since more ions and electrons can reach the electrodes. Until a certain voltage, all ion-pairs produced reach the electrodes and a saturated ionization current is obtained.

2. The saturated ionization current is increased with the rate of producing ion-pairs. Therefore, ionization chamber is suitable for detecting -particles and -particles since their ionizing powers are relatively strong. But the ionizing power of -rays is very weak; it cannot be measured by the ionization chamber.

source

source

V

Ionizing current (I)

Cloud Chamber (1)

The diagrams below show a diffusion cloud chamber and its structure.

Cloud Chamber (2)

The felt ring round the top of the chamber is soaked with alcohol.

The cooled chamber is full of alcohol vapour. A weak radioactive source inside the chamber

emits radiation that produces ions along its path. The alcohol vapour which diffuses downwards

from the top condenses around the ions. The resulting tiny alcohol drops show up as a

track in the bright light

Cloud Chamber Tracks (1)

radiation

Having a strong ionizing power, the heavy particles give straight and thick tracks of about the same length.

radiationThey are twisted because the particles are small in mass and bounce off from air molecules on collision.

Tracks of rays can hardly be seen.

Cloud Chamber Tracks (3)

Under diffusion cloud chamber, Alpha source gives thick , straight tracks ; Beta source produces thin, twisted tracks. They

are small in mass and so bounce off from air molecules on collision.

Gamma source gives scattered, thin tracks. Gamma rays remove electrons from air molecules. These electrons behave like beta particles.

GM Counter

When ionizing radiation enters the GM tube, ions and free electrons are formed.

A flow of charge takes place and causes a pulse of current.

The pulse of current is amplified and counted electronically.

GM tube

A GM tube is filled with argon gas and a high voltage (~ 400 V) is applied to the central wire.

radiation

Aluminium tube

Central wire as anode (+)

counter

400 V d.c

Argon gas at low pressure

Mica end-window

GM tube

When radiation enters the tube, it pulls an electron from an argon atom and produces an ion-pair.

The resulting electrons rapidly accelerated towards the anode and cause more ions formed as they collide with argon gas atoms.

In this way, one electron can lead to the release of 108 electrons. An avalanche of electrons is produced.

When the electrons reach the anode, a pulse is created and can be counted by the GM counter.

radiation

Aluminium tube

Central wire as anode (+)

counter

400 V d.c

Argon gas at low pressure

Mica end-window

1 Three types of decayAlpha decay

XAZ

A 4Z 2

Y + He42

Gamma emissionenerg

yX*A

ZAZX +

Beta decay

XAZ

AZ + 1

Y + e01

pne-

2p2n

Example 1

No. of throws No. of dice remaining

0 100

1 89

2 71

3 54

4 46

5 37

6 28

7 24

8 20

9 18

10 16

The points plotted do not fall exactly on the curve. The fluctuations are due to the random nature of dice throwing.

Radioactive decay is also random in nature because, like the dice ‘decay’ the chance of certain nuclei decaying at a particular time is random.

0250500750

100012501500175020002250

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

no. of undecayed nuclei

Time / s

Always decreasing.Always decreasing.Decrease rapidly in Decrease rapidly in the beginning.the beginning.Decreases gently Decreases gently finally.finally.Becomes zero after a Becomes zero after a long time.long time.

Activity of a radioactive isotope (1)

Let N(t) be the number of radioactive nuclei in a sample at time t.

)()(

tkNdt

tdN

The decay rate is directly proportional to N(t).

dt

tdN )(Decay rate (Activity) =

The constant k is called the decay constant. A large value

of k corresponds to rapid decay.

The `-’ sign indicates that N(t) decreases with time

The SI unit of activity is the becquerel (Bq).

Activity of a radioactive isotope (2)

k can be interpreted as the probability per unit time that any individual nucleus will decay.

kNdt

dN

Ndt

dN

k

From ,

0250500750

100012501500175020002250

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

no. of undecayed nuclei

Time / s

0 – 4 s: 2000 0 – 4 s: 2000 1000 10004 – 8 s: 1000 4 – 8 s: 1000 500 5008 – 12 s: 500 8 – 12 s: 500 250 25012 – 16 s: 250 12 – 16 s: 250 125 125Half lifeHalf life = 4 s = 4 sIt takes 4 s for It takes 4 s for half to decayhalf to decay

Half life t1/2

Activity A

A0

½A0

⅛A0

¼A0

Half-life t½

Half-life (1)

The graph shows the number of remaining nuclei N(t) as a function of time.

Half-life (2)

The half-life t1/2 is the time required for the number of radioactive nuclei to decrease to one-half the original number No.

At t = t1/2, N(t) = No/2, obtaining2

12/1 kte

Taking logarithms to base e, gives

kkt

693.02ln2/1

Cloud Chamber Tracks (2)

Rate of decay

undecayed nucleus

decayed nucleus

)()(

tNdt

tdN

dN(t)dt

= -kN(t)

Radiation hazard

Ionizing effect can destroy or damage living cells.

Radioactive gas and dust cannot be removed once taken in.

Gamma rays are dangerous due to strong penetrating power

Background radiation

Cosmic rays 12%

Radioactive material in rocks and soil 15%

Radioactive gases 40%

Living bodies, and food and drinks 15%

Medical practice 17%

Nuclear discharge 1%

Hazards due to sealed and unsealed sources (1)

Hazards due to sealed sources α-particles usually do not present any external

radiation hazard because they are unable to penetrate to dead layer of skin. But, extremely precautions must be taken to prevent α-emitters from getting into the body.

β-particles never constitute a whole-body external radiation hazard due to their short range in tissue.

γ-rays have very high penetrating power and require greater care to avoid receiving excess dosage.

Hazards due to sealed and unsealed sources (2)

Hazards due to unsealed sources Unsealed sources usually constitute some kind

of internal hazard. This is the absorption and retention of radionuclides into specific organs of the body through intake of the materials present in air and in water.

The radionuclides may be rapidly absorbed by the organs causing damage to these organs.

Radioactive doses

The radiation emitted transfers energy to the organs and causes damage.

The level of damage depends on

1. energy absorbed by the body

2. type of radiation

3. the parts of human body

Effective dose

= Absorbed dose x Radiation weighting factor x tissue weighting factor

Handling precautions

The weak sources used at school should always by lifted with forceps.

The sources should never by held near the eyes. The source should be kept in their boxes (lead

container) when not in use. Take great care not to drop the sources when

handling them. Carefully plan the experiments to minimize the

time the source is used.

Uses of radioisotopes

Medical uses Treatment of body cancer Investigation of Thyroid Gland ( 甲狀腺 )

Radon-222 ( emitted) Iodine-131 ( emitted)

Industrial uses

Application Type of radiation

Half life

Thickness gauge / / Long / short

Checking oil leakage

/ / Long / short

Smoke detector / / Long / short

Archaeological Use (Carbon-14 dating)

Carbon-14 exists due to formation by bombardment of nitrogen-14 in atmosphere by neutrons ejected from nuclei by cosmic rays ( ) and this forms radioactive carbon dioxide.

Living plants or trees absorb and give out carbon dioxide, so the percentage of C-14 in their tissue remains unchanged.

After death, no fresh CO2 taken in.

C-14 starts to decay with a half-life of 5.7 103 years.

By measuring the activity of C-14 ( ), the age of carbon containing material (e.g. wood, linen, charcoal) can be estimated.

714

01

614

11N n C H

eNC 01

147

146

End of this chapter

Alpha-Scattering Experiment (1)

A beam of -particles was directed at a thin sheet of gold-foil and the scattered -particles were detected using a small zinc sulphide screen viewed through a microscope in a vacuum chamber. Side

view

To vacuum pump

Evacuated metal box

-source

Gold foilZinc sulphidescreen

microscope

Alpha-scattering Experiment (2)

From the experiment it was found that

a few were deflected at very large angles,some were nearly reflected back in the direction from which they had come.

most of the -particles passed through the foil unaffected,

Rutherford’s atomic model

Rutherford’s assumptions: All the atom’s positive charge is concentrated

in a relatively small volume, called the nucleus of the atom

The electrons surround the nucleus at relatively large distance. Most of the atom’s mass is concentrated in its nucleus.

10-15 m

10-10 m

Difficulties of Rutherford’s model

The Rutherford model was unable to explain why atoms emit line spectra. The main difficulties are: It predicts that light of a

continuous range of frequencies will be emitted;

It predicts atoms are unstable—electrons should quickly spiral into the nucleus.

Mass and Energy

The mass-energy relationship Einstein showed that mass and energy are

equivalent. E = mc2

Mass defect The difference between the mass of an atom and

the mass of its particles taken separately is called the mass defect (Δm).

Δm = Zmp +Nmn- Mnucleus

The mass defect is small compared with the total mass of the atom.

Unified Atomic Mass Unit

The unified atomic mass unit (u) is defined as one twelfth of the mass of the carbon atom which contains six protons, six neutrons and six electrons.

1 u = 1.660566 × 10-27 kg Energy equivalence of mass

1 u = 931.5 MeV It is a useful quantity to calculate the energy

change in nuclear transformations.

Binding Energy (1)

The energy required to just take all the nucleons apart so that they are completely separated is called the binding energy of the nucleus.

Binding Energy (2)

From Einstein’s mass-energy relation, the total mass of all separated nucleons is greater than that of the nucleus, in which they are together. The difference in mass is a measure of the binding energy.

According to relativity theory, total binding energy = Δmc2

where Δm is the mass defect of the nucleus.

Binding Energy (3)

Binding energy of Helium

m = 4.0330 u - 4.0026 u = 0.0304 u

E = 28.3 MeV

Binding energy per nucleon = 7.08 MeV per nucleon

Binding Energy (4)

The values of the binding energy varies from one nuclear structure to another.

The greater the binding energy per nucleon, the more stable the nuclei.

Binding Energy Curve (1)

The graph shows the variation of the binding energy per nucleon among the elements.

Fission

Fusion

Binding energy Curve (2)

The important features of the binding energy curve: Maximum binding energy per nucleon is at about

nucleon number A = 50. Maximum binding energy per nucleon corresponds to the most stable nuclei.

Either side of maximum binding energy per nucleon are less stable.

Binding Energy Curve (3)

When light nuclei are joined together, the binding energy per nucleon is also increased. So energy is released when light nuclei are fused together.

When a big nucleus disintegrates, the binding energy per nucleon increases and energy is released. So fission or radioactive decay both lead to an increase of binding energy per nucleon and hence to release energy as KE of the product.

Principles of Nuclear Fission (1)

Nuclear fission is a decay process in which an unstable nucleus splits into two fragments of comparable mass.

nKrBanU 10

8936

14456

10

23592 3 + energy released

nSrXenU 10

9438

14054

10

23592 2 + energy released

Two typical nuclear fission reactions are:

Principles of Nuclear Fission (2)

Further investigations showed that several neutrons are released with the fission

fragments, many fission products are possible when U-235 is

bombarded with neutrons, the products themselves are radioactive, slow neutrons are more effective in fissioning U-235

than fast neutrons, energy is released on much greater scale than is

released from chemical reaction.

Chain Reactions

Fission of uranium nucleus, triggered by neutron bombardment, released other neutrons that can trigger more fission. Chain reaction is said to occur.

http://www.smartown.com/sp2000/energy_planet/en/trad/fission.html#

Nuclear Power Plant

A power plant with cooling tower

Nuclear Reactor (1)

The schematic diagram of a nuclear reactor is shown below:

http://www.ae4rv.com/games/nuke.htm

Nuclear Reactor (2)

Enriched uranium is used as the fuel. The fuel is in the form of rods

enclosed in metal containers. A moderator is used to slow down

fission neutrons. Control rods are used to absorb

neutrons to maintain a steady rate of fissioning.

A coolant is pumped through the channels in the moderator to remove heat energy to a heat exchanger.

Processes inside the Nuclear Reactor

Each fission of U-235 nucleus produces fission fragments including neutrons. The fission fragments carry away most of the KE and transfer the KE to other atoms that they collide with. So the fuel pin get very hot.

The fission neutrons enter the moderator and collide with moderator atoms, transferring KE to these atoms. So the neutrons slow down until the average KE of a neutron is about the same as that of a moderator atom.

Slow neutron re-enter the fuel pins and cause further fission of U-235 nuclei.

Important features in the design of a nuclear reactor (1)

The critical mass of fuel required The critical mass of fuel is the minimum

mass capable of producing a self-sustaining chain reaction.

The fission neutrons could be absorbed by the U-238 nuclei without producing further fission.

The fission neutron could escape from the isolated block of uranium block without causing further fission.

Important features in the design of a nuclear reactor (2)

The choice of the moderator The atoms of an ideal moderator should have the

same mass as a neutron. So a neutron colliding elastically with a moderator atom would lose almost all its KE to the moderator atom.

In practice, graphite or heavy water (D2O) is chosen as the moderator.

The moderator atoms should not absorb neutrons but should scatter them instead.

Important features in the design of a nuclear reactor (3)

The choice of control rods The control rods absorb rather than scatter

neutrons. Boron and cadmium are very suitable

elements for control rods. Control rods are operated automatically.

Important features in the design of a nuclear reactor (4)

Coolants should ideally have the following properties: The coolant must have high heat transfer coefficient. The coolant must flow easily. The coolant must not be corrosive. Coolant atoms may become radioactive when they pass

through the core of the reactor. So the coolant must have low induced radioactivity.

The coolant must be in a sealed circuit.

Important features in the design of a nuclear reactor (5)

The treatment of waste The fuel rods are stored in containers in cooling

ponds until their activity has decreased and they are cooler.

The spent fuel is removed from the cans by remote control. The fuel is then reprocessed to recover unused fuel.

the unwanted material is then stored in sealed containers for many years until the activity has fallen to an insignificant.

Nuclear Fusion

Fusion is combining the nuclei of light elements to form a heavier element. This is a nuclear reaction and results in the release of large amounts of energy!

Energy is released due to the increase in binding energy of the product of the reaction.

In a fusion reaction, the total mass of the resultant nuclei is slightly less than the total mass of the original particles.

Example of Nuclear Fusion

An example of nuclear fusion can be seen in the Deuterium-Tritium Fusion Reaction.

MeVnHeHH 6.1710

42

31

21

Conditions for a Fusion Reaction (1)

Temperature Fusion reactions occur at a sufficient rate only at very high

temperature. Over 108 oC is needed for the Deuterium-Tritium reaction.

Density The density of fuel ions must be sufficiently large for fusion

reactions to take place at the required rate. The fusion power generated is reduced if the fuel is diluted by impurity atoms or by the accumulation of Helium ‘ash’ from the fusion reaction.

As fuel ions are burnt in the fusion process they must be replaced by new fuel and the Helium ash must be removed.

Conditions for a Fusion Reaction (2)

ConfinementThe hot plasma must be well isolated away from material surfaces in order to avoid cooling the plasma and releasing impurities that would contaminate and further cool the plasma.

In the Tokamak system, the plasma is isolated by magnetic fields.

Advantages of Nuclear Fusion

Abundant fuel supply No risk of a nuclear accident No air pollution No high-level nuclear waste No generation of weapons material

Nuclear Waste

Some waste is stored on asphalt pads in drums.

Storage Tanks for Nuclear Waste

These storage tanks were constructed to store liquid, high-level waste. After construction was completed, the earth was replaced to bury the tanks underground.

Nuclear Stability (1)

The Segrè chart below shows neutron number and proton number for stable nuclides.

For low mass numbers, NZ.The ratio N/Z increases with A.Points to the right of the stability region represents nuclides that have too many protons relative to neutrons.To the left of the stability region are nuclides with too many neutrons relative to protons.

Nuclear Stability (2)

Nuclear Stability (3)

Deflection of α, β and γ rays in electric and magnetic fields (1)

α

-

+

β

γ

Deflection in electric field Deflection in magnetic field

Deflection of α, β and γ rays in electric and magnetic fields (2)

Under the effect of electric field or magnetic field, (in the direction of going into the paper); α-ray shows small deflection in an upward direction; β-ray shows a larger deflection than that of alpha

ray, and in a downward direction; γ-ray shows no deflection.

Penetrating Power

The diagram below shows the apparatus used to deduce the penetrating abilities of α, β and γ radiations.

Moderator

Use materials that slow the neutrons down to such low energies at which the probability of causing a fission is significantly higher. These neutron slowing down materials are the so called moderators.

http://www.npp.hu/mukodes/lancreakcio-e.htm

umM

Mmv

umM

mV

2

Control rods

reactor core at the bottom of a 5 m deep tank of very pure water

Reactor core glowing at full licensed power

Heating of Plasma (1)

Ohmic Heating and Current Drive Currents up to 7 million amperes (7MA) flow in the

plasma and deposit a few mega-watts of heating power.

Neutral Beam Heating Beams of deuterium or tritium ions, accelerated by a

potential of 140,000 volts, are injected into the plasma.

Radio-Frequency Heating The plasma ions and electrons rotate around in the

magnetic field lines of the tokamak. Energy is given to the plasma at the precise location where the radio waves resonate with the ion rotation.

Heating of Plasma (2)

Current Driven by Microwaves 10 MW of microwaves at 3.7 GHz accelerate

the plasma electrons to generate a plasma current of up to 3MA.

Self Heating of Plasma The helium nuclei (alpha-particles)

produced when deuterium and tritium fuse remain within the plasma's magnetic trap. Their energy continues to heat the plasma to keep the fusion reaction going.

JET Tokamak

During operation large forces are produced due to interactions between the currents and magnetic fields. These forces are constrained by the mechanical structure which encloses the central components of the machine.