The Same Radiation That Treats Cancer Can Also Cause It
Sort the common radioisotopes into alpha, beta and gamma emitters and give a use for each, learn why strontium behaves like calcium and lodges in bone, account for the radiation always around us, and separate nuclear fission from fusion with balanced equations.
How can something used to cure cancer also be the thing that causes it?
A beam of gamma radiation from cobalt is aimed at a tumour to destroy it. The same radiation, falling on healthy tissue, damages that tissue in exactly the same way. There is no difference in the radiation — only in where it is pointed and how much of it arrives.
That single fact runs through the whole of this part of the chapter. Radiation damages living cells, and every use of a radioisotope is a deliberate trade between the damage done and the good achieved.
- In treatment, the beam is aimed and its dose is calculated so that the tumour receives far more than the surrounding tissue
- In diagnosis, a very small quantity of a short-lived isotope is used, so that the information gained is worth the tiny dose
- In industry, the source is sealed and shielded so that no one receives any dose at all
And that is why the choice of isotope is never accidental. An alpha emitter, a beta emitter and a gamma emitter behave so differently — from the previous part, in penetration and in ionising power — that each has jobs the others cannot do.
- An alpha emitter cannot get out of a small sealed chamber, so it is used where the radiation must stay put
- A beta emitter passes through thin sheets but not thick ones, so it is used to measure thickness
- A gamma emitter passes right through a body or a steel casting, so it is used to see inside things
The chapter then widens twice. First to the radiation that is always present around us, from the rocks, the sky, our food and our own bodies — the background against which any measurement must be made. Then to the two nuclear processes that release energy on an industrial scale, fission and fusion, which are opposites in almost every respect and which between them explain both a power station and the sun.
And it ends with the awkward question that fission raises and fusion does not: what to do with the waste.
This page covers the second part of the ICSE Class 10 Physics chapter on modern physics: the uses of radioisotopes, the hazards of radiation and the precautions against them, background radiation, and nuclear fission and fusion.
That single fact runs through the whole of this part of the chapter. Radiation damages living cells, and every use of a radioisotope is a deliberate trade between the damage done and the good achieved.
- In treatment, the beam is aimed and its dose is calculated so that the tumour receives far more than the surrounding tissue
- In diagnosis, a very small quantity of a short-lived isotope is used, so that the information gained is worth the tiny dose
- In industry, the source is sealed and shielded so that no one receives any dose at all
And that is why the choice of isotope is never accidental. An alpha emitter, a beta emitter and a gamma emitter behave so differently — from the previous part, in penetration and in ionising power — that each has jobs the others cannot do.
- An alpha emitter cannot get out of a small sealed chamber, so it is used where the radiation must stay put
- A beta emitter passes through thin sheets but not thick ones, so it is used to measure thickness
- A gamma emitter passes right through a body or a steel casting, so it is used to see inside things
The chapter then widens twice. First to the radiation that is always present around us, from the rocks, the sky, our food and our own bodies — the background against which any measurement must be made. Then to the two nuclear processes that release energy on an industrial scale, fission and fusion, which are opposites in almost every respect and which between them explain both a power station and the sun.
And it ends with the awkward question that fission raises and fusion does not: what to do with the waste.
This page covers the second part of the ICSE Class 10 Physics chapter on modern physics: the uses of radioisotopes, the hazards of radiation and the precautions against them, background radiation, and nuclear fission and fusion.
Which radioisotopes are used for what, and why does the choice matter?
The kind of radiation an isotope emits decides what it can be used for, because the three kinds differ so sharply in how far they travel.
Alpha emitters, whose radiation is stopped within a few centimetres of air.
- Americium-241 — used in smoke detectors. Its alpha particles ionise the air inside a small chamber so that a current flows; smoke entering the chamber absorbs them, the current falls and the alarm sounds
- Radium-223 — used in the treatment of bone cancer. It behaves chemically like calcium, so it collects in bone, where its short-range alpha radiation destroys the cancerous cells nearby without reaching far into healthy tissue
- Polonium-210 — used to remove unwanted static electric charge from machinery, from paper in printing presses and from photographic film. Its alpha particles ionise the surrounding air, which then conducts the charge away
Notice how the short range is the point in every case. The smoke detector needs radiation that cannot escape the device; bone-cancer treatment needs radiation that acts only where it is deposited; static removal needs the air right next to the surface to be ionised. An alpha emitter is chosen precisely because its radiation goes nowhere.
Beta emitters, whose radiation passes through thin material but not thick.
- Strontium-90 — used to measure and control the thickness of paper, plastic and metal sheets during manufacture. A beam is passed through the moving sheet and the amount getting through is measured; a thicker sheet lets less through, so the reading controls the rollers automatically
- Carbon-14 — used to estimate the age of ancient wood, bones, charcoal and fossils. Living things take up carbon from their surroundings, and the proportion of the radioactive isotope in their remains falls steadily after death, so measuring it gives the age
Again the choice follows the physics. Thickness gauging needs radiation that is partly absorbed by the sheet — alpha would be stopped completely and gamma would pass through almost unaffected, so neither would give a useful reading. Only beta radiation is sensitive to a thin sheet.
Gamma emitters, whose radiation passes through thick material.
- Cobalt-60 — used in the treatment of cancer, in sterilising surgical instruments and dressings inside their sealed packets, and in preserving food by killing the organisms that cause spoiling. It is also used to detect flaws and cracks inside metal castings and welded joints
- Technetium-99m — used for imaging internal organs. It is injected in a small quantity, collects in the organ being studied, and its gamma radiation is detected from outside the body to build an image
- Sodium-22 — used as a tracer in medicine, for example to study the circulation of blood, since its radiation can be followed from outside the body
- Iodine-131 — used in the diagnosis and treatment of disorders of the thyroid gland. Iodine collects naturally in the thyroid, so the isotope goes there of its own accord and its radiation acts on that gland alone
The two that work by chemistry rather than by physics are worth noting together. Radium-223 collects in bone because it behaves like calcium; iodine-131 collects in the thyroid because the body treats it as ordinary iodine. In both cases the body delivers the isotope to the right place, and the physics only supplies the radiation once it is there.
A tracer, defined. A tracer is a small quantity of a radioisotope introduced into a system so that its progress can be followed from outside by detecting its radiation. Tracers are used in medicine to study organs and circulation, in agriculture to study the uptake of fertilisers by plants, and in engineering to detect leaks in underground pipes.
And one requirement that every medical isotope must meet. It must have a short active life, so that it decays away quickly once the examination is over and does not go on irradiating the patient. Technetium-99m is used so widely for exactly that reason — a long-lived isotope would give the same picture and a far larger dose.
Alpha emitters, whose radiation is stopped within a few centimetres of air.
- Americium-241 — used in smoke detectors. Its alpha particles ionise the air inside a small chamber so that a current flows; smoke entering the chamber absorbs them, the current falls and the alarm sounds
- Radium-223 — used in the treatment of bone cancer. It behaves chemically like calcium, so it collects in bone, where its short-range alpha radiation destroys the cancerous cells nearby without reaching far into healthy tissue
- Polonium-210 — used to remove unwanted static electric charge from machinery, from paper in printing presses and from photographic film. Its alpha particles ionise the surrounding air, which then conducts the charge away
Notice how the short range is the point in every case. The smoke detector needs radiation that cannot escape the device; bone-cancer treatment needs radiation that acts only where it is deposited; static removal needs the air right next to the surface to be ionised. An alpha emitter is chosen precisely because its radiation goes nowhere.
Beta emitters, whose radiation passes through thin material but not thick.
- Strontium-90 — used to measure and control the thickness of paper, plastic and metal sheets during manufacture. A beam is passed through the moving sheet and the amount getting through is measured; a thicker sheet lets less through, so the reading controls the rollers automatically
- Carbon-14 — used to estimate the age of ancient wood, bones, charcoal and fossils. Living things take up carbon from their surroundings, and the proportion of the radioactive isotope in their remains falls steadily after death, so measuring it gives the age
Again the choice follows the physics. Thickness gauging needs radiation that is partly absorbed by the sheet — alpha would be stopped completely and gamma would pass through almost unaffected, so neither would give a useful reading. Only beta radiation is sensitive to a thin sheet.
Gamma emitters, whose radiation passes through thick material.
- Cobalt-60 — used in the treatment of cancer, in sterilising surgical instruments and dressings inside their sealed packets, and in preserving food by killing the organisms that cause spoiling. It is also used to detect flaws and cracks inside metal castings and welded joints
- Technetium-99m — used for imaging internal organs. It is injected in a small quantity, collects in the organ being studied, and its gamma radiation is detected from outside the body to build an image
- Sodium-22 — used as a tracer in medicine, for example to study the circulation of blood, since its radiation can be followed from outside the body
- Iodine-131 — used in the diagnosis and treatment of disorders of the thyroid gland. Iodine collects naturally in the thyroid, so the isotope goes there of its own accord and its radiation acts on that gland alone
The two that work by chemistry rather than by physics are worth noting together. Radium-223 collects in bone because it behaves like calcium; iodine-131 collects in the thyroid because the body treats it as ordinary iodine. In both cases the body delivers the isotope to the right place, and the physics only supplies the radiation once it is there.
A tracer, defined. A tracer is a small quantity of a radioisotope introduced into a system so that its progress can be followed from outside by detecting its radiation. Tracers are used in medicine to study organs and circulation, in agriculture to study the uptake of fertilisers by plants, and in engineering to detect leaks in underground pipes.
And one requirement that every medical isotope must meet. It must have a short active life, so that it decays away quickly once the examination is over and does not go on irradiating the patient. Technetium-99m is used so widely for exactly that reason — a long-lived isotope would give the same picture and a far larger dose.
What harm does radiation do, and what precautions are taken against it?
Radiation ionises the atoms of living cells, damaging or killing them, and the damage can appear both in the person exposed and in their descendants.
The mechanism. Alpha, beta and gamma radiations all ionise the atoms they pass, tearing electrons away from them. In a living cell that breaks the molecules the cell is made of, including the ones that carry its instructions for dividing. A damaged cell may die, may fail to work properly, or may begin to divide uncontrollably.
The harmful effects, which is a standard list question:
- Burns and blisters on the skin, from a large external dose
- Loss of hair
- Sterility, from damage to the reproductive organs
- Blood disorders including leukaemia, from damage to the bone marrow where blood cells are made
- Cancer of various kinds, from cells that begin dividing uncontrollably
- Genetic mutations, which are changes in the instructions carried in the reproductive cells and can therefore be passed on to later generations
- Damage to the eyes, including cataract
Strontium-90 deserves separate attention, because the reason it is dangerous is chemical rather than nuclear.
- Strontium lies just below calcium in the periodic table, so it behaves chemically very like calcium
- The body therefore treats it as calcium and deposits it in the bones, where calcium belongs
- Once there it cannot easily be removed, and it goes on irradiating the bone marrow and the surrounding tissue from inside
- It causes bone cancer and cancer of the eye
That is a general warning and not a fact about one isotope. An isotope that the body mistakes for a nutrient is far more dangerous than one it does not, because it is carried to exactly the tissue that radiation damages most and it stays there. Iodine-131 concentrating in the thyroid is the same effect used deliberately, which is why the same behaviour can be a treatment or a hazard depending on the quantity.
The safety precautions, each with its reason:
- Never handle a radioactive substance with bare hands. Use long tongs or remote handling equipment, so that the distance reduces the dose received
- Store sources in thick lead containers, because lead absorbs gamma radiation strongly
- Work behind a lead or thick concrete shield, and use lead glass to see through
- Wear lead-lined aprons and gloves when working with radiation
- Wear a film badge or a dosimeter, which records the total dose received so that it can be checked against the safe limit
- Never eat, drink or smoke in a radiation area, so that no source can be taken into the body — where even an alpha emitter becomes extremely dangerous
- Keep the exposure time as short as possible and the distance as large as possible
- Mark every radiation area with the standard warning sign, and keep unauthorised people out
- Dispose of radioactive waste only by the approved method, never with ordinary rubbish
The three principles behind all of those, which is the compact way to remember them. Time, distance and shielding. Reduce the time of exposure, increase the distance from the source, and put absorbing material in between — and every precaution on the list is one of those three applied to a particular situation.
One boundary case that explains why eating in the area is forbidden. From the previous part, an alpha source outside the body is nearly harmless because its radiation cannot penetrate skin. Swallowed or inhaled, the same source is the most dangerous of the three, because it deposits all its energy directly in living tissue with nothing in the way. So the rule about eating and drinking is not about tidiness; it is the most important rule on the list.
The mechanism. Alpha, beta and gamma radiations all ionise the atoms they pass, tearing electrons away from them. In a living cell that breaks the molecules the cell is made of, including the ones that carry its instructions for dividing. A damaged cell may die, may fail to work properly, or may begin to divide uncontrollably.
The harmful effects, which is a standard list question:
- Burns and blisters on the skin, from a large external dose
- Loss of hair
- Sterility, from damage to the reproductive organs
- Blood disorders including leukaemia, from damage to the bone marrow where blood cells are made
- Cancer of various kinds, from cells that begin dividing uncontrollably
- Genetic mutations, which are changes in the instructions carried in the reproductive cells and can therefore be passed on to later generations
- Damage to the eyes, including cataract
Strontium-90 deserves separate attention, because the reason it is dangerous is chemical rather than nuclear.
- Strontium lies just below calcium in the periodic table, so it behaves chemically very like calcium
- The body therefore treats it as calcium and deposits it in the bones, where calcium belongs
- Once there it cannot easily be removed, and it goes on irradiating the bone marrow and the surrounding tissue from inside
- It causes bone cancer and cancer of the eye
That is a general warning and not a fact about one isotope. An isotope that the body mistakes for a nutrient is far more dangerous than one it does not, because it is carried to exactly the tissue that radiation damages most and it stays there. Iodine-131 concentrating in the thyroid is the same effect used deliberately, which is why the same behaviour can be a treatment or a hazard depending on the quantity.
The safety precautions, each with its reason:
- Never handle a radioactive substance with bare hands. Use long tongs or remote handling equipment, so that the distance reduces the dose received
- Store sources in thick lead containers, because lead absorbs gamma radiation strongly
- Work behind a lead or thick concrete shield, and use lead glass to see through
- Wear lead-lined aprons and gloves when working with radiation
- Wear a film badge or a dosimeter, which records the total dose received so that it can be checked against the safe limit
- Never eat, drink or smoke in a radiation area, so that no source can be taken into the body — where even an alpha emitter becomes extremely dangerous
- Keep the exposure time as short as possible and the distance as large as possible
- Mark every radiation area with the standard warning sign, and keep unauthorised people out
- Dispose of radioactive waste only by the approved method, never with ordinary rubbish
The three principles behind all of those, which is the compact way to remember them. Time, distance and shielding. Reduce the time of exposure, increase the distance from the source, and put absorbing material in between — and every precaution on the list is one of those three applied to a particular situation.
One boundary case that explains why eating in the area is forbidden. From the previous part, an alpha source outside the body is nearly harmless because its radiation cannot penetrate skin. Swallowed or inhaled, the same source is the most dangerous of the three, because it deposits all its energy directly in living tissue with nothing in the way. So the rule about eating and drinking is not about tidiness; it is the most important rule on the list.
What is background radiation, and where does it come from?
Background radiation is the low-level radiation always present in our surroundings, from natural sources and from a few of our own making.
Why it matters practically. Every radiation detector registers a reading even with no source anywhere near it. That reading is the background, and it must be measured and subtracted before any experimental result means anything. A measurement of a weak source is otherwise a measurement of the room.
The natural sources:
- Cosmic rays — highly energetic radiation reaching the earth from space, mostly from the sun and from beyond the solar system. Much of it is absorbed by the atmosphere, so the intensity is greater at high altitude and on an aeroplane than at sea level
- Rocks and soil — many contain small quantities of naturally radioactive uranium, thorium and potassium, and their decay is a steady source
- Radon gas — a radioactive gas produced by the decay of the uranium in rocks and soil. It seeps out of the ground and can collect inside buildings, especially in poorly ventilated basements, and it is inhaled
- Building materials — granite, some concretes and some bricks are made of rock and carry its natural radioactivity with them
- Food, water and our own bodies — all living things contain small amounts of radioactive potassium and radioactive carbon, taken up from their surroundings. Every person is very slightly radioactive, and always has been
The sources produced by human activity:
- Medical X-rays and radiation treatment, which are the largest artificial contribution for most people
- Radioactive fallout — radioactive material released into the atmosphere by accidents at nuclear installations and by the testing of nuclear weapons, which then settles on the ground and enters the food chain
- Nuclear power stations and the transport of nuclear fuel and waste, which release very small quantities under normal operation
- Industrial and research uses of radioisotopes
The distinction to keep clear. Most of the background is natural and has always been present; the artificial part is an addition to it. A question asking for sources of background radiation expects both kinds to be named, and expects cosmic rays and rocks to appear before anything artificial.
Why fallout is treated separately from the rest. Fallout consists of radioactive material spread into the environment, where it can be taken up by plants, eaten by animals and concentrated as it passes along a food chain.
- Strontium-90 in fallout is absorbed by grass, eaten by cattle and passed into milk, from which it reaches human bone by the calcium route described in the previous section
- Radioactive iodine in fallout concentrates in the thyroid by the same kind of route
- So fallout is dangerous out of proportion to its quantity, because biological processes gather it up and deliver it to the tissues that are most vulnerable
And that is the same concentrating effect met in the environment chapter. A substance the body cannot distinguish from a nutrient, and cannot easily excrete, accumulates — which is why a small amount widely scattered can end up as a large amount in one organ.
One reassuring qualification and one caution. The natural background is small, and life has developed in its presence throughout its history. But the effects of radiation are cumulative, so an additional dose adds to what a person has already received, and there is no level at which an extra dose does no harm at all. That is why medical X-rays are taken only when they are needed, and why a radiographer stands behind a screen while taking one, even though the patient does not.
Why it matters practically. Every radiation detector registers a reading even with no source anywhere near it. That reading is the background, and it must be measured and subtracted before any experimental result means anything. A measurement of a weak source is otherwise a measurement of the room.
The natural sources:
- Cosmic rays — highly energetic radiation reaching the earth from space, mostly from the sun and from beyond the solar system. Much of it is absorbed by the atmosphere, so the intensity is greater at high altitude and on an aeroplane than at sea level
- Rocks and soil — many contain small quantities of naturally radioactive uranium, thorium and potassium, and their decay is a steady source
- Radon gas — a radioactive gas produced by the decay of the uranium in rocks and soil. It seeps out of the ground and can collect inside buildings, especially in poorly ventilated basements, and it is inhaled
- Building materials — granite, some concretes and some bricks are made of rock and carry its natural radioactivity with them
- Food, water and our own bodies — all living things contain small amounts of radioactive potassium and radioactive carbon, taken up from their surroundings. Every person is very slightly radioactive, and always has been
The sources produced by human activity:
- Medical X-rays and radiation treatment, which are the largest artificial contribution for most people
- Radioactive fallout — radioactive material released into the atmosphere by accidents at nuclear installations and by the testing of nuclear weapons, which then settles on the ground and enters the food chain
- Nuclear power stations and the transport of nuclear fuel and waste, which release very small quantities under normal operation
- Industrial and research uses of radioisotopes
The distinction to keep clear. Most of the background is natural and has always been present; the artificial part is an addition to it. A question asking for sources of background radiation expects both kinds to be named, and expects cosmic rays and rocks to appear before anything artificial.
Why fallout is treated separately from the rest. Fallout consists of radioactive material spread into the environment, where it can be taken up by plants, eaten by animals and concentrated as it passes along a food chain.
- Strontium-90 in fallout is absorbed by grass, eaten by cattle and passed into milk, from which it reaches human bone by the calcium route described in the previous section
- Radioactive iodine in fallout concentrates in the thyroid by the same kind of route
- So fallout is dangerous out of proportion to its quantity, because biological processes gather it up and deliver it to the tissues that are most vulnerable
And that is the same concentrating effect met in the environment chapter. A substance the body cannot distinguish from a nutrient, and cannot easily excrete, accumulates — which is why a small amount widely scattered can end up as a large amount in one organ.
One reassuring qualification and one caution. The natural background is small, and life has developed in its presence throughout its history. But the effects of radiation are cumulative, so an additional dose adds to what a person has already received, and there is no level at which an extra dose does no harm at all. That is why medical X-rays are taken only when they are needed, and why a radiographer stands behind a screen while taking one, even though the patient does not.
How do nuclear fission and nuclear fusion differ, and what happens to the waste?
Fission splits a heavy nucleus; fusion joins light ones. Both release enormous energy, and almost everything else about them is opposite.
Nuclear fission. Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei of comparable mass, with the release of a large amount of energy and two or three neutrons, when the heavy nucleus is struck by a slow neutron.
The standard example, with the numbers balanced:
Check both conservation rules. Mass numbers: on the left, and on the right. Atomic numbers: , and . Both balance.
The chain reaction. The two or three neutrons released can each strike another heavy nucleus and cause it to fission, releasing more neutrons still.
- In a nuclear reactor the chain reaction is controlled, so that on average exactly one neutron from each fission goes on to cause another. Control rods of cadmium or boron absorb the surplus neutrons, and a moderator of graphite or heavy water slows the fast neutrons down so that they can be captured
- In a nuclear weapon the chain reaction is uncontrolled, so the number of fissions multiplies rapidly and all the energy is released at once
Nuclear fusion. Nuclear fusion is the combining of two light nuclei to form a heavier nucleus, with the release of a large amount of energy.
Two standard examples:
Check: mass numbers and ; atomic numbers and . Both balance.
Check: mass numbers ; charges . Both balance. This second reaction is the kind that supplies the energy of the sun and the other stars.
Why fusion needs such extreme conditions. Two nuclei are both positively charged, so they repel each other strongly, and that repulsion grows as they approach. To get close enough for the short-range attractive force to take over, they must be moving extremely fast — which means a temperature of millions of degrees, together with an enormous pressure to keep them close enough for long enough.
The differences, point by point, which is the most examined part of this section:
- Starting material: fission needs a heavy nucleus; fusion needs light nuclei
- Conditions: fission proceeds at ordinary temperature once triggered by a neutron; fusion needs extremely high temperature and pressure
- Chain reaction: fission is a chain reaction, since it produces the neutrons that continue it; fusion is not
- Products: fission produces highly radioactive fragments; fusion produces little or no radioactive waste
- Energy per unit mass: greater in fusion than in fission
- Control: fission has been controlled and is used to generate electricity; fusion has not yet been controlled for power generation
So fusion is in every respect the more attractive source and the harder one to achieve, and that combination is why it remains a subject of research while fission already runs power stations.
The disposal of nuclear waste, which is the price fission carries. The fragments left after fission are intensely radioactive, and some of them remain active for a very long time.
Waste is classified by how active it is:
- Low-level waste — contaminated clothing, tools, paper and packaging. It may be compacted, sealed in drums and buried in shallow, monitored landfill sites
- Intermediate-level waste — reactor components and chemical residues. It is mixed with concrete and sealed in steel drums before being stored
- High-level waste — the spent fuel itself, which is both intensely radioactive and hot
How high-level waste is handled, in order:
- It is first stored under water in cooling ponds at the site for a long period, until its activity and its heat output have fallen substantially. The water both cools it and absorbs the radiation
- It is then vitrified — mixed into molten glass, which sets into a solid block that will not dissolve in water and from which the radioactive material cannot leach out
- The blocks are sealed inside containers of steel and concrete
- The containers are buried deep underground in stable rock formations, chosen to be far from any source of ground water and away from regions of earthquakes
And the rules about what must not be done: nuclear waste must never be dumped into the sea, discharged into rivers, or put into ordinary landfill, because the radioactive material would enter the water supply and the food chain.
One honest observation about the difficulty. The waste must be kept isolated for far longer than any structure yet built has lasted, so the problem is not one of engineering a container but of guaranteeing that the container remains undisturbed for an enormous time. That is why the disposal question, rather than the physics, is the hardest part of fission power — and why fusion, which barely produces such waste, is worth the trouble of pursuing.
Nuclear fission. Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei of comparable mass, with the release of a large amount of energy and two or three neutrons, when the heavy nucleus is struck by a slow neutron.
The standard example, with the numbers balanced:
Check both conservation rules. Mass numbers: on the left, and on the right. Atomic numbers: , and . Both balance.
The chain reaction. The two or three neutrons released can each strike another heavy nucleus and cause it to fission, releasing more neutrons still.
- In a nuclear reactor the chain reaction is controlled, so that on average exactly one neutron from each fission goes on to cause another. Control rods of cadmium or boron absorb the surplus neutrons, and a moderator of graphite or heavy water slows the fast neutrons down so that they can be captured
- In a nuclear weapon the chain reaction is uncontrolled, so the number of fissions multiplies rapidly and all the energy is released at once
Nuclear fusion. Nuclear fusion is the combining of two light nuclei to form a heavier nucleus, with the release of a large amount of energy.
Two standard examples:
Check: mass numbers and ; atomic numbers and . Both balance.
Check: mass numbers ; charges . Both balance. This second reaction is the kind that supplies the energy of the sun and the other stars.
Why fusion needs such extreme conditions. Two nuclei are both positively charged, so they repel each other strongly, and that repulsion grows as they approach. To get close enough for the short-range attractive force to take over, they must be moving extremely fast — which means a temperature of millions of degrees, together with an enormous pressure to keep them close enough for long enough.
The differences, point by point, which is the most examined part of this section:
- Starting material: fission needs a heavy nucleus; fusion needs light nuclei
- Conditions: fission proceeds at ordinary temperature once triggered by a neutron; fusion needs extremely high temperature and pressure
- Chain reaction: fission is a chain reaction, since it produces the neutrons that continue it; fusion is not
- Products: fission produces highly radioactive fragments; fusion produces little or no radioactive waste
- Energy per unit mass: greater in fusion than in fission
- Control: fission has been controlled and is used to generate electricity; fusion has not yet been controlled for power generation
So fusion is in every respect the more attractive source and the harder one to achieve, and that combination is why it remains a subject of research while fission already runs power stations.
The disposal of nuclear waste, which is the price fission carries. The fragments left after fission are intensely radioactive, and some of them remain active for a very long time.
Waste is classified by how active it is:
- Low-level waste — contaminated clothing, tools, paper and packaging. It may be compacted, sealed in drums and buried in shallow, monitored landfill sites
- Intermediate-level waste — reactor components and chemical residues. It is mixed with concrete and sealed in steel drums before being stored
- High-level waste — the spent fuel itself, which is both intensely radioactive and hot
How high-level waste is handled, in order:
- It is first stored under water in cooling ponds at the site for a long period, until its activity and its heat output have fallen substantially. The water both cools it and absorbs the radiation
- It is then vitrified — mixed into molten glass, which sets into a solid block that will not dissolve in water and from which the radioactive material cannot leach out
- The blocks are sealed inside containers of steel and concrete
- The containers are buried deep underground in stable rock formations, chosen to be far from any source of ground water and away from regions of earthquakes
And the rules about what must not be done: nuclear waste must never be dumped into the sea, discharged into rivers, or put into ordinary landfill, because the radioactive material would enter the water supply and the food chain.
One honest observation about the difficulty. The waste must be kept isolated for far longer than any structure yet built has lasted, so the problem is not one of engineering a container but of guaranteeing that the container remains undisturbed for an enormous time. That is why the disposal question, rather than the physics, is the hardest part of fission power — and why fusion, which barely produces such waste, is worth the trouble of pursuing.
Exam tip
Which points does an examiner look for in a modern physics answer?
Name the isotope, say which radiation it emits, and then give the use — in that order. A use without the kind of radiation loses the reasoning mark.
- Classify before you use: alpha emitters for smoke detectors, bone-cancer treatment and static removal; beta for thickness gauging and age estimation; gamma for imaging, sterilising and treatment
- Give the reason the radiation suits the job — short range for a smoke detector, partial absorption for thickness, deep penetration for imaging
- For strontium-90, say it behaves chemically like calcium and therefore lodges in bone
- Give the three principles of protection — time, distance and shielding — and then the specific precautions
- Say why eating and drinking are forbidden in a radiation area, and connect it to the danger of an internal alpha source
- Name both natural and artificial sources of background radiation, with cosmic rays and rocks first
- Balance both the mass numbers and the atomic numbers in every fission or fusion equation, and show the check
- Give the differences between fission and fusion as paired statements, not as two separate paragraphs
- For waste, name the three levels and describe vitrification and deep burial for the high-level kind
- Say what must never be done with nuclear waste
The misconception to name. A radioisotope used in medicine is not chosen for being weak. It is chosen for emitting the right kind of radiation and for decaying away quickly, so that the dose to the patient stops soon after the examination. A long-lived isotope of the same element would give an identical image and go on irradiating the patient for years — which is why technetium-99m is used so widely and why "short active life" belongs in the answer.
A second trap. Writing that fusion is used in power stations. Fission is controlled and used to generate electricity; fusion has not yet been controlled for that purpose, although it is the source of the sun's energy. The two are also confused in the other direction — fusion needs millions of degrees while fission needs only a slow neutron — and an answer that swaps the conditions has the physics exactly backwards.
- Classify before you use: alpha emitters for smoke detectors, bone-cancer treatment and static removal; beta for thickness gauging and age estimation; gamma for imaging, sterilising and treatment
- Give the reason the radiation suits the job — short range for a smoke detector, partial absorption for thickness, deep penetration for imaging
- For strontium-90, say it behaves chemically like calcium and therefore lodges in bone
- Give the three principles of protection — time, distance and shielding — and then the specific precautions
- Say why eating and drinking are forbidden in a radiation area, and connect it to the danger of an internal alpha source
- Name both natural and artificial sources of background radiation, with cosmic rays and rocks first
- Balance both the mass numbers and the atomic numbers in every fission or fusion equation, and show the check
- Give the differences between fission and fusion as paired statements, not as two separate paragraphs
- For waste, name the three levels and describe vitrification and deep burial for the high-level kind
- Say what must never be done with nuclear waste
The misconception to name. A radioisotope used in medicine is not chosen for being weak. It is chosen for emitting the right kind of radiation and for decaying away quickly, so that the dose to the patient stops soon after the examination. A long-lived isotope of the same element would give an identical image and go on irradiating the patient for years — which is why technetium-99m is used so widely and why "short active life" belongs in the answer.
A second trap. Writing that fusion is used in power stations. Fission is controlled and used to generate electricity; fusion has not yet been controlled for that purpose, although it is the source of the sun's energy. The two are also confused in the other direction — fusion needs millions of degrees while fission needs only a slow neutron — and an answer that swaps the conditions has the physics exactly backwards.
Did you know
Why does the sun run on the harder of the two processes?
Fusion needs temperatures of millions of degrees and pressures no laboratory can sustain, which is why no fusion power station exists. The sun manages it continuously, and has done throughout its existence. It has two advantages no laboratory can copy.
The first is gravity. The sun's own weight compresses its core to a pressure and density far beyond anything achievable on earth, and that compression is free and permanent. Nothing has to be built to hold it in; the sun holds itself in.
The second is size. The core is enormous, so even a slow reaction rate per cubic metre adds up to a colossal total output. A fusion reactor on earth has to make the reaction go fast because it is small, which means reaching conditions the sun's core does not even need.
So the sun is not doing fusion better than we can — it is doing it in a very much easier place.
And the energy released has a definite origin, which is worth stating. In both fission and fusion, the products together have slightly less mass than the starting material. That missing mass has become energy. The amount lost is a tiny fraction of the total, and yet it produces an output far beyond anything chemical, because the conversion factor between mass and energy is so enormous.
Which is why a nuclear fuel goes so much further than a chemical one. Burning coal rearranges electrons; fission rearranges nuclei. The energies involved in the nucleus are millions of times larger, so a kilogram of nuclear fuel replaces an enormous quantity of coal — and that ratio is the entire attraction of nuclear power.
The same comparison explains the fission fragments' radioactivity. A heavy nucleus splits into two pieces that have inherited its high proportion of neutrons, which is too high for nuclei of their new, smaller size. So they are unstable and decay, mostly by beta emission, until the ratio is right. The waste problem is a direct consequence of splitting a heavy nucleus rather than an accident of engineering — and it is exactly the problem fusion does not have, because joining two light nuclei gives a product whose ratio is already suitable.
One last observation about the isotopes in this chapter. Carbon-14 is present in every living thing because living things constantly take up carbon from their surroundings. Once an organism dies it stops taking any up, and what it contains decays away steadily — so the proportion remaining is a measure of how long ago the exchange stopped. The method works on wood, bone, charcoal and cloth, and it works for exactly the same reason that makes strontium dangerous: the body cannot tell one isotope of an element from another, so it takes up whichever is offered. One consequence is a dating technique and the other is a hazard, and both come from the same indifference of chemistry to the nucleus.
The first is gravity. The sun's own weight compresses its core to a pressure and density far beyond anything achievable on earth, and that compression is free and permanent. Nothing has to be built to hold it in; the sun holds itself in.
The second is size. The core is enormous, so even a slow reaction rate per cubic metre adds up to a colossal total output. A fusion reactor on earth has to make the reaction go fast because it is small, which means reaching conditions the sun's core does not even need.
So the sun is not doing fusion better than we can — it is doing it in a very much easier place.
And the energy released has a definite origin, which is worth stating. In both fission and fusion, the products together have slightly less mass than the starting material. That missing mass has become energy. The amount lost is a tiny fraction of the total, and yet it produces an output far beyond anything chemical, because the conversion factor between mass and energy is so enormous.
Which is why a nuclear fuel goes so much further than a chemical one. Burning coal rearranges electrons; fission rearranges nuclei. The energies involved in the nucleus are millions of times larger, so a kilogram of nuclear fuel replaces an enormous quantity of coal — and that ratio is the entire attraction of nuclear power.
The same comparison explains the fission fragments' radioactivity. A heavy nucleus splits into two pieces that have inherited its high proportion of neutrons, which is too high for nuclei of their new, smaller size. So they are unstable and decay, mostly by beta emission, until the ratio is right. The waste problem is a direct consequence of splitting a heavy nucleus rather than an accident of engineering — and it is exactly the problem fusion does not have, because joining two light nuclei gives a product whose ratio is already suitable.
One last observation about the isotopes in this chapter. Carbon-14 is present in every living thing because living things constantly take up carbon from their surroundings. Once an organism dies it stops taking any up, and what it contains decays away steadily — so the proportion remaining is a measure of how long ago the exchange stopped. The method works on wood, bone, charcoal and cloth, and it works for exactly the same reason that makes strontium dangerous: the body cannot tell one isotope of an element from another, so it takes up whichever is offered. One consequence is a dating technique and the other is a hazard, and both come from the same indifference of chemistry to the nucleus.
Exam relevance
How do radioisotopes and nuclear reactions feed into JEE and NEET?
This is foundation work for Class 12 Nuclei, examined in JEE Main, JEE Advanced and NEET Physics, and it also feeds Class 11 and 12 Chemistry and Biology.
Where the fission and fusion equations lead. Class 12 computes the energy released from the mass defect, using the difference between the total mass of the reactants and that of the products, converted through the mass-energy relation. The balanced equations you write here are exactly what those calculations start from, and a candidate who cannot balance the mass numbers and charges cannot begin. JEE Main sets energy-release numericals of precisely this kind.
Where the binding-energy reasoning leads. Class 12 plots binding energy per nucleon against mass number and reads both processes off the curve: heavy nuclei release energy by splitting and light ones by joining, because both move toward the peak of the curve. Your qualitative statement that fusion releases more energy per unit mass is read directly from that graph, and it is a recurring assertion-reason item.
Where the chain-reaction material leads. Class 12 describes the reactor with its moderator, control rods, coolant and shielding, and asks for the function of each. The names and roles you learn here are the ones expected, and the distinction between a controlled and an uncontrolled chain reaction is asked directly.
Where the radioisotope uses lead. Class 12 covers them again under applications, and NEET Biology uses tracers throughout — in the study of photosynthesis, of translocation in plants, of metabolic pathways and of thyroid function. The fact that a tracer must have a short active life is expected in those answers too.
Where the strontium-and-calcium argument leads. Chemistry explains it through the periodic table: strontium is in the same group as calcium, so its chemistry is similar and the body cannot distinguish them. That is a group-trend argument of exactly the kind the periodic-properties chapters treat, and NEET sets questions linking chemical similarity to biological uptake.
Where background radiation leads. It appears in Class 12 in the context of measurement and of safe limits, and the requirement to subtract the background from every count is assumed in any activity numerical.
Where the biological-concentration idea leads. It is biomagnification, treated in Class 12 Biology under environmental issues, and the strontium-in-milk pathway is a standard example of it.
Question types to expect. At this level: classify isotopes and give uses, list harmful effects and precautions, name sources of background radiation, balance fission and fusion equations, and give the differences between the two processes. In competitive papers: mass defect and energy release, binding-energy curve reasoning, reactor components, half-life and activity with background subtraction, and tracer applications in Biology.
The single trap that costs marks. Swapping the conditions for the two processes. Fission is triggered by a slow neutron at ordinary temperature; fusion needs millions of degrees and enormous pressure — and an answer that reverses them, or that says fusion powers our electricity, has the whole comparison backwards. The check is the physical reason: two positive nuclei must overcome their mutual repulsion, and only extreme speed can do that.
A second trap. Failing to balance a nuclear equation. Both the mass number and the charge must agree on the two sides, and in the fission equation the three neutrons on the right are easy to leave out — which makes instead of and reveals the omission at once. Showing the check is what earns the mark.
Board versus competitive emphasis. The ICSE paper marks the classification, the named use with its reason, the listed precautions, the balanced equation and the paired differences; a competitive paper marks an energy release, a reactor component's function or a tracer's suitability. The transferable habit is asking what kind of radiation a job needs before choosing an isotope — because that single question decides the answer here, in Class 12, and in every biological tracer experiment you will read about.
Where the fission and fusion equations lead. Class 12 computes the energy released from the mass defect, using the difference between the total mass of the reactants and that of the products, converted through the mass-energy relation. The balanced equations you write here are exactly what those calculations start from, and a candidate who cannot balance the mass numbers and charges cannot begin. JEE Main sets energy-release numericals of precisely this kind.
Where the binding-energy reasoning leads. Class 12 plots binding energy per nucleon against mass number and reads both processes off the curve: heavy nuclei release energy by splitting and light ones by joining, because both move toward the peak of the curve. Your qualitative statement that fusion releases more energy per unit mass is read directly from that graph, and it is a recurring assertion-reason item.
Where the chain-reaction material leads. Class 12 describes the reactor with its moderator, control rods, coolant and shielding, and asks for the function of each. The names and roles you learn here are the ones expected, and the distinction between a controlled and an uncontrolled chain reaction is asked directly.
Where the radioisotope uses lead. Class 12 covers them again under applications, and NEET Biology uses tracers throughout — in the study of photosynthesis, of translocation in plants, of metabolic pathways and of thyroid function. The fact that a tracer must have a short active life is expected in those answers too.
Where the strontium-and-calcium argument leads. Chemistry explains it through the periodic table: strontium is in the same group as calcium, so its chemistry is similar and the body cannot distinguish them. That is a group-trend argument of exactly the kind the periodic-properties chapters treat, and NEET sets questions linking chemical similarity to biological uptake.
Where background radiation leads. It appears in Class 12 in the context of measurement and of safe limits, and the requirement to subtract the background from every count is assumed in any activity numerical.
Where the biological-concentration idea leads. It is biomagnification, treated in Class 12 Biology under environmental issues, and the strontium-in-milk pathway is a standard example of it.
Question types to expect. At this level: classify isotopes and give uses, list harmful effects and precautions, name sources of background radiation, balance fission and fusion equations, and give the differences between the two processes. In competitive papers: mass defect and energy release, binding-energy curve reasoning, reactor components, half-life and activity with background subtraction, and tracer applications in Biology.
The single trap that costs marks. Swapping the conditions for the two processes. Fission is triggered by a slow neutron at ordinary temperature; fusion needs millions of degrees and enormous pressure — and an answer that reverses them, or that says fusion powers our electricity, has the whole comparison backwards. The check is the physical reason: two positive nuclei must overcome their mutual repulsion, and only extreme speed can do that.
A second trap. Failing to balance a nuclear equation. Both the mass number and the charge must agree on the two sides, and in the fission equation the three neutrons on the right are easy to leave out — which makes instead of and reveals the omission at once. Showing the check is what earns the mark.
Board versus competitive emphasis. The ICSE paper marks the classification, the named use with its reason, the listed precautions, the balanced equation and the paired differences; a competitive paper marks an energy release, a reactor component's function or a tracer's suitability. The transferable habit is asking what kind of radiation a job needs before choosing an isotope — because that single question decides the answer here, in Class 12, and in every biological tracer experiment you will read about.
Key takeaways
What must you be able to do from this part?
Three classes of emitter, three principles of protection and two opposite nuclear processes.
- Alpha emitters — Americium-241 in smoke detectors, Radium-223 for bone cancer, Polonium-210 for removing static charge. Chosen because the radiation has a very short range
- Beta emitters — Strontium-90 for measuring the thickness of paper, plastic and metal sheets; Carbon-14 for estimating the age of ancient wood, bone and fossils. Chosen because a thin sheet partly absorbs beta radiation
- Gamma emitters — Cobalt-60 for cancer treatment, sterilising instruments, preserving food and detecting flaws in castings; Technetium-99m for organ imaging; Sodium-22 as a tracer; Iodine-131 for thyroid diagnosis and treatment. Chosen because gamma radiation penetrates deeply
- Radium-223 goes to bone because it behaves like calcium; Iodine-131 goes to the thyroid because the body treats it as iodine
- A tracer is a small quantity of a radioisotope followed by its radiation, used in medicine, agriculture and leak detection, and it must have a short active life
- Radiation ionises the atoms of living cells, causing burns, hair loss, sterility, leukaemia, cancer, cataract and genetic mutations passed to later generations
- Strontium-90 lodges in bone because it behaves chemically like calcium, and causes bone cancer and eye cancer
- The three principles of protection: reduce the time, increase the distance, and add shielding
- Precautions: long tongs or remote handling, thick lead containers, lead or concrete shields, lead-lined aprons and gloves, a film badge, no eating or drinking in a radiation area, and approved disposal only
- Background radiation comes naturally from cosmic rays, rocks and soil, radon gas, building materials, and food, water and our own bodies; and artificially from X-rays and medical treatment and radioactive fallout
- The background must be subtracted from any measurement of a weak source
- Fission splits a heavy nucleus into two comparable lighter nuclei with two or three neutrons and a large energy release: , balancing and
- A chain reaction is controlled in a reactor by cadmium or boron control rods and a graphite or heavy-water moderator, and uncontrolled in a weapon
- Fusion joins light nuclei: , and , which powers the sun
- Fusion needs millions of degrees and enormous pressure, to overcome the repulsion between two positive nuclei
- Fission uses heavy nuclei, needs only a slow neutron, is a chain reaction, produces highly radioactive waste and is controlled; fusion uses light nuclei, needs extreme conditions, is not a chain reaction, produces little waste, releases more energy per unit mass and is not yet controlled
- Waste is low, intermediate or high level; high-level waste is cooled under water, vitrified into glass, sealed in steel and concrete, and buried deep in stable rock far from ground water — and never dumped at sea or in ordinary landfill
The sharpest self-test is a matching exercise you set yourself. Write the nine isotopes in one column and their uses, shuffled, in another, then pair them — and for each pair say in one clause why that kind of radiation is the one the job needs.
- Alpha emitters — Americium-241 in smoke detectors, Radium-223 for bone cancer, Polonium-210 for removing static charge. Chosen because the radiation has a very short range
- Beta emitters — Strontium-90 for measuring the thickness of paper, plastic and metal sheets; Carbon-14 for estimating the age of ancient wood, bone and fossils. Chosen because a thin sheet partly absorbs beta radiation
- Gamma emitters — Cobalt-60 for cancer treatment, sterilising instruments, preserving food and detecting flaws in castings; Technetium-99m for organ imaging; Sodium-22 as a tracer; Iodine-131 for thyroid diagnosis and treatment. Chosen because gamma radiation penetrates deeply
- Radium-223 goes to bone because it behaves like calcium; Iodine-131 goes to the thyroid because the body treats it as iodine
- A tracer is a small quantity of a radioisotope followed by its radiation, used in medicine, agriculture and leak detection, and it must have a short active life
- Radiation ionises the atoms of living cells, causing burns, hair loss, sterility, leukaemia, cancer, cataract and genetic mutations passed to later generations
- Strontium-90 lodges in bone because it behaves chemically like calcium, and causes bone cancer and eye cancer
- The three principles of protection: reduce the time, increase the distance, and add shielding
- Precautions: long tongs or remote handling, thick lead containers, lead or concrete shields, lead-lined aprons and gloves, a film badge, no eating or drinking in a radiation area, and approved disposal only
- Background radiation comes naturally from cosmic rays, rocks and soil, radon gas, building materials, and food, water and our own bodies; and artificially from X-rays and medical treatment and radioactive fallout
- The background must be subtracted from any measurement of a weak source
- Fission splits a heavy nucleus into two comparable lighter nuclei with two or three neutrons and a large energy release: , balancing and
- A chain reaction is controlled in a reactor by cadmium or boron control rods and a graphite or heavy-water moderator, and uncontrolled in a weapon
- Fusion joins light nuclei: , and , which powers the sun
- Fusion needs millions of degrees and enormous pressure, to overcome the repulsion between two positive nuclei
- Fission uses heavy nuclei, needs only a slow neutron, is a chain reaction, produces highly radioactive waste and is controlled; fusion uses light nuclei, needs extreme conditions, is not a chain reaction, produces little waste, releases more energy per unit mass and is not yet controlled
- Waste is low, intermediate or high level; high-level waste is cooled under water, vitrified into glass, sealed in steel and concrete, and buried deep in stable rock far from ground water — and never dumped at sea or in ordinary landfill
The sharpest self-test is a matching exercise you set yourself. Write the nine isotopes in one column and their uses, shuffled, in another, then pair them — and for each pair say in one clause why that kind of radiation is the one the job needs.