10th Science Unit 6 Nuclear Physics Book Back Answers

Making light elements radioactive by artificial or induced methods is called artificial radioactivity, also named induced (man-made) radioactivity.

So both 'induced' and 'artificial' name the same phenomenon.

Radioactivity is measured in curie, rutherford and becquerel (SI unit); roentgen measures exposure of $\gamma$ and X-rays.

All the listed names are units connected with radioactivity.

Artificial (induced) radioactivity was discovered in 1934 by Irene Curie and F. Joliot.

In $\beta$ decay the mass number stays the same and only the atomic number increases by 1.

In $\gamma$ decay only the energy level changes; atomic number and mass number remain the same.

In $\alpha$ decay mass number falls by 4, so it is not included.

Radio cobalt (Co-60) along with radio gold (Au-198) is used in the treatment of skin cancer.

Gamma rays have very high penetrating power and pass through the body.

They trigger mutation in human genes and cause genetic transform diseases.

Radioactive materials are stored in thick-walled lead containers, and lead coated aprons and lead gloves are worn while handling them.

Alpha particles are helium nuclei, not photons, so (i) is wrong.

Gamma rays have very high penetrating power, so (ii) is wrong and (iv) is right.

Ionising power of $\alpha$ rays is 100 times that of $\beta$ and 10,000 times that of $\gamma$ rays, so (iii) is right.

In the proton–proton chain, lighter hydrogen nuclei combine to form a heavier nucleus (helium) with release of energy.

Combination of lighter nuclei into a heavier nucleus is nuclear fusion — the source of stellar energy.

By Soddy–Fajan's law, in $\alpha$ decay mass number falls by 4 and atomic number falls by 2.

Mass number $A = 12 - 4 = 8$; atomic number $Z = 6 - 2 = 4$.

Kamini is a research reactor of the nuclear establishment at Kalpakkam, Tamil Nadu.

Chain reaction occurs both in a nuclear reactor and in an atom bomb, so (i) is correct and (iv) is wrong.

In a reactor the chain reaction is controlled by control rods, so (ii) is correct and (iii) is wrong.

Roentgen (R) is the unit used to measure the exposure of $\gamma$-rays and X-rays, not the number of disintegrations.

The textbook defines it as the quantity of radioactive substance which produces a charge of $2.58 \times 10^{-4}$ coulomb in 1 kg of air under standard pressure, temperature and humidity.

Positron is listed along with photon, meson and neutrino as one of the elementary particles discovered.

It carries a positive charge and is the antiparticle of the electron; it is emitted in nuclear fusion and artificial radioactivity.

Radio isotopes are used in medicine for diagnosis and therapy.

Radio-iron ($\mathrm{Fe^{59}}$) is used to diagnose anaemia and also to provide treatment for it.

ICRP is the body that recommends the maximum permissible exposure limits to radiation.

Its safe limit of overall exposure is 20 milli sievert per year.

A dosimeter is a device used to detect the level of exposure to ionizing radiation.

A pocket dosimeter gives the wearer an immediate reading of his/her exposure to X-rays and $\gamma$ rays, so users in nuclear plants and imaging centres wear it.

Alpha rays have low penetrating power (stopped even by thick paper); beta rays can pass through a thin metal foil.

Gamma rays have very high penetrating power, greater than beta rays, and can penetrate thick metal blocks.

Here the mass number A does not change but the atomic number increases by 1 unit.

By Soddy–Fajan's displacement law, this is beta decay, so the emitted particle X is a beta particle (electron).

In this reaction both the mass number A and the atomic number Z remain the same; only the energy level of the nucleus changes.

Such a change takes place in gamma decay.

In nuclear fusion two lighter nuclei combine to form a heavier nucleus, e.g. $_{1}H^{2} + {}_{1}H^{2} \rightarrow {}_{2}He^{4} + Q$.

The average energy released in each such fusion reaction is $3.84 \times 10^{-12}$ J.

The positively charged nuclei repel each other, so very high kinetic energy is needed to push them together.

This is possible only at a temperature of the order of $10^{7}$ to $10^{9}$ K with high pressure, hence fusion is called a thermonuclear reaction.

Radio isotopes are widely used in agriculture.

The radio isotope of phosphorous, $\mathrm{P^{32}}$, helps to increase the productivity of crops.

The safe limit of receiving radiation is about 100 mR per week.

An exposure of 100 R may cause fatal diseases like leukemia (death of red blood corpuscles) or cancer; about 600 R leads to death.

A fissionable (fissile) material is one that undergoes fission in a sustained manner when it absorbs a neutron.

The textbook lists $U^{235}$ and plutonium ($Pu^{239}$ and $Pu^{241}$) as fissionable materials.

Elements whose atomic number is more than 82 are heavy, unstable nuclei; they undergo spontaneous radioactivity and nuclear fission, not fusion.

Nuclear fusion is the combination of two lighter nuclei (like deuterium $_1H^2$) to form a heavier nucleus.

Nuclear fission emits alpha, beta and gamma radiations; the gamma radiation triggers mutation in human genes and causes genetic disorders.

In nuclear fusion only light and heat energy are emitted, so it is comparatively less harmful.

Natural uranium contains 99.28 % of $_{92}U^{238}$ and only 0.72 % of $_{92}U^{235}$, and $U^{238}$ does not undergo fission.

Only $U^{235}$ is fissionable, so enriched uranium rich in $U^{235}$ is used as the fuel in a nuclear reactor.

The moderator (graphite, heavy water) only slows down the fast neutrons; it does not limit their number.

Control rods of boron or cadmium absorb the extra neutrons and keep only one neutron for further fission, making the chain reaction controlled.

Without control rods the chain reaction becomes uncontrolled, which is the principle of the atom bomb.

In the fission of $_{92}U^{235}$ by a neutron, the nucleus splits into $_{56}Ba^{141}$ and $_{36}Kr^{92}$ with the release of 3 neutrons and energy.

On an average about 2 to 3 neutrons are released in each fission, which makes a chain reaction possible.

In both fission and fusion, the mass of the products is less than the mass of the parent nuclei; this difference is the mass defect.

This missing mass is converted into energy by Einstein's mass–energy equivalence relation $E = mc^2$.

BARC (Bhabha Atomic Research Centre) is at Mumbai, Maharashtra.

India's first atomic power station is Tarapur Atomic Power Station.

IGCAR (Indira Gandhi Centre for Atomic Research) is at Kalpakkam, Tamil Nadu.

Apsara was the first nuclear reactor built in India (and in Asia).

Fuel – uranium, a fissile material commonly used in reactors.

Moderator – heavy water, which slows down fast neutrons (graphite is also used).

Control rods – cadmium rods (or boron), which absorb the extra neutrons.

Shield (protection wall) – lead (thick concrete-lead wall) that stops harmful radiation escaping.

Soddy and Fajan framed the radioactive displacement law (1913).

Irene Curie, with F. Joliot, discovered artificial (induced) radioactivity in 1934.

Henri Becquerel discovered natural radioactivity in uranium in 1896.

Albert Einstein proposed mass–energy equivalence, $E = mc^2$, in 1905.

Uncontrolled fission chain reaction is the principle of the atom bomb.

Fertile materials (U-238, Th-232, Pu-240) are converted into fissionable material in a breeder reactor.

Controlled fission chain reaction takes place in a nuclear reactor to give energy steadily.

Fusion reaction is the principle of the hydrogen bomb.

Radio-iron (Fe-59) is used to diagnose and treat blood disorders like anaemia and leukemia.

Radio-iodine (I-131) is used to cure goitre, a thyroid disease.

Radio-sodium (Na-24) is used to check the effective functioning of the heart.

Carbon-14 is used in radio carbon dating to find the age of fossils, old paintings and monuments.

Alpha rays have the lowest penetrating power – they are stopped even by a thick paper.

Beta rays penetrate more than alpha rays; they can pass through a thin metal foil.

Gamma rays have a very high penetrating power, greater than beta rays; they can pass through thick metal blocks.

Cosmic rays are high energy radiations from outer space and have penetrating power even greater than gamma rays.

Radioactivity – discovered by Henri Becquerel in 1896 from a uranium compound.

Discovery of radium – Marie Curie and Pierre Curie separated radium from pitchblende in 1898.

Artificial radioactivity – discovered by Irene Curie and F. Joliot in 1934.

Nuclear reactor – the first nuclear reactor was built in 1942 at Chicago, USA.

Natural radioactivity is a spontaneous process — the element disintegrates on its own.

When light elements are made radioactive by bombarding them with particles like alpha particles, the disintegration is an induced process.

This induced (man-made) emission is called artificial radioactivity.

Nuclear fusion needs an extremely high temperature (of the order of $10^7$ to $10^9$ K) and high pressure to push the nuclei together.

Nuclear fission does not need such conditions; a slow neutron is enough to split the heavy nucleus.

So fission can be carried out at room temperature.

Radio phosphorous (P-32) is used in agriculture to increase the productivity of crops.

In medicine, radio sodium (Na-24) is used for the effective functioning of the heart.

Alpha rays are positively charged, so they are deflected by electric and magnetic fields.

Gamma rays are neutral electromagnetic waves (photons) with zero charge.

Hence gamma rays are not deflected by electric or magnetic fields — null deflection.

Given: parent nucleus $\mathrm{{}_{88}Ra^{226}}$, mass number $A = 226$, atomic number $Z = 88$; it undergoes 3 alpha decays.

Soddy–Fajan law: in each $\alpha$-decay the mass number falls by 4 and the atomic number falls by 2.

For 3 alpha decays: mass number of daughter $A = 226 - (3 \times 4) = 226 - 12 = 214$.

Atomic number of daughter $Z = 88 - (3 \times 2) = 88 - 6 = 82$, so the daughter nucleus is $\mathrm{{}_{82}Pb^{214}}$.

Number of neutrons $=$ mass number $-$ atomic number $= 214 - 82 = 132$.

Given: activity $= 75.6$ millicurie per second; $1$ curie $= 3.7 \times 10^{10}$ Bq.

Convert millicurie to curie: $75.6 \text{ mCi} = 75.6 \times 10^{-3} \text{ Ci}$.

Multiply by the conversion factor: activity $= 75.6 \times 10^{-3} \times 3.7 \times 10^{10}$ Bq.

$= 279.72 \times 10^{7} = 2.7972 \times 10^{9}$ Bq (nearly $2.8 \times 10^{9}$ disintegrations per second).

Assertion is true: $_{92}U^{235} + {}_0n^1 \rightarrow {}_{56}Ba^{141} + {}_{36}Kr^{92} + 3\,{}_0n^1 + Q$ — this is the fission reaction given in the text.

Reason is true: a fissile (fissionable) material is one that undergoes fission in a sustained manner when it absorbs a neutron, and $U^{235}$ is such a material.

The splitting into barium and krypton happens exactly because $U^{235}$ is fissile, so the reason correctly explains the assertion.

In $\beta$-decay the mass number stays the same and the atomic number increases by 1 (e.g. $_{15}P^{32} \rightarrow {}_{16}S^{32} + {}_{-1}e^0$).

Number of neutrons = mass number − atomic number. Since $A$ is unchanged and $Z$ rises by 1, the neutron number falls by 1. So the assertion is true.

The reason (atomic number increases by one) is true and is exactly why the neutron number decreases by one.

Assertion is true: nuclear fusion is possible only at extremely high temperature of the order of $10^7$ to $10^9$ K along with high pressure, hence it is called a thermonuclear reaction.

Reason is also true: in fusion two lighter nuclei combine to form a heavier nucleus with the release of a large amount of energy.

But the high temperature is needed to give the nuclei enough kinetic energy to overcome the electrostatic repulsion between the two positively charged nuclei — not because energy is released.

So the reason is a true statement but is not the correct explanation of the assertion.

Assertion is true: control rods (boron or cadmium) absorb neutrons, and hence they are called 'neutron seeking rods'.

Reason is also true: control rods keep the number of neutrons at one per fission so that the chain reaction is sustained and controlled in the reactor.

However, the name 'neutron seeking rods' comes from their neutron-absorbing nature; sustaining the fission is their purpose, not the explanation of the name.

Hence both statements are true, but the reason does not explain the assertion.

In 1896, the French physicist Henri Becquerel found that a uranium compound kept near a photographic plate exposed the film on its own.

This spontaneous emission of radiation was named radioactivity.

Pitchblende is an ore of uranium.

Marie Curie and Pierre Curie separated a new, more intensely radioactive substance from it and named it radium.

Artificial radioactivity is induced in lighter elements by bombarding them with alpha particles.

Boron and aluminium are such light elements.

Alpha and beta rays are particles, but gamma rays are electromagnetic waves consisting of photons.

Hence gamma rays are the electromagnetic radiation emitted during natural radioactivity.

By Soddy–Fajan's law, in $\alpha$-decay the mass number falls by 4 and the atomic number falls by 2.

So for the parent A: atomic number $= 104 + 2 = 106$ and mass number $= 259 + 4 = 263$.

Each fission of a uranium nucleus releases about 200 MeV of energy.

In joules this average energy is $3.2 \times 10^{-11}$ J.

Nuclear fission emits gamma radiation.

Gamma radiation triggers mutation in human genes and causes genetically transformed diseases.

Exposure of 100 R may cause fatal diseases like leukemia or cancer.

When the body is exposed to about 600 R, it leads to death.

A nuclear reactor produces energy by controlled fission.

The first one was built in the year 1942 at Chicago, USA.

Becquerel is the SI unit of radioactivity.

It is defined as the quantity of one disintegration per second.

Radioactive materials are kept in thick walled lead containers.

Lead coated aprons and gloves, and a thick concrete–lead protection wall around a reactor, stop harmful radiation.

Origin: Natural radioactivity is the emission of radiation due to self-disintegration of a nucleus; artificial radioactivity is emission due to disintegration of a nucleus through an induced process.

Radiations emitted: In natural radioactivity alpha, beta and gamma radiations are emitted; in artificial radioactivity mostly elementary particles such as neutron and positron are emitted.

Nature of process: Natural radioactivity is a spontaneous process and cannot be controlled; artificial radioactivity is an induced process and can be controlled.

Elements involved: Natural radioactivity is exhibited by elements with atomic number more than 82, while artificial radioactivity is exhibited by elements with atomic number less than 82.

During fission some neutrons escape (leakage) and some are absorbed by non-fissionable material, so the chain reaction can stop.

To sustain the chain reaction the rate of production of neutrons must be more than the rate of their loss, which needs a certain optimum mass of fissile material.

Definition: The minimum mass of a fissile material necessary to sustain the chain reaction is called critical mass ($m_c$).

It depends on the nature, density and size of the fissile material. Less than $m_c$ is subcritical; more than $m_c$ is supercritical.

Roentgen (R) is the unit used to measure the radiation exposure of $\gamma$ rays and X-rays.

One roentgen is defined as the quantity of radioactive substance which produces a charge of $2.58\times10^{-4}$ coulomb in 1 kg of air under standard conditions of pressure, temperature and humidity.

Soddy and Fajan framed these displacement laws in 1913 for the daughter nucleus produced in alpha and beta decay.

Alpha emission: When a radioactive element emits an alpha particle, the daughter nucleus formed has a mass number less by 4 units and an atomic number less by 2 units than the parent nucleus.

Beta emission: When a radioactive element emits a beta particle, the daughter nucleus has the same mass number, but its atomic number is more by 1 unit than that of the parent nucleus.

Example: $_{92}U^{238} \rightarrow {}_{90}Th^{234} + {}_2He^4$ and $_{15}P^{32} \rightarrow {}_{16}S^{32} + {}_{-1}e^0$.

Control rods are one of the essential components of a nuclear reactor.

They control the number of neutrons in the reactor so that a sustained (controlled) chain reaction takes place.

They do this by absorbing the extra neutrons, leaving only one neutron to cause further fission.

Boron or cadmium rods are mostly used as control rods.

Atom bombs were exploded at Hiroshima and Nagasaki in Japan during World War II.

During such an explosion a tremendous amount of heat, light and hazardous radiation like $\gamma$ rays is released.

Gamma radiation triggers mutation in the human gene, so a person exposed to radiation can be affected genetically.

These genetic changes are passed on, and hence some new born children in Japan still have congenital diseases.

X-rays are harmful radiations; close or long exposure puts a person at greater health risk and can affect him genetically.

So he must wear lead coated aprons and lead gloves while working with radiation, as lead absorbs the radiation.

He should also wear a dosimeter to check his level of exposure, and the safe limit is about 100 mR per week (20 milli sievert per year).

He should avoid eating while handling radioactive materials and stand behind the protective lead wall while taking X-rays.

Stars like our Sun emit a large amount of energy in the form of light and heat; this energy is called stellar energy.

All stars contain a large amount of hydrogen and their temperature is very high (of the order of $10^7$ to $10^9$ K).

This high temperature and pressure induce fusion of hydrogen nuclei in the core of the stars.

The enormous energy released in this fusion (thermonuclear) reaction is the stellar energy, the source of light and heat of the Sun and other stars.

The radio isotope of phosphorus (P-32) is used to increase the productivity of crops.

Radiations from radio isotopes kill insects and parasites and thus prevent wastage of agricultural products.

Perishable cereals exposed to radiation remain fresh beyond their normal life, increasing the storage time.

Very small doses of radiation prevent sprouting and spoilage of onions, potatoes and gram.

{'step': 'X-rays are high energy radiations with very high penetrating power, so they easily pass through the body tissues.'}

{'step': 'They ionise the atoms of the living cells and damage them; a person exposed very closely or for a long duration is at greater health risk.'}

{'step': 'Repeated exposure adds up. The safe limit fixed by ICRP is only about 100 mR per week (20 milli sievert per year); crossing it can cause fatal diseases like leukemia or cancer, and can affect a person genetically.'}

{'step': 'Hence X-rays should be taken only when a doctor finds it necessary, and not often.'}

{'step': 'Cell phone towers continuously emit electromagnetic radiation, which is one of the man-made sources of radiation exposure around us.'}

{'step': 'The intensity of this radiation is greatest near the tower and decreases as the distance from it increases.'}

{'step': 'People living very close to a tower are exposed closely and for a long duration, so they receive a dose above the safe permissible limit.'}

{'step': 'Such prolonged exposure is harmful to the human body and can even affect people genetically. Hence towers are placed far away from residential areas.'}