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Correct answer is (b) boron

Moderator like heavy water contains a large number of protons. When fast neutrons are passed through it, they make elastic collisions with its protons, which have much smaller velocity. After few interactions, the velocities of neutrons get interchanged with those of protons. The final velocities of the neutrons are equal to the random velocities of the molecules of the moderator. The neutrons so obtained are called thermal neutrons.

ISOBARS : “Atoms of different elements having same MASS NUMBER and different ATOMIC NUMBER are called ISOBARS.”

Example : Sodium and Magnesium both have atomic Mass 23 but different Atomic numbers of 11 and 12. ₁₁Na²³ ₁₂Mg²³.

α-particles has wide spectrum.

α-particle is the nucleus of helium which is double ionized particle.
_ZX^A → _{Z – 2}Y^{A – 4} + ₂He⁴
Example : ₉₂U²³⁸ → ₉₀Th²³⁴ + ₂He⁴
It is clear that the emission of α-particle reduces the atomic number of the original nucleus from two and the mass number is 4 that means the atomic number of the product nucleus decreases by 2 and the mass number is four, it is called α-decay.

Exposure from the nucleus of α-particles can be interpreted on the basis of quantum mechanics rather than coherent principles. According to that α-particle occurs only in the nucleus and by the tunnel effect it comes out of then nucleus. The spectrum of energy of α-particles is discrete and linear discrete energy spectrum demonstrates that nuclear power level is available in the nucleus as well as nuclear.

NUCLEONS : “The protons and neutrons inside the nucleus are called NUCLEONS.” 

NUCLIDE : “Representation of an atom along with its ATOMIC NUMBER and MASS NUMBER is called 

NUCLIDE” e.g. ZX A is the nuclide of atom X. 

ANTINEUTRINO : “An uncharged particle having negligible small mass emitted along with P-particle is called ANTI NEUTRINO.” 

1. ATOMIC NUMBER : “The number of protons in the nucleus is called ATOMIC NUMBER -Z”. 

2. MASS NUMBER : “The number of protons and neutrons present in nucleus is called MASS NUMBER-A.”

This mass is received from the binding energy of the nucleus.

In α-decay, the daughter nucleus has atomic number less by 2. For hydrogen isotope this is not possible.

(c) Decomposition of neutron in nucleus
Due to decomposition of neutron in nucleus

The distance between molecules is of the order of Å, while nuclear forces act in distance of fermi order.

(b) F_e > > F_n
F_e >> F_n

(b) 4(X₁ – X₁)
₁H² + ₁H² → ₂Hi⁴ + Q
Q = 2X₁ + 2X₁ – 4X₂
Q = 4X₁ – 4X₂
Q = 4(X₁ – X₂)

From lead it is easy. Per nucleon binding energy of lead is less than iron.

Mass number of ₈₈Ra²²⁶ is 266 

Atomic number of ₈₈Ra²²⁶ is 88 

Mass number of α = 4

Atomic number of α = 2 

After 3 a decay 

Mass number of the daughter element is 

= 226 – (3 × 4) 

= 226 – 12 

= 214 

Atomic number of the daughter element is

= 88 – (3 × 2) 

= 88 – 6 

= 82 

Number of neutrons is 

= 214 – 82 

= 132

(i) Penetrating power varies inversely as mass of the particle. But mass of alpha particles is very much greater than mass of beta particles. 

(ii) Hence, penetrating power of α – particle is less than that of β – particles.

Since heavy water (D₂O) is rich in hydrogen, it is used to slow down the fast moving neutrons.

Control rods are used to control the number of neutrons in order to have a sustained chain reaction. Mostly boron or cadmium rods are used as control rods. They absorb the neutrons.

A coolant is used to remove the heat produced in the reactor core, to produce steam. This steam is used to run a turbine in order to produce electricity. Water, air and helium are some of the coolants.

Correct answer is (d) Am²⁴¹

Correct answer is (c) cadmium

Similarities : 

1. Both y -rays and X-rays are the electromagnetic waves. 

2. Both travel with speed 3 ×10⁸ms^-1 in air (or vacuum). 

Dissimilarities : 

1. The wavelength of y -rays is shorter than that of X-rays. 

2. y -rays are more penetrating than X-rays.

γ-radiations are electromagnetic waves like light and are not charged particles. Hence can not be deflected by ELECTRIC or MAGNETIC field and cannot be detected.

(b) β-particles

(c) ϒ -radiation 

Beta particle particle has -e charge.

It’s charge is +2e.

Gamma ray  posses minimum ionising power .

Beta ray  travels with speed about 9/10 time that of light .

Alpha ray has greatest ionising power

Nuclear power station is example of nuclear fission. Controlled chain reaction takes place and electricity is produced.

²³⁵₉₂U + ¹₀n → ¹³⁹₅₆Ba + (⁹⁴₃₆Kr) + 3¹₀n.

Splitting of nucleus into two nearly lighter nuclei is called nuclear fission. 

Enriched Uranium ²³⁵ ₉₂U is used as fuel in nuclear reactor.

(d) Density
Density does not depend upon mass number

(a) the mass number remains unchanged, but atomic number increases by 1 amu.

(c) the nucleus of the atom

(a) α-particles 

Mass of nuclear particles is expressed in 

u → Unified atomic mass unit 

1 u = 1.6603 x 10^-27kg = 931 MeV in terms of energy

According to Einstein’s mass-energy equivalence 

E = mc² 

E = 1kg x [3 x 10⁸ms^-1]² 

Energy released E = 1 x 3 x 10⁸ x 3 x 10⁸ = 9 x 10¹⁶ J

E = mc² is Einstein’s mass-energy relation. 

1. Although both the infrared and y -rays are electromagnetic radiations, y-rays have much shorter wavelength (10^-13m) as compared to infrared whose wavelength is nearly 10^-6m or more. 

2. γ -rays are much more penetrating as compared to the infrared radiations.

NUCLEAR ENERGY : “Energy obtained from the nucleus of atom i.e. from defect mass is called NUCLEAR ENERGY.”

α, β and γ radiations. 

1. α and β radiations are charged. 

2. γ (Gamma Radiations) are the most penetrating. 

3. γ radiations travel with the velocity of light.

The shadow of wood is faint because only the α-radiations are stopped by wood (since α-radiations are least penetrating). The shadow of lead is dark because β and γ -radiations are also stopped by lead. (If wood is replaced by aluminium or any other light metal, the appearance on screen and reason will be the same), i.e. shadow will be dark.

Properties of nuclear forces:

(i) Nuclear forces are independent on electric charges. For a given system, the force between learn about charge distribution in the nucleus. Similarly, we get to know about the nuclear force (and not the nuclear charge) from the neutron-nucleus scattering experiment, from which we know about the mass distribution in the nucleus.

(ii) The range of nuclear forces is very small, of the order of nuclear size (of few femtometre), but from such type of scattering experiments, we inside this range, the nuclear force is much stronger than the electric force (50 ~ 60 times). Outside this range, nuclear forces are not two protons and two neutrons and that between 1 neutron and 1 proton is almost equal in magnitude. Electrons are not affected by the nuclear force. That is why in the scattering experiments by high energy electrons, electrons are scattered by the nuclear charge only and effective.

The answer is (d)
₂₆Fe⁵⁶ as maximum binding energy per nucleon.

According to Einstein, this mass (Δm) turns into energy. This energy is called binding energy. This energy binds all nucleons of the nucleus in the form of nucleus. Δm means that when the protons and neutrons together from the nucleus then Am mass disappears and its equivalent energy is released. By this energy the protons and neutrons are bound to the nucleus. It is obvious that breaking the proton and neutron of the nucleus will require the same outer energy. Thus it becomes clear that when the protons and neutrons from the nucleus then the some energy is released, which is called the binding energy. It is also clear from this fact that if the same energy is given to the nucleus then all nucleons will be free. Thus binding energy can also be defined as “The binding energy is the amount of energy that release all of the nucleons when giving it to the nucleus.” So, the binding energy of nucleus.
ΔE = Δmc²
= [{Zm_p + (A – Z)m_n} – m]uc² …………. (2)
If the nucleon’s number divided in binding energy of nucleus then we get the binding energy per nucleon. Binding energy demonstrates the stability of the atom.
∴ Binding energy per nucleon = \(\frac{\Delta E}{A}\)

(c) γ rays
γ-rays are electromagnetic waves.

After γ-decay, there is no change in atomic number and mass number. But there is change of energy state.

Radioactivity:

In the beginning of this chapter, it was mentioned that Henry Becquerel discovered radioactivity in 1896. Heavy elements like uranium, thorium, etc. particles or radiations on their own and undergoes a decay. In this process of decay, new atoms (elements) are formed that themselves exhibit radioactivity and this process continues till a stable element is formed.

The discovery of radioactivity by Henry Becquerel was purely accidental. He observed that when uranium salt-crystals or uranium-potassium sulphate were illuminated with visible light, they emitted some invisible radiations that blackened the photographic plate placed near it which was covered by a light resistant cover. After this, scientists Marie Curie and Pierre Curie discovered the new elements radium and polonium from the pitch blend are which were more radioactive than uranium. Through the experiments, Rutherford showed that radioactive radiations are of three type which he called alpha, beta and gamma rays. The experiments performed later showed that-α (alpha) rays are helium nuclei, β (beta) rays are electrons or positrons and γ (gamma) rays are photons of high energy. Experiments also showed that the radioactivity is a result of decay of unstable nuclei. This way, we can say that radioactivity is a nuclear phenomenon. Some important facts related to radioactivity are as follows :

(i) External conditions like pressure temperature and state of radioactive substance (solid, liquid or gas) has no effect on radioactivity. Radioactivity is also unaffected by the chemical reactions or chemical combinations (like uranium or any of its compounds is radioactive). As there is contribution of only the outermost electrons in a chemical combination. Thus, the electronic configuration of atoms is not related to the radioactivity. Also, α-rays, β-particles of very high energy or emission of γ-ray photons is not possible from the emission of the outer part of atom. Thus, radioactivity is purely a nuclear phenomenon.

(ii) In the radioactive decay of nuclei, with the laws of conservation of mass, energy, charge linear momentum, angular momentum and the conservation of number of nucleons should be followed.

(iii) A nucleus say X is unstable for the decay of α and β if its mass is more than the sum of its product constituents.

(iv) In the process of radioactive decay, emission energy per atom is the order of few MeV where as it of the order of few eV for chemical reactions.