Chapter 25: Nuclear Physics and Particle Physics (Set-3)
A nucleus has mass defect 0.02 u. What is its binding energy roughly?
A 18.6 MeV
B 0.02 MeV
C 200 MeV
D 931 MeV
Binding energy ≈ (mass defect in u) × 931 MeV. So 0.02 × 931 ≈ 18.6 MeV. This energy is released when nucleons bind to form the nucleus.
Why is nuclear force called “short range” in nuclei?
A Acts only far
B Depends on charge
C Acts within few fm
D Depends on temperature
Nuclear force is significant only over distances of about 1–2 femtometres. Beyond this range it becomes negligible, which is why nucleons interact mainly with nearest neighbors.
Which graph best indicates whether fission releases energy?
A Atomic radius curve
B Activity-time curve
C Density-radius curve
D Binding-energy curve
The binding energy per nucleon curve shows heavy nuclei have lower binding energy per nucleon than medium nuclei. Splitting heavy nuclei increases binding energy per nucleon, releasing energy.
A stable even-even nucleus generally has nuclear spin?
A Usually 0
B Always 1/2
C Usually 1
D Always 2
In even-even nuclei, protons and neutrons pair up with opposite spins, canceling angular momentum. Hence total nuclear spin is commonly zero, making such nuclei very stable.
What does the “saturation property” mainly imply for nuclei?
A Force grows with A
B Density increases with A
C Binding per nucleon nearly constant
D Radius decreases with A
Saturation means each nucleon interacts strongly only with a few neighbors. So binding energy grows roughly proportional to A, making binding energy per nucleon nearly constant for many nuclei.
In semi-empirical mass formula, Coulomb term mainly reduces?
A Neutron number
B Binding energy
C Nuclear radius
D Decay constant
Coulomb repulsion among protons reduces nuclear binding energy. This term increases with proton number and makes very heavy nuclei less stable against fission and radioactive decay.
Which factor explains why heavy nuclei need more neutrons for stability?
A Neutron charge
B Electron shielding
C Gamma emission
D Coulomb repulsion
As Z increases, proton–proton repulsion increases strongly. Extra neutrons add attractive nuclear force without adding repulsion, helping heavy nuclei remain bound and stable.
Which decay increases atomic number by 1 but keeps mass number same?
A Alpha decay
B Beta plus decay
C Beta minus decay
D Electron capture
In β⁻ decay, a neutron converts to a proton, emitting an electron and antineutrino. So Z increases by 1 while A remains unchanged.
Which decay decreases atomic number by 1 without changing mass number?
A Beta plus decay
B Gamma decay
C Alpha decay
D Neutron emission
In β⁺ decay, a proton changes into a neutron, emitting a positron and neutrino. Thus Z decreases by 1 and A stays the same.
Electron capture is most likely from which shell?
A Valence shell
B K shell
C Outer shell
D Conduction band
Electron capture usually involves an inner K-shell electron because it has the highest probability of being found near the nucleus, where capture can occur.
For radioactive decay, decay constant λ has unit of?
A second
B joule
C s⁻¹
D kilogram
Decay constant represents probability per unit time for a nucleus to decay. Therefore its unit is inverse time, commonly s⁻¹.
If half-life is 10 days, mean life is closest to?
A 14.4 days
B 6.9 days
C 10 days
D 20 days
Mean life τ = 1/λ and half-life T½ = 0.693/λ. So τ = T½/0.693 ≈ 10/0.693 ≈ 14.4 days.
In a nuclear reaction, Q-value is positive when reaction is?
A Endothermic
B Elastic only
C Exothermic
D Forbidden
Positive Q-value means initial mass-energy is greater than final mass-energy. The difference appears as released kinetic energy, so the reaction is exothermic.
Which condition must be satisfied for any allowed nuclear reaction?
A Z increases always
B Radius conserved
C Half-life conserved
D Charge conserved
Electric charge conservation is fundamental. Total proton number (charge) before and after reaction must match, even if neutrons, gamma rays, or other particles are produced.
Threshold energy is required mainly for reactions with?
A Q positive
B Q negative
C Q maximum
D Q zero
When Q is negative, reaction needs minimum kinetic energy to supply required energy. This minimum depends on masses and momentum conservation.
A slow neutron is more effective for U-235 fission because?
A Higher momentum
B Lower binding energy
C Higher capture probability
D Larger nuclear radius
U-235 has a much higher fission cross section for thermal (slow) neutrons. Slow neutrons are captured more easily, increasing chance of fission.
“Critical reactor” means multiplication factor k is?
A k = 1
B k < 1
C k > 1
D k = 0
In a critical reactor, neutron production exactly balances neutron losses. Power remains steady because each generation produces the same number of neutrons as previous generation.
Which device gives strong gas amplification but not full discharge like GM?
A Ionization chamber
B Cloud chamber
C Bubble chamber
D Proportional counter
Proportional counters operate in region where gas multiplication occurs and pulse size is proportional to deposited energy, unlike GM where pulse size is nearly same for all events.
Which detector is best suited for precise gamma spectroscopy?
A GM tube
B Cloud chamber
C Semiconductor detector
D Ion chamber
Semiconductor detectors (especially HPGe) provide excellent energy resolution. They can distinguish close gamma energies, which GM tubes cannot due to poor energy information.
Detector efficiency mainly depends on?
A Geometry and absorption
B Sample color
C Nuclear spin only
D Electron mass
Efficiency depends on how many emitted particles enter the detector and get absorbed to produce signal. Shape, distance, detector material, and thickness strongly affect it.
Cyclotron frequency is independent of speed only when particles are?
A Relativistic
B Non-relativistic
C Neutral
D Unstable
Cyclotron frequency f=qB/2πmf=qB/2πm assumes mass constant. At relativistic speeds, effective mass increases, frequency changes, so simple cyclotron condition fails.
Which accelerator is most affected by relativistic mass increase?
A Cyclotron
B Van de Graaff
C Linear accelerator
D Ion chamber
In cyclotron, fixed frequency matches particle revolution only if mass constant. At high speeds, relativistic mass increase breaks synchronism, limiting maximum energy.
A baryon has baryon number:
A 0
B −1
C +1
D +2
By definition, baryons like proton and neutron have baryon number +1. Antibaryons have baryon number −1. This quantity is conserved in typical reactions.
A meson has baryon number:
A +1
B 0
C −1
D +2
Mesons are quark–antiquark pairs. Their baryon numbers cancel (+1/3 and −1/3), giving total baryon number zero.
Which pair are leptons?
A Proton, neutron
B Pion, kaon
C Proton, pion
D Electron, neutrino
Leptons include electron, muon, tau and their neutrinos. They do not participate in strong interaction, unlike hadrons such as protons and pions.
Which interaction changes flavor in beta decay?
A Strong
B Electromagnetic
C Weak
D Gravitational
Beta decay is a weak interaction process where neutron–proton conversion occurs through W boson exchange, changing quark flavor and violating parity.
Angular momentum conservation in decay includes?
A Spin plus orbital
B Only orbital
C Charge only
D Mass only
Total angular momentum includes intrinsic spin and orbital components of all particles. Nuclear transitions must satisfy angular momentum selection rules accordingly.
Parity operator mainly performs?
A Time reversal
B Charge reversal
C Space inversion
D Spin flip
Parity transforms spatial coordinates as (x,y,z)→(−x,−y,−z)(x,y,z)→(−x,−y,−z). If physics remains unchanged, parity is conserved; weak interaction breaks this symmetry.
CP symmetry means applying which operations together?
A P and T
B C and T
C T and spin
D C and P
CP combines charge conjugation (particle ↔ antiparticle) and parity (spatial inversion). Some weak processes show CP violation, important in particle physics.
CPT theorem suggests antiparticle and particle must have equal?
A Radius and density
B Mass and lifetime
C Charge and spin
D Binding and Q
CPT invariance implies particle and antiparticle share same mass and lifetime, and opposite charges. It is a deep consequence of relativistic quantum field theory assumptions.
Isospin projection distinguishes proton from neutron mainly by?
A Charge difference
B Mass difference
C Spin difference
D Size difference
Isospin treats proton and neutron as two states of one nucleon. Their isospin projections differ, matching their electric charge difference under emphasizing strong interaction similarity.
Strangeness is conserved in which interaction?
A Weak only
B Weak always
C Strong only
D Gravity only
Strangeness is conserved in strong interactions, explaining production of strange particles in pairs. In weak decays, strangeness can change, causing longer lifetimes.
Associated production of strange particles occurs because?
A Strangeness must vanish
B Strangeness conserved strongly
C Charge not conserved
D Parity always violated
Strong interactions conserve strangeness, so strange particles are produced with another particle carrying opposite strangeness, keeping total strangeness unchanged.
In quark model, strange quark charge is?
A −1/3e
B +2/3e
C +1e
D 0
Strange quark has electric charge −1/3e, like down quark. It contributes to strangeness quantum number and helps classify kaons and hyperons.
A kaon is classified as which hadron type?
A Baryon
B Lepton
C Meson
D Boson only
Kaons are mesons made of a quark and an antiquark, often involving strange quark. They have baryon number zero and participate in strong interactions.
Gell-Mann–Nishijima formula uses I3 and?
A Mass number
B Decay constant
C Binding energy
D Hypercharge
The relation Q=I3+Y/2Q=I3+Y/2 connects charge to isospin projection and hypercharge. Hypercharge includes baryon number and strangeness.
Hypercharge Y is commonly defined as?
A Z + N
B B + S
C A − Z
D Q − I3
For many hadrons, hypercharge is Y=B+SY=B+S, where B is baryon number and S is strangeness. It helps organize hadrons into SU(3) multiplets.
Pair annihilation of e⁻ and e⁺ usually produces?
A Alpha particles
B Neutrons
C Two gamma photons
D Neutrinos only
Electron and positron annihilate converting rest mass into energy. To conserve momentum, typically two gamma photons are produced in opposite directions.
Which unit measures equivalent dose for biological risk?
A Sievert
B Gray
C Curie
D Becquerel
Sievert accounts for biological effect by weighting absorbed dose (Gy) with radiation type and tissue sensitivity. It reflects risk better than gray alone.
Which radiation has highest ionizing power but least penetration?
A Gamma rays
B X-rays
C Beta particles
D Alpha particles
Alpha particles are heavy and doubly charged, producing dense ionization in short path. They are stopped by paper or skin but are dangerous if inhaled/ingested.
A neutron has magnetic moment because it has?
A Net charge
B Large radius
C Internal structure
D High mass
Neutron is neutral but composed of charged quarks. Their motion and spins create a net magnetic moment, showing neutron has internal substructure.
Shell model explains magic numbers mainly due to?
A Surface tension
B Energy gaps
C Coulomb term
D Neutron moderation
In shell model, nucleons occupy quantized levels. Large energy gaps after certain numbers produce closed shells, leading to extra stability and magic numbers.
Nuclear deformation mainly affects which observable strongly?
A Rotational levels
B Atomic spectra
C Electron affinity
D Chemical bonding
Deformed nuclei show rotational energy levels and characteristic spectra. These patterns support collective motion like rotation, beyond simple spherical shell predictions.
In fission of U-235, chain reaction is sustained using mainly?
A Alpha particles
B Positrons
C Photons
D Neutrons
Fission releases neutrons, which can induce further fission events. If enough neutrons cause new fissions, the reaction becomes self-sustaining.
Reactor coolant is mainly used to?
A Absorb neutrons
B Increase fission
C Remove heat
D Create gamma
Coolant carries away heat produced by fission to prevent overheating. It transfers thermal energy to steam generators or turbines for electricity production.
In beta minus decay, emitted antineutrino has lepton number?
A −1
B +1
C 0
D +2
Electron has lepton number +1. In β⁻ decay, electron is produced, so an antineutrino with lepton number −1 is emitted to keep total lepton number conserved.
A neutrino is electrically neutral, so it does NOT interact via?
A Weak force
B Electromagnetic
C Gravity
D Weak neutral
Neutrinos have no charge, so they do not undergo electromagnetic interactions. They interact mainly through weak force and gravity, making detection very difficult.
In particle classification, “resonance” particles are generally?
A Very long-lived
B Always stable
C Very short-lived
D Non-interacting
Resonance particles exist for extremely short times and appear as peaks in scattering cross-section versus energy. They decay rapidly into other particles.
Which interaction is responsible for most nuclear binding?
A Strong interaction
B Weak interaction
C Electromagnetic
D Gravitational
Nuclear binding arises from strong interaction between nucleons (residual strong force). It is much stronger than electromagnetic repulsion at short distances.
Which statement best fits “confinement” in QCD?
A Quarks free outside
B Quarks isolated easily
C Neutrinos hold quarks
D Hadrons hold quarks
Confinement means quarks cannot be isolated; they remain bound inside hadrons. Pulling quarks apart increases energy until new quark–antiquark pairs form.