Chapter 25: Nuclear Physics and Particle Physics (Set-4)
A nucleus has radius 4.8 fm. What is the approximate mass number A?
A 64
B 27
C 125
D 216
Use R=R0A1/3R=R0A1/3 with R0≈1.2R0≈1.2 fm. Then A1/3=4.8/1.2=4A1/3=4.8/1.2=4, so A=43=64A=43=64. This radius–mass scaling is a standard nuclear property.
Which nucleus is most likely to have spin 1/2?
A Even-even nucleus
B Odd-odd nucleus
C Odd-A nucleus
D Doubly magic nucleus
Odd-A nuclei have one unpaired nucleon. Its angular momentum often sets the total nuclear spin, commonly producing half-integer spins like 1/2, 3/2, etc.
Why does binding energy per nucleon decrease for very heavy nuclei?
A Nuclear force weakens
B Neutron mass decreases
C Radius becomes constant
D Coulomb repulsion grows
As proton number increases, repulsive Coulomb energy rises strongly and reduces net binding per nucleon. This makes heavy nuclei less stable and more likely to undergo fission.
Which term in mass formula explains extra stability near N≈Z for light nuclei?
A Coulomb term
B Surface term
C Asymmetry term
D Pairing term
Asymmetry term penalizes imbalance between neutrons and protons. Light nuclei are most stable when N≈Z, giving smaller asymmetry penalty and higher binding energy.
A beta-minus decay changes neutron number N by?
A Increases by 1
B Decreases by 1
C No change
D Doubles N
In β⁻ decay, a neutron converts to a proton, so neutron number decreases by 1 while proton number increases by 1. Total mass number stays unchanged.
For a radioactive sample, if λ doubles, half-life becomes?
A Halves
B Doubles
C Same
D Becomes zero
Half-life T1/2=ln2/λT1/2=ln2/λ. If decay constant doubles, half-life becomes half. Higher λ means faster decay and quicker reduction of nuclei.
In alpha decay, Q-value appears mainly as?
A Gamma energy
B Rest mass increase
C Kinetic energies
D Electron binding
Q-value becomes kinetic energy shared by alpha particle and daughter nucleus. Due to momentum conservation, alpha gets most kinetic energy because it is much lighter.
Why is gamma emission often preceded by other decays?
A Changes Z directly
B Reduces mass number
C Creates neutrinos
D Removes excitation energy
After alpha or beta decay, the daughter nucleus can be left in excited state. Gamma emission releases this extra energy without changing A or Z.
Which conservation law helps explain continuous beta spectrum?
A Energy conservation
B Charge conservation
C Lepton number
D Parity conservation
Energy is shared among electron, neutrino, and recoil nucleus. Because neutrino carries varying energy, electron energy varies continuously while total energy remains conserved.
A reaction a+A→b+Ba+A→b+B has negative Q. Threshold depends on?
A Only Q-value
B Both masses and Q
C Mass ratio
D Nuclear radius
Threshold energy arises from momentum conservation and energy requirement. It depends not just on Q but also on masses of target and products, affecting recoil and kinetic distribution.
In reactor, moderation increases fission chance mainly by increasing?
A Neutron energy
B Proton number
C Neutron capture probability
D Gamma intensity
Thermal neutrons are captured more easily by fissile nuclei like U-235, raising fission cross section. Moderation slows neutrons, improving probability of fission.
Why does a GM counter not measure particle energy accurately?
A Same pulse size
B No gas inside
C No electric field
D No dead time
In GM region, avalanche spreads through tube producing nearly identical large pulses for different energies. Hence it counts events but provides poor energy information.
Which detector gives pulse height proportional to energy deposited?
A Cloud chamber
B Proportional counter
C GM tube
D Bubble chamber
Proportional counters operate in region where gas multiplication is proportional to primary ionization. Thus pulse height relates to energy deposited, unlike GM tube.
Energy resolution is best defined as ability to?
A Detect low activity
B Increase count rate
C Separate close energies
D Reduce dead time
High energy resolution means detector can distinguish two nearby radiation energies as separate peaks. Semiconductor detectors typically offer very good energy resolution.
Electrons reach relativistic speeds quickly due to small mass. Their effective mass changes, breaking resonance with fixed cyclotron frequency, making acceleration inefficient.
A “generation” in leptons refers to groups like?
A e, μ, τ families
B Proton-neutron pair
C Pion-kaon family
D Photon-gluon pair
Leptons occur in three generations: (electron, electron neutrino), (muon, muon neutrino), and (tau, tau neutrino), each with increasing mass.
Which pair are baryons?
A Electron, muon
B Pion, kaon
C Proton, neutron
D Photon, gluon
Baryons are hadrons with three quarks and baryon number +1. Proton and neutron are the simplest baryons forming atomic nuclei.
Which statement best distinguishes bosons from fermions?
A Bosons have charge
B Fermions have integer spin
C Bosons have integer spin
D Fermions mediate forces
Bosons have integer spin and can share quantum states, often acting as force carriers. Fermions have half-integer spin and obey Pauli exclusion principle.
Parity conservation is strongly valid in which process?
A Beta decay
B Strong interaction scattering
C Neutrino emission
D Muon decay
Strong interaction conserves parity very well. Weak processes like beta decay violate parity, which is why left-right asymmetry appears in weak decays.
CP violation is observed mainly in?
A Strong interaction
B Gravitational interaction
C Classical mechanics
D Weak interaction
CP violation occurs in certain weak decays, notably in neutral kaon and B-meson systems. It is important for explaining matter–antimatter imbalance.
If baryon number is conserved, proton decay would be?
A Forbidden normally
B Allowed always
C Fast process
D Electromagnetic
Proton decay would violate baryon number conservation. In the Standard Model, baryon number is effectively conserved, so proton is extremely stable experimentally.
In β⁻ decay, which particle ensures lepton number conservation?
A Photon
B Proton
C Antineutrino
D Alpha particle
β⁻ decay produces an electron with lepton number +1. To conserve total lepton number, an antineutrino with lepton number −1 is emitted.
Isospin treats nucleons similarly under which interaction?
A Weak interaction
B Strong interaction
C Electromagnetic
D Gravity
Strong interaction acts nearly the same on proton and neutron. Isospin symmetry models them as two states of nucleon, useful in classifying nuclear and hadron reactions.
Which particle is “strange” because it contains s-quark?
A Proton
B Electron
C Photon
D Kaon
Kaons contain a strange quark or antiquark, giving nonzero strangeness. They are produced strongly but decay weakly, explaining longer lifetimes.
In strong interactions, strangeness in reactions is typically?
A Not conserved
B Always decreases
C Conserved
D Always increases
Strangeness is conserved in strong interactions. Therefore strange particles are often created in pairs so total strangeness remains unchanged in production processes.
The quantity I3I3 in Gell-Mann–Nishijima is?
A Isospin projection
B Mass number
C Decay constant
D Binding energy
I3I3 is the third component of isospin, distinguishing members in an isospin multiplet. It helps calculate electric charge using hypercharge relation.
Using Q=I3+Y/2Q=I3+Y/2, proton has I3=+1/2I3=+1/2 and Y=1. Q equals?
A 0
B 2
C -1
D 1
Substitute I3=+1/2I3=+1/2 and Y=1Y=1. Then Q=1/2+1/2=1Q=1/2+1/2=1. This matches proton charge +1e, confirming the scheme.
CPT invariance implies reversing C, P, and T gives?
A Different physics
B Same physics
C Only weak changes
D Only strong changes
CPT theorem states combined transformation of charge conjugation, parity inversion, and time reversal leaves fundamental laws unchanged in relativistic quantum field theory.
Helicity refers to direction of?
A Charge flow
B Mass change
C Spin along motion
D Energy loss
Helicity is projection of particle’s spin along its momentum direction. Neutrinos are observed mainly left-handed, linking helicity to parity violation in weak interactions.
V–A theory in weak interactions implies weak currents are?
A Vector minus axial
B Vector only
C Axial only
D Scalar only
Weak interaction has V–A structure, meaning it couples to left-handed particles strongly. This explains parity violation and observed handedness of neutrinos.
Quark model assigns baryons as combinations of?
A Two quarks
B One quark
C Three quarks
D Four quarks
Baryons are made of three quarks, giving baryon number +1. Antibaryons are made of three antiquarks. Mesons are quark–antiquark pairs.
Which property is introduced to satisfy Pauli principle in baryons?
A Electric charge
B Color charge
C Nuclear radius
D Hypercharge
Color charge allows quarks to exist in different quantum states even if flavors and spins match, ensuring baryon wavefunction respects Pauli principle.
Confinement implies attempting to separate quarks results in?
A New hadrons form
B Force decreases
C Quarks become free
D Charge vanishes
Pulling quarks apart increases energy until a new quark–antiquark pair forms, creating new hadrons. This prevents observation of isolated quarks.
Which reaction is an example of pair production?
A e⁻ emits photon
B p → n + e⁺
C γ → e⁻ e⁺
D α → γ + γ
High-energy gamma photon can convert into an electron–positron pair near a nucleus to conserve momentum. Minimum energy required is 2mec22mec2.
In annihilation, electron–positron rest mass converts mainly into?
A Alpha particles
B Neutrons
C Protons
D Gamma photons
In e⁻–e⁺ annihilation, mass-energy converts to photons. Typically two gamma rays are produced in opposite directions to conserve momentum.
Which unit corresponds to “decays per second”?
A Gray
B Becquerel
C Sievert
D Coulomb
Becquerel (Bq) measures activity as one decay per second. It describes how quickly a radioactive sample disintegrates, independent of biological effects.
Equivalent dose differs from absorbed dose because it includes?
A Mass number
B Nuclear radius
C Radiation weighting
D Electron density
Absorbed dose (Gy) is energy per mass. Equivalent dose (Sv) multiplies absorbed dose by radiation weighting factor to reflect biological damage from different radiations.
Which detector visually shows tracks by condensation of vapor?
A Cloud chamber
B GM counter
C Scintillator
D Semiconductor
Cloud chamber contains supersaturated vapor that condenses along ionization tracks of charged particles, allowing visual observation of paths and curvature in magnetic fields.
Bubble chamber works best for observing particles in?
A Solid crystals
B Vacuum gas
C Semiconductor slab
D Superheated liquid
Bubble chamber uses superheated liquid; ionizing particles create bubble trails. It provides detailed track images and helped discover many short-lived particles.
Nuclear scattering cross-section is measured typically in?
A Tesla
B Joule
C Barn
D Coulomb
Cross section is an effective area representing interaction probability. Unit barn equals 10−28 m210−28m2, commonly used in nuclear and particle scattering data.
A “resonance peak” in cross section indicates formation of?
A Stable nucleus
B Intermediate state
C Electron cloud
D Magnetic domain
Resonance peak occurs when projectile energy matches an excited state of compound nucleus or resonance particle, temporarily formed and then decaying into products.
Why do many nuclei undergo beta decay instead of alpha?
A To change A quickly
B To emit gamma always
C To adjust N/Z ratio
D To reduce radius
Beta decay changes neutron-to-proton ratio without changing mass number. It moves nucleus toward stability line when it has too many neutrons or too many protons.
In β⁺ decay, why must nucleus have enough energy?
A Positron mass creation
B Neutron binding
C Gamma absorption
D Proton disappears
β⁺ decay creates a positron, requiring at least 2mec22mec2 energy difference between parent and daughter atoms (considering atomic electrons). Otherwise electron capture occurs.
Semi-empirical formula “surface term” exists because?
A Protons repel less
B Neutrons are charged
C Shell gaps vanish
D Surface nucleons fewer neighbors
Nucleons on surface have fewer nearby nucleons to bond with, reducing binding energy. Hence surface term subtracts binding proportional to surface area.
Which nuclear model best explains fission barrier concept?
A Shell model
B Quark model
C Liquid drop model
D Bohr model
Liquid drop model describes nucleus deformation and competition between surface tension and Coulomb repulsion. It naturally explains fission barrier and why heavy nuclei can split.
Which interaction conserves parity and strangeness both?
A Strong interaction
B Weak interaction
C Electromagnetic
D Beta interaction
Strong interaction conserves parity and strangeness. Weak interaction can violate parity and change strangeness, which is why strange particle decays are weak and slower.
Which particle is a mediator of electromagnetic force?
A Gluon
B Photon
C W boson
D Neutron
Photon is gauge boson of electromagnetic interaction. It mediates forces between charged particles and is massless, allowing long-range electromagnetic effects.
Which particle mediates strong interaction between quarks?
A Photon
B Neutrino
C Gluon
D Electron
Gluons carry color charge and mediate strong force between quarks in quantum chromodynamics. They keep quarks confined inside hadrons.
Weak interaction mediator responsible for beta decay is mainly?
A W boson
B Photon
C Gluon
D Higgs boson
Beta decay occurs via exchange of W bosons, converting quark flavor and producing leptons. This explains neutron-proton conversion and parity violation in weak processes.
A key difference between strong and weak interaction is that weak interaction?
A Conserves strangeness always
B Has infinite range
C Has strongest force
D Violates parity
Weak interaction violates parity, showing left-right asymmetry in processes like beta decay. Strong interaction conserves parity and is much stronger but short-ranged at nuclear scale