Chapter 25: Nuclear Physics and Particle Physics (Set-5)
Two nuclei have radii ratio R2/R1=2R2/R1=2. What is A2/A1A2/A1?
A C. 8
B A. 2
C B. 4
D D. 16
Nuclear radius follows R∝A1/3R∝A1/3. So (R2/R1)3=A2/A1(R2/R1)3=A2/A1. With ratio 2, cube gives 8. This links size directly to mass number.
If nuclear density is nearly constant, which ratio stays nearly constant?
A A. R/AR/A
B C. R3/A2R3/A2
C B. A/R3A/R3
D D. Z/RZ/R
Constant density means mass (∝A) divided by volume (∝R³) is constant. Hence A/R3A/R3 stays nearly constant for nuclei, consistent with R∝A1/3R∝A1/3.
Which combination best supports “saturation” of nuclear forces?
A A. B∝A2B∝A2
B C. R∝AR∝A
C D. Density increases
D B. B/AB/A ~ constant
Saturation means each nucleon binds with only nearby nucleons, so total binding energy BB grows roughly proportional to A. That makes B/AB/A roughly constant for many nuclei.
A nucleus has T1/2=6T1/2=6 h. After 18 h, remaining fraction is?
A C. 1/8
B A. 1/2
C B. 1/4
D D. 1/16
18 h equals 3 half-lives. Remaining fraction is (1/2)3=1/8(1/2)3=1/8. This uses exponential decay property without needing decay constant explicitly.
Which is true for mean life τ and half-life T1/2T1/2?
A A. τ = T1/2T1/2
B C. τ > T1/2T1/2
C B. τ < T1/2T1/2
D D. τ = 2T1/2T1/2
T1/2=0.693/λT1/2=0.693/λ and τ=1/λτ=1/λ. Hence τ=T1/2/0.693≈1.44T1/2τ=T1/2/0.693≈1.44T1/2. So mean life is always greater than half-life.
In α decay, why does α particle get most kinetic energy?
A A. Higher charge
B C. Larger radius
C B. Smaller mass
D D. Lower binding
Momentum conservation gives equal and opposite momenta to α and daughter nucleus. Kinetic energy K=p2/2mK=p2/2m is larger for smaller mass, so α gets most energy.
In β⁺ decay, why is electron capture often preferred when energy is low?
A B. Positron needs energy
B A. Needs photons
C C. Proton disappears
D D. Neutron emitted
β⁺ decay must create a positron, requiring at least 2mec22mec2 energy difference (atomic masses). If insufficient, nucleus can still reduce Z by capturing an inner electron.
Continuous β spectrum mainly implies existence of?
A A. Proton recoil only
B C. Nuclear rotation
C D. Pairing term
D B. Neutrino particle
If only electron and nucleus were produced, electron energy would be fixed. The observed continuous spectrum is explained by a neutrino sharing energy and momentum with the electron.
For reaction a+A→b+Ba+A→b+B, which is always conserved?
A A. Rest mass
B B. Binding energy
C C. Charge
D D. Packing fraction
Electric charge conservation is strict in all known interactions. Nuclear reactions can change rest mass and binding energy, but total charge before and after must remain equal.
A reaction has Q = −2 MeV. Which statement is correct?
A B. Needs ≥2 MeV input
B A. Releases 2 MeV
C C. Always forbidden
D D. Always spontaneous
Negative Q means the reaction absorbs energy. At least ∣Q∣∣Q∣ must be supplied, and the actual threshold projectile energy is typically higher because some energy must satisfy momentum conservation and recoil.
Which condition best defines “supercritical” reactor?
A A. k = 1
B B. k < 1
C C. k > 1
D D. k = 0
Multiplication factor k > 1 means each neutron generation produces more neutrons than the previous one. The chain reaction grows, increasing reactor power unless controlled.
Why are moderators usually light nuclei like H or D?
A A. High charge
B C. Efficient energy transfer
C B. Large neutron loss
D D. Strong absorption
In elastic collisions, maximum energy transfer occurs when masses are comparable. Light nuclei slow neutrons effectively with fewer collisions, without absorbing too many neutrons.
In a GM counter, dead time causes measured count rate to be?
A B. Lower than true
B A. Higher than true
C C. Exactly equal
D D. Negative value
During dead time, the detector cannot record events. Some particles arrive but are missed, so observed count rate is less than the actual rate, especially at high intensities.
Best reason semiconductor detectors have high resolution is?
A A. No dead time
B B. Large charge yield
C C. Small statistical spread
D D. Strong magnetic field
Semiconductor detectors create many charge carriers per unit energy with low fluctuations, giving narrow peaks. This reduces statistical noise and improves energy resolution for spectroscopy.
Why does cyclotron fail at very high energies for ions?
A A. Magnetic field disappears
B B. Frequency mismatch
C C. Charge becomes zero
D D. Mass becomes zero
At relativistic speeds, effective mass increases and particle takes longer per revolution. Cyclotron’s fixed-frequency electric field no longer stays in step, limiting energy gain.
In particle classification, which has integer spin and is a hadron?
A A. Proton
B C. Electron
C D. Neutrino
D B. Pion
Pion is a meson (hadron) made of quark–antiquark and has spin 0 (integer). Proton is baryon with spin 1/2, while electron and neutrino are leptons.
Which interaction conserves strangeness but not necessarily isospin perfectly?
A B. Electromagnetic
B C. Weak
C A. Strong
D D. Gravity
Strong interaction conserves strangeness and approximately conserves isospin. Electromagnetic interaction can break isospin due to charge differences, and weak changes strangeness.
Which process can change strangeness by ±1?
A C. Weak decay
B A. Strong scattering
C B. Electromagnetic decay
D D. Elastic collision
Weak interaction allows flavor change, so strangeness is not conserved and can change by ±1. That is why strange particles decay weakly and live longer.
A meson has quark content usˉusˉ. Its strangeness is?
A B. 0
B A. +1
C C. −1
D D. +2
Strange quark s has S = −1, so anti-strange sˉsˉ has S = +1. The meson usˉusˉ therefore carries strangeness +1.
For proton, using Q=I3+Y/2Q=I3+Y/2, with Q=+1Q=+1 and I3=+1/2I3=+1/2, Y is?
A A. 0
B C. 2
C B. 1
D D. −1
Rearranging gives Y=2(Q−I3)=2(1−1/2)=1Y=2(Q−I3)=2(1−1/2)=1. This matches proton hypercharge used in hadron multiplet classification.
Hypercharge Y for a baryon with B=1 and S=−1 equals?
A A. 0
B B. 1
C C. 2
D D. −1
Hypercharge is Y=B+SY=B+S for many hadrons in this scheme. With B=1 and S=−1, Y=0Y=0. This helps determine charge via Gell-Mann–Nishijima.
If CPT holds, which must match for particle and antiparticle?
A A. Same charge
B C. Same baryon number
C B. Same mass
D D. Same lepton number
CPT invariance implies particle and antiparticle have identical mass and lifetime, while internal quantum numbers like charge, baryon number, and lepton number reverse sign.
Parity violation is most directly indicated by observing?
A B. Left-right asymmetry
B A. Energy change
C C. Mass difference
D D. Charge conservation
If a process behaves differently under mirror reflection, it shows left-right asymmetry, which is direct evidence of parity violation seen in weak decays of polarized nuclei.
A neutrino being left-handed means its spin is?
A A. Along momentum
B B. Opposite momentum
C C. Zero always
D D. Random direction
Left-handed helicity means spin projection is opposite to momentum direction. Weak interaction couples mainly to left-handed neutrinos, explaining strong parity violation.
Why was “color” introduced in quark model?
A A. Explain gravity
B C. Explain beta decay
C D. Explain half-life
D B. Avoid Pauli violation
Some baryons need identical quarks in same state. Color adds an extra quantum number so overall wavefunction becomes antisymmetric, satisfying Pauli exclusion principle for fermionic quarks.
Which statement best matches confinement?
A A. Quarks free at low energy
B C. Gluons uncharged always
C B. Quarks never isolated
D D. Leptons confined too
Confinement means quarks cannot exist alone as free particles. Attempting separation increases energy until new hadrons form, so only color-neutral hadrons are observed.
In annihilation e−+e+e−+e+, why two γ photons are common?
A B. Conserve momentum
B A. Save charge
C C. Increase mass
D D. Reduce energy
With initial system nearly at rest, one photon cannot conserve momentum. Two photons emitted in opposite directions conserve momentum and energy, making two-gamma annihilation typical.
A radiation unit Gy measures?
A A. Decays per second
B B. Energy per mass
C C. Biological weighting
D D. Electric charge
Gray is absorbed dose: energy deposited per unit mass (J/kg). It does not include biological effectiveness; that is accounted by sievert using weighting factors.
Which radiation gives highest dose equivalent for same absorbed dose?
A C. Alpha
B A. Gamma
C B. Beta
D D. X-rays
Alpha particles have high linear energy transfer and cause dense ionization. For same absorbed dose (Gy), their biological effect is higher, giving larger equivalent dose (Sv).
A “resonance” in scattering indicates?
A A. No interaction
B C. Constant cross section
C B. Temporary state formed
D D. Zero momentum
Resonance peaks occur when incoming energy matches an excited compound state. The intermediate state exists briefly and decays, producing sharp increase in cross section at that energy.
Shell model predicts extra stability mainly due to?
A A. Surface tension
B C. Coulomb term
C D. Neutron moderation
D B. Closed shells
When proton or neutron shells are filled, nuclei are especially stable (magic numbers). Large energy gaps reduce likelihood of excitation, decay, or deformation.
Liquid drop model best explains which feature?
A B. Average fission behavior
B A. Magic numbers
C C. Quark confinement
D D. Neutrino helicity
Liquid drop model treats nucleus as charged liquid with surface tension. It explains general fission trends, deformation, and energy considerations, though it cannot explain shell effects well.
Which term in SEMF increases binding for even-even nuclei most?
A B. Coulomb term
B C. Surface term
C A. Pairing term
D D. Asymmetry term
Pairing term adds extra binding for nuclei with both Z and N even, because nucleons form pairs with opposite spins. Odd-odd nuclei get reduced binding.
A nucleus far above stability line usually undergoes?
A B. β⁺ decay
B A. β⁻ decay
C C. α decay only
D D. γ decay only
Above the stability line means neutron-rich. β⁻ decay converts neutron to proton, reducing N/Z ratio and moving nucleus toward stable region.
A nucleus far below stability line usually undergoes?
A B. β⁺ decay
B A. β⁻ decay
C C. Neutron emission
D D. Gamma emission
Below stability line means proton-rich. β⁺ decay or electron capture converts proton to neutron, increasing N/Z ratio and moving nucleus toward stability.
In β⁻ decay, which conservation is checked using neutrino?
A A. Charge only
B C. Mass number
C B. Lepton number
D D. Nuclear radius
Electron has lepton number +1. To conserve total lepton number, an antineutrino with −1 is emitted. This bookkeeping is essential in weak interaction processes.
Which particle is a lepton but not charged?
A A. Proton
B B. Neutrino
C C. Pion
D D. Kaon
Neutrinos are leptons with zero electric charge and tiny mass. They interact only weakly, making them difficult to detect and important in beta decay and astrophysics.
If activity is 800 Bq, number of decays per second is?
A B. 800
B A. 80
C C. 8000
D D. 8
Becquerel equals one decay per second. So 800 Bq means 800 nuclear decays occur each second. This unit measures activity directly, not energy or dose.
Which instrument directly measures charge collected from ion pairs?
A B. GM counter
B C. Bubble chamber
C A. Ionization chamber
D D. Cloud chamber
Ionization chamber measures small current due to ion pairs created by radiation. It has no gas multiplication, giving stable output useful for dose rate measurements.
In scintillation detector, light output is converted to signal by?
A A. Control rod
B C. Cyclotron magnet
C D. Moderator block
D B. Photomultiplier
Scintillator emits light flashes. Photomultiplier tube converts light into electrons and multiplies them, producing measurable electrical pulses proportional to deposited energy.
Which is most accurate statement about weak interaction parity?
A C. Violated in many cases
B A. Always conserved
C B. Always violated
D D. Undefined quantity
Weak interaction can violate parity, shown in beta decay and neutrino helicity. Strong and electromagnetic interactions conserve parity to very high accuracy.
CP symmetry violation does NOT automatically imply violation of?
A A. C symmetry
B C. CPT symmetry
C B. P symmetry
D D. Weak interaction
CP violation can occur while CPT remains conserved. CPT theorem is more fundamental; experiments strongly support CPT invariance even when CP is violated in weak interactions.
Which force mediator is electrically neutral and massless?
A B. Gluon
B C. W boson
C A. Photon
D D. Z boson
Photon has zero rest mass and no electric charge, mediating electromagnetic force with infinite range. Gluons are massless but carry color; W and Z are massive.
Which mediator carries color charge and causes confinement effects?
A B. Gluon
B A. Photon
C C. Neutrino
D D. Higgs
Gluons carry color charge and self-interact, unlike photons. This property leads to confinement and the strong binding of quarks inside hadrons.
A hadron with three quarks must be classified as?
A A. Meson
B C. Lepton
C D. Boson only
D B. Baryon
Three-quark combinations form baryons with baryon number +1. Mesons are quark–antiquark pairs with baryon number zero.
Which combination is consistent with baryon number conservation?
A A. p → e⁺ + π⁰
B B. n → p + e⁻ + ν̄
C C. p → γ + γ
D D. e⁻ → ν + γ
In β⁻ decay, baryon number remains 1 on both sides (neutron and proton). The other options either violate baryon number or represent impossible processes.
In a fission reactor, why are control rods inserted deeper?
A A. Increase fission
B C. Increase moderation
C B. Absorb neutrons
D D. Create fuel
Control rods absorb neutrons, reducing the number available for further fission. Deeper insertion lowers multiplication factor, controlling power and preventing runaway chain reaction.
In SEMF, why does volume term increase binding with A?
A B. More neighbor interactions
B A. More surface area
C C. More Coulomb repulsion
D D. More electron shielding
Volume term assumes each nucleon contributes nearly constant binding due to short-range nuclear forces with nearby nucleons. As A increases, total binding rises roughly proportional to A.
Which decay can occur without changing nucleus composition but changing its energy state?
A A. Alpha decay
B C. Beta decay
C B. Gamma decay
D D. Electron capture
Gamma decay is de-excitation of a nucleus. It emits a photon to lose excess energy while A and Z remain unchanged, only nuclear energy level changes.
A nucleus undergoing β⁺ decay must reduce Z. Which conservation remains strict?
A A. Parity
B B. Lepton number
C C. Strangeness
D D. Isospin
In β⁺ decay, a positron (lepton number −1) and neutrino (+1) are emitted so total lepton number remains conserved. Parity may be violated in weak processes.