Chapter 25: Nuclear Physics and Particle Physics (Set-1)
What mainly determines an approximate nuclear radius?
A Number of neutrons
B Atomic number Z
C Mass number A
D Electron shell size
Nuclear radius follows R=R0A1/3R=R0A1/3. This shows nuclei grow slowly with mass number because nucleons pack in a roughly constant-density volume.
Which relation best describes nuclear radius scaling?
A R∝AR∝A
B R∝A2/3R∝A2/3
C R∝Z1/3R∝Z1/3
D R∝A1/3R∝A1/3
The nucleus behaves like nearly incompressible matter with almost constant density, so volume ∝A∝A. Therefore radius ∝A1/3∝A1/3.
Nuclear density is nearly constant because of which property?
A Saturation of forces
B Electron shielding
C Coulomb repulsion
D Large nuclear size
Nuclear forces are short-range and saturating, so each nucleon interacts strongly with only nearby nucleons. This makes density roughly constant across different nuclei.
What is the mass defect of a nucleus?
A Electron mass loss
B Missing nucleon count
C Mass difference value
D Neutron-proton ratio
Mass defect is the difference between the sum of free nucleon masses and the actual nuclear mass. This “missing mass” corresponds to binding energy via E=mc2E=mc2.
Binding energy represents which physical meaning?
A Coulomb potential only
B Energy to separate nucleus
C Energy to add electron
D Energy to ionize atom
Binding energy is the energy required to completely separate a nucleus into free protons and neutrons. Larger binding energy means greater nuclear stability.
Which quantity best indicates nuclear stability?
A Binding energy per nucleon
B Atomic radius
C Packing fraction
D Electron affinity
Binding energy per nucleon compares stability across nuclei. It peaks near iron, showing intermediate-mass nuclei are most tightly bound and stable.
A nucleus with same Z but different A is called?
A Isobar
B Isotone
C Isotope
D Isomer
Isotopes have the same atomic number (same protons) but different mass number due to different neutrons. Chemical properties stay similar, nuclear properties differ.
Nuclei with same A but different Z are called?
A Isotopes
B Isotones
C Leptons
D Isobars
Isobars share the same mass number AA but have different proton numbers ZZ. They are different elements but have equal total nucleons.
Nuclei with same neutron number N are called?
A Isotones
B Isobars
C Isotopes
D Isoelectronic
Isotones have the same number of neutrons NN but different ZZ. They can show similar nuclear trends linked to neutron shell filling.
The “stability line” mainly relates which two numbers?
A Z and A
B A and density
C N and Z
D N and radius
The stability line plots stable nuclei in neutron number NN versus proton number ZZ. Heavy stable nuclei require more neutrons to reduce Coulomb repulsion.
What is nuclear spin mainly due to?
A Electron spin
B Orbital electrons only
C Nuclear radius changes
D Nucleon angular momenta
Nuclear spin comes from the vector sum of intrinsic spins and orbital angular momenta of protons and neutrons. Pairing often cancels contributions in even-even nuclei.
Q-value of a nuclear reaction represents?
A Energy released/absorbed
B Reaction cross section
C Neutron moderation rate
D Half-life duration
Q-value equals the change in rest-mass energy: Q=(minitial−mfinal)c2Q=(minitial−mfinal)c2. Positive Q releases energy; negative Q requires energy input.
Which model treats nucleus like a drop of fluid?
A Shell model
B Quark model
C Liquid drop model
D Bohr atom model
The liquid drop model explains nuclear binding and fission by assuming nuclear matter behaves like an incompressible fluid with surface tension and Coulomb effects.
Magic numbers are explained well by which model?
A Liquid drop model
B Shell model
C Classical gas model
D Rutherford model
The shell model predicts extra stability at certain proton or neutron numbers (magic numbers) due to closed shells, similar to electron shells in atoms.
Which set is a common magic number list?
A 1, 2, 3, 4
B 5, 10, 15, 25
C 2, 8, 20, 28
D 7, 14, 21, 35
Magic numbers like 2, 8, 20, 28, 50, 82, 126 correspond to closed nucleon shells with higher stability, supported by nuclear spectra and abundances.
Semi-empirical mass formula mainly estimates?
A Electron binding energy
B Atomic radius
C Photon energy
D Nuclear binding energy
The semi-empirical mass formula (Weizsäcker formula) estimates nuclear binding energy using volume, surface, Coulomb, asymmetry, and pairing terms.
Which term accounts for odd-even stability in nuclei?
A Pairing term
B Surface term
C Coulomb term
D Asymmetry term
The pairing term adds extra binding for even-even nuclei and reduces it for odd-odd nuclei, reflecting the tendency of nucleons to form stable pairs.
Binding energy per nucleon is highest near which region?
A Very light nuclei
B Very heavy nuclei
C Around iron region
D Around uranium only
The binding energy per nucleon peaks around A≈56A≈56 (iron/nickel region). This explains why fusion releases energy for light nuclei and fission for heavy nuclei.
Nuclear fission means?
A Two nuclei combine
B Heavy nucleus splits
C Nucleus emits electron
D Proton changes to neutron
Fission is the splitting of a heavy nucleus into two medium nuclei plus neutrons and energy. It is driven by competition between surface tension and Coulomb repulsion.
Nuclear fusion means?
A Heavy nucleus splits
B Gamma emission only
C Light nuclei combine
D Electron capture process
Fusion combines light nuclei into a heavier nucleus. Because binding energy per nucleon increases for light nuclei, fusion can release large energy, requiring high temperature to overcome repulsion.
Radioactive decay law states activity decreases how?
A Exponentially with time
B Linearly with time
C As square of time
D Randomly increasing
Radioactive decay follows N=N0e−λtN=N0e−λt. The probability of decay per nucleus per unit time remains constant, giving an exponential decrease.
Half-life is the time for?
A All nuclei to decay
B Activity to double
C N to become half
D Mass to become zero
Half-life T1/2T1/2 is the time for the number of undecayed nuclei to drop to half. It relates to decay constant by T1/2=ln2/λT1/2=ln2/λ.
Mean life ττ relates to λλ as?
A τ=λτ=λ
B τ=ln2/λτ=ln2/λ
C τ=λ2τ=λ2
D τ=1/λτ=1/λ
Mean life is the average lifetime of a radioactive nucleus and equals 1/λ1/λ. Half-life is shorter: T1/2=0.693/λT1/2=0.693/λ.
Alpha decay emits?
A Electron
B Positron
C Helium nucleus
D Gamma photon
Alpha decay emits an αα particle, which is a 4He4He nucleus (2 protons + 2 neutrons). It reduces mass number by 4 and atomic number by 2.
Beta minus decay converts?
A Neutron to proton
B Proton to neutron
C Proton to alpha
D Neutron to alpha
In β−β− decay, a neutron changes into a proton, emitting an electron and an antineutrino. Atomic number increases by 1, mass number stays same.
Beta plus decay emits?
A Electron and neutrino
B Alpha particle only
C Positron and neutrino
D Gamma photon only
In β+β+ decay, a proton converts to a neutron, emitting a positron and a neutrino. Atomic number decreases by 1, mass number remains unchanged.
Electron capture means nucleus captures a(n)?
A Proton
B Inner electron
C Neutron
D Gamma photon
In electron capture, the nucleus captures an orbital electron (often K-shell). A proton turns into a neutron and a neutrino is emitted, lowering atomic number by 1.
Gamma decay changes which nuclear numbers?
A A and Z unchanged
B A decreases
C Z decreases
D A increases
Gamma decay is de-excitation of a nucleus by emitting a gamma photon. It changes energy state but does not change mass number AA or atomic number ZZ.
SI unit of activity is?
A Curie
B Gray
C Becquerel
D Sievert
Activity is decays per second. The SI unit is becquerel (Bq), where 1 Bq=11Bq=1 decay per second. Curie is an older large unit.
1 Curie corresponds approximately to?
A 3.7×1063.7×106 Bq
B 3.7×1033.7×103 Bq
C 3.7×10123.7×1012 Bq
D 3.7×10103.7×1010 Bq
Curie (Ci) was defined from radium activity. Numerically, 1 Ci≈3.7×10101Ci≈3.7×1010 disintegrations per second, i.e., becquerels.
In reaction notation ZAXZAX, A means?
A Proton number
B Neutron number
C Mass number
D Atomic radius
AA is mass number, total nucleons (protons + neutrons). ZZ is atomic number (protons). Neutron number is N=A−ZN=A−Z.
Conservation laws in nuclear reactions include?
A Energy and momentum
B Mass alone
C Charge alone
D Color charge only
Nuclear reactions obey conservation of energy, momentum, angular momentum, and charge. Rest mass may change, but total energy including mc2mc2 is conserved.
Threshold energy is relevant when Q-value is?
A Positive
B Zero
C Negative
D Infinite
If Q is negative, the reaction needs a minimum kinetic energy to occur. This minimum is called threshold energy and depends on masses and reaction kinematics.
Cross section in nuclear physics indicates?
A Detector voltage
B Nuclear radius only
C Half-life measure
D Reaction probability
Cross section measures likelihood of a nuclear interaction, like scattering or capture. Larger cross section means higher probability of reaction for incident particles.
A chain reaction in fission needs?
A Fast electrons
B Self-sustaining neutrons
C Gamma-only emission
D High magnetic field
In fission, emitted neutrons can cause further fissions. A chain reaction occurs when, on average, at least one neutron from each fission causes another fission.
Neutron moderator is used to?
A Increase neutron energy
B Absorb all neutrons
C Slow down neutrons
D Create gamma rays
Moderators like graphite or heavy water slow fast neutrons to thermal energies, increasing the chance of fission in fuels like U-235.
Control rods in a reactor mainly do?
A Reflect neutrons
B Cool the core
C Increase fuel mass
D Absorb neutrons
Control rods (boron, cadmium) absorb neutrons to regulate the fission rate. Inserting rods reduces reactivity and power; withdrawing increases it.
A Geiger–Müller counter detects radiation by?
A Crystal diffraction
B Magnetic resonance
C Gas ionization pulses
D Thermal expansion
In a GM tube, incoming radiation ionizes gas and triggers a strong discharge, producing pulses counted as events. It is good for detection, not precise energy measurement.
A scintillation detector works using?
A Light flashes in material
B Gas expansion bubbles
C Cloud droplet tracks
D Electric spark gaps
Scintillators emit tiny light flashes when radiation deposits energy. A photomultiplier or photodiode converts light into electrical signals, allowing counting and energy estimation.
Semiconductor detectors are valued for high?
A Smoke production
B Water moderation
C Energy resolution
D Neutron reflection
Semiconductor detectors (like Si, Ge) produce charge carriers proportional to deposited energy, giving excellent energy resolution for spectroscopy compared to GM counters.
Cyclotron accelerates charged particles using?
A Constant electric field
B Alternating electric field
C Strong gravity field
D Laser cooling only
A cyclotron uses a perpendicular magnetic field for circular motion and an alternating electric field across the gap to accelerate particles each time they cross.
Van de Graaff accelerator mainly provides?
A Very low pressure
B High nuclear density
C Neutron moderation
D Very high voltage
A Van de Graaff generator builds a large electrostatic potential using a moving belt, accelerating charged particles through high voltage, useful for early nuclear experiments.
Leptons include which particle?
A Electron
B Proton
C Pion
D Neutron
Leptons are fundamental particles not affected by strong interaction, such as electron, muon, tau, and their neutrinos. Protons and pions are hadrons.
Hadrons are particles that feel?
A Only gravity
B Only weak force
C Strong interaction
D Only electromagnetic
Hadrons participate in strong interactions and are made of quarks. They include baryons (like proton, neutron) and mesons (like pions, kaons).
Baryons are hadrons made of?
A Three quarks
B One quark
C Two quarks
D Quark-gluon plasma
Baryons consist of three quarks (or three antiquarks for antibaryons). Example: proton (uud) and neutron (udd). This explains baryon number assignment.
Mesons are composed of?
A Three quarks
B Two neutrons
C Quark–antiquark pair
D Electron–positron pair
Mesons are hadrons made from one quark and one antiquark, such as pion and kaon. They carry integer spin and can mediate nuclear forces effectively.
Parity is best described as symmetry under?
A Time reversal
B Space inversion
C Charge change
D Energy shift
Parity refers to how physical laws behave under spatial inversion (x,y,z)→(−x,−y,−z)(x,y,z)→(−x,−y,−z). Many strong and electromagnetic processes conserve parity.
Which interaction famously violates parity?
A Strong interaction
B Electromagnetic
C Weak interaction
D Gravitational
Weak interactions violate parity, shown experimentally in beta decay (Wu experiment). This means left-right symmetry is not preserved in certain weak processes.
Baryon number of a proton is?
A 0
B -1
C 2
D 1
Baryon number BB is +1 for baryons (proton, neutron) and -1 for antibaryons. It is conserved in typical reactions, helping track particle transformations.
Electron lepton number of an electron is?
A 0
B 2
C 1
D -1
Electron family lepton number is +1 for electrons and electron neutrinos, and -1 for their antiparticles. This bookkeeping explains neutrino/antineutrino emission in beta decay.