Nuclear Physics Flashcards
What are 3 interpretation of results of particle scattering experiment?
- most particles pass thru undeflected/ w small deflect n
- since vv few particle scattered thru large angles, prob of particle getting close to centre of +ve charge small
=> atom cosist of mostly empty space - small fraction (<1%) of α-particle deflected thru large angle
- to produce large deflect n, must hv large force
- all +ve charge in atom concentrated as nucleus in small region of space vs diameter of atom - few particles reflected backwards, thru angle close to 180 deg
- nucleus is vv small, massive (vv large mass in small space)
Give order of magnitude of nuclear and atomic diameters
- nuclear: ~10E-15 m
- atomic: ~10E-10 m
Define proton/atomic number, nucleon/mass number
- proton/atomic no. , Z, give no. of proton in nucleus
- nucleon/mass no. , A, give total no. nucleons (proton + neutron) in nucleus, A=Z+N
Describe nuclide notation
- notat n for nucleus X, atomic no. Z, mass no. A is
A
X
Z - a nuclide is a particular species w unique pair values A, Z. It is represented by notat n above where X is chem symbol of element
Define isotope
atoms w same no. proton but diff no. of neutron
Name some common isotopes
- H: hydrogen, deuterium, tritium
- C: carbon-12, carbon-14
Define unified atomic mass unit
- one unified atomic mass unit is 1/12 mass of carbon-12 atom
thus,
1u = 1.66E-27 kg
Define relative atomic mass, m r
m r is of atom is ratio of mass of atom to unified atomic mass unit
m r = mass of atom / 1/12 mass of carbon-12 atom
Describe mass-energy equivalence
mass can b ‘created’ or destroyed’; when this happen, equivalent amt energy simultaneously vanish or come into being. Energy E produced by change of mass m is given by mass-energy equivalence relat n:
E = mc²
Define electron-volt (Nuclear Physics)
1 electron-volt is energy gained by charge equal to that of e- in moving thru pd of 1 volt
1eV = 1.60E-19J
Define mass defect of a nucleus. What is the formula?
diff btw mass of separated nucleon & combined mass of nucleus
Δm = sum of m (nucleons) - m (nucleus)
* if u calculate mass defect of ATOM,
Δm = sum of m (nucleons+e-) - m nucleus
Define binding energy (BE). What is the formula?
nuclear BE of nucleus is min energy to completely separate nucleus into constituent neutron, proton
BE = Δmc²
Explain important feature of graph of BE/nucleon against mass number
- high rate +ve graph that bcome gentler until ard Fe, then gently -ve graph
impt feature:
- except for lighter nuclei, avg BE/nucleon abt 8MeV
- Fe nucleus located close to peak w BE/nucleon ~ 8.8MeV, one of most stable nuclide that exist
- nuclei w vv low, high mass no. hv lower BE/nucleon & less stable
- nuclei w low mass no. located to left side of peak may undergo nuclear fusion, so final pdt may hv greater BE/nucleon (more stable)
- nuclei w high mass no. located to right of peak may undergo nuclear fission to form daughter nuclei w greater BE/nucleon
Define nuclear fission
disintegrat n of heavy nucleus into 2 lighter nuclei of approx equal mass
Define nuclear fusion
combining of 2 light nuclei to produce heavier nucleus
What are conserved in all nuclear processes?
- nucleon no.
- proton no. (Charge)
- linear momentum
- mass-energy (same both sides)
Describe some problems of nuclear reactors
- disposal of radioactive fission fragments
- accidental release of highly radioactive fission fragments into atmos
Define radioactive decay. Elaborate
- spontaneous emis n particles (α, β particle) and/or radiat n (gamma ray) fr unstable nucleus so that it bcome more stable
- radioactive decay is spontaneous & random
- spontaneous bcos not affected by external condit n (eg physical factor eg Pa, temp, B, E-fields)
- random bcos impossible predict which nucleus decay next. There is const prob that nucleus decay in any fixed period of time
Explain evidence of randomness of radioactive decay
- random nature of radioactive decay can b determined by observe fluctuat n in count rate
- when Geiger-Muller (GM) tube is near radioactive source, irregularity of counts & fluctuat n in count rate recorded by GM tube show randomness of radioactive decay
NOTE: GM tube is a device to measure count rate (& hence activity) of radioactive source
What are the types of radiation (Nuclear Physics)?
- alpha particle
- beta particle
- gamma ray
Explain alpha particles
- helium-4 nuclei
- typically hv energy in range of few MeV
- speed of order 10^7
- deflected by strong B field (being +ve charge)
- oso deflected by e- field
- high ionising pwr (can remove e- fr nearby species effectively), produce large no. of ion along path
- range in air abt 3-4cm, easily stopped by piece of paper
- cause substance eg zinc sulfide to fluoresce, blacken photographic plate
Explain beta particles
- high speed e-, emitted when neutron in nucleus decay into proton
- emitted w range of speeds, can travel up to 50% light speed
- pdt of beta decay process cnt just consist daughter nuclide & beta particle as no definite speed for both pdt for linear momentum b conserved
- oso observed that daughter nuclide no recoil back in straight line but at angle
- by conserv n law energy, momentum, there must b 3rd particle produced in process
=> neutrino (mass-less, charge-less, so hard to detect) - easily deflected by B, e-field (so low mass)
- ionising pwr abt 1/10 of alpha particle (so lower charge)
- range of beta particles in air abt 10 times that of alpha particle; stopped oni by few mm thick Al
Explain gamma rays
- EM waves (shorter wavelength than X ray)
- electrically neutral, not deflected by e-, B field
- strongest penetrate pwr, stopped by lead of few cm thick
- ionisat n pwr abt 1/10 000 of alpha particle
- emis n no accompany any change in nuclear structure; nucleus just descend to lower energy state
- gamma decay represent emis n energy fr nucleus returning to ground state
excited nucleus -> more stable nucleus + gamma ray
Define decay constant of nucleus
nucleus’ probability of decay per unit time