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
Give equations involving decay constant, activity and count rate
N = N0e^(-λt) OR N = N0(0.5)^n
A = A0e^(-λt) OR A = A0(0.5)^n
A = -dN/dt = λN
C = C0e^(-λt) OR C = C0(0.5)^n
where
N is no. of radioactive nuclide,
A is activity (rate decay parent nuclei),
C is corrected count rate (detected decay),
0 is initial value,
λ is decay const,
t is time taken,
n is no. of half lives passed
Define activity of radioactive source
no. of nuclear decay per unit time occurring in source
Describe graphical representation of decay of parent nuclide
- N vs t graph give decrease exponential graph
N = N0e^(-λt) - ln N vs t graph give w grad -ve straight line graph
ln N = lnN0 -λt
Describe cancer treatment with radiation
- amt irradiat n used in each treatment of patient usually same
=> for freshly prepared sample, durat n of treatment is shorter - if same sample used later, durat n of treatment is longer for patient to receive same dose
Since, amt radiat n (dose) = At, can form
A1t1 = A2t2 to find new duration of treatment
Define half-life (nuclear physics)
time taken for half og no. of radioactive nuclei to decay
Explain background radiation
- to determine measuremt of rad n fr radioactive scs, we must consider backgrd rad n (ionising rad n emitted fr variety of natural, artificial rad n scs)
- backgrd rad n come fr environ ard us, present in small amt
- lvl of rad n vary fr place to place
- accidents at nuclear pwr station or any form of nuclear dumping cause increase backgrd rad n
- backgrd rad n come fr scs eg,
air (cosmic ray), building material, soil (+ rock containing radioactive isotope), water, human body, old coal fired pwr station, medical scs)
Explain effects of radiation on living organisms
- hazards to human beings arise fr: exposure of body to external rad n & ingest n/inhalat n of radioactive matter
- alpha particle: slight hazard (unless enter body) since cnt penetrate outer layer of skin
- beta particle: generally more penetrating than alpha; most of their energy absorbed by surface tissues, few mm of Al suff to provide protect n
- gamma ray: highly penetrating; can penetrate deeply into body, may require few cm lead or concrete shielding
- rad n can cause immediate severe damage to body tissue eg rad n burn, by damage structure of molecules, cause malfunct n, death of living cells (some cell recover but others cnt, effect on tissues r cumulative)
- delayed effects eg cancer, eye cataract may appear many yr later
- hereditary defects may oso occur in succeeding gen due to genetic damage
- chromosome r sensitive to ionising rad n at moment of cell divis n causing genetic mutat n likely harmful; such can cause birth defect if unborn child &/or mother exposed
Name and explain uses of radiation
- tracers
- used to follow path of cpd thru system
eg leak in undergrd pipe carrying water, oil can b detected by inject radioactive tracer into flow
- geiger tube on surface r then used to detect leakage - medical, biological uses
- immature cell, cell growing/dividing most rapidly r most sensitive to rad n; this is made use of in rad n treatmt of cancer
- often, cancer cell growing rapidly, so more likely killed by high dose gamma rad n fr cobalt-60 scs vs normal cell dividing less frequently - Archaeological dating (carbon dating)
- atmos contain small proport n radioactive carbon-14, absorbed by living plant, trees during photosynthesis
- half-life of C-14 is 5700 yr, so there is negligible disintegrat n over lifetime of most plant
- BUT, once plant dies, no further carbon-14 is taken in, so proport n of carbon-14 in plant starts decrease as it decay
- after 1 half-life, 50% of carbon-14 remain
- since activity is proportional to no. of C-14 atom left, measuring activity enable age (time since death) of dead sample to b calculated)
- measured activity is compared w activity of same mass of living wood, then using value of half-life of C-14, age can b determined (up to abt 20 000 yr limit)
Explain safety precautions for radiation
- handling
- minimise time contact w radioactive material
- solid scs most easily handled, shd b manipulated remotely fr dist eg use tong, in glove-box, etc
- avoid any ingest n eg radioactive particles lodged in lungs much more danger, harmful than outside body
- rad n worker given protective clothing, regular test monitor dosage they receive
- limit amt rad n received fr X-ray equipmt - storage
- penetrating pwr of diff type rad n give clue to safe practice; pure alpha particle present little hazard when enclosed in container; but since most alpha scs oso emit gamma rad n, lead-lined container needed. same is true for beta scs
- generally, keep all radioactive material in lead container when not in use - disposal
- radioactive waste pdt must b quickly, safely disposed of; they can b encased in concrete, sealed in steel tanks, then buried undergrd
