Nuclear Physics

Question 1

What are 3 interpretation of results of particle scattering experiment?

Answer 1

1. 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
2. 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
3. few particles reflected backwards, thru angle close to 180 deg
   - nucleus is vv small, massive (vv large mass in small space)

Question 2

Give order of magnitude of nuclear and atomic diameters

Answer 2

• nuclear: ~10E-15 m
• atomic: ~10E-10 m

Question 3

Define proton/atomic number, nucleon/mass number

Answer 3

• 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

Question 4

Describe nuclide notation

Answer 4

• 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

Question 5

Define isotope

Answer 5

atoms w same no. proton but diff no. of neutron

Question 6

Name some common isotopes

Answer 6

• H: hydrogen, deuterium, tritium
• C: carbon-12, carbon-14

Question 7

Define unified atomic mass unit

Answer 7

• one unified atomic mass unit is 1/12 mass of carbon-12 atom
  thus,

1u = 1.66E-27 kg

Question 8

Define relative atomic mass, m r

Answer 8

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

Question 9

Describe mass-energy equivalence

Answer 9

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

Question 10

Define electron-volt (Nuclear Physics)

Answer 10

1 electron-volt is energy gained by charge equal to that of e- in moving thru pd of 1 volt

1eV = 1.60E-19J

Question 11

Define mass defect of a nucleus. What is the formula?

Answer 11

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

Question 12

Define binding energy (BE). What is the formula?

Answer 12

nuclear BE of nucleus is min energy to completely separate nucleus into constituent neutron, proton

BE = Δmc²

Question 13

Explain important feature of graph of BE/nucleon against mass number

Answer 13

• 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

Question 14

Define nuclear fission

Answer 14

disintegrat n of heavy nucleus into 2 lighter nuclei of approx equal mass

Question 15

Define nuclear fusion

Answer 15

combining of 2 light nuclei to produce heavier nucleus

Question 16

What are conserved in all nuclear processes?

Answer 16

• nucleon no.
• proton no. (Charge)
• linear momentum
• mass-energy (same both sides)

Question 17

Describe some problems of nuclear reactors

Answer 17

• disposal of radioactive fission fragments
• accidental release of highly radioactive fission fragments into atmos

Question 18

Define radioactive decay. Elaborate

Answer 18

• 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

Question 19

Explain evidence of randomness of radioactive decay

Answer 19

• 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

Question 20

What are the types of radiation (Nuclear Physics)?

Answer 20

• alpha particle
• beta particle
• gamma ray

Question 21

Explain alpha particles

Answer 21

• 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

Question 22

Explain beta particles

Answer 22

• 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

Question 23

Explain gamma rays

Answer 23

• 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

Question 24

Define decay constant of nucleus

Answer 24

nucleus’ probability of decay per unit time

Question 25

Give equations involving decay constant, activity and count rate

Answer 25

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

Question 26

Define activity of radioactive source

Answer 26

no. of nuclear decay per unit time occurring in source

Question 27

Describe graphical representation of decay of parent nuclide

Answer 27

• 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

Question 28

Describe cancer treatment with radiation

Answer 28

• 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

Question 29

Define half-life (nuclear physics)

Answer 29

time taken for half og no. of radioactive nuclei to decay

Question 30

Explain background radiation

Answer 30

• 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)

Question 31

Explain effects of radiation on living organisms

Answer 31

• 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

Question 32

Name and explain uses of radiation

Answer 32

1. 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
2. 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
3. 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)

Question 33

Explain safety precautions for radiation

Answer 33

1. 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
2. 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
3. disposal
   - radioactive waste pdt must b quickly, safely disposed of; they can b encased in concrete, sealed in steel tanks, then buried undergrd