Physics 123

1
Radiation and its detection

• Physics 123

2
Binding energy per nucleon
3
Isotopes

• Same chemical element (ZNp) can have different
  number of neutrons isotopes
• Carbon Z6?6 p, but it can have
• 5n 11C, 6n 12C, 7n 13C, 8n 14C, 9n 15C,
  10n 16C
• Only 12C is stable (98.9 of C on Earth)
• Unstable isotopes radioactively decay

4
Radiation

• Types of radioactive particles
• a radiation (He nuclei 2p2n, charge 2e)
• b radiation (this is just an electron, charge
  -e)
• g radiation (these are photons, charge0)
• Neutrons (charge0)
• Sources of radiation
• Naturally radioactive elements (e.g. plutonuium,
  uranium),
• Induced radiation (by bombarding with energetic
  particles),
• Human activity (nuclear power plants, X-rays)
• Cosmic rays (atmosphere shields most of it)
• Atomic bomb

5
a-radiation

• 22688Ra?22286Rna
• a-particle nucleus of He 2p2n
• Z decreases by 2 when a-particle is emitted, A
  decreases by 4
• Why not just ps and ns separately?
• Because of binding energy m(a)lt2m(p)2m(n)
• Why not then larger nuclei?
• a-particle is the most tightly bound nucleus.
  Just like atoms nuclei have energy shells
• Protons and neutrons are fermions like electron
  Pauli exclusion principle works as well
• In the ground state we can have 2p (spin up, spin
  down) and 2n (spin up, spin down)
• Protons and neutrons are not identical particles
  (e.g. different electric charge), so Pauli
  principle does not apply to a proton and neutron

6
a-radiation

• If this decay is energetically favored why does
  not it happen immediately?
• Tunneling through a barrier
• Probability depends exponentially on the
  thickness of the barrier
• That is why lifetimes of radiation materials
  differ by many orders of magnitude 1ms-1010 years

7
b-decay

• 146C?147Ne-ne
• In b-decay nucleus charge (Z) changes by 1,
  while mass (and A) essentially stays the same
  meltltmp, mnltlt me (though not zero!!! as the book
  says the book is old, new discoveries were made)
• Electron is not one of the atomic orbital
  electrons, it was created in the neutron decay
• n?pene
• Neutrino has no electric charge and does not
  participate in strong interactions its chances
  of interaction with matter are rather low was
  not detected until 1956
• Its existence was predicted by Pauli in 1930
  based on energy conservation in b-decays

8
b -decay

• 1910Ne?199Feene
• In b -decay nucleus charge (Z) changes by -1,
• e - is a positron antipartner of an electron
• Positrons dont live long, when matter meets
  antimatter they annihilate. There are plenty of
  atomic electrons to annihilate with
• e-e ?gg
• Electron capture (from innermost shell
  K-shell)
• 74Bee-?73Lin

9
g-decay

• g-rays are just very energetic photons (keV, MeV)
• Because nuclei energy is much larger than atomic
  energy, the spacing between energy levels is
  larger as well (though space-wise nucleus is much
  smaller than atom), hence photons emitted in
  nuclear transitions are a lot more energetic
• Charge and mass stays the same in this transition

10
Radioactive decay law

• Nuclei decay is a random process, number of
  particles DN decaying in interval of time Dt is
  proportional to the total number of particles
• DN -lNDt
• Differential equation

11
Rate of decay

• Number of particles decaying per unit of time

12
Half-life

• Half-life period of time T1/2 in which the
  number of particles decreases by a factor of 2
  NN0/2

13
Radioactive dating

• The majority of carbon atoms are 126C, but small
  fraction (1.3x10-12) is radioactive isotope 146C
  (half-life 5730 yr)
• This ratio in atmosphere is constant for many
  thousand years
• Carbon is absorbed by living organisms in CO2
  during the life process, after death it remains
  fixed
• The age of remains can be determined by the ratio
  of 146C to 126C

14
Example of radioactive dating

• An animal bone fragment has carbon mass 200g. It
  registers an activity of 16 decays/s. What is the
  age of the bone?

15
Decay series
16
Detection of radiation

• Different types of radiation interact differently
  with matter (atomic structures)
• Charged particles ionize atoms ionizing
  radiation
• Photons can also ionize atoms Compton effect
  pair production
• Alpha particles and neutrons can displace nuclei
  from crystal structure interact with nuclei
  strongly
• Radiation dose amount of energy deposited per
  kg of mass
• 1 rad 10-2J/kg

17
Magnetic force on moving charge

• Charged particles bend in magnetic field, there
  charge to momentum ratio can be determined from
  the radius of curvature
• Magnetic force Centripetal force
• FqvB
• Centripetal acceleration
• av2/R
• Newtons second law
• Fma
• qvBmv2/R

In experiments B is known, qe most of the time,
Measure R- measure mv For a given v measure m
magnetic spectrometer
18
Detection of radiation
19
p-n diode

• The current flows through p-n junction if
  electrons have vacancies to jump to, it does not
  flow in the opposite direction
• When ionizing particle goes through p-n diode it
  produces e-hole pairs current starts flowing
  detect the particle with high position precision

P-type vacancies

P-type
current
No current

-
--- --- ---
-
Electron flow
--- --- ---

n-type electrons
n-type
Reverse bias
Forward bias
20
Radiation effect on humans

• Effects on humans are different from different
  types of radiation ? quality factor (QF)
• QF(g) QF(b) 1 QF(a) 20.
• Effective dose (in rem) dose (in rad) x QF
• Levels of radiation
• Natural background 0.36 rem /year
• Medical X-ray 0.040 rem /year
• US recommended upper limit 0.5 rem/year
• Radiation workers 5rem /year
• Radiation damage in biological organisms
  alteration in DNA
• Somatic (any part of body, but reproductive)
• First affect the blood cells (shortest
  regeneration time)
• Can lead to cancer (by altering the DNA)
• Genetic (reproductive organs)
• Leads to mutations, smaller dose is harmful
• Rate matters
• Short dose of 1000rem is fatal
• 400 rem over short period of time 50 fatal
• 400 rem over several weeks usually not fatal

21
Radiation in medicine

• Radiation therapy
• Focus radiation on cancer cells (kill the bad
  guys)
• Medical imaging
• Use radioactive isotopes to tag molecules
• PET (positron emission tomography)
• ee-2g
• Photons travel in a
• straight line
• easy to find their source