Charged Particles

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Charged Particles
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Nuclear Physics

• Charged particles can come from nuclear decay.
• Nuclear physics figures into particle detection.
• Use terminology from nuclear physics.
• Isotopes share Z
• Isotones share N

• Nucleus consists of protons and neutrons.
• Protons Z (atomic number)
• Neutrons N
• Nucleons A Z N (atomic mass)
• Full notation shows A, Z

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Energy Measurement

• Energy measurements for nuclear an particle
  physics are built on the electron volt (eV)
• Energy to move one electron through a volt
• 1 eV 1.6 ? 10-19 J
• Mass is expressed in terms of the rest energy
• Also atomic mass unit (u)
• 1 u 931.5 MeV/c2

• Proton, p
• 938.3 MeV/c2
• 1.007 u
• Neutron, n
• 939.6 MeV/c2
• 1.009 u
• Electron, e
• 0.511 MeV/c2
• 5.546 ? 10-4 u

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Mass Difference

• The mass (M) in u is nearly equal to the atomic
  number (A).
• Tables of isotope data frequently list D M A.
• Often converted into MeV
• Data used to calculate energy of decay products.

• 1H D 7.29 MeV
• 4He D 2.42 MeV
• 56Fe D 60.60 MeV
• 214Pb D 0.15 MeV
• 218Po D 8.38 MeV
• 222Rn D 16.39 MeV
• 226Ra D 23.69 MeV

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Alpha Particles

• Alpha particles are 4He nuclei.
• Mass approximately 4 AMU
• Charge is 2
• Generally from the decay of heavy nuclei
• The energy of the alpha particle is due to the
  mass difference of the daughter nuclei.

• Typical Problem
• Calculate the energy of the alpha particle from
  222Rn.
• Answer
• Get the reaction equation.
• The energy released is
• Q MRn222-MPo218-MHe4
• Q 12.89 MeV
• Most will go to the alpha.

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Beta Particles

• Electron decay
• Nucleus emits an electron and antineutrino
• Atomic number increases
• Energy goes to e and n
• Some include photon as well

• Positron decay
• Nucleus emits a positron and a neutrino
• Atomic number decreases
• Kinematics like electron decay
• Same result as electron capture no beta out

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Table of Isotopes
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Decay Rates

• The number of particles decaying in a short
  period of time is proportional to the number of
  particles.
• The decay constant is l.
• The decay rate or activity is the rate of change.
• Activity decreases as time increases

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Half-Life

• The differential equation for decay gives rise to
  an exponential relation.
• Decay constant is fixed for a decay reaction
• Decay is usually expressed as a half-life.
• Time for half a sample to decay
• Remains constant

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Measured Activity

• The SI unit of activity is the Becquerel (Bq).
• equals one decay/sec (s-1)
• The older unit is the curie (Ci).
• Based on the decay of 226Ra
• Once activity of one gram
• Now defined by Bq
• 1 Ci 3.7 ? 1010 Bq

• Typical Problem
• A source of 24Na is marked at 1.16 MBq. How many
  24Na atoms are there in the sample?
• Answer
• First thing is to look up the half-life for 24Na
  
• T 15 h 5.4 ? 104 s

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Specific Activity

• Physical variables are often normalized to the
  mass.
• Described as specific
• Specific activity is the activity of a sample
  divided by the mass.
• Units Bq g-1 or mCi g-1
• In solution expressed per unit volume pCi L-1

• For a pure radionuclide
• Normal soil has a few pCi/g
• Drinking water has a recommended limit of 5 pCi/L
  of 226Ra 228Ra.

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Particle Physics

• Charged particles are measured in particle
  physics.
• Energy scale gt 1 GeV
• Energetic particles are the results of
  acceleration or decays.

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Particle Lifetime

• Unstable particles have a characteristic
  lifetime.
• The lifetime t is related to the probability that
  a particle will survive a given period of time.
• The survival time is affected by relativity.

• The probability is an exponential relation

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Quarks

• Quarks are fundamental building blocks, but are
  not detected directly.
• Binding force too great
• Stable quarks bind to others
• Quarks exist in hadrons.
• Baryons are three quarks
• Mesons are a quark-anti quark pair.

• Some baryons
• proton, p uud
• neutron, n udd
• lambda, L0 uds
• lambda-b, Lb0 udb
• Some mesons
• pi-minus, p- ud
• k-plus, K us
• J/psi, Y cc

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Hadrons

• Protons are stable hadrons.
• Charged particles
• Interact strongly
• Easy to detect
• Any other baryon will eventually decay into a
  proton and other particles.

• Charged pions are unstable, but relatively
  long-lived hadrons.
• Lifetime 26 ns
• Interact strongly
• Detectable like protons
• Pions frequently accompany the decay of other
  hadrons.

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Jets

• Hadrons that collide at high energy can eject a
  quark.
• When the quark emerges it hadronizes forming a
  jet of particles.
• Most emerging particles are pions

High energy pion interaction, Fermilab 1973
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Leptons

• Leptons are fundamental particles.
• Interact weakly
• Able to exist in isolation
• Detection of charged leptons is important in many
  particle physics experiments.

• Charged leptons
• electron, e- 0.511 MeV/c2 1/1836 mp
• muon, m- 0.1057 GeV/c2 1/9 mp
• tau, t- 1.776 GeV/c2 1.9 mp

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Electrons

• Electrons are perhaps the most important particle
  for detection.
• Stable
• Charged
• Lightest charged particle
• Electrons result from nuclear and particle decays.

• Electron from W decay

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Muons

• Muons are charged, long-lived and weakly
  interacting.
• Lifetime 2.2 ms
• Heavy version of the electron.
• Mass provides greater penetration
• Muons are naturally created by cosmic rays.

• Muon from top decay