Neutral Particles. Neutrons Neutrons are like neutral protons. –Mass is 1% larger –Interacts...
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Transcript of Neutral Particles. Neutrons Neutrons are like neutral protons. –Mass is 1% larger –Interacts...
Neutral Particles
Neutrons
• Neutrons are like neutral protons.
– Mass is 1% larger
– Interacts strongly
• Neutral charge complicates detection
• Neutron lifetime is long
– = 624 s
eepn
Nuclear Reaction Notation
• Nuclear reactions usually involve light particles (p, n, , ) colliding with a nucleus.
– Light particle will carry most of the energy
• Use a notation that avoids arrows and plus signs.
– Indicate incident and exiting particles
• X(a, b)Y
– X, Y are nuclei
– a, b are light particles
• Examples
– 7Li(p, n)7Be
– 12C(n, )13C
bYXa
np BeLi 74
73
CC 136
126n
Cross Sections
• The cross section measures the likelihood of a reaction.
– Effective area of a particle
– 1 barn = 10-24 cm2
• Assume a set of particles interacting with a target.
– N0 initial particles
– dN particles interacting
– n particle density
– A target area
– dx target thickness
nAdx
dxnA
dxnA
N
dN
xneNN 0
dxnA
nuclei in target
effective exposed area
Neutron Energies
• Neutrons for detection have distinct ranges of energy.
– Slow or thermal neutrons with energies under 1 eV
– Fast neutrons with energies from 100 keV to 10 MeV
– Relativistic neutrons with energies over 1 GeV
Useful Fact• What is the kinetic energy of a
thermal neutron?
• It must be about kT.
– At 20 °C, kT = 1/40 eV
• Better is (3/2)kT
– 3 degrees of freedom
– K = 0.038 eV
Reactor Sources
• Nuclear reactors are rich sources of neutrons.
• Nuclear fission of 235U produces multiple neutrons per reaction.
• Neutron energy is important to reaction.
– 235U uses thermal neutrons
– 238U absorbs fast neutrons
• Typical fission:
– Releases 208 MeV
nn 3KrBaU 9236
14156
23592
Moderators and Absorbers
• Neutrons produced in reactors are generally fast.
– A few MeV
• Some reactions and detectors require slow or fast neutrons.
– Moderators slow down fast neutrons
– Absorbers capture neutrons
Typical Problem• Calculate the neutrons captured
per second by aluminum 0.50 mm thick with = 2.0 mb for a flux of 5.0 x 1012 /cm2s
Answer• The reaction is 27Al(n,)28Al.
• The density of Al is 2.7 g/cm3
– n = NA/A = 6.02 x 1028
– dN/N = ndx = 6.0 x 10-6
– Rate R = 3.0 x 1012 /cm2s
Accelerator Sources
• Accelerators can create neutrons by spallation.
– Incident proton or deuteron
– Knock out neutrons from target
• Proton or deuteron beams used.
– Light targets preferred
– Avoid excited nuclear states
• Neutron beam at Fermilab
– 66 MeV protons
– Beryllium target
np BBe 95
94
Neutrinos
• Neutrinos are leptons
– Neutral partners of e, , – Very light mass
– Stable particles
• Produced with lepton partner or during partner decay or interaction.
• Neutrinos mix with each other.
• Electron neutrino, e
– Mass < 2.8 eV
• Muon neutrino,
– Mass < 0.19 MeV
– m2 = 0.002 eV2 (m < 3.5 eV)
• Tau neutrino,
– Mass < 18.2 MeV
Missing Energy
• The neutrino is very difficult to detect.
– No charge
– Low mass (> 0 in 1998)
– Weakly interacting
• Detection is by inference.
– Energy and momentum must be conserved
Neutrino Observatories
• Neutrino detection is also by interaction.
– Collision with nucleon
– Creation of charged lepton
• Low cross section requires large volume.
Photons
• The photon is the gauge boson of the electromagnetic force.
– Massless
– Stable
– Interacts with charged particles.
• Photon energy ranges of interest:
– Visible light – 1 to 3 eV
– X-rays – 100 eV to 1 MeV
– rays – over 30 keV
Useful Conversion• hc = 1.240 keV nm
X-Rays
• X-rays are associated with energetic transitions in atoms.
• Continuous spectra result from electron bombardment.
– Peak energy (kVp) depends on beam energy.
• Discrete spectra result from electron transitions with an atom.
target
electrons
x-ray
Synchrotron Radiation
• A bending beam of electrons will emit photons.
– Energy lost from electrons
• Insertion device will create sinusoidal field.
– More bends in short distance
Gamma Rays
• Gamma rays are photons associated with nuclear or particle processes.
– Energy range overlaps: soft gamma equals hard x-ray
• Nuclear gamma emissions are between isomers.
– A and Z stay constant
– Distinct energies for transitions
Nuclear Gammas
• Nuclear decay can leave a nucleus in an excited state.
– Many possible states may be reached
– Lifetime typically 10-10 s
• Excess energy may be lost as a photon or electron.
– Single gamma
– Series of gamma emissions
– Internal conversion beta
Ra22688
4.785 MeV
Rn22286
0.186 MeV
0 MeV
94.4%
5.5%
2.2% 3.3%
Bremsstrahlung
• Acceleration of a charged particle is associated with a photon.
– Bremsstrahlung means braking radiation
– Electrons passing through matter
• Continuous spectrum x-rays are also bremsstrahlung
e
e
Z
Particle Gammas
• Gammas are emitted in many elementary particle decays.
– Charge constant
– Lepton/baryon numbers constant
• Gammas appear in production reactions.
• Direct decays
• Resonance decays
0
0
KK )892(*
)1(/)1(2 SJPc