Neutral Particles

Neutrons Neutrons are like neutral protons. –Mass is 1% larger –Interacts strongly Neutral charge complicates detection Neutron lifetime is long – = 624 s

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 – 7 Li(p, n) 7 Be – 12 C(n,  ) 13 C

Cross Sections The cross section  measures the likelihood of a reaction. –Effective area of a particle –1 barn = cm 2 Assume a set of particles interacting with a target. –N 0 initial particles –dN particles interacting –n particle density –A target area –dx target thickness 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 = eV

Reactor Sources Nuclear reactors are rich sources of neutrons. Nuclear fission of 235 U produces multiple neutrons per reaction. Neutron energy is important to reaction. – 235 U uses thermal neutrons – 238 U absorbs fast neutrons Typical fission: –Releases 208 MeV

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 /cm 2 s Answer The reaction is 27 Al(n,  ) 28 Al. The density of Al is 2.7 g/cm 3 –n = N A  /A = 6.02 x –dN/N = n  dx = 6.0 x –Rate R = 3.0 x /cm 2 s

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

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 –  m 2 = eV 2 (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 = 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 s Excess energy may be lost as a photon or electron. –Single gamma –Series of gamma emissions –Internal conversion beta MeV 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