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