Sources of Radiation

Professor Jasmina Vujic

Lecture 4Nuclear Engineering 162

Department of Nuclear EngineeringUniversity of California, Berkeley

SOURCES OF RADIATION

• NATURAL RADIATION SOURCES– Radioactive Sources in the Ground– Cosmic Ray Interactions in the Atmosphere– Natural Radioactive Sources within the Body

• MAN-MADE RADIATION SOURCES– Medical and Dental X-Ray Machines– Industrial X-Ray Machines– Accelerators– Man-made Radioisotopes– Fallout from Past Bomb Tests– Nuclear Power Reactors

Radioactive Sources in the Ground

• Primordial Radionuclides: 238U, 232Th and 235U radioac tive decay chains; 40K and 87Rb 22Na

• Cosmogenic radionuclides: 14C, tritium (3H), 7Be, and 22Na

Cosmic Ray Interactions in the Atmosphere

Cosmic Ray Interactions in the Atmosphere

• Radon as a source of internal exposure: 238U chain, 226Ra decays into 222Rn. The significant dose is from the decay products of radon:

Natural Radioactive Sources within the Body

• Potassium-40: 140 g of potassium in a man of 70 kg (i.e. 0.1 /μCi)

• Carbon and hydrogen in the biosphere contain 14C and 3H

• Radon and its decay products • 137Cs, 131I, 90Sr

The Contribution of Natural Radiation Sources

SOURCES OF X-RAYS

• Two principal methods for generating X-rays:– The rearrangement of atomic electron

configurations • "Characteristic X-Rays"

– The deflection of charged particles in the vicinity of the atomic nucleus

• "Continuous X-Rays or Bremsstrahlung

CHARACTERISTIC X-RAYS

• Characteristic X-rays are emitted from the atomic shells, when electrons jump from the shells at higher energy levels (with Iower binding energy) to the vacancies in the shells at lower energy lev els (with higher binding energies).

• The binding energy of the K-shell electron is the largest in an atom (for example, it is 13.6eV for H and up to 115keV for U)

• The energy of the emitted X-ray is determined by

where Em is the upper energy level and En is the lower energy level.

m nhv E E= −

CONTINUOUS X-RAYS

• In addition to loosing its kinetic energy in collisions with the atomic electrons causing ionization or excitation of the atoms along its path, a charged particle (in our case an electron) gives up its kinetic energy by a photon emission as it is deflected (or accel erated) in the electric field of nuclei.

• The emitted EM radiation has a continuous energy spectrum from 0 to Ek, where Ek is the kinetic energy of a charged particle.

• For Ek < 100 keV, radiation is emitted at 900 to the direction of the charged particle. For higher Ek the direction of the emitted radiation shifts toward the forward-peaked direction.

X-RAY MACHINES

• Made of a glass vacuum tube with two electrodes: cathode and anode:

• Cathode with tungsten wire (filament) is heated and electrons are emitted

• Electrons are accelerated by a large potential difference (high voltage)

• The focusing cup concentrates electrons into the target oh the anode

• After hitting the target, electrons are abruptly brought to rest with the lost of their kinetic energy

• Only about 1% of electron kinetic energy is emitted as EM radiation, 99% is lost in electronic collisions and converted to heat (anode must be cooled)

Medical and Dental X-Ray Machines

Medical and Dental X-Ray Machines

Fallout from Past Bomb Tests

Fallout from Past Bomb Tests

Fallout from Past Bomb Tests

Nuclear Power Reactors

Nuclear Power Reactors

Radionuclides used in Diagnostics and Therapy

Choice of Radioisotopes for Imaging

• The physical characteristics that are desirable for nuclear medicine imaging include:– A suitable physical half-life– Decay via photon emission– Photon energy high enough to penetrate the

body tissue with minimal tissue attenuation– Photon energy low enough for minimal

thickness of collimator speta– Absence of particulate emission

Important Nuclides of Biomedical Uses

Production of Radionuclides

• There are several ways by which radionuclides are produced– Neutron capture (neutron activation)– Nuclear fission– Charged-particle bombardment– Parent decay (radionuclide generator)

Radionuclides produced by neutron absorption

Radionuclides produced by nuclear fission

Radionuclides produced by charged particle bombardment

Generator-produced radionuclides