Rad Physics and Detection

mass number is where

TOP

mass number is

protons AND neutrons

atomic number is where

BOTTOM

Atomic number is

the elemental identity

Isotones

same neutron number (A-Z), different proton number

Isotone example

C14 and N15

isotopes

same atomic number (protons), different neutron number

Isotope example

C12, C13, C14

isobars

same mass number, different atomic and neutron numbers

Isobar example

C14 and N14

Isomers

same mass number, different energy states

Isomer example

Tc99 and Tc99m

Line of stability

1n to 1p, then ratio increases

How are X-rays produced

electrons interact with an atom’s shell

X ray types

Bremsstrahlung and characteristics

Bremsstrahlung radiation

Energetic electrons pass very near the nucleus of the atom

The strong force of the positively charged atom causes the negatively charged electron to decelerate

The path of the electron is significantly changed and some kinetic energy (KE) is lost

The KE lost is converted into X-rays

Characteristic Radiation

Ionization of an atom that leaves an inner shell

Empty outer-shell electrons that drop in to fill the inner-shell vacancy must loose energy

This energy lost is released in the form of X-rays

Energy is directly emitted as a photon (characteristic) or transferred to another electron (Auger).

Stability ratio can predict

decay type

Isobaric transitions (mass number stays the same)

Beta, Position, Electron capture

Isomeric transmission (energy changes)

Gamma emission or internal conversion

Tc99m decays by

isomeric transition

gamma decay

unstable nucleus dumps excess energy in the form of a gamma ray

alpha decay

unstable, proton and neutron rich nucleus decrease atomic number by ejecting an alpha particle (2 protons and 2 neutrons)

beta decay

unstable, proton and neutron rich nucleus. Exchanges a neutron for a proton, beta and antineutrino emitted

positron emission

unstable proton rich nucleus, exchanges proton for neutron, positron and neutrino ejected (Over 1.022 MeV)

electron capture

proton rich nucleus, under 1.022 MeV, electron is pulled from K shell, exchanges proton for neutron

isomeric transition

Excess nuclear energy, decays to ground state, emitting gamma ray/internal conversion

Internal conversion

nuclear energy exceeds binding energy of valance electron, liberty, conversion electron goes to lower energy state. Results in X-ray or Auger electrons.

X rays result from what process

Internal conversion

Gamma rays result from what process

Isomeric transition

transient equilibrium

parent 10-50x longer

secular equilibrium

parent more than 100x longer

Transient generator

Mo/Tc

Secular generator

Sr/Rb

Types of photon interaction with matter

Classical scatter, photoelectric absorption, Compton scatter, pair production, Rayleigh (coherent) scatter

What is photoelectric absorption

Incident photon energy is absorbed by atom, resulting in ejection of inner shell electron

Characteristic x-ray results from what interaction?

Photoelectric absorption

What is Compton scatter?

Medium-energy photon ejects an outer shell electron with lower binding energy. Lower-energy scattered photon is emitted (likely photoelectrically absorbed during next interaction)

Instruments with NaI(Tl) crystals

well counter, uptake probe, scintillation detectors, multi-channel analyzer

Instruments with ionization chambers

Portable ionization meters, GM counter

Well counter downfall

Activities >2uCi can cause coincidence loss (deadtime)

Well counter used for

Used to measure very, very low radioactivity - wipes / blood volume studies

Well counter QC

Background: ensures no contamination. Daily

Constancy: measures same known activity day to day. Daily

Energy Resolution: ability to distinguish between different energies. Quarterly. FWHM

Reproducibility: ensures measurement is consistent with known randomness or Poisson statistics. Quarterly

NaI(Tl) crystal benefit

can determine photopeak - thus isotope

Portable Ionization Meters best used for

measuring HIGH radiation exposure rates

What device would you find most useful in therapies?

Portable Ionization Meters

What is a solid state detector?

converts ionization to electrical signal. CZT and D-SPECT. Compact, immune to magnetic field. Electrical signal fluctuates with temperature.

GM counter is best used for

measuring radiation exposure and detecting contamination