Introduction To Radiation Physics, Quantities And Units

Introduction to RadiationPhysics, Quantities and UnitsRobert E. Reiman, MSPH, MD, Duke University Medical CenterCenter for Medical CountermeasuresAgainst Radiation

Course ObjectivesParticipants Should Be Able To: Understand the basic physics of theelectromagnetic and particulate forms ofionizing radiation. Understand the distinctions between the unitsof radiation quantity, exposure and dose. Be familiar with some of the methods used tomeasure radiation dose.

Physics froma Doctor’sPoint of View

What is “Radiation”? Radiation can be thought of as thetransmission of energy through space. Two major forms of radiation:– Electromagnetic (EM) radiation– Particulate radiation Both forms can interact with matter,and transfer their energy to the matter.

yHigher Frequenciesand EnergiesVisibleUVInfraredMicrowaveRadioLower Frequenciesand Energies

Electromagnetic Radiation Electromagnetic radiation has no mass,and moves through space at the speed oflight (3.0 x108 meters per second). Electromagnetic radiation can bedescribed by two models:– Wave Model– Photon Model

EM Radiation: Wave Model EM radiation is a pair of perpendicular, timevarying electric and magnetic fields travelingthrough space with the velocity of light (c). The distance between maxima of the EM fieldsis the wavelength (λ). The frequency (ν) of the wave is given by:ν c/λ

EM Radiation: Photon ModelE hc/λElectromagnetic radiation can also be described asdiscrete packets of energy called photons. The energy (E)is related to the wavelength (λ) in the wave model throughPlanck’s Constant (h) and the speed of light (c).

Ionizing EM Radiation EM radiation with wavelengths shorter than 100nanometers can remove electrons from theouter atomic shells. This process produces ions. Ions can interact with living tissue to producebiological damage. A major source of ionizing radiation is nucleartransformation.

Human Transformation- Δm

Nuclear Transformation- ΔmRadioactiveStableIonizing Radiation: α, β, or γ

Nuclear TransformationEnergyE1E1 Excited StateE0 Ground StateET E1 – E0ET Transformation EnergyE0

Gamma RaysZ, MZ, MγGamma rays are electromagnetic radiationresulting from nuclear transformation.

Particulate Radiation Charged particles are emitted from the atomicnucleus at high energy in some nucleartransformations. These include alpha andbeta particles. Uncharged particles (neutrons) are producedby fission or other nuclear reactions. Both types of particles produce ionization.

Alpha ParticlesZ - 2, M - 4 α24Z, MAlpha Particle(Helium Nucleus)

Beta Particles0ν0AntineutrinoZ 1, MZ, M β 10Beta Particle

Production of X-RaysElectronor betaX-RayTarget Nucleus(Heavy metal)X-rays are produced when a charged particles(electrons or betas) are decelerated by a strongelectrostatic field, such as that found near the nucleiof heavy metals (tungsten, lead).

Physical Half-life Radioactive nuclei undergo disintegration at arate that is proportional to the number ofuntransformed nuclei present. The physical half-life is the time required forone-half of the remaining nuclei to transform. The half-life is characteristic of theradionuclide.

Simple Model of thePhysical Half-Life of aRadionuclideT 016 ParentsAfter One Half-life8 Parents, 8 DaughtersAfter Two Half-lives4 Parents, 12 Daughters

Radioactive 32 yearsCesium-13730 yearsCobalt-605.3 yearsIridium-19274 daysIodine-1318 days

Radiation Exposure Exposure is an index of the ability of aradiation field to ionize air. Radiation passing through a gas liberatesion pairs. If the gas is in an electric field, movementof ion pairs can be measured as a current,which is proportional to exposure rate.

Quantity of RadioactiveMaterial Quantity of radioactive material isexpressed as the number of nucleartransformations (or disintegrations) thatoccur in a sample per unit time. The term for quantity of radioactivematerial is activity.

Radiation Absorbed Dose Absorbed Dose is a measure of theenergy imparted to matter when anionizing radiation field interacts withmatter. Absorbed dose is expressed as energyabsorbed per unit mass of material.

Equivalent Dose For the same absorbed dose (depositedenergy) in tissue, different forms ofionizing radiation can have differentbiological effects. “Equivalent Dose” attempts to normalizethese differences.

Equivalent Dose Equivalent Dose is the product of thedose and a modifying factor calledthe quality factor (QF), which reflectsthe relative biological effectivenessof the radiation:HT D x QF

Quality Factors (QF) QF are indices of the “relative biologicaleffectiveness” (RBE) of a radiation. RBEis a complicated function of type ofradiation, energy and the biologicalsystem under consideration. QF are not measured. They aredetermined by a committee.

Some Values of QFRadiationQF (ICRP 60)Photons, electrons (all energies)1Thermal neutrons ( 10 keV) andneutrons 20 MeV5Neutrons 10 keV – 200 keVNeutrons 2 – 20 MeV10Alphas, neutrons (100 keV- 2 MeV),protons, fission fragments20

Effective Dose Equivalent Effective Dose Equivalent (EDE) isintended to reflect the total biologicaleffect of a given exposure on a human. Itis a weighted average of the individualdoses to a number of important tissues:HE Σ (HT x WT)(sum is over all tissues)

Effective Dose Equivalent Effective Dose Equivalent (EDE) is a derivedquantity, not a measurable quantity. Applies to situation where irradiation of organsand tissues is non-uniform. EDE yields the same “radiation detriment” as anumerically-equivalent whole-body dose. WT values are assigned by a committee.

Some Tissue Weighting FactorsTissue / OrganICRP 26ICRP 60*Gonads0.25.20Red hyroid0.03.05Bone Surfaces0.03.01*When ICRP 60 weighting factor and algorithm are used, result isexpressed as “effective dose” as opposed to “effective doseequivalent” in the ICRP scheme.

Radiation Units Two systems are in common use:– Special Units– System Internationale (SI) Units Special units are used by most regulatoryagencies in the U.S. SI units and are used in the rest of theworld, and are based on “MKS”

Units of Exposure and QuantitySpecial UnitsRoentgen (R)2.58 x 10-4coulombs /kg dry air atSTPCurie (Ci)Disintegrations persecond in 1 gmradium (3.7 x 1010dps)SI UnitsBecquerel (Bq)1.0 dps

Units of Absorbed DoseSpecial UnitsSI Unitsradgray (Gy)radiationabsorbed dose(100 erg/gm)S.I. unit: 1.0J/kg (100 rads)

Units of Equivalent Dose and EDESpecial UnitsSI Unitsrem (rem)sievert (Sv)roentgen equivalent man(rad x quality factor)Gy x qualityfactor

Computing Exposure Rate If the activity of a source of gamma raysis known, the exposure rate as a givendistance from the source can becomputed. Exposure rate at 1 centimeter and activityare related by a quantity called thespecific gamma constant (Γ). Assumes that source is a point source.

Computing Exposure RateR ΓA /2rR exposure rate (roentgens/hr)Γ specific gamma constant (R/hr-mCi at 1 cm)A source activity (mCi)R distance from source (cm)

Half Value Layer (HVL) Is the thickness of a material required toreduce the transmitted exposure rate (R)to one half the incident exposure rate(R0). HVL depends upon the material’s atomicnumber and density, and upon the energyspectrum of the incident photons.

Photon Attenuation by Adding HVLs8 R/min16 R/minHVL4 R/min16 R/min2 R/min16 R/min1 R/min

Half Value Layer (HVL)Energy 000.0241.5241250.0272.0321500.0292.235

Attenuation of Photons byShieldingR R0 ( exp ( - 0.693 t / HVL ) )R Attenuated exposure rateR0 Primary Exposure Ratet thickness of shielding (cm)HVL “Half Value Layer” (cm)

Attenuator Blocks to Modify Irradiator Dose Rate“Stacking” lead attenuator blocks can incrementally reduce thedose-rate and shape the dose profile inside the irradiation chamber

Calorimetric Dosimetry Energy released in a medium by ionizingradiation ultimately degraded to thermalenergy. Thermal energy will raise thetemperature of the medium. For water, 1.0 Gy increases thetemperature by 0.24 mK (0.00024 degreecentigrade)

Thermoluminescence DosimetryRadiation produces free electrons in the crystal, which fall into “traps” at thesites of lattice imperfections. Later, the crystal is heated, which liberates the“trapped” electrons. This process releases light, in proportion to the originalradiation dose.“Glow” Curve“Trap”Ionizing RadiationLuminescentphotonLithium Fluoride LatticeImpurity Atom

TLD “Chips” are Tissue Equivalent and Can be Miniaturized5 mm

Optically Stimulated Luminescence DosimetryRadiation produces free electrons in the crystal, which fall into “traps” at thesites of lattice imperfections. Later, the crystal is exposed to a burst of laserlight, which liberates the “trapped” electrons. This process releases light, inproportion to the original radiation dose.“Trap”Stimulating Laser BeamRadiationAluminum OxideImpurity AtomLuminescentphoton

Polyacrylamide Gel DosimetryWhen irradiated, polyacrylamidepolymer gels change chemicalcharacteristics. Tubes have beenirradiated with 0 (left) to 11 (right)Gy.Chemical changes can bequantified by MRI scanning.Changes in T1 can calibrateabsorbed dose.Source: Prague 3D Gel Dosimetry Group (http://3dgeldos.fjfi.cvut.cz/results/)

Polyacrylamide Gel DosimetrySource: Prague 3D Gel Dosimetry Group (http://3dgeldos.fjfi.cvut.cz/results/)

Polyacrylamide Gel DosimetrySource: Prague 3D Gel Dosimetry Group (http://3dgeldos.fjfi.cvut.cz/results/)

MOSFET Dosimetry7 mmMOSFETMOSFET detectors are semiconductors that generate measurableelectric current when irradiated. Current is proportional to dose rate.

Course Objectives Understand the basic physics of the electromagnetic and particulate forms of ionizing radiation. Understand the distinctions between the units of radiation quantity, exposure and dose. Be familiar