1 Introduction ¾ A more general title for this course might be “Radiation Detector Physics” ¾ Goals are to understand the physics, detection, and applications of ionizing radiation The emphasis for this course is on radiation detection and applications to radiological physics However there is much overlap with experimental astro-, particle and nuclear physics And examples will be drawn from all of these fields
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IntroductionA more general title for this course might be “Radiation Detector Physics”Goals are to understand the physics, detection, and applications of ionizing radiation
The emphasis for this course is on radiation detection and applications to radiological physicsHowever there is much overlap with experimental astro-, particle and nuclear physicsAnd examples will be drawn from all of these fields
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IntroductionWhile particle and medical radiation physics may seem unrelated, there is much commonality
Interactions of radiation with matter is the sameDetection principals of radiation are the sameSome detectors are also the same, though possibly in different guises
Advances in medical physics have often followed quickly from advances in particle physics
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IntroductionRoentgen discovered x-rays in 1895 (Nobel Prize in 1901)A few weeks later he was photographing his wife’s handLess than a year later x-rays were becoming routine in diagnostic radiography in US, Europe, and JapanToday the applications are ubiquitous (CAT, angiography, fluoroscopy, …)
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IntroductionErnest Lawrence invented the cyclotron accelerator in 1930 (Nobel Prize in 1939) Five years later, John Lawrence began studies on cancer treatment using radioisotopes and neutrons (produced with the cyclotron)Their mother saved from cancer using massive x-ray dose
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IntroductionImportance and relevance
Radiation is often the only observable available in processes that occur on very short, very small, or very large scalesRadiation detection is used in many diverse areas in science and engineeringOften a detailed understanding of radiation detectors is needed to fully interpret and understand experimental results
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IntroductionApplications of particle detectors in science
Particle physicsATLAS and CMS experiments at the CERN LHCNeutrino physics experiments throughout the world
Nuclear physicsALICE experiment at the CERN LHCUnderstanding the structure of the nucleon at JLAB
Astronomy/astrophysicsCCD’s on Hubble, Keck, LSST, … , amateur telescopesHESS and GLAST gamma ray telescopesAntimatter measurements with PAMELA and AMS
Variety of experiments using synchrotron light sources throughout the world
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IntroductionApplications of radiation/radiation detectors in industry
Medical diagnosis, treatment, and sterilizationNuclear power (both fission and fusion)Semiconductor fabrication (lithography, doping)Food preservation through irradiationDensity measurements (soil, oil, concrete)Gauging (thickness) measurements in manufacturing (steel, paper) and monitoring (corrosion in bridges and engines)Flow measurements (oil, gas)Insect control (fruit fly)Development of new crop varieties through genetic modificationCuring (radiation curing of radial tires) Heat shrink tubing (electrical insulation, cable bundling)
Huge number of applications with hundreds of billions of $ and millions of jobs
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Introduction
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Introduction
Cargo scanning using linear accelerators
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RadiationDirectly ionizing radiation (energy is delivered directly to matter)
Charged particlesElectrons, protons, muons, alphas, charged pions and kaons, …
Indirectly ionizing radiation (first transfer their energy to charged particles in matter)
PhotonsNeutrons
Biological systems are particularly sensitive to damage by ionizing radiation
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Electromagnetic Spectrum
Our interest will be primarily be in the region from 100 eV to 10 MeV
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Electromagnetic SpectrumNote the fuzzy overlap between hard x-rays and gamma raysSometimes the distinction is made by their source
X-raysProduced in atomic transitions (characteristic x-rays) or in electron deacceleration (bremsstrahlung)
Gamma raysProduced in nuclear transitions or electron-positron annihilation
The physics is the same; they are both just photons
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Nuclear TerminologyNuclear species == nuclide
A nucleons (mass number), Z protons (atomic number)N neutrons (neutron number)A = Z+N
Nuclides with the same Z == isotopesNuclides with the same N == isotones
Nuclides with the same A == isobarsIdentical nuclides with different energy states == isomers
Metastable excited state (T1/2>10-9s)
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Table of Nuclides
Plot of Z vs N for all nuclidesDetailed information for ~ 3000 nuclides
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Table of NuclidesHere are some links to the Table of Nuclides which contain basic information about most known nuclides
Binding EnergySeparation energies can also be calculated as
Q, the energy released, is just the negative of the separation energy S
Q>0 => energy released as kinetic energyQ<0 => kinetic energy converted to nuclear mass or binding energy
Sometimes the tables of nuclides give the mass excess (defect) Δ = {M (in u) – A} x 931.5 MeV
( ) ( ) ( )( ) ( ) ( )( ) ( ) ( )XMHeMXMS
XMHMXMS
XMnMXMS
AZ
AZ
AZ
AZp
AZ
AZn
−+=
−+=
−+=
−−
−−
−
422
111
1
α
Note these areatomic masses
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Example
Is 238U stable wrt to α decay?Sα = B(238U) - B(234Th) - B(4He)Sα = 1801694 – 1777668 – 28295 (keV)Sα = -4.27 MeV => Unstable and will decay
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Radioactivity
Radioactive decay law
Nomenclatureλ in 1/s = decay rateλ in MeV = decay width (h-bar λ)τ in sec = lifetimeYou’ll also see Γ = λ
( ) ( ) ( )
( ) ( ) lifetimemean theis 1 where0
tat timenumber theis where0
/
λτ
λ
τ
λ
==
=
−=
−
−
t
t
eNtN
tNeNtNNdtdN
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Radioactivityt1/2 = time for ½ the nuclei to decay
( )
λτ
τ
τ
2ln2ln
21ln
2
2/1
/0
0
==
−=
== −
t
t
eNNtN t
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RadioactivityIt’s easier to measure the number of nuclei that have decayed rather than the number that haven’t decayed (N(t)) The activity is the rate at which decays occur
Measuring the activity of a sample must be done in a time interval Δt << t1/2
Consider t1/2=1s, measurements of A at 1 minute and 1 hour give the same number of counts
( ) ( ) ( )
00
0
NA
eAtNdt
tdNtA t
λ
λ λ
=
==−= −
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Radioactivity
Activity unitsbequerel (Bq)
1 Bq = 1 disintegration / sCommon unit is MBq
curie (C)1 C = 3.7 x 1010 disintegrations / sOriginally defined as the activity of 1 g of radiumCommon unit is mC or μC
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Radioactivity
Often a nucleus or particle can decay into different states and/or through different interactions
The branching fraction or ratio tells you what fraction of time a nucleus or particle decays into that channel
A decaying particle has a decay width ΓΓ = ∑Γi where Γi are called the partial widthsThe branching fraction or ratio for channel or state i is simply Γi/Γ
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RadioactivitySometimes we have the situation where
The daughter is both being created and removed
PoRnRa 218222226
32121
→→
→→λλ
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RadioactivityWe have (assuming N1(0)=N0 and N2(0)=0)