Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
1
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
2
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
3
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, …)
4
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
5
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
6
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
Condensed matter/material science/ chemistry/biology
Variety of experiments using synchrotron light sources throughout the world
7
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
8
Introduction
9
Introduction
Cargo scanning using linear accelerators
10
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
11
Electromagnetic Spectrum
Our interest will be primarily be in the region from 100 eV to 10 MeV
12
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
13
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)
14
Table of Nuclides
Plot of Z vs N for all nuclidesDetailed information for ~ 3000 nuclides
15
Table of NuclidesHere are some links to the Table of Nuclides which contain basic information about most known nuclides
http://www.nndc.bnl.gov/nudat2/chartNuc.jsphttp://www.nndc.bnl.gov/chart/http://ie.lbl.gov/education/isotopes.htmhttp://t2.lanl.gov/data/map.htmlhttp://yoyo.cc.monash.edu.au/~simcam/ton/
16
Table of Nuclides~3000 nuclides but only ~10% are stableNo stable nuclei for Z > 83 (bismuth)Unstable nuclei on earth
Naturally found if τ > 5x109 years (or decay products of these long-lived nuclides)
238U, 232Th, 235U (Actinium) series
Laboratory producedMost stable nuclei have N=Z
True for small N and ZFor heavier nuclei, N>Z
17
Valley of Stability
18
Valley of StabilityTable also contains information on decays of unstable nuclides
Alpha decay
Beta (minus or plus) decay
Isomeric transitions (IT)
Spontaneous fission (SF)
HeThU 42
23490
23892 +→
γ+→ TcTcm 9943
9943
HeThU 42
23490
23892 +→
eveBaCs ++→ −13756
13755
nPdXeFm 411246
14054
256100 ++→
19
Valley of Stability
20
Binding EnergyThe binding energy B is the amount of energy it takes to remove all Z protons and N neutrons from the nucleus
B(Z,N) = {ZMH + NMn - M(Z,N)}M(Z,N) is the mass of the neutral atomMH is the mass of the hydrogen atom
One can also define proton, neutron, and alpha separation energies
Sp = B(Z,N) - B(Z-1,N)Sn = B(Z,N) - B(Z,N-1)Sα = B(Z,N) - B(Z-2,N-2) - B(4He)
Similar to atomic ionization energies
21
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
22
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
23
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
24
Radioactivityt1/2 = time for ½ the nuclei to decay
( )
λτ
τ
τ
2ln2ln
21ln
2
2/1
/0
0
==
−=
== −
t
t
eNNtN t
25
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
=
==−= −
26
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
27
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
28
RadioactivitySometimes we have the situation where
The daughter is both being created and removed
PoRnRa 218222226
32121
→→
→→λλ
29
RadioactivityWe have (assuming N1(0)=N0 and N2(0)=0)
( ) ( )
( ) ( ) ( )
( )12
12max
12
20222
12
102
22112
111
/lnat activity maximum and
then
21
21
λλλλ
λλλλ
λλλ
λλλ
λλ
λλ
−=
−−
==
−−
=
−=−=
−−
−−
t
eeAtNtA
eeNtN
dtNdtNdNdtNdN
tt
tt
30
RadioactivityCase 1 (parent half-life > daughter half-life)
This is called transient equilibrium
( )
( ) ( )
( )( )
12
2
1
2
12
2
11
22
12
102
01
21
12
21
1
1
becomes
λλλ
λλλ
λλ
λλλ
λλ
λλ
λλ
λ
−≈
−−
=
−−
=
=
<
−−
−−
−
AA
eNN
eeNtN
eNtN
t
tt
t
31
RadioactivityTransient equilibrium
A2/A1=λ2/(λ2-λ1)Example is 99Mo decay (67h) to 99mTc decay (6h)Daughter nuclei effectively decay with the decay constant of the parent
32
RadioactivityCase 2 (parent half-life >> daughter half-life)
This is called secular equilibriumExample is 226Ra decay
( ) ( )
( ) ( )( )
12
1022
2
102
12
102
21
2
21
1
becomes
AANtN
eNtN
eeNtN
t
tt
≈≈
−≈
−−
=
<<
−
−−
λλλλ
λλλ
λλ
λ
λλ
33
RadioactivitySecular equilibrium
A1=A2
Daughter nuclei are decaying at the same rate they are formed