introduction - university of arizonaatlas.physics.arizona.edu/~shupe/physics_courses/... · 7...

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