Introduction ¾A more general title for this course might be “Radiation Detector Physics” ¾Goals 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

1 Introduction ¾While particle and medical radiation physics may seem unrelated, there is much commonality „ Interactions of radiation with matter is the same „ Detection principals of radiation are the same „ Some detectors are also the same, though possibly in different guises ¾Advances in medical physics have often followed quickly from advances in particle physics 2 Introduction

¾ Roentgen discovered x-rays in 1895 (Nobel Prize in 1901) ¾ A few weeks later he was photographing his wife’s hand ¾ Less than a year later x-rays were becoming routine in diagnostic radiography in US, Europe, and Japan ¾ Today the applications are ubiquitous (CAT, angiography, fluoroscopy, …)

3 Introduction

¾ Ernest Lawrence invented the cyclotron accelerator in 1930 (Nobel Prize in 1939) ¾ Five years later, John Lawrence began studies on cancer treatment using radioisotopes and neutrons (produced with the cyclotron) ¾ Their mother saved from cancer using massive x- ray dose

4 Introduction ¾Importance and relevance

„ Radiation is often the only observable available in processes that occur on very short, very small, or very large scales

„ Radiation detection is used in many diverse areas in science and engineering

„ Often a detailed understanding of radiation detectors is needed to fully interpret and understand experimental results

5 Introduction ¾Applications of particle detectors in science „ Particle physics Š ATLAS and CMS experiments at the CERN LHC Š Neutrino physics experiments throughout the world „ Nuclear physics Š ALICE experiment at the CERN LHC Š Understanding the structure of the nucleon at JLAB „ Astronomy/astrophysics Š CCD’s on Hubble, Keck, LSST, … , amateur telescopes Š HESS and GLAST gamma ray telescopes Š Antimatter measurements with PAMELA and AMS „ Condensed matter/material science/ chemistry/biology Š Variety of experiments using synchrotron light sources throughout the world

6 Introduction ¾ Applications of radiation/radiation detectors in industry „ Medical diagnosis, treatment, and sterilization „ Nuclear power (both fission and fusion) „ Semiconductor fabrication (lithography, doping) „ Food preservation through irradiation „ Density 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 modification „ Curing (radiation curing of radial tires) „ Heat shrink tubing (electrical insulation, cable bundling) ¾ Huge number of applications with hundreds of billions of $ and millions of jobs 7 Introduction

8 Introduction ¾Cargo scanning using linear accelerators

9 Radiation ¾Directly ionizing radiation (energy is delivered directly to matter)

„ Charged particles Š Electrons, protons, muons, alphas, charged pions and kaons, … ¾Indirectly ionizing radiation (first transfer their energy to charged particles in matter)

„ Photons

„ Neutrons ¾Biological systems are particularly sensitive to damage by ionizing radiation

10 Electromagnetic Spectrum ¾Our interest will be primarily be in the region from 100 eV to 10 MeV

11 Electromagnetic Spectrum ¾Note the fuzzy overlap between hard x-rays and gamma rays ¾Sometimes the distinction is made by their source

„ X-rays Š Produced in atomic transitions (characteristic x-rays) or in electron deacceleration (bremsstrahlung)

„ Gamma rays Š Produced in nuclear transitions or electron-positron annihilation ¾The physics is the same; they are both just photons

12 Nuclear Terminology ¾Nuclear species == nuclide

„ A nucleons (mass number),

„ Z protons (atomic number)

„ N neutrons (neutron number)

„ A = Z+N ¾Nuclides with the same Z == isotopes ¾Nuclides with the same N == isotones ¾Nuclides with the same A == isobars ¾Identical nuclides with different energy states == isomers -9 „ Metastable excited state (T1/2>10 s) 13 Table of Nuclides

¾Plot of Z vs N for all nuclides ¾Detailed information for ~ 3000 nuclides 14 Table of Nuclides ¾Here are some links to the Table of Nuclides which contain basic information about most known nuclides

„ http://www.nndc.bnl.gov/nudat2/chartNuc.jsp

„ http://www.nndc.bnl.gov/chart/

„ http://ie.lbl.gov/education/isotopes.htm

„ http://t2.lanl.gov/data/map.html

„ http://yoyo.cc.monash.edu.au/~simcam/ton/

15 Table of Nuclides ¾~3000 nuclides but only ~10% are stable ¾No stable nuclei for Z > 83 (bismuth) ¾Unstable nuclei on earth 9 „ Naturally found if τ > 5x10 years (or decay products of these long-lived nuclides) Š 238U, 232Th, 235U (Actinium) series

„ Laboratory produced ¾Most stable nuclei have N=Z

„ True for small N and Z

„ For heavier nuclei, N>Z

16 Valley of Stability

17 Valley of Stability

¾ Table also contains information on decays of unstable nuclides 238 234 4 92U → 90Th+2 He „ Alpha decay 238 234 4 92U → 90Th+2 He „ Beta (minus or plus) decay 137 137 − 55 Cs→ 56 Ba + e + ve „ Isomeric transitions (IT) 99m 99 43Tc→ 43Tc + γ

„ Spontaneous fission (SF)

256 140 112 100 Fm→ 54 Xe+ 46 Pd + 4n

18 Valley of Stability

19 Binding Energy ¾The 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)} Š M(Z,N) is the mass of the neutral atom

Š MH is the mass of the hydrogen atom ¾One 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) 4 „ Sα = B(Z,N) - B(Z-2,N-2) - B( He) Š Similar to atomic ionization energies

20 Binding Energy ¾Separation energies can also be calculated as

A−1 A Sn = M ( Z X )+ M (n)− M (Z X ) Note these are A−1 1 A atomic masses S p = M ()()()Z −1 X + M H − M Z X ()()()A−2 4 A Sα = M Z −2 X + M He − M Z X

„ Q, the energy released, is just the negative of the separation energy S Š Q>0 => energy released as kinetic energy Š Q<0 => kinetic energy converted to nuclear mass or binding energy ¾Sometimes the tables of nuclides give the mass excess (defect) „ Δ = {M (in u) – A} x 931.5 MeV 21 Example ¾Is 238U stable wrt to α decay? 238 234 4 „ Sα = B( U) - B( Th) - B( He)

„ Sα = 1801694 – 1777668 – 28295 (keV)

„ Sα = -4.27 MeV => Unstable and will decay

22 Radioactivity ¾Radioactive decay law dN = −λNdt λ N()t = N 0 (e− )t where N t is the () number at time t () ( ) τ 1 N t = N 0 e−t /τ where = is the mean lifetime λ ¾Nomenclature

„ λ in 1/s = decay rate

„ λ in MeV = decay width (h-bar λ)

„ τ in sec = lifetime

„ You’ll also see Γ = λ 23 Radioactivity

¾t1/2 = time for ½ the nuclei to decay N N ()t = 0 = N e−t /τ 2 0 1 t ln = − 2 τ ln 2 t = τ ln 2 = 1/ 2 λ

24 Radioactivity

¾ It’s easier to measure the number of nuclei that have decayed rather than the number that haven’t decayed (N(t)) ¾ The activity is the rate at which decays occur dN (t) A()t = − = N ()t = A e−λt dt 0 λ A0 = λN0

„ Measuring the activity of a sample must be done in

a time interval Δt << t1/2

Š Consider t1/2=1s, measurements of A at 1 minute and 1 hour give the same number of counts 25 Radioactivity ¾Activity units

„ bequerel (Bq) Š 1 Bq = 1 disintegration / s Š Common unit is MBq

„ curie (C) Š 1 C = 3.7 x 1010 disintegrations / s Š Originally defined as the activity of 1 g of radium Š Common unit is mC or μC

26 Radioactivity ¾Often 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 ¾A decaying particle has a decay width Γ

„ Γ = ∑Γi where Γi are called the partial widths

„ The branching fraction or ratio for channel or state i is simply Γ /Γ i 27 Radioactivity ¾Sometimes we have the situation where λ 1 λ2 1→2→3 226Ra→222Rn→218Po ¾The daughter is both being created and removed

28 Radioactivity ¾We have (assuming N (0)=N and N (0)=0) λ 1 0 2 dN = −λ N dt 1 λ 1 1 dN2 = 1N1dt − 2 N2dt λ then λ λ ()λ 1 ()− 1t − 2t N2 t = Nλ0 e − e 2 − 1 λ λ A ()t = N t = ()λA 2 ()e− 1t − e−λ2t 2 2 2 0 − λ 2 1 and maximum activityλ at λ ln()2 λ/ 1 tmax = λ 2 − λ1 29 Radioactivity ¾Case 1 (parent half-life > daughter half-life)

„ This is called transient equilibrium λ

λ1 < λ2

− 1t N1()t = N0e λ λ () 1 − 1t λ − 2t N2 t λ= N0 ()e − e λ 2 − 1 λ becomes

λ λ λ N −()−λ t 2 2 λ= 2 ()1− e 2 1 1N1 2λ− 1 A λ 2 ≈ λ 2 A1 2 − λ1 30 Radioactivity ¾Transient equilibrium

„ A2/A1=λ2/(λ2-λ1) 99 „ Example is Mo decay (67h) to 99mTc decay (6h)

„ Daughter nuclei effectively decay with the decay constant of the parent

31 Radioactivity ¾Case 2 (parent half-life >> daughter half-life)

„ This is called secular equilibrium 226 „ Example is Ra decay λ << λ 1 λ2 λ 1 − 1t − 2t N ()t =λ N ()e −λ e 2 0 λ 2 − 1 becomes λ N ()t ≈ N 1 ()1− e−λ2t λ 2 0λ 2

2 N2 ()t ≈ N0λ1 A ≈ A 2 1 32 Radioactivity ¾Secular equilibrium

„ A1=A2 „ Daughter nuclei are decaying at the same rate they are formed

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