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 ATLAS and CMS experiments at the CERN LHC Neutrino physics experiments throughout the world Nuclear physics ALICE experiment at the CERN LHC Understanding the structure of the nucleon at JLAB Astronomy/astrophysics CCD’s on Hubble, Keck, LSST, … , amateur telescopes HESS and GLAST gamma ray telescopes Antimatter measurements with PAMELA and AMS Condensed matter/material science/ chemistry/biology 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 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 Produced in atomic transitions (characteristic x-rays) or in electron deacceleration (bremsstrahlung) Gamma rays 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) 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)} M(Z,N) is the mass of the neutral atom 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) 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 Q>0 => energy released as kinetic energy 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 N()t = N (0 )e−λt where N ()t is the number at time t 1 N()t = N (0 )e−t /τ where τ = 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 = − = λN ()t = A e−λt dt 0 A0 = λN0 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 25 Radioactivity ¾Activity units bequerel (Bq) 1 Bq = 1 disintegration / s Common unit is MBq curie (C) 1 C = 3.7 x 1010 disintegrations / s Originally defined as the activity of 1 g of radium 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 λ1 λ2 1→2→3 226Ra→222Rn→218Po ¾The daughter is both being created and removed 28 Radioactivity ¾We have (assuming N1(0)=N0 and N2(0)=0) dN1 = −λ1N1dt dN2 = λ1N1dt − λ2 N2dt then λ1 −λ1t −λ2t N2 ()t = N0 ()e − e λ2 − λ1 λ2 −λ1t −λ2t A2 ()t = λ2 N2 ()t = A0 ()e − e λ2 − λ1 and maximum activity at ln()λ2 / λ1 tmax = λ2 − λ1 29 Radioactivity ¾Case 1 (parent half-life > daughter half-life) This is called transient equilibrium λ1 < λ2 −λ1t N1()t = N0e λ1 −λ1t −λ2t N2 ()t = N0 ()e − e λ2 − λ1 becomes λ N λ 2 2 = 2 ()1− e−()λ2 −λ1 t λ1N1 λ2 − λ1 A λ 2 ≈ 2 A1 λ2 − 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 λ1 << λ2 λ1 −λ1t −λ2t N2 ()t = N0 ()e − e λ2 − λ1 becomes λ1 −λ2t N2 ()t ≈ N0 ()1− e λ2 λ2 N2 ()t ≈ N0λ1 A ≈ A 2 1 32 Radioactivity ¾Secular equilibrium A1=A2 Daughter nuclei are decaying at the same rate they are formed 33.