Introduction

1. Introduction • A 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 physics • However there is much overlap with experimental astro-, particle and nuclear physics • And examples will be drawn from all of these fields

2. Introduction • While 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 • Advances in medical physics have often followed quickly from advances in particle physics

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

4. Introduction • Ernest 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. Introduction • Importance 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

6. Introduction • Applications 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

7. Introduction • 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) • Huge number of applications with hundreds of billions of $ and millions of jobs

8. Introduction

9. Introduction • Cargo scanning using linear accelerators

10. Radiation • Directly ionizing radiation (energy is delivered directly to matter) • Charged particles • Electrons, protons, muons, alphas, charged pions and kaons, … • Indirectly ionizing radiation (first transfer their energy to charged particles in matter) • Photons • Neutrons • 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 Spectrum • Note the fuzzy overlap between hard x-rays and gamma rays • Sometimes 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 • The physics is the same; they are both just photons

13. Nuclear Terminology • Nuclear species == nuclide • A nucleons (mass number), • Z protons (atomic number) • N neutrons (neutron number) • A = Z+N • Nuclides with the same Z ==isotopes • Nuclides with the same N ==isotones • Nuclides with the same A ==isobars • Identical nuclides with different energy states ==isomers • Metastable excited state (T1/2>10-9s)

14. Table of Nuclides • Plot of Z vs N for all nuclides • Detailed information for ~ 3000 nuclides

15. Table of Nuclides • Here are some links to the Table of Nuclides which contain basic information about most known nuclides • http://www.nndc.bnl.gov/nudat2 • http://atom.kaeri.re.kr/ton/ • http://ie.lbl.gov/education/isotopes.htm • http://t2.lanl.gov/data/map.html • http://yoyo.cc.monash.edu.au/~simcam/ton/

16. Table of Nuclides • ~3000 nuclides but only ~10% are stable • No 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 produced • Most stable nuclei have N=Z • True for small N and Z • For heavier nuclei, N>Z

17. Valley of Stability

18. Valley of Stability • Table also contains information on decays of unstable nuclides • Alpha decay • Beta (minus or plus) decay • Isomeric transitions (IT) • Spontaneous fission (SF)

19. Valley of Stability

20. Binding Energy • The 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 • 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 Energy • Separation 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 energy • Q<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 Note these are atomic 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 • Nomenclature • λ in 1/s = decay rate • λ in MeV = decay width (h-bar λ) • τ in sec = lifetime • You’ll also see Γ = λ

24. Radioactivity • t1/2 = time for ½ the nuclei to decay

25. Radioactivity • It’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

26. Radioactivity • Activity 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

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 Γ • Γ = ∑Γi where Γi are called the partial widths • The branching fraction or ratio for channel or state i is simply Γi/Γ

28. Radioactivity • Sometimes we have the situation where • The daughter is both being created and removed

29. Radioactivity • We have (assuming N1(0)=N0 and N2(0)=0)

30. Radioactivity • Case 1 (parent half-life > daughter half-life) • This is called transient equilibrium

31. Radioactivity • Transient equilibrium • A2/A1=l2/(l2-l1) • Example is 99Mo decay (67h) to 99mTc decay (6h) • Daughter nuclei effectively decay with the decay constant of the parent

32. Radioactivity • Case 2 (parent half-life >> daughter half-life) • This is called secular equilibrium • Example is 226Ra decay

33. Radioactivity • Secular equilibrium • A1=A2 • Daughter nuclei are decaying at the same rate they are formed

34. Radioactivity • Case 3 (parent half-life < daughter half-life) • What happens?

35. Units • Sometimes I will slide into natural units used in particle physics • Then at the end of the calculation or whatever we’ll insert h-bar’s and c’s to make the answer dimensionally correct • And while it might not come up so often

36. Electromagnetic Spectrum • What part of the EM spectrum has a physiological effect on the human body?

37. Radioactivity • Case 3 (parent half-life < daughter half-life) • What happens? • Parent decays quickly away, daughter activity rises to a maximum and then decays with its characteristic decay constant

38. Electromagnetic Spectrum • What part of the EM spectrum has a physiological effect on the human body?

39. Electromagnetic Spectrum • Photon energy is given by

40. Constants and Conversions