1. Ionizing Radiation radioactivity measurements
High energy particles and photons that ionise atoms and molecules along their tracks in a medium are called ionizing radiation. For example, a, b, g, cosmic rays and X-rays are ionizing radiation.
Most radioactive measurement are based on their ionizing effect.
Ionizing radiation causes illness such as cancer and death. Radiation effect is a health and safety concern.
Ionizing radiation can also be used in industry for various purposes.
Light and microwaves that do not ionize atoms and molecules are called non-ionizing radiation.
Ionizing Radiation

2. Discovery of Ionization by Radiation
X-rays and radioactivity discharged a charged electroscope. Curie and Rutherford attributed the discharge to the ionization of air by these rays.
Ionizing Radiation

3. Ionization Energy of Gases
The minimum energy required to remove an outer electron from atoms or molecules is called ionization potential. Ionizing radiation also remove electrons in atomic inner shell, and the average energy per ion pair is considered ionization energy
He + 25 eV  He+ + e-
He eV  He2+ + e-
Ionization energy (IE eV) per ion pair of some substances Material Air Xe He NH3 Ge-crystal Average IE
Ionizing Radiation

4. Primary and Secondary Ion Pairs
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+-ooooooooooooooo
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Primary ion pairs are caused directly by radiation. Secondary ion pairs are generated by high-energy primary electrons.
Molecular density (molecules/mL) air = 2.7e19 water = 3.3e22
Ionizing Radiation

5. Interaction of Heavy Charged Particles with Matter
Fast moving protons, 4He, and other nuclei are heavy charged particles.
Coulomb force dominates charge interaction.
They ionize and excite (give energy to) molecules on their path.
Ionizing Radiation

6. Energy Loss of Heavy Charged Particles in Matter
Stopping power is the rate of energy loss per unit length along the path.
Stopping power is proportional to the mass number A, and to the square of atomic number, Z2, of a medium, but inversely proportional to the energy of the particle E.
The surges of ion density before they stop give the Bragg peaks.
Ionizing Radiation

7. Range of Heavy Charged Particles in a Medium
Particles lose all their energy at a distance called range.
 source
Shield
Ionizing Radiation

8. Range of Heavy Charged Particles in a Medium
The range can be used to determine the energy of the particles and the radiation source.
Ionizing Radiation

9. Speed of Particles Speed of 1 MeV a particle
1.6e-13 J = (½) m v = (½)(41.66e-27 kg) v2
Solving for v v2 = 4.82e13 (m/s) v = 6.9e6 m/s
Speed of 1 MeV (= 1.6e-13 J) electron
1.6e-13 J = (1/2) m v2 = (½) 9.1e-31 kg v2 Solving for v, v2 = 3.52e17 (m/s)2 or v = 5.9e8 m/s.
exceeds c (=3e8 m/s), the speed limit Proper evaluation method shown next
Still reasonable
Ionizing Radiation

10. Proper Evaluation of Particle Speed
The relativity mass m of a particle of kinetic energy Ek is the sum of the rest mass and its kinetic energy
m = Ek MeV MeV (rest mass of electron)
For an electron with Ek = 1.0 MeV, m = 1.51 MeV
The speed of an electron with a kinetic energy 1.0 MeV is evaluated by applying the Einstein’s equation:
m = mo / (1-(v/c)2)
This speed is a 80% of c, the speed of light.
Ionizing Radiation

11. Scattering of Electrons in a Medium
Fast moving electrons are light charged particles.
They travel at higher speed., but scattered easily by electrons.
 source
Shield
Ionizing Radiation

12. Range of Light Charged Particles in a Medium
Range of b particles is not as well defined as heavy charged particles, but measured range is still a useful piece of information.
Ionizing Radiation

13. Braking Radiation of b particles Influenced by Atom
Bremsstrahlung (braking) radiation refers to photons emitted by moving electrons when they are influence by atoms.
Ionizing Radiation

14. Interaction of Beta particles with Matter
Ionization
Braking radiation
Annihilation
Beta particles interact with matter mainly via three modes:
Ionization (scattering by electrons)
Bremsstrahlung (braking) radiation
Annihilation with positrons
Ionizing Radiation

15. Interaction of Photons with Matter
Photon Energies
Visible red light eV visible blue light 3.0 eV
UV few eV-hundreds eV
X-rays to 60 keV
Gamma rays keV - some MeV
Interactions of gamma rays with matter:
photoelectric effect
Compton effect
Pair productions
Ionizing Radiation

16. Compton Effect of Gamma Rays
When a photon transfers part of its energy to an electron, and the photon becomes less energetic is called Compton effect.
Ionizing Radiation

17. The animated Compton effect
Ionizing Radiation

18. Pair Production of Gamma Rays
Gamma photons with energy greater than 1.02 MeV produce a electron-positron pair is called pair production.
Ionizing Radiation

19. Gamma-ray Three Modes of Interaction with Matter
Photoelectric effect Compton scattering pair production
Ionizing Radiation

20. Attenuation of Gamma Rays by Matter
Gamma-ray intensity decreases exponentially as the thickness of the absorber increases.
I = Io e–c x
I: Intensity at distance x c: absorption constant x: thickness
Ionizing Radiation

21. Ionizing Radiation Measurements
Radiation Detectors - overall view
electroscopes ionization chambers proportional counters Geiger-Muller counters
solid-state detectors
photographic films and photographic emulsion plates
bubble chambers and cloud chambers
scintillation counters and fluorescence screen
Ionizing Radiation

22. Ionization Chambers
Current (A) is proportional to charges collected on electrode in ionization chambers.
Ionizing Radiation

23. Proportional Counters
Gas Multiplication –+ 
–+–+–+
–+–+–+–+–+–+–+–+–+
–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+–+
X00 V
Proportional counters
Gas multiplication due to secondary ion pairs when the ionization chambers operate at higher voltage.
Ionizing Radiation

24. Geiger-Muller Counters
1X00 V
Every ionizing particle causes a discharge in the detector of G-M counters.
Ionizing Radiation

25. Solid-state Detectors
A P-N junction of semiconductors placed under reverse bias has no current flows. Ionizing radiation enters the depleted zone excites electrons causing a temporary conduction. The electronic counter register a pulse corresponding to the energy entering the solid-state detector.
depleted
P N
+ + zone
Negative
Positive
electronic counter
See: bo.iasf.cnr.it/ldavinci/programme/Presentazioni/Harrison_cryo.pdf
Ionizing Radiation

26. A simple view of solid-state detectors
Solid-state detectors are usually made from germanium or cadmium-zinc-telluride (CdZnTe, or CZT) semiconducting material. An incoming gamma ray causes photoelectric ionization of the material, so an electric current will be formed if a voltage is applied to the material.
Digirad has developed and made commercially available the world's first solid-state, digital gamma camera for the nuclear medicine imaging market. Our proprietary, solid-state imaging technology is based on a patented, silicon photodiode technology that replaces the vacuum photomultipier tubes (PMTs) used in all other gamma cameras. These photodiodes are coupled to individual scintillation crystals to create a unique detection element for each addressable spatial location of the camera's head. We call this Digital Position Sensing™ technology. It provides images with excellent contrast and spatial resolution.
Ionizing Radiation

27. Scintillation Counters
Photons cause the emission of a short flash in the Na(Tl)I crystal. The flashes cause the photo-cathode to emit electrons.
Ionizing Radiation

28. Scintillation Detector and Photomultiplier tube
Ionizing Radiation

29. Gamma ray spectrum of 207mPb (half-life 0.806 sec)
207mPb Decay Scheme
13/2+____________ keV
- Intensity (log scale)
1063
-1e4
569
5/2-____________569.7 keV
-1e3
1/2-____________0.0 stable
-1e2
Energy
-ray spectrum of 207mPb
Ionizing Radiation

30. Fluorescence Screens
Fluorescence materials absorb invisible energy and emit visible light.
J.J. Thomson used fluorescence screens to see electron tracks in cathode ray tubes. Electrons strike fluorescence screens on computer monitors and TV sets give dots of visible light.
Röntgen saw the shadow of his skeleton on fluorescence screens.
Rutherford observed alpha particle on scintillation material zinc sulfide.
Fluorescence screens are used to photograph X-ray images using films sensitive visible light.
Ionizing Radiation

31. Cloud and Bubble Chambers
The ion pairs on the tracks of ionizing radiation form seeds of gas bubbles and droplets. Formations of droplets and bubbles provide visual appearance of their tracks, 3-D detectors.
C.T.R. Wilson shared the Nobel prize with Compton for his perfection of cloud chambers.
Ionizing Radiation

32. Image Recorded in Bubble Chambers
Charge exchange of antiproton produced neutron-antineutron pair.
p + p  n + n (no tracks)
Annihilation of neutron-antineutron pair produced 5 pions.
n +n  3p+ + 2p- + ?
Only these tracks are sketched.
Ionizing Radiation

33. Bubble Chambers
The Brookhaven 7-foot bubble chamber and the 80-inch bubble chamber 
Ionizing Radiation

34. Image from bubble chamber
This image shows a historical event: one of the eight beam particles (K- at 4.2 GeV/c) which are seen entering the chamber, interacts with a proton, giving rise to the reactions
K– p  – K+ K0
K0  + –
–  0 K–
K+  + 0
0  p –
Ionizing Radiation

35. Photographic Emulsions and Films
Sensitized silver bromide grains of emulsion develope into blackened grains. Plates and films are 2-D detectors.
Roentegen used photographic plates to record X-ray image.
Photographic plates helped Beckerel to discover radioactivity.
Films are routinely used to record X-ray images in medicine but lately digital images are replacing films.
Stacks of films record 3-dimensional tracks of particles.
Photographic plates and films are routinely used to record images made by electrons.
Ionizing Radiation

36. Overall View
Ionizing radiation interacts with matter in various ways: ionization (photoelectric effect), excitation, braking radiation, Compton effect, pair production, annihilation etc.
Mechanisms of interaction are utilized for the detection of ionizing radiation.
Function and principles of electroscope, ionization chambers, proportional chambers, Geiger-Muller counters, solid-state detectors, and scintillation counters, bubble chambers, and cloud chambers have been describe.
Ionizing Radiation

37. The Sudbury Neutrino Observatory
SNO will contain 1000 tonnes of heavy water, held in a 12-m diameter spherical acrylic vessel. It has the ability to detect all three types of neutrinos.
Ionizing Radiation