Dr Paul Sellin Page 1 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Radiation Dosimetry and Detectors: Part 1 Dr Paul Sellin

Page 2 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October Interactions of Radiation with Matter Basic definitions of nuclear radiation:  particles: helium nucleus containing 2 protons and 2 neutrons: Sometimes written as: or: Note: It is unusual to write the neutron number N and the electronic charge, and there numbers are often not shown The alpha particle is a helium nucleus with no electrons  the 2 electrons are removed, so the electronic charge is 2+ Mass of an  = atomic mass units (amu) = x x kg = 6.644x kg atomic number Z = number of protons atomic mass A = number of protons + neutrons neutron number N electronic charge

Dr Paul Sellin Page 3 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Alpha particle decay Alpha particles can be emitted spontaneously by a range of heavy nuclei: where X and Y are the parent and daughter nuclei. A common example of an alpha emitter is 241 Am (241-Americium), where the decay sequence is: See the free interactive Segre chart: e.kr/index.html e.kr/index.html  decay   decay   decay

Dr Paul Sellin Page 4 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Beta emission  particles come in two types:   - which are electrons: charge = -1   + which are positron: charge = +1 Most laboratory sources are  - sources Beta decay similarly comes in two types:  - decay – emission of an electron (and an antineutrino)  + decay – emission of a positron (and a neutrino) The radioactive decay process for these two decay modes can be written as: Note that the neutrino/anti-neutrino are not detectable using conventional radiation detectors. Example of a ‘pure’  - emitter:

Dr Paul Sellin Page 5 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006  - decay of 36 Cl The maximum energy of the emitted  particle is the decay Q value In the detected  spectrum, this is the ‘end point energy’

Dr Paul Sellin Page 6 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006  decay energy spectrum  The Q value of the beta decay reaction is shared between the electron and the antineutrino (in the cased of  - emission)  Because the antineutrino is not detected, the measured energy is only that of the electron E e, which has a characteristic spectral shape:  When E e = 0  the antineutrino ‘takes’ all the decay energy  When E e = Q  the antineutrino takes no a negligible energy – this is the endpoint energy of the pulse height spectrum Calculated  detected energy spectrum for Q=2.5 MeV

Dr Paul Sellin Page 7 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Photons: X-rays and  -rays The electromagnetic spectrum covers a wide range of photon energies, from ionising radiation to radio waves, and including visible light: increasing photon energy

Dr Paul Sellin Page 8 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Nature of gamma rays Gamma rays are photons emitted from excited nuclear states: Excited nuclear states can be produced:  in the daughter nucleus of a preceding alpha or beta decay  from the excited nuclei resulting from fission  from excited nuclei resulting from nuclear reactions In the laboratory,  -,  + sources are often used as gamma ray sources, where the beta-decaying parent nucleus is long lived. The  -emitting daughter states decay very quickly (picoseconds):   energy is the difference between the states involved in the transition  long-lived  -emitting states are called isomers Typical gamma ray energies are ~ 100 keV – 10 MeV NB: 241 Am has a ‘famous’ low energy gamma ray at 59.5 MeV

Dr Paul Sellin Page 9 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Gamma spectra: 60 Co and 137 Cs Decay schemes and end-point energies for the common gamma ray emitters 60 Co, 137 Cs:

Dr Paul Sellin Page 10 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October  + decay which causes a positron to be emitted 2. pair production from high energy gamma rays, which produces an electron/positron pair Annihilation Radiation produces a very characteristic signature: two 511 keV photons are emitted back-to-back X-rays and Annihilation radiation Characteristic X-rays, produced from atomic transitions within the atom. The X-ray energy is characteristic of the element. Typical characteristic X-ray energies are <1 keV to ~80 keV Bremsstrahlung X-rays, these are X-rays generated from electron tubes, with a continuous spectrum of photon energies Annihilation Radiation is an additional source of photons due to the annihilation of a positron with an electron. This can occur from:

Dr Paul Sellin Page 11 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Positron emitter 22 Na: a positron and  -ray source 22 Na is a common  + source: Note that 22 Na is commonly used as a laboratory ‘gamma’ source: MeV annihilation photons MeV gamma ray

Dr Paul Sellin Page 12 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Production of neutrons Within the laboratory, neutrons are generated by two main mechanisms: (a) Spontaneous fission sources Many heavy nuclei have a significant decay probability for spontaneous fission - parent nucleus emits one or more neutrons per fission Eg. 252 Cf decays by: (1)  decay to 248 Cm (2) spontaneous fission Dominant decay mode is  : 32x the fission rate Neutron rate is still very large: 1  g 252 Cf produces 2.3x10 6 n/s (b) Radioisotope ( ,n) sources Mixture of an alpha emitting isotope with a suitable target material  undergoes a nuclear reaction with the target to produce a fast neutron. Eg. 241 Am/Be Common combination: Be has a large positive Q-value Max neutron energy = Q + E  = 11.2 MeV Most alphas a stopped in the source material (typical range ~ 20  m):  1 in 104 alphas react tp produce a neutron  n yield: 80 per 10 6 primary alphas

Dr Paul Sellin Page 13 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Interactions of Radiation with Matter - Overview Nuclear radiation can be grouped into 4 main types, based on their method of interaction with matter: Charge particle radiations Uncharged radiations Heavy charged particles ( , protons, fission fragments)  Neutrons Electrons  Photons: X-rays, gamma rays charged particle interactions: High stopping power Scatter non-charged interactions: Scatter Absorption Reactions (neutrons)

Dr Paul Sellin Page 14 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Interactions of Heavy Charged Particles Heavy charged particles interact with matter by Coulomb scattering from atomic electrons in the material:  many interactions, each losing a small amount of energy  the particle stops quickly, with a ‘short’ range  the particle travel in a straight line – no scatter angles Heavy charge particles mean: alpha particles, protons, fission fragments, heavy ions Particle Range The range depends on the Linear Stopping Power, S, which is defined as the energy loss per unit length travelled:  a high dE/dX means that the particle loses energy more rapidly  shorter range  a low dE/dx means that the particle loses energy more slowly  longer range BUT: dE/dx changes as the particle slows down, so estimating the range is difficult!

Dr Paul Sellin Page 15 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October Alpha particle ranges* Published range curves give estimates for particle ranges. The range depends on:  particle type (eg.  )  particle energy  material type Example: find the range for 5.5 MeV alphas in Si. (Density of Si  = 2.33 g/cm 3 ) Note the quantity displayed on the y-axis: ‘range x density’ with units of mg/cm 2 Divide by density, , to obtain range in cm

Dr Paul Sellin Page 16 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Alpha particle penetration High energy loss (dE/dx) of alphas means they are easily stopped:  Range of 5.5 MeV  in air: ~ 4cm   stopped by almost all thin coatings, eg a sheet of paper   particle sources must be ‘open’:   -emitting isotopes evaporated onto the surface of a metal disk  even varnish or a similar layer will cause the  ’s to lose a lot of energy Detection of  particles: Requires a thin detector with good near-suface sensitivity Detectors with a surface dead-layer are not suitable  Silicon detectors or surface-sensitive scintillators are preferred Contamination with  -emitting nuclei (U, Th, Am):   particles only penetrate micrometers into the skin – all dose is localised  but the dose received is very high – due to  ’s high energy (MeV)

Dr Paul Sellin Page 17 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Interactions of Electrons Electrons (and positrons) interact through two mechanisms: Coulomb Scattering with atomic electrons:  because the electron is light, it is scattered by large angles  overall path of the electron is the sum of many scatters – may not be a straight overall path  energy loss dE/dx is less per collision – electron range is longer Bremsstrahlung, or radiative scattering:  de-acceleration of high energy (relativistic) electrons produces Bremsstrahlung X-rays  only important for high energy electrons produced in X-ray tubes Total linear stopping power S is given by the sum of these 2 terms: Only important for high energy electrons and high-Z materials

Dr Paul Sellin Page 18 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Electron range curves Plot of electron range x density vs energy in silicon and sodium iodide scintillator To a good approximation, most materials follow the same range x density function.

Dr Paul Sellin Page 19 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Interactions of X-rays and gamma rays Photons (X-rays and gamma rays) interact in a material by: (1) Absorption (2) Scattering A photon only interacts at a few discrete points in a material – or can pass through without interaction Three primary photon interaction processes are possible:  photoelectric absorption  Compton scattering  pair production There are other photon interaction mechanisms which are generally less important and outside the scope of this course The relative likelihood of each process occurring depends on: 1.the photon energy 2. the atomic number Z of the material

Dr Paul Sellin Page 20 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Summary plot of photon interactions increasing photon energy makes Compton scatter and pair production more likely increasing Z of the material makes photoelectric absorption more likely

Dr Paul Sellin Page 21 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Attenuation Coefficient* A beam of photons is exponentially attenuated as it passes though a piece of material:  fgdgdg where: and  is the linear attenuation coefficient (units of m -1 ) Note that  varies with:  photon energy  material (Z number) Example: Calculate the % of 1 MeV gamma rays which are transmitted through a 7 mm thick aluminium sheet. Note:  = 0.35 cm -2 for Al at 1 MeV

Dr Paul Sellin Page 22 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Mass Attenuation Coefficient Photon attenuation is often expressed in terms of the mass attenuation coefficient: mass attenuation coefficient = where  is the material density The units of are cm 2 /g If the mass attenuation coefficient is known, then the transmitted photon intensity I is given by: To calculate I/I 0 knowing the mass attenuation coefficient, the material density must also be known

Dr Paul Sellin Page 23 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Total attenuation coefficient The overall attenuation coefficient  is given by the reciprocal sum of the attenuation coefficients for the 3 main processes: Example figure shows the mass attenuation coefficient for lead:

Dr Paul Sellin Page 24 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Interactions of Neutrons Neutrons are classified as fast, epithermal, or thermal: Fast Neutrons (eg. energies up to MeV)  mainly interact by scattering, particularly of light nuclei  proton recoil scattering is commonly used for detection: eg. in organic scintillators Thermal/Epithermal Neutrons (eg no kinetic, only thermal, energy)  mainly interact by reactions  several common isotopes have very large cross-sections for thermal neutrons, commonly used in neutron detectors: 10 B: 10 B(n,  ) 7 Li reactionQ=2.79 MeVs=3840 barns* 6 Li: 6 Li(n,a) 1 H reactionQ=4.78 MeVs=940 barns 3 He: 3 He(n,p) 3 H reactionQ=0.76 MeVs=5330 barns *1 barn is a measure of cross section = m -2

Dr Paul Sellin Page 25 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Detector Types (1): Silicon and Germanium Detectors can be grouped into different families depending on type: 1. Semiconductor detectors: measure charge from the radiation event (a) Silicon (‘planar’ or ‘surface’ barrier) – high energy resolution, thin (300  m – 2mm) Ideal for high resolution ,  particle spectroscopy. Very poor detection efficiency for X-rays and gamma rays (b) Germanium (with ‘co-axial’ or ‘planar’ geometry) – large volume, high efficiency for X-rays and gamma rays HPGe (high purity germanium) has the highest energy resolution All Ge detectors need cooling: liquid nitrogen or a ‘cryo-cooler’ Less common detector types: (c) Si(Li) Lithium-drifted silicon detector Thick silicon detector, high resolution and good efficiency for X-rays (d) CdZnTe/CdTe cadmium zinc telluride or cadmium telluride A new detector technology for X-rays and gamma rays Small volume, reasonably high resolution, operates at room temperature (b) (a)

Dr Paul Sellin Page 26 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Detector Types (2): Scintillators 2. Scintillator detectors: measure light from the radiation event, which is converted into a current pulse via a photocathode  Inorganic scintillators: good gamma efficiency, moderate/good energy resolution Eg: Sodium Iodide (NaI), Barium Fluoride (BaF 2 )  Organic scintillators: commonly called ‘plastic’:  large volume, cheap, very fast pulses  poor energy resolution and low gamma efficiency Eg. Plastic scintillator (BC501), liquid scintillator New Lanthanum Chloride scintillators by St Gobain: BrilLanCe 350: 3.8% FWHM at 662 keV

Dr Paul Sellin Page 27 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Detector Types (3): Gas Detectors 3. Gas Detectors: (a) GM Tube (Geiger-Muller tube) Very common radiation monitor Produces pulses but no energy information – a counter Efficient for beta and gamma particle (b) Neutron detectors ( 3 He, BF 3 tubes) Both helium-3 and boron have a very high detection efficiency for thermal neutrons Fast neutrons are detected by surrounding the tube is a moderating plastic (eg. Bonner sphere)

Dr Paul Sellin Page 28 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Personal Dosimetry Detectors Thermo-luminescent detector (TLD badge) Plastic scintillator dose rate meter Thermo Corporation Electronic Personal Dosimeter (EPD)

Dr Paul Sellin Page 29 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Instrumentation Chain Spectroscopic detectors require instrumentation to convert the event energy into a pulse height spectrum:  preamplifier, positioned close to the detector, produces small long-tailed pulses  shaping amplifier, filters and amplifiers the signal, producing Gaussian pulses  Multi-channel analyser (MCA) produces a pulse height spectrum Portable digital MCA system: Canberra NaI scintillator system Amptek portable MCA Conventional MCA card, embedded inside a PC Standard setup for a Ge detector

Dr Paul Sellin Page 30 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Spectral Resolution The resolution of a detector system defines how well close-spaced peaks can be detected:  resolution = Full Width Half Maximum peak centroid  small (‘high’) resolution  narrow peaks Note the same gamma spectrum taken with a CdTe detector (high resolution) and a NaI scintillator (low resolution)

Dr Paul Sellin Page 31 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Summary  The nature and origin of the major radiations have been summarised:   are heavy charged particles, containing 2 protons and 2 neutrons       are light charged particles, either electrons or positrons  X-rays and gamma rays are photons, with low and high energies respectively  Neutrons are uncharged heavy particles  The interaction of radiation with matter has been described:   particles have a short straight paths in matter, typically  m range   particles travel are scattered in matter, travelling ~mm  X-rays and gamma rays photons interact through a combination of photon absorption and scatter.  Neutrons interact through either scatter or nuclear reactions  The major detector systems have been introduced:  Silicon surface barrier detectors for a, b particles  Germanium detectors for high resolution X-ray, gamma ray spectroscopy  Scintillation detectors for lower resolution gamma ray spectroscopy  Gas detectors, principally GM tubes and neutron detectors  Si(Li) for X-ray detection  CdTe/CdZnTe new technologies for room temperature gamma ray detection