![Thallium doped, sodium iodide [NaI(TI)]:
✓ Sodium iodide crystal doped with a very small amount of thallium
[NaI(Tl)] is most commonly used.
✓ detection of gamma rays in the energy range E = 0.1 - 100 MeV•
✓ The detection efficiency of NaI(TI) detectors generally improves
with increasing crystal volume, whereas the energy resolution is
largely dependent on the crystal growth conditions.](https://image.slidesharecdn.com/presentation1-250131020245-0e7f3289/85/Presentation1-vector-model-of-a-one-electron-atom-on-atomic-and-nuclear-35-320.jpg)
Presentation1.vector model of a one electron atom on atomic and nuclear
- 3. Detectors ❖ “A device used to indicate the presence of fast-moving charged atomic or nuclear particles by observation of the electrical disturbance created by a particle as it passes through the device known as radiation detector.”
- 4. TYPES OF RADIATION MEASURING INSTRUMENTS Four basic types of radiation measuring instrument may be used in the workplace: (A) Dose rate meters (measure the potential external exposure). (B) Dosimeters (indicate the cumulative external exposure) (C) Surface contamination meters (indicate the potential internal exposure when the radioactive material is distributed over a surface ➢ (D) Airborne contamination meters and gas monitors : (indicate the potential internal exposure when a radioactive material is distributed within the atmosphere.
- 6. MECHANISMS USED FOR DETECTING RADIATION ❖There are several effects caused by ionizing radiation which allow us to detect and measure the radiation and these are as follows: ❖Ionization; ❖Scintillation; ❖Thermoluminescence; ❖Chemical mechanisms; ❖Heating; and ❖Biological mechanisms.
- 7. ❖ Ionization is caused directly by alpha and beta radiation ❖ and indirectly by x-rays, gamma and neutron radiation. ❖ The ion pairs which are produced can be collected, and the number of ion pairs collected can be related to the amount of radiation causing the ionization. Ionization:
- 8. ❖ Scintillation is the production of light following the movement of electrons from high energy levels orbits to lower energy levels within an absorbing material. ❖ The electrons have moved into higher energy orbits by the process of excitation. ❖ The light released can be converted to an electrical signal. ❖ The size of the electrical signal depends on the number of electrons moved into higher energy orbits and can therefore be related to the amount of radiation causing the scintillation. Scintillation:
- 9. ❖ When electrons in certain materials absorb energy they will move into higher energy levels or ‘forbidden bands’. They remain trapped in these bands until the material is heated to a certain temperature. ❖The heat energy releases the electrons and the material emits light as the electrons move back to their original level. ❖ The light is converted to an electrical signal which can be related to the amount of incident radiation. Thermoluminescence
- 10. ❖ Ionizing radiation can cause chemical changes. ❖ The chemical process that converts the latent image into a visible image with a range of densities, or shades of gray. ❖ Film density is produced by converting silver ions into metallic silver, which causes each processed grain to become black. ❖ This effect is observed in the use of photographic film for personal dosimetry, medical x-rays and industrial radiography. Chemical Mechanisms
- 11. ❖Ionizing radiation can increase the temperature of the absorbing medium and careful measurement of this increase can give a radiation dose measurement. ❖This technique known as calorimetry. Heating
- 12. ❖ High doses of radiation can cause biological changes in ving cells. Biological changes are only used for dose estimation in extreme circumstances where personnel are suspected of having accidentally received a high dose. li Biological Mechanisms
- 13. Mechanism Main Use Type of Instrument Detector Ionization Radiation monitoring instruments 1. Ion chamber 2. Proportional counter 3.Geiger-Müller counter 4. Solid state 1. Gas-filled 2. Gas-filled 3. Gas-filled 4. Semiconductor Scintillation Radiation monitoring instruments Scintillation counter Crystal or liquid Thermoluminescence Personal dosimetry Thermoluminescent Dosimeter (TLD) Crystal Chemical Personal dosimetry Photographic film Photographic emulsion Heating Primary standard & instrument calibration Calorimeter Solid or liquid Biological Accident situations Biological tissue Biological tissue Summary
- 14. Radiation Misconceptions. . . . . . Thanks
- 16. Gas-filled Detectors Gas-filled detectors consist of a chamber filled with a gas (often air) and two voltage plates known as electrodes. The positive electrode is called the anode and is often in the centre of the chamber. It is electrically insulated from the outer casing. The outer casing of the chamber is often the negative electrode or cathode. Incoming radiation interacts with the walls of the chamber or the gas particles and produces ion pairs. When a voltage is applied between the electrodes, positive ions are attracted towards the negatively charged cathode, and the electrons are attracted towards the positively charged anode. A charge builds up on the anode, causing a voltage change in the circuit. This change in voltage is referred to as a pulse, and the presence of this pulse causes a current to flow in the external circuit. By detecting either this pulse or current, we can detect the presence of ionizing radiation.
- 17. Types of gas-filled detectors
- 18. ❖ These detectors can be classified into three general groups: • Ion chamber • Proportional counter • GM tube
- 19. ✓ An ionization chamber must have a sufficient potential difference across the anode and cathode to collect all the free electrons produced. ✓ If the potential difference is insufficient, recombination occurs and information is lost. ✓ If the potential difference is too great, additional free electrons can be produced by secondary interaction within the chamber. ✓ The region between too much and too little voltage is the ideal ion chamber region. ✓ If recombination of free electrons and positively charged atoms is eliminated, then the current flow is a direct measure of the amount of radiation incident on the chamber ❖ Ionization chambers
- 20. A Typical Beta Gamma Radiation Monitoring Instrument Incorporating an Ion Chamber Detector ✓ If the instrument is to be used to measure alpha or beta radiation, the chamber must have thin walls or a thin end window. ✓ By sliding this plate over the end window of the ion chamber, you can distinguish between beta and gamma radiation. ✓ The gamma efficiency of these detectors is only a few percent (as determined by the wall absorption), while the beta response is near 100% for beta particles entering the detector.
- 21. ❖ Proportional counters ✓ Proportional counters are more sensitive than ionization chambers. (low intensity radiation fields) ✓ The gas multiplication may increase the number of ions produced by 104. ✓ High pulse rates can be counted. ✓ The total pulse size is proportional to the energy deposited by the radiation. ✓ Proportional counters can be used with a pulse height discrimination circuit to distinguish between the types of radiation on the basis of their ionizing ability. ✓ And can also be used to distinguish between the different energies of the incoming radiation (i.e perform spectroscopy).
- 22. ❖ Geiger-Müller counters ✓ The height of the output pulse is independent of the energy of the ionizing particle. This means that it is impossible to distinguish electronically between alpha and beta radiation, or discriminate between the energies of the incoming radiation. ✓ Because of the large charge amplification (9 to 10 orders of magnitude), GM survey meters are widely used at very low radiation levels. ✓ They are considered ‘indicators’ of radiation, whereas ionization chambers are used for more precise measurements.
- 23. ✓ Geiger-Müller counters can be made in a variety of shapes and sizes. ✓ One of the disadvantages of a G-M counter is their long resolving time. This is usually of the order of 100 to 300 microseconds which means that this counter is not suitable for high counting rates where pulses are forming very quickly. ✓ A portable GM survey meter may become paralyzed in a very high radiation field and yield a zero reading (foldback ) where initial pulse are therefore too small to be registered. ✓ One advantage of G-M counters is that the output pulse is in the order of a few volts, so the signal does not require pre-amplification.
- 24. Summary of gas-filled detectors Detector Type Efficiency Comments Ionization Chambers Alpha High (with suitably thin end window) Used for counting and spectroscopy. Beta Moderate (with suitably thin end window) Used in portable radiation monitoring instruments. Gamma <0.1% Used in portable radiation monitoring instruments. X-rays Depends on window thickness Useful for most energies encountered in radiation protection. Proportional Counters Alpha High (with suitably thin end window) Used for counting and spectroscopy. Beta Moderate (with suitably thin end window) Used for counting all energies. Gamma <1% Can be used for spectroscopy with energies >200 keV. X-rays Depends on window thickness Geiger-Müller Counters Alpha Moderate (with suitably thin end window) Cannot discriminate between energies. Beta Moderate (with suitably thin end window) Cannot discriminate between energies. Gamma <1% Cannot discriminate between energies but used (with suitable energy compensation) in portable radiation monitoring instruments X-rays Depends on window thickness Cannot discriminate between energies but used (with suitable energy compensation) in portable radiation monitoring instruments
- 25. Radiation Misconceptions. . . . . . Thanks
- 26. Solid State Conductivity Detectors
- 27. ✓ A semiconductor is a material with properties somewhere in between insulators and conductors. It is typically made of silicon or germanium ✓ Solid state conductivity detectors are so named because they consist of semiconducting crystalline solids. ✓ Ionizing radiation can give enough energy to an electron in a semiconducting crystalline solid to move it from its usual energy level (in the valence band) through normally forbidden levels (in the forbidden band) and up into a higher energy state (known as the conduction band). ✓ When ionizing radiation interacts with these solids, the overall conductivity of the material is increased. If this increase is then measured, it can be related to the amount of incident radiation.
- 28. There are many different types of solid state conductivity detectors available for detecting ionizing radiation. The types of solid state conductivity detectors considered in this module are: • Diffused junction diodes; • Surface barrier detectors; • Ion implantation detectors; • Lithium drifted detectors; and • High purity germanium detectors. Types of solid state detectors
- 29. ❖ High Purity Germanium Detectors ✓ The HPGe detector acts as an efficient gamma detector with excellent energy resolution. ✓ The detectors require cooling with liquid nitrogen for efficient operation but one advantage is that it may be stored at room temperature when not in use. ✓ Requires bigger active volume.
- 30. Detector Main Uses Advantages Disadvantages Diffused Junction Diode Charged particle detection • More rugged than surface barrier • Lower energy particles not detected Surface Barrier Alpha and beta spectroscopy • Efficient at detecting charged particles. • Very good energy resolution Surface must be kept very clean Very sensitive to light Ion Implantation Alpha spectroscopy Low energy beta monitoring • Less likely to be affected by environmental conditions • Very rugged Lithium Drifted Germanium Ge(Li) Gamma spectroscopy • Efficient detectors of gamma radiation • Excellent energy resolution • Must be kept at liquid nitrogen temperatures at all times Lithium Drifted Silicon Si(Li) Beta, gamma and x-ray spectroscopy • Good detectors for very low energy gamma rays ( 150 keV), x-rays and beta particles • Can be operated at room temperature • Less likely to interact with gamma radiation than Ge(Li) detectors • Should be cooled to liquid nitrogen temperatures during operation High Purity Germanium (HPGe) Gamma spectroscopy • Efficient detectors of gamma radiation • Excellent energy resolution • May be stored at room temperature when not in use • should be cooled to liquid nitrogen temperatures during operation Summary of Solid State Conductivity Detectors
- 31. Solid State DetectorsVersus Gas-filled Detectors ❑ Solid state detectors have a number of advantages over gas-filled detectors and these are as follows: • Solid state conductivity detectors are much smaller. • They have much better energy resolution for all radiation types. • They have much higher efficiency for gamma radiation. • The sensitive volume of the detector can be chosen to suit the application. ❑ The main disadvantages of solid state detectors are that: • They may need to be cooled to liquid nitrogen temperatures for operation. • They are sometimes less portable than gas-filled detectors.
- 32. DETECTORS BASED ON SCINTILLATION
- 33. Scintillation detectors rely on the fact that some materials (known as phosphors) will emit visible light when electrons change energy levels. PMTs absorb the light emitted by the scintillator and re-emit it in the form of electrons via the photoelectric effect. The number of photons of light emitted, and therefore the intensity of the light, is proportional to the energy of the incoming radiation. can be used for spectroscopy purposes
- 34. ❑ The types of scintillation detectors discussed in this module are as follows: ➢Zinc sulphide detectors; ➢Sodium iodide detectors; ➢Plastic organic scintillators; and ➢Liquid organic scintillators
- 35. Thallium doped, sodium iodide [NaI(TI)]: ✓ Sodium iodide crystal doped with a very small amount of thallium [NaI(Tl)] is most commonly used. ✓ detection of gamma rays in the energy range E = 0.1 - 100 MeV• ✓ The detection efficiency of NaI(TI) detectors generally improves with increasing crystal volume, whereas the energy resolution is largely dependent on the crystal growth conditions.
- 36. Detector Main Uses Advantages Disadvantages Zinc Sulphide Detection of alpha particles and heavy ions • Efficient for detecting alpha particles and heavy ions • Thin layer can easily be pierced by sharp objects Sodium Iodide Gamma spectroscopy Gamma detection • More efficient for detecting gamma radiation than solid state conductivity detectors • Does not need to be cooled • Poorer energy resolution than solid state conductivity detectors Plastic Organic Monitoring alpha and beta radiation • Cheap • Can be manufactured in different shapes and sizes Liquid Organic Monitoring alpha and low energy beta radiation • High detection efficiency when contaminant is mixed with the scintillant Summary of Scintillation Detectors
- 38. Criteria of the neutron detectors : There are several factors to be taken into account when designing a suitable neutron detector: • Moderating material must be used to slow down fast neutrons (without absorbing them) so that they will interact with the detector material. • The detector material must have a high cross section (i.e. a high possibility) for the particular reaction to occur so that detectors can be built which are not too large. • The heavy charged particles formed during the interaction with the detecting material must all be stopped within the active volume of the detector.
- 39. Four types of neutron detectors which fit this criteria are as follows: • Boron trifluoride proportional counters. • Helium proportional counters. • Gas recoil proportional counters. • Bubble detectors. Types of Neutron Detectors
- 40. Boron trifluoride proportional counters ✓ Proportional counters filled with boron trifluoride (BF3) gas. ✓ To improve the detection efficiency, the BF3 is enriched in B-10, to 96%. ✓ This gas provides the filling gas for the detector as well as the target for incoming thermal neutrons. ✓ The nuclear reaction which occurs in the detector is given by: 10B + n → 7Li + ✓ The lithium nucleus and the alpha particle both have sufficient energy to cause secondary ionization in the filling gas. ✓ The production of secondary ionization events can then be detected.
- 41. ✓ some neutron interactions produce a 0.48 MeV gamma ray, Hence, a suitable discrimination circuit is necessary to distinguish between the incoming neutrons and resultant gamma rays. ✓ It can be used to detect intermediate and fast neutrons (up to 10 MeV). in this case, the detector must be surrounded by a moderating material such as polyethylene to slow down the neutrons before capture. ✓ boron trifluoride proportional counters can be used for neutron spectroscopy purposes. Proportional counter filled with Boron-10 enriched BF3 gas
- 42. Detector Main Uses Comments Boron trifluoride proportional counters • Detection of thermal neutrons • Can be used to detect neutrons up to 10 MeV with suitable moderator • Neutron spectroscopy • Gamma rays may be produced so a discriminator circuit is necessary Helium proportional counters • Detection of thermal neutrons • Can be used to detect neutrons up to 10 MeV with suitable moderator • Neutron spectroscopy • Gamma rays may be produced so a discriminator circuit is necessary Gas recoil proportional counters • Detection of fast (< 500 keV) neutrons Bubble detectors • Personal dosimetry • Environmental monitoring Summary of Neutron Detectors
- 43. Radiation Detection and Safety Personal dosimetry electronic dosimeter thermo luminescent dose meter (TLD) film badge finger ring (TLD) Dose limits recommended by the ICRP (1991): Occupational: 100mSv in 5 years, 50mSv maximum in any year Public: 5mSv in any 5 consecutive years
- 48. Radiation Misconceptions. . . . . . Thanks
- 49. Thank you