L10 Radiation Monitoring & Measurement (1) PDF

MRD 441: RADIATION BIOLOGY & SAFETY RADIATION MONITORING  OUTLINE Radiation Detection and Measurements. 9.1 Methods of detection. 9.2 Radiation detectors and measurement. 9.4 Personnel dosimetry. 9.4.1 Monitoring. 9.4.2 Wearing personnel dosimeter. 9.4.3 Dosimetry report. ...

MRD 441: RADIATION BIOLOGY & SAFETY RADIATION MONITORING  OUTLINE Radiation Detection and Measurements. 9.1 Methods of detection. 9.2 Radiation detectors and measurement. 9.4 Personnel dosimetry. 9.4.1 Monitoring. 9.4.2 Wearing personnel dosimeter. 9.4.3 Dosimetry report.  Introduction The pro gramme covers bot h radiation and co nt amin at ion and may co nsi st s of t h e f ollow ing c o m po n en t s :  Personnel monitoring  Work place monitoring  Environmental monitoring  Introduction Personnel monitoring and work places (environmental) monitoring are carried out:  To control occupational exposure of working personnel.  Work place (environmental) monitoring is more toward controlling public exposure.  Monitoring Instrument The basic interaction of radiation with matter is the excitation or ionization of an atom or a molecule. All detectors of ionizing radiation make use of ionization and excitation process.  Monitoring Instrument There are direct or indirect measurements of ionization. Selection of a specific measuring device depends on several factors including:  Relative intensity of the radiation.  Required measurement accuracy.  Monitoring Instrument Measurement Device X-ray beam exposure Ionization chambers Environment exposure Survey meters Personnel exposure Film badges Thermoluminescene Dosimeters Radioactivity Scintillation detectors Activity calibrators  Monitoring Instrument Types of Radiation Detector Classified according to the medium of interactions of the radiation:  Gas- filled Detector  Scintillation Detector  Solid State Detector  Gas-filled Detectors 1. Gas-filled Detectors Used for charged particles producing ionization in gas filled chamber. 1. Gas-filled Detectors Main differences among the three types of gas-filled detectors lie in:  Gas used.  Pressure at which the gas is maintained within the chamber. Voltage level that is maintained between the central electrode and walls of the chamber. The different region of operation of gas filled detector for two different energies 1. Ionization Chamber B e st u s e d a s p h o t o n m e a s u r i n g i n st r u m e n t s b u t can be modified to monitor for alpha, beta, and e ve n n e u tr o n r a d i at i o n. L ess se nsi ti vi ty co mpa re d to G-M co un ter b ut ca n b e u s e d i n h i g h c o u n t i n g r a te s i t u a ti o n s. Have good energy dependence characteristics. Examples are Condensed r-Meter, fluoroscopic survey meter and “Cutie Pie”. Ionization chamber  Ionization Chamber radiation amplifier display detector signal processor Ionization Chamber Ionization Chamber Ionization Chamber Ionization Chamber 2. Proportional Counters  A type of gas- filled detector Pr o po rti o n al tu be s a re a lmo st a lw ay s o pe ra ted i n p ul se mode.  Rely on the phenomenon of gas multiplication to amplify the charge represented by the original ion pairs created within the gas.  One important application is the detection and spectroscopy of low energy X-radiation.  Widely applied in the detection of neutrons. Proportional Counter 3. Geiger-Muller (GM) Counter Useful for monitoring low-level beta and gamma radiation. High sensitivity. Meters of choice for monitoring contamination and searching of lost radiation sources. Geiger-Muller Tube  Scintillation detectors Scintillation can be produced using solid medium and this is the underlying mechanism for the sodium iodide thallium-activated detector, NaI(Tl). Measures the light released by a crystal after an interaction with radiation. Consists of:  Single crystal [NaI(Tl), anthrac ene]  Photomultiplier tubes [PMT’s] Photon crystal light PMT electrical pulse When excited by an X-ray or gamma ray photon, the crystal produces a burst of light. PMT detects that light and generates a corresponding electrical signal. Photomultiplier tube Vacuum tube light pulse of charge PMT is a v acuu m tube, wh en exp osed to a very faint flash of light, will generates a pulse of charge. Photocathode Converts light to electron light photon photocathode photoelectron photoelectric interaction When light photon of sufficient energy strikes the photocathode, it ejects a photoelectron due to the photoelectric effect. Photocathode material is usually a mixture of alkali metals, which make the PMT sensitive to photons throughout the visible region of the electromagnetic spectrum. Crystals and PMT’s  Dynodes multiply the electrons. Eve ry ph oto el ectro n i s acce le ra ted to the fi rst d yn od e - e j ect s se ve r al se co n da r y e l ect ro n s. Secon dary el ectron heads for the secon d dynode, w h e r e t h e p r o c e s s r e p e a ts i t s e l f. Amplification depends on the number of dynodes and the accelerating voltage. A mp l i fi e d e l e ct r i cal si g na l i s co l l ec ted a t a n a no d e at gro und pot en ti al , which can be measured.  Solid state detectors Maximize ionizing radiation capture.  Use of devices employing semiconductors. E l e c t r o n - h o l e p a i r s c r e a t e d a l o n g t h e p a t h t a k e n b y the charged particle (primary radiation or secon dary particle) through the detector. M o t i o n i n a n a p p l i e d e l e c t r i c f i e l d g e n e r a t e s t h e b as ic e l ect ri ca l si g na l fr o m th e d ete ctor. Schematic of Semiconductor Detector  Silicon Detector Germanium Detector Lithium-Drifted Silicon Detector  Silicon (Si) Detector  Atomic number 14.  Extremely low noise, which results from the use of high- resistivity Si substrates.  Alow-leakage-current fabrication proces s. Germanium (Ge) Detector  E x c el l ent en er g y r e s ol ut i on , pot en t ial ly hi gh s pat i al resolu tion, large active volumes leading to high detector efficiencie s, simplified fabrication, and enabling unique de te ctor ge om etr ie s an d de te ction schem es. Advantages: Long life expectancy. Improved reliability- the readings more stable than other detector. Improved maintainability -semiconductor detectors operate at a fixed. L ow vo l ta ge , th e n ee d fo r w or k i n a dj u sti n g th e d et ec to rs i s greatly reduced. Miniaturization - semiconductor detectors are small - makes it possible to reduce the size of the detector equipment, thereby both saving space and reducing. C ompac t de s ign. Disadvantages: Lower sensitivity. Poor energy resolution, scatter rejection. Poor spectral performance. Voltage supplied must large enough. Too high energy photon.  Type of Personal Radiation Monitoring Devices Film Badge Thermoluminecents Dosimeter (TLD) Optically stimulated luminescent dosimeter (OSLD) Pocket Dosimeter Analog Digital Ring Dosimeter