![The method makes use of electrons trapped between the valence and conduction bands in
the crystalline structure of certain minerals (most commonly quartz and feldspar).[1] The
trapping sites are imperfections of the lattice — impurities or defects.
The ionizing radiation produces electron-hole pairs: Electrons are in the conduction band and
holes in the valence band. The electrons that have been excited to the conduction band may
become entrapped in the electron or hole traps. Under the stimulation of light, the electrons
may free themselves from the trap and get into the conduction band. From the conduction
band, they may recombine with holes trapped in hole traps. If the centre with the hole is a
luminescence center (radiative recombination centre), emission of light will occur.
The photons are detected using a photomultiplier tube. The signal from the tube is then used to
calculate the dose that the material had absorbed.](https://image.slidesharecdn.com/measurementofradiationdose-240106042731-31d55858/85/MEASUREMENT-OF-RADIATION-DOSE-pptx-34-320.jpg)
MEASUREMENT OF RADIATION DOSE.pptx
- 1. DEVICES TO MEASURE RADIATION DOSE DR. GADHA B DNB RESIDENT
- 2. DOSIMETRY Study in physics that deal with measurement of radiation Contains the required quantitative methods which are used to determine dose of radiation.
- 4. DOSIMETERS Dosimeters are a form of devices to monitor ionising radiation. They are used regularly for a number of key roles including both patient and personal dosimetry, environmental monitoring, spectroscopy, radiopharmaceutical and equipment checking. The 2 parts of radiation measuring system are: Detector Measuring apparatus The interaction of radiation with the system takes place in the detector while the measuring apparatus takes the output and performs the function required to accomplish the measurements.
- 5. Common output modalities are: •Dose rate: gives an instantaneous value over time • often used in environmental, radiopharmaceutical and equipment checks •Total dose: the amount of activity over a given time frame • patient or personal dosimetry
- 8. PROPERTIES OF A USEFUL DOSIMETER •High accuracy and precision •Linearity of signal with dose over a wide range •Small dose and dose rate dependence •Flat energy response ( quality dependence) •Small directional dependence •High spatial resolution
- 9. TYPES OF DOSIMETERS Passive Dosimeters - A radiation-induced signal is produced by a passive dosimeter and stored in the unit. The data of the dosimeter is then processed and analysed.The Thermo Luminescent Dosimeter (TLD) and the film badge are two widely used passive dosimeters.They can operate without any built in electronics. Typically, passive dosimeters provide monthly or quarterly accumulated dose. They are best suited for monitoring anyone who could be exposed to radiation over a period of time. •Active Dosimeters - An active dosimeter generates a radiation-induced signal and provides a real-time reading of the dose or dose rate observed. An example is electronic personal dosimeter. Some models also have some additional functions such as alarm threshold settings for dose or dose rate values, as well as audible and visual indication of the dose rate level. These give an immediate indication to the wearer that something could be wrong.
- 10. There are three main types of radiation detectors: •Gas ionization detectors: These include: • Ionization chamber: A detector that can measure photons, beams, and alpha, beta, and neutron particles. It's less sensitive than a Geiger-Muller counter, but can be used in high counting rate sources. • Proportional counter: Operates in linear mode, so the energy pulse measured is proportional to the energy of the radioactive particle. • Geiger–Müller counter: A gas-filled detector that's commonly used by professionals. •Scintillation detectors: These detectors illuminate when an ionized particle is present. •Solid state detectors : These include semiconductor detectors, which use a reverse-bias p-n junction diode.
- 11. 1)GAS FILLED COLLECTORS This detector works on the principle that - as the radiation pass through air or specific gas, ionization occurs in air or molecules. The ion-pairs produced are collected and measured as current or pulses. These are generally cylindrical in shape with two electrodes, the central electrode, and the outer sheath, separated by an insulator. When a variable voltage is applied across the electrode, the positive ions will be attracted to outer electrode (the cathode) and the negative ion will travel to the positive electrode (the anode). The charges collected by anode and cathode form a very small current across the detector. By placing a sensitive current meter across the cathode and anode, the small current can be measured and displayed as a signal. More the radiation entering the chamber, more the current is displayed by the instrument.
- 13. If one measures the number of ion pairs collected across the detector at different applied voltages, the following six different regions of response would be noticed
- 14. REGION-1(REGION OF RECOMBINATION) : At low applied voltages some of the ion pairs recombine to form neutral atoms. The process of recombination decreases as the applied voltage is increased. Hence, the number of ion pairs collected increases initially with applied bias. REGION-2(IONISATION CHAMBER REGION) : With further increase in applied voltage, all the ion- pairs are collected and no re-combination of ion-pair takes place. In a limited voltage range, the number of ion-pairs produced and collected remains independent of applied voltage but dependent on the energy deposited in the detector volume by the radiation. The detectors operating in this region are called Ionization chambers.
- 15. REGION-3( REGION OF PROPORTIONALITY) : The negative ions (usually electrons) in their path towards the central electrode are acce1erated because of higher electrical field. In the vicinity of the central electrode the electrons gain sufficient energy to produce secondary ionization when they interact with gas molecules. These results in an increase in the number of ion pairs collected which is proportional to the energy dissipated by the incident particle inside the detector. The constant of proportionality (i.e. amplification) increases with increase in applied voltage and may be as high as 104. In this region (at a given voltage) the size of pulses produced by a particle is proportional to the number of primary ions generated by the radiation which in turn depends on the energy of the particle. Hence, by operating the detector in this region one can distinguish radiations of different energies and of different types. The detectors operating in this region are called proportional counters.
- 16. REGION-4( REGION OF LIMITED PROPORTIONALITY) : In this region the amplification is not constant because the space charge alters the shape of electric field. This effect marks the onset of the region of limited proportionality where the pulse height does not increase in a linear fashion with the energy deposited. This region is not used for radiation detection. REGION-5( GEIGER MULLER REGION ) : As the voltage is increased further the sensitive region spreads over entire length of the chamber. There is no difference in the pulse height for particles of different energies and types. But, since the number of ion pairs produced are much more, the pulse height is greater compared to that in ionization chamber or proportional counter. The amplification factor may be as high as 108 and the detectors operating in this region are very sensitive. The detectors operating in this region are called Geiger Muller counters.
- 17. REGION-6( REGION OF CONTINUOUS DISCHARGE ) : At further high voltages, the positive ions bombard the cathode. As a result, ultra-violet radiation and electrons are emitted from the cathode surface, which causes further ionization. At this stage, a continuous discharge occurs and sparking takes place. This region is not suitable for radiation detection. IONISATION, PROPORTIONAL AND GM regions are commonly used regions of radiation detectors. The detectors used in nucleonic gauges are one of the above detectors.
- 18. GAS FILLED RADIATION DETECTORS IONISATION CHAMBERS GEIGER MULLER COUNTER PROPORTIONAL COUNTER
- 19. IONISATION CHAMBER The ionization chamber is a very versatile device. It can be designed in many shapes and sizes and with different filling gases. The electrode material used in the ionization chamber, the type of gas filled and its pressure and the size of the ionization chamber etc., depend on the radiation intensity to be measured. The gas used in ionization chamber for radiation monitoring is air, filled generally at atmospheric pressure. The wall material have radiation absorption and scattering characteristics identical to that of air i.e. the effective atomic number of the wall material should be close to that of air ( Z= 7.64). An instrument with such an ionization chamber will give energy independent response. Several materials such as graphite, bakelite, and teflon satisfy this requirement of air equivalence in atomic number.
- 20. GEIGER MULLER COUNTER In this detector, any particle that produces ionization in the gas will produce a discharge, i.e. an avalanche of ion-pairs, even if the primary ionization consists of one ion-pair. The resulting pulse size is independent of primary ionization. As a result, the detector has high sensitivity. However, it cannot be used for the determination of radiation energy. After an avalanche in the counter, the electrons, because of their large mobility and proximity to the collecting electrode, are immediately collected by the central anode, whereas the heavier positively charged ions in the same duration would have hardly moved. Because of this slow moving positive ion sheath, the electric field near the central wire is lowered and no further pulse formation is possible until the positive ion sheath reaches a certain critical distance. This period is called dead time. This is followed by a recovery time during which all the positive ions reach the cathode and the field near the wire is restored to its original value.
- 21. During this period, the pulse may not be of sufficient height to actuate the counting equipment. Hence, on account of finite dead-time and recovery time of the order of hundreds of microseconds, all the pulses are not recorded. Hence loss of counts occurs at high count-rates and thus the GM counter fails to register in very high radiation fields. The probability of a beta particle generating at least one ion-pair along its path through the sensitive volume of a counter is high. Hence, the efficiency of GM counter for beta particles is nearly 100%. In case of gamma radiation, the probability of secondary electron production in the counter gas is very small. All the ionisation produced in a counter by gamma radiation is, substantially, the result of the secondary electrons ejected due to the interaction of gamma radiation with the wall or widow material. Therefore, the counter efficiency for gamma radiation is of the order of 1%.
- 23. PROPORTIONAL COUNTERS Proportional counter are gas filled chambers operated in the proportional region, wherein the electrons released in the initial ionization are multiplied as a result of repeated collisions with gas molecules and production of further ion-pairs. The number of electrons collected at the anode and the resultant pulse height are, therefore, proportional to the number of ion-pairs in the initial ionization or in other words proportional to the radiation energy dissipated in the detector medium. These characteristics facilitate the use of proportional counters for identifying different types of radiation and also to study the energy spectrum by the pulse height analysis. Secondly, proportional counters have smaller resolving time compared to GM counter (detector), say, a micro-seconds and hence are capable of responding to high radiation intensities and high exposure rates.
- 24. 2)SCINTILLATION DETECTORS The scintillation detector measures radiation by detecting tiny flashes of light which radiation produces in certain materials. These light flashes, called scintillation, are converted to electrical pulses and, when fed into suitable electronics, can discriminate between different types of radiation and even between different energies of the same radiation. There are several types off scintillation counters, but their detector systems always consist of two components which are optically coupled. The first is a scintillator. This is a solid or liquid which emits light pulses when radiation deposits energy in it. This is called the scintillation 'phosphor’. The second component is a photomultiplier tube (PMT) which converts this light pulse into a pulse of electric current.
- 25. There are scintillation detectors for alpha, beta, gamma and neutron radiation. Incoming radiation interacts with a scintillating material and a portion of/or the total energy is transferred to the scintillating material. The excited scintillating molecules produce light photons during the de-excitation process. A NaI(Tl) detector is commonly used for gamma scintillation detection and gamma analysis. Due to the high sensitivity, NaI(Tl) detectors give high background radiation levels. The detector is shielded to reduce background radiation level before use.
- 26. 3)SOLID STATE DETECTORS Solid state detectors are modern semiconductor instruments which use a reverse-bias p-n junction diode. As a particle passes through the junction, the resultant excitation of electrons causes the formation of holes in the valence band. The electrons are then attracted to the n material, while the holes move toward the p material. This creates a current pulse which can be measured using a meter or counter. Semiconductor detectors essentially operate in a similar manner to G-M tubes, except that the electrons and ions used as charge carriers in a gas detector are replaced by electrons and holes. They are superior to both gas and scintillation types in nearly every application.
- 33. OPTICALLY STIMULATED LUMINESCENCE An OSL dosimeter most commonly uses Beryllium Oxide (BeO) to absorb x-ray energy. It then releases it and measures the precise dose of ionizing radiation received. Beryllium Oxide is commonly used because it is extremely durable, sensitive, and resistant to environmental influences and fading. Some OSL dosimeters use Aluminum Oxide (Al2O3:C) instead of BeO. OSL dosimeters are generally more sensitive than TLD dosimeters, with a lower limit of detection( 1 mrem).
- 34. The method makes use of electrons trapped between the valence and conduction bands in the crystalline structure of certain minerals (most commonly quartz and feldspar).[1] The trapping sites are imperfections of the lattice — impurities or defects. The ionizing radiation produces electron-hole pairs: Electrons are in the conduction band and holes in the valence band. The electrons that have been excited to the conduction band may become entrapped in the electron or hole traps. Under the stimulation of light, the electrons may free themselves from the trap and get into the conduction band. From the conduction band, they may recombine with holes trapped in hole traps. If the centre with the hole is a luminescence center (radiative recombination centre), emission of light will occur. The photons are detected using a photomultiplier tube. The signal from the tube is then used to calculate the dose that the material had absorbed.
- 37. PHOTOGRAPHIC FILMS These films are identical to x-ray films and consist of a sensitive layer of silver halide crystals in gelatin spread on cellulose acetate base. The thickness of the emulsion layer ranges from 10-25 µm. On exposing to the nuclear radiation or light, a latent image is formed on the film. Radiation exposure causes ionization in the silver bromide crystals (grains). A group of silver clumps containing several silver atoms are formed on the surface of the crystal. During development, each exposed grain is reduced to metallic silver. The developer serves merely as reducing agent. The unaffected, undeveloped silver halide crystals are dissolved by immersing the film in fixer solution.
- 38. The processed film shows blackening and the amount of blackening are related to the quantity of radiation recorded. The blackening is measured in terms of optical density. It is related to the quantity of radiation absorbed in the film. The optical density is measured using an instrument known as densitometer. Optical Density(O.D) = log10 (I0/I) Where, Io - is light intensity without the processed film, and I - is light intensity through the processed film Photographic films are used in industrial radiography for revealing defects in castings, welds, forgings, etc. photographic films are also used for personnel monitoring, by loading personnel monitoring films in a film cassettes containing different metallic filters designed for monitoring different types of radiations.