TOPIC 15 Pengesanan Dan Pengukuran Radioaktiviti
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TOPIK 15 15.1Pengesanan & pengukuran radioaktiviti 15.2 Kesan biologi kerana Radiasi 15.3 Aplikasi Kimia Nuklear
Pengesan Radioaktiviti 1. Pengesan fotografi2. Tiub Gieger-Muller3. Elektroskop Kerajang Emas 4. Kebuk Awan (Cloud Chamber)5. Pembilang bunga api (Spark counter)
Instrumen 1: Pengesan fotografiKesan radiasi terhadap filem fotografi digunakan oleh pekerja di loji radiasi untuk memantau aras pendedahan mereka terhadap radiasi.Pekerja yang terdedah kepada radiasi dikehendaki memakai lencana yang mengandungi filem fotografi.
Figure 5 Schematic Diagram of a Gas-Filled Detector
Anode-collects negative charges; Cathode-collects positive charges. A voltage is applied to the anode and the chamber walls. The resistor in the circuit is shunted by a capacitor in parallel, so the anode is at a positive voltage with respect to the detector wall. As a charged particle passes through the gas-filled chamber, it ionizes some of the gas (air) along its path of travel. The positive anode attracts the electrons, or negative particles. The detector wall, or cathode, attracts the positive charges. The collection of these charges reduces the voltage across the capacitor, causing a pulse across the resistor that is recorded by an electronic circuit. The voltage applied to the anode and cathode determines the electric field and its strength.SummaryThe central electrode, or anode, attracts and collects the electron of the ion-pair. The chamber walls attract and collect the positive ion. When the applied voltage is high enough, the ion pairs initially formed accelerate to a high enough velocity to cause secondary ionizations. The resultant ions cause further ionizations. This multiplication of electrons is called gas amplification.
Instrument 2:Gold Leaf ElectroscopeDeveloped in 1787 by British clergyman and physicist Abraham Bennet. A charged electroscope used to detect alpha particles. The more intense the radiation, the faster the leaf falls.It not suitable for detecting beta & gamma radiation as these cause weak ionization of the air.
If a charged object is brought near the electroscope terminal, the leaves also diverge-electric field of the object causes the charges in the electroscope rod to separate.
Charges of the opposite polarity to the charged object are attracted to the terminal, while charges with the same polarity are repelled to the leaves, causing them to spread.
If the electroscope terminal is grounded while the charged object is nearby, by touching it momentarily with a finger, the same polarity charges in the leaves drain away to ground, leaving the electroscope with a net charge of opposite polarity to the object.
The leaves close because the charge is all concentrated at the terminal end. When the charged object is moved away, the charge at the terminal spreads into the leaves, causing them to spread apart again.Instrument 3: Geiger-Muller Tube (GMT) a particle detector designed to detect ionizing radiation, such as alpha and beta as well as gamma radiationinvented by the German physicist Hans Geiger one of the most famous radiation detectors, mostly due to its simplicity and the distinctive audible clicks produced with the detection of individual particles.main element of a Geiger counter a chamber filled with inert gas or a mix of organic vapor and halogens. The tube contains two electrodes, the anode and the cathode, which are usually coated with graphite. The anode is represented by a wire in the center of the cylindrical chamber while the cathode forms the lateral area. One end of the cylinder, through which the radiation enters the chamber, is sealed by a mica window.As ionizing radiation coming from the surrounding medium passes through the mica window and enters the Geiger-Muller tube, it ionizes the gas inside, transforming it into positively charged ions and electrons. The electrons eventually migrate towards the anode of the tube detector, while the positively charged ions accelerate towards the cathode. As the positive ions move towards the cathode, they collide with the remaining inert gas thus producing more ions through an avalanche effect. When this happens an electrical current is established between the two electrodes.This current can then be easily collected, amplified and measured or counted and played in the form of an acoustic signal made out of clicks, each of which should correspond to the detection of a single ion (most of the times prevented by secondary avalanche processes). To improve the detection, multiple discharge stopping techniques can be used, either by removing the high voltage from the electrodes or by inserting additional organic or halogen gases in the inert mix.Mica is generally preferred in the construction of Geiger-Muller tubes mostly because it is able to detect alpha particles, as opposed to glass, but it is also much more fragile than glass and can be easily damaged. Although it is said that Geiger counters detect ionizing radiation, most of them are not sensitive to neutrons. Nevertheless, the Geiger-Muller tube can be modified accordingly by coating it with boron or by using boron trifluoride gas or helium-3.
Instrument 4: Diffusion Cloud Chamberalso known as the Wilson chamberis used for detecting particles of ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supercooled, supersaturated water or alcohol vapor. When an alpha particle or beta particle interacts with the mixture, it ionizes it. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes. An alpha particle's track is broad and straight.An beta particles track is thinner and wavy tracks.An gamma particles track is short, irregular and thin.
Natural Sources of RadiationCosmic raysextremely energetic particles, primarily protons, which originate in the sun, other stars and some of the violent cataclisms which occur in the far reaches of space.
Cosmogenic radiation-produced by the interaction of cosmic rays with gases in the upper atmosphere. Terrestrial radiationdue to the remnants of radioactive elements that were present on the primordial Earth and their decay products.
Natural Radioactivity in the Body-come mainly from radioactivity present in minute quantities in the food we eat.
Radon Progeny- As the Rn decays its progeny, which are not gases, can attach themselves to particulates in the air, and these particulates may be trapped in the lungs of people breathing the air.Artificial Sources of RadiationX rays- used for diagnostic purposes in medicine and dentistryTelevision screens and computer monitors- also produce X raysLuminescent paints for watch dials- used radium, a highly toxic alpha emitter if ingested by those painting the dials.
Radiation Detection Instruments Instrument TypesDetection PrincipleApplicationsIon chamber (IC)Ionization of air (or other gases)Direct measurement of exposure or exposure rates, with minimal energy dependence.Geiger-Mueller (GM) Proportional counter (PC)Ionization of gas with multiplication of electrons in detectorDetection of individual events, i.e. alpha or beta particles & secondary electrons, for measuring activity (in samples or on surfaces) & detecting low intensities of ambient x or gamma radiation; precautions required due to energy dependence.Solid state diodesIonization of semiconductorDetection & energy measurement of photons or particles; primarily for laboratory use.ScintillatorsIonization & excitation followed by light emissionDetection of individual events;Photographic filmIonization of Ag BrPersonal exposure monitoring.Thermoluminescent detector (TLD)Excitation of crystal; light release by heatingPersonal and environmental exposure monitoring.Radiation Detection Instruments Biological effect of Radiation Biological effect: Radiation Because of the many factors involved in radiation exposure (length of exposure, intensity of the source, energy and type of particle), it is difficult to quantify the specific dangers of one radioisotope versus another.
Radiation doses of 600 rem and higher are invariably fatal, while a dose of 500 rem kills half of the exposed subjects within 30 days.
Smaller doses (< 50 rem) appear to cause only limited health effects, may cause long-term health problems such as cancer.The tissues most affected by large, whole-body exposure are bone marrow, intestinal tissue, hair follicles and reproductive organs, all which contain rapidly dividing cells.The suscepceptibility of rapidly dividing cells to radiation exposure explains why cancers are often treated by radiation. The effects of a single radiation dose on a 70 kg humanDose, remSymptoms/ Effects300Permanent sterility in women>500 Fatal to 50% within 30 days.>3000Fatal within hoursAppliacation:Nuclear ChemistryApplication: Nuclear Chemistry Radioactive Tracers Agriculture: To detect how readily a plants takes in phosphate andidentify which part of the plant accumulate the phosphates. Medicine: To detect suspected brain tumours and blood clots.Radio therapy for cancers and X-rays. To provide diagnostic information about the functioning of a person's specific organs, or to treat them. (technetium-99)
Isotopes used in Medicine: RadiotherapyType of Isotopes Uses Bismuth-213 (46 min)Used for targeted alpha therapy (TAT), especially cancersChromium-51 (28 day) Used to label red blood cells and quantify gastro-intestinal protein loss.Cobalt-60 (5.27 yr) Used for sterilising bandages and dressings. Dysprosium-165 (2 hr) Used as an aggregated hydroxide for syno