Friday, 5 August 2011

Physics: Introduction to radiation

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X-ray: it’s produced when electrons are accelerated through high p.d. and hit the heavy metal (like Pb). It has a higher frequency than UV and visible light, and it has a certain ionizing power, which enables X-ray to remove one or more electrons from the atom and the atom is said to be ionized. (In fact, X-ray gives enough energy which is larger than the ionization energy, to knock out an electron.)
Usage of X-rays: it has a certain penetrating power that it passes through lighter elements like C, H, O and being absorbed by heavier elements like Ca, Pb. Therefore it can be used in photographic imaging or checking stuffs carried by a human body for security use.
Substances that spontaneously emit high energy radiations are called radioactive materials, which are discovered by Becquerel in 1896. There are three types of radiation called α, β and γ respectively. Some of them are from artificial source (e.g. electrical appliances like TV and making photographic film), but some of them are from the natural environment called background radiation, including cosmic ray from the universe and the radiation from radon gas.
1)       α-radiation: it’s a Helium-4 nuclei (4He) which carries net charge +2e (e is the magnitude of charge of 1 electron), with mass 4a.m.u.. Since it’s massive and it carries high k.e., it has the highest ionizing power (about 104 atoms/mm) but the lowest penetrating power (stopped by a thin paper).
2)       β-radiation: it’s fast moving electron (-1e) which carries net charge –e with mass 1/1800 a.m.u. or 9.11*10-31kg. It has medium ionizing power (100atoms/mm) and medium penetrating power (stopped by 5mm Al sheet).
3)       γ-radiation: it’s gamma ray which has no charge and mass. It has lowest ionizing power (1atom/mm) but highest penetrating power; it can only be halved by 25mm Pb block.
-          Photographic effect depends in penetrating power so γ>β>α.
-          Fluorescent effect depends on ionizing power so α>β>γ.
-          In B and E field α, β is deflected in opposite direction but β deflects more because it’s lighter. γ gives no deflection since it has no charge.
Detectors of radiation
1)       Photographic film: amount of blackening = amount of exposure in radiation, but it can’t tell which type of radiation it is.
2)       Spark counter: applying high p.d. across the counter, and when radiation enters and ionize atoms in air, it completes the circuit and high current cause sparking. It only detects α because only this type of radiation has enough ionizing power.
3)       Geiger-Muller (GM) tube also make use of ionization power but enough more sensitive. It contains Ar(g) in low pressure. When radiation goes into the tube, it ionize some gas atom, and those Ar ion further ionize other Ar atoms, as a result, a pulse of current is recorded by the scaler to measure the amount of radiation. Theoretically it’s best to detect β but it can detect all kinds of radiations (except very energetic γ as they pass through the tube without ionization.
4)       Diffusion cloud chamber: it contains some dry ice at bottom and supersaturated alcohol vapour at the top. Without radiation, alcohol vapour has no where to condense. When radiation ionizes the air, it condenses around the ionized ion and the path of radiation if shown. α radiation has straight, thick and short path; β radiation has twisted, long path while γ radiation has thin, twisted and fork-shaped path.
Application
-          Radiotherapy: killing cancer cells by gamma rays (e.g. from 60Co)
-          Tracers like 25Na in tracing blood clots in human bodies, 30P for tracing fertilizers in crop.
-          Carbon dating: Human keeps constant amount of 14C while living, and when they dead, it decays in the body. By finding amount of 14C in the dead body we can know their age.
-          Sterilization on medical supplies and food (before flights)
-          Industrial use like controlling thickness of metal block.
Safety hazards
-          Damaging cells like ionizing atoms in cells and cause burning or cancer (genetic changes) in long term.
-          People handling radioactive materials should use forceps and the source should be kept in Pb box when not in use.
-          Equivalent dose and effective dose describes the biological effect on a organ and on the whole human body respectively, with unit Sievert (Sv), or commonly mSv.

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