O Level Physics Chapter :24: Radiation and Half Life Prepared By: Shakil Raiman

24.1: Becquerel (Bq):  The Becquerel (Bq) is a measure of how many unstable nuclei are disintegrating per second.  One Becquerel means a rate of one disintegration per second.  kBq = 1000 Bq (1000 disintegration per second)  MBq = Bq ( disintegration per second)

24.2: Detecting ionizing radiation:  We can detect ionizing radiation by using:  Photographic film  Geiger-Müller tube

24.2.1: Photographic Film:  Photographic film is used to detect radioactivity. Scientists who work with radioactive materials wear a strip of photographic film in a badge. If the film becomes fogged it shows that the scientist has been exposed to a certain amount of radiation. These badges are checked regularly to ensure that the safety limit for exposure to ionizing radiation is not exceeded.

24.2.2: Geiger-Müller tube:  Figure shows the basic construction of a Geiger-Muller (GM tube). It is a glass tube with an electrically conducting coating on the inside surface.

24.2.2: Geiger-Müller tube:  The tube has a thin window made of mica. The tube contains a special mixture of gases at very low pressure. In the middle of the tube, electrically insulated from the conducting coating, there is an electrode. This electrode is connected, via a high value resistor, to a high- voltage supply, typically V.

24.2.2: Geiger-Müller tube:  When ionizing radiation enters the tube it causes the low pressure gas inside to form ions. The ions allow a pulse of current to flow from the electrode to the conducting layer. This is detected by an electronic circuit.  The GM tube is usually linked up to a counting circuit. This keeps a count of how many ionizing particles (or how much gamma radiation) have entered the GM tube.

24.2.2: Geiger-Müller tube:  Sometimes GM tubes are connected to rate meters. These measure the number of ionizing events per second, and so give a measure of the radioactivity in becquerels.  Rate meters usually have a loudspeaker output so the level of radioactivity is indicated by the rate of clicks produced.

24.3: Background Radiation:  Background radiation is low- level ionizing radiation that is produced all the time due to natural and artificial sources.  Some sources of background radiation are: radon gas, ground and buildings, cosmic rays and nuclear power.

24.4: Radioactive Decay:  Radioactive decay is a random process. It is impossible to tell which nuclei will disintegrate at a particular time.  Experimental Demonstration of Nuclear Decay:  Take 1000 coins and toss them. Then remove all the coins that comes down heads, note the number of coins remaining and then repeat the process.  If we dis this for six or more trials we would begin to see the trend. A set of typical results is shown in the following table and in Figure.

24.4: Radioactive Decay:  This graph follows a rule: the smaller the quantity, the more slowly the quantity decreases. This quantity here is the number of coins still in the experiment. The name for this kind of decrease proportional to size is called exponential decay.  If one of the atoms is disintegrated it is out of the games like the coins facing head down.

24.5: Half Life:  The half-life of a radioactive sample is the average time taken for half the original mass of the sample to decay.  The half-life is different for different radioactive isotopes.

24.6: Half Life: Uses:  Isotopes with short half-lives are suited to medical use. This is because the activity of a source will rapidly become very small as the isotope decays quickly.  Isotopes used for dating samples of organic material need to have very long half-lives. This is because the activity will become difficult to measure accurately if it is drops below a certain level.

24.7: Half Life: Calculation

24.7.1: Half Life: Calculation

24.7.2: Half Life: Calculation

24.7.3: Half Life: Calculation

Thank You All  Wish you all very good luck and excellent result.