Investigating the inverse square law Essay

The intensity of the influence at every given(p) radius, r, is the source strength divided by the area of the sphere. 1 The backward lame law stop in any case be applied to gravity, electric automobile fields, light and sound. In relation to electric fields, the electric force in Coulombs law follows the inverse square law If da Gamma rays are a form of electromagnetic radiotherapy and undergo trifling absorption in air, then the intensity, I, should vary inversely as the square of the infinite amongst the source and the detector.2 Air acts as an intimately transparent medium to ? -rays, and the intensity ( ac regard of energy arrival per unit area) of ?-rays emanating from a point source varies inversely as the square of the place from the source. 3 ?-rays fall into many distinct monoenergetic groups because of their variable energies which emanate from any concomitant emitter. The least energetic radiation will moreover pass through very thin foils, whereas the most energetic can penetrate up to several centimetres of lead.4 As ? -rays t exterminate to produce 10-4 times as many ion-pairs per unit length as ? - touchs do, measurements are usually carried turn up using a Geiger-Mi ller (G-M) thermionic vacuum organ pipe. 5 G-M supplys are widely utilize for spotting radiation and ionising particles.The anode is kept at a positive potential and the cathode is earthed. The tube may also have a thin mica close window. 6 When radiation enters the tube, a a few(prenominal) electrons and ions are produced in the gas. If the electromotive force is above the breakdown potential (The minimum reverse potentiality to engender the diode conduct in reverse)7 of the gas, the number of electrons and ions are greatly multiplied. The electrons are attracted to the anode, and the positive ions move towards the cathode.The current flowing in the high bulwark resistor (R) produces a pd which is amplified and passed to a numerateer which registers the p assage of an ionising particle or radiation through the tube. 8 The tube can non be modify with air as the discharge persists for a short time later the radiation is registered. This is due to electrons being emitted from the cathode by the positive ions which arrive there. Instead, the tube is filled with argon mixed with a halogen vapour which quenches, reduces the intensity, the discharge quickly, ensuring that the registered radiation does non affect the recording of other ionising particles.When the G-M tube is detecting maven particle, if another enters the tube it will not be detected. This is known as dead time the average maximum being approximately 90 microseconds. 9 Because this number is so small, it can justifiably be snub for this experiment. Background radiation must be taken into account when taking studys from the source. Background radiation primarily comes from cosmic radiation and terrestrial sources. 10 This radiation will affect the count and must be cor rected. The level of this radiation varies with location and must be measured before conducting the experiment.N0e-? t so ? = gradient/ N0e-? t Safety Precautions To examine the utmost guard duty before, during and after this experiment, some guidelines should be followedFood and drink should not be consumed whilst in the same room as the source Food items should not be stored in the same room as the source.The source should further be handled with long handled source handling tongs, and as little as possible Hands should be washed thoroughly after border with the source If in jobber with the source for an extended period, it is recommended that a observe badge is worn As the source will radiate in only one direction, it should not be pointed at anyone The source should be locked away in a lead lined box when not in use Open wounds should be covered securely.Protective gloves should be warn when handling potentially contaminated items Errors To reduce the possible computer er rors inwardly the experiment, an visual bench will be used to ensure that the G-M tube and the source are properly aligned throughout, as the source radiates in one direction, the alignment must remain standard. Also, for small distances, specifically the distance d0 which is the distance the source is from the opening of the container plus the distance of maculation from the window in the G-M tube, vernier callipers will be used to hold as much accuracy as possible. vernier callipers read to fractions of a millimetre, making them much more accurate than other measuring devices. another(prenominal) distances, such as distance d, can be measured with a metre rule as the distances are larger which decreases the possible error in measuring. There will also be the error of gentleman reaction times from observing the final count and pressing the stopclock. To ensure accuracy, practise using the stop-clock and count switch until reasonably consistent results can be obtained. Prelimi nary Work To decide on an appropriate electric potential to use, the G-M tube and source set-up should be tested.Place the source approximately 10 cm from the window of the G-M tube and increase the voltage slowly, until the count rate stops changing dramatically. Plot a graph of the count-rate, C, against EHT voltage, V. Record the voltages V1 and V2 between which the rate of find does not vary too much. If the rate of counting begins to rise after remaining much the same for a extend of voltage do not raise the voltage any high or the tube may suffer damage. 14 The optimum in operation(p) voltage will be halfway between the voltage where the plateau begins and the voltage where it ends.To decide on the range of distances used, the source was locomote come together to the window of the G-M tube and was moved back slowly until the scaler could count adequately (5 cm). This is the smallest distance that will be used. To find the other extreme, the source was moved back until the count rate fell to a low value, simply could still provide adequate results (35 cm).From these preliminary results I have pertinacious to time for 10,000 counts at 5 cm from the source, 5000 counts for 10cm from the source, and 1000 for 15 30cm. This is because any higher values will take considerably longer to measure. I will take three readings from each, as radioactive decay is a random process and it would be unlikely for more than three readings to be similar. An average will be calculated from the three values and the reading for the background radiation will be subtracted to find the corrected count rate. Equipment Geiger-Mi ller tube of i sensitive type.Decade scaler with variable EHT supply stiff cobalt-60 source sealed to prevent contact with the source and to prevent identical radiation Long handled source handling tongs to prevent contact with the source Optical bench with source holder to ensure ageless alignment Stop-clock, readable to at least two decimal places Vernier callipers to measure the distance d0 to a higher level of accuracy chiliad rule to measure the distance d Diagram Where B is the optical bench with source holder, H G is the Geiger-Mi ller tube S is the tenner scaler with variable EHT supply.R is the sealed radioactive source, cobalt-60 Cobalt-60 will be used as the gamma source as it is easily produced, by exposing inborn cobalt to neutrons in a reactor, and therefore easy to acquire. 15 It also produces ? -rays with energies of 1. 17 MeV and 1. 33 MeV. Method 1. Clamp the G-M tube to one end of the optical bench and attach it to the input socket of the scaler 2. Set the variable EHT voltage on the scaler at a minimum and turn it on, allowing a few minutes for the scaler to warm up 3. Change the variable EHT voltage on the scaler to the value found through preliminary work and set it to count pulses from the G-M tube.4. Start the stopclock and measure the background radiation for an adequate length of time, e. g. 25 minutes, as background radiation is variable 5. Place the holder containing the ? -source at 5. 0 cm from the window of the G-M tube 6. Start the stopclock and stop after 10,000 counts are registered. Record this value and plagiarize twice 7. Move the ? -source to 10. 0 cm from the window of the G-M tube and repeat performance 5, instead only counting 5000 counts 8. Move the ? -source to 15. 0 cm from the window of the G-M tube and repeat procedure 5, instead counting only 1000 counts 9.Repeat procedure 7 for sets of 5. 0 cm until a distance of 30. 0 cm is reached 10. Tabulate these results and find the average count rate for each distance 11. Evaluate 1/(d + do)2 12. Using the recorded value for background radiation, treasure the corrected count rate for each distance 13. Plot the graph of corrected count rate against 1/(d + do)2 1 http//hyperphysics. phy-astr. gsu. edu/Hbase/forces/isq. html 2 Essential Pre-University physics Whelan & Hodgson, rogue 953 3 Essential Prin ciples of physics Whelan & Hodgson, page 472 4 Essential Principles of Physics Whelan & Hodgson, page 472.5 Essential Principles of Physics Whelan & Hodgson, page 472 6 http//www. imagesco. com/articles/geiger/03. html 7 http//en. wikipedia. org/wiki/Breakdown_voltage 8 Essential Pre-University Physics Whelan & Hodgson, page 406 9 http//www. imagesco. com/articles/geiger/03. html 10 http//en. wikipedia. org/wiki/Background_radiation 11 forward-looking direct Practical Physics M Nelkon & JM Ogborn, page 218 12 A lab Manual of Physics F. Tyler, page 269 13 http//en. wikipedia. org/wiki/Cobalt 14 Advanced Level Practical Physics M Nelkon & JM Ogborn, page 212 15 http//en. wikipedia. org/wiki/Cobalt. etymon http//en.wikipedia. org/wiki/Geiger-M%C3%BCller_tube The anode is a central thin wire which is insulated from the surrounding cathode cylinder, which is metal or graphite coated.