Measuring Build Up Factor-Physics-Lab Report, Exercises of Advanced Physics

Download Measuring Build Up Factor-Physics-Lab Report and more Exercises Advanced Physics in PDF only on Docsity! The purpose of the experiment is to determine buildup factor for given source. The experiment involves firing a narrow beam of gamma-rays at a material and measuring how much of the radiation gets through. We vary the type of absorbing material as well as its thickness and density by interposing iron, aluminum, and copper absorbers of different thicknesses t between the source and the detector. Buildup factor is determined by using two geometries, first the counts are taken for good and bad geometry and then buildup factor is determined from it. Another method used for measuring build up factor is determining buildup factor by using multi channel analyzer. Build up factor:- The factor by which the total value of the quantity being assessed at the point of interest exceeds the value associated with only primary radiation. The total value includes secondary radiations especially scattered radiation. The gamma-ray buildup factors for point isotropic sources in infinite homogeneous media have been widely used in gamma-ray shielding calculations. Considerable effort has been devoted to developing methods of calculating buildup factors taking into account multiple scattering of gamma-ray. The data set of gamma-ray buildup factors was first developed by Goldstein theoretical value by some factor called build up factor. This increase in intensity of gamma rays at observation point is due to many effects one of which is scattering of gamma rays from some material. Theoretical value by some factor called build up factor. This increase in intensity of gamma rays at observation point is due to many effects one of which is scattering of gamma rays from some material. Buildup factors vary with a number of parameters such as the distance of penetration through the attenuating medium; the geometric configuration of the attenuating medium, source and detector position; the composition of the medium; the detector response function; and the energy and direction of emission of the source photons, ideally taken to be monoenergetic and isotropic. Why Build Up Factor Is Needed:- when radiation is incident on some attenuator material then beside some other process also take place. The collision processes depend very much on the type of particles involved in the collision. Heavily ionizing particles such as, alpha particles or protons are very easily stopped by a small amount of material because they leave a dense trail of ions. They are not generally removed by a single collision but slowed with energy going into the ionizing process. On the other hand, electrons scatter off other electrons and in this process, lose energy and produce a gamma. Subsequently, the gamma may react with another electron to produce an electron and gamma. This process is called a gamma cascade which is complicated to calculate. Due to these many processes radiation passed through attenuator is not only given by attenuation constant but also by another constant called build up factor. This build up factor has a number of uses in radiation shielding. Attenuation:-. When radiation falls on some material then some of radiation is blocked by material by some processes like stopping or blocking by its nucleus. This property is used in radiation shielding of material. Material having higher cross section of radiation stopping is used in radiation shielding. docsity.com For a material with buildup factor µ and I0 intensity is incident on that material with thickness of L then intensity passed is given as The attenuation coefficient µ can be considered as the fraction of photons that interact with the shielding medium per centimeter of shielding. This coefficient assumes that all photons that interact are removed. But here we are ignoring some other processes like Compton scattering and pair production photons. These processes ignoring underestimates the shielded dose rate and the shielding required. It is also known as narrow beam conditions because the source and detector are assumed to be collimated and the measurement made at a short distance. No photons are scattered. The only way to make this happen is to side and back shield the source and the detector (collimate). This only applies at close distances though. Further away, the air scatters the photons in real life. This is idealistic and without the collimation or at a longer distance, the dose-rate is underestimated. In real there are some processes making the process some complicated. These are Compton scattering and pair production. These processes decrease number of photons in radiation reaching the detector and cause attenuation. Simple Scatter (Rayleigh scattering):-. This process is always present and causing attenuation everywhere. Rayleigh scattering (named after Lord Rayleigh) is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases. Photo electric effect:-. If energy of photon is such that it can eject an electron from surface of a material, then photons of such energy are stopped. For this process photon must have threshold energy of φ or above it. Threshold energy φ is minimum energy required to eject an electron from surface of material. For f frequency photon we have energy of ejected electron as Where h is Planck’s constant. This process also contributes to attenuation. X-ray production:-. Some of photons are not stopped by photoelectric effect and Compton Effect but they penetrate into electronic configuration of atom of shield and they are eject docsity.com 8. CRO 9. Delay amplifiers 10. Multi channel analyzer Fig. 3 good geometery Procedure:-. Using different materials in bad geometry and good geometry we can count number of particles radiation passed through these material for different thickness and from these count rates we can calculate attenuation coefficient and build up factor for different energies and thickness. As docsity.com Taking ln of both sides we get ( ) ( ) So measuring slope of line given by above equation give attenuation constant. ( ) ( ) By this formula we can calculate attenuation constant. Similarly build up factor is measured by Or Ibg= bad geometry intensity. Igg = good geometry intensity 1. Good geometery:-. Let use different materials 1. Iron: Tabulated Data: Plate No X1 X2 X3 X=(X1+X2+X3)/3 Count rate1 Count rate2 Average ln(Average) 0 0 0 0 0 162346 162330 162338 11.9974359 4 6.8 7.225 7.075 7.03333333 114775 115232 115003.5 11.6527178 2 6.37 7.075 6.4 6.61666667 81395 81650 81522.5 11.3086343 13 6.25 6.25 6.275 6.25833333 57664 57827 57745.5 10.9638007 10 6.27 6.25 6.35 6.29166667 58244 41304 49774 10.615248 9 6.37 6.175 6.45 6.33333333 29613 30220 29916.5 10.3061654 docsity.com FIG. 4 for attenuation constt of Iron The value of attenuation coefficient is 0.3404 mm -1 2. For Aluminum: Tabulated Data: Plate No X1 X2 X3 X=(X1+X2+X3)/3 Count rate1 Count rate2 Average ln(Average) 0 0 0 0 0 162346 162330 162338 11.9974359 24 6.3 6.25 6.125 6.225 141789 141201 141495 11.8600197 27 6.175 6.075 6.25 6.16666667 125131 124378 124754.5 11.7341031 25 6.15 6.175 6.25 6.19166667 111015 108782 109898.5 11.6073125 30 6.175 6.15 6.225 6.18333333 97344 98014 97679 11.4894419 20 6.3 6.343 6.2 6.281 86740 87000 86870 11.372168 docsity.com 10 6.275 6.25 6.35 6.29166667 437779 438181 437980 9 6.375 6.175 6.45 6.33333333 332560 334614 333587 Since attenuation coefficient is already calculated so we do not need to calculate it again. Build up factor measurement:-. The buildup factor corresponding to different value of thickness for the three materials is tabulated and is graphically represented in the following paragraphs. Buildup Factor by SCA a) Iron Graphical Representation: No. of Plates Accumulative Thickness Good Geometry Bad Geometry Buildup Factor(Ibg/Igg) 0 0 162338 1101770 6.786889 1 7.033333 115003.5 907346.5 7.889729 2 13.65 81522.5 725823 8.903346 3 19.90833 57745.5 566383 9.808262 4 26.2 49774 437980 10.46237 5 32.53333 29916.5 333587 11.1506 docsity.com Fig. 6, build factor versus thickness b) Aluminum No. of Plates Accumulative Thickness Good Geometry Bad Geometry Buildup Factor 0 162338 1101770 6.786889 1 141495 1033014 7.30071 2 124754.5 967058.5 7.751692 3 109898.5 902632 8.213324 4 97679 838968.5 8.589037 5 86870 776028 8.933211 Graphical Representation: Fig. 7, build factor versus thickness c) Copper No. of Plates Accumulative Thickness Good Geometry Bad Geometry Buildup Factor 0 0 162338 1101770 6.74947 1 12.65 72457 683853.5 9.43806 2 25.28333 33584 374116 11.13971 3 37.86667 15608 195376.5 12.51772 4 50.50667 7472.5 100341.5 13.4281 5 62.90333 3467 51988.5 14.99524 docsity.com Graphical Representation: Fig. 8, build factor versus thickness Buildup Factor by MCA a) Iron Plate No. Accumulative Thickness Gross Counts(Cg) Photo Peak Counts(Cp) Buildup Factor(Cg/Cp) 0 0 13689873 4524507 3.02571595 4 7.033333 10722789 3715680 2.88582144 2 13.65 3893642 1326438 2.93541198 13 19.90833 2739625 771904 3.5491784 10 26.2 1974753 700118 2.82060024 b) Aluminum Plate No. Accumulative Thickness Gross Counts(Cg) Photo Peak Counts(Cp) Buildup Factor(Cg/Cp) 0 0 813859 287179 2.83397811 8 6.225 379861 133301 2.84964854 4 12.39167 176542 54688 3.22816706 1 18.58333 89346 29119 3.06830592 6 24.76667 46004 13229 3.47751153 docsity.com