Topic 5 - Norriza Practical on System of dose limitation - STD.pptx
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Topic 5: SYSTEM OF DOSE LIMITATION To understand time, distance and shielding concepts in radiation protection Background Radiation Measurement Gamma-ray Measurement Time Concept in Radiation Protection Distance Concept in Radiation Protection Shielding Concept in Radiation Protection ...
Topic 5: SYSTEM OF DOSE LIMITATION To understand time, distance and shielding concepts in radiation protection Background Radiation Measurement Gamma-ray Measurement Time Concept in Radiation Protection Distance Concept in Radiation Protection Shielding Concept in Radiation Protection Activity Formula At = A0 exp (-λT) A0 = A0 exp(-λT½) 2 ½ = exp(-λT½) ln(½) =ln (exp(-λT½) -0.693 = -λT½ -0.693 = -λ (30 yrs) 0.693 = λ 30 yrs 0.0231 = λ At = A0 exp (-λT) At = (100mCi) exp (-0.0231)(35yrs) At = (100mCi) exp (-0.7854) At = 44.553mCi At = 44.55mCi (2DP) Cs-137, (T½=30yrs) A0=100mCi(1988) At = ? (2023) T=2022-1988 = 35yrs Sources of Background Radiation Cosmic radiation Radionuclides in the shielding materials, environment and human body Electronic drift of the survey meter Leakage radiation from the source container Shutter Source Distance Concept in Radiation Protection To determine the value of gradient, m from the graph of log net reading versus log distance (based on inverse square law theory) X 1/d2 X = k/d2 log X = log k - 2 log d (y = mx + c) Gradient, m = -2; if hold two conditions: Using point source Using narrow beam condition Shielding Concept in Radiation Protection To determine the HVL from the graph of net reading versus shielding thickness HVL = thickness of shielding material required to reduce the intensity of the beam to the half of its initial value Distance Concept in Radiation Protection Inverse square law: X 1/d2 = k/d2 where X is doserate reading, d distance and k constant. log X = log k + log 1/d2 = log k + log d-2 = log k – 2 log d Linear Equation!! y = mx + c Gradient, m = -2 Distance Concept in Radiation Protection Graph Log Net Reading versus Log Distance 5.5 Gradient = 5.20 – 4.05 = - 1.15 - 1.74 1.74 – 2.40 0.66 Log Net Reading 5 4.5 4 3.5 1.50 1.60 1.70 1.80 1.90 2.00 2.10 Log Distance 2.20 2.30 2.40 2.50 Discussion: 1. What is the gradient of the graph? 2. Explain in more detail the answer in (1) 3. What is the significant of distance in radiation protection? Discussion: Inverse square law: X 1/d2 = k/d2 where X is doserate reading, d distance and k constant. log X = log k + log 1/d2 = log k + log d-2 = log k – 2 log d log X = log k -2 log d y=mx+c Linear Equation!! Gradient = -2 Answer Q2 • < -2 ( near -2) • Base on inverse square law ……show the gradient of graph should -2 (theory) • The result is not =-2 because in the practical condition, the source that we use is not a point source & the beam is not narrow beam (broad beam) • The beam in practical is a broad beam which contributed from scattered radiations. Answer Q3 • The further the distance, the much lower the dose produced . X∞(1/d2) X=k(1/d2); k=constant Xd2 =k=constant X1d12=x2d22 X=dose rate d= distance Answer Q3 • Keep max. possible distance from the source. • Based on inverse square law you can estimate dose rate at certain distance if you know the dose rate at other distance. X1d12=x2d22 Shielding Concept in Radiation Protection Half-value Half-valuelayer: layer:HVL HVL HVL is the thickness of shielding material required to reduce the intensity of the beam to the half (1/2) of its initial value Tenth-value Tenth-valuelayer: layer:TVL TVL TVL is the thickness of shielding material required to reduce the intensity of the beam to the 1/10 of its initial value Shielding Concept in Radiation Protection Graph Net Reading versus Shielding Thickness Net Reading (cpm) 1,000,000 500,000 400,000 300,000 200,000 138,350 cpm (100%) 100,000 50,00 40,00 0 030,00 020,00 50% of 138,350 = 69,175 cpm HVL for Pb = 0.69 cm 0 10,000 0 0.1 0.2 0.3 0.4 0.5 Shielding Thickness (cm) 0.6 0.7 0.8 Discussion 1. Plot a graph of exposure versus shielding thickness 2. Determine HVL from the graph and explain on how to determine the HVL value from the graph?. 3. Why do you need to measure the background radiation? 4. Why do you need to repeat the measurements? Answer Q2 • Explain Definition HVL • Determine the initial value (e.g X0) a point on the graph where plotted line cross the Y-axis) • Calculate half of the initial value (e.g X0/2). • Find the Xo/2 on Y-axis at the graph • Draw a dotted straight line until it is reach the plotted line (intercept point) –at that point draw down a straight line until reach the xaxis. • The point where the line reach on x-axis is HVL value. Answer Q3 • To Get net reading • Background reading come from the cosmic ray, electronic noise, internal radiation from human body, radon from the room (decay of Uranium from rock) & etc. Answer Q4 The nature of ionization radiations, the dose rate reading are always fluctuate (not constant). More repeat , better average, final results are more accurate & precise (especially for low dose rate reading) Practical Examination: Accomplishment of data form (20 marks) Graph: linearity, labeling including title, labels of X and Y axis, used ¾ graph paper (40 marks) Written examination (40 marks) RPO Question Format A. Basic Theory:Answer 40 objective questions for all level I,II & III B. For specific level either (I,II,III): Answer 20 objective questions Select 5 from 8 essay questions C. Practical for all level I,II & III :Answer all 6 essay questions * Passing Mark for every section A, B & C ~ 70%
