Radiation Physics –Practical (Experimental No. 1) PDF

Radiation physics –practical Experimental No.1 Linear and mass absorption coefficients Asst.Lec.Dumoaa Haider Shakir  1.1 The objectives of this experiment:  1) Verification of the absorption law of Gamma radiation.  2) Determination: a) The linear absorption coeffici...

Radiation physics –practical Experimental No.1 Linear and mass absorption coefficients Asst.Lec.Dumoaa Haider Shakir  1.1 The objectives of this experiment:  1) Verification of the absorption law of Gamma radiation.  2) Determination: a) The linear absorption coefficient (𝜇). b) The mass absorption coefficient μm. c) The half value thickness of the absorbing material (𝑿𝟏 𝟐 )  1.2 Apparatus  GM Tube and stand (Counter box, power supply – transformer, GM Tube, shelf stand, USB cable, and a source holder for the stand) – shown in Figure 1.  Source of radiation.  Sheets of different absorbing materials (Aluminum and Lead).  1.3 Theory  When Gamma radiation passes through matter, it undergoes attenuation primarily by Compton, photoelectric and pair production interactions. The intensity of the radiation is thus decreased as a function of thickness of the absorbing material. The mathematical expression for intensity ( I ) is given by the following expression: 𝑰 = 𝑰𝟎 𝒆−𝝁𝒙 … … … ….. …. (𝟏) where, 𝐼0 the original intensity of the beam. 𝐼 is the intensity transmitted through an absorber thickness X. μ is the linear absorption coefficient for the absorbing material  If we rearrange eq.(1) and take the natural logarithm of both sides, the expression becomes, 𝑰 𝐥𝐧 =−𝜇X …….. (2) 𝑰𝑶  The half value layer (HVL) of the absorbing material is defined as that thickness 𝑿𝟏 𝟐 which will decrease the initial intensity by half. That is, I= 𝐼0/2. If we substitute this into eq.(2) we get: ln (2)=𝜇𝑿𝟏 𝟐 ….(3)  Rearranging eq.(3) we get: 𝑿𝟏 𝟐 = ln (2)/ 𝜇 …. (4) Or 𝑿𝟏 𝟐 = 0.693/ 𝜇  The factors which effects on the Linear and mass absorption coefficients  1. Effect of Atomic Number  2. Effect of Density  3. Effect of Thickness  4. Effect of Gamma-Ray Energy  the mass absorption coefficient (µm) another useful concept is defined by dividing the linear absorption coefficient by the material density µm= μ /ρ …. (5) Where ρ is the density of the absorber material.  1.4 Procedure  1. Connect the plugs of the electric mains.  2. Set the timer to 60s and the operating voltage to 380 V  3. Record the count rate per one minute for the background (IB.G).  4. Put the source in front of the GM tube.  5. Record the count rate (𝐼0).  6. Place (Al) sheet midway between the source and the GM tube.  7. Record the count rate ( I1 and I2) and then find IAvg.  8. Repeat steps 6 and 7 with increasing the thickness of the absorbing material.  9. Place (Pb) sheet and repeat steps 6,7 and 8.  10. Plot a graph between In(𝐼0/I) and thickness X, if the relation is a straight line, then the absorption law is verified.  11. Find the slope from the graph, this is equal to the linear absorption (or Attenuation) coefficient.  12. Calculate the mass absorption (or Attenuation) coefficient.  13. Plot a graph between (I) and thickness (X), then find the value of the half thickness graphically.  14. Calculate the half thickness theoretically by using eq.(4).