Lab 3 Detector Efficiency PDF

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Radiochemistry and Detection with Lab...

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Lab #3a: Efficiency of GM Detector Objective: You will determine the efficiency of your Geiger-Müller counter for various types of radiation.

Introduction: If A is the activity of the radioactive source in disintegrations per second or Bq then the count rate R (in counts per second) recorded by the radiation detector is equal to:

where is the absolute efficiency of the detector for the ionizing particle ( eg. , or ) and I is the intensity or probability that the ionizing particle that you are detecting is emitted in a single disintegration of the radioactive atom. The absolute efficiency will be less than one because not every particle emitted by the radioactive source will interact with the detector and because not every particle that strikes the detector will be recorded by the detector. In this experiment, you will measure the absolute efficiency of your GM tube counting system for different isotopes by comparing the measured count rate to the disintegration rate (activity) of the source.

Equipment: The ST-360 Counter Radioactive Sources Po-210 (, Sr-90 (, Co-60 ()

Procedure: 1. Setup the Geiger counter as you have in the previous experiments. 2. Measure the background count rate of the detector using a five minute count. 3. Place the Po-210 source on the top shelf of the counter and record three, twominute counts. Record the activity (A) of the source and calibration date. 4. Place the Sr-90 source on the third shelf of the counter and record three, twominute counts. Record the activity (A) of the source and calibration date. 5. Place the Co-60 source on the third shelf of the counter and record three, twominute counts. Record the activity (A) of the source and calibration date.

Data Analysis: 1. Calculate the average count rate of each radioactive source in counts per second. 2. Correct the average measured count rate of each source for detector dead time. 3. Correct the average measured count rate of each source for background count rate. 4. Use the calibration date and source activity to calculate the activity ( A) of each source in disintegrations per second or Bq.

5. Use an intensity of Iα of 1 (or 100%) for Po-210, Iβ of 1 (or 100%) for Sr-90, and Iγ of 2 (or 200%) for Co-60 to calculate the absolute efficiency of your GM counter for each type of ionizing radiation. (Note: Co-60 emits two gamma-rays of near equal energy per decay so Iγ is 2. The radiation intensities for radionuclides can be found at http://nucleardata.nuclear.lu.se/nucleardata/toi/nucSearch.asp) 6. Calculate the uncertainty in the measured absolute efficiency for each type of ionizing radiation.

Post-Lab Questions: 1. Is the efficiency of the GM counter the same for alpha particles, beta particles, and gamma rays? If not, discuss the observed differences. 2. If a different shelf is used, will the absolute efficiency change? Explain your answer. 3. Why was the alpha source counted on shelf 1? (You are welcome to try your own experiment to answer this question.)

Lab #3b: Detector Solid Angle and Inverse Square Law Objective: You will verify the inverse square relationship between the source-to-detector distance and detector count rate.

Introduction: The absolute efficiency of a detector is the ratio of the count rate to the emission rate (A x I) of the particles from the radioactive source:

The intrinsic efficiency of a detector is the ratio of the count rate to the number of particles per unit time striking the active volume of the detector:

The solid angle ( ) that the detector subtends gives the ratio of the number of particles striking the detector to the number of particles emitted by the radioactive source. The absolute efficiency is related to the detector solid angle and intrinsic efficiency through:

The detector count rate is then given by:

A good approximation of the detector solid angle for point radioactive sources is the ratio of the area that the detector subtends on the surface of a sphere whose radius is equal to the source-to-detector distance. For the case of your GM counter, the solid angle is equal to:

where r is the radius of the detector window and d is the source-to-detector distance. In this lab, you will demonstrate that the count rate of the radioactive source follows the or inverse square law as predicted by the above equations.

Equipment: The ST-360 Counter with GM Tube and stand.

Tl-204 source

Procedure: 1. Setup the Geiger counter. 2. Take a 5 minute background count on the detector. 3. Place the radioactive source in the top shelf and take a 1 minute count. The source-to-detector distance of this shelf is 2 cm. 4. Move the source down one shelf at a time and record a 1 minute count. The shelves are 1 cm apart. Take a count from all ten shelves.

Data Analysis: 1. 2. 3. 4.

Correct your measured count rate from each shelf for detector dead time. Correct your measured count rate from each shelf for detector background. Use Excel to make a plot of corrected count rate versus Use Excel to fit a line to the data in your plot and determine the correlation coefficient for the line.

Post-Lab Questions: 1. Is your data well described by the inverse square law? Explain. 2. A radioactive source gave a count rate of 1200 counts per minute. What would be the count rate from this source if the source-to-detector distance was doubled? 3. Based upon what you learned in this lab, how can you reduce the magnitude of the detector dead time correction when counting high activity radioactive sources?...