In this experiment you will measure the transmission of gamma rays through different absorbers. Theoretically, there should be an exponential decrease of the transmitted counts with thickness of absorber, determined by a mass absorption coefficient. You will determine mass absorption coefficients and evaluate how they vary as a function of gamma energy and of the atomic number of the absorber.

Equipment: NaI scintillation detector, amplifier, pulse-height analyzer; gamma sources, sets of absorber plates.

Readings: Interactions of photons with matter, concept of crosssections, sect. 5.2.1 and 5.2.5; Absorption measurements, sect. 5.4.3.

Key concepts: Absorption and scattering crosssections for photons; absorption lengths, mass absorption coefficients.

4.1 Transmitted counts vs. absorber thickness

Absorbers of Al, Cu, Cd and Pb are available in plates that can be stacked to produce a range of thicknesses.   Various gamma sources are available, including 137Cs (662 keV), 60Co (1.17 and 1.33 MeV) , 57Co (122 keV), 22Na (511 keV, 1.27 MeV) , and 241Am (59.7 keV) may be available.   Also, some sources emit x-rays of lower energy, e.g. K x-rays of Ba followind decay of 137Cs.  For various paired choices of source and absorber, make measurements of transmitted counts as a function of absorber thickness.  Choose a range of absorber thicknesses that result in a large change in the number of transmitted counts and make a series of measurements to help experimentally to determine how the number of transmitted counts decreases with absorber thickness . Measure absorber thicknesses with a micrometer or vernier caliper.

An important issue is to determine the "background" counting rate that is irrelevant to your measurements.  Background radiation may come from cosmic rays and environmental radioactivity, but also, e.g., from scattering of radiation from your source off of nearby objects into the detector.  You need to use judgement to determine best the background counting rate.  After subtraction of background counting rates, plot count rates on semi-log paper versus absorber thickness.  Determine absorption lengths and coefficients from slopes using the theory of gamma ray absorption (exponential absorption law).

Compare your measured coefficients with those obtained from the attached graph of mass absorption coefficients or from the net (e.g. see http://www.csrri.iit.edu/periodic-table.html or other links on the course home page).  Ascertain whether or not your coefficients are consistent with those reported elsewhere.  If they are not consistent, try to figure out why and to explain how better values might be obtained.

4.2 Mass absorption coefficient versus gamma energy

Using one absorber determine absorption coefficients for a wide range of gamma energies.  Al, Cu or Cd is recommended over Pb.   Plot coefficients versus energy and try to establish a qualitative or quantitative dependence,

4.3 Mass absorption coefficients vs. absorber atomic number Z

Using one gamma source (preferably 57Co or 241Am) determine absorption coefficients for absorbers having a wide range of atomic numbers Z.  Plot mass absorption coefficients versus Z and try to establish a qualitative or quantitative dependence.

Questions and Considerations

a. How can you determine the "true" background counting rate?  By interposing a very thick absorber?  By moving the source far away?  Are there contributions to "background" from the source itself?  If so, could they be reduced through a better experimental design?

b. The total crosssection for removal of photons from the beam, or 'extinction' crosssection, is the sum of crosssections for photoelectric absorption, Compton scattering and pair production.  How might you try to distinguish between these contributions experimentally?

c. Consider the simple pulse-height spectrum of 137Cs, which emits a single gamma ray with an energy of 662 keV.  To determine the counting rate, you have the choice of integrating counts over the entire spectrum or only over the photopeak.  What are the tradeoffs in each choice?

Diagram of Apparatus

Mass absorption coefficients for selected elements
Energy units BeV= Billion of electron-volts= GeV.  Note that lower curves are extensions of the upper curves to higher energies and that there are different energy scales at top and bottom of the graph..

Copyright Gary S. Collins, 1997-2002..