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Absorbtivity thickness conversion

  1. Nov 8, 2018 #1

    I'm working on a lab report centered on Beta decay and I am confused how absorber thickness is obtained. We used varying thickness aluminum plates spaced 2cm from a Sr-90 source taking measurements with an ST350 counter/GM tube/software.

    By my understanding the absorber thickness [g or mg/cm^2] is calculated by getting the product of the material's density and thickness.

    Density of Material x Thickness

    Is this correct? The lab asks a question regarding fluctuating count values higher than 1400 mg/cm^2 thickness however my maximum thickness is 857 mg/cm^2.

    Thank you.
  2. jcsd
  3. Nov 8, 2018 #2


    Staff: Mentor

    I don't know the lab setup, but maybe you can estimate the expected rate at 1400 mg/cm^2 and then make statements based on that.
  4. Nov 8, 2018 #3
    I feel like maybe the lab was designed to use even thicker Aluminum plates. I wish my scanner worked so I could post it here.

    My E value also has a huge 57.5% error.

    Experimental .2124 MeV vs True Value Sr-90 E of .546 MeV

    I may just have to roll with these numbers and say something was WAY off.
  5. Nov 10, 2018 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    The accepted mean energy of betas from 90Sr is 195.8 keV (from NNDC, BNL). So if one measured 212.4 keV, the error would be 8.5%.
    Ref: https://www.nndc.bnl.gov/chart/decaysearchdirect.jsp?nuc=90Sr&unc=nds

    The maximum energy of a beta from 90Sr is 0.546 MeV, and the most probable energy is about 1/3 of the maximum. An anti-neutrino takes some of the energy.

    Find a beta spectrum for 90Sr and other beta emitters.

    The following paper reports a mean energy of 196.1 keV for the average beta energy.
    Robert J. Budnitz, STRONTIUM-90 AND STRONTIUM-89: A REVIEW OF MEASUREMENT TECHNIQUES IN ENVIRONMENTAL MEDIA, Lawrence Berkeley Laboratory - https://www.osti.gov/servlets/purl/4236199/

    J. Mantel, "The Beta Ray Spectrum and Averege Energy of Several Isotopes of Interest in Medicine and Biology", Int. J. Appl. Radiat. Isotopes 23, 407 (1972).

    Note that the decay product of 90Sr is 90Y, which has a beta of maximum energy 2.28 MeV, and an average energy of ~933.1 keV, so some activity one counts would be coming from 90Y. The beta from Y would increase the apparent energy of the less energetic beta from Sr. Decay of 90Y is more complicated.

    In designing shielding for a given isotopic source, one must consider the decay chain and gamma radiation in addition to betas.
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