Absorbtivity thickness conversion

[PlasMav]

Hello,

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.

 

[mfb]

Density of Material x Thickness

Is this correct?

Yes.

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.

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.

 

[PlasMav]

Yes.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.

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.

 

[Astronuc]

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

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.
https://www.nndc.bnl.gov/chart/decaysearchdirect.jsp?nuc=90Y&unc=nds

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