Radiation Exp: Converting μCi to dps & Understanding Half-Lives

[Von Neumann]

Question:

Recently I've conducted an experiment measuring the dose rate (in μR/h) from various samples (Ba-133, Cs-137, Co-60), at varying distances. In determining the efficiency of paper as a shield, it is required to convert the activity of the source to units of dinsintegrations/second (dps). On each of the sources was a label that stated the half-life. Additionally, each source said "1 μCi". So therefore, since 1 μCi = 3.7 x 10^4 dps, that would be the activity in dps. Correct? However, I don't understand how all of the sources could have the same activity in dps, but have different half-lives.

Note: the half-lives given on the sources for Ba-133, Cs-137, and Co-60 are 10.5, 30.2, and 5.27 years, respectively.

 

[mfb]

In determining the efficiency of paper as a shield, it is required to convert the activity of the source to units of dinsintegrations/second (dps).

Why? It should be sufficient to know the measured rate with and without shield.

However, I don't understand how all of the sources could have the same activity in dps, but have different half-lives.

Simple: they contain a different number of radioactive atoms.

Additionally, each source said "1 μCi". So therefore, since 1 μCi = 3.7 x 10^4 dps, that would be the activity in dps. Correct?

If the sources are new. Activity goes down with time, while the labels do not change.

 

[Von Neumann]

Yes that does seem to be quite obvious, sorry.

Furthermore, I am looking to estimate my total extra exposure in mR (dose above background) due to performing the experiment by taking into account how long I was near each source, the strength of the source, and its approximate distance from me.

If I take my distance as 0 inches (since I was holding each source throughout experiment) and the time as roughly 10 minutes per source, how would I go about doing this? Perhaps, since I have the measured values of the dose rate at 0 inches from each source, could I multiply the dose rate by the amount of time in order to get this result?

 

[mfb]

That should give a reasonable upper limit on your dose rate. Even if you touched the radioactive part of the probe (I hope you did not), not all decay products have hit you.

 

[Von Neumann]

Yes, I was holding the sources. However, I don't believe my professor would have the class conduct a lab that would endanger our health in any way. The sources were in specially prepared cases from Oak Ridge National Laboratory.