Re: Halflife source: Bi-212 demo

["rick strickert" <rick_strickert@pc.radian.com>]

Claude Epperson of the University of Arkansas wrote to request a reference
for making a thorium lantern mantle/nickel half-life source.

In a previous response on radioactive sources for cloud chambers I pointed
out:

    Rather than hassle with dry ice and alcohol, I have done high school
    radiochemistry demonstrations using the thorium lantern mantles,
    muriatic acid (HCl, for non-chemists) and a US nickel coin.  The
    thorium decay product, Bi-212, self-deposits on the nickel (a good
    chemistry lesson on why) and one can count the decay using a simple
    thin-window geiger counter (and do half-life calculations).  What's
    nice is that the 1-hour half-life radioactivity on the nickel is gone
    in a day, and the acid solution containing the lantern mantle is ready
    to be "milked" for another demo!

There are no specific references that I am aware of on the demonstration
with Bi-212.  W.T. Smith and J.H. Wood (J. Chem. Ed. 36 (1959), p.492)
discussed a student experiment depositing Bi-210 on nickel foil and
measured its 5-day half-life.   The technique is based on the known
behavior of lead, bismuth and polonium to self-deposit on metals surfaces
(such as nickel) that are more electropositive than the element.  The
EPA Method 111 (40 CFR 61, App.B) uses a similar technique to deposit
polonium on silver disks.  Another reference to Bi-210 and Pb-210
deposition is R.L.Blanchard, Analytical Chem. 38 (1966), p.189.  The
demonstration can also be done by students themselves as a chemistry or
physics lab experiment if sufficient equipment is available or can be
shared.

For my demonstration a thorium lantern mantle is placed in a small wide-
mouth polyethylene bottle containing 40 mLs of 1 to 3 M HCl (don't forget
safety precautions).  A couple of hours before a class demo, I put a clean
nickel in the bottle (heating in hot water helps; so does coating one side
of the nickel with epoxy paint).  The nickel is then removed with coated
tweezers, washed with water, gently dried, and counted repeatedly over 2-3
hours with a thin window alpha detector (e.g., GM, scintillation, etc.)
attached to a counter (or a computer - see B.E.Taylor, Am.J.Phys 57 (1989),
p.1051, or E.Vitz and S.Reinhard, J.Chem.Ed. 70 (1993), p.758).  Depending
on the activity, 2-4 nickels can be alternately counted on one detector.

I initially get a few hundred cpm with an alpha scintillation detector.
You should explain to the students that they are also counting alphas from
the 0.3-usec Po-212 daughter of Bi-212.  There also appears to be a slight
"tailing" after several hours, probably caused by some deposition of the
11-hour Pb-212 on the nickel - a chance to explain decay schemes and
composite decay curves to any young radiochemist proteges, and why the
solution (and the nickel) can be reused after a day or so.  If you do the
demo often (or leave the nickel in the solution too long), an observant
student may question you about the greenish tinge to the mantle solution.
I also wrote a BASIC program to plot the data on the screen (on semi-log
scale) and do a linear least squares fit/half-life determination.  You can
also use a spare lantern mantle, a piece of paper, and a thin-window GM to
distinguish alpha, beta and gamma activity to a class.

With all the chemistry, physics, and math that can be discussed, students
have the opportunity to get a lot more than a nickel's worth out of
counting on a nickel!

If anyone comes up with any unusual variations, I would be interested in
adding them to my demo notebook.

Rick

Richard G. Strickert, Ph.D.          |    "Science is the belief in
Radian Corporation, Austin, TX       |     the ignorance of experts."
Internet:rick_strickert@radian.com   |           -  Richard Feynman
 ---> "All written IMHO." <---       |