Nuclear Spectroscopy

DATA ANALYSIS
Record the photopeak energies and net counts for all
significant peaks that you observe in your spectra. Do
you observe the gammas from the daughters of either
radium isotope? Do you see evidence for radium in
both the branch and the leaves? Why or why not? Can
you suggest other biological or geological sources
that concentrate calcium and possibly radium?
Biological uptake of iodine, cesium, barium, and
strontium radioisotopes from fission products can be
health-threatening to many forms of life, including
human, depending upon the total exposure from the
isotope.
Exercise 14.3
Gamma Spectra from Liquids
Liquids need not mean just water, but for the sampling
suggested here it does. Whatever is in the air can
also become included in the rain and collected from
rainwater. Water filtered from rivers and streams
includes not only the rain’s contributions, but the
radioisotopes provided by land runoff and factory or
sewer discharges. Standard filter paper will remove
some of the insoluble material of diameters greater
than 0.1 µm.
For rainwater and snow you may expect to see radionuclides
from cosmic-ray production, radon decay
products ( 222 Rn and 220 Rn), and some nuclear fission
decays, especially 137 Cs. Recent volcanic activity may
add thorium-series, uranium-series, and potassium
radioisotopes. Water from a sewage treatment facility
or from a river downstream from such a plant may
have measurable levels of 99m Tc or 131 I, if there are
hospitals using these isotopes and discharging them
through the treatment plant you are observing. Fertilizer
runoff from agricultural lands can give an
increased yield of 40 K.
OBJECTIVE
Observe the radioactive decay of isotopes filtered
from rainwater, melted snow, or other effluents. Fresh
rainwater and snow will be sources similar to atmospheric
samples. Water from sewage treatment facilities
will provide sources of medical and nuclear industrial
wastes.
SUPPLIES
•coffee can or other large cylinder with detachable
lid
•filter paper (e.g. #41 Whatman)
•permission to acquire water samples from private
property
SUGGESTED EXPERIMENTAL PROCEDURES
1. Cover one end of your container with a perforated
cap that holds your filter. About 10-15% of
the cap area should be perforated.
2. Run 200-1,000 liters of water or melted snow
through your filter. Rainwater collection is enhanced
by placing the filter can under a downspout.
3. Start the MCA and check your calibration.
4. Dry, cut, and stack the filter paper to fit neatly in
your sample holder. Line your holder with plastic
wrap to prevent its contamination. Acquire a
spectrum for about 1 hour.
5. Measure a background spectrum.
6. Repeat your spectral measurement three days
later. Record the spectrum for two hours.
DATA ANALYSIS
Blending of gamma lines can sometimes be resolved
by taking advantage of the different half-lives of the
gamma emitters. Your ability to measure the halflives
will depend upon the span of time you measure
and your ability to reduce the background radiation.
Short half-lives (< a few hours) can be measured, but
it requires a short sample collection time, and immediate
counting of the sample.
Isotope concentrations in water samples (Bq/l or
pCi/l) provide important environmental measures
of waste discharge or runoff. What concentrations
did you find? How good are your measurements, and
what measurement errors may limit their accuracy?
Assume that your filtering is 100% efficient.
47