Additional
Resources
Hands-On Investigation: Measuring the Half
Live of a Radioactive Sample
Teacher Notes
Introductory description of the investigations:
In the following investigation, the students will be directly measuring the
half-life of a radioactive material. The half-life of a material is unique,
so careful determination of the half-life of a sample can help identify
what isotope is present in a material. If multiple radioactive isotopes are
present in a sample, determining how much of each type is present can
help identify the origin of the sample as well as how it has been used.
Let students know that in these explorations they will use the same
digital radiation monitors and stopwatches that they used in earlier
explorations.
Teacher Notes
Student Handout
Student Solution
Detector Setup
Guide
Detector
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Radiation Safety
Poster
HPS Guidelines
In this experiment, you will be working with an isotope generator. The
Health Physicists
isotope generator is a ceramic material that contains cesium-137 (137Cs).
Society
Cesium is a very reactive element and it is embedded in the ceramic in
order to allow you to safely handle the material. Cesium beta decays to an excited state of
barium-137 (137Ba). These excited barium atoms can be washed into a liquid (eluted) and this
liquid then placed on a wipe or tissue in a sealed baggie. The excited barium nucleus eventually
decays into its ground state by emitting a photon. This photon is the gamma radiation that your
students will be detecting with the digital radiation monitor.
The radioactive barium decays in a predictable way. The probability that any nucleus will decay
in a given second is called the decay constant (k) and so the activity of the sample is proportional
to the probability a nucleus will decay and to the number of nuclei in the sample. As time
passes, the number of radioactive barium nuclei in the sample decreases, so the activity must also
decrease. Activity is typically indicated by the symbol R and its standard unit is a Becquerel,
where 1 Bq = 1 decay per second.
After half of the original sample of excited barium nuclei has emitted γ-radiation and decayed to
its ground state, the activity of the sample will also be half of its original value. This time period
for half of the sample to decay is called the half-life ( t 1 / 2 ) of the nucleus. In this investigation,
you will measure the activity of the barium sample over 20 minutes and use that information to
determine the half-life of this nucleus.
Preparing:
Read, prepare for, and discuss the safety procedures involved with the isogenerator. The
safety guidelines below are quoted from the Health Physics Society Safety document
included in an earlier lesson, which can also be found at:
http://hps.org/sciencesupport/documents/hps_scisupport_guidance_exempt-quantities-school.pdf
Special Precautions for Isogenerators
Isogenerators typically contain cesium-137, which is radioactive and decays to barium-137,
which is also radioactive. Barium-137 has a half-life of 2.5 minutes, emitting gamma radiation,
and then becomes stable. When an eluting solution flows through the cesium, barium attaches to
the eluting solution and the liquid leaving the solution then contains some radioactive barium.
Because of the short half-life, the barium activity decreases to less than one percent of its
original activity in a little more than 15 minutes. When using an isogenerator:
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Be certain that the eluting solution enters the proper side of the isogenerator so that the
cesium-137 is not eluted in addition to barium-137.
Do not let anyone ingest any of the liquid.
Wipe up spills with a paper towel and dispose of it in the trash.
If any liquid solution splashes onto clothes, use a damp cloth to wipe off as much as possible.
The remaining radioactivity in the solution will decay rapidly, thus producing minimal
radiation exposure.
Wipe sample holders with a paper towel after use and dispose of towels in the trash.
Wash hands after handling.
Disposal
Radioactive material decays away with a characteristic half-life. Therefore, some sources lose
enough of their radioactivity that they become useless for teaching demonstrations. If you have
borrowed sources from somebody, the owner will handle the disposal when the source becomes
invaluable for any purpose. If you own an exempt quantity source that has become useless, you
may dispose of it in the regular trash. You must remove labels and other identifying marks prior
to disposal to avoid any confusion if found by someone else.
Materials needed:
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Radiation monitors, preferably digital radiation monitors
Cs-Ba isotope generator
Snack sized plastic bags
Small tissues
Stop watches
Classroom Handling Procedures for Radioactive Sources handouts (in Introduction to
Ionizing Radiation section).
Preparing radioactive samples:
To produce a sample of barium isotopes for your students to work with, you will pour a small
amount of weak acid through the ceramic material. After passing through the ceramic, the liquid
will contain a small amount of radioactive barium. (Cesium is not extracted from the ceramic by
this very weak acid.) Your isotope generator should have instructions for how to pass the acid
through the ceramic. One suggestion is to allow the liquid to drop onto a tissue inside of a small
bag, rather than into a dish. Once you seal the bag, it is safe for students to handle and you do
not have to worry about inadvertent spills.
Should any liquid be spilled during sample preparation, simply wipe it up and dispose of it in a
garbage can. There are no chemical safety concerns and the half-life of the material is quite
short. By the time 15 minutes have passed, the radiation levels should be close to background
levels.
Preparing the students:
If you haven’t conducted the hands-on investigations “Using a GM Radiation Monitor” and
“Measuring low-levels of different types of radiation” in the Introduction to Ionizing Radiation,
you may want to use some of the introductory information provided in those handouts to prepare
your students for working with the materials listed above. In particular, you will want to remind
them how to set the monitors up to take timed measurements.
Remind students to take care with the radiation monitors and keep all materials from touching
the mica window. In addition, in this activity they should follow the ALARA principles of
keeping their exposure to radiation “As Low As Reasonably Achievable.”
Review with the class the basic ALARA principles:
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To minimize exposure when handling a radioactive source, keep it at an arms length when
possible.
Wash your hands after working with radioactive sources.
Pick up solid sealed sources holding their sides. Be careful not pierce the plastic with a
fingernail.
Place solid sealed sources face down (label side up), and at a distance from live organisms
when not in immediate use.
Do not put any sources in your pockets.
Handle sources with respect.
Remind students about background radiation and the actual values they obtained in their
classroom.