CHEM 122L
General Chemistry Lab
Revision 3.4
Determination of the Half-Life of m137Ba
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To learn about Transmutation of Atomic Nuclei.
To learn about Radioactive Decay of Atomic Nuclei.
To learn about the Kinetics of Decay Processes.
To learn about the Measurement of Radioactive Decay Processes.
In this laboratory exercise we will measure the Half-Life (1/2) of an unstable Isotope of Barium;
m137
Ba. This isotope is itself a nuclear decay product of 137Cs. Although nominally the province
of physicists, this type of nuclear decay process is of interest to chemists because it frequently
involves the Transmutation of an atom of one element into an atom of another.
The alchemists' dream of transmuting base metals into gold was not realized until, in the early
20th century, physicists found they could transmute one element into another. The transmutation
of Mercury into Gold was accomplished in the 1940’s by bombarding 198Hg nuclei with neutrons
generated in a cyclotron to produce 198Au. However, this induced transmutation occurs on such
a limited scale, and requires such expense, that the conversion of base metals into gold is not of
practical value.
The natural transmutation of many elements is a consequence of the Radioactive Decay of their
atom's nucleus. The discovery of radioactive decay occurred by accident. In 1896, the French
physicist Antoine-Henri Becquerel discovered that Uranium minerals, even when wrapped in
paper and stored in the dark, emit a penetrating radiation that can produce bright images on a
photographic plate. Two years later, Marie Sklodowska Curie began a search for other minerals
that emitted radiation. She found that Thorium minerals also emit radiation. Curie named the
emissions Radioactivity.
Photographic plate exposed to a Uranium Salt by
Becquerel. A Maltese Cross has been place
between the salt ant the plate in the lower image.
(http://en.wikipedia.org/wiki/File:Becquerel_plate.jpg)
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The radioactivity of these elements, and numerous others, is due to decay of the atomic nucleus
within the atom. The nucleons, protons and neutrons, in the nucleus are held together as a result
of the Strong Nuclear Interaction, which is a residual effect of the Color Force acting between
the quarks that comprise the nucleons. For instance, two positively charged protons within the
confines of the nucleus experience a mutually attractive interaction due to this Strong Interaction.
However, this interaction is effective over only relatively short distances; distances of less than
~10-15m. Countering this attractive force is the electrostatic repulsion between protons, because
of their like charge. This repulsion, although weaker than the Strong Interaction, operates over
much longer distances.
So, if the nucleons are close together, the Strong Interaction dominates and the nucleus holds
together. If the protons are far apart, as they would be in a large nucleus, the electrostatic
repulsion between them dominates and the nucleus becomes unstable. Atoms with nuclei larger
than Bismuth (Atomic Number 83) are all unstable and undergo radioactive decay.
This “size” effect is only one source of instability in atomic nuclei. Some generalizations
concerning nuclear stability are:
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
Very Large Nuclei Are Unstable
Even Numbers of Nucleons Are Stable
Nuclei With Magic Numbers of Nucleons Are Stabl
If an atomic nucleus is inherently unstable, it will undergo radioactive decay. The most common
types of decay are:
-Particle Emission
An alpha particle is a high energy helium nucleus; 42He.
226
88Ra
222
86Rn
+
4
2He
(Eq. 1)
This type of decay typically occurs if the nucleus is too large; i.e., has a very large
atomic number.
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-Particle Emission
A beta particle is a high energy electron; 0-1 or 0-1e.
63
28Ni
63
29Cu
+
0
-1
(Eq. 2)
This type of decay typically occurs if the nucleus lies above the belt of stability.
Positron Emission
A positron is an anti-electron; 01 or 01e.
11
6C
11
5B
+
0
1
(Eq. 3)
This type of decay typically occurs if the nucleus lies below the belt of stability.
K-electron Capture
A K-electron is a core electron, which lies very close to the nucleus.
55
26Fe
+
0
-1e
55
25Mn
(Eq. 4)
This type of decay typically occurs if the nucleus lies below the belt of stability.
-Particle Emission
A gamma particle is a very high energy photon; 00.
m137
56Ba
137
56Ba
+
0
0
(Eq. 4)
This type of decay can occur if the daughter isotope of a given nuclear reaction has its
nucleus left in a high energy state. When it "relaxes", the energy given off is
frequently in the form of a gamma particle. Additionally, gamma particle emission
frequently accompanies other nuclear reactions, and is simply omitted from the
nuclear equations.
A couple of points must be kept in mind concerning these decay processes. The daughter isotope
of a nuclear decay is frequently itself unstable. Being unstable it will decay further. Also, many
isotopes have multiple decay pathways available, with the probability for a given pathway being
well defined. Europium-152, for example, can undergo either -particle emission or K-electron
capture:
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Finally, it is found these processes occur with First Order kinetics:
ln (N) = ln (No) - k t
(Eq. 5)
where N represents the number of nuclei present. The decay of nuclei in a sample can be
followed by Counting the number of decay events which occur in a small period of time. One
method of counting the decay events is to count the decay products given off. This is typically
done using a Geiger-Muller Tube. The emission products pass through a window in the GM
Tube, ionizing Argon gas within the Tube. The ionized gas then generates a voltage which is
registered by a Counter.
The number of Counts will be proportional to the number of Nuclei in the sample.
Counts ~ N
(Eq. 6)
provided the counting time is sufficiently short. Hence,
ln (Counts) = ln (Counts)o - k t
(Eq. 7)
Typically, radioactive decay processes are characterized by their Half-Life, 1/2; the time required
for half the sample's nuclei to decay. The half-life is related to the rate constant, k, by:
1/2 = ( ln 2 ) / k
(Eq. 8)
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In this laboratory, we will measure the Half-Life of meta-stable Barium-137. This isotope
decays by gamma particle emission:
m137
56Ba
137
56Ba
+
0
0
(Eq. 9)
We will Count the gamma particles emitted using a Geiger-Muller Tube connected to a standard
Counter. We will follow these Counts over a 10 minute period; counting for one minute periods,
followed by one minute of waiting.
The question becomes, what is the source of our m13756Ba. This isotope is the daughter isotope of
137
55Cs. Cesium-137 decays by beta particle emission, generating the meta-stable Barium-137:
137
55Cs
m137
56Ba
+
0
-1
(Eq. 10)
From a practical point of view, the Cesium-137 is contained in a "Cow" in which the isotope is
fixed in a fritted, or porous, disk. An elutant is passed through the Cow, carrying the Barium
isotope with it. In this way, we Milk the Cow for the desired isotope.
Thus, we Milk the Cow, transfer the elutent to a sample tray below a GM-Tube connected to a
Counter, and immediately begin Counting emitted gamma particles. This will allow us to
measure the Counts per minute as a function of time, from which we can determine the Rate
Constant k for the decay process. And, this gives us the Half-Life 1/2 for this Barium isotope.
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Pre-Lab Safety Questions
1.
What is the SI unit of measurement for Effective Radiation Dose?
2.
What is the NRC's definition of a lethal dose of radiation? What is this level in units
defined above.
3.
To an order of magnitude, what is background radiation; reported in units defined above?
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Procedure
Kinetic Measurements
Caution: The Membrane on the Face of the Geiger-Muller Tube is very, very sensitive.
Do not touch it.
1.
Set-up the Geiger Tube / Counter System as pictured below:
i)
ii)
iii)
iv)
Plug-In the Counter with the available Power Supply.
Attach the Coax from the Geiger-Muller Tube to the Counter.
Turn on the Power on the Counter.
Set the Counter to the "High Voltage" setting. Increase the voltage setting to 900
Volts.
v) Set the Counter to the "Count" setting.
vi) Start the Counter by depressing the "Count" button.
vii) Test the system using the sample provided by your instructor.
viii) When the system is working correctly, "Stop" the Counter and "Reset" the count.
ix) Slip the Cardboard cover off the GM Tube. (Caution: The membrane on the face
of the GM Tube is very, very sensitive. Do not touch it.) Slip the GM Tube into
the top of the Sample Rack.
2.
Determine the level of Background Radiation:
i)
With no Radiation Sources nearby, start the "Count" for 1 minute. Record the value.
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ii)
iii)
3.
Wait 1 minute.
Repeat twice more, for a total of 3 Background Radiation readings.
Determine the Half-Life of m137Ba:
i)
ii)
iii)
iv)
v)
vi)
Obtain a metal Sample Holder. Cut out a piece of filter paper, such that it fits into the
Sample Holder.
Place the filter paper into the Sample Holder. Obtain about 4 drops of Elutant from
the 137Cs Cow. This contains the m137Ba Parent Isotope.
Immediately place the Sample Holder onto the Sample Tray and slide this into the top
position of the Sample Rack.
Start the "Count" for 1 minute. Record the value. Reset the Counter.
Wait exactly 1 minute.
Repeat the counting procedure until a total of 9 minutes worth of data has been
obtained; alternating counting for 1 minute and waiting for 1 minute.
4.
Place the spent Sample and Holder into a Waste Beaker.
5.
Take the GM Tube out of the Sample Rack and replace the Cardboard Cover. (Caution:
Be very careful with the face of the GM Tube as it is very, very sensitive.)
6.
Turn the system off.
Observations of Some Ores and Compounds
1.
Ores are rock that contain components or minerals such that the rock is commercially
profitable when mined. This is as true of Uranium based ores as it is of other metals.
Samples of some of these ores are provided:
Ore
Pitcheblende
Carnotite
Autunite
Formula of Mineral Component
UO2
K2(UO2)2(VO4)2•3H2O
Ca(UO2)2(PO4)2•10-12H2O
Make observations of each. (Do not handle the samples.)
2.
Compounds of a number of Radioactive metals have also been prepared. Samples of a few
of these are also available:
Compound
Thorium Oxide
Uranyl Acetate
Uranyl Nitrate
Formula
ThO2
UO2(CH3CO2)2•2H2O
UO2(NO3)2•6H2O
Make observations of each. (Do not handle the samples.)
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Data Analysis
1.
Using Excel or some other graphical software package, prepare a plot of ln(Counts) vs.
time.
2.
Determine the Rate Constant k for this nuclear decay product.
3.
Determine the Half-Life 1/2 for the m137Ba isotope.
4.
Consult a CRC Handbook of Chemistry and Physics to determine the Accepted Half-Life of
m137
Ba. Calculate the Percentage Error for your determination.
5.
Determine the Oxidation State of the Uranium or Thorium in each of the Ores and
Compounds observed.
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Addendum
Penetrating Power of , , and  Particles
Because of differences in charge and mass, and  particles behave distinctly differently.
Particle



Mass Num.
4
0
0
Charge
+2
-1
0
For instance, when passing through a magnetic field,  particles will be deflected in one
direction,  particles in an opposite direction and  particles will pass through undeflected.
The interaction of these particles with matter is also distinctly different. When  particles pass
through matter, their positive charge will occasionally attract valence shell electrons away from
atoms they pass, ionizing the atoms and slowing the  particles. Additionally, inelastic
collisions between  particles and the matter’s atoms will also slow the particles somewhat.
Because they are charged,  particles will also interact with matter via coulomb forces.
However, because of their much smaller mass,  particles tend to move with much higher speeds
than  particles, and hence are more penetrating. Further, they are scattered much more
significantly.  particles, on the other hand, are not charged and so interact with matter quite
differently. These uncharged particles are the least interacting with matter and so are more
highly penetrating. Their interactions with matter occur only via a photoelectric effect, Compton
Scattering and Pair formation. In general, the penetrating power of and  particles is
inversely related to the mass and charge of the particle.
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In this short exercise, we will examine the penetrating effects of each of these particles. We will
not directly compare the penetrating effects of the different types of radiation. This is because
factors other than penetrating power influence the penetration measurements. For instance,
because they are more massive,  particles tend to move through absorbing matter undeflected
and can be measured directly. However,  particles scatter significantly and so can be scattered
out of the measured beam, making it appear they have not penetrated the absorbing matter.
Instead, we will examine the penetrating power of each type of radiation independently.
Procedure
 Particle Penetration of Air
1.
Place the  source (210Po) with the window up (label down) in the sample tray and place
the sample tray in the first shelf closest to the GM tube. Measure the GM counts for 1
minute. Note, Air is the “material” which the alpha particles are penetrating.
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2.
Move the sample tray down one shelf and repeat the measurements. We are effectively
increasing the thickness of the “material” to be penetrated.
3.
Do this until the counts drop to that of background radiation.
 Particle Penetration of Aluminum
1.
Place the  source (90Sr) with the window up in the sample tray and place the sample tray in
the second shelf down from the GM tube. Measure the GM count for 1 minute.
Aluminum is now the material to be penetrated by the beta particles.
2.
Place an Aluminum absorber 0.02m thick (G) in the shelf between the GM tube and the
source. Measure the GM counts for 1 minute.
3.
Repeat this with Aluminum absorbers I, K, M, O.
 Particle Penetration of Lead
1.
Place the  source (60Co) with the window up in the sample tray and place the sample tray
in the second shelf down from the GM tube. Measure the GM count for 1 minute. Lead is
the material that can stop gamma particles.
2.
Place the thinest Lead absorber in the shelf between the GM tube and the source. Measure
the GM counts for 1 minute.
3.
Repeat this with the other Lead absorbers.
Data Analysis
1.
Write nuclear decay reactions for each of the sources.