Another Lab Report. Gamma Ray Spectroscopy.

1. Gamma Ray Spectroscopy Report
Jennifer S. Nalley
Lab Partner: Chris G. Cumby
February 20, 2007
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2. Abstract
The Gamma Ray Spectroscopy lab allowed us to experimentally verify
the nature, behavior, and patterned phenomenon associated with
gamma ray emission. Using a number of radioactive samples, in
conjunction with a sodium-iodide (NaI) crystal detector and computer
spectrometer interface, we observed and recorded the energies of the
gamma rays emitted by these isotopes. The relationship between
energy and wavelength can be stated as
E = h /λ.
where h is Planks constant. This relationship allows us to state that
electromagnetic radiation with shorter wavelengths, have larger
energies than their shorter wavelength counterparts. This lab deals
with gamma rays, which have very short wavelengths (picometer
scale), which implies a large energy.
Electromagnetic radiation can react with matter in various ways. Best
known of these matter-electromagnetic interactions include the
photoelectric effect, Compton Effect, and pair production. The
photoelectric effect and Compton Scattering differ in that the Compton
Effect deals with a significantly higher energy than that of the
photoelectric effect. In this lab, with the exception of pair production,
each of these interaction results took place. Pair production requires
gamma ray energy equaling two times that of an electron at rest. This
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3. experiment did not allow energies of this magnitude. Because
Compton Scattering was to be included in this experiment, it is
reasonable that radioactive substances be used, because gamma rays
have extremely high intensity and short wavelength, which suggests
the possibility for a high energy.
During the lab, related phenomenon, showed up in the form(s) of
photo peak energy, Compton edges, and back scattering (Compton
scattering). The initial calibration, which was used as a reference
point, was set using Cs-137 and Co-60. The final data, which was
comprised primarily of each isotopes channel number and energy level
was analyzed and graphed. The linear equation corresponding to the
data points from each isotope was 222.237664.0 += xy . In the
formula, x=channel number, and y= energy in KeV.
Introduction and Theory
The gamma ray spectroscopy lab consisted of an experiment that
allowed us to better understand radioactivity, gamma rays, and how
the energy emitted from gamma rays is distributed. Reading up on
topics, such as Compton Scattering, the Photoelectric Effect, and pair
production, proved to be helpful in the understanding of this lab.
During the lab, we were able to visually observe the energy peaks of
individual isotopes. This was accomplished by the use of purpose
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4. specific equipment and eight radioactive samples with which we were
provided (one of which was unknown). To briefly state the actual
process within the device, a gamma ray would hit the NaI crystal,
which in turn would eject an electron, which would then return to its
initial state within the crystal, thereby emitting a photon. This happens
because, as something returns to a lower state, it must release energy
to do so. The release of a photon may be considered a disposal of
excess energy. Next, the photon would be caught by the photocathode
of the photomultiplier where once again electrons are ejected, this
time due to the photoelectric effect.
With the samples and equipment, we were able to visually view the
energies of each of the samples respectively, (not including Cd 109,
which we omitted due to lack of time). Generally the energies from the
gamma rays that were emitted from the isotopes showed up on the
screen in such a way that would allow one to determine how, and to
what degree, each of the samples was emitting energy. The plots
shown on the screen were actually the kinetic energies of the
photoelectrons, energies created by the interactions with gamma rays
as described above. In other words, our data was obtained by indirect
means.
As stated above, this experiment, the gamma rays did not have
sufficient energy (1.022 MeV) to give us any readings for pair
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5. production. As expected, during Compton Scattering, the electron
absorbed the bulk of the energy, and “scattered” at a 180° angle
(bounced back). Visually, Compton Scattering was recognized by its ill-
defined peak, as it occurred for a wide range of energies. As the name
indicates, it was scattered. The photo peak was a well defined peak
because it was there that the photoelectrons were totally absorbed by
the detector.
After taking into account our calibration, we obtained a channel
number along with a corresponding energy (keV) for each isotope. The
relationship between these two variables served as our data.
Apparatus and Procedure
Equipment:
Included in the main unit:
• NaI –(sodium iodide) crystal detector
• UCS 20 spectrometer interface software
• Photomultiplier
• Radioactive samples (table below)
Isotope Half-life Type
Co-60 5.27 years Gamma
Ba-133 10.5 years Gamma
Co-57 271 days Gamma
Mn-54 313 days Gamma
Cd-109 464 days Gamma
Cs-137 30.2 years Gamma/Beta
Na-22 2.6 years Gamma
*All samples were dated January 2004
*All samples were labels 1.0μ
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6. Before beginning the actual procedure, we inserted the Cs-137 sample
into the second slot (2 cm from top) of the sensor. The sensor was
attached to an oscilloscope. From this we could view voltage pulses
and polarity. While watching the attached oscilloscope, starting with
800 V, we slowly decreased the voltage. In terms of a Cartesian
coordinate graph, the visuals on the oscilloscope originated at the x=0
line. They appeared as curved lines reaching down into the negative x
quadrants. As the voltage was decreased (thereby current decreased),
the lines seen on the oscilloscope retracted back towards their origin.
This was done in order to get an initial feel for the behavior of the
phenomenon at hand.
Oscilloscope View
During the remainder, which was the bulk of the experiment, we used
a computer opposed to an oscilloscope. This particular setup was
initially calibrated using the Co-60 and Cs-137 samples. We believe
the reason that these two particular isotopes were used for the
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7. calibration, relates to the fact that Co-60 and Cc-137 have the shortest
(271 days) and longest (30.2 years) half-lives of the samples
respectively. When the necessary peaks came into view, the
calibration was then set. We followed the recommendation of leaving
the isotope beneath the detector for about 10 minutes for this
calibration. The calibration was set as the table below reads. After
calibration, we kept the voltage constant. Doing otherwise would have
given us skewed data.
Next, individual readings for each isotope were taken. There was a
definite connection between an individual isotopes’ half-life, and the
time it took to get a complete reading for it. Those samples who had
the longer half-lives’, took a significantly longer time to produce an
energy reading. This intuitively makes sense considering the definition
of half-life.
The software used for the main part of the experiment was able to
give numerical values, as well as a visual representation of both the
High Voltage 760
Channels (used to
calibrate)
1611
Energies (used to
calibrate)
1332
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8. energy levels and the number of counts per channel for each isotope.
The sodium-iodide detector/ spectrometer to UCS interface gave the
energy counts numerically and visually. A graphical representation for
each isotope’s energy is attached
Data Analysis
After obtaining the data, my instinct was to immediately categorize the
isotopes, hoping to have an easier time in observing any immediate
patterns. This is where it was noticed that the two isotopes used for
the calibration, Cs-137 and Co-60, happened to be the isotopes with
the shortest and longest half-lives respectively. For the duration of the
data analysis, the isotopes were kept in the order of shortest half-life
to longest half-life. The channel number and energy level for each
sample were then entered, and graphed. This is where the most
striking observation was made. For every isotope, there was an
undeniable correlation between any samples channel number and
energy level. When these two variables, (channel number and energy),
were graphed on the same line, this relationship was visually obvious.
When these two variables were then graphed against each other, the
result was what appeared to be a nearly perfect linear fit. The
equation yielded was 222.237664.0 += xy , where y=energy in KeV.
Basically this told us that if a channel number for any particular
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9. sample is noted and plugged into x, the energy can be solved for. The
uncertainty for the channels turned out to be 14.14± , which can also
be considered our propagated error for the energy in KeV, as there is a
linear relationship. Additionally, it is notable that the 2
X appearing
on our graph(s) (actually as 2
R ) is very close to unity. 9999.02
=X .
The unknown isotope was determined to have an energy level of 655.5
KeV. Taking into account the uncertainty mentioned above, we
concluded that our unknown isotope was Cs- 137.
I, for one, was initially confused, and was in disagreement with the
identification of the unknown. While comparing the graph of the
unknown to the others, I noticed that the graph and data for Mn-54
was incredibly similar to that of the unknown, and believed it to be the
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10. correct unknown. See the picture.
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11. *Notice the various KeV, channel, and energy values. Also note the
peak placement.
Error
Once again, we were unable to obtain data for isotope Cd-109 due to
time constraints. Many of the samples had very long half-lives, and the
sample set was considerably old. When considering this, and then
factoring in the time it took to figure out how to use the equipment, it
seems as though time constraints alone could be considered a
contribution towards possible error.
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12. In addition, there was the risk of over saturating the detector, and
possibility that we did not always allow ample time to allow it to
“clear”.
When we were using the oscilloscope at the beginning of the
experiment, it was noticed that when the box of samples sat as far as
2 feet away from the apparatus, the gamma rays were still being
detected. The great sensitivity of the sensor, (although desirable under
the right conditions), could have contributed to experimental error.
Greater precautions on our part could have been taken.
Conclusion
The quantified energy for gamma rays (and all electromagnetic
radiation) is particularly intriguing when displayed in a visual manner.
In this experiment, there was a definite linear between the channel
number and energy level. Although the procedure for completing this
lab assumed some previous knowledge on the subject, it was
undoubtedly a worthy experiment for exhibiting the nature of gamma
rays and phenomenon related to them. If we were to do the
experiment again in a more meticulous manner, greater accuracy
could have been achieved.
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13. References
(1) NUCSAFE Inc- Gamma interactions and id.
http://www.nusafe.com.technology/gamma_interactions_and_spect
roscopy.htm
(2) Nonclassical Physics- Ray Harris pp. 77-93
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