Published on January 7, 2009
Gamma Ray Spectroscopy Report Jennifer S. Nalley Lab Partner: Chris G. Cumby February 20, 2007 I
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 II
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 y = 0.7664 x + 23.222 . In the formula, points from each isotope was 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 III
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 production. As expected, during Compton Scattering, the electron IV
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 V
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 calibration, relates to the fact that Co-60 and Cc-137 have the shortest (271 days) and VI
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. High Voltage 760 Channels (used to 1611 calibrate) Energies (used to 1332 calibrate) 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 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 VII
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 y = 0.7664 x + 23.222 , where y=energy in KeV. Basically this told us that if a channel number for any particular 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. VIII
Additionally, it is notable that the X 2 appearing on our graph(s) (actually as R 2 ) is very close to unity. X 2 = 0.9999 . 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 IX
correct unknown. See the picture. X
*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. 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”. XI
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. References NUCSAFE Inc- Gamma interactions and id. (1) XII
http://www.nusafe.com.technology/gamma_interactions_and_spectr oscopy.htm (2) Nonclassical Physics- Ray Harris pp. 77-93 XIII
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