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Gamma Ray Spectroscopy

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Abstract

Spectroscopy is the study of how radiation of different wavelengths interacts with matter. Gamma ray spectroscopy specifically deals with the spectroscopy of radioactive material. The equipment used to analyze the gamma rays emitted by various radioactive materials is a sodium iodide scintillation detector. Data collected by the detector is used to produce a gamma energy spectrum.

Introduction

Many radioactive materials have a property of producing gamma rays at different energies and different intensities. In order for a gamma spectrum to be collected a proper detection system needs to be constructed in order for the rays to be captured. This is done by using a radiation detector which in this case is a sodium iodide (NaI) scintillation detector. The detector is then connected to a multi-channel analyzer (MCA) or a pulse sorter; the function of the MCA is controlled by a computer program which eventually produces the desired gamma spectrum. This spectrum is analyzed and origins of the gamma rays are determined. Also, the interaction mechanisms that are responsible for a gamma ray to lose energy in the crystal are discussed. These mechanisms are the photoelectric effect, the Compton Effect and pair production.

Theory

The Photoelectric Effect

The photoelectric effect is a natural occurring phenomenon where electrons are ejected from matter after the energy from a form of electro-magnetic radiation has been absorbed by the material. This is one of the three processes in which the gamma ray of a specific radioactive material might loose its energy. The incoming gamma ray will interact with the crystal. Based on the structure of the crystal and the bond lengths the gamma ray will loose energy since it would have to interact with the particles/electrons of the crystal. So, energy from the gamma ray will be depleted a certain amount since it would have to excite an electron from the crystal. In other words, most of its energy would be transferred to the electron.

The Compton Effect

More specifically, the Compton Effect or Compton scattering is attributed to the fact that an incoming form of radiation (X-ray or Gamma ray) looses energy upon interacting with matter. This effect usually deals with the fact that the individual photons which constitute the radiation loose energy. This process can be thought of as an inelastic collision where the photon of the incoming radiation interacts with the valence electrons of the material and not all the energy of the photon is transferred to the electron. Hence, there are lurking photon(s) within the material that still have energy and will interact with more particles within the crystal. This the second process in which a gamma ray might loose its energy in the sodium iodide crystal.

Pair Production

Pair production occurs when a photon (more energetic than photoelectric effect and Compton Effect) interacts with matter and causes for an electron and positron to be created at the same time. The only way this can happen is if there is a sufficient amount of energy available to create the particle/anti-particle pair. The manner in which the gamma ray in this experiment would loose energy due to this phenomenon is that its initial energy would have to be split between the electron/positron pair in order for energy to be conserved. Upon producing the pair, the gamma ray or gamma photon undergoes an inelastic scattering process where the newly deflected and less energetic photon is free to interact with other parts of the crystal or simply leave the crystal. This is the third process by which the gamma rays in this experiment might loose energy within the crystal.

Scintillator

A scintillator is a substance that has a property of luminescence. Luminescence is basically light that is radiated at low temperatures. This can be caused by for example stresses on a crystal. The material exhibits this property when it is excited by some form of ionizing radiation and in this case this form of radiation is gamma rays. So, the scintillator in this case is a sodium iodide crystal which in principle exemplifies the property of luminescence when it comes into contact with a gamma photon. Essentially, the crystal absorbs the energy of the gamma photon and “scintillates”; basically re-emitting the absorbed energy from the photon in a different form of radiation.

The detection of gamma rays directly involves the use of a scintillator coupled with a multi-channel analyzer. A scintillator is basically a material, in this case an alkali halide salt such as sodium iodide, which emits low-energy photons after it has been bombarded with a high-energy charged particle. The intriguing part is that the gamma rays are not directly observed by the scintillator. The gamma rays are responsible for producing charged particles inside the crystal (sodium iodide) which interact with the crystal itself which eventually emit photons. Now these low energy photons are captured by what are called photomultiplier tubes (constitute of a multi-channel analyzer). Moreover, it can be noted that the crystal used in the scintillator is inorganic in nature. Hence, the interactions due the incoming gamma rays that bombard the crystal are due to the electronic band structure of the crystal.

Further, the manner in which low energy photons are captured by the photomultiplier tubes is that the tubes absorb the emitted light of the scintillator and amplify this radiation in order for individual photons to be detected. The purpose of the photomultiplier is to detect the light emitted by the scintillator and translated into a signal that can be understood by a computer program in order for a spectrum to be produced.

Procedure

The procedure for this experiment can be found in the Physics 360B lab handout, titled “Gamma Ray Spectroscopy”

Results

The theoretical gamma energies of the various samples studied in this experiment are summarized in the table below:

Radioactive Elements

Gamma Ray Energies (keV) http://www.tutormenow.ca

Cs137

661.650

Co60

1332.50, 1332.50, 1173.20

Na22

1274.54

Ba133

12.3000, 356.010

Bi207

569.700, 1063.60, 1770.20

Table 1: Theoretical Gamma Ray Energies for various Radioactive Elements

The experimental gamma energies of the various samples studied in this experiment are summarized in the table below:

Radioactive Elements

Gamma Ray Energies (keV)http://www.tutormenow.ca

Cs137

657.600

Co60

1333.01, 1163.75

Na22

1271.39

Ba133

372.160

Bi207

570.310, 1051.45, 1796.94

The percent difference between the theoretical and experimental value is given by:

http://www.tutormenow.ca

The percent difference between the theoretical value and experimental value and calculated and summarized in the table on the next page:

Radioactive Elements

Percent Difference (%)

Cs137

0.612000

Co60

0.0383000, 0.805000

Na22

0.247000

Ba133

4.54000

Bi207

0.107000, 1.14000, 1.51000

 

Discussion

The spectrum produced by the Cs137 isotope yields a gamma energy peak at approximately 657.600 keV. From the spectrum it can be noted that from 42.3800 keV to about 646.640 keV there is evidence of Compton scattering occurring. Based on the angle that the scattering is occurring the excited electrons will have different energies therefore corresponding to different heights on the graph. Passed the point of 464.640 keV a full energy peak is obtained and that is where the gamma line is observed. For the case of Co60 two gamma lines are observed at approximately 1333.01 keV and 1163.75 keV.

Moreover, the gamma lines observed for Na22 are 515.170 keV and 1271.39 keV. It should be noted that since Na22 emits positrons and gamma photons there is pair production occurring within the crystal since there is enough energy being emitted by the Na22 source. Hence, the two distinct peaks are attributed to the fact of the gamma ray loosing its energy to pair production. Ba133 produced a spectrum with quite a few peaks. The main regions of interest were roughly around 372.160 keV.

Finally, Bi207 produced a rather interesting spectrum with three distinct peaks. The Compton distribution is obvious between all the peaks. The peaks occurred at approximately 570.310 keV, 1051.45 keV and 1796.94 keV respectively. These values were consistent with those predicted by theory. Since bismuth produced the most intense gamma rays it can be seen that the manner in which its gamma photons lost energy can be attributed to the fact that photoelectric effect, Compton Effect and pair production were occurring within the crystal.

Conclusion

In conclusion, the principles of operation of the sodium iodide scintillator were discussed and its coupling with a photomultiplier was also discussed. The manner in which a gamma ray or gamma photon can loose energy in a crystal can be attributed to three phenomena known as the photoelectric effect, the Compton Effect and pair production. These phenomena were observed in the data and were analyzed in the spectrum produced.

References

“Experiment #16 Analysis of Waves”, University of Waterloo, Physics 360B, 2009
Website: http://en.wikipedia.org/wiki/Scintillator#Inorganic_scintillators
Website: http://en.wikipedia.org/wiki/Gamma_spectroscopy#Scintillation_detectors
Textbook: Quantum Chemistry & Spectroscopy - Engel