|Department of Physics, Middlebury College||1992-93|
|Modern Physics Laboratory|
Thallium doped, sodium iodide [NaI(T)] scintillation detectors are used for detection of gamma rays in the energy range E = 0.1 - 100 MeV, when high efficiency ( 10 - 60%) and moderate energy resolution (E/E = 5 - 15%) are required.1,2 The detection efficiency of NaI(T) detectors generally improves with increasing crystal volume, whereas the energy resolution is largely dependent on the crystal growth conditions. Lithium-drifted germanium [Ge(Li)] detectors are typically used to detect photons at lower energies with lower efficiencies, but with a gamma ray energy resolution of E/E = 0.1 - 1%. The higher energy resolution is essential in radioactive counting situations where a large number of lines are present in a gamma ray spectrum. It should be mentioned that a Ge(Li) detector is far more expensive to purchase and to maintain than a NaI(T) detector. This is because the Ge(Li) detector crystal must be kept at liquid nitrogen temperature (77 K) at all times to maintain the density profile of the lithium drifting in the Ge crystal. As can be seen in Fig. 1, a liquid nitrogen dewar makes up the greater part of the weight and volume of a Ge(Li) detector; the Ge detector crystal is only about the size of a walnut.
Many radionuclides can be identified by examining the characteristic gamma rays emitted in the decay of the radioactive "parent" to a "daughter" nucleus. For example, two characteristic gamma rays occur in the decay of the radionuclide 22Na, as shown in Fig. 2.3 The 22Na decay occurs by one of two independent mechanisms.
(1) + Decay. In each of the two beta decay branches
22Na --> 22Ne(g.s.) + e+Ne + (0.06% of all decays)
22Na --> 22Ne*(1.275 MeV) + e+Ne + (90.4% of all decays)
a positron and a neutrino are emitted, and the net nuclear charge changes from Z = 11 to Z = 10. The 22Ne ground state is stable; however, the first excited state of 22Ne at 1.275 MeV decays with a lifetime of 3.7 ps in the gamma decay process
22Ne*(1.275 MeV) --> 22Neg.s. + 1.275 ,
which gives rise to a characteristic gamma ray with energy 1.275 MeV. The positrons slow rapidly in the radioactive source material and disappear in the annihilation process
e+ + e- --> 2 0.511 MeV ,
producing two characteristic 0.511 MeV annihilation gamma rays.
(2) Electron Capture. In this decay process, an atomic electron is captured by the 22Na nucleus in the reaction
22Na + e- --> 22Ne*(1.275 MeV) + (9.5% of all decays)
and a monoenergetic neutrino is emitted. The electron capture process populates only the first excited state of 22Ne at 1.275 MeV and therefore characteristic 1.275 MeV gamma rays result. Annihilation gamma rays at 0.511 MeV are not produced in electron capture because positrons are not created.
Sample NaI(T) and Ge(Li) gamma ray spectra of a 22Na radioactive source are given in Fig. 3. In addition to the expected peaks at 0.511 and 1.275 MeV, two Compton "edge" structures labeled CE1 and CE2 are seen in the spectra of both detectors. A count at CE2 corresponds to an event in which a single gamma ray: (a) enters a detector with 1.275 MeV, (b) reverses its direction completely in a Compton backscattering collision with an electron in the detector, and (c) exits the detector volume without further loss of energy. The energy lost by the backscattered gamma ray is transferred to the recoiling electron which slows in the detector, producing the observed signal. An analogous Compton process occurs for 0.511 MeV gamma rays, resulting in the Compton edge CE1. The 0.511 MeV gamma ray peak actually sits on the Compton "plateau" generated by 1.275 MeV gamma rays.
A NaI(T) scintillation crystal gives a light output that is proportional to the gamma ray energy. The height of the electronic pulse produced in a Ge(Li) detector also is proportional to gamma ray energy. Therefore, the horizontal scales of Figs. 3(a) and 3(b) can be calibrated directly in MeV energy units using known 22Na gamma ray energies. Once calibrated, NaI(T) and Ge(Li) detector systems can be used to determine the energies of gamma rays from other radioactive sources.
Browse through a copy of the Quantum 8 manual5 and gain some familiarity with the many functions that can be controlled from the front panel of the Quantum 8. Obtain a copy of the Q8 program4 diskette and insert it into the A: disk drive (leftmost drive) of the IBM PC/XT computer. When you turn on the main power to the computer you should find that the Q8 program starts automatically. Explore the many menu options of the Q8 program. With a 22Na source, practice setting the Quantum 8 amplifier gain until your spectra look approximately like those shown in Fig. 3. You should be aware that the energy calibration option in the Q8 program works in units of keV (1000 keV = 1 MeV), not MeV.
When you are thoroughly familiar with the Q8 program, take 1024 channel spectra of the 22Na, 60Co, and X sources using the NaI(T) and Ge(Li) detectors. All spectra taken with a given detector should be at the same amplifier gain setting so that they can be calibrated together with the gamma ray energies of the 22Na source. When all data have been written to disk and an appropriate energy calibration procedure has been undertaken, print all six spectra for inclusion in your lab report.
(1) 22Na. Use the 0.511 MeV and 1.275 MeV peaks to calibrate your NaI(T) and Ge(Li) spectra with the Q8 program. You will use this calibration for 60Co and Source X, so take care to do it correctly. Determine the energies CE1 and CE2 from your NaI(T) and Ge(Li) spectra and compare these values to what you expect for gamma ray backscattering governed by the Compton equation6
= ( h/mc ) (1 - cos)
with = 180o. Remember that the NaI(T) and Ge(Li) detectors actually detect the electron recoil energy, not the photon recoil energy, in a Compton edge scattering process.
From your spectra, determine the fractional energy resolution, in percent, for detection of 1.275 MeV gamma rays by the NaI(T) and Ge(Li) detectors.
(2) 60Co. In your 60Co spectra you should see two main gamma ray peaks above 1 MeV. Determine the centroid energies of these peaks and their associated uncertainties. An energy level diagram showing the 60Co decay scheme is given in Fig. 4. Although Fig. 4 is a bit complex, you can use it to check that your energies for the two 60Co peaks are correct.
(3) Source X. From your energy calibrations for 22Na, determine the centroid energies of any major peaks you may see in this gamma ray spectrum. Estimate the uncertainties associated with these energy determinations. With this information, identify Source X using the Chart of the Nuclides 7 and perhaps the Table of Isotopes.3 (Hint: The half-life of Source X is greater than 10 years and therefore the upper half of its square in the Chart of the Nuclides is light blue.)
1. G.F. Knoll, Radiation Detection and Measurement (John Wiley & Sons, New York, 1979), pp. 239-271.
2. R.E. Lapp and H.L. Andrews, Nuclear Radiation Physics (Prentice-Hall, Englewood Cliffs, NJ, 1972), pp. 50-54.
3. Table of Isotopes, 7th ed., edited by C.M. Lederer and V.S. Shirley (John Wiley & Sons, New York, 1978).
4. O.G. Berkes, The Quantum 8 Interface Program, B.A. thesis, Middlebury College, 1985.
5. Quantum 8 Preliminary Operation Manual (The Nucleus, Oak Ridge, TN,1982).
6. R. Eisberg and R. Resnick, Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles, 2nd ed. (John Wiley & Sons, New York, 1985), p. 37.
7. Chart of the Nuclides, 12th ed. (General Electric Company, Schenectady, NY, 1977)
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