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1、3-13. NUCLEAR SPECTROSCOPY(Updated by Scott Shelley and Suzanne Amador Kane, May 2005)References:1. Compton scattering is covered in Eisberg and Resnick, Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles (and the collision problem is worked out on) pp. 34-37, and in Bernstein, Fishba
2、ne, and Gasiorowicz Modern Physics on pp. 114-115. Pair production is also discussed in both texts. Passage of EM radiation through matter is discussed in Melissinos, Experiments in Modern Physics, pp. 165-169. You will need a basic understanding of the photoelectric effect, Compton scattering, and
3、pair production.2.Scintillation Counters are discussed in Experiments in Modern Physics, Melissinos, 2003.3.Radiation Safety: Experiments in Modern Physics, Melissinos, 2003 Chapter 8 (page 295), Chapter 9 (page 367) and Appendix B (page 485). Memorize the meaning of microcurie and millirad.4.Multic
4、hannel analyzer (MCA) in lab manual.The instrumentation and experimental methods in this lab are reminiscent of those used throughout particle physics and medicine. These techniques are also important in radiation safety and in the uses of radioactivity for dating in archaeology and other fields. In
5、 this experiment, you will measure the energies of gamma rays emitted in the process of nuclear decays. Measurements of this nature have been used to determine the internal structure of nuclei, much as optical spectroscopy was used to determine atomic structure. Since the energies corresponding to n
6、uclear excitations are so large, one can easily detect a single nuclear decay. You will calibrate the detection system using radioactive sources with gamma rays of known energy and then you will measure the energies of the gamma rays of an unknown source.The basic instrumentation is as follows. You
7、are provided with several gamma-ray sources. Emitted gamma rays are converted to optical photons in a scintillator of NaI. These optical photons produce an electrical pulse in a photomultiplier tube (PMT). The electrical pulses are amplified and shaped in a preamplifier and then sent to a multichann
8、el analyzer (MCA) board in a microcomputer. The MCA records the number of pulses at each digitized pulse height and the result is a spectrum of emitted gamma rays.RADIATION DOSAGESBeginning with the definitions of the curie and the rad, determine the absorbed dose (in millirads) for your entire body
9、 for one afternoon spent 1 meter from a radioactive sample with a source activity (# decays/sec) of 1 microcurie of 137Cs (E=0.663MeV). To do this you will need to make a rough estimate of the size of your body (cross sectional area and thickness). You will also need to estimate the fraction of inci
10、dent gamma ray energy that is absorbed by a given thickness of tissue. This can be done using the relation3-2 Nuclear Spectroscopy (1) I I0emxwhere is the fraction of gamma rays that penetrate (i.e., are not absorbed by) a thickness x, m is the 0IImass absorption coefficient, and is the density of t
11、he absorbing medium (e.g., your body or lead shielding). Assume that m and for your body are the same as that of water (see enclosed graph of m). How does the answer change if you are 2 m away?Here is some useful information about units of radiation:1 Gray (Gy) = 100 rad = 1 J/kg (a unit of absorbed
12、 dose; Grays are now preferred) (2)1 Curie (Ci) = 3.7 x 1010 decays/sec (a unit of source activity)1 Becquerel = 1 decay /sec (a unit of source activity)Now suppose that you are 2 meters away and that in addition the source is surrounded by 4 centimeters of lead (see enclosed graph for m of lead). E
13、stimate your total exposure for one afternoon in lab. Compare this to the natural background radiation of about 150 millirads per year (or 0.5 mr per day). Your additional dose should be negligible. Show your results to the instructor and discuss with him/her the issue of whether the experiment is s
14、afe. Include your calculations, results and a discussion of what they mean in your lab report.SCINTILLATION COUNTERSThe scintillator you will use consists of a sodium iodide crystal attached to a photomultiplier tube. Small quantities of thallium (0.1% to 1%) have been introduced into the crystal st
15、ructure as a photosensitive impurity. Incident gamma rays produce a high-energy electron in the crystal, generally through the photoelectric effect. This high-energy electron travels through the crystal, producing an ionization track consisting of a huge number of electrons in the conduction band of
16、 the material. Since each one has an energy of only about 10 eV, while the primary high energy electron may have an energy of 1 MeV, there may be 105 or so of these secondary electrons, which then interact with the Thallium impurity atoms, raising them to an excited state. When these excited atoms r
