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Transcript
A Measurement of the Energy of Internal Conversion Electrons from 207Bi Introduction The electromagnetic process by which nuclei make transitions from an excited state to a state of lower energy is often accompanied by the emission of a (gamma ray) photon. The energy of the emitted photon is equal to the transition energy less the recoil energy of the nucleus in the lower energy state. However, there is a competing process called internal conversion (IC) whereby the nucleus, in making the transition from the excited state to the state of lower energy releases this de-excitation energy not to an emitted photon, but to an atomic electron. Most often, this energy imparted to the atomic electron is sufficient to drive the electron off the atom, i.e., the atom becomes ionized and there is a vacancy in one of the atomic states. The kinematics for this process is quite straightforward. You begin with an atom at rest in the initial state. The final state is an emitted electron and a recoiling atom. Simple kinematics will allow you to solve for the expected kinetic energy of the emitted electron if you know (a) the de-excitation energy in the nucleus, (b) the binding energy of the electron to the atom in that state, and (c) the mass of the atom which is to recoil. You should solve this problem as part of the laboratory analysis. The physics concept of internal conversion can be qualitatively appreciated if you examine the radial probability density distributions of the atomic electrons as predicted for a hydrogen atom from the solution to the Schrödinger equation. It is noted that there is a non-zero probability that the electron can be found within the physical boundary of the nucleus. The likelihood that this electron might interact via the electromagnetic force with the nucleus so as to absorb the energy released in the nuclear de-excitation is also non-zero and this process competes with the process of photon emission from the nucleus. Two points are to be stressed here. The first is that this process is not a two step process whereby an emitted gamma ray is absorbed on an atomic electron causing the electron to leave the atom. The second is that the electrons emitted in this process are not those electrons formed inside the nucleus in decay. The goal of the experiment is to measure the energy of the emitted IC electrons along with the decay of 207Bi and to account for the physical processes by a careful analysis. It is not surprising that if the process leaves a vacancy in one of the electron states which is normally occupied, atomic electron transitions will follow. These atomic transitions will emit photons and for higher Z elements, the energy of these emitted photons will be in the X-ray range of the EM spectrum. One might expect that these X-rays could be detected and their energies measured using a Si(Li) detector. If time permits, we might carry out this measurement. Experimental Method Measurements of charged particle energies (at energies of several MeV or less) can be made in silicon surface barrier detectors. (Consult Krane: chapter 7 and/or consult the ORTEC description of these detectors.) It is assumed that you have had experience with these detectors or you will learn the proper procedures to assure that the detector will be used in a safe manner to avoid detector damage. The electronics required is simple and is shown in Fig. 1. To calibrate the detector, it is proposed that you use an alpha particle source provided by your instructor. Request that your instructor handle the alpha particle source for reasons of personnel safety. The kinetic energy of the alpha particle will be substantially larger than that of the IC electrons. It may be required that you adjust the gain on the amplifier after calibration with the alpha particle source to position the IC spectrum properly in the dynamic range of the MCA. If you choose to do so, you can adjust the gain in a calibrated fashion by first positioning a pulser peak on the alpha particle peak by setting the helipot to read exactly the alpha particle energy and using the attenuation switches and the trimpot adjustment to set overlap the pulser peak on the alpha particle peak channel. Then, if you lower the pulser peak to some value on the helipot and raise the amplifier gain to position the pulser peak to the same channel as its initial location, the relative gain change of the amplifier can be computed to give a new calibration scale for the MCA. Acquire the electron spectrum from the source for sufficient time to clearly and cleanly identify all relevant peaks, find their energies and width s, and the number of counts under each peak after subtracting the nearby (flat) background. Analysis To understand the spectrum observed it is necessary to understand the nuclear decay process in detail. For this, you are to consult the Table of Nuclides link from the PHYS-430 home page, and/or Lederer's reference. In addition, it may be helpful to know the energies required to free the electrons from their bound states on the atom. These may be found either in Lederer or the Handbook of Chemistry and Physics or from the Table of Isotopes link from the PHYS-430 home page. Your analysis should result in a quantitative description of the processes and appropriate diagrams to make these processes clear. Pulser Source MCA Vacuum Si Detector Preamp Amp Bias Fig. 1. Diagram of electronics setup for internal conversion electron energy measurement.