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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.