Alpha Particle Induced X-ray Emission in the Classroom
Jorge A. Lopeza), Mario F. Borundab), and Jaime Moralesc)
Department of Physics, University of Texas at El Paso, El Paso, Texas 79968
Abstract. We report on an experimental demonstration in an introductory modern physics course to elucidate the X-ray
line spectra, and how they arise from transitions of electrons to inner shells. We seek to determine the effect of limited
use of an interactive component as a supplement to a traditional lecture, and how it would improve the student
achievement. In this preliminary study the students were exposed to traditional lectures on X-ray production and Bohr’s
model, they then were given a homework on the abc of X-ray spectra, after which they were given a pre-test on the
materials, followed by an in-class demonstration, and a final post-exam. The gain, as measured from pre- to post-exams
appears to remark the differences in how students approached the subject before and after the use of the demonstration.
This initial study shows the validity of in-class demonstrations as teaching tools and opens a wide new area of research
in modern physics teaching.
elements, in contrast to discussion of the emission spectra of
the hydrogen atom. At UTEP the use of gas tubes filled with
elements such as helium and mercury vapor are often
presented in class along with diffraction gratings to provide
the student with a hands-on view of atomic emission lines.
However, in such demonstrations actual measurement of
characteristic wavelengths and quantitative connection to the
atomic structure of the gas in question is ignored. We are
developing an in-class demonstration, where the atomic
structure of tangible elements such as iron and copper is
explored via x-ray fluorescence. This activity includes
quantitative calculations the student can perform using their
knowledge of the Bohr hydrogen atom, to explore atomic
structure via Kα x-ray emissions using Alpha-particle
Induced X-ray Emission (α-PIXE). We present an outline of
this class activity along with preliminary results in
evaluating its impact on students understanding of atomic
structure.
INTRODUCTION
One way to enhance science instruction in modern physics
courses is to actively engage the student with an
experimental activity illustrating a particular concept. Many
studies have shown that students who go through activeengagement activities do better than those who go through
traditional instruction [1]. In engineering programs of study
at the University of Texas at El Paso (UTEP) students are
required to take three semesters of physics: two-semesters of
classical physics, which have laboratory sections to
supplement class instruction; followed by a one-semester
course surveying topics in modern physics. Unlike the
introductory courses in classical physics, the course on
modern physics has no laboratory engagement, as is the case
at other colleges and universities.
Modern physics
experiments, which could be used to aid in modern physics
instruction, are often costly and require a fair amount of a
priori knowledge in physics and laboratory techniques.
Nevertheless, it is possible to construct activities that
actively engage the student without obfuscating concepts
with details irrelevant to the crux of the principle being
illustrated.
ALPHA–PIXE
In α–PIXE, α–particles from a radioactive source are
used fluoresce x-rays from target atoms. The typical energy
of alpha particles is between 4 to 8 MeV. These energies are
sufficient to knockout inner shell electrons in some atoms,
thereby leading to the emission of characteristic x-rays.
When presented to many students, atomic structure is a
topic that can be difficult to grasp in the context of a
semester survey course. Classroom aids and textbooks often
present slides showing optical emission and absorption
spectra to illustrate the electronic structure of various
a)
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c)
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b)
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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The above source holder is held with laboratory clamps so
that the emerging alpha beam is horizontal to the lab table.
The sample to be analyzed is then held within the 4.4 cm
range using laboratory clamps. The sample is held such that
an angle of ~ 40º is made with the axis of the incident beam
and the samples surface. The X-ray detector is then placed a
distance of 5 cm from the point where the alpha beam axis
comes into contact with the sample to detect any fluoresced
x-rays. Since the intensity of any fluoresced x-rays is
inversely proportional to distance, our x-ray detector is
placed approximately at 1cm from the sample. The close
proximity of the setup requires suitable shielding of the
source from the detector, as some of the 241Am alphas from
decay fluoresce x-rays from source, e.g. 241Am decay
product 237Np, whose Lα and Lγ x-ray spectra is present. If
one does not have adequate shielding this x-ray spectra
would dominate any observed spectra with the setup.
Along with other forms of particle and ion induced x-ray
emission, such as Proton Induced X-ray Emission, PIXE is
often used as a nondestructive means of performing
quantitative elemental analysis down to the ppm level (or
better) [2]. There are many texts discussing underlying
theory on the production of x-rays using α−particles and
other ions [2]. We present here a general description of the
theory and our experimental setup as it pertains to the use of
PIXE as a class activity.
Experimental Setup
The demonstration uses a setup designed and tested in a
master’s thesis [3]. It uses a 0.5 mCi Americium-241 as a
source of α–particles. The source was taken from a
Rutherford scattering experiment presented in a senior
laboratory course for physics majors. Americium-241 has a
half-life of 435 years and undergoes radioactive decay by the
emission of a 5.6 MeV α−particle with an associated gamma
emission at 59.5 keV. The source is a “pellet” source,
where the Am is deposited on the surface of a metallic disk
13.5 mm in diameter and 5.0 mm in height. To house the
source we fabricated an aluminum-lead cylindrical housing
for the source, consisting of a hollowed aluminum cylinder
6.2 mm in thickness and 40 mm in length, which holds the
disk. This casing then slips into a larger cylindrical lead
housing, 3.5 mm thick. The casings have a small aperture
7.3mm in diameter to allow the α–particles to escape the
housing. The source sits flush with against the aluminum
aperture. At 5.6 MeV the α–particles have a maximum
range of approximately 4.4 cm in air. Since α–particles are
mono-energetic, their relative count rate is constant
throughout this range and sharply to zero beyond it. Health
physics issues with this source are addressed with the lead
casing, which absorb most of the 59.5 keV gammas.
Laboratory tongs are used when loading the source into the
Al-Pb chamber to prevent direct contact with the 241Am
deposit. Latex gloves are also worn as a precaution when
working near the source, i.e. when placing samples in front
of the source. Further precautions are taken by making sure
aperture points away from all persons and a 3/4 inch thick
4x4 in. lead plate is placed 6 inches from the aperture to stop
any gamma rays that pass through our samples as a final
precaution. Once placed in the above source holder health
physics issues considerations are not of great concern to the
students or the instructor.
Demonstration and Assessment
The following is a description of the activity presented
during a one-semester course in Modern Physics instruction
at the UTEP during the Fall 2002 semester. The activity was
administered after the students were given traditional
instruction on the atomic structure of the hydrogen atom.
Prior to the class activity, a homework set was administered
to the students in which they were presented with a crude
model of energy levels in multi-electron atoms based on
Bohr’s model of the Hydrogen atom. The students were
given the resulting expression for the energy levels in an
atom of atomic number Z,
2
Z
E n = −13.61  .
n 
(1)
They were then asked to explore the energy levels in Iron
(Z=26). The students were asked to identify ionization
energies of the various energy levels and to calculate photon
emission energies in transitions between the L- and K-shell
electrons (n=2 and n=1 respectively). The students had to
identify these photons as x-rays. They were then introduced
to an expression, which is a better approximation in
calculating Kα x-ray energies,
E K α −photon = 10.21( Z − 1)2 .
(2)
They repeated their calculation for Iron using Eq. (2), and
asked to calculate the Kα x-ray energies for calcium and
potassium. A graph of the X-ray spectrum of calcium was
then presented to them as that from an unknown atom. The
Kα peak on the graph was labeled and the students were
asked to identify which element was producing the spectra
by reading the energy from the graph and using Eq. (2) to
determine Z (students could also infer this from their
previous calculation of the Ca Kα X-ray energy).
In this setup we used the Amptek XR-100T x-ray detector
and PX2T power supply and amplifier. The XR-100K
employs a Silicon-PIN photodiode X-ray detector and
preamplifier assembly in a relatively small package, which is
thermoelectrically cooled, and a resolution around 200 eV.
This setup was used in conjunction with the Amptek
MCA8000A pocket multi-channel analyzer (MCA). This
entire assembly is compact and ideal for setup on a small
cart, which can be brought into the classroom. A PC is used
to with the supplied software to display data from the MCA
through a video projector onto an overhead screen in the
classroom in real-time.
The day of the activity a pre-test assignment was issued in
which students were given a set of multiple-choice
questions. The questions were constructed to gauge the
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students’ knowledge of atomic structure, focusing on Kα Xray emissions. After about 10 minutes the students handed
back the pretest. The students were then given a 20-minute
introduction to α−PIXE, during which they examined the
equipment, and were handed a table of Kα x-ray energies for
various elements. Copper was then fluoresced and its x-ray
spectra projected in real time. The class was then shown the
peak corresponding to a Kα emission, the energy is read off
and they then compare it to the listed energy in their table.
Zinc was fluoresced, as another instructional example, and
the students observed the Kα X-ray peak from Zinc and
again identify the observed energy with their handout. A
sample of Iron is then fluoresced as an unknown substance,
that is the students are not told the spectrum being shown is
for Iron, the Kα peak is highlighted by the instructor,
indicating the Kα x-ray energy, and the students are issued a
posttest evaluation, where they will be asked to identify the
element responsible for this emission.
70%
60%
50%
40%
30%
20%
10%
0%
1
2
3
4
5
6
7
Average
-10%
FIGURE 1. The percentage gain is plotted per question.
The vertical axis represents the percentage increase or
decrease in post-evaluation. The horizontal axis indicates
the concept being questioned. The average gain is indicated
in the far right.
We must note that the above results are preliminary and
many more samples are needed to gain a better statistical
picture of the α−PIXE demonstration’s impact. In light of
the above results, we are currently working to improve the
activity for future administration and assessment. These
preliminary results are encouraging and will be useful in
designing similar activities to facilitate modern physics
instruction at UTEP.
In the posttest a set of questions, which are similar to those
presented in the pretest, are administer as open-ended essay
questions. The idea is to gauge the students’ knowledge of
atomic spectra after the α−PIXE demonstration has been
presented. We then compare these posttest responces to
those in the pretest.
Results and Conclusions
ACKNOWLEDGMENTS
The α−PIXE demonstration was administered to a onesemester course in modern Physics at UTEP in fall 2002. In
this class 38 students participated in the activity. Using the
above methodology. The pretest and posttests consisted of
seven questions, focusing on fundamental aspects of atomic
structure and x-ray spectra as follows:
1. Quantization of the electron energies in atoms.
2. The energy scale of Ka x-ray emissions for Z=27,
3. The relationship between binding energy and
ionization energy.
4. Photon emission.
5. Production of Kα x-rays.
6. Uniqueness of electron structure for a given
element.
7. The comparison of binding energy between the
ground state and an excited state.
As mentioned the topics questioned in the posttest are
identical to those in the pretest. When scoring the pre-post
exams, a correct result was scored as a 1 and an incorrect
result as a 0. For the pre- and post-exams a table was
constructed containing the average score of the class per
question. We defined the gain as the difference between
these averages in the pre- and post- evaluations. A positive
gain for a particular question (concept) indicates that a larger
number of students answered correctly in posttest evaluation,
as compared to their pretest results. A negative gain would
indicate fewer correct responses for a given question in
posttest evaluation. Our results are indicated in Figure (1).
The results appear to indicate the α−PIXE demonstration
had a favorable impact on the students’ concept knowledge
of atomic structure.
We are indebted to the Louis Stokes Alliance for Minority
Participation for sponsoring this work. We would like to
thank Jerome Duggan for all his intellectual feedback.
REFERENCES
1. R. R. Hake, Am. J. Phys. 66, pp. 64-74, (1998).
2. Johansson, S., Campbell, J., and Malmqvist, ParticleInduced X-ray Emission Spectrometry, John Wiley &
Sons, Inc (1997).
3. Mu X., Alpha Particle Induced X-ray Emission, UTEP
MS Thesis, (1998).
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