X-ray Fluorescence to Determine Composition
Purpose:
The purpose of this activity is to use an Americium-241 source to create X-ray fluorescence
spectra to identify the components of a meteorite sample.
Background:
When electrons make transitions from one atomic orbit to another that is lower
energy it releases x-ray photons. In this experiment we will generate these transitions by
sending in a higher energy gamma ray into our sample. The gamma ray has higher energy
than the binding energy of an electron in that atom. This gamma ray can knock an inner
electron out of its orbital creating a hole, where a higher level electron can drop down to fill
this vacancy, releasing x-rays of a characteristic spectrum. This is called fluorescence.
In your chemistry class you learned have quantum numbers, including the principal
quantum number (n), angular momentum quantum number( l ) and the magnetic quantum
number (m l ) and spin. The total angular momentum (j) of an energy level is equal to l ± _.
The K shell designates the n=1 electrons, L the n=2 and M the n=3. Electrons transitions into
the K shell are called K x-rays, and transitions into the L shell are L x-rays. Our experiment
today will deal primarily with identifying two types of K x-rays, those from the L shell,
designated Kα, and those from the M shell labeled K_. There are smaller divisions within the
Kα and K_ but these are beyond the limits of the particular detector that we are using today.
(Image from Rech, Lee, et. Al)
Today we will be using two kinds of electromagnetic radiation to perform our
analysis, x-rays and gamma rays. We will use gamma ray photons to excite the inner shell
electrons from our sample releasing x-rays when a different electron moves to that lower
state. Our source of gamma rays will be a sealed source of Americium-241. Americium
undergoes alpha decay by the following process 241Am  237Np + 4α + 5.6MeV. The 237Np
then undergoes gamma decay releasing a 59.537 keV gamma ray, which we will use to
fluoresce the samples. 241Am can only fluoresce elements with biding energies lower than
this energy.
(Image from www.lbl.gov)
PROCEDURE
1.) Turn on the detector to get a long background count. As you learned earlier today
sources of radiation are all around, we will have to subtract out this background from
our sample.
2.) Place sample in the shielded area, the x-rays will begin to be detected by the Ortec
Germanium detector. These different energies will be sorted using a software called
GammaVision.
3.) The computer will select some of the largest peaks that are not part of our
background. We can then select these and have them plotted by themselves for
further analysis.
4.) If we locate the energy of the peaks of interest we can then look up this value to
determine what the identity of the unknown substance might be.
Procedure in case you don’t have X-ray fluorescence at school…go to
http://ie.lbl.gov/xray
The periodic table here has the standard x-ray spectra for each element. There are
also several unknowns for you to test your skills, using the tables provided.
DATA
Sample
ID
Single Element
Peak #1
Max
Kα
Element
ID
Peak #2
Max
K_
If a second element is contributing
Element
ID
Peak #1
Max
Kα
Element
ID
Peak #1
Max
K_
Element
ID
Questions
1.) Which elements on the reference table on the next page cannot be examined by our
Americium-241 source? Why?
2.) Why do different elements have different spectra?
3.) Why would an astrophysist (like you) want to know the composition of a meteor?
4.) What are some precautions we would take to make sure we were operating safely?
Reference Table of Average K x-rays
Weighted Average
Weighted Average
Energy (keV)
Energy (keV)
Element
Element
Kα
K_
Kα
K_
1H
26 Fe
6.400
7.059
2 He
27 Co
6.925
7.649
3 Li
0.052
28 Ni
5.411
8.265
4 Be
0.110
29 Cu
8.041
8.907
5B
0.185
30 Zn
8.631
9.572
6C
0.282
31 Ga
9.243
10.263
7N
0.392
32 Ge
9.876
10.984
8O
0.523
33 As
10.532
11.729
9F
0.677
34 Se
11.210
12.501
10 Ne
0.851
35 Br
11.907
13.296
11 Na
1.041
36 Kr
12.630
14.120
12 Mg
1.254
37 Rb
13.375
14.971
13 Al
1.487
38 Sr
14.142
15.894
14 Si
1.740
1.838 39 Y
14.933
16.754
15 P
2.015
2.142 40 Zr
15.746
17.687
16 S
2.307
2.468 41 Nb
16.584
18.647
17 Cl
2.622
2.817 42 Mo
17.443
19.633
18 Ar
2.957
3.191 43 Tc
18.327
20.647
19 K
3.312
3.589 44 Ru
19.235
21.687
20 Ca
3.690
4.012 45 Rh
20.167
22.759
21 Sc
4.088
4.459 46 Pd
21.123
23.859
22 Ti
4.508
4.931 47 Ag
22.104
24.987
23 V
4.949
5.427 48 Cd
23.109
26.143
24 Cr
5.411
5.947 49 In
24.139
27.382
25 Mn
5.895
6.492 50 Sn
25.193
28.601
Weighted Average
Energy (keV)
Element
Kα
K_
51 Sb
26.274
32.437
52 Te
27.380
31.128
53 I
28.512
32.437
54 Xe
29.669
33.777
55 Cs
30.854
35.149
56 Ba
32.065
36.553
57 La
33.302
37.986
58 Ce
34.569
39.453
59 Pr
35.864
40.953
60 Nd
37.185
42.484
61 Pm
38.535
44.049
62 Sm
39.914
45.649
63 Eu
41.323
47.283
64 Gd
42.761
48.949
65 Tb
44.229
50.650
66 Dy
45.728
52.384
67 Ho
47.257
54.155
68 Er
48.818
55.963
69 Tm
50.410
57.806
70 Yb
52.035 59.687*
71 Lu
53.693
61.607
72 Hf
55.382
63.562
73 Ta
57.106
65.556
74 W
58.864
67.586
75 Re
60.655
69.659
Below is a sample of a commercial X-ray fluorescence scan of lower energy lines than our
detector.
Teacher Notes
Points to Ponder
• It is important to mention that the 59.6 keV is from the source and other x-rays
detected are from the excited sample.
• X-rays come from electrons in atomic orbitals and gamma rays come from nuclear
shells.
• These spectrum determined on earth help scientists to determine x-ray spectra for
astronomy such as Chandra X-ray telescope. These standard values can also show red
shift. Data is available from http://chandra.harvard.edu.
Questions
1.) Which elements on the reference table on the next page cannot be examined by our
Americium-241 source? Why?
Hydrogen and helium cannot be detected because they have no outer shell
electrons to decay.
Any elements whose binding energies are very close or greater the 59.6keV of
our source. Any K alphas > Z=74 or betas Z > 70.
2.) Why do different elements have different spectra?
Each nucleus will have a characteristic binding energy and electron structure.
3.) Why would an astrophysist (like you) want to know the composition of a meteor?
Knowing the composition of a meteor can give clues as to its origin and age
depending upon its “metallicity.” (If it is unfractionated)
Knowing x-ray spectra on Earth can help create standards for comparison to
x-ray astronomy.
4.) What are some precautions we would take to make sure we were operating safely?
ALARA (means As Low As Reasonably Achievable)
The source is SHEILDED using lead bricks
The source DISTANCE IS MAXIMIZED from contact with students
TIME is MINIMIZED around the source.