Bryn Mawr College
Department of Physics
Undergraduate Teaching Laboratories
Alpha Particle Spectroscopy
Introduction
This experiment is designed to study the detection of charged-particle radiation and the
attenuation of such radiation during interaction with matter. Specifically, the experiment
studies alpha particles which are helium-4 nuclei (two protons and two neutrons).
Alphas are commonly emitted decay products of unstable heavy isotopes. The alphaparticles are detected with a silicon surface-barrier detector. Alpha particles produced
from nuclear decays generally have energies between 4 and 10 MeV, and can only travel a
few centimeters in air at atmospheric pressure. So this experiment requires working at
varying degrees of vacuum to permit them to travel from the source to the detector. The
source is Americium-241 ( 241 Am), which is used in common household smoke alarms.
There will be interludes while the data is being acquired during which you can do some
of the necessary research and reading. Do not get bogged down in a lot of reading in the
laboratory before getting the experiment underway. The first four pages of the EG&G
Instruction Manual for Silicon Charged Particle, Radiation Detectors are worth reading
for information about the specific detector (which is an A-series detector) and for
instructions as to operating and handling procedures. Other papers and potentially
useful information are contained in this folder. Quickly familiarize yourself with the
entire folder. Depending on how much time you are going to devote to this experiment,
you are encouraged to do independent research and to perform independent theoretical
analyses.
Experimental Set-up
The surface barrier detector and the
241
Am alpha source are contained in the
stainless steel vacuum chamber. Please do not touch it. Please read the few pages in the
detector manual before proceeding. The main thing to remember is that before turning
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the detector voltage all the way off at the end of the day or at the end of the experiment,
let some air into the chamber.
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THREE IMPORTANT POINTS TO REMEMBER:
(1) Never power the detector when it is exposed to light.
(2) Never change the pressure in the chamber when the detector is powered.
(3) Always change the chamber pressure slowly.
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The electrical connections have been set up. Review them as summarized below.
(a) The laptop computer contains the data acquisition software (Ortec MAESTRO), and
connects to the multi-channel analyzer (Ortec EASY-MCA) with a USB cable. The NIM
(nuclear instrumentation module) bin contains the Ortec Model 460 Delay Line Amplifier
(DLA) and the Ortec Model 428 (Detector Bias Supply) to power the surface barrier
detector in the chamber. Note the Ortec Model 142A Preamplifier, a small brown box in a
clamp stand, located behind the computer. A high-voltage cable connects the Channel A
output of the Ortec 428 to the “Bias” connector on the Ortec 142 Preamplifier via a digital
voltmeter. These are special cables in that they have SHV connectors at one end (the
Supply and the Preamplifier) and MHV connectors at the other end (the voltmeter).
(b) The grey power cable for the 142 Preamplifier plugs into the rear of the Ortec 460
Delay Line Amplifier (DLA) in the NIM bin. The plug is a multipin-connector (a 9-pin Dsubminiature connector).
(c) The "Input" on the 142 Preamplifier is connected to the detector via a BNC connector
on the top of the vacuum chamber. A short cable is used to minimize noise.
(d) There is a 50 termination on the "T" connector of the Preamplifier. A 93  BNC
cable (it’s thicker than the normal 50  cables and likely has the markings “RG-62” on it)
connects the "E" output of the Preamplifier to the oscilloscope, and then to the “Input” of
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the Ortec 460 Dely Line Amplifier. The “Unipolar Output” of the Ortec 460 DLA is
connected to the “Input” port of the Ortec EASY-MCA multi-channel analyzer via channel
2 of the oscilloscope. The “Test” connector on the Preamplifier is unused.
The various steps below are numbered so you can keep track of where you are from day
to day.
(0)
Turn on the voltmeter and ensure that the detector bias voltage is set to zero (likely
the NIM crate is off, but it may not be). If the voltage is not zero, SLOWLY ramp down
the voltage by turning the Channel A dial on the detector bias supply (the Ortec 428).
Leave the voltmeter on at all times when this experiment is in use.
(1)
Turn on and configure the oscilloscope:
Ch 1 coupling:
AC
Trigger source:
CH 1
Trigger slope:
Rising
Time/div:
0.5 – 2.0 ms
Volts/div:
0.1
Trigger mode:
AUTO
(2)
Configure the Delay Line Amplifier. Set the Coarse Gain to 20 and the Fine Gain to
0.8. Ensure that that POS/NEG switch is set to “POS”, and that the “INTEG usec” switch
is set to 0.04.
(4)
Inspect the vacuum system and make a diagram. The pressure is measured by a
Hastings Model 760 Vacuum Gauge. Atmospheric pressure is about 760 Torr and a
vacuum is about zero. This gauge will be left on at all times when this experiment is being
used. Note the state of the vacuum chamber.
(5)
We begin by evacuating the chamber. This entails several steps. Get help if this is
your first time. Hint: valve handles are oriented such that the valve is open if the handle
runs parallel to the hose/tubes, and closed if the handle is perpendicular to the
hose/tube. (A) The green valve just above the vacuum pump (labeled “#1”) should at
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4
this stage be open. That is, the green handle should be vertical. (B) Close this valve by
rotating it 90 degrees to the horizontal position. (C) Turn the pump on. After a bit of
gurgling, it should settle down. You are now pumping just up to the closed green valve.
(D) Make sure the black-handled valve just after the red hose (labeled “#2”) is closed. (E)
open the green valve (#1) so that you are now pumping up to valve #2. Let the pump
settle down again. (F) Ensure that the second black-handled valve (#3) is closed. This
valve is used to let air into the system (to “vent” the system). There is a cigarette filter in
this opening to slow down the air intake when it is open. (G) Open valve #2 to evacuate
the chamber. When doing this, look and listen. Listen to the sound of the pump (is it
settling down?) and look at the pressure gauge (is the pressure falling). If not, then gently
push down on the chamber lid to get it to seal. If this doesn't work, close Valve #1 and
Valve #2 and get an instructor.
(6)
When the system reads zero Torr, slowly raise the detector voltage from zero to
+20V by turning the Channel A dial of the Ortec Model 428 Detector Bias Supply module
in the NIM crate. Note that this requires much less than one full turn! Monitor your
progress on the voltmeter. If there is a difference between the reading on the dial pot and
the voltmeter, trust the voltmeter. As you raise the voltage, pay attention to the noise on
the scope traces. The noise should decrease as you increase the bias voltage (e.g. see Figure
4 in the Ortec instruction manual on page 1-11). Once you are satisfied that the voltage
setting is reasonable, slowly increase the voltage to our operating target of +125 V.
(7)
Observe the data pulses on the oscilloscope. Ensure that you see traces from both
Channels 1 and 2. You should see pulses on Channel 1 that rise quickly then decay
exponentially with a time constant of ~100 microseconds. Now change the trigger mode
from AUTO to NORM (make sure you know the difference between these two settings)
and adjust the variable trigger level to trigger on the pulses. What is displayed are the
charge pulses from the detector which have been integrated by the preamplifier and
which then are presented as exponentially decaying pulses. The "information" (that is,
how much charge was generated) is given by the peak height of the pulse.
Alpha particle spectroscopy
(8)
5
Now change the time-scale to zoom in on the channel 2 pulse (should be roughly 1
microsecond per division). The decaying signal in channel 1 now looks like the first part
of a square-wave pulse (that is the amplitude looks quite flat after the fast rise). Channel 2
should show a pulse that rises and falls with a nearly flat top. This is the output of the
Delay Line Amplifier which “shapes” the preamp pulse. Why do you think this amplifier
is beneficial when measuring the energy spectrum of the alpha particles (think about
measuring the peak amplitude of channel 1 vs. that of channel 2). Note also that the
amplitude of the Channel 2 pulse jumps around a bit. What does that mean physically?
Wouldn’t you like something to keep track of all of those amplitudes? That’s the role of
the MCA.
(9)
Launch the MCA software program – The Unipolar output signal of the DLA is sent
to the input of the free-standing MCA box (Ortec EASY-MCA), where the pulses are
categorized according to their voltage height into N bins, where N is determined in
software. Now let’s open the software that controls the MCA. On the laptop, find a link
to “Ortec MAESTRO” on the Desktop or on the Taskbar (alternatively, the program is
located in C:\Program Files (x86)\Maestro\Mca32.exe). Start the software. From the
“Display” menu, select “Detector…” and then in the “Pick Detector” dialog box, choose
“00001 PHYSICS-PC MCB 129” (it should be the only option). This corresponds to the
EASY-MCA box on your desk. Now we will configure the MCA for your needs.
(10)
Setup the MCA for data acquisition – First, we’ll select the number of channels that
the MCA will use to sort the amplitudes of the incoming pulses (N in the previous step).
From the “Acquire” menu, select “MCB Properties.” A dialog window will appear with
four tabs. Select the “ADC” tab. Here, “Conversion Gain” means N. For now, set it to
256. Ensure that the “Lower Level Disc” is set to 5 and that the “Upper Level Disc” is set
to 255. These tell the MCA to ignore all events that fall below bin 5 and all events that fall
above bin 255. If the full-range of the EASY-MCA is 0 to 10 V, and we have divided this
range into 256 bins, what voltage does bin 5 correspond to? How about bin 255? On the
“Presets” tab, set the “Live Time (s)” to 30. Click OK to save changes and exit.
Alpha particle spectroscopy
(11)
6
Acquire a Spectrum – Now, clear any existing data (“Acquire  Clear” or find the
corresponding button on the toolbar), and then begin your run (“Acquire  Start” or use
the green “Go” button on the toolbar). If all went well, you should see data arriving, with
a strong peak near channel 150. Since the pulse height is an indication of the energy
deposited in the detector, this so-called Pulse Height Analysis of the MCA results in a
spectrum where energy is displayed along the horizontal axis and the frequency of
occurrence of a pulse height is displayed along the vertical axis.
(12)
Note that with only 256 bins (and with only 30 seconds of data) you cannot see all
the important structure in the energy spectrum. So we would like to take a highresolution (and longer) run. Increase the number of bins to the maximum (8192) and take
another dataset (be sure to clear any existing data). If you see a large number of counts at
low energy (small bin number), you can adjust the LLD (Lower Level Discriminator)
accordingly.
(13)
The energy calibration along the horizontal axis in terms of N bins or channel
numbers will need to be established as part of this experiment. The vertical axis displays
the number of counts in each energy channel and can be increased by simply acquiring
data for a longer time interval.
(14)
You can save your data in a text format for further analysis using KaleidaGraph or
Matlab. Select “File” then “Save As” and choose the “ASCII SPE” format. Make a new
directory on the Desktop (you could name it after yourself) and store the SPE file there.
Open this file with the Windows text-editing program Notepad (or even better, you can
use Emacs if you prefer). Look at the contents. There are several lines of header data that
contain information about the MCA and information about when the data was taken (the
timestamp). We call this “meta-data” (i.e. data about the data). We would like to
massage this data into a format that is easily read in by Kaleidagraph or Matlab (or other
programs). The most straightforward format would be two columns of data, where the
first column is the channel number and the second column is the number of counts in that
channel. To accomplish this task, see, the Python program on the Desktop called
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easymca_dataconverter.py. Drag and drop your SPE file onto the
easymca_dataconverter.py icon. Alternatively, open up a cmd terminal, navigate to the
Desktop and type “python easymca_dataconverter.py path_to_your_spe_file” where you
should replace “path_to_your_spe_file” with the actual location of your SPE file (relative
to the Desktop).
Either way (drag-and-drop or command-line), you should see a new file
in your directory with the same name as the SPE file, but now the extension “.SPE” has
been replaced with “.dat” Open this file in Notepad (or emacs) and verify that it contains
two columns of data as expected, and that there are as many rows as expected.
(15)
Take a deep breath. Congratulations, you now have the detector setup and ready
to take your data!
Experiment One: The Am-241 Spectrum and Using the MCA
(16)
This is a "quick-and-dirty" experiment. You want the alpha spectrum to be near
the right-hand edge of the MCA screen but with a good baseline to the right of it. On the
normal scale, you will just see what looks like a sharp single peak for the alpha spectrum.
(You can expand it later to look at the details.) There will also be some electronic noise
near zero energy. If you do not see a peak, adjust the Coarse and Fine gains on the Ortec
460 DLA.
(17)
At this point, play with the mouse cursor to learn information about your scan.
Figure out how to select specific features, determining their channel number and number
of counts, also be able to change what is displayed in the scan window. Also look for
electronic noise in the spectrum near zero energy. Under the "Acquire  MCB
Properties" menu (ADC tab) set the lower level discriminator (LLD) until most of the
noise is eliminated. If not, adjust the LLD value and take scans until you have little low
energy noise in your spectrum.
(18)
The spectrum you see is dominated (85%) by a peak at 5.486 MeV on the right-
hand side of the spectrum. Look at the materials in the back of the folder for a schematic
of the expected alpha emission spectrum of the
241
Am source including a list of the alpha
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particle energies emitted. The 5.443 MeV (12.8%) and 5.388 MeV (1.3%) alpha peaks are
much less intense and may not be evident unless you collect enough data and look closely
at the spectrum. There are also two much smaller-intensity lines at 5.513 MeV (0.12%)
and 5.545 MeV (0.35%) which you likely will not see. You can control in what channels (08191) along the horizontal axis the peaks occur by changing the Fine gain knob on the
Ortec 460 DLA. Try this and provide an explanation of what happens as a result of
changing this control in your notebook.
(19)
When the LLD and the ADC Fine gain are set to your liking, collect data for five
minutes. Set the live time to 300 seconds. After the data has been collected use the
Expand On/Off feature to look at the spectrum on a greatly expanded horizontal scale.
How many peaks can you see? How well are they resolved?
(20)
For now, you want four pieces of information from this spectrum. Both the location
in channel number of the largest peak (1) and (2) the number of counts at the peak
maximum can be read directly from the MCA when you have the cursor on the peak.
Make sure you are in expanded mode. You also want (3) the Full Width at Half
Maximum (FWHM) of this peak; that is, the width of the largest peak at half its maximum
peak height. Normally, to do this, you would note the channel numbers above and below
the peak where the number of counts is half of that at the peak. But there is a problem
here. You need to make sure that the number of counts in the channel at half-height is not
influenced by other energy peaks. This shouldn't be a problem at the high-energy side but
may be a problem on the low-energy side. Different half-widths on the high and lowenergy sides may indicate this as a problem. You may have to just measure the highenergy half-width and multiply by two. Use the expanded data window feature. Finally,
you want (4) the total number of alphas particles represented in the spectrum regardless
of their energy. Investigate the software to find a way to integrate over the whole
spectrum to get the total number counts in the spectrum. Hint—explore the Region of
Interest (ROI) capabilities of the software. Is there a procedure where you can subtract
the noise showing up as a background in the spectrum to obtain a more accurate result?
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Experiment Two: Surface Barrier Detector Operation
The purpose of this experiment is to determine the effect of the bias voltage on the
operation of the surface barrier detector. If you have not already done so from the
previous experiment, adjust the DLA gain so that the peak of the data occurs near full
scale with the detector bias voltage of +125V. Reduce the detector bias supply voltage
(slowly) to 25 V. Take a five-minute spectrum and record the four pieces of information
discussed in the previous experiment. Repeat this procedure for bias voltages of +60 V
and +95 V. In KaleidaGraph or Matlab, plot and look at the data sets (125, 95, 60, and 25
V) together on the same scale to get a qualitative feel for the effect of bias voltage. Be sure
the pressure remains constant at essentially zero (you can keep the pump running during
these measurements). Discuss the data of (1) "channel number of the peak" height versus
bias voltage, (2) the full width at half maximum (FWHM) versus bias voltage, and (3) the
total count versus bias voltage. Observe those factors, which affect the operation of the
detector. The manufacturer recommends operation of the detector at 125 V. Can you
justify this based on your results?
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This is a natural place to take a break. If you are going to spend less than about 30 more
minutes in the lab today, proceed to Experiment Three. If you are going to spend more
than about 30 more minutes in the lab today, quickly read through Experiment Three to
see what it is about, but then proceed to Experiment Four and setup Experiment Three
just before leaving.
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Experiment Three: Data Acquisition Techniques
The purpose of this experiment is to become familiar with the apparatus, to perform an
energy calibration, to investigate the resolution of the apparatus, and, perhaps, to perform
a detailed analysis of the spectrum. Take a good alpha spectrum with 8192 channels.
This should be between five and ten hours. Any longer will actually result in a poorer
spectrum because of slow drifts in various parameters that determine the calibration.
Consider three things to remember: (1) Remember you cannot change any settings (like
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DLA gain, detector bias, etc.) while acquiring this spectrum; it will affect the calibration.
(2) Make sure the MCA timer is set appropriately. Ten hours is 36,000 seconds.
Save your long-time, +125V data. We will use this data to learn some advanced curvefitting procedures. The next several paragraphs deal solely with data manipulation. You
can be doing this while you are running Experiment Four below. You may want to start
that now and come back to this data manipulation while those experiments are running.
If you are doing a mini-version of this experiment, you may not want to get involved with
this data manipulation. Seek guidance on the matter.
We want the peak positions (with uncertainties) for the four peaks. The problem is that
while the dominant high-energy peak can be characterized fairly accurately, each of the
lower energy peaks are superimposed on the larger peak to their right (higher energy).
The best way to proceed is to use a program to simultaneously fit three or four Gaussians
at once. You are highly encouraged to do this in Matlab (rather than Kaleidagraph), but
the choice is yours. A procedure is outlined below to do this. Feel free to explore other
methods using the data selection tool in K-graph and/or writing a custom multiple
Gaussian fit function. Ask for help if you want to try this.
A note on KaleidaGraph. Perform constant save updates but save the plot, not the data.
If you are working on the data and want to save, click the plot update at the upper-left of
the data file (it's best to do it twice). This re-associates the data with the plot (even if
nothing different is being plotted). Then go to the plot and save it. Do not save data files
that are already associated with plots. That is, answer "no" when Kaleidagraph asks if
you want to save the data. From the plots you will be able to extract the original data if
you need to. This will cut down on the number of files you have to keep track of. Do not
have two Kaleidagraph plots and their data files open at the same time.
One can perform a fitting-subtraction procedure using KaleidaGraph. (1) Assuming the
channel number is in the left-hand column of your KaleidaGraph file (column zero or C0)
and the number of counts is in C1, copy those data entries from C1 to C2 that you think
Alpha particle spectroscopy
11
are unaffected by the lower three peaks. [There has been a hint of late, of an additional,
small high-energy peak of unknown origin. You must not use this part of the spectrum
either.] You can change the copied range of channels later in an iterative way if it turns
out you made a bad choice. Do not clear any data from C1. The entire spectrum remains
in C1. [Remember, update the plot and save it.] (2) Fit the large peak (C2) to a Gaussian
function. Write your own KaleidaGraph Gaussian fit as outlined in the instructions
provided. This fit will provide you with the peak channel number, the standard deviation
and a normalization factor, all with uncertainties. If you have chosen data badly, change
the entries in C2. (3) Using the Formula Entry feature of KaleidaGraph, make a new data
column (C3) with the fitted Gaussian. (4) Subtract this fitted high energy peak from the
original data (C2) and put the difference in C4. (5) Move, to C5, those data in C4 that you
think are not affected by the third peak to the left and so on and so forth. Repeat the
process until you have all the peaks characterized by Gaussians.
How do the fitted peak channels (and their uncertainties) compare with what you would
determine (including uncertainties) from simply looking at the data? Compare the fitted
four channel numbers of the peaks with what you might expect from a simple analysis
using Poisson statistics. If there are N counts in any channel then the uncertainty is √N.
Looking directly at the data on the MCA, use this fact to help you determine the
uncertainty in the channel number of each of the peaks. In determining the overall
uncertainty of the widths of overlapping peaks, visually estimate the effect of the rising
baseline on the width uncertainties.
Knowing the energy of the three (or four) peaks, you can calibrate the MCA. These
calibration points are very close together. Use KaleidaGraph and include the
uncertainties as error bars. You can also plot the total counts and half-width for each of
the four peaks from the parameters obtained from the Gaussian fits. Compare their
standard deviations. What trends do you observe? Are they what you expected?
Experiment Four: Energy Loss of Charged Particles
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Alpha particles are easily stopped by air and we want to investigate this process. [To start
with, don't take time to do any calculations needed to answer questions posed here. File
them away for later. Get on with the experiment.] The alphas (helium-4 nuclei) from the
241
Am source are in the 5 MeV range. When these massive, high-energy particles hit
oxygen and nitrogen molecules in air, they will either ionize the molecule or, excite an
electron to a higher energy state. Either way, they will continue on largely undeflected, at
least until they have lost most of their energy. (Occasionally, alphas can hit an oxygen or
nitrogen nucleus. How likely do you think this is? How big is the nucleus compared
with the average separation of molecules?) The average ionization potential for air is
about 32.5 eV. (How many air molecules can a 5 MeV alpha particle ionize before coming
to rest?) Much more theory is presented in various articles in this folder. You are
encouraged to peruse them as you develop the experimental plan described below.
The goal in this experiment is to determine all the appropriate properties of the spectrum
(as discussed above) as a function of air pressure and see if we can make sense of the data
in terms of collisions between alphas and air.
Look at the vacuum system, including the chamber that houses the source and the
detector. Practice increasing and decreasing the pressure.
You should do a quick-and-dirty scan of pressure, spending five or ten minutes at each
pressure. Remember to cover the 760 Torr pressure range in very large steps at first.
Make qualitative quick-and-dirty plots (sketches) of the pressure dependence of (1) peak
energy (or channel number), (2) total counts in a peak, and (3) half-width. Do you care
about the number of counts at the peak? Above what pressure do you no longer care that
there are four individual peaks and why? Above some pressure, do you expect to see any
counts at all? You can use standard fitting routines in KaleidaGraph to fit some of your
data. At this time, the pressure dependence of the total counts makes sense but we do not
have a good theory for the pressure dependence of the peak energy and the half width.
Alpha particle spectroscopy
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Plan your own strategy for a more careful experiment. This will depend, in part, how
long you have to work on this experiment, whether you can come in once a day, and so
on. In the end, of course, all one wants to do is make sense out of the data and generate a
good model.
expt14_alphaSpec_2013.docx