Download Physics PHYS 276 Experimental Physics Laboratory Alpha Particle Spectroscopy

Physics PHYS 276
Experimental Physics Laboratory
Alpha Particle Spectroscopy
I. Introduction
In this experiment we will be looking at the decay chains of 210Po, 241Am, 238U and
isotopes present in natural uranium) by examining their alpha particle emissions.
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U (the primary
Unlike gamma rays, alpha particles loose energy as they travel through matter, therefore, a large sample
cannot be used. For example, a very thin coating of Uranyl Acetate on a microscope slide will be the
natural uranium source. In order to measure the alpha particle spectra a silicon surface barrier (SSB)
detector will be used. These detectors are very efficient, and, since the alpha particle background rate is
very low, an alpha spectrum can be measured even for a very weak source.
This measurement will be carried out using the multichannel analyzer (MCA). This type of measurement
of the total energy of a particle is called calorimetry, and is an important technique in most nuclear
physics experiments.
II. Setup: In this lab we will measure the alpha energy spectrum using a Multichannel Analyzer
(MCA).
Amp
MCA
III. Procedure
A. Wear gloves when handling these radioactive sources. Do not touch the radioactive
surface.
B. Make sure you understand the SSB detector and the circuits above. Do not touch the surface of
the detector.
C. Make sure you understand how the pre-amplifier, amplifier and single channel analyzer work.
Read the manuals for the pulser, preamp and the bias supply.
D. Set up the vacuum system.
E. Set up the electronics.
F. Place the 210Po source in the holder, close the chamber and evacuate. Let the pressure stabilize
before turning up the bias voltage. Remember, never change the pressure in the chamber
with the SSB bias voltage turned up!
G. Turn the bias voltage slowly up to +50V. Be careful to use POSITIVE voltage! As you turn
up the voltage, watch the output of the SSB detector with the oscilloscope to make sure
everything is okay. Never turn up the voltage when light can enter the detector.
H. Look at the signals from each stage of the circuit. Adjust the gain so that the 5.305 MeV alpha
particles fall within the 0-12V output of the amplifier.
I. Collect the spectrum using the MCA.
J. Remember, the alpha particles emitted by the 210Po source have a kinetic energy of 5.305
MeV. Using the 210Po peak, and the pulser, calibrate the energy scale of the MCA. Match the
pulser output pulses to the same channel as the 210Po peak, then use the attenuated output to
make peaks of “known” energy.
K. Replace the 210Po source with the 241Am source. Be sure you turn the voltage down before
allowing air back into the chamber. Collect an energy spectrum.
L. Replace the 210Po source with the uranium source. Be sure you turn the voltage down
before allowing air back into the chamber.
M. Collect an energy spectrum for the natural uranium source.
N. Try to identify the peaks in the spectrum as belonging to daughter nuclides in the uranium
decay chain. Use the Handbook of Chemistry and Physics and the handout.
IV. Logbook
In addition to the items discussed earlier and any ideas of your own, you should have the
following in your logbook:
A. Sketches of the signals after each stage of the electronics.
B. A plot showing the number of counts as a function of pulse height as measured using the
MCA, for the 210Po calibration energy spectrum, the 241Am spectrum and the natural uranium
energy spectrum.
C. List of any peaks found in the uranium spectrum.
D. Identification of spectral peaks.
V. Questions to ponder
A. Are there alpha energies you expected to see but did not? If not, can you explain why?
B. Do you see any peaks that you cannot identify? What do you suppose is their source?
C. Why do the peaks have such a strange shape? What is the best way to determine the energy of
the peak?
D. Why is it that you can see pulses coming out of the detector even when the bias supply is
turned off? Since this is the case, what is the advantage of even using the bias supply?
E. What is the difference between a SSB detector and an ordinary silicon diode? Could you use a
silicon diode as a particle detector? Why or why not?
F. Does the distance between the source and detector change the spectrum?
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Appendix – Decay Series
A very useful tool is the “Decay Chain Explorer” at http://spice.duit.uwa.edu.au/samples/ast0197/
(taken from http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/radser.html )
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