Alpha Particle Spectroscopy
The Alpha particle contains two protons and two neutrons identical to the structure of Helium-4 nuclei (so alpha particle has a +2 overall charge), which is generally produced in the process of Alpha decay. Alpha particles are commonly emitted by all of the larger radioactive nuclei. The atomic number goes down by exactly 2, as the result of losing two protons.They are highly ionizing form of particle radiation, and have low penetration depth. Thus, they are easily stopped by a few centimeters of air.
The purpose of this lab is to investigate the interaction of alpha particles and air molecules at different atmospheric pressures by observing the alpha spectra emitted by Americium-241( the source of our experiment). The alpha particles are detected by a silicon surface barrier detector placed in a vacuum chamber. A vacuum gauge will monitor the pressure inside the chamber.
Two major elements will affect the gain (resolution of the spectrum) of the detector. First is the pressure in the chamber and the other is the voltage applied to the detector. The setup of the experiment is indicated as in Figure 1 and 2 and most of equipment are labeled. The Am-241 source is placed in a stainless steel vacuum chamber, a pump is used to evacuate the air inside the chamber because the alpha particles has very low penetration depth. The pressure inside of the chamber has to be around 0 Torr in order to get a high resolution spectrum. In this case, we fix the atmospheric pressure to 0 Torr, and applied different voltage to the detector. An Ortec 428 detector bias supply is used to apply voltages for the Detector. Data was taken at three different voltages: 25V, 60V, 95V and 125V. Based on the data, the spectrum is cleanest at 125V. In other word, under 125 voltage, the spectrum has the most distinguishable peaks and the FWHM is the smallest. So the voltage for taking data acquisition is fixed to 125V from now on.
The major experiment consists two part. First, we want to correlate the channel number in MCA with energy using the 10-hour-data acquisition for Am-241 and its characteristic peaks: 5.38 MeV, 5.44 MeV, and 5.48 MeV with their corresponding channel numbers. Second is to investigate the energy loss when increase the pressure inside the chamber.
Following diagram shows the three characteristic peaks for Am-241 for 10 hours data acquisition of alpha spectrum:
Using Gaussian distribution we can do the fitting. We are able to approximate pretty accurate mean for each peaks which indicate the channel number corresponding to each characteristic peaks. For 5.38 MeV, 5.44 MeV, and 5.48 MeV peaks, they have corresponding channel number 7377(2), 7451(2), and 7495(2). A linear line for correlating the channel number and the energy can be plotted:
The equation of the linear fit is:
Channel Number =1184.2*Energy (MeV)+1006.8.
Since the Energy is already correlated with Channel Number, we can investigate the energy of alpha particle detected at different atmospheric pressures. The alpha spectra of Am-241 taken at different levels of vacuum are shown in the following figure (each peak is collected with 5 minutes data acquisition) :
The pressure is increased at a step around 30 Torr. The peaks along the x-axis corresponds to the pressure ranging between 420 to 33 Torr. We can plot a graph to show the relationship between the peak energy versus the pressure to study the interaction between the alpha particle and the air molecules:
As we can see from the figure above, the higher the pressure in the chamber gets the lower the peak energy is. So, only the alpha with very low energies made it to the detector.
Also, we can investigate the energy of alpha radiation detected under different thickness of air. The absorber thickness is defined as the distance between the detector (5.9 cm) and the source multiplied by the ratio of chamber pressure to the air pressure(720 Torr):
The following figure shows the absorber thickness versus the energy detected alpha radiation:
Form the figure above, we see the expected trend: as the the absorber thickness increases, the energy of alpha particle detected decreases.
The stopping power curve (dE/dX versus absorber thickness) thus can be plotted using the adjacent figure above. The difference between each x value is dx, and the difference between each correspond E is dE. Thus, the dE/dX can be calculated. The stopping power curve is displayed following:
The result is consistent with the previous report value of dE/dX which is around 2.
I should take more data at higher pressure next time to get a more complete power stopping curve.