DUNPL Preliminary Energy Calibration for Proton Detection DUNPL Professor Alexander Komives By: Josh Wyant and Andrew Bever Report Number: TR-PD01 April 28, 2005
Throughout the semester we have been working with radioactively decaying sources in the hopes that we might become better acquainted with the various aspects and equipment of our long-term experiment. Our main focus has been on detecting gamma rays from decaying sources with a sodium iodide detector (NaI) (Figure 6). In doing so we are able to work with a multi channel analyzer, pre-amplifier, amplifier, oscilloscope, vacuum chamber, and surface barrier detector (our future proton detector). In working with all of this equipment we have been attempting to make an energy calibration for an 241 Am source that is decaying to its ground state of 237 Np. To make this calibration we have tested many decaying sources such as 137 Cs and 133 Ba (the graphs of which are similar to those below) in order to determine their decay schemes. The data we collected told us how many times a gamma ray with a certain energy hit the detector. These gamma rays had a corresponding channel so given the channel and number of hits for a given energy we were able to make a graph of the decay scheme. The peaks of the graphs correspond to a higher number of counts for that given energy level as can be expected. So, to determine what energy the gamma rays of the peaks had we turned to a website that lists the common energies of radioactive emissions from a given source (ie.lbl.gov/education/isotopes.htm). Once we found the most common x-ray and gamma ray energies for a given source we were able to correlate these energies to a particular channel number. Throughout the process we kept a constant gain so that the correlation was not skewed. Using the peaks from 137 Cs (Figure 1) and 133 Ba (Figure 2) we were able to make a graph based upon the channel number and energy level. This graph turned out to be a straight line with a slope of one corresponding to a one to one relationship between the energy level and channel number. With this in mind, we were able to make a decay scheme for 241 Am (Figure 3) and know that channel 60 corresponded to the 60KeV gamma ray peak. Of course, we had to keep the gain at a constant for each one of the decay schemes we made or else the calibration would not be correct. In knowing the channel number for this energy peak we can now progress to the next part of our experiment in which we will determine the half life of the 241 Am. We will take the alpha particle emitted by the source as a start signal and the gamma ray as the stop signal. Based upon the decay scheme of 241 Am we know it is much more likely to emit a 60 KeV gamma ray in its decay to the ground state, thus it is necessary to know which channel this gamma ray corresponds to. Through this process we have not only been making energy calibrations, though. We have most recently been testing the various amplifiers that we have in the lab so that we might know which ones work and which ones do not. This will be a very important thing to know as we further our experiment and begin to measure the half-life of our 241 Am. In order to know which amplifiers do work we made a decay scheme for 60 Co with the sodium iodide detector (NaI). This gave us the basis for what the graph should look like. With this in mind, we tested all of the amplifiers connected to the NaI detector to determine which amplifiers would give us the same graph as the one we knew to be correct. We only found one to be in working order, that being the Ortec 716a bipolar output. With this information we will be able to further pursue our half-life of 241 Am experiment using this amplifier. The two 60 Co graphs (Figures 4 & 5) we made are shown below.
We have also been working recently on making a table for our germanium detector that will be arriving within the next month. This detector will be able to show us sharper energy peaks for a given source. An energy resolution tells us how accurate the peaks are that we find in a given source and can be determined by measuring how many channels across the middle of the peak is. With this in mind a good energy resolution will result from a thinner peak, thus a more sensitive detector is necessary. The germanium detector is a much more sensitive device and will give us the thinner peaks we are looking for, so when it arrives we will be ready to incorporate it into our experiment. One difficulty with the germanium detector is the fact that they must be kept at low temperatures because they have a tendency to heat up rather quickly. To cope with this dilemma we will have to constantly cool it with liquid nitrogen, but this is a minor inconvenience for having superior data. So, until the arrival of our new detector we will continue using the surface barrier detector to set up the experiment such as the diagram below, and when the new detector arrives we will be able to use it in place of the surface barrier detector.
Figure 1. Resultant Graph of Channel number versus Count number for 137 Cs Figure 2. Resultant Graph of Channel number versus Count number for 133 Ba
Figure 3. Resultant Graph of Channel number versus Count number for 241 Am Figure 4. Resultant Graph of Channel number versus Count number for 60 Co; the amplifier built with the MCA was used
Figure 5. Resultant Graph of Channel number versus Count number for 60 Co; the bipolar channel of the Ortec 716a Amplifier was used Figure 6. Set up for the experiment.