Week 7 Blog

Hi Everybody!

Welcome to week 7 of my senior project blog! This week, I discuss radioactive spectra and progress on the project.

Nuclear Physics Weekly

Last week, we looked at ways radioactive spectra are generated—with scintillators or HPGe. This week, we’re taking a closer look at the radioactive spectrum itself. Specifically, why does it look the way it does? What determines where the peaks show up?

What Is a Radioactive Spectrum?

By now, you all are probably familiar with the term “radioactive spectrum” and what it dictates. In case you need a quick refresher though, a radioactive spectrum is just a plot showing how many radioactive emissions we detect at different energy levels. The horizontal axis shows energy (usually in units like keV or MeV), and the vertical axis shows how many times radiation of that energy was detected (more on this later)—basically, a count of how often particles with that energy showed up.

These spectra are almost never flat, and today’s article discusses the common shapes you’ll see in a typical radioactive spectrum.

Peaks: The Indicators of Decay
Let’s start with the sharp peaks found im a spectrum. When a gamma ray emitted by a nucleus deposits all (or close to all) of its energy in a detector (like an HPGe or NaI detector), that energy shows up as a spike. These are especially important because each radioactive isotope emits gamma rays of very specific energies. That means every peak is like a fingerprint, helping scientists identify which isotopes are present in a sample.

Notably, a couple of the common isotopes we see in most spectra are Potassium-40 at 1460keV, Bismuth-214 at 609keV, and Thorium-238 at 2300keV. Potassium is found in all plants and animals, while bismuth and thorium are found in dirt and dust. So, these peaks usually stand out in a typical spectrum.

Compton Continuum: The Sloping Background
However, not every gamma ray gives up all its energy in one clean hit. Sometimes it bounces off an electron inside the detector, transferring only part of its energy before exiting the detector. This process is called Compton Scattering, and it creates a sloped, continuous region in the spectrum called the Compton Continuum.

Think of it like this: every scattered gamma ray gives up a random fraction of its energy, depending on the angle it bounces, which is recorded as a smooth downward sloping background. Specifically, the shape of the Compton Continuum is an exponential decay curve.

We can actually derive this shape relatively simply, although it requires some basic calculus. First, let N(E) is the function describing the number of counts for an energy E. Assuming a uniform number of actual radioactive particles at every energy, the number of particles that have an energy greater than a certain value increases as we consider higher energies. Put more simply, let’s say our energy range is 20keV to 2500keV. Any gamma with energy 1000keV-2000keV could contribute to Compton noise at 1000keV. However, any gamma with energy 500keV to 2500keV can contribute to Compton noise at 500keV (considerably more gammas than for 1000keV). Since we assume uniform gammas at all energies, this relation can be described with the equation dN/N = -dE. All this equation is saying is that the probability of there being a Compton gamma decreases as energy increases. Solving for N(E) yields the exponential decay we seek.

Project Progress

This week was again dedicated to making my code work on a real time system. A lot of time was first spent on tweaking the threshold for my code to kick in—that is how smooth the data should be before I start applying the data analysis method. As of now, approximately 30 minutes of data is sufficient, although I want to try and decrease that number even more.

Also, I worked on peak recognition software, which initially stores a list of common radioactive decayed and matches them to peaks found in the spectrum. That way, the user knows what they’re looking at, opposed to just some numbers.

That’s it for this week! Tune in text week to learn more about radiation and my project!