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The Experiment

In this experiment, gamma spectroscopy data was collected from a lab assignment performed at UCDenver. The lab involved taking data using a spectrometer which was fed from a scintillator and a photomultiplier tube (PMT) to record the energy levels emitted by various radioactive sources. Let's go over the types of radioactive decay and describe what the related instruments are doing so that you can have a better understanding of the data that we're looking at when we start plotting it.

Radioactive Decay

There are three main types of radioactive decay associated with emissions, alpha (\(\alpha\)), beta (\(\beta\)), and gamma (\(\gamma\)), named in increasing order of their ionizing properties (\(\alpha > \beta > \gamma\)). These three types of emissions are notable also for their ability to be separated from each other using an electromagnetic field - meaning they all have different types of charge.

Starting from the element bismuth (Bi-83), every isotope of every element is at least slightly radioactive. In other words, no element has any stable isotopes after bismuth. From bismuth up, the strong force which holds the nucleus together struggles to reach across the distance of the nucleus, meaning the electric force which wants the positively charged protons to repel each other is able to produce more force than is required to hold the nucleus together. For elements lighter than bismuth, every element has at least one stable isotope, but there are still many radioactive isotopes such as Carbon-14 or Potassium-40.

Alpha (\(\alpha\)) Decay

This type of decay releases a positively charged alpha particle (sometimes represented as \(^4_2\alpha\)), consisting of two protons and two neutrons (a helium nucleus) being ejected from the atom, reducing its atomic number (Z) by two. It is also the least penetrating of these forms of radiation and can be stopped by just a few centimeters of air, but it has the power to be the most ionizing and destructive of the types we'll cover here.

Because of the low penetrating power of alpha particles, they are generally only harmful to life if the radioactive source is swallowed, inhaled, or held closely to the body for prolonged periods of time.

NOTE: Don't do that.

Beta (\(\beta\)) Decay

This type of decay releases a beta particle, which can either be a high-speed electron (\(\beta^-\) or beta minus decay), or its antimatter counterpart the positron (\(\beta^+\) or beta plus decay). \(\beta^-\) decay occurs when a neutron decays into a proton, releasing an electron and an antineutrino to conserve charge, while \(\beta^+\) decay occurs when a proton decays into a neutron, releasing a neutrino and positron to conserve charge. So, an atom which undergoes beta decay can gain or lose one atomic number depending on whether it is \(\beta^-\) or \(\beta^+\), but will have an unchanged mass number (A).

This type of radiation is in between the other two in terms of both ionization and penetrative power. It is able to pass several millimeters into aluminum, but is generally considered a mild hazard. If exposed to beta radiation for long enough however, it is possible to develop burns similar to those caused by heat. As with alpha radiation, these effects can be exacerbated if the source is swallowed, inhaled, or held for long periods of time.

Gamma (\(\gamma\)) Decay

Unlike the other two, gamma decay doesn't release a particle with mass, instead releasing electromagnetic radiation in the form of a chargeless photon of light. Gamma radiation differs from other more familiar forms of light (such as visible light or x-rays), in that it has the shortest wavelengths and thus the highest energies of light (\(E=\frac{hc}{\lambda}\)). Gamma radiation has many possible sources, but in particle physics it normally occurs after a nucleus undergoes either alpha or beta decay. This decay leaves the nucleus in an excited state, and when the nucleons transition to a lower energy state, it releases one of these high energy photons.

This transition is much like the process through which the more common types of light are emitted, where an electron will transition between energy levels, releasing a photon in the process. However, gamma decay transitions involve the strong nuclear force rather than the electromagnetic, producing a photon orders of magnitude more energetic. The typical photon released through electronic relaxation will be in the eV range, usually less than 100eV, while the typical photon released from an atomic nucleus will range from around 1keV to 10MeV of energy.

Gamma radiation has the most ability to penetrate materials but is also the least ionizing of these three types of radiation. Natural exposure to gamma radiation is in the range of 1-4 mSv (milli-Seivert) per year, while the lower end of exposure which can begin to cause harmful effects such as cancer are estimated to be around 100 mSv. For reference, your typical chest x-ray will deliver around 5-8 mSv, and a PET/CT scan will deliver between 14-32 mSv. At 1 Sv, the effects of acute radiation sickness can start to be observed, and the dosage at which approximately 50% of those exposed to radiation will die as a result of the exposure (the \(LD_{50}\)) is around 3-5 Sv.

Instrumentation

Now that we've gone over the different types of radiation, let's go over how we can detect gamma decay experimentally, and then how we can use these detections to calibrate our instruments and determine the properties of an unknown isotope.

Scintillator

The scintillator is the first step in our instrumentation. These devices utilize the properties of certain materials to luminesce when excited by radiation, causing them to re-emit light at a lower and more easily detectable wavelength. For gamma ray detection, photons Compton scatter off of electrons in the structure of the scintillator.

Compton scattering describes the effect where high-energy photons interact with loosely-bound valence shell electrons, giving them enough energy to be released from their atoms and ionizing the atom in the process. Those free electrons can then scatter off of other electrons, spreading the energy of the initial gamma ray across multiple electrons. As each of these electrons are recaptured by the ionized atoms, they then release photons at a lower energy.

It is by this process that one high energy gamma ray can be turned into several lower energy photons which can then be detected using the second instrument in the experiment.

Photomultiplier Tube (PMT)

A PMT is an incredibly sensitive photon detector, and they are normally designed to detect light specifically in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum (which is why we need the scintillator to step-down our high gamma energies).

They are typically constructed using an evacuated glass housing with a photocathode on one end, which is engineered to use the photoelectric effect. This again uses energy from light to release electrons, but now in the presence of an electric field. The electric field accelerates the released electrons towards the back of the PMT, and along the way the electrons will strike against arrays of dynodes.

Each dynode has a higher potential difference (voltage) than the previous one, and they are designed so that with each electron that strikes them, they release several more. This causes an exponential cascade of electrons to flow through the system until they finally reach the anode at the end of the PMT. Here, the cascade of electrons results in a current which can be detected using a device such as an oscilloscope or a spectrometer, and the results can be recorded. In this experiment, a spectrometer was used to record the data.

Spectrometer

"Spectrometer" is a broad class of scientific instruments which can be used for detecting and measuring the spectral components of light. The one used for this experiment was a Universal Computer Spectrometer (UCS). The UCS was the final component in the detection chain and serves as the interface between the analog signal produced by the PMT and the digital data we will analyze. This type of spectrometer is called "universal" as it can be configured for a wide range of radiation detection experiments, and "computer" as it can be integrated with a digital system capable of binning, processing, and storing pulse data.

It works by monitoring the pulses of current generated at the anode of the PMT. Each pulse corresponds to a single detected photon event detected by the scintillator and converted to a current by the PMT. The one used in this experiment contained 2048 channels, with higher channels corresponding to higher detected currents.

Data Analysis and Calibration

The next step in this process is to take the data from the radioactive sources and feed them into a computer using software. As decay events occur in relation to the associated element's half life, it is necessary to run the detection software for several minutes for each element in order to collect as much data as possible. Without running the data for enough time, it may become difficult or impossible to distinguish the signal from the noise. The longer the data is collected, the smaller the margin of error.

After an event is measured in a specific channel, that channel's count number is increased by one. This eventually results in noticeable peaks which correspond with the isotope's emission energy. Every isotope has a unique emission energy, allowing the detection of a photon and the calculation of its energy to function like taking the isotope's fingerprint.

However, due to the steps of the scintillator and PMT, it is not possible to immediately directly correlate a specific channel with a specific emission energy. It is because of the steps required to be able to detect the gamma rays in the first place that we must also perform significant data analysis to associate channel numbers with emission energies. The resulting relationship between channel numbers and emission energies however should be a linear one. Each channel should correspond to a specific range of gamma energies. The more channels that a spectrometer has, the higher its effective resolution.

This system is further limited by the geometry of the setup. The radioactive sources are placed underneath the detector and then radiate equally in every direction, meaning only a fraction will interact optimally with the detector. Some of these emissions will hit the detector with a glancing blow, meaning that they'll have less detected energy than the others, while a small amount of photons actually will go straight into the detector but will then have much of their energy reflected directly out. This distribution of energies will result in a Gaussian curve, where the center of the curve will correspond to the average detected energies of the emitted particles. It will also result in other features which will become visible when plotted which we will go over in the next section, called the Compton continuum and the Compton edge.

So, in order to determine what element our unknown radioactive source is, it will be necessary to first perform a curve fit for each one of the peaks detected, and then use the known values for their emissions to perform a second curve fit to relate the channel numbers with their actual energies.

This entire process is known as calibration. By using known isotopes, we are testing the parameters of our instruments, allowing us to accurately assign a particle's emission energy levels to a specific range of data channels. Calibration describes the process of establishing a relationship between values using measurements and their uncertainties. This lesson is showing specifically how to calibrate a spectrometer, but you should be thinking about how this process can be generalized and applied to other types of scientific recording.

Let's get going, then! Click here to continue to the next section where we'll get started by isolating the photopeaks in our data.