Gamma spectroscopy

Gamma-ray spectroscopy is a way to study the energy levels of gamma rays, which are high-energy waves released by radioactive materials. This tool is important in many areas, such as the nuclear industry, studying rocks and minerals, and even looking at stars and other objects in space.

Many radioactive substances give off gamma rays with different amounts of energy and strength. By using special tools to catch and look at these rays, scientists can create a picture called a gamma-ray energy spectrum.

Looking closely at this picture helps scientists figure out which radioactive elements are in a sample and how much of them there is. This is very useful for measuring radioactive materials. Just like how a regular spectrometer can tell what a material is made of by looking at light, a gamma-ray spectrum can show what radioactive elements are present.

Gamma ray characteristics

Gamma rays are the highest-energy type of electromagnetic radiation. They are similar to other types of radiation like X-rays, visible light, and radio waves, but they have more energy because their wavelength is shorter. Scientists can use special tools called gamma-ray spectrometers to measure the energy of each gamma ray.

Many radioactive materials, called radionuclides, release gamma rays with energies ranging from a few thousand electronvolts up to about 10 million electronvolts. These gamma rays often have specific energies, creating what is called a "line spectrum." In fields like space science and particle physics, gamma rays can have even higher energies, sometimes over a trillion electronvolts. While gamma rays and X-rays are both types of electromagnetic radiation, gamma rays come from changes in the nucleus of an atom and usually have a single, specific energy, while X-rays come from changes in the electrons around the atom and can have a range of energies.

Components of a gamma spectrometer

A gamma spectrometer has key parts that work together to study gamma rays. The main parts are a special detector that can sense energy and electronic devices that look at the signals from the detector, like a pulse sorter (also called a multichannel analyzer). Other parts might include signal amplifiers, tools to measure rates, and devices to handle data.

Laboratory equipment for determination of γ-radiation spectrum with a scintillation counter. The output from the scintillation counter goes to a Multichannel Analyzer which processes and formats the data.

Detector

Detectors for gamma spectroscopy are materials that can catch gamma rays. Important ways they work include the photoelectric effect, the Compton effect, and pair production. These processes turn the energy of the gamma ray into a voltage signal. Common detector materials are sodium iodide (NaI) scintillation counters, high-purity germanium detectors such as Bismuth germanate, and more recently, GAGG:Ce.

Data acquisition

Pulse-Height Analyzer Principle: Three pulses, 1, 2, and 3 are detected at different times t. Two discriminators emit a counting signal if their set voltage-level is reached by a pulse. Pulse 2 triggers the Lower Level EL but not the Upper Level EU. Pulse 2 is thus counted into the spectral region denoted as P. The anti-coincidence counter prevents a pulse from being sorted into more than one region

The signals from each gamma ray are looked at by a multichannel analyzer (MCA). The MCA changes the shape of the signal and turns it into a digital form using a fast analog-to-digital converter (ADC). The MCA sorts the signals by their strength into specific groups, or channels. Each channel shows a range of energy, and the number of signals in each channel shows how strong the radiation is in that energy range. The MCA can send this data to a computer for storage, display, and more analysis. Software tools help with tasks like measuring energy and calculating results.

A USB sound card can act as an affordable ADC, a method started by Marek Dolleiser. Special software on a computer can analyze the digital signals, creating a complete MCA. Sound cards have fast but lower-resolution ADC chips, which work well for low to medium counts of signals. The "sound card spectrometer" has been improved by both hobbyists and professionals.

Detector performance

Gamma spectroscopy systems are chosen based on how well they can detect and measure gamma rays. Two key features are how clearly the system can separate different gamma rays (resolution) and how often it can detect gamma rays (efficiency).

When gamma rays are detected, they create peaks in the spectrum. The sharpness of these peaks shows how good the detector is. A sharper peak means the system can tell apart gamma rays that have very similar energies. The width of these peaks is often measured using a value called full width at half maximum (FWHM).

Another important feature is efficiency, which tells us how likely a gamma ray is to be detected. Bigger detectors usually catch more gamma rays, but the material of the detector also matters. Efficiency can be described in exact numbers or compared to other detectors. The energy of the gamma rays also affects how well they are detected.

Scintillation detectors

Scintillation detectors use special crystals that glow when gamma rays hit them. The brightness of the glow usually tells us how much energy the gamma ray had. One common example is looking at gamma rays from a material called caesium ¹³⁷
Cs. This material gives off gamma rays with a specific energy of 662 keV. These gamma rays actually come from a related material called ^137m
Ba, which is created when ¹³⁷
Cs breaks down.

Figure 2: Sodium iodide gamma spectrum of cobalt-60 (60Co); see also a different measurement

When we measure these gamma rays, we can see different groups or “peaks” in the data. These peaks show us information like low-energy radiation, scattered gamma rays, and the main glow at 662 keV. Sometimes, extra gamma rays can make it harder to read the data clearly, but special tools can help reduce these extra rays.

Another example is gamma rays from cobalt ⁶⁰
Co, which has two gamma rays with energies of 1.17 MeV and 1.33 MeV. These appear as two separate peaks in the data. However, temperature changes can shift these peaks, so special stabilizers are used to keep the measurements accurate.

Semiconductor-based detectors

Semiconductor detectors, also called solid-state detectors, work in a special way compared to other types. They detect tiny particles called electrons and holes that are created when gamma rays hit the material.

When a gamma ray hits the detector, it gives an electron enough energy to move from one place to another. This moving electron leaves behind a space called a "hole," which another electron can fill. The movement of these electrons and holes creates an electric signal that helps scientists study the gamma rays. Common types of these detectors include germanium, cadmium telluride, and cadmium zinc telluride. Germanium detectors are very good at measuring energy but need to stay very cold, usually with liquid nitrogen, to work properly.

Interpretation of measurements

Scintillation gamma spectrum of a radioactive Am-Be-source. Visible are the main photopeak of 12C neutron excitation and the two escape peaks associated with it.

When scientists measure gamma rays, they sometimes see special patterns in the data called peaks. One of these is the backscatter peak. This happens when gamma rays bounce off materials around the detector before reaching it. These bounced rays have less energy, creating a wide peak at lower energy levels, usually below 250 keV.

Another pattern comes from very high-energy gamma rays. These can create pairs of particles that quickly disappear, producing two photons with 511 keV each. Depending on whether both, one, or none of these photons reach the detector, scientists can see peaks at the original energy, or lower energies by 511 keV or 1022 keV. This is called the single escape peak or double escape peak.

Calibration and background radiation

When we use a gamma spectrometer to find out what’s in a sample we don’t recognize, we first need to adjust the machine so it reads the right energy levels. We do this by using a sample with known energy levels, like caesium-137 or cobalt-60. This helps us change the machine’s reading from numbers to actual energy levels.

There is always some natural radioactivity around us, called background radiation. When we measure something, we need to take this natural radiation into account. We can do this by measuring the background radiation when no special sample is present and then subtracting it from our results. We can also use lead around the machine to help lower the background radiation.