Gamma spectroscopy

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

Many radioactive substances give off gamma rays with different energy levels and strength. Scientists use special tools to catch and study these rays. They can make a picture called a gamma-ray energy spectrum.

By looking at this picture, scientists can find out which radioactive elements are in a sample and how much of them there is. This helps in measuring radioactive materials. Just like 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 like other types of radiation such as X-rays, visible light, and radio waves, but they have more energy because their wavelength is shorter. Scientists 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 space science and particle physics, gamma rays can have even higher energies, sometimes over a trillion electronvolts. Gamma rays and X-rays are both types of electromagnetic radiation, but gamma rays come from changes in the nucleus of an atom and usually have a single, specific energy. 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 important 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. 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.

Detector performance

Gamma spectroscopy systems are picked based on how well they find and measure gamma rays. Two main things matter: how well the system can tell apart different gamma rays (resolution) and how often it finds gamma rays (efficiency).

When gamma rays are found, they make peaks in the spectrum. How sharp these peaks are shows how good the detector is. A sharp peak means the system can tell apart gamma rays with very close energies. The width of these peaks is often measured using a value called full width at half maximum.

Another important thing 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 shown with numbers or by comparing it to other detectors. The energy of the gamma rays also changes how well they are detected.

Scintillation detectors

Scintillation detectors use special crystals that glow when gamma rays hit them. The brightness of the glow tells us about the energy of the gamma ray. One common example is gamma rays from a material called caesium ¹³⁷
Cs. This material gives off gamma rays with a specific energy. These gamma rays 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 about the energy of the gamma rays. Sometimes, extra gamma rays can make it harder to read the data clearly, but special tools can help.

Another example is gamma rays from cobalt ⁶⁰
Co, which has two gamma rays with different energies. These appear as two separate peaks in the data. Special tools are used to keep the measurements accurate.

Semiconductor-based detectors

Semiconductor detectors, also called solid-state detectors, work in a special way. They detect tiny particles called electrons and holes when gamma rays hit the material.

When a gamma ray hits the detector, it gives an electron energy to move. This moving electron leaves a space called a "hole." The movement of these electrons and holes creates an electric signal. This signal helps scientists study the gamma rays. Common types of these detectors include germanium, cadmium telluride, and cadmium zinc telluride. Germanium detectors are 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.

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

Calibration and background radiation

When we use a gamma spectrometer to learn about an unknown sample, we first need to adjust the machine. We do this by using a sample with known energy levels, like caesium-137 or cobalt-60. This helps us change the machine’s numbers into real energy levels.

There is always some natural radioactivity around us, called background radiation. When we measure something, we need to think about this natural radiation. We can do this by measuring the background radiation with no special sample and then taking it away from our results. We can also use lead around the machine to help lower the background radiation.