Gamma-ray (γ-ray) Spectroscopy

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Gamma-ray spectroscopy is a systematic study of energy spectra of sources of gamma radiation like in nuclear industries as well as geochemical research and the field of astrophysics.

The majority of radioactive sources emit the gamma radiation, which is with varying intensities and energies. If these emission are investigated and detected with an spectroscopy device the gamma-ray spectrum is produced.

An in-depth analysis of the spectrum is often utilized to identify the identity and amount of gamma emitters in a gamma source and is a crucial instrument for radiometric tests. This spectrum of gamma is the characteristic of the nuclides emitting gamma within the source. as in an optical spectrometer The optical spectrum is a characteristic of the materials that comprise the sample.


Gamma-ray (γ-ray) spectroscopy Principle

The majority of radioactive sources emit Gamma rays that are of various intensities and energies. If these emission are detected and examined using the help of a spectroscopy instrument and a gamma-ray energy spectrum is produced. In gamma-ray spectrum, the energy of the gamma-rays emitted is measured using the detector.

Through comparing the measured energies against the measured energy and the known energy of radioisotopes generating gamma-rays the specific emitter can be identified. A thorough examination of this spectrum is often utilized to identify the identity and amount of gamma emitters that are present in a gamma-based source. It is an essential instrument in the radiometric assay. The spectrum of gamma is typical of the gamma emitting nuclides that are present inside the source.

Characteristics of Gamma ray

Gamma rays are the most powerful form of electromagnetic radiation being physically identical to the other types (e.g. the X-rays visible light infrared, radio) but possessing (in general) greater energy from photons because of their shorter length. Due to this, the energy of gamma-ray particles can be analyzed individually and a gamma-ray spectrumrometer can be used to measure and display the energy of the gamma-ray light particles that are identified.

Nuclei that are radioactive (radionuclides) generally emit gamma radiation in the range of energy from just a few keV to around 10 MeV which corresponds to the typical energies of nuclei with relatively long lifespans. They typically emit the gamma-ray “line spectra” (i.e. that many photons are released at different energies) while high energy levels (upwards to 1 TeV) could be observed in the continuum spectrum observed in astrophysics as well as elementary particle Physics. The line between gamma rays and X rays can be obscure, since X rays usually refer to the high-energy electron emission from atoms that can exceed 100 keV. On the other hand, the smallest energy emission of nuclei are generally referred to as Gamma rays even though their energy levels may be less than 20 keV.


  • To introduce students to the most basic techniques for measuring Gamma Rays.
  • To gain experience in energy calibration and analysis of gamma ray spectrums to identify the isotopes emitting gamma radiation.

Equipment Needed

  • 60Co radioactive source (PASCO).
  • NaI(Tl) scintillation detector with photomultiplier tube(REXON Components Inc.).
  • UCS30 Universal Computer Spectrometer (SPEC TECH).
  • Position stand with sample tray and lead shield.
  • BNC-6 Signal Cable (Advanced Digital Cables, Inc.), and MHV-6 High voltage cable (CAROL).

Instrumentation/Components of Gamma-ray (γ-ray) spectroscopy

The equipment used for the gamma spectrum comprises:

1. An energy-sensitive radiation detector

The detectors that are commonly used can be any of the two;

Scintillation detector

A scintillation detector can be one of many possible ways of the detection of ionizing radiation. Scintillation refers to the method by which the substance, whether it’s a liquid, solid, or gas emits light when exposed to ionizing radiation that is incidentally. In reality, it’s employed in the form of one crystal made of sodium Iodide, which is doped by a small amount of thallium, which is referred to by NaI(Tl). The crystal is connected to a tube for photomultiplier which transforms the tiny light flash into an electrical charge by an effect called photoelectric. The electrical signal will be identified by the computer.

Semiconductor detector

A semiconductor can achieve the same thing as a scintillation detector the conversion of gamma radiation to electrical impulses, however, it does so by a different method. In the semiconductor, there’s an energy gap of a tiny size between the valence bands of electrons as well as the conduction band. If a semiconductor is struck by gamma-rays that is emitted by the gamma ray can be enough to move electrons into their conduction bands. This alteration in conductivity could be observed and the signal generated to reflect the change. Germanium crystals that are doped with lithium Ge(Li) or high-purity germanium (HPGe) detectors are among the most popular kinds.

Advantages and disadvantages of detector:

Each type of detector comes with its own pros and drawbacks.

  • NaI(Tl) NaI(Tl) detectors typically less effective than Ge(Li) and HPGe detectors, in many ways however, they is superior Ge(Li) as well as HPGe sensors in terms of cost and ease of use and long-term durability.
  • Germanium-based detectors typically have greater quality that NaI(Tl) detectors. Small photopeaks are often invisible when using NaI(Tl) detectors. They are clearly visible in germanium detectors.
  • In reality, Ge(Li) detectors must be kept at a temperature of cryogenic throughout their lifespan or else they will become not able to function as a gamma-ray detection device.
  • Sodium iodide detectors are smaller and portable, and can possibly be utilized in the field since they don’t require cryogenic temperatures as they are able to be used as long as the photopeak which is being studied can be distinguished from the surrounding photopeaks.

2. Electronics

It is responsible for processing signals from the detector produced from the device. For instance. A pulse sorter (i.e., multichannel analyzer)

3. Associated amplifiers and data readout devices

They assist in the creation, display and storage of the spectrum.

Features of Gamma spectrum

There are a variety of distinctive aspects that are visible in the Gamma spectrum. The main feature that will be noticed will be the photopeak. The photopeak occurs when a gamma-ray has been completely absorption into the detector. Larger and more dense detector sizes enhance the chance of absorbing the gamma-ray.

The other major aspect to be observed is The Compton edge, and the distribution. The Compton edge occurs due to Compton Effect which is where some energy of the gamma ray is transfered to the detector for semiconductors or scintillator. This happens when the high energy gamma ray is struck by an electron that is relatively low in energy. There is a sharp edge that is a Compton edge that represents the highest amount of energetic energy transferable to an electron through this kind of scattering. 

The broad peak with lower energies than that of it is the Compton edge is known as the Compton distribution. It is a reflection of the energy levels generated by a range different scattering angle. A characteristic of Compton distributions includes that of the peak in backscatter. This peak is the outcome of the exact phenomenon however it is the smallest energy quantity of energy transferred. A sum of energy that are reflected by both the Compton edge as well as the peak of backscatter ought to give that energy as the photopeak.

Another set of characteristics in an gamma spectrum include high-frequency peaks associated with pair production. This is the process through the gamma ray with sufficient energies (>1.022 MeV) can create the electron and positron pairs. The electron and the positron could be annihilated and create two 0.511 MeV photons called gamma. If all three gamma-rays that are the original, with its energy diminished by 1.022 MeV, as well as the two annihilation gamma-rays are simultaneously detected the full energy peak can be observed. In the event that one of annihilation-gamma rays are not absorbed into the detection system, an energy peak equal to the full energy but less 0.511 MeV is observed. This is referred to as an escape peak. If both annihilation gamma radiations escape, then a complete energy peak of less than 1.022 MeV is observed. This is referred to as an escape peak with double-escape.

Gamma-ray (γ-ray) spectroscopy Applications

  • This instrument is used to conduct the analysis of elemental and isotopic elements of the airless body in the solar system particularly mars and the moon.
  • GRS instruments offer data about the abundance and distribution of chemical elements
  • They are extensively used in the study of: Nuclear structure, Nuclear changes and, Nuclear reactions
  • Space research, for instance the detection of water on planets


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