Gamma-ray Spectroscopy - Definition, Principle, Parts, Uses

Gamma-ray Spectroscopy is a non-destructive analytical technique used for qualitative and quantitative study of gamma-ray energy spectra.

It is used to study gamma rays emitted from radioactive source. The unstable atomic nuclei emit gamma-ray photons at specific energy level. This occurs when the nuclei pass into more stable ground state.

These gamma-ray emissions are different for different radionuclides. So it works as a fingerprint of the radioactive isotope.

In Gamma-ray Spectroscopy, the intensity of gamma radiation is measured with the energy of each photon. A spectrum is formed. The spectrum has energy peaks.

The energy peaks are matched with known gamma-ray energy values. From this the unknown radioactive material in the sample is identified. The amount of the isotope can also be measured.

The main parts of the system are energy-sensitive radiation detector and multichannel analyzer (MCA). The detector may be sodium iodide scintillator or high-purity germanium detector.

When gamma ray interacts with detector material, its energy is absorbed. The interaction takes place by photoelectric effect, Compton scattering, or pair production. This absorbed energy is changed into electrical voltage pulse.

The height of the pulse depends on the energy deposited by gamma ray. The MCA measures these pulse heights and arranges them into channels. Then a visual histogram or spectrum is produced.

It is used for study of isotopic and elemental composition of sample. It is also used in astrophysics, planetary science, national security, and clinical medicine.

Gamma-ray (γ-ray) spectroscopy Principle

Principle of Gamma-ray (γ-ray) Spectroscopy is based on the emission of gamma rays from unstable radioactive nuclei at specific energy level.

Unstable radioactive nuclei change into more stable state by emitting gamma-ray photons. These photons are emitted with fixed and discrete energy. The energy value is not same for all radioisotopes.

Each radioisotope gives its own characteristic gamma-ray energy. So the emitted radiation acts as a fingerprint of that radioactive isotope.

When the emitted gamma ray falls on an energy-sensitive detector, it interacts with detector material. The detector may be a scintillation crystal or a solid-state semiconductor detector.

During this interaction, the energy of the gamma ray is absorbed by the detector. This absorbed energy is converted into an electrical voltage pulse.

The height of the electrical pulse depends on the energy deposited by the gamma ray. A gamma ray of higher energy produces a higher pulse, while lower energy gamma ray produces lower pulse.

These voltage pulses are passed into multichannel analyzer (MCA). The MCA sorts the pulses according to their pulse height and counts the number of pulses in each channel.

A gamma-ray energy spectrum is formed from these counts. The spectrum shows different peaks at different energy positions.

The position of the peak is compared with known gamma-ray energy values. From this comparison, the radioactive isotope present in the sample is identified.

The area under each peak represents total number of counts. It is proportional to the activity or amount of that particular isotope present in the sample.

Instrumentation of Gamma-ray (γ-ray) spectroscopy

The Gamma-ray (γ-ray) spectroscopy system is made up of different parts which are used to detect and measure the energy of gamma rays. These parts convert the energy of incident γ-rays into electrical signal and finally produce a gamma-ray energy spectrum.

The following are the important parts of Gamma-ray spectroscopy–

Energy-sensitive radiation detector
It is the main part of Gamma-ray spectroscopy. It interacts with the incident gamma rays and converts their deposited energy into electrical pulses. The common types are scintillation detectors and semiconductor detectors.
Example- Sodium Iodide (NaI) detector with photomultiplier tube (PMT), High-Purity Germanium (HPGe) and CZT detector.
High voltage power supply
It is also called bias supply. It is used to provide the necessary operating voltage to the detector or to the photomultiplier tube. This voltage helps in proper working of the detector system.

Preamplifier
The preamplifier is generally placed close to the detector. It collects the first charge from the detector and converts it into a small voltage pulse. It is kept near the detector to reduce the noise in signal.
Spectroscopy amplifier
It is also called pulse-shaping amplifier. It receives the signal from the preamplifier and increases the size of the pulse. It also reshapes the pulse into Gaussian or trapezoidal shape and removes electronic noise.
Analog-to-Digital Converter (ADC)
The ADC is used to convert the analog voltage signal into digital values. These digital values represent the pulse height. The pulse height is related with the energy of the incident γ-ray.

Multichannel Analyzer (MCA)
The Multichannel Analyzer (MCA) is also called pulse sorter. It sorts and counts the digital pulses according to their height or energy. These pulses are kept in different bins called channels. This finally forms the histogram of the gamma-ray energy spectrum.
Data processing and readout equipment
It includes computer interface, memory and display system. It is used to record, visualize and analyse the obtained gamma-ray spectrum. The peaks in the spectrum are used for the identification and measurement of radioactive substances.
Ancillary equipment
These are the supporting parts of the spectroscopy system. It includes cables, tripods, rate meters, peak position stabilizers and shielding materials. Heavy shielding like lead (Pb) is used to block background radiation.

Steps of Gamma-ray (γ-ray) spectroscopy

The Gamma-ray (γ-ray) spectroscopy is done by detecting the emitted gamma rays from a radioactive source and converting their energy into electrical signal. This signal is amplified, converted into digital value and analysed to form the gamma-ray energy spectrum.

In this step, gamma rays emitted from radioactive source enter into the detector. The detector may be Sodium Iodide (NaI) scintillation detector or High-Purity Germanium (HPGe) semiconductor detector.
The gamma rays interact with the detector material mainly by photoelectric effect, Compton scattering or pair production. During this process, the energy of γ-rays is transferred to electrons.

In scintillation detector, the energized electrons excite the crystal and visible light is produced. This light strikes the photomultiplier tube (PMT) and electrons are released and multiplied to produce a current pulse.
In semiconductor detector, the gamma ray directly produces electron-hole pairs. These pairs move towards the electrical contacts and produces an electrical signal.
The first electrical charge is collected by the preamplifier. It converts the charge into a small voltage pulse.

The height of this voltage pulse is directly proportional to the energy deposited by the γ-ray inside the detector.
The voltage pulse is then passed to the amplifier. The amplifier increases the size of the pulse by gain.
The amplifier also shortens the long tails, rounds off the leading edges and reshapes the pulse into proper form. Random electronic noise is also filtered out.

The shaped analog voltage pulse is passed to the Multichannel Analyzer (MCA). In this part, Analog-to-Digital Converter (ADC) measures the peak height of the pulse.
The ADC converts the measured analog pulse height into a discrete digital value.
The MCA sorts the digitized pulses according to their size into specific bins called channels.

When thousands or millions of pulses are sorted in these channels, a histogram is formed. This histogram is the gamma-ray spectrum, where number of counts are plotted against gamma-ray energy.
The system is calibrated by using reference sources having known gamma-ray energies. This calibration gives the relation between MCA channel number and actual energy values.
The energy values are generally measured in keV or MeV.

Finally, the spectrum is analysed by spectrometry software. The energy peaks are identified and the specific radionuclides responsible for those peaks are determined.
Background radiation is subtracted from the spectrum and the total area of the peaks is calculated. This is used to determine the quantity or activity of the isotopes present.

Applications of Gamma-ray (γ-ray) spectroscopy

Gamma-ray (γ-ray) spectroscopy is used in different fields for detection, identification and measurement of radioactive materials. It is used to identify elements, isotopes and radioactive source present in different samples.

The following are the applications of Gamma-ray spectroscopy–

It is used in space exploration for determining the elemental and isotopic composition of planets, moons and asteroids. It is also used for detecting water on airless bodies and for studying solar physics and cosmic phenomena like supernovae, neutron stars and black holes.
It is used in scientific research for investigating nuclear structures, nuclear transitions and nuclear reactions. It is also used in geosciences for dating and determining the accumulation rates of sediments like marine deposits, ferromanganese nodules and phosphorites, and for identifying material composition of different samples.
It is used in medicine and healthcare for diagnostic imaging, mainly Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT). It is also used in cancer treatment and radiotherapy to target and destroy malignant cells, and for sterilization of medical equipment and pharmaceuticals by highly penetrating gamma rays.

It is used in industrial and engineering fields for non-destructive testing and inspection of structural components such as welding seams and castings. It is also used for real-time troubleshooting of distillation columns and process vessels to detect tray damage, blockages or flooding without shutting down production, and for pipe scanning to identify blockages, scale, coke buildup or corrosion.
It is used in oil and gas well logging for characterization of subsurface rock formations. It helps to identify clay types and locate potential hydrocarbon reservoirs.
It is used for sterilization of food products. The gamma rays destroy bacteria, viruses and pathogens present in food materials.

It is used in national security and safeguards for cargo inspection and border security. It helps to detect illicit radioactive materials and Special Nuclear Material (SNM).
It is used in nuclear forensics to characterize unknown radioactive sources and nuclear debris. It also helps to determine the origin, age and production reactor of the material.
It is used in Radio-Isotope Identification Devices (RIIDs) to quickly distinguish between legitimate commercial radioactive materials or naturally occurring radioactive materials and potential threats. It also helps to prevent false alarms from disturbing commerce.

It is used in arms control and nuclear disarmament verification by confirming the presence of specific isotopes such as plutonium.

Advantages of Gamma-ray (γ-ray) spectroscopy

Gamma-ray (γ-ray) spectroscopy has many advantages because it can detect and measure radioactive isotopes without destroying the sample. It is used in laboratory, medical, industrial and field analysis.

The following are the advantages of Gamma-ray spectroscopy–

It is a non-destructive and non-invasive technique. It can analyse the internal condition of objects, equipment or human organs without destroying the sample, without physical disassembly and without invasive surgery.

It can identify and measure many gamma-emitting radioactive isotopes in the same sample at the same time. Each isotope gives a specific energy peak, which acts like its unique energy fingerprint.
It has high penetrating power because gamma rays have high energy and short wavelength. So, they can pass through dense materials and helps in inspection of structures, packaging and even steel and cement walls of cased boreholes.
It is a fast analytical method and gives quick result. It generally needs very simple sample preparation and does not require long chemical processing.

It is often less expensive than traditional radiochemistry. It also helps to detect problems early, so it can reduce industrial downtime and avoid unnecessary medical procedures.
It can be performed remotely in some field conditions. In this case, radioactive materials can be detected from a distance without collecting physical samples.
It is a highly versatile technique. It is used in different fields such as astrophysics, environmental monitoring, national security and medical imaging.

Limitations of Gamma-ray (γ-ray) spectroscopy

Gamma-ray (γ-ray) spectroscopy has some limitations because it can detect only those radionuclides which emit gamma rays. The accuracy also depends on detector type, background radiation, counting rate and proper shielding.

The following are the limitations of Gamma-ray spectroscopy–

It cannot directly detect radionuclides which do not emit gamma rays. Radionuclides such as H-3, C-14, P-32, Sr-90 and Pu-239 cannot be directly identified or measured by this method.
It is often less sensitive than radiochemical analysis. It has higher minimum detectable concentration and usually needs larger amount of sample for proper measurement.

Sodium Iodide (NaI) detectors have poor energy resolution. So, it becomes difficult to separate isotopes having very close energy peaks and also difficult to analyse complex mixtures.
High-Purity Germanium (HPGe) detectors give very good energy resolution, but they need continuous cryogenic cooling like liquid nitrogen. For this reason, they are costly, bulky and difficult to carry and maintain in field condition.
Room temperature semiconductor detectors like Cadmium Zinc Telluride (CZT) have better resolution than NaI detector, but they have small active crystal volume. They also suffer from excessive charge trapping and mechanical cracking.

Scintillation detectors like NaI(Tl) are affected by environmental temperature changes. Due to temperature fluctuation, the energy spectrum may shift and stabilizers are required.
The analysis may be affected by background radiation, X-ray fluorescence and Compton continuum. These signals can hide the required energy peaks and make the spectrum difficult to analyse.
At very high radiation intensity, the detector may become saturated. This causes dead time, where some pulses are missed, and gain shift may also occur.

At very low count rate, statistical noise becomes more. So, long counting time is required to get proper result.
In industrial equipment scanning, it gives only one-dimensional density profile. The resolution of signal may be reduced by scattering in dense or multi-phase materials.
It needs proper radiation safety during use because active radioactive sources are often used for calibration or scanning. So, strict safety rules, licensed personnel and heavy shielding are required.

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Gamma-ray Spectroscopy – Definition, Principle, Parts, Uses