Developed by
Department of Chemistry
Radiochemistry
University of Helsinki

By performing this exercise, students will learn how to analyse qualitatively a gamma spectrum containing multiple gamma sources. They will be familiarised with identification of gamma emitters based on their energy, intensity, and half-lives by using nuclear databases, books and/or decay charts.

Gamma radiation is generated when a nucleus having excess energy transfers to the ground energy state. Beta and alpha decay processes do not necessarily lead to ground state of the daughter nuclide but instead, to its excited state or states. These excited states are relaxed by emission of one or several gamma quanta. In addition to beta and alpha decay processes, gamma radiation is formed in fission and in nuclear reactions induced by particle bombardments with particle accelerators and nuclear reactors.

All atoms have their characteristic energy state structure, similarly to the energy states of electrons. Energy state structure is dependent on the number of protons and neutrons, as well as on the shape and motion state of the nucleus. Figure below shows the decay schemes of two nuclides, ⁵⁷Co and ⁵⁸Co, decaying by electron capture and β+ decay to their daughter nuclides ⁵⁷Fe and ⁵⁸Fe, respectively.

Beta decay/electron capture of ⁵⁷Co and ⁵⁸Co for excited states of ⁵⁷Fe and ⁵⁸Fe (horizontal lines). In the figure, the diagonal dashed arrows represent beta decays/electron captures (the probability of decay next to the percentage) and the vertical arrows the gamma radiations generated by the relaxation of the daughter nuclides. The energy of the gamma radiation equals to the difference of the energies between the excitation states - the energies of the excitation states are given on the left side of the states. The probabilities of gamma decays have been omitted due to space constraints.

As seen from the figure above, the decay to different excited states occurs at different probabilities and their de-excitation to ground state or to other excited states of the daughter nuclide also occurs at different probabilities, resulting in the formation of gamma rays. The energy of a gamma ray is the energy of the excited state if de-excitation takes place to the ground state or difference of the energies of the excited states if the decay takes place between the excited states. Since each nuclide has its own characteristic energy state structure and thus a characteristic gamma ray spectrum, radionuclides can be identified based on their measured gamma spectra. In addition, when the probabilities (intensities) of gamma transfers, corresponding to each gamma peak, and the counting efficiency of the spectrometer system are known, the activities of the radionuclides can be determined. Thus, gamma spectrometry enables both qualitative and quantitative analysis of radionuclides. Furthermore, as gamma radiation is very penetrating the analysis can be most often carried directly from the sample without radiochemical separations.

It should be noted, that gamma radiation is usually referred to belong to the parent nuclide, even though they are transitions between energy states of the daughter nuclide. However, the de-excitations usually occur in such a short time (t1/2 < ns) that they can be considered to be part of the parent nuclide decay. Thus, the parent nuclide identification and its activity determination are based on the gamma rays emitted by the daughter nuclide. In this exercise, a gamma spectrometer is first calibrated with respect to energy using several gamma- emitting standard sources, then unknown radionuclides are identified in a few samples and, finally, a background gamma spectrum is examined.

Gamma spectrometry – spectrometer and formation of spectrum Gamma spectrometry is performed with a HPGe (high purity germanium) semiconductor detector. The advantage of HPGe detector is its good energy resolution, that is, its ability to distinguish radiation with energies close to each other. HPGe detector is connected to a high voltage source and signal processing electronics, from which the spectrum is read with computer in a spectrum analysis program. A schematic diagram of the equipment is shown in the following figure.

A gamma spectrometer setup. The open arrows indicate the path of the radiation-induced pulse in the equipment, while the filled arrows indicate the operating voltage paths (solid line - high voltage for the detector, dashed line - operating voltage of a pre-amplifier).

Gamma radiation causes ejection of electrons in the detector material, and these electrons are collected with electric field. The number of electrons ejected is directly proportional to the energy of the radiation: with higher energy, more electrons are ejected. All three interaction types of gamma ray with detector material - photoelectric effect, Compton effect and pair production - result in the formation of electrons in the detector. A voltage pulse proportional to the number of electrons is generated in pre-amplifier, which is amplified in linear amplifier, still in proportion. Finally, in analog-digital converter (ADC) the pulse is converted into a binary number so that each pulse height correlates with a binary number. The information about the pulse is then stored in the multichannel analyzer (MCA) in the channel corresponding to the binary number. As the measurement progresses, each channel registers the number of pulses of a certain height. As a result, a spectrum with MCA channel number in X-axis and the number of recorded pulses on Y-axis is produced. To determine the activities of radionuclides, the sum of pulses in the peak, which corresponds to the amount of photoelectron formation caused by the radiation, is used.

Energy calibration
The peaks observed in a gamma spectrum do not give directly the energies of the radiation but they correspond certain channels in a multichannel analyzer. These channels can be however, related to the radiation energies. The relation between the energy and the channel number is determined by performing an energy calibration. This is carried out by measuring gamma sources of known radionuclides with characteristic radiation and determining which channel corresponds to which energy.

In this work, we measure several radionuclides, who all emit gamma radiation of a single energy and utilize the energy calibration function of the spectrum analysis program of the spectrometer. Single radionuclide (standard) sources containing ²⁴¹Am, ¹³⁷Cs, ⁵⁴Mn, ²²Na, or some other suitable and available radionuclides fit for this purpose.

• Before starting the exercise, familiarize yourself with the instrument and identify the components.
• Before work, find out the gamma radiation energies emitted by the used radionuclides.
• During the work, measure each standard sample for a couple of minutes, identify the centermost channel of the photopeak in the spectrum and calibrate the energy with the spectrum analysis program on the computer. The Lab Supervisor helps with using the program.

Analysis of unknown samples
Supervisor gives 2-3 different samples with unknown radionuclide composition. Place each sample close to the detector and measure the spectra. Identify the energies of the photopeaks found in spectra and based on these identify the nuclide(s) in the sample.

To identify the radionuclides, use available gamma radiation databases and books as references for photopeak energy, probability and intensity. It is best to start from the most intense peaks of the spectrum: from the literature, look for the radionuclides, which emit radiation at this energy. Remember that the uncertainty in energy calibration can be even half keV, so it is a good idea to expand the search a bit from the exact measured energy value. When potential nuclides have been found, check if those nuclides have other intense gamma energies and see if you can find them in the spectrum. Finding these supports the identification. After spotting all the peaks of the nuclide identified, look if there are more unidentified peaks and continue until all the peaks are identified.

In uncertain cases, one should also use the knowledge about the half-life of the nuclide and the intensity of the radiation:

• is the lifetime of the nuclide long enough considering what the sample is?
• is the intensity high enough, that it is realistic to observe this gamma emission in a short measurement time and relatively low activities used in the samples?

After identification of the nuclides in the samples, determine the count rates (pulses/s) of each radionuclide, by determining the peak areas of their intensive photopeaks, and dividing them by the counting times. With this nuclide wise count rate information, you can compare the activities of the same nuclides in different samples, and evaluate isotopic composition of gamma emitters in different samples. The activity ratios thus obtained are only estimates, since the self-absorption may differ from sample to another.

Examination of background spectrum
Both natural and anthropogenic gamma emitters exist in environment and they cause background events in a gamma spectrum. Especially when measuring low activities, it is important to be familiar with the background radiation in the laboratory and for the instrument, so that the background can be subtracted from the measured activity of the sample. Background radiation also affects the limit of detection, that is, the smallest activity still measurable with the instrument: the higher the background, the higher the detection limit. Background radiation can be diminished by protecting the detector with thick lead or steel shielding which absorbs as much background radiation as possible. It is also beneficial if the laboratory has been built in a manner that prevents background radiation from passing to gamma measurement room. Effects from background radiation can be mitigated by, e.g., careful selection of building materials, protection from cosmic radiation and radon gas.

Study the laboratory background radiation by opening the latest background spectrum of the instrument and identify the nuclides in the spectrum. (Hint: in many countries, the majority of background radiation comes from the natural decay series of uranium and thorium. For example, nuclides like ²³⁵U, ²³⁸U, ²³²Th, and their decay products.)

Determine also the count rates of gamma emitters in background radiation, in the same way as with the unknown samples.

Work report is written in a form of a scientific essay, starting with the description of the exercise topic with background information of the work and progress of the experiments.

After this, present the results. First, the energy calibration procedure is described and produced energy calibration spectrum is presented.

After this, present the identification of radionuclides in the unknown samples: what nuclides were found, what where the approximate activities of radionuclides in the samples, were the isotopic ratios of gamma emitters comparable with each other in unknown samples, and would it be possible to make conclusions about the origins of the radionuclides, based on the isotopic composition.

In the end, explain which nuclides contribute to the background radiation of the laboratory and how large its influence can be expected to be to the measured sample spectra (compare the count rates of the background spectrum to those in the unknown sample spectra).

Remember to present necessary figures, gamma spectra, tables and equations.

Gamma radiation from standard samples and unknown samples will not cause significant radiation exposure when they are handled appropriately, i.e. with tweezers. Otherwise, there are no specific safety concerns regarding the exercise.

For executing the lab exercise, you need to gather and prepare beforehand

• HPGe gamma detector with spectrum analysis software
• several calibration standards, e.g., ²⁴¹Am, ¹³⁷Cs, ⁵⁴Mn, ²²Na, or other available gamma emitters
• two-three unknown samples, containing gamma emitters with adequate activity level for less than one hour measurement per sample
• book(s) containing gamma decay data and/or access to gamma data search in internet

Leave feedback in comments