The task of a gamma detector is to convert the gamma radiation that hits it, which is electrically neutral, into a measurable electrical signal. This utilizes the fact that gamma radiation is a type of ionizing radiation: It removes electrons from the atoms of the detector material through ionization. The amount of charge produced, i.e., the number of electrons – to reiterate: these are negatively charged particles – is proportional to the energy of the gamma radiation.
The charge produced in the detector is collected in a preamplifier and converted into a voltage pulse. The height of the voltage pulse corresponds to the respective amount of charge. The further processing of this voltage pulse takes place in various analog - or more recently predominantly digital - components. We will initially consider these as a black box, where the voltage pulse enters on one side and is processed “somehow” and comes out on the other side. Ultimately, the processed voltage pulse “lands” in a multi-channel analyzer.
To illustrate, a multi-channel analyzer consists of a large number of “bins,” called channels. Each channel corresponds to the height of a voltage pulse. When a voltage pulse of a certain height is transmitted to the multi-channel analyzer, the number of already registered voltage pulses in the corresponding channel is increased by one.
Schematic representation of the detection of gamma radiation. The gamma radiation hits a detector and generates a charge amount through ionization that is equivalent to the energy of the gamma radiation. The charge amount is converted in the preamplifier into a voltage pulse, the height of which corresponds to the charge amount. The voltage pulse is further processed in a “black box” and subsequently distributed to the appropriate channels in a multi-channel analyzer.
This way, the multi-channel analyzer (MCA) obtains a distribution that indicates how many voltage pulses have been registered in each channel.
Example of an MCA distribution (channels), measured for the radioactive isotope 137Cs. The characteristic line emitted during the decay of the isotope with a gamma energy of 661 keV is clearly visible (the smaller lines that are also visible originate from the background present at the measurement location at the time of measurement).
As we already know, the gamma radiation emitted during the decay of an isotope has certain (discrete) energies, which are characteristic for each isotope, i.e., they provide a kind of fingerprint. This means that primarily charge amounts are generated in the detector that correspond to the energies of these characteristic lines. These are converted in the preamplifier into voltage pulses with corresponding heights, further processed in the black box, and the result of this processing is handed over to the multi-channel analyzer. There, the content of the respective channel, a numeric value indicating how often voltage pulses in the current measurement have already been assigned to this channel, is increased by one. This results in a distribution in the multi-channel analyzer over time, known as a spectrum. The time it takes for such a spectrum, with the characteristic lines, to build up depends on various factors. These include, for example:
- the amount of radioactive decay described by the activity of the isotope,
- the probability that gamma radiation of a characteristic line is produced during a radioactive decay (this does not generally occur in every decay event),
- the distance of the detector from the sample containing the isotope (the further the sample is from the detector, the less gamma radiation hits the detector, as gamma radiation is emitted in all directions),
- the efficiency of the detector (the proportion of gamma radiation that hits the detector and is detected (measured) depends on the shape, size, and material of the detector),
- the energy of the gamma radiation (if the energy is too low, gamma radiation cannot even enter the detector, and as energy increases, the likelihood that the gamma radiation simply passes through the detector without being detected, i.e., without converting its energy into a corresponding charge amount, increases),
- etc.
This incomplete list already demonstrates that measuring gamma radiation is a non-trivial task and performing an actual measurement requires good planning. This includes choosing a suitable gamma detector, of which there are many different types.
All the points briefly mentioned in this section, such as the black box, various types of detectors, ionization, etc., will be discussed in depth in the Advanced section.
However, we now want to continue with the (measured) distribution in the multi-channel analyzer, the gamma spectrum.
