Pandemonium effect
Encyclopedia
The Pandemonium effect is a problem that may appear when high resolution detectors
(usually germanium detectors) are used in beta decay studies
. It can affect the correct determination of the feeding to the different levels of the daughter nucleus
. It was first introduced in 1977 paper by J.C. Hardy et al.
The general picture of the problem is schematically this: when a parent nucleus decays into its daughter, the energy levels of the daughter can be populated in two ways:
When one of these levels decays, the total gamma rays
emitted by the level (IT) should be equal to the sum of these two contributions (neglecting internal conversion
). As the magnitude that can be measured are the gamma intensities (ΣIi and IT), the beta feeding (that is, how many times a level is populated by direct feeding) has to be extracted indirectly by subtracting the contribution from de-excitations of higher energy levels (ΣIi) to the total gamma intensity that leaves the level (IT), that is: Iβ = IT - ΣIi. But sometimes the daughter nucleus has a large Q value
, allowing the existence of many levels. This means that the total feeding will be fragmented, as it will spread over all of them (with a certain distribution given by the strength, the level densities, the selection rules
, etc). Then, the gamma intensity emitted from the less populated levels will be weak, and it will be weaker as we go to higher energies where the level density can be huge. Also, the energy of the gammas de-excitating this high density level zone can be high.
Measuring these gamma rays with high resolution detectors may present two problems:
The consequence of these two effects is that much of the beta feeding at high excitation energy is not detected so less ΣIi is subtracted from the IT, and the levels are incorrectly assigned more Iβ than they really have. When this happens, the low-lying energy levels are the more affected ones. Some of the level schemes of nuclei that appear in the nuclear databases suffer from this Pandemonium effect and are not reliable until better measurements are done. The knowledge of the beta feeding, (Iβ) is important for different applications, like for example, the calculation of the residual heat
in nuclear reactors.
One possible solution is to use a calorimeter like the Total Absorption Spectrometer (TAS). It has been shown that even with a high efficiency array of Germanium detectors in a very close geometry (the CLUSTER CUBE), about 57 % of the total B(GT) observed with the TAS technique is lost.
Semiconductor detector
This article is about particle detectors. For information about semiconductor detectors in radio, see Diode#Semiconductor_diodes, rectifier, detector and cat's-whisker detector....
(usually germanium detectors) are used in beta decay studies
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
. It can affect the correct determination of the feeding to the different levels of the daughter nucleus
Decay product
In nuclear physics, a decay product is the remaining nuclide left over from radioactive decay. Radioactive decay often involves a sequence of steps...
. It was first introduced in 1977 paper by J.C. Hardy et al.
The general picture of the problem is schematically this: when a parent nucleus decays into its daughter, the energy levels of the daughter can be populated in two ways:
- either by direct feeding from the decay (Iβ),
- or by de-excitation of higher energy levels also beta-populated in the decay (ΣIi).
When one of these levels decays, the total gamma rays
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
emitted by the level (IT) should be equal to the sum of these two contributions (neglecting internal conversion
Internal conversion
Internal conversion is a radioactive decay process where an excited nucleus interacts with an electron in one of the lower atomic orbitals, causing the electron to be emitted from the atom. Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but...
). As the magnitude that can be measured are the gamma intensities (ΣIi and IT), the beta feeding (that is, how many times a level is populated by direct feeding) has to be extracted indirectly by subtracting the contribution from de-excitations of higher energy levels (ΣIi) to the total gamma intensity that leaves the level (IT), that is: Iβ = IT - ΣIi. But sometimes the daughter nucleus has a large Q value
Q values
In nuclear physics and chemistry, the Q value for a reaction is the amount of energy released by that reaction:Q = E\left - E\left,...
, allowing the existence of many levels. This means that the total feeding will be fragmented, as it will spread over all of them (with a certain distribution given by the strength, the level densities, the selection rules
Selection rule
In physics and chemistry a selection rule, or transition rule, formally constrains the possible transitions of a system from one state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions...
, etc). Then, the gamma intensity emitted from the less populated levels will be weak, and it will be weaker as we go to higher energies where the level density can be huge. Also, the energy of the gammas de-excitating this high density level zone can be high.
Measuring these gamma rays with high resolution detectors may present two problems:
- First, these detectors have a very low efficiency of the order of 1-5%, and will be blind to a weak radiation in most of the cases.
- Second, the efficiency curve drops to very low values as it goes to higher energies, starting from energies of the order of 1-2 MeV. This means that most of the information coming from the high energy gamma rays will be lost.
The consequence of these two effects is that much of the beta feeding at high excitation energy is not detected so less ΣIi is subtracted from the IT, and the levels are incorrectly assigned more Iβ than they really have. When this happens, the low-lying energy levels are the more affected ones. Some of the level schemes of nuclei that appear in the nuclear databases suffer from this Pandemonium effect and are not reliable until better measurements are done. The knowledge of the beta feeding, (Iβ) is important for different applications, like for example, the calculation of the residual heat
Decay heat
Decay heat is the heat released as a result of radioactive decay. This is when the radiation interacts with materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.-Natural occurrence:...
in nuclear reactors.
One possible solution is to use a calorimeter like the Total Absorption Spectrometer (TAS). It has been shown that even with a high efficiency array of Germanium detectors in a very close geometry (the CLUSTER CUBE), about 57 % of the total B(GT) observed with the TAS technique is lost.
External links
- "Conquering nuclear pandemonium", by Krzysztof P. Rykaczewski