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High energy calibration of scintillating detectors with thermal neutrons
Hochenergetische Energiekalibrierung von Szintillationsdetektoren mit thermalen Neutronen
Bachelor-Thesis von Alexander K. Blecher
Technische Universität Darmstadt
Institut für Kernphysik
The NEPTUN tagger is a setup which uses LaBr3 detectors. The calibration of them is challenging since the channel-to-energy relation is not linear and energy drifts can occur. The quality of the calibration increases by using high-energy γ-ray sources. Neutron capture reactions (n,γ) with lathanum, bromine and chlorine which provide energies up to 8.5MeV are investigated with experiments and simulations. A moderator can deaccelerate the neutrons and that is why it increases the cross section for these reactions. Therefore the moderator thickness is analysed for an polyethylene exemplary. A thickness between around 5 cm is indicated as best choice. The lanthanum and bromine in the detector provides sharp peaks, as shown in the experiments. Chlorine target adds additional peaks to the spectrum. It is now possible to build an efficient and compact calibration setup.
Der NEPTUN-Tagger ist ein Messaufbau, der LaBr3-Detektoren benutzt. Die Kalibrierung derer ist herausfordernd, da die Kanal-zu-Energie-Relation nicht linear ist und Energieverschiebungen auftreten können. Die Kalibrierungsqualität kann über hochenergetische -Strahlen-Peaks gesteigert werden. Neutroneneinfangreaktionen (n,γ) mit Lanthan, Brom und Chlor, welche Energien von bis zu 8.5MeV liefern, werden mittels Experimenten und Simulationen untersucht. Ein Moderator bremst Neutronen und kann daher den Wirkungsquerschnitt der Reaktionen erhöhen. Aus diesem Grund wurde die Dicke des Moderators am Beispiel von Polyethlen untersucht. Es zeigte sich, dass eine Dicke von etwa 5 cm die beste Wahl ist. Lanthan und Brom im Detektor erzeugen scharfe Peaks wie die Experimente zeigten. Zusätzliche Peaks im Spektrum lieferte das Chlor-Target. Nun ist es möglich, eine effiziente und kompakte Kalibrierungsquelle zu bauen.
1 Motivation ... 6
1.1 NEPTUN experiment ... 6
1.2 Calibration problems ... 7
2 Nuclear physics background ... 8
2.1 Neutrons ... 8
2.2 Neutron sources ... 8
2.3 Moderation of neutrons ... 9
2.4 Neutron capture reactions and high energy calibration sources ... 10
2.5 High energy γ-ray production ... 11
2.6 Detectors ... 11
2.6.1 γ-ray and neutron scintillators ... 11
2.6.2 High Purity Germanium detectors ... 12
3 Simulations ... 13
3.1 Moderation of neutrons ... 13
3.1.1 Moderation with a fixed position of the detector ... 14
3.1.2 Moderation with detector bordering on moderator ... 15
3.2 Neutron and γ-ray detector response ... 17
3.2.1 Spectrum of the detector response ... 18
3.2.2 Isotopes produced in neutron capture ... 18
4 Experiment ... 21
4.1 Measurements ... 21
4.2 Calibration ... 21
4.2.1 Identifying known peaks in spectra ... 23
4.2.2 Calibration function ... 23
4.3 Analysis of the detector-response linearity ... 25
4.4 Energy deviations of identified peaks ... 26
4.5 Comparison to other measurements ... 27
4.6 Analysis of moderation ... 27
4.7 Possible effect of the setup related to the moderation analysis ... 29
5 Conclusion and Outlook ... 31
5.1 Moderation behaviour ... 31
5.2 Detector response ... 32
5.3 Quality of high energy peak calibration ... 33
5.4 Supposed further investigations for the construction of a handy calibration setup.34
6 Appendix ... 36
6.1 Peak specifications of the experiment ... 36
6.2 Data used in graphics ... 42
List of Figures ... 45
List of Tables ... 46
References ... 48
All figures and tables in this paper not attributed to a third party are the author's own.
The motivation for this work is the more accurate calibration of the lanthanum bromine detectors used in the NEPTUN setup. The focus goes on different aspects of the detector response which is analysed with simulations and experiments. Close look is taken at the physical properties of neutrons and their measurement to create detailed simulations. Based on the simulation results we can build an accurate calibration method. Neutrons are moderated to reduce the velocity and to increase the cross section for capture reactions. These reactions create high energy γ-ray lines. Investigations for the optimal moderator thickness and the analysis of the experimental and simulated spectra are necessary.
1.1 NEPTUN experiment
Figure 1.1: Principle of tagging: By measuring the remaining energy of the producing electron the energy of the bremsstrahlung photons is calculated, taken from Savran et al. .
[Figure not part of this preview]
NEPTUN is a photon tagging spectrometer at the superconducting Darmstadt electron linear accelerator (S-DALINAC) at Technische Universität Darmstadt. The setup, shown in Figure 1.1, is designed to measure the dipole response of atomic nuclei, especially the pygmy dipole resonance. The S-DALINAC produces an electron beam with an energy up to 90MeV. Continouos bremsstrahlung is made via shooting an elelectron beam on a gold target. By using a thin gold radiator it is assured that most of the scattered electrons produce one photon. The energy of a bremsstrahlung photon is given by the difference of the incoming electron energy E0 and the remaining energy of the scattered electron Ee: Eγ = E0 - Ee. The remaining energy is measured via the bending radius in a magnetic dipole field. A focal plane detector array detects the electrons with 128 scintillating fibers.
The bremsstrahlung photons interact on different secondary targets to measure the dipole response. The measurements of the secondary γ-rays take place with High Purity Germanium detectors (HPGe) and an array of 16 lanthanum bromine scintillators (LaBr3). For further details about the detectors see Section 2.6. Together with the focal plane detector array the coincidence is determined. The γ-ray detector can be positioned in different incident angles and positions [22, 23].
1.2 Calibration problems
The NEPTUN secondary target produces γ-rays which energies vary from few hundreds of keV up to tens of MeV. For this setup it is important to have an exact calibration of the used detectors over the whole energy range. Most standard calibration sources emit γ-rays with maximum energy up to around 4 MeV. But it is necessary to have fix points at the top of the energy scale to reduce the uncertainties of the calibration. Especially since, the detectors do not have a linear channel-to-energy correlation . Polynoms are then a good choice as calibration function but require sources with high energies.
An additional problem are energy drifts of the detectors, which are caused by a variety of factors, e.g. temperature, humidity, count rate and magnetic field influence. Both problems, drifts and non-linear calibration, make the calibration very challenging. With the method investigated in this work and presented in Section 2.4, we can probe the response of detectors up to around 8 MeV.
2 Nuclear physics background
The following chapter describes the physical background of the method e.g. moderation, neutron capture and sources, the mode of operation of scintilling detectors and energy calibration.
Neutrons are together with protons the components of nuclei. Their shortcut is n and the mass of 939.56MeV is slightly higher than the proton mass 938.27MeV. A free neutron decays with a half life time of 880 s. They do not interact via the coulomb force, because they are not charged. Though neutrons can interact with the electron shell, because of their magnetic moment. This interaction is very weak . Therefore only nuclei scattering and absorption of neutron are relevant processes.
2.2 Neutron sources
There are different ways to get free neutrons. Many heavy isotopes decay by spontaneous fission. They emit two heavy fission products and several γ-rays and neutrons. A good example for those isotopes is 252Cf. But for experimental purposes it is nonpractical: The nucleus has only a probability of about 3% to decay with neutron emission. It mostly emits α-particles. As well the half life time of 2.65 years has to be considered . In a nuclear reactor a spallation source produces neutrons via 7Li(p,n)7Be or 2H(2H,n)3He reactions. Spallation sources are difficult to build and not accessible in our case.
Another possibility are sources based on beryllium or other light elements. Based on (γ ,n) and (α,n) reactions they are portable and handy neutron sources with a long half time for activation analysis, as calibration source and for industrial purposes. A widespread combination are 239Pu9Be and 241Am9Be sources : 239Pu and 241Am emits α-particles. Excited carbon nuclei are produced in a 9Be(α,n)12C* reaction. The state has a short lifetime of 61 fs and a deexcitation energy of Eγ = 4.44MeV. Additional γ-ray peaks are observed at energies of 3.9MeV (single escape peak, SEP) and 3.4MeV (double escape peak, DEP). Single and double escape peaks are the result of pair production.
A positron-electron pair can be produced in the detector material for γ-ray energies higher than 1.022MeV. The positron annihilates with an electron in two γ-rays. One (SEP) or both (DEP) γ-rays can leave the detector without being detected. The resulting peaks have a reduced energy of 511 keV or twice 511 keV. This is the energy shift of the original peak (full energy peak, FEP). Beside these γ-ray emissions the neutrons resulting from the reaction have a continuous energy distribution. The AmBe spectrum obtained from an example source is shown in Figure 2.1. Thermal neutrons are emitted from the nucleus. The higher the energy above 4.5MeV the less is the number of emitted neutrons. The supreme limit of the neutron energy is 11MeV . These high energy neutrons are not wanted because they have a low cross section for neutron capture reactions. Therefore neutrons are deaccelerated.
Figure 2.1: Neutron energy spectrum from a 370GBq AmBe neutron source normalised to unit flux, used in Section 3.1 to analyse neutron moderation, taken from Marsh et al. .
[Figure not part of this preview]
2.3 Moderation of neutrons
Table 2.1: Specifications of neutron energy distribution ranges, taken from .
[Table not part of this preview]
A moderator is a material which reduces the speed of (fast) neutrons mainly via elastic scattering. The energy of the neutron is emitted as heat in the medium. Because of the physics of collisions it works best with hydrogen because of the similar mass of the proton and the neutron. Since gaseous hydrogen is difficult to implement light or heavy water can be used as e.g. in nuclear power plants. For small experimental setups polyethylene is very handy. It is made of long chains of H-C-H components connected on carbon.
Neutrons have different mean energies. In the following work some specific terms are used for neutron energy ranges. An overview is given in Table 2.1. Thermal neutrons can also be referred as the term for the whole left column of the table without making a difference of this fine ranges.
Figure 2.2: Cross section behavior of the neutron capture reaction 139La(n,γ )140La depending on the neutron energy, taken from the Atlas of Neutron Capture Cross Sections .
[Figure not part of this preview]
2.4 Neutron capture reactions and high energy calibration sources
Neutron capture is a nuclear reaction. The mass number increases by one. Since neutrons carry no electric charge it is easy for them to enter the nuclei. Because of the fact, that no matter is emitted, the added binding energy is mostly emitted via gamma rays, sometimes with a beta decay or with fission. The less the neutron energy, the greater are the observed neutron cross sections [1, 2]. An exemplary cross section of 139La(n,γ )140La is shown in Figure 2.2. A good example for those thermal neutron capture reactions is 79Br(n,γ )80Br and 81Br(n,γ )82Br reactions. Through this reactions the nuclei gain an energy of 7892 keV 80Br and 7593 keV 82Br , respectively. With the ability to detect these lines an easy accessible high energy calibration point is given. The importance of this reaction is that the LaBr3 detector itself generates two high energy peak which is unique.
Besides of the complete emission of the gained energy bromine is also possible to de-excites through a cascade of states. There are not many information about these de-excitation and the cascade of states. This is a problem for later done simulation in the software package Geant4 .
Another example are chlorine capture reactions. 35Cl is a stable istope of chlorine. A neutron capture causes the reaction 35Cl(n,γ )36Cl with an energy of Eγ = 8580 keV . This high energy γ-ray serves as a high energy calibration point, too. Since chlorine is gaseos it is difficult to build a target. A very handy and cheap possibility is using dishwasher salt which contains sodium chloride (NaCl). Nickel can be used as well. The capture 58Ni(n,γ )59Ni results in γ-rays with Eγ = 8999.28 (5) keV. 60Ni(n,γ )61Ni emits an energy of Eγ = 7820.11 (5) keV .
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