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Collective Structure of Neutron-Rich Rare-Earth Nuclei and Development of Instrumentation for Gamma-Ray Spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Nuclear Physics. (Nuclear structure group)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Neutron-rich rare-earth nuclei are among the most collective nuclei that can be found in nature. In particular, the doubly mid-shell nucleus 170Dy is expected to be the nucleus where the collective structure is maximized. This has implications for the astrophysical r-process, since it has been suggested that the collectivity maximum plays an important role in the abundances of the rare-earth elements that are created in supernova explosions. In this work, the collective structure of the five nuclei 168,170Dy and 167,168,169Ho are studied and different theoretical models are used to interpret the evolution of collectivity around the mid-shell. In order to produce and study even more neutron-rich nuclei in this mass region, new radioactive ion beam facilities will be a valuable tool. These facilities, however, require advanced instruments to study the weak signals of exotic nuclei in a high background environment. Two of these instruments are the γ-ray tracking spectrometer AGATA and the neutron detector array NEDA. For AGATA to work satisfactorily, the interaction position of the gamma rays must be determined with an accuracy of at least five millimetres. The position resolution is measured in this work using a model independent method based on the Doppler correction capabilities of the detector at two different distances between the detector and the source. For NEDA, one of the critical parameters is its ability to discriminate between neutrons and γ rays. By using digital electronics it is possible to employ advanced and efficient algorithms for pulse-shape discrimination. In this work, digital versions of the common analogue methods are shownto give as good, or better, results compared to the ones obtained using analogue electronics. Another method which effectively distinguishes between neutrons and γ rays is based on artificial neural networks. This method is also investigated in this work and is shown to yield even better results.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2011. , 122 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 818
Keyword [en]
nuclear structure, collective models, 150 < A < 189, AGATA, HPGe detectors, gamma-ray tracking, NEDA, pulse-shape discrimination, neutrons, neural network
National Category
Subatomic Physics
Research subject
Physics with specialization in Nuclear Physics
Identifiers
URN: urn:nbn:se:uu:diva-149772ISBN: 978-91-554-8056-1OAI: oai:DiVA.org:uu-149772DiVA: diva2:407152
Public defence
2011-05-20, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2011-04-29 Created: 2011-03-22 Last updated: 2011-05-05Bibliographically approved
List of papers
1. Spectroscopy of neutron-rich Dy-168,Dy-170: Yrast band evolution close to the NpNn valence maximum
Open this publication in new window or tab >>Spectroscopy of neutron-rich Dy-168,Dy-170: Yrast band evolution close to the NpNn valence maximum
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2010 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 81, no 3, 034310- p.Article in journal (Refereed) Published
Abstract [en]

The yrast sequence of the neutron-rich dysprosium isotope Dy-168 has been studied using multinucleon transfer reactions following collisions between a 460-MeV Se-82 beam and an Er-170 target. The reaction products were identified using the PRISMA magnetic spectrometer and the gamma rays detected using the CLARA HPGe-detector array. The 2(+) and 4(+) members of the previously measured ground-state rotational band of Dy-168 have been confirmed and the yrast band extended up to 10(+). A tentative candidate for the 4(+) -> 2(+) transition in Dy-170 was also identified. The data on these nuclei and on the lighter even-even dysprosium isotopes are interpreted in terms of total Routhian surface calculations and the evolution of collectivity in the vicinity of the proton-neutron valence product maximum is discussed.

National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-136596 (URN)10.1103/PhysRevC.81.034310 (DOI)000276769400026 ()
Available from: 2010-12-13 Created: 2010-12-13 Last updated: 2011-05-05Bibliographically approved
2. The neutron rich isotopes 167,168,169Ho studied in multi-nucleon transfer reactions
Open this publication in new window or tab >>The neutron rich isotopes 167,168,169Ho studied in multi-nucleon transfer reactions
(English)Manuscript (preprint) (Other academic)
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-149771 (URN)
Available from: 2011-03-22 Created: 2011-03-22 Last updated: 2011-05-05
3. Interaction position resolution simulations and in-beam measurements of the AGATA HPGe detectors
Open this publication in new window or tab >>Interaction position resolution simulations and in-beam measurements of the AGATA HPGe detectors
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2011 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, Vol. 638, no 1, 96-109 p.Article in journal (Refereed) Published
Abstract [en]

The interaction position resolution of the segmented HPGe detectors of an AGATA triple cluster detector has been studied through Monte Carlo simulations and in an in-beam experiment. A new method based on measuring the energy resolution of Doppler-corrected γ-ray spectra at two different target to detector distances is described. This gives the two-dimensional position resolution in the plane perpendicular to the direction of the emitted γ-ray. The γ-ray tracking was used to determine the full energy of the γ-rays and the first interaction point, which is needed for the Doppler correction. Five different heavy-ion induced fusion-evaporation reactions and a reference reaction were selected for the simulations. The results of the simulations show that the method works very well and gives a systematic deviation of in the FWHM of the interaction position resolution for the γ-ray energy range from 60 keV to 5 MeV. The method was tested with real data from an in-beam measurement using a 30Si beam at 64 MeV on a thin 12C target. Pulse-shape analysis of the digitized detector waveforms and γ-ray tracking was performed to determine the position of the first interaction point, which was used for the Doppler corrections. Results of the dependency of the interaction position resolution on the γ-ray energy and on the energy, axial location and type of the first interaction point, are presented. The FWHM of the interaction position resolution varies roughly linearly as a function of γ-ray energy from 8.5 mm at 250 keV to 4 mm at 1.5 MeV, and has an approximately constant value of about 4 mm in the γ-ray energy range from 1.5 to 4 MeV.

Keyword
gamma-ray tracking, AGATA, Monte Carlo simulations, HPGe detectors, Fusion-evaporation reactions
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-106820 (URN)10.1016/j.nima.2011.02.089 (DOI)000290082600015 ()
Available from: 2011-03-22 Created: 2009-07-05 Last updated: 2013-05-17Bibliographically approved
4. Digital pulse-shape discrimination of fast neutrons and gamma rays
Open this publication in new window or tab >>Digital pulse-shape discrimination of fast neutrons and gamma rays
2008 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, Vol. 594, no 1, 79-89 p.Article in journal (Refereed) Published
Abstract [en]

Discrimination of the detection of fast neutrons and γ rays in a liquid scintillator detector has been investigated using digital pulse-processing techniques. An experimental setup with a 252Cf source, a BC-501 liquid scintillator detector, and a BaF2 detector was used to collect waveforms with a 100 Ms/s, 14 bit sampling ADC. Three identical ADCs were combined to increase the sampling frequency to 300 Ms/s. Four different digital pulse-shape analysis algorithms were developed and compared to each other and to data obtained with an analogue neutron–γ discrimination unit. Two of the digital algorithms were based on the charge comparison method, while the analogue unit and the other two digital algorithms were based on the zero-crossover method. Two different figure-of-merit parameters, which quantify the neutron–γ discrimination properties, were evaluated for all four digital algorithms and for the analogue data set. All of the digital algorithms gave similar or better figure-of-merit values than what was obtained with the analogue setup. A detailed study of the discrimination properties as a function of sampling frequency and bit resolution of the ADC was performed. It was shown that a sampling ADC with a bit resolution of 12 bits and a sampling frequency of 100 Ms/s is adequate for achieving an optimal neutron–γ discrimination for pulses having a dynamic range for deposited neutron energies of 0.3–12 MeV. An investigation of the influence of the sampling frequency on the time resolution was made. A FWHM of 1.7 ns was obtained at 100 Ms/s.

Keyword
digital pulse-shape discrimination, fast-neutron detection, liquid scintillator, BC-501, sampling ADC
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-17672 (URN)10.1016/j.nima.2008.06.004 (DOI)000259021800014 ()
Available from: 2008-08-14 Created: 2008-08-14 Last updated: 2011-05-05Bibliographically approved
5. An artificial neural network based neutron-gamma discrimination and pile-up rejection framework for the BC-501 liquid scintillation detector
Open this publication in new window or tab >>An artificial neural network based neutron-gamma discrimination and pile-up rejection framework for the BC-501 liquid scintillation detector
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2009 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, Vol. 610, no 2, 534-539 p.Article in journal (Refereed) Published
Abstract [en]

BC-501 is a liquid scintillation detector sensitive to both neutrons and gamma rays. As these produce slightly different signals in the detector, they can be discriminated based on their pulse shape (Pulse Shape Discrimination, PSD). This paper reports on results obtained with several PSD techniques and compares them with a method based on artificial neural networks (NN) developed for this application. Results indicated a large performance advantage of NN especially in the region of small deposited energy which typically contains the majority of the events. NN were also applied for discrimination of pile-up events with good results. This framework can be implemented on some of the most recent programmable data acquisition cards and it is suitable for real-time application.

Keyword
BC-501, NE213, BC-501A, Neural networks, PSD, Pulse shape discrimination, Neutron gamma discrimination, Liquid scintillator, Cf-252, Time of flight
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-148206 (URN)10.1016/j.nima.2009.08.064 (DOI)000273240800008 ()
Available from: 2011-03-03 Created: 2011-03-03 Last updated: 2011-05-05Bibliographically approved

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