Alan Owens

Publications (290) View all

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  Available from: Alan Owens

  Article: Picosecond dynamics of hot carriers and phonons and scintillator non-proportionality

  A. Kozorezov, J. K. Wigmore, A. Owens
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  ABSTRACT: We have developed a model describing the non-proportional response in scintillators based on non-thermalised carrier and phonon transport. We show that the thermalization of e-h distributions produced in scintillators immediately after photon absorption may take longer than the period over which the non-proportional signal forms. The carrier and LO-phonon distributions during this period remain non-degenerate at quasi-equilibrium temperatures far exceeding room temperature. We solve balance equations describing the energy exchange in a hot bipolar plasma of electrons/holes and phonons. Taking into account dynamic screening we calculate the ambipolar diffusion coefficient at all temperatures. The non-proportional light yields calculated for NaI are shown to be consistent with experimental data. We discuss the implications of a non-equilibrium model, comparing its predictions with a model based on the transport of thermalised carriers. Finally, evidence for non-equilibrium effects is suggested by the shape of non-proportionality curve and wide dispersion in data observed in K-dip spectroscopy near the threshold. A comparison of the predicted curves shows good agreement for deformation potential value in the range 7-8 eV.
  Journal of Applied Physics 09/2012; 112(5):053709. · 2.17 Impact Factor
• Article: Neutron sources for test and calibration of neutron detectors for space research

  C. Granja, M. Kralik, Z. Kohout, P. Masek, S. Pospisil, M. Solar, J. Solc, Z. Vykydal, A. Owens, J. Vacik, D. Chvatil, P. Bem, P. Krist, J. Stursa, V. Rypar
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  ABSTRACT: This work reports on the current status of neutron sources in the Czech Republic as calibrated and ESA compliant stations for space related applications such as the testing and calibration of neutron detectors and neutron sensitive devices as well as for studies of radiation effects of electric and electronic components. The work was carried out as part of the preparatory accession activities of the Czech Republic with ESA. The goal and one of the tasks is to test and evaluate neutron sensitive devices, both conventional and prototypes, developed at ESA and at the IEAP CTU Prague. The facilities consist of both fast and thermal neutron sources providing testing and absolute calibration of a wide range of neutron detectors. The evaluation and calibration of the sources were carried out in cooperation with the Nuclear Physics Institute, Academy of Sciences of the Czech Republic, the Research Center Rez, and the Czech Metrology Institute in Prague
  AIP Conference Proceedings 01/2012; 1423:446.
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  Available from: Conny C.T. Hansson

  Conference Proceeding: Development of low noise scintillator crystals for planetary space missions

  C.C.T Hansson, A. Owens, B. Shortt, P. Dorenbos, F. Quarati, R. Williams, D. Hahn, T. Toepfer, L. Pathier, P. Schotanus, J. v.d. Biezen, K. N. O'Neill, C. Jackson, L.Wall
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  ABSTRACT: The recent development of large volume lanthanum halides (LaBr3 and LaCl3), has made scintillator based detectors a viable option for use as high resolution Ȗ-ray spectrometers for a large numberof space applications. The main drawback of both LaCl3 and LaBr3 is the high background noise inherent to the crystal due to the decay of radioactive 138La, which comprises 0.09% of naturally occurring stable lanthanum (139La). This paper describes the development of a low noise equivalent to LaBr3, namely CeBr3. Crystal sizes up to 3” have successfully been grown. The energy resolution of CeBr3(4.1% FWHM at 662keV) was found to be slightly worse than that observed for LaBr3 (2.8% FWHM at 662keV) but a measured relative decrease of almost 30 times in internal activity makes it an attractive alternative when low noise operation is required. In parallel to the CeBr3 development, a SiliconPhoto Multiplier (SPM) aimed at having a response matched to both LaBr3 and CeBr 3 has also been developed. Preliminary results are presented.
  IEEE Nuclear Science Symposium 2012, Anaheim, CA, USA; 01/2012
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  Available from: Alan Owens

  Article: Wide energy range gamma-ray calibration source

  M Kroupa, C Granja, Z Janout, M Kralik, F Krejci, A Owens, S Pospisil, F Quarati, J Solc, M Vobecky
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  ABSTRACT: Calibration source with monoenergetic gamma-ray lines in wide energy range designed for gamma-ray detector energetic calibration and testing has been built. Gamma-rays are obtained from thermal neutron capture, which is a suitable and cost efficient way how to provide discrete gamma-ray lines with energies above 3 MeV with reasonable intensity. With appropriate and interchangeable targets the source can generate different gamma-ray spectra with energy up to 10 MeV. We present the data obtained with neutron capture on chlorine, but other elements with high thermal neutron capture cross-section such as chrome, iron, nickel and titanium can be used as well. As neutron source we employ radionuclide sources (252Cf or 241Am-Be) with emission rate about 106 neutrons/s. The emitted fast neutrons are moderated by a moderator made of light materials such as graphite, standard water or heavy water. Performance of the source is demonstrated by calibration spectra measured by HPGe and scintillation detectors (LaBr3, NaI(Tl)).
  Journal of Instrumentation 11/2011; 6(11):T11002. · 1.87 Impact Factor
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  Available from: Alan Owens

  Article: Polarization effects in thallium bromide x-ray detectors

  A Kozorezov, V Gostilo, A Owens, F Quarati, M Shorohov, M A Webb, J K Wigmore
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  ABSTRACT: We present the results of a detailed experimental study of polarization effects in thallium bromide planar x-ray detectors. Measurements were carried out in the range 10–100 keV by scanning a highly focused x-ray beam, 50 m in diameter, from a synchrotron source across the detector. Above a certain radiation threshold the detector response showed a systematic degradation of its spectroscopic characteristics, peak channel position, peak height, and energy resolution. Using a pump-and-probe technique, we studied the dynamics of spectral degradation, the spatial extent and relaxation of the polarized region, and the dependence of the detector response on bias voltage and temperature. For comparison, we modeled polarization effects induced by the charging of traps by both electrons and holes using a model based on recent theoretical work of Bale and Szeles. We calculated the charge collection efficiency and spectral line shapes as functions of exposure time, beam position, count rate, and photon energy, and obtained credible agreement with experimental results. © 2010 American Institute of Physics.
  Journal of Applied Physics 03/2010; 108:064507. · 2.17 Impact Factor