Hans Kraus

Abstract:

We studied the feasibility of using sensitization for the improvement of the VUV-to-visible conversion efficiency of red-emitting pentaborate phosphor. It is shown that Tb 3+ ions that exhibit a strong absorption throughout the 160-230 nm wavelength range facilitate enhancement of Eu3 emission in TbM gB 5O 10-Eu at excitation in this spectral range. The maximum increase of the Eu 3+ emission efficiency in a pentaborate host at excitation with 150 nm photons is ca.10%. It is also shown that substitution of Zn for Mg in Gd(Mg,Zn)B 5O 10- Mn phosphor results in strong (up to 2.4 times) enhancement of the intensity of Mn 2+ emission. A possible reason for the observed host sensitization effect is discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

The possibility to use Tb3+ as luminescence sensitizer for enhancement of the conversion efficiency of vacuum-ultraviolet (VUV) radiation into visible light was examined. We studied the luminescence properties of K3Tb(PO4)2 and Ba3Tb(PO4)3 activated by Eu3+, and of SrAl12O19 co-doped with Mn2+ and Tb3+ at excitation over the 120 to 300nm wave-length range. It is shown that Tb3+ ions, exhibiting a strong absorption band in the VUV, can provide efficient sensitization of Eu3+ and Mn2+ emissions for excitation in this spectral range, giving rise to intense red and green luminescence, respectively. This study provides a proof for the concept of VUV sensitization, which enables the engineering of luminescence materials with improved efficiency for excitation from a noble gas discharge. © Taylor & Francis Group, LLC.

Abstract:

An increasing number of applications of scintillators at low temperatures, particularly in cryogenic experiments searching for rare events, has motivated the investigation of scintillation properties of materials over a wide temperature range. This paper provides an overview of the latest results on the study of luminescence, absorption and scintillation properties of materials selected for rare event searches so far. These include CaWO4, ZnWO4, CdWO4, MgWO4, CaMoO4, CdMoO4, Bi4Ge3O12, CaF2, MgF2, ZnSe and Al2O3-Ti. We discuss the progress achieved in research and development of these scintillators, both in material preparation and in the understanding of scintillation mechanisms, as well as the underlying physics. To understand the origin of the performance limitation of self-activated scintillators we employed a semi-empirical model of conversion of high energy radiation into light and made appropriate provision for effects of temperature and energy transfer. We conclude that the low-temperature value of the light yield of some modern scintillators, namely CaWO4, CdWO4 and Bi4Ge312, is close to the theoretical limit. Finally, we discuss the advantages and limitations of different materials with emphasis on their application as cryogenic phononscintillation detectors (CPSD) in rare event search experiments. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

Future cryogenic experiments searching for rare events require reliable, efficient and robust techniques for the detection of photons at temperatures well below that to which low-temperature photomultipliers (PMT) were characterised. Motivated by this, we investigated the feasibility of a low-temperature PMT for the detection of scintillation from crystalline scintillators at T=6 K. The scintillation module was composed of a CaWO 4 scintillator and a low-temperature PMT D745B from ET Enterprises. The PMT responsivity was studied at T=290, 77 and 6 K using γ-quanta from 241Am (60 keV) and 57Co (122 and 136 keV) sources. We have shown that the low-temperature PMT retains its single photon counting ability even at cryogenic temperatures. At T=6 K, the response of the PMT decreases to 51±13% and 27±6%, when assessed in photon counting and pulse height mode, respectively. Due to the light yield increase of the CaWO 4 scintillating crystal, the overall responsivity of the scintillation modules CaWO4PMT is 94±15% (photon counting) and 48±8% (pulse height) when cooling to T=6 K. The dark count rate was found to be 20 s-1. The energy resolution of the module remains similar to that measured at room temperature using either of the detection modes. It is concluded that commercially available low-temperature PMT are well suited for detection of scintillation light at cryogenic temperatures. © 2010 Elsevier B.V. All rights reserved.

Abstract:

The current status of the direct Dark Matter experiments CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) and the planned EURECA (European Underground Rare Event Calorimeter Array) is presented. Both experiments are aimed at the direct detection of WIMPs (Weakly Interacting Massive Particles), potential candidates for the Dark Matter in the universe. New design developments of the cryogenic detectors operated at mK temperatures are investigated to optimize detector performance and to simplify mass production. Thus, CRESST is also providing a basis for the EURECA project, aimed at a ton of cryogenic detectors with a multi-material target. © 2010 Elsevier B.V. All rights reserved.