Eu2 Doped CSBal3 and CsBa215 Scintillators for Gamma-Ray Spectroscopy
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AbstractThe proliferation of weapons of mass destruction such as nuclear missiles and “dirty bombs” is a serious threat in the world today. Preventing the spread of these nuclear weapons has reached a state of heightened urgency in recent years, more so since the events on September 11, 2001 and its aftermath. Gamma-ray spectrometers are an important tool in monitoring the proliferation of nuclear weapons. Important requirements for the gamma-ray spectrometers used for nuclear non-proliferation include high energy resolution, high detection efficiency, low cost and reasonably fast response. Currently, there are three classes of devices that are used for gammaray spectroscopy: 1) high purity germanium (HPGe) detectors, 2) room temperature semiconductors, and 3) inorganic scintillation crystals. Without doubt, HPGe detectors provide excellent energy resolution and are available in large sizes to provide good detection efficiency. However, HPGe detectors require cooling down to 100 K. Unfortunately, this makes gamma-ray spectroscopy systems based on HPGe detectors quite cumbersome, increases their cost, and makes them not readily portable. Room temperature semiconductors include detector materials such as cadmium telluride (CdTe) and cadmium zinc telluride (CZT). These detectors operate at room temperature and with unipolar charge sensing designs with 3-D correction, they can provide high-energy resolution (<1% FWHM at 662 keV) [He]. Despite decades of research, however, the uniformity of CZT crystals is still not very good, especially for large detector volume. As a result, the typical size of CZT spectrometers is generally limited to 1 - 3 cm3 which is barely adequate for many energetic radiations. Also, the cost per unit volume of CZT detectors is still quite high. Inorganic single crystal scintillators such as NaI:Tl and CsI:Tl provide reasonably high light yields and can be obtained in large sizes at moderate cost, but their energy resolution is poor, limited mostly by their highly non-proportional response [Dorenbos 01, Mengesha, Moses]. The lanthanide trihalide based materials (such as LaBr3:Ce, CeBr3 and LaCl3:Ce) while having the desired luminosity [van Loef01, Shah03 & 05, Menge] have proven difficult to produce at reasonable cost in the large sizes due to their intrinsic brittleness [Zhou] and highly anisotropic nature [Menge]. Also, the proportionality of these lanthanide halides is poor at lower energies, which degrades their energy resolution for gamma-ray emissions below 200 keV [Cherepy]. Recently, europium-doped barium bromo-iodides (BaBrI:Eu) and cesium-barium iodides (CsBaI3:Eu, CsBa2I5:Eu) have emerged as very promising scintillators for gamma-ray spectroscopy [Bourret-Courchesne, Bizzari, van Loef08, Glodo, Shah10]. These three scintillators have high densities and effective Z for the efficient detection of gamma-rays, see Table 1. Additionally, they exhibit very high light yields of up to 90,000 ph/MeV and have a reasonably fast scintillation decay time of less than 1 μs due to the d-f transition of Eu2+. Also, BaBrI:Eu, CsBaI3:Eu, and CsBa2I5:Eu show exceptionally good proportionality (better than LaBr3:Ce and NaI:Tl crystals) and consequently have very good energy resolution (<4% FWHM at 662 keV). The peak emission wavelength of BaBrI:Eu, CsBaI3:Eu, and CsBa2I5:Eu occurs at 413, 430, 435 nm, respectively, which is well-matched to bialkali photomultiplier tubes and silicon photodetectors. For all three compositions, their performance at energies below 200 keV is superior to that of NaI:Tl and LaBr3:Ce (e.g. at 60 keV, the energy resolution of CsBa2I5:Eu is 7% FWHM while that of LaBr3:Ce is 9.5% FWHM and NaI:Tl is >11% FWHM). In fact, the performance of these scintillators approaches that of semiconductor detectors such as CZT.Ultimately, as samples with higher optical quality are produced, we expect the energy resolution of these novel scintillators to supersede LaBr3:Ce over the entire energy range of interest for nuclear non-proliferation monitoring (few keV to >1 MeV). With respect to the crystal growth aspects, all three compositions have melting points well below 1000°C (see Table 1) but only BaBrI:Eu and CsBa2I5:Eu melt congruently while CsBaI3:Eu appears to melt incongruently. This implies that large single crystals of BaBrI:Eu and CsBa2I5:Eu can be grown relatively easily by melt based crystal growth techniques such as vertical Bridgman method. In contrast, the growth of CsBaI3:Eu would require techniques that support phase formation below its melting such as flux growth or ceramic consolidation. Note that for operating temperature below 1000°C standard crystal growth equipment can be used that is fitted with low-cost SiC or Nichrome heating elements and type K thermocouples. Higher melting temperatures would have required more expensive MoSi heating elements in combination with platinum thermocouples. Based on these considerations, we believe that BaBrI:Eu and CsBa2I5:Eu are the most attractive scintillation materials for gamma-ray spectroscopy studies (of the three compositions). As a result, the goal of the proposed Phase II effort is to grow large diameter (>1”) BaBrI:Eu and CsBa2I5:Eu crystals using the vertical Bridgman technique. Subsequently, the scintillation properties of BaBrI:Eu and CsBa2I5:Eu will be characterized. Based on the results of the 1st year effort, we will select the composition that appears to be more promising for further optimization and scale-up in the 2nd year of the Phase II effort. Finally, detector units employing the selected crystal material will be built and tested. Dr. Stephen Derenzo and Dr. Edith Bourret-Courchesne of Lawrence Berkeley National Laboratory (LBL) will participate in the characterization of these new scintillators in the Phase II effort on a fee-for-service basis.
* information listed above is at the time of submission.