Eu2 Doped CSBal3 and CsBa215 Scintillators for Gamma-Ray Spectroscopy
The 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.
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