BGO Properties of Bismuth Germanate - BGO Density [g/cm3] 7.13 Melting point [K] 1323 Thermal expansion coefficient [K-1] 7 x 10-6 Cleavage plane none Hardness (Mho) 5 Hygroscopic no Wavelength of emission maximum [nm] 480 Lower wavelength cutoff [nm] 320 Refractive index at emission maximum 2.15 Primary decay time [ns] 300 Temperature Response 1% K-¹ Light yield [photons/MeVg] 8 - 10 x 103 Photoelectron yield [% of NaI(Tl)] (for γ-rays) 15 - 20 General description Bismuth Germanate of the composition Bi4Ge3O12, also called BGO, is a high Z, high density scintillation material. Due to the high atomic number of bismuth (83) and its high density, BGO is a very efficient γ-ray absorber. Due to the high Z value of the material, the photofraction for γ-ray absorption is high, which results in a very good photopeak to Compton ratio. Fig. 4 shows the relative content of the total absorption photopeak as a function of the γ-ray energy for 38 mm diameter, 38 mm high (1.5" x 1.5") NaI(Tl) and Bismuth Germanate crystals. The scintillation emission maximum of BGO is situated at 480nm. Fig. 1 shows the emission spectrum. The scintillation light can be detected with standard photomultiplier tubes. The light yield (photons/MeV γ) is about 20-25% of NaI(Tl), but, since the emission is partly in the area above 500 nm where PM tubes are less sensitive, the relative photoelectron yield compared to NaI(Tl) amounts to 10-15%. Fig. 3 shows a pulse height spectrum obtained by irradiating a BGO crystal with 662 keV γ-rays. Due to its high density, the material is used where high stopping power is required. It is possible to read out Bismuth Germanate crystals with silicon photodiodes but due to the moderate light output, this is only useful for the detection of high energy particles, or photons of more than a few MeV. The scintillation intensity of Bismuth Germanate is a strong function of the temperature. Fig. 2 shows the relation. At room temperature, the gradient is approx. 1% K-1. The decay time of BGO is about 300 ns at room temperature, which is comparable to that of NaI(Tl). BGO scintillation crystals are susceptible to radiation damage starting at radiation doses between 1 and 10 Gray (102 - 103 rad). The effect is largely reversible. Since the radiation damage to BGO crystals depends on the presence of sub ppm impurities, large differences between individual crystals can occur. BGO is relatively hard, rugged, non-hygroscopic crystal which does not cleave. The material does not show any significant self absorption of the scintillation light. The crystal housing can be kept simple since no hermetic air-tight sealing is required. BGO can be machined to various shapes and geometries. BGO scintillation crystals are used in applications where a high photofraction is required (as in PET scanners) or because of its high density (as in Compton suppression spectrometers). Bicron has developed a proprietary technique to join or "weld" together two pieces of BGO. The joint has high mehanical strength and excellent optical transmission without the performance penalty of a glue joint. Long bars or extra wide sheets become available with this technology. Bismuth Germanate - BGO PDF The following abstract is reprinted from Nuclear Instruments and Methods with permission from Elsevier Science. This paper compares nine irradiated BGO test crystals from several different sources, including Crismatec. The conclusion drawn was that Crismatec crystals were the radiation hardest for all types of radiation. They also recover much faster to their original performance. Please refer to Nuclear Instruments and Methods in Physics Research A 413 (1998) 50-58. The complete paper can be ordered through a document service such as EmDocs. Influence of radiation damage on BGO scintillation properties R. Georgiia*, R. Meibla, W. Hajdasb, H. Henschelc, H.D. Grafd, G.G. Lichtia, P. von Neumann-Coseld, A. Richterd, V. Schonfeldera aMax-Planck Institut fur Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany bPaul-Scherrer Institut, CH-5232 Villingen, Switzerland cFraunhofer-Institut fur naturwissenchaftlich-technische Trendanalyse, D-53879 Euskirchen, Germany dInstitut fur Kernphysik, Technische Universitat Darmstadt, D-64289 Darmstadt, Germany Received 9 March 1998 Abstract Aboard INTEGRAL, the next medium-size gamma ray mission of the European Space Agency (ESA), a high-resolution Ge-spectrometer array with a BGO anticoincidence shield and imaging capability will be flown. The influence of the radiation damage on the photoelectron yield of the BGO scintillators due to the radiation environment in the orbit, i.e. gamma-rays electrons and protons from the radiation, belts and the cosmic diffuse radiation, was investigated. Irradiation tests with doses equivalent to the orbit conditions were performed and the photoelectron yields of different BGO crystals were measured. It was found that for equal doses the reduction of the photoelectron yield varies strongly for the crystals of different manufacturers. Furthermore, electromagnetic radiation affects the photoelectron yield much stronger than particle radiation. A possible explanation is given by interpreting the effect due to the gamma-rays primarily as damage of the electronic structure of the BGO, whereas the particle radiation damages mainly the crystal structure. ®1998 Elsevier Science B.V. All rights reserved. Keywords: INTEGRAL; gamma-rays; BGO scintillator; Radiation damage; Space environment; Photoelectron yield [ Scintillator options available ] Organic Products | Inorganic Scintillation Products | Crystal Products Bicron RMP | Optics and Oxides | Crismatec | Product Search/Faq Company Profile | News | Contact Us | NE Technology | Home