BGO Crystals Research Articles

Arbitrary-to-linear or linear-to-arbitrary polarization controller based on Faraday and Pockels effects in a single BGO crystal

We propose an arbitrary-to-linear or linear-to-arbitrary polarization controller based on the mutual action of Faraday and Pockels effects in a single Bi(4)Ge(3)O(12) (BGO) crystal after the wave coupling theory describing these two effects. It is demonstrated that, the expected conversion of arbitrary-to-linear or linear-to-arbitrary polarization state of light can be realized by adjusting the applied electric and magnetic fields.

High temperature Raman spectra and micro-structure of Bi4Ge3O12 crystal and its melt

The High temperature Raman spectra of BGO crystal and melt were studied. The structures of BGO crystal and melt were analyzed. The rule of structure change of BGO crystal with increasing temperature was analyzed. The results show that the peaks of spectra move to the low wavelength and broaden in different degrees. The intensities of spectra peaks decrease, too. And there exit ［GeO4］ and ［BiO6］ bond structure in the melt of BGO crystal, while the bond between ［GeO4］ and ［BiO6］ bond structures disappear, so those two structures are unattached in the melt.

Development of next-generation GRB detector based on the silicon drift detector and scintillator

We propose a next-generation GRB detector combining the Silicon Drift Detector (SDD) array and scintillators with broad-band coverage (0.5-1000 keV or more), high energy resolution (2-10%) and high time resolution (∼ μs). To realize such compact high-performance detector without photomultiplier tubes, we constructed a proto-type model using KETEK SDD with a detection area of 1cm 2 and BGO crystal. Signals from both detectors are clearly separated by the double-integration method. The detector shows a very good performance, although further refinement is in progress. Obtained FWHM energy resolution was 200 eV at 5.9keV in the SDD, while 6.5% at 662 keV in the BGO at -30 degree C.

Design considerations and construction of a small animal PET prototype

We are developing a small animal PET scanner consisting of two block detectors, each made of 216 BGO crystals of dimensions 3.75 mm×3.75 mm×20 mm, cylindrically arranged and coupled to a position-sensitive photomultiplier tube (R2486 PSPMT). Our design was based on a very detailed Monte Carlo, that simulates the function of a PET scanner from the system level down to the individual γ-ray detectors. We have made laboratory measurements of the individual detector performance as well as measurements of characteristics of the PSPMTs. The two detector blocks which will form the basic tomographic unit have been assembled. We are developing electronics to individually process (amplify and digitize) anode signals, and use field programmable gate arrays (FPGAs) in the position determination and energy measurement of the γ-rays. At present, as an intermediate step, we are using the electronics supplied from Hamamatsu to study various aspects of the system and produce initial images.

Optical properties of single crystalNd3+-dopedBi4Ge3O12: Laser transitions at room and low temperature

A ${\mathrm{Nd}}^{3+}$-doped ${\mathrm{Bi}}_{4}{\mathrm{Ge}}_{3}{\mathrm{O}}_{12}$ (BGO) single crystal was grown by the modified Czochralski method. Using the $4f$-energy matrix diagonalization procedure various interaction parameters viz. Slater-Condon (${F}^{2}$, ${F}^{4}$, and ${F}^{6}$), spin-orbit $(\ensuremath{\xi})$, two body interaction ($\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$), Judd parameter (${T}^{2}$, ${T}^{3}$, ${T}^{4}$, ${T}^{6}$, ${T}^{7}$, and ${T}^{8}$), spin-other-orbit parameters (${M}^{0}$, ${M}^{2}$, and ${M}^{4}$) and electrostatically correlated spin-orbit interaction parameters (${P}^{2}$, ${P}^{4}$, and ${P}^{6}$), and the crystal-field parameters $({B}_{q}^{K})$ were evaluated. The crystal-field analysis has also been carried out for ${\mathrm{Nd}}^{3+}:{\mathrm{Bi}}_{4}{\mathrm{Si}}_{3}{\mathrm{O}}_{12}$, ${\mathrm{Eu}}^{3+}:\mathrm{BGO}$, and ${\mathrm{Er}}^{3+}:\mathrm{BGO}$ systems. The potential of a BGO crystal as a laser crystal for 1064.4 and $1341.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ was established by quantitative analysis of the absorption, emission spectrum, and fluorescence decay characteristics. Judd-Ofelt parametrization was employed to compute the radiative spectral parameters such as radiative transition probabilities, fluorescence branching ratios, stimulated emission cross sections, and quantum efficiencies of the observed bands in the fluorescence spectrum. Using the measured radiative properties, 100% quantum efficiency was obtained for the principal emission band at $1064.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ with an effective stimulated emission cross section of $1.34\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$. The high stimulated emission cross section, fluorescence branching ratio, and quantum efficiency indicate that the ${\mathrm{Nd}}^{3+}$-doped BGO crystal can be a suitable host for 1064.4 and $1341.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ emission wavelengths in $Q$-switched laser applications.

Preliminary evaluation of four-layer BGO DOI-detector for PET

We found that Bi/sub 4/Ge/sub 3/ O/sub 12/ (BGO) scintillator can be elements of a four-layer depth of interaction (DOI) detector and it was proved with a 12/spl times/12/spl times/4 array of BGO crystals in dimensions of 2.9 mm/spl times/2.9 mm/spl times/7.5 mm coupled to a 256-channel flat panel position sensitive photomultiplier tube. Appropriate reflector insertion in the array makes all crystal identification possible on one position histogram. Despite the large refractivity and small light output of BGO, the four-layer BGO detector showed no significant variation in the full energy peaks among all crystal elements. When no optical grease was used in the construction of the BGO DOI-block and irradiated with gamma-rays from /sup 137/Cs, a top layer crystal has 80% of light output relative to the bottom layer. The obtained two-dimensional position histogram by the irradiation was clear enough to allow identification of the crystals of interaction. Profiles of the histogram show peak-to-valley ratio of 1.9:1 for the top layer crystals and larger ratio for other layer crystals in the experiment.

Development of the HXD-II wide-band all-sky monitor onboard Astro-E2

The hard X-ray detector (HXD-II) is one of the three scientific instruments onboard Japanese X-ray astronomy satellite Astro-E2 scheduled to be launched in 2005. This mission is very unique in a point of having a lower background than any other past missions in the 10-600 keV range. In the HXD-II, the large and thick BGO crystals are used as active shields for particle and gamma-ray background to the main detector. They have a wide field of view of ~2pi and a large effective area of 400 cm2 even at 1 MeV. Hence, the BGO shields have been developed as a wide-band all-sky monitor (WAM) with a broadband coverage of 50-5000 keV. In this paper, overall design and performance of the HXD-II/WAM based on the results of preflight calibration tests carried out in June 2004 are described. By irradiating various radio isotopes with the WAM flight model, we verified that it had comparable capabilities with other gamma-ray burst detectors

In-Flight status and performances of the detectors on board the INTEGRAL spacecraft

INTEGRAL is an observatory of the European Space Agency (ESA) dedicated to the study of the Gamma Ray sky. It features many different detector technologies: imaging micro-strip gas chambers, germanium crystals, cadmium telluride pixels, cesium iodide pixels, BGO crystals, silicon diodes, CCD optical camera. The measured stability and performances of those detectors have been for the moment according specifications. The noise level is slightly worse then expected. All the surrounding infrastructure of the detectors (orbit, satellite services, communications, ground segment) has exceeded specifications.

Influence of non‐stoichiometry and doping on scintillating response of PbWO 4 crystals

Systematic scintillation response studies dealing with photoelectron yield (N p h e l s (E)), energy resolution (En.Res.(E)) and radio-luminescence (RL) measurements were accomplished on non-stoichiometric and/or (Ba, Mo) doped PbWO 4 single crystals. We used a new experimental set-up with a Hybrid Photomultiplier (HPMT) to measure γ-ray pulse height spectra of some lines in the energy range 500 - 1300 keV. Scintillation responses of PbWO 4 crystals were compared with that of a BGO crystal of the same shape and dimensions. PbWO 4 crystals, which show mainly blue emission, (λ p 410 nm) provide N p h e l s (E) between 2.4 - 4.4 % of BGO. In non-stoichiometric PbWO 4 ( 49 % of PbO in the melt) green emission (λ p 500 nm) dominates and N p h e l s (E) reaches 8 % of BGO. Non-stoichiometric crystals doped with Mo show the highest N p h e l s (E) (almost 10 % of that of BGO) while En.Res. at 662 keV reaches 20 %.

Stoichiometry and doping induced modifications in the properties of Bi 12SiO 20 single crystals

Bismuth silicon oxide (Bi 12SiO 20) single crystals were grown from the melts with different SiO 2 mole percentages and with different dopants by Czhochralski technique. The absorption coefficient for these crystals depends on the stoichiometry of the melt composition. Photoluminescence emission spectra for the grown crystals were measured from 80 to 293 K. The spectra present a broad structureless peak at 80 K. The interband transitions for different stoichiometric crystals were analysed and the behavior seems to be independent of composition unlike the bismuth germanate crystals. The three level interband model proposed for the BGO crystals completely explains the interband transitions in BSO crystals also.

Study of large area Hamamatsu avalanche photodiode in a [formula omitted]-ray scintillation detector

We have carried out study of a large area ( 10 × 10 mm 2 ) , reverse-type avalanche photodiode (APD) recently developed by Hamamatsu photonics. It has low dark current of 3 nA at room temperature, and the gain stability was almost the same as prototypical APDs reported in our previous paper. We studied the performance as a γ -ray detector with four scintillators, CsI(Tl), BGO, GSO(Ce), and YAP(Ce) crystals. For example we obtained the best energy resolution of 4.9 ± 0.2 % (FWHM) for 662 keV γ -rays, as measured with a 10 × 10 × 10 mm 3 CsI(Tl) crystal. The minimum detectable energy was as low as 10 keV at 20 ∘ C and 3.1 keV at - 20 ∘ C . Thanks to its large effective area, this APD can effectively read out photons from larger size scintillators. When coupling to a 300 × 48 mm 2 BGO plate of 3 mm thickness, an FWHM energy resolution of 20.9 ± 0.2 % was obtained for 662 keV γ -rays, with the minimum detectable energy of about 60 keV at - 15 ∘ C . These results suggest that our prototype APD can be a promising device for various applications replacing traditional PMTs such as use in space for Japan's future X-ray astronomy mission NeXT.

A fast and compact electromagnetic calorimeter for the PANDA detector at GSI

PANDA is a 4π detector planned for the antiproton storage ring at the proposed International Accelerator Facility for Beams of Ions and Antiprotons at the GSI. Experiments foreseen with the antiproton beam of extremely high luminosity and excellent beam quality up to 15GeV/c beam momentum require an electromagnetic calorimeter of high granularity and resolution to measure photons of several GeV down to 10MeV energy. The scintillator materials being investigated for this calorimeter are BGO and new improved PbWO4 crystals. For the detection of the scintillation light Avalanche Photo Diodes are being tested. The concept of the calorimeter as well as initial results from the R&D program are presented.

Radiation resistance of heavy scintillators to low-energy gamma-rays

Radiation resistance of BGO, fluoride (BaF 2, CeF 3) and tungstate (PbWO 4, CdWO 4) heavy scintillation crystals due to low-energy gamma-rays has been studied for doses up to 10 5 Gy. Results are discussed in terms of degradation of transmission and appropriated induced absorption coefficients. The recovery time has been estimated from measurement of transmission spectra during 0.5–15 days after irradiations.

Intrinsic energy resolution and light yield nonproportionality of BGO

The intrinsic energy resolution and nonproportionality of the light yield as a function of gamma ray energies, in the energy range of 14 keV to 1.33 MeV, were studied for small BGO (Bi/sub 4/Ge/sub 3/O/sub 12/) crystals at room and liquid nitrogen (LN/sub 2/) temperatures. The study showed that the intrinsic resolution of BGO and the light yield nonproportionality as a function of energy do not depend on the crystal temperature. High light outputs of 14000/spl plusmn/300 electron-hole pairs and energy resolution of 6.5% /spl plusmn/ 0.2% for 662keV gamma rays were measured with the 9 mm diameter, 4 mm thick crystals, coupled to large area avalanche photodiodes and cooled down to LN/sub 2/ temperature. Special attention was paid to analyzing the energy resolution of the escape peaks, which were well separated from the full-energy peaks due to the good energy resolution of BGO at LN/sub 2/ temperatures and the energy of bismuth KX-rays. The intrinsic energy resolution of the BGO crystal for escape peaks does not show deviations from analysis of total energy absorption photopeaks. This is in spite of the fact that the contributions of X-rays and Auger electron cascade in creation of escape peaks are much smaller than in full-energy peaks. In the small volume crystal, mainly electrons produced in the photoelectric absorption create the escape peaks.

The influence of temperature gradient on energy resolution of Bi 4Ge 3O 12 (BGO) crystal

In this work, we grew Bi 4Ge 3O 12 (BGO) crystals of two types by Bridgmen–Stockbarger technology in different temperature gradients and compared their optical transmissions and energy resolutions before and after ultraviolet irradiation. It was found that the optical transmissions and energy resolutions of the two types of BGO crystals are similar before UV-irradiation. After UV-irradiation, the optical transmissions at 480 nm of the first type of BGO crystals which were grown in higher temperature gradient cut down about 20% and their energy resolutions obviously deteriorated from about 12–18%. But the optical transmissions and energy resolutions of the second type of BGO crystals which were grown in lower temperature gradient keep unchanged on the whole. After 250 °C/4 h annealing, the optical transmissions and energy resolutions of the second type of BGO crystals can be completely recovered, whereas the optical transmissions and energy resolutions of the first type of BGO crystals cannot be completely recovered until through 600 °C/8 h annealing. The reason for the difference between the first and second type of BGO crystals is discussed.

INTEGRAL Spectrometer SPI's GRB detection capabilities

The spectrometer SPI, one of the two main instruments of the INTEGRAL spacecraft, offers significant gamma-ray burst detection capabilities. In its 35° (full width) field of view SPI is able to localise gamma-ray bursts at a mean rate of ~0.8/month. With its large anticoincidence shield of 512 kg of BGO crystals SPI is able to detect gamma-ray bursts quasi omni-directionally with a very high sensitivity. Burst alerts of the anticoincidence shield are distributed by the INTEGRAL Burst Alert System. In the first 8 months of the mission about 0.8/day gamma-ray burst candidates and 0.3/day gamma-ray burst positions were obtained with the anticoincidence shield by interplanetary network triangulations with other spacecrafts.

Comparison of LuYAP, LSO, and BGO as scintillators for high resolution PET detectors

For high resolution positron emission tomographs based on scintillation detectors a fast, dense, and bright scintillator is required. A sample of a new scintillator, Lu/sub 0.8/Y/sub 0.2/AlO/sub 3/:Ce (LuYAP) with a density of 7.7 g/cm/sup 3/ and a scintillation decay time of 20 and 160 ns is compared with LSO and BGO crystals of the same size to estimate the potential of the crystal for high resolution PET detectors. Special attention was paid to use an application specific measurement setup with respect to high resolution PET. The light yield of the crystals using polished crystals as well as PTFE tape and BaSO/sub 4/ as reflector material and the temperature dependence of the light yield are measured using crystals of 2 /spl times/ 2 /spl times/ 10 mm/sup 3/. At room temperature, the light yields of BGO : LuYAP : LSO are 1 : 1.5 : 4.5 with energy resolutions of 27.4% (BGO), 20% (LuYAP) and 15% (LSO), respectively. LuYAP shows an increase in light yield with increasing temperature whereas both BGO and LSO show a decrease.

Feedback controlled periodic states for different kinds of photorefractive nonlinear response

We investigate analytically and numerically the influence of the type of the photorefractive nonlinear response on the periodic states (attractors) which occur during feedback controlled 2W-coupling and correspond to almost 100% diffraction efficiency of the dynamic index grating. In addition to the case of the local response typical, for example, for LiNbO3 crystals we consider the cases of nonlocal (diffusive) response (BaTiO3, SBN) and resonant response (DC-biased BSO, BTO, and BGO crystals). It is shown that the conditions for the transition to the periodic states and their apparent characteristics are strongly different for the two limiting cases above.

A systematic study of proton capture reactions in the SeSb region at energies relevant to the p process

A systematic investigation of (p,γ) cross sections of nuclei from Se to Sb is presented. In-beam cross section measurements were carried out at E p = 1.4–5 MeV by using an array of 4 HPGe detectors of 100% relative efficiency shielded with BGO crystals. The S factors obtained are compared with the predictions of the statistical model code MOST.

Variations of solid–liquid interface in the BGO low thermal gradients Cz growth for diffuse and specular crystal side surface

A global analysis of heat transfer in growing BGO (Bi 4Ge 3O 12) crystals was carried out to explain the significant variation of the solid–liquid interface shape observed in practice during crystal growth. Special attention was given to the accurate treatment of radiative heat transfer in crystal and the gap between crystal and crucible wall. Crystal side surface was assumed to be transparent and diffuse or specular (Fresnel) reflective while melt was opaque. Spectral absorptivity of crystal was approximated by a three-band model. It is shown that rotationally driven vortex dominates under the solid–liquid interface during the whole growth process and no reconstruction of flow pattern, which could lead to interface inversion, takes place. As a result, in the case of diffuse crystal surfaces, the calculated deflection of the crystallization front does not exceed several millimeters and that completely disagrees with the observations. In contrast, for the specular crystal surface the solid–liquid interface turns out to be deeply convex toward the melt at the initial stage of the growth, and then, as the crystal is pulled, its convexity strongly diminishes. Such behavior is in good agreement with the experiment and mainly determined by specular reflection at the conical part of the crystal (its shoulder) while the effect of specular reflectivity of the cylindrical part and melt-free surface is significantly less. It is also shown that radiation transfer in the wavelength band of crystal opacity in conjunction with the specular reflection of radiation in the band of crystal transparency result in the appearance of “convex–concave” isotherms in the upper part of the crystal.