BGO is a high Z, high-density scintillation material. Due to the high atomic number of bismuth (83) and the material's high density of 7.13 g/cm3, it is a very efficient gamma-ray absorber. Given the high Z value of the material, the photo fraction for gamma-ray absorption is high and as a result, very good peak-to-total ratios are observed.
It is a relatively hard, rugged, non-hygroscopic crystal which does not cleave. The material does not show any significant self-absorption of the scintillation light.
The scintillation emission maximum is situated at 480nm. The light emission in photons/keV is about 15-20% of NaI(Tl); but, since the emission is partly in the area above 500nm where phototubes are less sensitive, the relative photoelectron yield of a bialkali PMT compared to NaI(Tl) amounts to 10-15%.
Due to the high Z value of the material, the photofraction for γ-ray absorption is high; and BGO scintillation crystals are used in applications where a high photofraction is required (for example, PET scanners) or because of its high detection efficiency (for example, Compton suppression spectrometers). It is a combination of properties that make BGO the material of choice for neutron activation analysis.
The decay time of BGO is about 300ns at room temperature, which is comparable to that of NaI(Tl). As there is no slow component in BGO and the rise time is quite fast (intrinsic scintillator), it is possible to get good timing <2ns with 3” thick crystals.
The scintillation intensity of BGO is a strong function of the temperature. At room temperature, the rate of change with temperature is approximately -1.2%/C. The radioactivity in BGO can make it unacceptable for some applications. We have developed a production process that significantly reduces the natural background, making our BGO well-suited for most applications.
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 with time or annealing. Since the radiation damage to BGO crystals depends on the presence of sub ppm impurities, large differences between individual crystals can occur.
| Density [g/cm3] | 7.13 |
| Melting point [K] | 1323 |
| Thermal expansion coefficient [C-1] | 7 x 10-6 |
| Hardness (Mho) | 5 |
| Hygroscopic | No |
| Wavelength of emission max. [nm] | 480 |
| Lower wavelength cutoff [nm] | 320 |
| Refractive index @ emission max | 2.15 |
| Primary decay time [ns] | 300 |
| Light yield [photons/keVγ | 8 - 10 |
| Photoelectron yield [% of NaI(Tl)] (for γ-rays) | 15 - 20 |
| Temperature coefficient of light yield | 1.2%C-1 |
| Neutron capture cross-section | 1.47b |
| Afterglow @ 20ms | 150ppm |
BGO can be machined to various shapes and geometries. Since BGO is not hygroscopic it does not require a hermetic package. Arrays can be produced for imaging applications such as medical PET and security.
For Physics experiments such as anti-Compton shields, crystals are typically packaged into larger annulus type detectors with multiple readout devices such as photomultiplier tubes.
It is possible to read out BGO 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.
- Available in large sizes and variable shapes
- Typical detector sizes range from 1" to 5" diameter and thicknesses up to 5"
- Other shapes; cube, hexagon
- Pixels (minimum 0.5mm square) or when used in an array down to 0.3mm
