BGO Detector Research Articles

Testing and assessment of a large BGO detector for beach monitoring of radioactive particles

The Beach Monitoring Steering Group (BMSG) was set up by UKAEA to explore whether improved systems for beach monitoring of radioactive particles are available. The BMSG commissioned the British Geological Survey (BGS) and the Nuclear Geophysics Division of the Kernfysisch Versneller Instituut (KVI/NGD), and other companies, to test their most sensitive system. This paper presents the results of trials in a specially created test facility at UKAEA Harwell with a large BGO detector. The detector's size and weight mean that it would be suitable for vehicle deployment but would be too large and heavy to carry in areas that could not be accessed by a vehicle. However, it would be possible to use the same methodology that is described here with a smaller detector capable of being carried in a backpack, albeit with reduced sensitivity for particle detection. The approach that we present is also applicable, with modifications, to the detection of offshore particles using a towed seabed detector.

The feasibility of well-logging measurements of arsenic levels using neutron-activation analysis

Arsenic is an extremely toxic metal, which poses a significant problem in many mining environments. Arsenic contamination is also a major problem in ground and surface waters. A feasibility study was conducted to determine if neutron-activation analysis is a practical method of measuring in situ arsenic levels. The response of hypothetical well-logging tools to arsenic was simulated using a readily available Monte Carlo simulation code (MCNP). Simulations were made for probes with both hyperpure germanium (HPGe) and bismuth germanate (BGO) detectors using accelerator and isotopic neutron sources. Both sources produce similar results; however, the BGO detector is much more susceptible to spectral interference than the HPGe detector. Spectral interference from copper can preclude low-level arsenic measurements when using the BGO detector. Results show that a borehole probe could be built that would measure arsenic concentrations of 100 ppm by weight to an uncertainty of 50 ppm in about 15 min.

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.

New technique to trace [15O]water uptake in a living plant with an imaging plate and a BGO detector system

New technique to trace [15O]water uptake in a living plant with an imaging plate and a BGO detector system

New technique to trace [<Superscript>15</Superscript>O]water uptake in a living plant with an imaging plate and a BGO detector system

We present a new trial to measure real time water movement in a living plant using the positron emitting radionuclide, 15O. 15O was prepared by 14N(d,n)15O reaction and 10 ml of 15O labeled water (2 GBq) was supplied from the root of a soybean plant. To detect activity, an imaging plate (IP) as well as a BGO detector system were used. Since the half-life of 15O is extremely short, (T1/2= 122 s), water uptake measurement was performed only for 20 minutes. In order to get [15O]waterimage, an IP was exposed to the plant for 1 minute for two times. Since the exposure to an IP requires dark condition, a BGO detector system was developed to measure [15O]waterunder light condition. A couple of BGO probes was set at the lowest stem and the gamma-rays (0.511 MeV) emitted from the radionuclide were measured through coincidence counting and compared with the radioactivity measured from an IP image. Using this system, we have found that the water uptake activity of the plant was drastically reduced under high humidity (99%) and dark condition.

Experience with scintillators for PET: towards the fifth generation of PET scanners

Since the ECAT 2, the first commercial positron emission tomograph developed by EG&G ORTEC, four generations of scanners can be identified. The first such scanners were based on sodium iodide (NaI(Tl)) scintillators, although as early as 1978 the transition to bismuth germanate (BGO) detectors had begun. By 1981, second-generation PET scanners with up to four rings of BGO detectors were available commercially. The BGO block detector appeared in 1985, initiating the third generation of PET scanners with the potential to increase the axial coverage in a cost-effective manner. As with the second generation, the third generation of PET scanners incorporated lead septa to collimate the annihilation photons within transverse planes, thereby reducing the acquisition of scattered and random coincidences and limiting detector dead time. The fourth generation of PET scanners offered up to 15 cm axial coverage and incorporated retractable septa that permitted both 2D and 3D acquisition within the same scanner. Therefore, fifth-generation scanner should be a fully 3D system with no septa, a 25–30 cm axial field-of-view, and a spatial resolution approaching the limits set by the physics of positron emission. Rather than using septa, limitation of randoms and scatter should be achieved directly at the detector by using a scintillator with high-light output, good energy resolution, and a fast scintillation decay time. The fast decay time will ensure low deadtime. The recent development of lutetium oxyorthosilicate (LSO), a scintillator with the required properties, suggests that a fifth generation of positron emission tomographs can now be attained.

Detection of radioactive particles offshore by γ-ray spectrometry Part II: Monte Carlo assessment of acquisition times

This paper represents a supplementary study to Part I: Monte Carlo assessment of detection depth limits, aimed at estimating the acquisition times, required to detect radioactive particles offshore by towed γ-ray spectrometry. Using Monte Carlo simulations, sets of measuring conditions were covered, addressing different types and sizes of scintillation detectors as well as variations in source strength, source–detector geometry and intensity of the natural gamma background. The performance of a large-volume BGO detector ( 12.7 cm diameter, 15 cm length) was assessed, to further enhance the sensitivity of simulated towed measurements. The simulations indicate that this detector represent a major step forward in detecting the radioactive particles offshore by gamma-ray spectrometry, particularly in a variable naturally radioactive background. With this detector, particles with an activity as low as 10 5 Bq should be detectable, at a towing speed of 2 knots (1 m/ s) , with an acquisition time of 2 s . However, detection of such particles is limited to relatively shallow burial depths (∼10 cm) and small detector–particle distances (<30 cm) . Higher strength particles would be detectable at greater distance; the simulations indicate 50 cm for a 10 6 Bq source and 80 cm for 10 8 Bq at the same burial depth.

Characterization of Single and Multiple Scatter From Matter and Activity Distributions Outside the FOV in 3-D PET

Operation of PET in 3-D mode, without internal septa results in increased sensitivity but at the cost of increased scatter fraction. Significant amounts of scattered events in 3-D PET either originate from activity outside the FOV (OFOV) or scatter from matter outside the FOV, complicating scatter correction methods. We performed 3-D PET simulations with scanner geometry based on the GE Advance with several phantom geometries designed to characterize all scatter with particular attention to that from outside the FOV. Scatter from OFOV matter was deduced by performing simulations with matter and activity within the FOV only (short phantom). Studies were repeated with phantoms with identical characteristics inside the FOV but with additional OFOV material (long phantom) but no activity outside the FOV. The incremental effect of OFOV activity was then determined by repeating the long phantom studies with additional activity. Scatter was also classified as single or multiple. In summary, the data allow extraction of yields from unscattered and all types of scattered events. The effect of source position, both within and external to the FOV, on scatter was determined by performing a series of simulations with point sources and techniques similar to those described above. The effect of energy discriminator setting (with BGO detectors) was also studied and data sets were generated for five settings from 300 to 425 keV. Using a Zubal phantom with the lungs and heart in the FOV and the abdomen outside the FOV and with a 300 keV discriminator, we find a total scatter fraction of 57%. Of the total scatter, 7.4% comes from OFOV matter, and 24% from OFOV activity. If the discriminator is increased to 425 keV the scatter fraction drops to 36%. OFOV scatter, particularly from OFOV activity, becomes relatively less important with the OFOV matter contribution dropping to 5.3% and OFOV activity contribution to 17%. Additionally, the spatial distributions of scatter from external matter and external activity were found to be different.

Detection of low caloric power of coal by pulse fast-thermal neutron analysis

This paper introduces the analysis method and principle of pulse fast-thermal neutron analysis (PFTNA). A system for the measurement of low caloric power of coal by PFTNA is also presented. The 14 MeV pulse neutron generator and BGO detector and 4096 MCA were applied in this system. A multiple linear regression method applied to the data solved the interferential problem of multiple elements. The error of low caloric power between chemical analysis and experiment was less than 0.4 MJ/kg.

A BGO detector system for studies of neutron capture by radioactive nuclides

A 16-section BGO scintillation detector having a total volume of 8.54 ℓ and an associated 40 MHz flash-ADC-based data taking system were developed. This detector system is intended for measurements of neutron capture cross-sections of radioactive nuclei by the time-of-flight method. The detector response function and efficiency, gamma-ray and neutron energy resolution, and backgrounds were evaluated experimentally and by calculation. Neutron capture cross-section measurements with 10 B , 197 Au and 237 Np samples were carried out in the energy range of 1– 1000 eV to demonstrate the performance of the system.

A study of coincidence line spread function (CLSF) estimation for small scintillators using quadrant sharing technique

Inter-crystal scatter is one of the primary reasons to misplace the coincidence events in positron emission tomography (PET). The probability of Compton scattering of a 511 KeV photon within the crystal is more than 50% of all interactions. Unlike one-to-one scintillator-photomultiplier tube (PMT) coupled (discrete) detectors, detector blocks utilizing the light sharing technique must have an additional degradation on their spatial resolution due to shared scintillation photons within the block. This study focuses on an estimation of the coincidence line spread function (CLSF) for 7/spl times/7 BGO (bismuth germanate) detector blocks by comparing the simulation results for discrete and light shared detectors. Based on experimental measurements, decoding probabilities, which describe the uncertainty in positioning between neighboring crystals, were estimated from photoelectron statistics. The results were embedded into the Monte Carlo simulation to estimate the CLSF more accurately. In this study, two 7/spl times/7 BGO detector blocks (2.68 mm/spl times/2.68 mm/spl times/18 mm) with different surface treatments coupled to 4 PMTs were used to estimate the decoding probability values. For discrete detectors, the FWHM values of CLSF for 7/spl times/7 BGO block with crystal dimensions of 2.68 mm/spl times/2.68 mm/spl times/18 mm were approximately found 1.47 mm for discrete detectors and 1.51 mm for light shared detectors. The intrinsic spatial resolutions of 7/spl times/7 light shared BGO block with crystal dimensions of 2.68 mm/spl times/2.68 mm/spl times/18 mm for 20 cm, 40 cm and 80 cm detector ring diameters were projected to 1.57 mm, 1.75 and 2.30 mm, respectively. Making the assumption that LSO (lutetium orthosilicate) crystals have 5 times more light output than BGO, 14/spl times/14 LSO block with crystal dimensions of 1.32 mm/spl times/1.32 mm/spl times/20 mm can be decoded and intrinsic spatial resolutions for 20 cm, 40 cm and 80 cm detector ring diameters were expected to be 0.87 mm, 1.15 mm and 1.91 mm, respectively.

Performance characteristics of a new generation of processing circuits for PET applications

An electronics architecture for a PET camera is presented which utilizes a newly developed ASIC, CFD, and TDC to fully utilize the timing advantages of an LSO scintillation crystal. Measured intrinsic timing resolution for a LSO/PMT detector against plastic/PMT detector is 900 ps FWHM using the newly developed ASIC electronics. The energy and pixel location are derived using continuous sampling ADCs with FPGA digital processing algorithms. Sampled PMT signals are rescaled using the CFD trigger to recalibrate the energy and position since the ADCs are asynchronous with the events. The overall linearity of the energy channels is 1.8% for a 6:1 input dynamic range. The electronics are designed to be compatible with large and small area LSO or BGO detectors used in PET imaging.

In situ determination of sulphur in coal seams and overburden rock by PGNAA

The prompt gamma neutron activation analysis technique was tested successfully for the determination of sulphur in coal seams and overburden rock. The logging tool employs a 3 μg 252Cf neutron source and a 50 mm dia × 100 mm BGO detector.

A unit for inspection of materials using differential gamma-ray scattering technique

The main objectives of this research were to develop a prototype unit using the differential gamma-ray scattering technique (DGST) and to demonstrate its possible use in nondestructive inspection of materials. The unit consisted of a 5 mCi (185 MBq) 137Cs gamma-ray source positioned perpendicularly to a 5 cm × 5 cm BGO detector. The gamma-ray beam was collimated by a 5 cm thick lead collimator with 1 cm ∅ opening while the detector was only side shielded allowing scattered gamma-rays to reach the detector from different angles. The unit was then tested with 20 cm × 20 cm × 20 cm concrete mortar containing four rebars at its corners. It was found that the integral of the differential spectrum changed corresponding to the size and position of the rebar which was in front of the source and the detector. It was also found that the integral of the differential spectrum increased with increasing degree of corrosion of the rebar. The results indicated that a portable DGST unit could be designed to be used as a tool in nondestructive inspection but the interpretation of the differential spectrum still needs further investigation.

Signal characteristics of individual crystals in high resolution BGO detector designs using PMT-quadrant sharing

The PMT-quadrant sharing (PQS) detector design allows very high resolution detectors to be built with 70% fewer PMTs and lower cost. A common concern for the design is that there is a big gap (photo-insensitive area) between four circular PMTs and the photoelectron signal (pulse height) may be much lower for the central crystals. The concern increases with the use of smaller PMTs for high-resolution designs because small PMTs have relatively thicker walls and relatively larger tolerance spaces between them. The authors measured the pulse heights and energy resolution for each crystal in three different types of PQS blocks for 19 mm PMT. For a square 7 /spl times/ 7 block detector (2.66 mm /spl times/ 2.66 mm /spl times/ 18 mm BGO needles), the maximum photopeak signals occurred at the corner crystal of the block. The signals for the worst central five crystals (sitting on space with no PMT connection) had pulse heights 0.87 as high as that of the corner crystals. The 12 crystals (outside the central five) with coupling only to the glass wall but not to the photocathode had a relative pulse height of 0.92. The eight crystals with partial exposure to photocathodes had a 0.94 relative pulse height. The energy resolution for individual crystals was 22% - 30% with an average of 26%. Asymmetric photopeaks, especially for the corner crystals, were observed, and these were found to be the result of the depth-of-interaction effect. In the latest PQS design, extended blocks with asymmetric light distributions were used on the four edges and four corners of a large detector module so that the previously unused (wasted) half-row of peripheral PMT could be covered by crystals. An asymmetric block, single-extended (7 /spl times/ 8 crystals) was also tested. The pulse-height ratio between the worst and best group of crystals in the single-extended block was 0.72 and that of the double-extended block was also 0.72. In a more demanding, higher spatial resolution 8 /spl times/ 8 array (2.3 mm /spl times/ 2.3 mm /spl times/ 10 mm BGO) for mouse PET with shallower crystals, the pulse-height ratio was 0.73 with an average energy resolution of 20%. This study demonstrated that pulse height uniformity for the PQS design using circular PMT was excellent, better than the typical 3/1 pulse-height ratio in conventional block detectors.

Optimizing the signal-to-noise ratio for the PELAN system

Pulsed elemental analysis with neutrons is a portable system for the detection of explosives, weighing less than 45kg. It is based on the principle that explosives and other contraband contain various chemical elements such as H, C, N, O, etc. in quantities and ratios that differentiate them from other innocuous substances. Neutrons are produced with a pulsed 14MeV (d-T) neutron generator. Separate gamma-ray spectra from fast neutron, thermal neutron and activation reactions are accumulated and analyzed to determine elemental content. Currently, a 7.6cm×7.6cm BGO detector is incorporated into the design. However, the high γ-ray efficiency of this detector has a drawback in that it detects γ-rays from the environment surrounding the object under interrogation. These γ-rays from the environment are essentially noise that must be filtered from the signal from the object under interrogation. Since there is no practical way to focus the neutrons, the signal-to-noise ratio of the detector must be modified. In the past 2 years, we have tried several approaches to solve this problem.

Absolute full-energy peak efficiency calibration of a Clover–BGO detector system

Abstract An absolute full-energy peak efficiency calibration of a Clover-type composite germanium gamma-ray detector system is presented. The efficiency curve was measured in the energy range of 120 keV–12.2 MeV for the first time, using the combination of proton-capture nuclear reactions and radioactive sources, such as 56Co, 60Co, 66Ga and 152Eu. At high energies gamma-ray pairs from proton resonance capture on 27Al, 23Na, 39K and 11B were employed using evaporated Al, Na2WO4, K2SO4 and LiBO2 targets, respectively. The experimental data were fitted with a single polynomial function on log–log scale using a recently developed method. The add-back factor is found to increase quadratically with the energy, in contrast to the previously suggested linear relationship. The efficiency gain with respect to a single detector is about one order of magnitude at 10 MeV energy.

Development of a GSO detector assembly for a continuous blood sampling system

A new input function monitoring system has been developed using the GSO detector assembly for both PET and SPELT quantitative studies. Due to the rapid rise time of the pulse (about 10nsec), the coincidence time window can be reduced < 1nsec, reducing contribution of randoms associated with the high background activity surrounding the detector. Large light output improved the energy resolution of approximately 11% for 511keV photons, and 20% at 140 keV, as compared with the BGO detector, enabling the use of this system also in SPELT studies. The paired assembly of crystals provided the absolute sensitivity of approximately 7% for PET and 75% for PET tracers. Multiple arrangement of the paired detectors provided possibility of correcting for the transit time of radioactivity through the catheter tube. This study demonstrated that the present system can be of use in both clinical and small animal studies using SPECT and PET tracers.

A 10-mc/s, 0.5-/spl mu/m CMOS constant-fraction discriminator having built-in pulse tail cancellation

A 0.5-/spl mu/m, fully-integrated CMOS, constant-fraction discriminator (CFD) is presented for 10-Mc/s operation. The CFD utilizes a timing-shaping filter to provide amplitude insensitive time discrimination without delay lines or other external components. Continuous-time baseline restorers are used to cancel CMOS dc offsets and provide signal tail cancellation for both the arming and constant-fraction signals. Near total signal tail cancellation at 100 ns permits operation at count rates up to 10 Mc/s. Measured intrinsic timing resolution is 40 to 90 ps FWRM for a 3.7:1 input dynamic range and measured walk is 20 ps for a 2.7:1 dynamic range increasing to 80 ps for a 3.7:1 dynamic range. Measured tinting resolution for an LSO/PMT detector against a plastic/PMT detector is 1.00-ns FWHM at low count rates increasing to 1.27-ns FWRM at 6.0 Mc/s. The CFD is part of a 0.5-/spl mu/m CMOS front-end integrated circuit designed for large-area, high-count-rate LSO or BGO detectors used in PET medical imaging.

MCNP modelling of scintillation-detector γ-ray spectra from natural radionuclides

γ-ray spectra of natural radionuclides are simulated for a BGO detector in a borehole geometry using the Monte Carlo code MCNP. All γ-ray emissions of the decay of 40K and the series of 232Th and 238U are used to describe the source. A procedure is proposed which excludes the time-consuming electron tracking in less relevant areas of the geometry. The simulated γ-ray spectra are benchmarked against laboratory data.