Nuclear science

This paper presents the results of natural radioactivity measured for the soil of Qena using gamma-ray spectrometry at ZSR, Hanover University, Germany. Soil samples of radioactive concentrations of 238 U-series ( 234 Th, 214 Pb, and 214 Bi), 232 Th-series ( 228 Ac, 212 Pb, and 208 Tl) and 40 K were analyzed. Three objectives were set: (1) activity levels by surface soil sampling at 0~30 cm depth, (2) dose rates of gamma radiation, radium equivalent, index hazard, and effective dose, and (3) ambient dose rates.

_ Gd 2 SiO 5 :Ce (GSO:Ce) is a high-Z, nonhygroscopic fast emitting scintillator used in detectors of some positron tomography systems. The purpose of this study was to evaluate the luminescence of GSO:Ce scintillator at lower photon energies employed in other fields of medical imaging (e.g. x-ray computed tomography, general x-ray imaging, low energy gamma ray imaging). To this aim the absolute luminescence efficiency (emitted light flux over x-ray exposure), the matching factor (spectral compatibility to optical detectors) and the effective efficiency (combination of absolute efficiency with spectral compatibility) were experimentally determined. Various xray tube voltages ranging from 20 to 140 kV (employed in xray mammography, general radiography, and computed tomography) were used. In addition the behaviour of GSO: Ce under low energy gamma irradiation could be estimated from these data. Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The GSO:Ce optical emission spectrum was measured under x-ray excitation using an optical grating monochromator. The efficiency of GSO:Ce was found to increase with increasing x-ray tube voltage, while the GSO:Ce spectrum, peaking at 440 and 490 nm, was found compatible with most optical detectors (photodiodes, photocathodes, charge coupled devices). However the spectral compatibility (0.77) and the effective efficiency were found highest for GSO coupled to photodiode.

The MUSE program (multiplication with an external source) is in progress at the MASURCA critical facility at the Cadarache Research Center of the Commissariat à l'Energie Atomique in France. The program is dedicated to the physics studies of accelerator-driven systems in support of transmutation studies of minor actinides and long-lived fission products. It began in 1995 with the coupling of a Cf source in MASURCA and was followed by a commercial (d,T) source. In 2001, a specially constructed (d,D)/(d,T) neutron generator (GENEPI) was placed in MASURCA and the MUSE-4 program commenced.

We developed a cost-effective SPECT scanner prototype (rSPECT) for in vivo imaging of rodents based on small-area gamma cameras. Each detector consists of a position-sensitive photomultiplier tube (PS-PMT) coupled to a 30 30 NaI(Tl) scintillator array and electronics attached to the PS-PMT sockets for adapting the detector signals to an in-house developed data acquisition system. The detector components are enclosed in a leadshielded case with a receptacle to insert the collimators. System performance was assessed using 99m Tc for a high-resolution parallel-hole collimator, and for a 0.75-mm pinhole collimator with a 60 aperture angle and a 42-mm collimator length. The energy resolution is about 10.7% of the photopeak energy. The overall system sensitivity is about 3 cps Ci detector and planar spatial resolution ranges from 2.4 mm at 1 cm source-to-collimator distance to 4.1 mm at 4.5 cm with parallel-hole collimators. With pinhole collimators planar spatial resolution ranges from 1.2 mm at 1 cm source-to-collimator distance to 2.4 mm at 4.5 cm; sensitivity at these distances ranges from 2.8 to 0 5 cps Ci detector. Tomographic hot-rod phantom images are presented together with images of bone, myocardium and brain of living rodents to demonstrate the feasibility of preclinical small-animal studies with the rSPECT.

SCK•CEN, the Belgian Nuclear Research Centre, is designing an Accelerator Driven System (ADS), MYRRHA, that aims to serve as a basis for the European experimental ADS and is to provide protons and neutrons for various R&D applications. It consists of a proton accelerator delivering a 350 MeV * 5 mA proton beam to a liquid lead-bismuth eutectic (LBE) spallation target, which -in turn -couples to a LBE cooled, sub-critical fast-spectrum core in a pool type configuration. The liquid metal flow pattern in the lower part of the MYRRHA pool vessel needs to be investigated in order to assess the details of recirculation and stagnant zones for adequate coolant flow and sufficient physico-chemical mixing and to judge the scaling of the flow down to a model which can be handled experimentally. In view of this, three-dimensional Computational Fluid Dynamics calculations have been performed by NRG in collaboration with the MYRRHA team.

The development of an innovative detector technology for photon-counting in X-ray imaging is reported. This new generation of detectors, based on pixellated cadmium telluride (CdTe) and cadmium zinc telluride (CZT) detector arrays electrically connected to application specific integrated circuits (ASICs) for readout, will produce fast and highly efficient photon-counting and energydispersive X-ray imaging. There are a number of applications that can greatly benefit from these novel imagers including mammography, planar radiography, and computed tomography (CT). Systems based on this new detector technology can provide compositional analysis of tissue through spectroscopic X-ray imaging, significantly improve overall image quality, and may significantly reduce X-ray dose to the patient. A very high X-ray flux is utilized in many of these applications. For example, CT scanners can produce ~100 Mphotons/mm 2 /s in the unattenuated beam. High flux is required in order to collect sufficient photon statistics in the measurement of the transmitted flux (attenuated beam) during the very short time frame of a CT scan. This high count rate combined with a need for high detection efficiency requires the development of detector structures that can provide a response signal much faster than the transit time of carriers over the whole detector thickness. We have developed CdTe and CZT detector array structures which are 3 mm thick with 16×16 pixels and a 1 mm pixel pitch. These structures, in the two different implementations presented here, utilize either a small pixel effect or a drift phenomenon. An energy resolution of 4.75% at 122 keV has been obtained with a 30 ns peaking time using discrete electronics and a 57 Co source. An output rate of 6×10 6 counts per second per individual pixel has been obtained with our ASIC readout electronics and a clinical CT X-ray tube. Additionally, the first clinical CT images, taken with several of our prototype photon-counting and energy-dispersive detector modules, are shown.