I. Wilhelm

Development of low-pressure vapour-phase epitaxial GaAs for medical imaging

Abstract A summary is given of progress accomplished with the development of low-pressure vapour-phase epitaxial GaAs as a material for X-ray detectors. As the III–V concentration ratio is altered from Ga-rich to As-rich, the material is shown to improve from p-type, to n-type with compensation via deep levels, to n-type with a doping density of 1.7×1014 atoms cm−3. The measured barrier height is 0.8 V, as expected for the Ti contact used. Overdepletion was obtained before breakdown, enabling a layer thickness of 41 μ m to be deduced for the final sample. For the later samples, charge collection for 60 keV Am-241 gammas was bias independent at a value of 100±8%. Spectra were also obtained from Sr-90 electrons. The most probable value of the charge collected as a function of the bias reached a plateau and from this value a depletion width of 40 μ m was found for the final sample, equal to the epitaxial layer thickness. Results from detailed alpha and low-energy proton spectroscopy are shown for diodes fabricated from this material. A charge collection efficiency of 100% was obtained when the diode could be depleted sufficiently. The concept of a charge collection depth was introduced, since a significant amount of charge was collected without bias. The minimum depth of such a region was shown to be 10.8 μ m at 0 V reverse bias, far greater than the 1.1 μ m predicted for the depletion depth. Charge coupling between the guard ring and the pad was observed and successfully modelled.

Study of neutron response and n–γ discrimination by charge comparison method for small liquid scintillation detector

The study of the neutron response and n-gamma discrimination for small (18x26x8) mm3 liquid scintillator BC501A (Bicron) detector was carried out by digital charge comparison method. Three ranges of neutron energies were used: uniform distribution from 0.95 MeV to 1.23 MeV, continuous spectra of AmBe source and monoenergetic 16.2 MeV neutrons. The obtained results are compared with those for cylindrical liquid scintillation detector (40 mm diameter, 60 mm length) at the same energies of neutrons. A dramatic fall of the neutron response function at 400 keVee for small detector at 16.2 MeV neutron energy was measured. For (0.95 - 1.23) MeV neutron energy range such fall takes place at 260 keVee. The greater slope of neutron locus at (0.95 - 1.23) MeV neutron energy comparing to 16.2 MeV for both detectors is explained by longer tail of pulse from proton recoils within (0.1-1.23) MeV energy range.

Scanning of irradiated silicon detectors using alpha particles and low-energy protons

Abstract In a spectroscopic study of non-irradiated and proton-irradiated silicon diodes, the detectors were illuminated from the front side and from the rear side by various alpha particle sources (mainly ThC’) and by monoenergetic protons with energies from 1.0 to 2.5 MeV. Their response characteristics have been studied as a function of the incoming particle energy and the applied bias voltage. The charge collection efficiency was determined as a function of fluence.

Target with a frozen nuclear polarization for experiments at low energies

Abstract A target with a frozen spin polarization of protons in 1.2-propanediol with a paramagnetic Cr(V) impurity is described, intended for polarization parameter studies in np-scattering at approximately 15 MeV neutron energy. The target of cylindrical shape of 2 cm diameter and 6 cm long with an initial polarization of 95 ± 3%, obtainable by the dynamic polarization technique, is placed at a temperature about 20 mK in a magnetic field of 0.37 T generated by a magnetic system, which provides a large aperture for scattered particles. The relaxation time for the spin polarization in an experiment is about 1000 hours.

Production of monoenergetic neutrons from the T(d, n) α reaction with the associated particle method

Abstract An associated particle time-of-flight system based on the T(d, n)α reaction has been developed to study polarized neutron induced scattering and reactions between 14–17 MeV. A magnetic separator has been constructed to avoid elastically scattered deuteron pileup and a special seven-channel multiparameter data acquisition system together with powerful off-line corrections has been used. Typically 10 5 n/s in a well collimated beam with a time resolution better than 2.5 ns has been obtained.

STUDY OF GAAS DETECTOR RESPONSE WITH LOW-ENERGY PROTONS

Abstract Semi-insulating Czochralski-grown GaAs detectors were studied by monoenergetic protons with energies from 0.45 to 2.40 MeV. The response characteristics of the detector irradiated from the front side and the back side are shown. An attempt to account for the splitting of the observed peaks in terms of large scale potential fluctuations expected in this material, is presented.

Measurement of spin dependent total cross sections in neutron‐deuteron scattering

Since the beginning of nuclear physics, a central issue has been understanding the nucleon-nucleon (NN) force. Due to the progress in the development of new theoretical models as well as due to new experimental data our understanding of the NN system increased significantly during last 15 years. However, when these models were used to predict properties of three-nucleon (3N) system, they failed to provide a similarly exact picture.

CHARGE COLLECTION EFFICIENCY OF GAAS DETECTORS STUDIED WITH LOW-ENERGY HEAVY CHARGED PARTICLES

Abstract Epitaxially grown GaAs layers have recently been produced with sufficient thickness and low enough free carrier concentration to permit their use as radiation detectors. Initial tests have shown that the epi-material behaves as a classical semiconductor as the depletion behaviour follows the square root dependency on the applied bias. This article presents the results of measurements of the growth of the active depletion depth with increasing bias using low-energy protons and alpha particles as probes for various depths and their comparison to values extrapolated from capacitance measurements. From the proton and alpha particle spectroscopic measurements, an active depth of detector material that collects 100% of the charge generated inside it was determined. The consistency of these results with independent capacitance measurements supports the idea that the GaAs epi-material behaves as a classical semiconductor.

A multiparameter system for heavy nuclei fission study

A multiparameter system designed to measure as many as six parameters of a single fission event on line with an EC 1010 computer is described. The kinematics of emitted fragments and the calibration of light particle and fragment spectra are discussed. Fragment energy spectra from the thermal neutron induced fission of235U have been measured. Results of light particle emission from spontaneous fission of252Cf and from neutron induced fission of235U are presented.

Deuteron frozen-spin- polarized target for nd experiments at the VdG accelerator of Charles University

Abstract A frozen-spin- polarized deuteron target cooled by the 3 He/ 4 He dilution refrigerator is described. Fully deuterated 1,2-propanediol was used as a target material. Deuteron vector polarization about 40% was obtained for the target in the shape of a cylinder of 2-cm diameter and 6-cm length. The target is intended for a study of 3N interactions at the polarized neutron beam generated by the Van de Graaff accelerator at the Charles University in Prague.