Arnaldo Stefanini

Threshold π+ electroproduction at high-momentum transfer: A determination of the nucleon axial vector form factor

Abstract Threshold π+ electroproduction has been measured at momentum transfers ‖k2‖ of 0.45, 0.58 and 0.88 (GeV/c)2, extending the range of earlier measurements. Using PCAC and current algebra-based models we have deduced the axial vector form factor of the nucleon GA(k2), and find that the dipole parametrisation is favoured over the monopole. The value of MA in the dipole parametrisation is, in a weak PCAC model, 0.96 ± 0.03 GeV, in excellent agreement with neutrino measurements. The threshold cross section is also in excellent agreement with the predictions of a fixed-t dispersion relation model.

Measurements of threshold π+ electroproduction at low momentum transfer

Abstract The reaction e− + p → e− + π+ + n has been studied near threshold at momentum transfer |k2| of 0.078, 0.155, 0.233 and 0.311 (GeV/c)2. The slope of the total cross section evaluated at threshold has been compared with models based on PCAC and current algebra to deduce the axial vector form factor of the nucleon, GA(k2). In the dipole parametrisation GA(k2)/GA(0) = (1 + |k2|/MA2)−2 we find that the models give values of MA ranging between 1.0 and 1.2 GeV. The π+n angular distributions are compared with fixed t dispersion relations and with a pseudovector Born approximation and are found to be in good agreement.

Low contrast imaging with a GaAs pixel digital detector

A digital mammography system based on a GaAs pixel detector has been developed by the INFN (Istituto Nazionale di Fisica Nucleare) collaboration MED46. The high atomic number makes the GaAs a very efficient material for low energy X-ray detection (10-30 keV is the typical energy range used in mammography). Low contrast details can be detected with a significant dose reduction to the patient. The system presented in this paper consists of a 4096 pixel matrix built on a 200 /spl mu/m thick semi-insulating GaAs substrate. The pixel size is 170/spl times/170 /spl mu/m/sup 2/ for a total active area of 1.18 cm/sup 2/. The detector is bump-bonded to a VLSI front-end chip which implements a single-photon counting architecture. This feature allows to enhance the radiographic contrast detection with respect to charge integrating devices. The system has been tested by using a standard mammographic tube. Images of mammographic phantoms will be presented and compared with radiographs obtained with traditional film/screen systems. Monte Carlo simulations have been also performed to evaluate the imaging capability of the system. Comparison with simulations and experimental results will be shown.

Efficiency and spatial resolution measurements of a modular neutron detector in the kinetic energy range 15–120 MeV

Abstract The detection efficiency of a 3 × 3 matrix of NE 110 scintillator blocks, 153 × 153 × 270 mm long, has been measured in the neutron kinetic energy range 15–120 MeV for several thresholds, ranging from 2.80 to 15.75 MeV equivalent electron energy, and for various thicknesses of lead shielding in front of the counter. The probabilities of detecting a neutron in a block different from the one struck by the beam (‘mixing’) and in more than one block (‘multifiring’) have also been measured as a function of threshold and shielding. Comparisons have been made with a Monte Carlo program which accounts for the modular structure of the counter. The efficiency and the spatial resolution of a very large matrix have been obtained, using both experimental and Monte Carlo results.

A Medipix2-based imaging system for digital mammography with silicon pixel detectors

In this paper we present the first tests of a digital imaging system based on a silicon pixel detector bump-bonded to an integrated circuit operating in single photon counting mode. The X-rays sensor is a 300 /spl mu/m thick silicon, 14 by 14 mm/sup 2/, upon which a matrix of 256 /spl times/ 256 pixels has been built. The read-out chip, named MEDIPIX2, has been developed at CERN within the MEDIPIX2 Collaboration and it is composed by a matrix of 256 /spl times/ 256 cells, 55 /spl times/ 55 /spl mu/m/sup 2/. The spatial resolution properties of the system have been assessed by measuring the square wave resolution function (SWRF) and first images of a standard mammographic phantom were acquired using a radiographic tube in the clinical irradiation condition.

Experimental study of Compton scattering reduction in digital mammographic imaging

In mammography, the first cause of image contrast reduction arises from the photons scattered inside the examined organ. The amount of Compton scattering strongly depends on the irradiation area and on the distance between the organ and the X-ray detector. We have experimentally evaluated how these geometrical conditions affect the scattering fraction. Our experimental setup includes a single photon counting device based on a silicon pixel detector as X-ray sensor; a lucite cylinder to simulate the breast tissue, and a lead collimator to define the irradiation area. We have evaluated the contrast and the signal-to-noise ratio for images acquired in different conditions.

Calibration of a modular plastic scintillator neutron detector in the kinetic energy range 100–500 MeV

Abstract We describe the calibration of a modular neutron counter for neutrons and protons of between 100 and 500 MeV kinetic energy. We present results for the detection efficiency, time and spatial resolution, and multifiring. We emphasise the necessity of calibrating the crosstalk between neutron and photon identification.

Characterization of a mammographic system based on single photon counting pixel arrays coupled to GaAs x-ray detectors

The authors report on the imaging capabilities of a mammographic system demonstrator based on GaAs pixel detectors operating in single photon counting (SPC) mode. The system imaging performances have been assessed by means of the transfer functions: The modulation transfer function (MTF), the normalized noise power spectrum, and the detective quantum efficiency (DQE) have been measured following the guidelines of the IEC 62220-1-2 protocol. The transfer function analysis has shown the high spatial resolution capabilities of the GaAs detectors. The MTF calculated at the Nyquist frequency (2.94 cycles/mm) is indeed 60%. The DQE, measured with a standard mammographic beam setup (Mo/Mo, 28 kVp, with 4 mm Al added filter) and calculated at zero frequency, is 46%. Aiming to further improve the systems image quality, the authors investigate the DQE limiting factors and show that they are mainly related to system engineering. For example, the authors show that optimization of the image equalization procedure increases the DQE(0) up to 74%, which is better than the DQE(0) of most clinical mammographic systems. The authors show how the high detection efficiency of GaAs detectors and the noise discrimination associated with the SPC technology allow optimizing the image quality in mammography. In conclusion, the authors propose technological solutions to exploit to the utmost the potentiality of GaAs detectors coupled to SPC electronics.

Spectroscopic and imaging capabilities of a pixellated photon counting system

We are studying the performance of various thickness GaAs pixel detectors bump-bonded to a dedicated photon counting chip (PCC) for medical imaging applications in different energy ranges. In this work we present the experimental results obtained with a 600 μm thick pixel matrix (64×64 square pixels, 170 μm side) in the 60–140 keV energy range to evaluate the possible use of such a system in the nuclear medicine field. In particular, we have measured the spectroscopic properties of the detector (charge collection efficiency, energy resolution and detection efficiency) and evaluated the discrimination capability of the electronics. Then we have measured the imaging properties of the whole system in terms of Point Spread Function and using a home made thyroid phantom. We present also a comparison with a traditional gamma camera and an evaluation, made by both experimental measurements and software simulations, of the imaging characteristics related to the use of a collimation system.

A Silicon Vertex Detector for CDF

The major reason for building a vertex detector for CDF is the tagging of decay vertices of particles with lifetime in the 10-13/10-12 sec. range. This is a complementary approach to heavy flavour physics with respect to missing Et and large Pt leptons. The method can be best applied to tag hadronic decays of heavy flavours, which have the largest branching ratios, but have eluded any specific tagging until now. It also works, although with somewhat reduced efficiency, in events with a semileptonic decay. All in all it promises to be a powerful tool in the search of rather elusive processes like Higgs, top, or fourth generation quark production [1]. The additional information provided by the vertex detector will also improve significantly the resolution of the CDF central tracking system [2].