A. Di Fulvio

Characterization of Deuterated-Xylene Scintillator as a Neutron Spectrometer

We have experimentally characterized the neutron light output response functions of a deuterated-xylene scintillator for neutron energies lower than 10 MeV. We then used the response matrix to unfold the energy distribution of neutrons produced via several reactions, i.e., spontaneous fission, d(d,n)3He, 27Al(d,n)28Si, and 9Be(alpha,n)12C. Organic scintillators based on deuterated compounds show a fast response and good gamma-neutron discrimination capability, similar to or better than proton-based scintillators. Deuterated scintillators can also effectively provide neutron energy spectra by unfolding measured data with the detector response matrix, without the need of time-of-flight. Deuteron recoils, produced by elastic collisions between deuterium and impinging neutrons, are preferentially forward-scattered. This non-isotropic reaction results in distinct peaks in the response functions to monoenergetic neutrons. In this work, we evaluated a custom-fabricated 7.62 cm

Application of a Bonner sphere spectrometer for the determination of the angular neutron energy spectrum of an accelerator-based BNCT facility

\times 7.62

Fast multiplicity counter featuring stilbene detectors for special nuclear material assay

cm deuterated-xylene (EJ301D) liquid scintillator. This liquid has a low volatility and higher flash point, compared to benzene-based deuterated detectors, e.g., EJ315 and NE230. We measured the EJ301D detector neutron response matrix (up to 6 MeV neutron energy) using an intense 252Cf source and the time-of-flight technique. The number of response functions obtained using our method is only limited by counting statistics and by the experimentally achievable energy resolution. Multi-channel unfolding was then performed successfully for neutron sources with different energy spectra.

Application of the BINS superheated drop detector spectrometer to the 9Be(p,xn) neutron energy spectrum determination

Experimental activities are underway at INFN Legnaro National Laboratories (LNL) (Padua, Italy) and Pisa University aimed at angular-dependent neutron energy spectra measurements produced by the (9)Be(p,xn) reaction, under a 5MeV proton beam. This work has been performed in the framework of INFN TRASCO-BNCT project. Bonner Sphere Spectrometer (BSS), based on (6)LiI (Eu) scintillator, was used with the shadow-cone technique. Proper unfolding codes, coupled to BSS response function calculated by Monte Carlo code, were finally used. The main results are reported here.

Silicon Carbide Schottky Diodes for Alpha Particle Detection

A fast neutron multiplicity counting system based on organic scintillators, i.e. EJ-309 and stilbene, has been developed and experimentally tested at the University of Michigan. The system is able to detect correlated photon and neutron multiplets emitted by the fission reaction, within a gate time of tens of nanoseconds. This counting strategy is exploited to quantify fissile mass, without the need of complex electronic circuitry and unfolding procedures, otherwise required in moderated systems. Moderated systems are traditionally based on helium-3 detectors and feature a gate time of hundreds of microseconds. Measurement precision is thus negatively affected by accidental coincidences. A prototypal version of the proposed well-shaped counter was assembled and tested in the laboratory, using a spontaneous fission and an (α, n) neutron source, i.e. 252Cf and PuBe respectively. Measured results show excellent agreement with the simulated model of the system. The viability of the system to discriminate time-correlated fission neutrons from random, uncorrelated neutrons was proved. Preliminary results of an experimental campaign, carried out at INL (Idaho National Laboratory), to characterize plutonium metal samples are also shown. Results show a monotonic increasing trend for the range of measured 240-Pu effective masses, i.e. 0.024-0.5 kg, enabling the measurement of the mass of an unknown sample.

Advanced readout methods for superheated emulsion detectors

In the framework of TRASCO-BNCT project, a Bubble Interactive Neutron Spectrometer (BINS) device was applied to the characterization of the angle-and energy-differential neutron spectra generated by the 9Be(p,xn)reaction. The BINS spectrometer uses two superheated emulsion detectors, sequentially operated at different temperatures and thus provides a series of six sharp threshold responses, covering the 0.1-10 MeV neutron energy range. Spectrum unfolding of the data was performed by means of MAXED code. The obtained angle, energy-differential spectra were compared with those measured with a Bonner sphere spectrometer, a silicon telescope spectrometer and literature data.

Analysis of the response of innovative neutron detectors with monoenergetic neutron beams

The fabrication and characterization of SiC Schottky diodes for the detection of alpha particles at room temperature are described. A 5 × 5 matrix of diodes has been fabricated in order to verify the dependence of the device response on randomly distributed wafer defects. A dedicated exposure apparatus has been fabricated to test the detectors. Some preliminary alpha energy spectra obtained with the lowest reverse current diodes are shown.

Radiotherapy out-of-field dosimetry: Experimental and computational results for photons in a water tank

Superheated emulsions develop visible vapor bubbles when exposed to ionizing radiation. They consist in droplets of a metastable liquid, emulsified in an inert matrix. The formation of a bubble cavity is accompanied by sound waves. Evaporated bubbles also exhibit a lower refractive index, compared to the inert gel matrix. These two physical phenomena have been exploited to count the number of evaporated bubbles and thus measure the interacting radiation flux. Systems based on piezoelectric transducers have been traditionally used to acquire the acoustic (pressure) signals generated by bubble evaporation. Such systems can operate at ambient noise levels exceeding 100 dB; however, they are affected by a significant dead time (>10 ms). An optical readout technique relying on the scattering of light by neutron-induced bubbles has been recently improved in order to minimize measurement dead time and ambient noise sensitivity. Beams of infra-red light from light-emitting diode (LED) sources cross the active area of the detector and are deflected by evaporated bubbles. The scattered light correlates with bubble density. Planar photodiodes are affixed along the detector length in optimized positions, allowing the detection of scattered light from the bubbles and minimizing the detection of direct light from the LEDs. A low-noise signal-conditioning stage has been designed and realized to amplify the current induced in the photodiodes by scattered light and to subtract the background signal due to intrinsic scattering within the detector matrix. The proposed amplification architecture maximizes the measurement signal-to-noise ratio, yielding a readout uncertainty of 6% (±1 SD), with 1000 evaporated bubbles in a detector active volume of 150 ml (6 cm detector diameter). In this work, we prove that the intensity of scattered light also relates to the bubble size, which can be controlled by applying an external pressure to the detector emulsion. This effect can be exploited during the readout procedure to minimize shadowing effects between bubbles, which become severe when the latter are several thousands. The detector we used in this work is based on superheated C-318 (octafluorocyclobutane), emulsified in 100 μm ± 10% (1 SD) diameter drops in an inert matrix of approximately 150 ml. The detector was operated at room temperature and ambient pressure.

Superheated emulsions and track etch detectors for photoneutron measurements

Various neutron detectors are currently under development at the University of Pisa. The response of these devices is investigated using monoenergetic neutron beams produced at the CN accelerator of INFN Legnaro National Laboratories with thin lithium target bombarded by protons at different energies, exploiting the 7Li(p,n)7Be reaction.