A Bonner Sphere Spectrometer based on a large 6LiI(Eu) scintillator: Calibration in reference monoenergetic fields

A Bonner Sphere spectrometer employing a large, 11 mm diameter × 3 mm thickness, 6LiI(Eu) scintillator (LL-BSS), was assembled. The purpose was to produce a BSS similar in sensitivity to those based on 3He sensors, but using alternative sensors. With respect to the traditional BSS based on the 4 mm (diameter) × 4 mm (height) 6LiI(Eu), this new BSS is a factor of 3 more sensitive. LL-BSS response matrix, determined with MCNPX, was experimentally evaluated with monoenergetic reference neutron fields of 144 keV, 565 keV and 1.2 MeV available at NPL (Teddington, UK). The results of the experiment confirmed the correctness of the response matrix within an overall uncertainty lower than ±2%

Experimental characterization of HOTNES: A new thermal neutron facility with large homogeneity area

A new thermal neutron irradiation facility, called HOTNES (HOmogeneous Thermal NEutron Source), was established in the framework of a collaboration between INFN-LNF and ENEA-Frascati. HOTNES is a polyethylene assembly, with about 70 cmx70 cm square section and 100 cm height, including a large, cylindrical cavity with diameter 30 cm and height 70 cm. The facility is supplied by a 241Am-B source located at the bottom of this cavity. The facility was designed in such a way that the iso-thermal-fluence surfaces, characterizing the irradiation volume, coincide with planes parallel to the cavity bottom. The thermal fluence rate across a given isofluence plane is as uniform as 1% on a disk with 30 cm diameter. Thermal fluence rate values from about 700 cm−2 s−1 to 1000 cm−2 s−1 can be achieved. The facility design, previously optimized by Monte Carlo simulation, was experimentally verified. The following techniques were used: gold activation foils to assess the thermal fluence rate, semiconductor-based active detector for mapping the irradiation volume, and Bonner Sphere Spectrometer to determine the complete neutron spectrum. HOTNES is expected to be attractive for the scientific community involved in neutron metrology, neutron dosimetry and neutron detector testing

ETHERNES: A new design of radionuclide source-based thermal neutron facility with large homogeneity area

A new thermal neutron irradiation facility based on an 241Am-Be source embedded in a polyethylene moderator has been designed, and is called ETHERNES (Extended THERmal NEutron Source). The facility shows a large irradiation cavity (45cm×45cm square section, 63cm in height), which is separated from the source by means of a polyethylene sphere acting as shadowing object. Taking advantage of multiple scattering of neutrons with the walls of this cavity, the moderation process is especially effective and allows obtaining useful thermal fluence rates from 550 to 800cm-2s-1 with a source having nominal emission rate 5.7×106s-1. Irradiation planes parallel to the cavity bottom have been identified. The fluence rate across a given plane is as uniform as 3% (or better) in a disk with 30cm (or higher) diameter. In practice, the value of thermal fluence rate simply depends on the height from the cavity bottom. The thermal neutron spectral fraction ranges from 77% up to 89%, depending on the irradiation plane. The angular distribution of thermal neutrons is roughly isotropic, with a slight prevalence of directions from bottom to top of the cavity. The mentioned characteristics are expected to be attractive for the scientific community involved in neutron metrology, neutron dosimetry and neutron detector testing

Experimental study for improving the angle dependence of the response of PADC-based personal neutron dosemeters

Abstract The large angle dependence of the H p (10) response in PADC-based personal neutron dosemeters constitutes a serious concern in operational radiation protection dosimetry. For planar dosemeters, the typical H p (10) response falls by half or more when the incidence angle changes from 0° to 60°. To reduce this source of systematic uncertainty, configurations based on multiple detectors at different angles have been developed, but their complex geometries constitute an important obstacle to the implementation in routine services. This works proposes a simplified configuration, based on two orthogonal PADC detectors, which is suitable for the implementation in the routine service of INFN-LNF (Frascati). This system was tested, using a ISO slab standard phantom, in the following reference neutron fields: 241 Am–Be and 252Cf(D2O) available at ENEA-Bologna, and 1.2 MeV, 5 MeV and 14.8 MeV mono-chromatic beams available at PTB Braunschweig. Incidence angles of 0°, ±15°, ±30°, ±45° and ±60° were chosen. The sum of the track density in the PADC parallel to the phantom and that in the detector normal to the phantom face, was regarded as "dosemeter reading". On this basis the Hp(10) response was calculated for different energies and incidence angles. As expected, the angular response of the two-orthogonal-element dosemeter is highly improved with respect to that of a single planar PADC

Experimental test of a newly developed single-moderator, multi-detector, directional neutron spectrometer in reference monochromatic fields from 144 keV to 16.5 MeV

A new directional neutron spectrometer called CYSP (CYlindrical SPectrometer) was developed within the NESCOFI@BTF (2011–2013) collaboration. The device, composed by seven active thermal neutron detectors located along the axis of a cylindrical moderator, was designed to simultaneously respond from the thermal domain up to hundreds of MeV neutrons. The new spectrometer condenses the performance of the Bonner Sphere Spectrometer in a single moderator; thus requiring only one exposure to determine the whole spectrum. The CYSP response matrix, determined with MCNP, has been experimentally evaluated with monochromatic reference neutron fields from 144 keV to 16.5 MeV, plus a 252Cf source, available at NPL (Teddington, UK). The results of the experiment confirmed the correctness of the response matrix within an overall uncertainty of ±2.5%. The new active spectrometer CYSP offers an innovative option for real-time monitoring of directional neutron fields as those produced in neutron beam-lines

Experimental characterization of semiconductor-based thermal neutron detectors

In the framework of NESCOFI@BTF and NEURAPID projects, active thermal neutron detectors were manufactured by depositing appropriate thickness of 6LiF on commercially available windowless p–i–n diodes. Detectors with different radiator thickness, ranging from 5 to 62 μm, were manufactured by evaporation-based deposition technique and exposed to known values of thermal neutron fluence in two thermal neutron facilities exhibiting different irradiation geometries. The following properties of the detector response were investigated and presented in this work: thickness dependence, impact of parasitic effects (photons and epithermal neutrons), linearity, isotropy, and radiation damage following exposure to large fluence (in the order of 1012 cm−2)

A Detailed Analysis of a Cygnus Loop Shock-Cloud Interaction

The XA region of the Cygnus Loop is a complex zone of radiative and nonradiative shocks interacting with interstellar clouds. We combine five far ultraviolet spectral observations from the Hopkins Ultraviolet Telescope (HUT), a grid of 24 IUE spectra and a high-resolution longslit Halpha spectrum to study the spatial emission line variations across the region. These spectral data are placed in context using ground-based, optical emission line images of the region and a far-UV image obtained by the Ultraviolet Imaging Telescope (UIT). The presence of high-ionization ions (OVI, NV, CIV) indicates a shock velocity near 170 km/s while other diagnostics indicate v_shock=140 km/s. It is likely that a large range of shock velocities may exist at a spatial scale smaller than we are able to resolve. By comparing CIV 1550, CIII 977 and CIII] 1909, we explore resonance scattering across the region. We find that a significant column depth is present at all positions, including those not near bright optical/UV filaments. Analysis of the OVI doublet ratio suggests an average optical depth of about unity in that ion while flux measurements of [SiVIII] 1443 suggest a hot component in the region at just below 10^6K. Given the brightness of the OVI emission and the age of the interaction, we rule out the mixing layer interpretation of the UV emission. Furthermore, we formulate a picture of the XA region as the encounter of the blast wave with a finger of dense gas protruding inward from the pre-SN cavity.Comment: 21 pages, 9 figures, accepted by the Astronomical Journal, July 2001 Full resolution figures available at http://fuse.pha.jhu.edu/~danforth/xa

A complementary compact laser based neutron source

Several experiments of neutron generation using high intensity laser sources, with a power exceeding 10^19W/cm^2 via TNSA (Target Normal Sheath Acceleration) or other similar methods, have been performed in the past years in different laboratories. However, so far there is no one running neutron source based on such a technology. In the framework of the Conceptual Report Design of a new accelerator in the Eupraxia project we are studying the possibility to have a laser-based neutron source, not only by TNSA but also from self-injection schemes. We focus our attention on the applications in cultural heritage studies as well also on the complementary role that such a source can have in the framework of large facilities devoted to radiation production.Comment: 4 pages, two figures, 3rd European Advanced Accelerators Concept

Interaction of Supernova Ejecta with Nearby Protoplanetary Disks

The early Solar System contained short-lived radionuclides such as 60Fe (t1/2 = 1.5 Myr) whose most likely source was a nearby supernova. Previous models of Solar System formation considered a supernova shock that triggered the collapse of the Sun's nascent molecular cloud. We advocate an alternative hypothesis, that the Solar System's protoplanetary disk had already formed when a very close (< 1 pc) supernova injected radioactive material directly into the disk. We conduct the first numerical simulations designed to answer two questions related to this hypothesis: will the disk be destroyed by such a close supernova; and will any of the ejecta be mixed into the disk? Our simulations demonstrate that the disk does not absorb enough momentum from the shock to escape the protostar to which it is bound. Only low amounts (< 1%) of mass loss occur, due to stripping by Kelvin-Helmholtz instabilities across the top of the disk, which also mix into the disk about 1% of the intercepted ejecta. These low efficiencies of destruction and injectation are due to the fact that the high disk pressures prevent the ejecta from penetrating far into the disk before stalling. Injection of gas-phase ejecta is too inefficient to be consistent with the abundances of radionuclides inferred from meteorites. On the other hand, the radionuclides found in meteorites would have condensed into dust grains in the supernova ejecta, and we argue that such grains will be injected directly into the disk with nearly 100% efficiency. The meteoritic abundances of the short-lived radionuclides such as 60Fe therefore are consistent with injection of grains condensed from the ejecta of a nearby (< 1 pc) supernova, into an already-formed protoplanetary disk.Comment: 57 pages, 16 figure

A Comparison of Ultraviolet, Optical, and X-Ray Imagery of Selected Fields in the Cygnus Loop

During the Astro-1 and Astro-2 Space Shuttle missions in 1990 and 1995, far ultraviolet (FUV) images of five 40' diameter fields around the rim of the Cygnus Loop supernova remnant were observed with the Ultraviolet Imaging Telescope (UIT). These fields sampled a broad range of conditions including both radiative and nonradiative shocks in various geometries and physical scales. In these shocks, the UIT B5 band samples predominantly CIV 1550 and the hydrogen two-photon recombination continuum. Smaller contri- butions are made by emission lines of HeII 1640 and OIII] 1665. We present these new FUV images and compare them with optical Halpha and [OIII], and ROSAT HRI X-ray images. Comparing the UIT images with those from the other bands provides new insights into the spatial variations and locations of these different types of emission. By comparing against shock model calculations and published FUV spectroscopy at select locations, we surmise that resonance scattering in the strong FUV permitted lines is widespread in the Cygnus Loop, especially in the bright optical filaments typically selected for observation in most previous studies.Comment: 21 pages with 10 figures. See http://www.pha.jhu.edu/~danforth/uit/ for full-resolution figure