Vacuum laser acceleration of super-ponderomotive electrons using relativistic transparency injection.

Intense lasers can accelerate electrons to very high energy over a short distance. Such compact accelerators have several potential applications including fast ignition, high energy physics, and radiography. Among the various schemes of laser-based electron acceleration, vacuum laser acceleration has the merits of super-high acceleration gradient and great simplicity. Yet its realization has been difficult because injecting free electrons into the fast-oscillating laser field is not trivial. Here we demonstrate free-electron injection and subsequent vacuum laser acceleration of electrons up to 20 MeV using the relativistic transparency effect. When a high-contrast intense laser drives a thin solid foil, electrons from the dense opaque plasma are first accelerated to near-light speed by the standing laser wave in front of the solid foil and subsequently injected into the transmitted laser field as the opaque plasma becomes relativistically transparent. It is possible to further optimize the electron injection/acceleration by manipulating the laser polarization, incident angle, and temporal pulse shaping. Our result also sheds light on the fundamental relativistic transparency process, crucial for producing secondary particle and light sources.

Characterizing laser-plasma ion accelerators driving an intense neutron beam via nuclear signatures.

Compact, bright neutron sources are opening up several emerging applications including detection of nuclear materials for national security applications. At Los Alamos National Laboratory, we have used a short-pulse laser to accelerate deuterons in the relativistic transparency regime. These deuterons impinge on a beryllium converter to generate neutrons. During the initial experiments where these neutrons were used for active interrogation of uranium and plutonium, we observed ß-delayed neutron production from decay of 9Li, formed by the high-energy deuteron bombardment of the beryllium converter. Analysis of the delayed neutrons provides novel evidence of the divergence of the highest energy portion of the deuterons (i.e., above 10 MeV/nucleon) from the laser axis, a documented feature of the breakout afterburner laser-plasma ion acceleration mechanism. These delayed neutrons form the basis of non-intrusive diagnostics for determining the features of deuteron acceleration as well as monitoring neutron production for the next generation of laser-driven neutron sources.

Bright laser-driven neutron source based on the relativistic transparency of solids.

Neutrons are unique particles to probe samples in many fields of research ranging from biology to material sciences to engineering and security applications. Access to bright, pulsed sources is currently limited to large accelerator facilities and there has been a growing need for compact sources over the recent years. Short pulse laser driven neutron sources could be a compact and relatively cheap way to produce neutrons with energies in excess of 10 MeV. For more than a decade experiments have tried to obtain neutron numbers sufficient for applications. Our recent experiments demonstrated an ion acceleration mechanism based on the concept of relativistic transparency. Using this new mechanism, we produced an intense beam of high energy (up to 170 MeV) deuterons directed into a Be converter to produce a forward peaked neutron flux with a record yield, on the order of 10(10) n/sr. We present results comparing the two acceleration mechanisms and the first short pulse laser generated neutron radiograph.

New technology for transmission measurements in process pipes.

Transmission measurements of radiation through process pipes provide a non-intrusive method of determining the amount of product present in the pipes. The product could be a liquid, a slurry, or a gas, which is the most challenging because of the low density. Traditionally, these techniques have used a radioactive source that has to be replaced periodically. We have developed a transmission technique based on an X-ray tube instead of a decaying source. A notch filter is used to provide a narrow transmission line, and a thin silicon transmission detector is used to monitor the X-ray tube output. The transmitted X-rays are measured with a high-throughput gamma spectrometer that consists of a NaI(Tl) detector and an MCA with precise dead time correction. This spectrometer provides stable transmission measurements with an accuracy of a fraction of a percent. The shielding and collimator are made of machinable tungsten for thermal mechanical stability, as well low-cost, low-weight tungsten powder in polymer castings. We describe two methods of measuring the pipe wall thickness without evacuating the pipe. Our particular application was for enrichment monitors for UF(6) in process pipes. Enrichment monitors that are independent of the plant data require two measurements: a transmission measurement to determine the total amount of uranium in the pipe and a measurement of the 186-keV gamma-ray line to determine the amount of (235)U present. The ratio of these values gives the enrichment. Previous designs used a decaying radioactive source such as (57)Co (122 keV, T(½)=272 days) or (109)Cd (22 keV, T(½)=1.2 years). A major effort was required to access and periodically replace these sources in operating plants. In this report, we describe the use of an X-ray tube, which eliminated the source problem, and other innovations. Then we present data from an enrichment monitor that incorporates these innovations.

Thermoluminescence property of LiMgF3 erbium activated phosphor.

The perovskite-like LiMgF(3):ErF(3) pellets were obtained from the melt formed by LiF and MgF(2) mixed salts in the stoichiometric ratio. The perovskite material was doped with 1, 2 and 4 mol% of ErF(3) impurity. The pellets samples were (60)Co gamma irradiated and their thermoluminescence (TL) properties were analyzed, i.e., dose-response, fading at RT and under UV irradiation, TL signal reproducibility, and kinetic parameters. The intensity of the TL response against irradiation dose was increased remarkably by the high concentration of impurity, and a linear dose-response was observed in the range of 1-10 Gy. The fading observed at RT was about 10-30% after 24h from irradiation. All samples were exposed from 1 to 200 Gy gamma dose range. The TL glow peaks were found around 367-376, 438-447, 509-521, and 594-611 K, when the doped samples were 1, 2 and 4 mol% of the erbium impurity concentration. The thermoluminescence kinetics parameters of the glow curves have been analyzed using the Computerized Glow Curve Deconvolution (CGCD) method.

Investigation of the PGNAA using the LaBr3 scintillation detector.

The Joint Research Centre of the European Commission develops instrumentation for detection of hazardous materials. In relation to this a new experimental facility was constructed for research into methods applying the detection of characteristic gamma rays subsequent to neutron irradiation. This includes the detection of prompt gamma rays from neutron inelastic scattering and neutron capture. For this purpose the device employs LaBr(3) scintillation detectors. The paper investigates the applicability of the LaBr(3) scintillation detector to PGNAA.

Design and characterisation of a pulsed neutron interrogation facility.

The Joint Research Centre recently obtained a license to operate a new experimental device intended for research in the field of nuclear safeguards. The research projects currently being planned for the new device includes mass determination of fissile materials in matrices and detection of contraband non-nuclear materials. The device incorporates a commercial pulsed neutron generator and a large graphite mantle surrounding the sample cavity. In this configuration, a relatively high thermal neutron flux with a long lifetime is achieved inside the sample cavity. By pulsing the neutron generator, a sample may be interrogated by a pure thermal neutron flux during repeated time periods. The paper reports on the design of the new device and the pulsed fast and thermal neutron source. The thermal neutron flux caused by the neutron generator and the graphite structure has been characterised by foil activation, fission chamber and (3)He proportional counter measurements.

A feasibility study of a single event spectrometer based on semiconductor devices.

The electronics employed around particle accelerators can be disturbed or damaged because of single event effects (SEE). The most likely effect is the single event upset (SEU) which may affect all memory devices. In the case of high energy accelerators, SEUs are mostly produced by secondary charged particles generated by neutron interactions. The measurement of the energy and the lineal energy distribution of these neutron-induced charged particles was proposed. As a first approach, a commercial p-i-n photodiode was employed. This device was irradiated with thermal and monoenergetic fast neutrons. Some effects limiting the use of such a detector as a SEE spectrometer were observed, giving guidelines for the design of an application specific integrated circuit (ASIC). The possibility of creating a solid state microdosemeter by coupling the ASIC with a tissue-equivalent radiator is discussed. Moreover, the p-i-n photodiode covered with a hydrogenated plastic radiator may be employed as a proton-recoil spectrometer.

[A case of non-familial ulcer mutilating acropathy of the foot]. / Un caso di acropatia ulcero-mutilante non familiare del piede

A case of non-familial pseudo-syringomyelic acropathy is presented; in view of its rarity, numerous hypotheses had to be considered with regard both to diagnosis and to pathogenesis. It was first taken to be an undoubted case of osteomyelitis, but the rapid advance of the osteolytic form, which resisted all treatment both local and general, whether antibiotic, surgical or orthopaedic, led to consideration of the possibility of sarcoma or some other benign tumoral form. The conclusion was reached that doubtful diagnosis can be settled only by bioptic examination, whereas aetiopathogenetic doubts still remain within the field of hypotheses (Lattuada).