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Short pulse laser ablation of semiconductors and metals is studied by means of ultrafast timeresolved microscopy. The characteristic stages of the conversion of solid material into hot fluid matter undergoing ablation are identified. Initially metallic material transforms during the expansion into a transparent state with a high index of refraction. [S0031-9007(98)06502-8]

Thermal- and nonthermal melting in gallium arsenide after femtosecond laser excitation has been investigated by means of time resolved microscopy. Electronic melting within a few hundred femtoseconds is observed for rather strong excitation and the data reveal a distinct threshold fluence of $150{\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}$ for this nonthermal process. Below that threshold melting occurs on a 100 ps time scale and is of thermal nature. Using a simple numerical model we describe this type of the phase transition as heterogeneous melting under strongly overheated conditions.

Modification of the surface structure of solid materials by laser radiation involves a complex chain of processes. The first step is the deposition of a certain amount of optical energy in the material. The character of the material excitation is strongly dependent on the laser pulse duration. With the use of ultrashort laser pulses non-equilibrium energy distributions with large excess population in the excited states can be produced. The distinct physical processes which come into play in laser-solid interaction on the ultrafast time scale open new routes of modifying the structure and the morphology of materials and offer interesting perspectives in laser materials processing.

Femtosecond-laser-induced changes of the optical reflectivity and the reflected second harmonic are measured over a wide range of times and laser fluences. Changes of the linear and nonlinear optical properties suggestive of a transition to a metallic state have been observed. For relatively low fluences these changes take tens of picoseconds to develop. For higher fluences the transition takes place in just a few hundred femtoseconds. Our data suggest that, depending on the laser excitation conditions, two distinctly different types of phase transformations are observed.

We have investigated femtosecond laser-induced ablation of gallium arsenide using time-of-flight mass spectroscopy. At the ablation threshold, we estimated surface temperatures on the order of 3500 K. We observed a clear thresholdlike effect in the number of detected particles and with increasing fluence free flight desorption transforms into a collisional expansion process. Above the ablation threshold, the behavior of gallium particles can be quantitatively described through Knudsen-layer theory.

Time of flight (TOF) distributions of evaporated particles from the surface of GaAs have been measured in order to determine its maximum temperature during femtosecond irradiation (100 fs, 620 nm). A quadrupole mass spectrometer (QMS) provides the TOF distribution of a given particle mass after laser excitation.

Femtosecond laser induced ablation from solid surfaces has been investigated by means of time resolved microscopy. On transparent materials ablation is initiated by dielectric breakdown and formation of a dense and hot surface plasma. Measurements of the plasma threshold yield values of a few times 10(13) W/cm(2) with little variation among different materials. This indicates that microscopic surface properties are responsible for surface breakdown. On absorbing semiconductors and metals near-threshold ablation is brought about by hydrodynamic expansion of the lasergenerated hot and pressurized matter. Upon expansion into vacuum initially metallic materials transform into a transparent state with a high refractive index. The observed behavior is related to general properties of matter in the liquid-gas coexistence regime.

The Standard Radiation Environment Monitor (SREM) was designed in a cooperation between ESA-ESTEC, PSI and Contraves Space-the manufacturer. The monitor detects cosmic particles using 3 direction sensitive Si detectors. Total dose measurement is performed with internal and external RADFETs located at selected positions on the satellite. Their sensitivity towards protons and gammas was characterised for various energies, dose rates and exposure geometry.

Electronic boards for the High Energy Solar Spectroscopic Imager HESSI satellite contain almost exclusively COTS devices. Mission oriented components' qualification for total dose radiation hardness and single event phenomena were performed using proton beams. Results are reported for variety of low power parts including CCD, DSP and ADCs.

Irradiation facility for parasitic testing of electronic components was developed and is installed in PSI OPTIS area. Radiation effects experiments and testing are performed using proton beams with energies up to 65 MeV. The facility, constructed as a low energy extension to the PSI Proton irradiation Facility PIF, is designed in a similar, user friendly manner.

Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern. Optically sharp surface occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Velocity distributions of evaporated particles from laser irradiated GaAs and Silicon give a temperatures in excess of 3000 K at the ablation threshold. The results suggest that explosive boiling causes material removal.

Femtosecond laser induced ablation from solid surfaces has been investigated by means of time resolved microscopy. On transparent materials ablation is initiated by dielectric breakdown and formation of a dense and hot surface plasma. Measurements of the plasma threshold yield values of a few times 1013 W/cm2 with little variation among different materials. This indicates that microscopic surface properties are responsible for surface breakdown. On absorbing semiconductors and metals near-threshold ablation is brought about by hydrodynamic expansion of the laser generated hot and pressurized matter. Upon expansion into vacuum initially metallic materials transform into a transparent state with a high refractive index. The observed behavior is related to general properties of matter in the liquid-gas coexistence regime.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have investigated the dynamics of femtosecond laser-induced ablation from the surface of various materials. Combining pump-probe techniques with optical microscopy we have monitored the structural modifications of the irradiated surfaces both in space and time. In the fluence regime below the threshold for plasma formation ablation is caused by the hydrodynamic expansion of laser-heated material. Upon expansion into vacuum each of the investigated materials (Si, GaAs, Al, Au, Mg, Hg) evolves from the initial metallic state into an optically transparent phase with high index of refraction. (C) 1998 Elsevier Science B.V.

A prospective study has been carried out in 33 children with congenital heart disease, aged 5 days to 16 years, operated on with the aid of cardiopulmonary bypass (CPB), in order to evaluate intra and postoperative glycemia in relation to children's weight and intra-CPB perfusion temperature.Plasmatic levels of glucose, lactate, insulin, C-peptide, growth hormone (GH) and cortisol were measured at five different times, from pre-CPB up to 18 hours post-CPB. Thirteen children weighing less than 10 kilograms (group I) were compared to 20 children weighing more than 10 kilograms (group II), and 15 children operated on with perfusion temperature below 27 degrees C (group A) were compared to 18 with perfusion temperature above 27 degrees C.All the cases showed intraoperative hyperglycemia. In relation to weight, evolution of glycemia was similar in both groups but anaerobic glycolysis was significantly higher in group I. In relation to temperature, glycemia, lactic acidemia intra- and post-CPB and serum C-peptide post-CPB were significantly higher in group A. Intraoperative insulin was decreased and GH serum levels were also higher in group I and A, but fell drastically post-CPB in all groups.Perioperative control of glycemia is important in children operated on with the aid of CPB. Control of lactic acidemia is also expedient. Carbohydrates metabolism and its hormonal regulation are more disturbed in infants and in children operated on with low perfusion temperature.

Time-resolved optical microscopy during femtosecond laser irradiation of amorphous GeSb films provides evidence for a non-thermal phase transition occurring on a subpicosecond time scale.

Ultrafast time resolved microscopy has been used to study the dynamics of femtosecond laser induced ablation from the surfaces of various semiconductors and metals. Upon ablation initially metallic matter transforms into a transparent state with a high index of refraction.

Time resolved imaging has been used to analyze structural transformations induced by intense 100 fs laser pulses in amorphous GeSb films. Above a threshold of 19 mJ/cm2 the data show the formation of a transient nonequilibrium state of the excited material within 300 fs. The results are consistent with an electronically induced, amorphous-to-crystalline phase transition., J. S. acknowledges funding by the European Community (Training and Mobility of Researchers, ERB 40001GT954352). This work has been partially supported by CICYT (Spain, TIC93-0125).

The stent has demonstrated to be a useful device in the prevention of postangioplasty coronary restenosis and it is expected to have a favourable effect as an alternative or complementary treatment of stenotic lesions in arteries or veins associated with congenital defects. The aim of this study is to analyze our experience in this setting.From February 1992 to March 1996, 28 stenting procedures were performed in 26 patients (mean age: 8.6 +/- 0.7 years; mean weight: 26.2 +/- 3 kg). In 12 patients, stenting was single, and a iliac Palmaz stents were always used. Stenting location was: pulmonary artery branches in 17 patients, right ventricular outflow in 2 patients, in the junction of right atrium with pulmonary artery in 2 patients, systemic veins in 2 patients and in post Mustard intratrial channel stenosis in 2 patients. 25 patients had previously undergone at least one surgical procedure.The stenotic diameter of the treated lesions increased significantly after the procedure (4.4 +/- 0.3 mm before stenting vs 11.6 +/- 0.3 mm after stenting, p0.0001) and the transtenotic gradient decreased from 38.1 +/- 5 to 12 +/- 3.8 mmHg. Those changes were associated with a diminution of right ventricular pressure (81.6 +/- 3 vs 56.7 +/- 6 mmHg, p0.0001) in patients with pulmonary branch stenosis without septal defects. There was no mortality among the percutaneously treated patients and only one patient needed surgery. Nevertheless, one patient died after bilateral intraoperative stenting.The treatment of proximal or distal stenotic lesions in the pulmonary tree, systemic veins, and obstructed intraatrial channels with stents, can replace or complement conventional balloon angioplasty. It also offers a useful and effective alternative to surgery, when it is impossible or carries a risk.

Foreign body retrieval in the catheter room is a useful procedure at any age, but, although its interest, few reports of such technique have been reported in children.We review and present our experience in 8 children, aged 5 days to 11 years, five of them having congenital hearts defects.We retrieved 4 catheter fragments, 2 endocardial electrode catheter tips, 1 Rashkind 12 mm. PDA umbrella, and 1 detachable Jackson coil. All of them were placed in systemic veins, right heart chambers or pulmonary arteries. We used biplane fluoroscopy and percutaneous right femoral vein puncture in all cases. Goose-Neck (Microvena Corporation) snares were used in 5 patients, hand made snares in 2 and a Swan-Ganz catheter in one. In 4 cases, the snare was introduced trough a Mullins long sheath and the foreign body pulled into its distal end, in order to bring it out of the femoral vein. Six foreign bodies came off the femoral vein: 4 trough the puncture site and 2 needing a venous cut-down. The two remaining foreign bodies, stopped while pulling at the common iliac vein and a minor surgical procedure was needed for final extraction.Therapeutic catheterization is the technique of choice for intravascular foreign body retrieval in children.

During the past several years a number of reports have been published [1-5] suggesting that an ultrafast order-disorder transition can occur within approximately 100 fs when semiconductors (typically Si and GaAs) are irradiated with laser pulses 100 fs or less in duration. The evidence for such transitions was largely based on the observed changes of the optical reflectivity which were shown to be consistent with a transition from a covalent crystalline to a metallic liquid state.

A decrease of the reflected second harmonic and an increase of the optical reflectivity is observed in GaAs following intense femtosecond laser excitation. Our data indicate that a structural phase transformation from the crystalline state to a disordered metallic liquid takes place in less than one picosecond.

High-spin properties of 130Ce have been studied by using the reaction (12C, 6n). The γ-rays following this reaction were detected by six NaI(Tl) detectors in a two-dimensional coincidence arrangement. An experimental γ-ray energy-energy correlation spectrum was extracted from the original coincidence matrix. The matrix of the correlated spectrum shows a valley along the 45° diagonal extending up to Eγ ≈ 1.2 MeV, thus indicating a collective behaviour beyond the region of known discrete lines. From the width of the valley a collective moment of inertia is deduced.

High-spin properties of even 118–122Xe isotopes have been studied in experiments when a 118 MeV 12C beam impinged on 114Cd and 116Cd targets. The γ-rays emitted following the reactions were detected by six NaI(Tl) counters. The coincidence events from the counters were sorted into a two-dimensional matrices, from which the energy-correlation spectra were extracted. From the energy correlation spectra the quantities of the collective moment of inertia I (2) are deduced and compared with data obtained for the same nuclei in previous experiments, as well as with results from theoretical studies.

A 128-channel time-to-digital converter with a resolution from 0.5 μs up to 0.45 s per channel is described. The time base is taken from a crystal oscillator, which can be replaced by an RC-oscillator increasing the time width per channel up to 1000 s. The instrument is primarily designed for measurements of nuclear lifetimes longer than a few μs. The converter contains a read-out buffer which permits several time intervals to be measured with one common start pulse. A digital delay and a sampling function are incorporated in the instrument. Description and figures of merits are given. The measurement of a half-life in 180 W, produced in the pulsed beam from a cyclotron, is used as an illustration of the performance.

The ARC timing described has excellent timing properties even when using a wide energy range, e.g. from 10 keV to over 1 MeV. The detector signal from a preamplifier is accepted directly by the unit as a timing filter amplifier with a sensitivity of 1 mV is incoroorated. The adjustable rise time rejection feature makes it possible to achieve a good prompt time spectrum with symmetrical exponential shape down to less than 1/100 of the peak value. A complete block diagram of the unit is given together with results of extensive tests of its performance. For example the time spectrum for (1330 ± 20) keV of 60Co taken with a 43 cm3 Ge(Li) detector has the following parameters: fwhm = 2.2 ns, fwtm = 4.4 ns and fw (0.01) m = 7.6 ns and for (50 ± 10) keV of 22Na the following was obtained: fwhm = 10.8 ns, fwtm = 21.6 ns and fw (0.01) m = 34.6 ns. In another experiment with two fast plastic scintillators (NE 102A) and using a 20% dynamic energy range the following was measured: fwhm = 280 ps, fwtm = 470 ps and fw (0.01) m = 710ps.

A search for high-spin isomeric states in the 138,139Ce anc 141,142Nd nuclei has been made using (α, xn) reactions and the intrinsic pulsing of the 225-cm Stockholm cyclotron. In 138Ce, besides the known 7- isomeric state at 2 128.8 keV, another one at 3 538.5 keV, 10+, with T 1/2 = (78 ± 5) ns has been found. In 139Ce a new isomeric state at 2 631.5 keV, 19/2-, with T 1/2 = (70 ± 5) ns has been identified. Also in 141Nd a high-spin isomeric level exists probably at 3 104.9 keV with T 1/2 = (26 ± 5) ns. In 142Nd the level at 3 453.0 keV, with spin I = 6, shows a half-life T 1/2 = (1.3 ± 0.1) ns while that at 4 718.5 keV, I = 8, decays with a half-life of about 0.8 ns. The time distributions in 142Nd show that the 3 242.9 keV, I = 7, or the 3 456.2 keV, I = 8, level is also delayed and decays with T 1/2 = (1.6 ± 0.1) ns. The deduced transition probabilities are compared to theoretical predictions.

This paper describes multi-detector arrangements for measuring the number of quanta emitted in a cascade. Two set-ups are described, one with NaI(Tl) detectors and another with liquid scintillators. The latter system has the advantage of being able to separate between neutron and γ-ray events, which is of particular importance for reactions with a large number of neutrons emitted. The set-ups are used on-line the Stockholm cyclotron. Examples of experiments with 51 MeV α-particles and 118 MeV 12 C-ions are presented.

High-spin properties of even Pd and Cd isotopes have been studied by means of a 118 MeV 12 C beam impinging onto a 100 Mo target. The γ-rays following the reaction were recorded using six NaI(Tl) detectors in a two-dimensional coincidence arrangement. Energy-correlation spectra were extracted from the coincidence matrix and used to obtain information on the collective moment of inertia I . The gross properties of the behaviour of this moment of inertia are discussed within an extended Nilsson-Strutinsky model. The experimentally observed variation of the moment of inertia at high rotational frequencies is tentatively explained as being due to an increase of I as the band approaches its termination.

A constant fraction discriminator equipped with automatic controlled triggering point based on a feedback loop controller principle is described. The circuit can accept at the input dc (±150mB) and low frequency interferences (±100 mV, ⩾ 400 Hz) as well as high counting rates of change (0–10 MHz of 10 ns fwhm, −2 V pulses) without degradation of timing performance. The trigger was designed for signal rates up to 100 MHz and provides walk equal to ±100 ps over a dynamic range of 400. The timing performance of the discriminator compared to some commercially available ones is shown.

The idea of automatic walk stabilization for constant fraction discriminator based on compensated base line restoration is presented and tested. Comparison of this idea to gated base line restoration and other principles is discussed.

A fast single-channel analyser with built-in gate and adjustable delay for external timing pulses is described. Typical performance of a CFD/SCA-system is presented.

The γ-ray deexcitation of high spin states in 130,134,136Ce has been studied when targets of 122,128,130Te are bombarded with 12C-ions of 100 MeV. The γ-rays are detected in a coincidence arrangement consisting of six hexagonal NaI(Tl)-detectors and from the data recorded γγ-energy correlation matrices and γ-ray multiplicities are deduced. The two heavier isotopes 134,136Ce do not exhibit a clear rotation behaviour but with the light target a valley extending up to Eγ = 1.25 MeV is seen in the γγ-energy correlation matrix.

The dynamics of fs-laser ablation of graphite has been investigated experimentally and theoretically. The experimental observation of two different ablation mechanisms is supported by molecular dynamics calculations, which incorporate the changes of the interatomic potentials due to electronic excitation.