Hard X-ray Pulses Research Articles

Short-pulse, high-brightness X-ray production with the PLEIADES Thomson-scattering source

PLEIADES is a compact, tunable, high-brightness, ultra-short-pulse, Thomson-scattering X-ray source. Picosecond pulses of hard X-rays (10–200 keV) are created by colliding an ultra-relativistic (20–100 MeV), picosecond-duration electron beam with a high-intensity, sub-picosecond, 800-nm laser pulse. Initial operation of this source has produced 78-keV X-rays with 106 photons per pulse using a 57-MeV, 0.3-nC, 50-μm rms width electron beam and a 180-mJ, 15-μm rms width laser pulse. The angular distribution, energy, and energy spectrum of the source are found to agree well with theory and simulations. Source optimization is expected to increase X-ray output to between 107 and 108 photons per pulse with a peak brightness approaching 1020 photons/s/0.1% bandwidth/mm2/mrad2.

Hard X-Ray Emission from a Copper Target by Focusing a Picosecond Laser Beam at 3×1013 W/cm2

Hard X-ray pulses are generated by focusing a picosecond laser beam (25-ps, 1064 nm) on a copper target in air at an intensity of 3×1013 W/cm2. X-ray energy is measured using an X-ray charge-coupled device. The obtained spectrum consists of strong Kα and Kβ emissions and a weak continuum at an energy range of 4–10 keV. The photon numbers of the hard X-rays (4–10 keV) were estimated to be approximately 4000 photons/4πsr/pulse.

Hard x-ray Pulsations in the Initial Phase of Flares

When analyzing light curves of hard X-ray bursts recorded by the Hard X-Ray Spectrometer on board the MTI satellite, we have found three events (all associated with major solar flares, two of them in the same active region) which show pulsations in the very initial phase of the burst. Periods of the pulsations range from 25 to 48 s. We compare them with other observations of pulsations of radio waves and in X-rays and conclude that pulsations of this kind have not been observed before. We mention several possible causes and prefer interactions between current-carrying loops as the most likely interpretation of the observed variations.

D-D neutron yield in the 125 J dense plasma focus Nanofocus

We present here a very small transportable dense plasma focus with 125 J of energy able to be used mainly as an intense fast neutron source. The aim of this work was to design, construct and experimentally study a very compact nuclear fusion apparatus, at the lower energy limit, useful for multiple applications, such as soil humidity measurements, inspection of several materials metallic inclusions, medical neutron-therapies, etc. Besides, the possibility of using the same device as X-rays emitter has been explored. In a narrow range of deuterium filling pressure around 1 mbar, peaked Rogowski dips are observed. Correspondingly, strong neutron and hard X-ray pulses are measured. The neutron pulses last, in average, 50 ns, being about 106 the amount of neutrons per pulse. The performance of this device has shown to be higher than any other plasma focus apparatus, compar ed on the empirical scaling law of neutron yield vs. pinch current.

Metal nanoplasmas as bright sources of hard X-ray pulses.

We report significant enhancements in light coupling to intense-laser-created solid plasmas via surface plasmon and "lightning rod" effects. We demonstrate this in metal nanoparticle-coated solid targets irradiated with 100 fs, 806 nm laser pulses, focused to intensities approximately 10(14)-10(15) W cm(-2). Our experiments show a 13-fold enhancement in hard x-ray yield (10-200 keV) emitted by copper nanoparticle plasmas formed at the focal volume. A simple model explains the observed enhancement quantitatively and provides pointers to the design of structured surfaces for maximizing such emissions.

Characteristics of ultrafast K line hard x-ray source from femtosecond terawatt laser-produced plasma

Theoretical studies and analytical scalings were carried out to find the optimized laser parameters and target conditions so that ultrashort hard x-ray pulses and high x-ray power could be achieved. The dependence of laser intensity and wavelength on the yield of K-shell x-ray emission was studied. We propose an optimal design for a foil target for producing high-yield hard x-ray pulses of customizing duration.

A High‐Energy Study of the Geminga Pulsar

We present the results of deep X-ray and gamma-ray observations of the Geminga pulsar obtained in the final years of the ASCA and CGRO missions, and an upper limit from RXTE. A phase-connected ephemeris from the gamma-rays is derived that spans the years 1973-2000, after allowing for a minor glitch in frequency of Delta f/f = 6.2 x 10^-10 in late 1996. ASCA observations of the hard X-ray pulse profile in 1994 and 1999 confirm this glitch. An improved characterization of the hard X-ray pulse profile and spectrum from the long ASCA observation of 1999 confirms that there is a non-thermal X-ray component that is distinct from the gamma-ray spectrum as measured by EGRET. It can be parameterized as a power-law of photon index Gamma = 1.72 +/- 0.10 with a flux of 2.62 x 10^-13 ergs/cm^2/s in the 0.7-5 keV band and pulsed fraction 0.54 +/- 0.05, similar to, but more precise than values measured previously. An extrapolation of this spectrum into the energy band observed by the RXTE PCA is consistent with the non-detection of pulsed emission from Geminga with that instrument. These results are interpreted in the context of outer-gap models, and motivations for future X-ray observations of Geminga are given.

Discovery of a High-Latitude Accreting Millisecond Pulsar in an Ultracompact Binary

We have identified the third known accretion-powered millisecond pulsar, XTE J0929-314, with the Rossi X-Ray Timing Explorer. The source is a faint, high Galactic latitude X-ray transient (d ≳ 5 kpc) that was in outburst during 2002 April-June. The 185 Hz (5.4 ms) pulsation had a fractional rms amplitude of 3%-7% and was generally broad and sinusoidal, although occasionally double-peaked. The hard X-ray pulses arrived up to 770 μs earlier than the soft X-ray pulses. The pulsar was spinning down at an average rate of = (-9.2 ± 0.4) × 10-14 Hz s-1; the spin-down torque may arise from magnetic coupling to the accretion disk, a magnetohydrodynamic wind, or gravitational radiation from the rapidly spinning pulsar. The pulsations were modulated by a 43.6 minute ultracompact binary orbit, yielding the smallest measured mass function (2.7 × 10-7 M☉) of any stellar binary. The binary parameters imply an ≃0.01 M☉ white dwarf donor and a moderately high inclination. We note that all three known accreting millisecond pulsars are X-ray transients in very close binaries with extremely low mass transfer rates. This is an important clue to the physics governing whether or not persistent millisecond pulsations are detected in low-mass X-ray binaries.

Picosecond Time-Resolved X-ray Diffraction Using Laser-Induced X-ray Pulse.

Generation of ultrashort hard X-ray pulses using laser-driven electron X-ray sources and picosecond time-resolved X-ray diffraction measurements are reviewed. Hard X-ray pulses are characterized by measurements of energy spectrum and pulse duration. Picosecond lattice dynamics of laser-irradiated silicon crystal measured by time-resolved X-ray diffraction is described.

The timing of non-thermal soft X-ray emission line broadenings in solar flares

We study 59 solar limb flares using the Bragg Crystal Spectrometer (BCS) on Yohkoh and the Burst and Transient Source Experiment (BATSE) to investigate the relative timings between the Hard X-Ray (HXR) emission and the observed non-thermal broadenings of X-ray emission lines (Vnt). We show that the duration of the HXR flux rise to maximum emission aects the relative timing of the main Vnt peak with respect to the main HXR peak. In20% of the flares studied, secondary peaks in Vnt are observed. These are always associated with a strong HXR pulse and usually occur after the associated HXR pulse. There are also flares that show a relationship between the decay times of Vnt and HXR flux. These results are conducive to a causal relationship between the HXR flux and Vnt. We divided the sample of flares into two classes, gradual rise and impulsive rise, depending on the shape of the HXR lightcurve up to maximum emission. We show that the Vnt behaviour diers in the two classes. The implications are discussed with a view to understanding the mechanism of Vnt generation.

Reflection of atto-and femtosecond X-ray pulses from aperiodic multilayer mirror

The reflection of femto-and attosecond soft and hard X-ray pulses from multilayer structures (MSs) was investigated. The conditions are formulated under which the pulse shape and duration barely change upon reflection. The MSs are optimized for the reflection of extremely short pulses, and the possibility of compressing chirped pulses is considered. It is shown that the power reflection coefficient can be appreciably larger than unity. The aperiodic MSs capable of effectively reflecting pulses containing no more than three wave periods are calculated.

Observation of hard X-ray pulses with a highly sensitive streak camera

We have developed a highly sensitive X-ray streak camera system, which synchronously operates with the RF signal of the SPring-8 storage ring. The streak camera was installed at an undulator beamline of SPring-8, and the beam loading effect for various electron bunch structures (filling pattern) has been observed. The camera has also been operated as a timing monitor for a synchronization system of synchrotron radiation and a Ti:sapphire laser. The highly sensitive X-ray streak camera system can be used not only as an accelerator diagnosis, but also as a fast temporal detector for beamline applications, such as the observation of fast temporal transitions of diffraction images and relaxation process.

Laser produced X-ray source in the 10-60 keV range at 1 kHz. Modified irradiation schemes in order to reach medical imaging quality

By tightly focusing ultra-short pulses from a Ti:sapphire terawatt laser onto a high-Z metallic target, hard x-ray pulses of short duration are produced. In most of our previous work concerning x-rays, a 150 mJ laser pulse with a 110 A duration has been used. Using mostly tin and tantalum targets, hard x-rays in the 10-60 keV range have been produced and used in differential absorption imaging around the K-alpha absorption edge of a contrast agent and also in imaging employing gated viewing for suppression of scattered radiation. In order to increase the x-ray yield (shortening the acquisition time) an increase in the laser repetition rate is desirable while still staying in the K-alpha energy regime. We have used a I kHz repetition-rate laser delivering 35 fs pulses in order to work towards these goals. We have clear evidence of hard x-ray generation above 30 keV, even for low laser pulse energies. We also studied the effect of a fs prepulse. The medical imaging capability of the source was explored. The use of a prepulse has been optimized in order to improve the image quality as well as the overall x-ray generation yield.

Small plasma focus studied as a source of hard X-ray

An experimental study on hard X-ray pulses emitted by a small plasma focus is performed, regarding its application to biological radiographs in fast dynamic situations. It is found that the radiation intensity in a single shot is high enough to obtain fine resolution radiographs in very short exposures (about 10 ns). As an example, a radiograph on a live mouse is shown. It is found that the relation between responses of different detectors, i.e., thermoluminescence dosimeters and scintillator-photomultiplier, is linear.

Numerical studies on the properties of femtosecond laser plasma Kα sources

One of the features of ultra high intensity (UHI) short pulse laser–matter interactions is the prospect of creating a cheap, compact source of hard X rays with femtosecond pulse duration. The properties of such Kα sources are studied using analytical and numerical models of hot electron generation and subsequent transport in a range of materials (Reich et al., 2000). First, we find that there is an optimum laser intensity for Kα generation from bulk targets, which scales as Z4.4. Second, we show that efficient hard X-ray pulses with durations below 100 fs can be generated at intensities of ∼1016 W/cm2.

Instantaneous measurement system of the effective photon energy for a single shot x-ray pulse radiation

For a short pulse x-ray radiation with extremely broad spectrum such as Bremsstrahlung, the effective photon energy cannot be determined by conventional x-ray detectors with only a single x-ray pulse shot. An instantaneous measurement system capable of determining the energy with a single shot has been developed. The system is made of a metal–oxide–semiconductor linear array image sensor, a scintillation phospher, and an absorber material, and a control system of the sensor. The energy of a hard x-ray pulse with a short pulse duration of 500 ns is determined with this system. It is shown that the system can determine the energy more quickly and accurately than with the conventional method.

Discovery of Pulsed X-Ray Emission from the Small Magellanic Cloud Transient RX J0117.6−7330

We report on the detection of pulsed, broadband, X-ray emission from the transient source RX J0117.6-7330. The pulse period of 22 s is detected by the ROSAT/PSPC instrument in a 1992 September 30-October 2 observation and by the Compton Gamma-Ray Observatory/BATSE instrument during the same epoch. Hard X-ray pulsations are detectable by BATSE for approximately 100 days surrounding the ROSAT observation (1992 August 28-December 8). The total directly measured X-ray luminosity during the ROSAT observation is 1.0 × 10^(38) (d/60 kpc)^2 ergs s^(-1). The pulse frequency increases rapidly during the outburst, with a peak spin-up rate of 1.2 × 10^(−10) Hz s^(-1) and a total frequency change of 1.8%. The pulsed percentage is 11.3% from 0.1-2.5 keV, increasing to at least 78% in the 20-70 keV band. These results establish RX J0117.6-7330 as a transient Be binary system.

The Outbursts and Orbit of the Accreting Pulsar GS 1843−02 = 2S 1845−024

We present observations of a series of 10 outbursts of pulsed hard X-ray flux from the transient 10.6 mHz accreting pulsar GS 1843-02, using the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory. These outbursts occurred regularly every 242 days, coincident with the ephemeris of the periodic transient GRO J1849-03 (Zhang et al. 1996), which has recently been identified with the SAS 3 source 2S 1845-024 (Soffitta et al. 1998). Our pulsed detection provides the first clear identification of GS 1843-02 with 2S 1845-024. We present a pulse timing analysis which shows that the 2S 1845-024 outbursts occur near the periastron passage of the neutron star's highly eccentric (e = 0.88+-0.01) 242.18+-0.01 day period binary orbit about a high mass (M > 7 solar masses) companion. The orbit and transient outburst pattern strongly suggest the pulsar is in a binary system with a Be star. Our observations show a long-term spin-up trend, with most of the spin-up occurring during the outbursts. From the measured spin-up rates and inferred luminosities we conclude that an accretion disk is present during the outbursts.

Deconvolution of Directly Precipitating and Trap‐Precipitating Electrons in Solar Flare Hard X‐Rays. II.Compton Gamma Ray ObservatoryData Analysis

Based on the deconvolution method developed in the first paper of this series, we present here the data analysis of 20-200 keV hard X-ray (HXR) data from the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) recorded during 103 solar flares in 1991-1995. These are all of the flares simultaneously observed by CGRO with high time resolution (64 ms) and by Yohkoh in flare mode. The deconvolution method takes the measured HXR count rates as function of energy and time, I(ε, t), and computes the following self-consistently: the electron injection function n(E, t), the directly precipitating electron flux nprec(E, t), the trapped-precipitating flux ntrap(E, t), the fraction of directly precipitating electrons (qprec), the electron time-of-flight distance (lTOF), and the electron density at the loss cone site of the trap (ne). We find that the electron time-of-flight distances (lTOF = 20.0 ± 7.3 Mm) inferred with the deconvolution method are fully consistent with those obtained earlier using a Fourier filter method. The trap electron densities (ne = 1010.96±0.57 cm-3) obtained from deconvolving the e-folding decay times of HXR pulses (according to the trap model of Melrose & Brown) are found to be statistically a factor of 1.5 lower than those inferred from cross-correlation delays. The fraction qprec of directly precipitating electrons, measured for the first time here, is found to have a mean (and standard deviation) of qprec = 0.42 ± 0.16. Based on this precipitation fraction, we infer loss cone angles of α0 ≈ 20°-70° and magnetic mirror ratios of R = Bloss/Binj ≈ 1.2 - 3 (with a median value of Rmedian = 1.6) between the loss cone site and injection/acceleration site, assuming an isotropic pitch angle distribution at the injection site. The TOF distances and mirror ratios constrain magnetic scale heights in flare loops to λB = 10-150 Mm. The fact that this two-component model (with free-streaming and trapped electrons) satisfactorily fits the energy-dependent time delays in virtually all flares provides strong evidence that electron time-of-flight differences and collisional scattering times dominate the observed HXR timing, while the injection of electrons appears to be synchronized (independent of energy) and does not reveal the timing of the acceleration process.

Comparative study of ion, x-ray and neutron emission in a low energy plasma focus

In a low energy (2.3 kJ) Mather-type deuterium plasma focus, neutron and x-ray emission is investigated by time integrated and time resolved detectors. CR-39 nuclear track ion detectors are employed for measuring charged particle angular distribution. Correlation of charged particles with neutron and x-ray emission is also investigated. The neutron emission profile is found to be composed of two pulses, the intensity and anisotropy of which vary with the filling pressure. The charged particle flux is maximum with high fluence anisotropy for the pressure range 2.5-3.0 mbar which is also the optimum pressure for high neutron emission with low fluence anisotropy . The high neutron emission with low fluence anisotropy is attributed to the presence of trapped deuterons in an anomalous magnetic field. The relevant pressure range generates favourable conditions for plasma density and pinch filament diameter. X-ray emission is generally high at low pressure. For the pressure range of 2.5-4.0 mbar, the axial neutron detector registers a hard x-ray pulse, which may escape through a half inch thick Cu flange. These results suggest that at low pressures, the collapsing current sheath interacts with the anode end and causes intense low energy x-ray emission, but the neutron emission remains low. X-rays are dominantly Cu . In the narrow pressure regime 2.5-3.0 mbar, the current sheath forms a pinch filament leading to high neutron yield with low fluence anisotropy.