Generation of characteristic x rays by a terawatt femtosecond chromium-forsterite laser

Characteristic K _α x rays arising when a metallic target is irradiated by femtosecond infrared pulses that are generated by a terawatt chromium-forsterite laser system (1240 nm, 90 mJ, 80 fs) are studied. The absolute yield (up to 3 × 10⁸ photons/sr pulse) and the coefficient of the transformation of laser radiation to K _α radiation (maximum value ≈0.03%) are measured for an iron target. The dependence of the radiation intensity on the angle of incidence of p polarized laser radiation is analyzed. The mechanisms of the production of fast electrons responsible for generating characteristic x rays are discussed.     

Experimental study of nuclear processes in the presence of superstrong fields of a picosecond laser plasma

Nuclear processes in the presence of the superstrong laser fields of a picosecond laser plasma are experimentally studied at a radiation intensity of 2 × 10¹⁸ W/cm² on a Neodim laser setup with a power of 10 TW. Experimental data regarding neutron generation on the surface of a deuterated target (CD₂)_n owing to the thermonuclear fusion ²H(d,n)³He and the neutron generation on the Be target due to the photonuclear reaction ⁹Be(γ,n)2α are presented. Neutron yields Y _n of 10⁶ and 10³ per 4π sr per laser pulse are obtained for the (CD₂)_n and Be targets, respectively. The alpha-particle yield is measured for the first time in the neutron-free thermonuclear reactions ¹¹B + H → 3⁴He in the laser plasma on the surface of the composite B + (CH₂)_n targets. The alpha-particle yield is 10³ per 4π sr per laser pulse.     

Nuclear fusion from Coulomb explosions of deuterated methane clusters subjected to ultraintense femtosecond laser pulses

This paper reports that Coulomb explosions taken place in the experiment of heteronuclear deuterated methane clusters ((CD₄)₂) in a gas jet subjected to intense femtosecond laser pulses (170mJ, 70fs) have led to table-top laser driven DD nuclear fusion. The clusters produced in supersonic expansion had an average size of about 5nm in radius and the laser intensity used was 3×10¹⁷W/cm².The measured maximum and average energies of deuterons produced in the laser--cluster interaction were 60 and 13.5keV, respectively. From DD collisions of energetic deuterons, a yield of 2.5(±0.4)×10⁴ fusion neutrons of 2.45MeV per shot was realized, giving rise to a neutron production efficiency of about 1.5×10⁵ per joule of incident laser pulse energy. Theoretical calculations were performed and a fairly good agreement of the calculated neutron yield with that obtained from the present experiment was found.     

π° photoproduction on quasi-free neutrons in the reactionγd→π°np in theΔ region

The possibility of obtaining information about the reactionγn→π°n in theΔ resonance region from the reactionγd→π°np is analyzed. To this aim, the differential cross sectionsd ³ σ/dE _n dΩ _π dΩ _n both for unpolarized particles and in case of polarized photons and deuterons as well as the asymmetryΣ for linearly-polarized photons are calculated at photon energies from 250 to 400 MeV in the diagrammatic approach. The contributions from the pole diagrams as well as one-loop diagrams both withn-p andπ-N-rescattering are taken into account. The main contribution to the differential cross sections ofπ° photoproduction on the neutron in the quasi-free kinematics arises from the neutron pole diagram. The correction due ton-p rescattering decreases with increasing pion angle and becomes to be less than 8% atΘ _π ≥ 90°. The contributions of the proton pole diagram and the one ofπ-N rescattering were found to be negligible. Background effects are more pronounced in the case of the asymmetryΣ. However, they are also strongly suppressed in the neutron quasi-free kinematics at photon energies above 300 MeV.     

The e + e −→π0π0γ process below 1.0 GeV

The e ⁺ e ^?→π⁰π⁰ γreaction was studied in the energy range 0.36–0.97 GeV on a VEPP-2M e ⁺ e ^? collider with an SND detector. The decay probabilities were found to be B(ω→ π⁰π⁰γ)= (7.8 ±2.7±2.0)×10^?5 and B(π→ π⁰π⁰γ)= (4.8 _?1.8 ^+3.4 ±0.2)×10^?5.     

γpol+N→π±+N and the hadron weak current

It is pointed out that a very direct way of testing current ideas of the QCD renormalization of the weak neutral interaction between hadrons is to measure the asymmetry in yield induced by left- and right-polarized photons in the reactions γ_pol+N→π^±+N. Recent calculations predict the asymmetry to be in the range of 1.5 to 8.0 × 10^?7.     

Table-top kHz hard X-ray source with ultrashort pulse duration for time-resolved X-ray diffraction

The interaction of ultrashort laser pulses with solid state targets is studied concerning the production of short X-ray pulses with photon energies up to about 10 keV. The influence of various parameters such as pulse energy, repetition rate of the laser system, focusing conditions, the application of prepulses, and the chirp of the laser pulses on the efficiency of this highly nonlinear process is examined. In order to increase the X-ray flux, the laser pulse energy is increased by a 2nd multipass amplifier from 750 μJ to 5 mJ. By applying up to 4 mJ of the pulse energy a X-ray flux of 4×10¹⁰ Fe K _α photons/s or 2.75×10¹⁰ Cu K _α photons/s are generated. The energy conversion efficiency is therefore calculated to η _Fe≈1.4×10⁻⁵ and η _Cu≈1.0×10⁻⁵. The X-ray source size is determined to 15×25 μm². By focusing the produced X-rays using a toroidally bent crystal a quasi-monochromatic X-ray point source with a diameter of 56 μm×70μm is produced containing ≈10⁴ Fe K _α1 photons/s which permits the investigation of lattice dynamics on a picosecond or even sub-picosecond time scale. The lattice movement of a GaAs(111) crystal is shown as a typical application.     

Neutron production in a picosecond laser plasma at a radiation intensity of 3×1017W/cm2

Neutron production as a result of the reaction ²H(d, n)³He in a picosecond laser plasma is reported. A considerable neutron yield of 5×10⁴ per pulse is obtained for the first time in a picosecond laser plasma on the surface of a solid deuterated target at laser radiation intensity of 3×10¹⁷ W/cm².     

X-ray generation from slanting laser–Compton scattering for future energy-tunable Shanghai Laser Electron Gamma Source

There is great interest in the generation of energy-tunable, bright, short-pulse X/γ-ray sources, which are required in various research fields. Laser–Compton scattering (LCS) is considered to be one of the most promising methods to implement this kind of X/γ-ray source. At the 100-MeV LINAC of the Shanghai Institute of Applied Physics, a 2-J, 8-ns, 1064-nm, Q-switched Nd:YAG laser is brought to a slanting collision at 40° (44°) with an 112-MeV, 0.9-ns (rms) relativistic electron beam. We measured the LCS X-ray energy spectrum with a peak energy of 31.73±0.22_stat±1.64_syst keV and a peak width (rms) of 0.74±0.26_stat±0.03_syst keV. This preliminary investigation was carried out to understand the feasibility of developing an energy-tunable X/γ-ray source. Based on this study, the future Shanghai Laser Electron Gamma Source (SLEGS) at the Shanghai Synchrotron Radiation Facility (SSRF) can be constructed to be not only an energy-tunable γ-ray source by guiding the laser incident angle from laser–Compton scattering, but also a high flux (～10¹⁰ photons/s or even higher) γ-ray source by adding a laser super-cavity.     

Excitation of X rays by electrons accelerated in air in the wake wave of a laser pulse

The characteristics of X rays of a laser plasma generated in the interaction of a femtosecond pulse with solid targets in an air atmosphere have been investigated. It has been shown that the mechanism for the generation of X rays in the interaction of short intense laser pulses with solid targets in a gas atmosphere is attributed to the generation of fast electrons in the region of the filamentation of a laser pulse. It has been proven experimentally that under such conditions, the solid target irradiated by laser radiation of even a low density of about 10¹⁵ W/cm² very efficiently emits ∼10-keV photons. It has been shown theoretically that the maximum energy of accelerated electrons can reach ɛ_max ∼ 100–200 keV under these conditions. This means that the proposed method can provide characteristic radiation with the energy of photons much higher than 10 keV.     

New measurements of the radiative decay modes of the φ-meson

The radiative decay models of the φ-meson have been studied: e⁺e^?→φ→ηγ→3γ; e⁺e^?→φ→π^oγ→3γ. Cross sections σ_{φ→ηγ→3γ} and σ_{φ→π^oγ→3γ} have been measured at five energies in the φ-meson energy region and clearly show the φ-resonance in the ηγ → 3γ mode as well as in the π^oγ → 3γ mode. From a Breit-Wigner fit to the experimental data the values of the branching ratios are deduced: B_φ→ηγ = (1.5 ± 0.4) × 10^?2; B_φ→π^oγ = (1.4 ± 0.5) × 10^?3.     

Neutron yield upon the irradiation of thin TiD<Subscript>2</Subscript> foils with a superintense laser pulse

The yield of neutrons from the thermonuclear-fusion reaction D(d, n)³He induced in a thin skin layer by the interaction of a high-intensity laser pulse of picosecond duration with thin TiD₂ foils is calculated. A multiple ionization of titanium atoms at the leading edge of the laser pulse is considered. The heating of free electrons proceeds via induced inverse bremsstrahlung in elastic electron scattering on multiply charged titanium ions. The electron temperature is calculated. It proves to be about 10 keV at the laser-pulse intensity of 5×10¹⁸ W/cm² at the peak. The neutron yield is estimated at 10⁴ per laser pulse. These results are in qualitative agreement with experimental data.     

Search for rare φ decays in π+π−γ final state

A search for φ radiative decays has been performed using a data sample of about 2.0 million φ decays collected by the CMD-2 detector at VEPP-2M collider in Novosibirsk. From the selected e⁺e⁻→π⁺π⁻γ events the following results were obtained:B(φ→f₀(980)γ)<1×10⁻⁴ for destructive andB(φ→f₀(980)γ)<7×10⁻⁴ for constructive interference with the Bremsstrahlung process respectively,B(φ→γ→π⁺π⁻γ)<3×10⁻⁵ for E_γ>20 MeV,B(φ→ργ)<7×10⁻⁴. From the selected e⁺e⁻→μ⁺μ⁻γ eventsB(φ→μ⁺μ⁻γ)=(2.3±1.0)×10⁻⁵ has been obtained for E_γ>20 MeV. The upper limit on the P,CP-violating decay η→π⁺π⁻ has also been placed:B(η→π⁺π⁻)<9×10⁻⁴. All upper limits are at 90% C.L.     

Decay of the positronium molecule into a positronium atom and photons

Decays of the positronium molecule Ps₂ into para- or orthopositronium Ps in the ground state and photons are investigated. The differential probabilities of the decays are determined. The total probabilitiesw _2γ ^(Ps2) andw _3γ ^(Ps2) of Ps₂ annihilation with the production of two and three photons and positronium are calculated to be $$w_{2\gamma }^{(P_{S_2 } )} = 1.6 \cdot 10^{10} \sec ^{ - 1} ,w_{3\gamma }^{(P_{S_2 } )} = 0.43 \cdot 10^8 \sec ^{ - 1} $$ . The curve of the angular correlation of the γ rays on the decay of Ps₂ into two photons and parapositronium is studied. The width of this curve is Δ¦P¦=0.128 a.u. (¦P¦ is the total photon momentum), which corresponds to a deviation of the emission angle of the γ rays from π: θ ? 0.934 mrad. The maximum in the distribution of the photons with respect to the momenta ¦P¦ in the center of mass of the annihilating pair is attained at ¦P¦=0.175 a.u. The calculations were made on an M-222 computer, and their accuracy is determined by the choice of the wave function of the positronium molecule and the accuracy in the computer calculation of the integrals.     

Determination of Moisture Content in Coke with <Superscript>239</Superscript>Pu–Be Neutron Source and BGO Scintillation Gamma Detector

A series of experiments has been conducted at the Frank Laboratory of Neutron Physics (FLNP) of the Joint Institute for Nuclear Research (JINR) in order to study the possibility of determining the moisture content of coke using a standard neutron source. The proposed method is based on a measurement of the spectrum of prompt γ rays emitted when samples are irradiated by fast and/or thermal neutrons. The moisture content is determined from the area of the peaks of characteristic γ rays produced in the radiative capture of thermal neutrons by the proton (E_γ = 2.223 MeV) and inelastic scattering of fast neutrons by ¹⁶O (E_γ = 6.109 MeV). The ²³⁹Pu–Be neutron source (〈E _n 〉 ~ 4.5 MeV) with an intensity of ~5 × 10⁶ n/s was used to irradiate the samples under study. A scintillation detector based on a BGO crystal was used to register the characteristic γ radiation from the inelastic fast neutron scattering and slow (thermal) neutron capture. This paper presents the results of humidity measurement in the range of 2–50% [1, 2].     

The (γ, n) reaction on 16O between giant resonance and pion threshold

A new TOF facility has been built for measurements of differential (γ, n) cross sections to discrete final states of light nuclei in the photon energy range between giant resonance and pion threshold. The observed neutron angle θ_n can continuously be varied between 0° and 150°, and additionally measurements at 175° and 180° are possible. Differential cross sections for the reaction ¹⁶O(γ, n₀)¹⁵O are presented for E_γ = 60 MeV (40° ? θ_n ? 149°) and for θ_n = 90° (60 MeV ? E_γ ? 160 MeV). The results, combined with the corresponding (γ, p₀) cross sections, indicate an absorption mechanism of high energy photons by neutron-proton pairs.     

πo photoproduction on quasi-free neutrons in the reaction γd → πo np in the Δ region

The possibility of obtaining information about the reaction γn → π^on in the Δ resonance region from the reaction γd → π^onp is analyzed. To this aim, the differential cross sections d³σ/dE_ndΩ_πdΩ_n both for unpolarized particles and in case of polarized photons and deuterons as well as the asymmetry Σ for linearly-polarized photons are calculated at photon energies from 250 to 400 MeV in the diagrammatic approach. The contributions from the pole diagrams as well as one-loop diagrams both with n-p and π-N-rescattering are taken into account. The main contribution to the differential cross sections of π^o photoproduction on the neutron in the quasi-free kinematics arises from the neutron pole diagram. The correction due to n-p rescattering decreases with increasing pion angle and becomes to be less than 8% at Θ_π ≥ 90°. The contributions of the proton pole diagram and the one of π -N rescattering were found to be negligible. Background effects are more pronounced in the case of the asymmetry Σ. However, they are also strongly suppressed in the neutron quasi-free kinematics at photon energies above 300 MeV.     

Radiative tau decay into pion

The possibility of the experimental investigation of the radiative tau decay into pion is discussed including background conditions. The decay width is estimated to be Br(τ→v _τ π γ)=0.15% (at the minimal registered photon energy 50 MeV). The contribution of the structural component is 10% of the total radiation. The main background is produced by photons from the τ→v _τ π ^? π ⁰ decay. We investigate the angular and energy spectra of the decay products and analyse the background conditions. The kinematic region was found where the structural radiation dominates over background.     

Searches for rare and forbidden neutral decays of η mesons at the GAMS-4π facility

Searches for rare and forbidden neutral decays of η mesons are performed at the GAMS-4π facility. A charge-exchange reaction at an initial momentum of 32.5 GeV/c was used as a source of 3.7 × 10⁶ η mesons. The following upper limits are established at a 90% confidence level: Br(η → 3γ) < 1.6 × 10^?4, Br(η → 4γ) < 2.8 × 10^?4, Br(η → π⁰π⁰) < 3.5 × 10^?4, Br(η → π⁰π⁰γ) < 1.7 × 10^?3, Br(η → π⁰π⁰γγ) < 4.0 × 10^?3, Br(η → 3π⁰γ) < 2.4 × 10^?4, and Br(η → 4π⁰) < 2.0 × 10^?5.     

The dependence of the Fe <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> yield on the chirp of the femtosecond exciting laser pulse

The hard X-ray yield generated with femtosecond laser pulses is studied for differently chirped irradiating laser pulses. The radiation of a Ti:sapphire CPA laser system (29 fs, 750 μJ, 1 kHz) is focused onto an iron containing solid state target producing incoherent hard X-ray radiation, Bremsstrahlung as well as target-specific K_α and K_β lines. The hard X-ray yield has been optimized by introducing negative and positive group delay dispersion (GDD) and third order dispersion (TOD) to the femtosecond laser pulse. The K_α yield could be enhanced by a factor of 1.7 and reached 1.9×10⁸ Fe K_α photons/s in 4π with the laser pulse positively chirped, and 1.5×10⁸ Fe K_α photons/s with the pulse negatively chirped. When the pulse energy is lowered to about 400 μJ the yield maximum at negative chirp vanishes and only the maximum at positive chirp remains. We explain this behavior with an increased electron temperature caused by the induced GDD and TOD in the pulse. PACS 42.65.Re; 52.38.Ph; 52.50.Jm