High-pressure Gas Target Research Articles

Design and experimental validation of differential pumped vacuum system for HINEG windowless gas target

High Intensity Neutron Generator (HINEG) is a facility aiming for neutronics design validation, materials and components irradiation test, nuclear waste transmutation and nuclear technology applications, etc. A high-pressure windowless gas target system is adopted for HINEG to achieve a higher neutron yield. Therefore, a three-stage differential vacuum system has been developed to couple the pressure transition of 6 orders of magnitude to the 300 kV/50 mA high-intensity beam transport in a series of large-diameter pipes. A suitable pumping system has been developed to increases the target pressure which is used to generate D-T neutron yield of 2.1 × 1013 n/s. A windowless pressure transition from high gas pressure (1 × 103 Pa) in the gas target chamber to high vacuum (3 × 10−3 Pa) in the accelerator is reached. In this paper, a new differential pumped vacuum system is designed for HIENG. Innovatively, the newly designed differential vacuum system can not only meet the transport requirements of the high-current low-energy deuteron beam, but also couple the pressure transition of 6 orders of magnitude, which can solve the contradiction of the large-diameter pipe required for high-intensity beam transport and the small diameter pipe needed for the high-pressure difference.The experimental validation and design optimization of differential pumped vacuum system is introduced. The experiment results indicate the validity of the theoretical calculation. Compared to molecular pumps, roots pumps might be more suitable for windowless differential vacuum system coupled to large-diameter apertures and pipes. The design could be used for reference in other windowless gas target systems coupled to accelerator with high-current low-energy ion beam.

Phase-Matching for Generation of Isolated Attosecond XUV and Soft-X-Ray Pulses with Few-Cycle Drivers

Isolated attosecond pulses (IAPs) produced through laser-driven high-harmonic generation (HHG) hold promise for unprecedented insight into biological processes via attosecond x-ray diffraction with tabletop sources. However, efficient scaling of HHG towards x-ray energies has been hampered by ionization-induced plasma generation impeding the coherent buildup of high-harmonic radiation. Recently, it has been shown that these limitations can be overcome in the so-called 'overdriven regime' where ionization loss and plasma dispersion strongly modify the driving laser pulse over small distances, albeit without demonstrating IAPs. Here, we report on experiments comparing the generation of IAPs in argon and neon at 80 eV via attosecond streaking measurements. Contrasting our experimental results with numerical simulations, we conclude that IAPs in argon are generated through ionization-induced transient phase-matching gating effective over distances on the order of 100 $\mu$m. We show that the decay of the intensity and blue-shift due to plasma defocussing are crucial for allowing phase-matching close to the XUV cutoff at high plasma densities. We perform simulations for different gases and wavelengths and show that the mechanism is important for the phase-matching of long-wavelength, tightly-focused laser beams in high-pressure gas targets, which are currently being employed for scaling isolated attosecond pulse generation to x-ray photon energies.

Compact 200 kHz HHG source driven by a few-cycle OPCPA

We present efficient high-order harmonic generation (HHG) based on a high-repetition rate, few-cycle, near infrared (NIR), carrier-envelope phase stable, optical parametric chirped pulse amplifier (OPCPA), emitting 6 fs pulses with 9 μJ pulse energy. In krypton, we reach conversion efficiencies from the NIR to the extreme ultraviolet (XUV) radiation pulse energy on the order of ∼10−6 with less than 3 μJ driving pulse energy. This is achieved by optimizing the OPCPA for a spatially and temporally clean pulse and by a specially designed high-pressure gas target. In the future, the high efficiency of the HHG source will be beneficial for high-repetition rate two-colour (NIR-XUV) pump-probe experiments, where the available pulse energy from the laser has to be distributed economically between pump and probe pulses.

High-harmonic generation at 250 MHz with photon energies exceeding 100 eV

Ultrafast spectroscopy in the extreme ultraviolet demands for ever-higher pulse repetition rates and photon energies. Here, we drive cavity-enhanced high-order harmonic generation (HHG) at a repetition rate of 250 MHz, with 30 fs pulses and an average power of 10 kW. Employing an optimized cavity geometry and a high-pressure gas target, we couple out nanowatt-level harmonics at photon energies around 100 eV. This constitutes an improvement of more than two orders of magnitude over previous megahertz-repetition-rate HHG experiments and paves the way toward high-photon-energy frequency-comb spectroscopy and toward pump-probe photoelectron microscopy and spectroscopy at unprecedented repetition rates.

Helicity dependence of the γ 3He → πX reactions in the Δ(1232) resonance region

The helicity dependences of the differential cross sections for the semi-inclusive γ 3He → π0 X and γ 3He → π± X reactions have been measured for the first time in the energy region 200 < E γ 450 MeV. The experiment was performed at the tagged photon beam facility of the MAMI accelerator in Mainz using a longitudinally polarised high-pressure 3He gas target. Hadronic products were measured with the large-acceptance Crystal Ball detector complemented with additional devices for charged-particle tracking and identification. Unpolarised differential cross sections and their helicity dependence are compared with theoretical calculations using the Fix-Arenhovel model. The effect of the intermediate excitation of the Δ(1232) resonance can be clearly seen from this comparison, especially for the polarised case, where nuclear effects are relatively small. The model provides a better theoretical description of the unpolarised charged pion photoproduction data than the neutral pion channel. It does significantly better in describing the helicity-dependent data in both channels. These comparisons provide new information on the mechanisms involved in pion photoproduction on 3He and suggest that a polarised 3He target can provide valuable information on the corresponding polarised quasi-free neutron reactions.

First measurement of the helicity dependence of 3He photoreactions in the [formula omitted] resonance region

The first measurement of the helicity dependence of the total inclusive 3He photo-absorption cross section and of the partial cross sections for several reaction channels was carried out at MAMI (Mainz) in the photon energy range between 150 and 500 MeV. The experiment used the large acceptance Crystal Ball spectrometer, complemented by charged particle and vertex detectors, a circularly polarised tagged photon beam and a longitudinally polarised high-pressure 3He gas target. The results obtained give information on the GDH integral on 3He and on the neutron and allow an investigation of the modifications of nucleon properties inside 3He.

Helicity dependent meson photoproduction on 3He in the Δ-resonance region

The first measurement of the helicity dependence of the total inclusive photo-absorption cross section and of partial photon-induced reaction channels on 3He was carried out at MAMI (Mainz) in a photon energy range between 200 and 450 MeV. The experiment used the large acceptance Crystal Ball spectrometer, complemented by a charged particle and a vertex detector, a circularly polarized tagged photon beam and a longitudinally polarized high-pressure 3He gas target. The results obtained give information on the GDH sum rule on the neutron and allow an investigation of the modifications of nucleon properties inside 3He.

Pygmy dipole strength in86Kr and systematics ofN=50isotones

The dipole strength of the $N=50$ nucleus ${}^{86}$Kr was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.9 and 11.2 MeV delivered by the linear accelerator ELBE as well as using quasimonoenergetic and linearly polarized $\ensuremath{\gamma}$ rays of 10 energies within the range from 4.7 to 9.3 MeV delivered by the HI$\ensuremath{\gamma}$S facility. A high-pressure gas target was used. We identified 39 levels up to an excitation energy of 10.1 MeV. Simulations of $\ensuremath{\gamma}$-ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section derived in this way up to the neutron-separation energy is combined with the photoabsorption cross section obtained from a $(\ensuremath{\gamma},n)$ experiment at HI$\ensuremath{\gamma}$S. The enhanced $E1$ strength found in the range from 6 to 10 MeV is compared with the ones in the $N$ $=$ 50 isotones ${}^{88}$Sr, ${}^{90}$Zr, and ${}^{92}$Mo and with predictions of calculations within the quasiparticle-phonon model.

Biodistribution and dosimetry of 18F-EF5 in cancer patients with preliminary comparison of 18F-EF5 uptake versus EF5 binding in human glioblastoma

The primary purpose of this study was to assess the biodistribution and radiation dose resulting from administration of (18)F-EF5, a lipophilic 2-nitroimidazole hypoxia marker in ten cancer patients. For three of these patients (with glioblastoma) unlabeled EF5 was additionally administered to allow the comparative assessment of (18)F-EF5 tumor uptake with EF5 binding, the latter measured in tumor biopsies by fluorescent anti-EF5 monoclonal antibodies. (18)F-EF5 was synthesized by electrophilic addition of (18)F(2) gas, made by deuteron bombardment of a neon/fluorine mixture in a high-pressure gas target, to an allyl precursor in trifluoroacetic acid at 0° then purified and administered by intravenous bolus. Three whole-body images were collected for each of ten patients using an Allegro (Philips) scanner. Gamma counts were determined in blood, drawn during each image, and urine, pooled as a single sample. PET images were analyzed to determine radiotracer uptake in several tissues and the resulting radiation dose calculated using OLINDA software and standard phantom. For three patients, 21mg/kg unlabeled EF5 was administered after the PET scans, and tissue samples obtained the next day at surgery to determine EF5 binding using immunohistochemistry techniques (IHC). EF5 distributes evenly throughout soft tissue within minutes of injection. Its concentration in blood over the typical time frame of the study (∼3.5h) was nearly constant, consistent with a previously determined EF5 plasma half-life of ∼13h. Elimination was primarily via urine and bile. Radiation exposure from labeled EF5 is similar to other (18)F-labeled imaging agents (e.g., FDG and FMISO). In a de novo glioblastoma multiforme patient, focal uptake of (18)F-EF5 was confirmed by IHC. These results confirm predictions of biodistribution and safety based on EF5's characteristics (high biological stability, high lipophilicity). EF5 is a novel hypoxia marker with unique pharmacological characteristics allowing both noninvasive and invasive measurements.

Low-lyingE1,M1, andE2strength distributions inXe124,126,128,129,130,131,132,134,136: Systematic photon scattering experiments in the mass region of a nuclear shape or phase transition

Systematic nuclear resonance fluorescence (NRF) experiments on all nine stable (seven even-even and two odd-mass) Xe isotopes have been performed at the bremsstrahlung facility of the 4.3-MV Stuttgart Dynamitron accelerator. For the first time thin-walled, high-pressure gas targets (about 70 bar) with highly enriched target material were used in NRF experiments. Precise excitation energies, transition strengths, spins, and decay branching ratios were obtained for numerous states, most of them previously unknown. The systematics of the observed $E1$ two-phonon excitations (${2}^{+}\ensuremath{\bigotimes}{3}^{\ensuremath{-}}$) and $M1$ excitations to ${1}^{+}$ mixed-symmetry states in the even-even isotopes are discussed with respect to the new critical point symmetry E(5). The fragmentation of these fundamental dipole excitation modes in the odd-mass isotopes $^{129,131}\mathrm{Xe}$ is shown and discussed. In the even-even nuclei several low-lying $E2$ excitations were observed.

Measurement of the elastic magnetic form factor of (3)He at high momentum transfer.

New electron scattering measurements have been made that extend data on the (3)He elastic magnetic form factor up to Q(2) = 42.6 fm(-2). These new data test theoretical conjectures regarding non-nucleonic effects in the three-body system. The very small cross sections, as low as 10(-40) cm(2)/sr, required the use of a high-pressure cryogenic gas target and a detector system with excellent background rejection capability. No existing theoretical calculation satisfactorily accounts for all the available data.

Filling and Recycling Apparatus of a Cyclotron Target with Enriched Krypton for Production of Radiopharmaceuticals

An apparatus for multiple filling of a cyclotron target with enriched Kr gas is described. The system is based on recycling pressurized gas by cryogenic pumping between the target tube and storage containers. The design and construction makes use of previous experience in the construction and operation of two analogue apparatuses for Xe124 high pressure gas targets, but major modifications have been incorporated, evoked by the different physical properties of Kr, by the character of the nuclear reaction, and by the demand for automation from the side of the end user.

Performance of a plasma window for a high pressure differentially pumped deuterium gas target for mono-energetic fast neutron production – Preliminary results

The reactions D(d,n) 3He and T(d,n) 4He are frequently used for production of the mono-energetic or quasi mono-energetic neutron beams but successful applications are often limited by the intensity of the generated neutron beams. The development of a suitable neutron source for such applications as studies of resonance phenomena, fast neutron radiography, selective fast neutron activation, explosives and contraband detection and others, depends on the output ion current of the accelerator and the design of the target system. A practical solution for a high pressure gas target was previously developed and successfully implemented at De Beers Diamond Research Laboratory in Johannesburg (Guzek et al., 1999), but it is limited to applications using low (<20%) duty cycle accelerators. The concept of a plasma window for the separation of a high pressure gas target region and accelerator vacuum, that was originally developed by Hershcovitch (1995) for electron welding applications, may be suitable for operation with continuous wave accelerators at high particle current output. Preliminary test results, which have been performed with various gases (argon, helium and deuterium), indicate that implementation of the plasma window into a gas target system, for the production of intense mono-energetic fast neutron beams will be achievable.

11C-methane production in small volume, high pressure gas targets

The parameters affecting the production of11C-methane, in situ, in small volume, high-pressure gas targets include target chamber size and material. The results are based on experiments that varied the target gas composition, the target material and geometry. Methane production yields were typically 65% of the yields of11CO2in the same target chamber.

Gamow-Teller strength from the 20Ne(n,p)20F reaction at En=198 MeV.

Cross sections for the $^{20}\mathrm{Ne}$(n,p${)}^{20}$F reaction at ${\mathit{E}}_{\mathit{n}}$=198 MeV were measured at five angles using the TRIUMF charge-exchange facility and a novel high-pressure (20 atm, or 2.03 MPa) gas target. The L=0, L=1, and L\ensuremath{\ge}2 contributions to the cross sections were obtained from a multipole decomposition. Using an empirical proportionality between the cross section and Gamow-Teller (GT) strength from \ensuremath{\beta} decay, GT strength was deduced up to an excitation of 10 MeV in $^{20}\mathrm{F}$. Most of the strength is concentrated in a discrete state at ${\mathit{E}}_{\mathit{x}}$=1.0 MeV, with the extracted strength of the \ensuremath{\Delta}L=0 continuum at high excitation having a large error. These results are compared with measurements of GT strength using other probes. Comparison with the measured magnetic dipole (M1) strength in $^{20}\mathrm{Ne}$ shows that there is constructive interference between the spin part and a large orbital part of the M1 transition to the 11.26 MeV state in $^{20}\mathrm{Ne}$ (analog of the 1.0 MeV state in $^{20}\mathrm{F}$). If all the strength from the multipole decomposition is used, we find agreement of the total GT strength compared to untruncated sd shell-model calculations which use free-nucleon operators, although experimental and theoretical distributions differ in detail. The confirmation of strong orbital M1 contributions at the beginning of the sd shell together with systematics of GT quenching in heavier sd shell nuclei may provide evidence for meson exchange currents in M1 transitions.

Distribution of spin dipole transition strength in the 15N(n,p)15C reaction.

The reaction $^{15}\mathrm{N}$(n,p${)}^{15}$C was studied at a neutron energy of 288 MeV using the TRIUMF (n,p) charge-exchange facility and a high-pressure gas target. The angular distributions for spin dipole (\ensuremath{\Delta}L=1) transitions to the states in $^{15}\mathrm{C}$ at energies 0 and 0.740 MeV, as well as for higher excitation energies, were measured and the results were compared with distorted-wave impulse approximation calculations. The measured distribution of the spin dipole strength agrees well with shell-model predictions, indicating that a rather simple model provides a satisfactory description of the $^{15}\mathrm{N}$ ground state, and of positive-parity states in $^{15}\mathrm{C}$ up to about 18 MeV excitation. The magnitude of the peak cross sections (at \ensuremath{\simeq}7\ifmmode^\circ\else\textdegree\fi{}) is described well by the calculations when the theoretical cross section is renormalized by a factor of 0.7. The calculated cross sections near 0\ifmmode^\circ\else\textdegree\fi{} are generally smaller than experimental data. If this is a general feature of \ensuremath{\Delta}L=1 transitions, it suggests that estimates of Gamow-Teller strength based on a multipole decomposition of measured cross sections may be too high.

Optical studies of the influence of an intense ion beam on high-pressure gas targets

Optical studies of the penetration of protons, deuterons and α-particles (energy range 2–18 MeV) in helium, neon and argon gas targets have been made. The beam was photographed in the light emitted by atoms of the target gas. Through an interferometric arrangement, interferograms of the target gas were recorded showing variations in the refractive index of the gas caused by the particle beam. An asymmetric density reduction in the beam volume was seen in the form of an increased penetration range as a function of the vertical position in the beam. This density reduction was studied as a function of the beam current. The asymmetric density reduction is due to an upward gravitational transport of the heated gas in the target chamber. Estimates of the vertical temperature distributions were made from the recorded interferograms.

Elastic electric and magnetic e-d scattering at low momentum transfer

Up to q 2 = 4 fm −2 differential cross sections for elastic electron-deuteron scattering have been measured for a wide range of scattering angles. Using a combination of high pressure gas target and liquid target systems we have obtained high-accuracy data for the ratio e-p to e-d scattering and a small normalization error for backward scattering data. For extreme low momentum transfer the elastic structure deuteron rms radius has been extracted: r d = 1.9660 ± 0.0068 fm. This quantity, which is a characteristic parameter of the nucleon-nucleon potential was compared with the results of different models. The analysis of the isoscalar form factor indicates no significant contribution of a non-resonant three-pion state. The high-accuracy data of the electric neutron form factor could be extended to higher q 2. All nucleon form factors are described with a consistent ansatz. The magnetic scattering data indicate the important role of isobar configuration in nucleon-nucleon interactions.

Measurements of Short-Pulse Backscatter from a Gas Target

Measurements are reported of short-pulse (100 psec), 1.06-\ensuremath{\mu}m laser backscatter from a high-pressure nitrogen gas target. Spectral measurements clearly show the expected Brillouin red shift. The backscatter fraction saturated at \ensuremath{\sim}40% at 2\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{cm}}^{2}$ and what appears to be multiple Brillouin scattering in a long-gradient-length plasma is observed.

Absolute electron-proton cross sections at low momentum transfer measured with a high pressure gas target system

Absolute differential cross sections for elastic electron-proton scattering have been measured in a four-momentum transfer range up to 1.4 fm −2. Using a high pressure gas target system, we have obtained highly accurate data with a small normalization error of 0.5%. The electromagnetic form factors G E and G M have been extracted and the rms charge radius has been determined to be 〈r 2 E 〉 p 1 2 = 0.862±0.012 fm . The shape of the isovector spectral function near threshold shows a significant non-resonant contribution of the two-pion state. This enhancement is so strong that the derivative at q 2 = 0 differs considerably from the usual vector meson dominance model value. This result is in good agreement with theoretical predictions.