Ne Gas Target Research Articles

First direct limit on the 334 keV resonance strength in ^{22}Ne(alpha ,gamma )^{26}Mg reaction

In stars, the fusion of ^{22}Ne and ^4He may produce either ^{25}Mg, with the emission of a neutron, or ^{26}Mg and a gamma ray. At high temperature, the (alpha ,n) channel dominates, while at low temperature, it is energetically hampered. The rate of its competitor, the ^{22}Ne(alpha ,gamma )^{26}Mg reaction, and, hence, the minimum temperature for the (alpha ,n) dominance, are controlled by many nuclear resonances. The strengths of these resonances have hitherto been studied only indirectly. The present work aims to directly measure the total strength of the resonance at E_{text {r}} = 334 keV (corresponding to E_{text {x}} = 10949 keV in ^{26}Mg). The data reported here have been obtained using high intensity ^4He^+ beam from the INFN LUNA 400 kV underground accelerator, a windowless, recirculating, 99.9% isotopically enriched ^{22}Ne gas target, and a 4pi bismuth germanate summing gamma -ray detector. The ultra-low background rate of less than 0.5 counts/day was determined using 63 days of no-beam data and 7 days of ^4He^+ beam on an inert argon target. The new high-sensitivity setup allowed to determine the first direct upper limit of 4.0,times ,10^{-11} eV (at 90% confidence level) for the resonance strength. Finally, the sensitivity of this setup paves the way to study further ^{22}Ne(alpha ,gamma )^{26}Mg resonances at higher energy.

Direct measurement of low-energyNe22(p,γ)Na23resonances

The $^{22}$Ne(p,$\gamma$)$^{23}$Na reaction is the most uncertain process in the neon-sodium cycle of hydrogen burning. At temperatures relevant for nucleosynthesis in asymptotic giant branch stars and classical novae, its uncertainty is mainly due to a large number of predicted but hitherto unobserved resonances at low energy. Purpose: A new direct study of low energy $^{22}$Ne(p,$\gamma$)$^{23}$Na resonances has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA), in the Gran Sasso National Laboratory, Italy. Method: The proton capture on $^{22}$Ne was investigated in direct kinematics, delivering an intense proton beam to a $^{22}$Ne gas target. $\gamma$ rays were detected with two high-purity germanium detectors enclosed in a copper and lead shielding suppressing environmental radioactivity. Results: Three resonances at 156.2 keV ($\omega\gamma$ = (1.48\,$\pm$\,0.10)\,$\cdot$\,10$^{-7}$ eV), 189.5 keV ($\omega\gamma$ = (1.87\,$\pm$\,0.06)\,$\cdot$\,10$^{-6}$ eV) and 259.7 keV ($\omega\gamma$ = (6.89\,$\pm$\,0.16)\,$\cdot$\,10$^{-6}$ eV) proton beam energy, respectively, have been observed for the first time. For the levels at 8943.5, 8975.3, and 9042.4 keV excitation energy corresponding to the new resonances, the $\gamma$-decay branching ratios have been precisely measured. Three additional, tentative resonances at 71, 105 and 215 keV proton beam energy, respectively, were not observed here. For the strengths of these resonances, experimental upper limits have been derived that are significantly more stringent than the upper limits reported in the literature. Conclusions: Based on the present experimental data and also previous literature data, an updated thermonuclear reaction rate is provided in tabular and parametric form. The new reaction rate is significantly higher than previous evaluations at temperatures of 0.08-0.3 GK.

Probing astrophysically important states in theMg26nucleus to study neutron sources for thesprocess

The $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction is the dominant neutron source for the slow neutron capture process ($s$-process) in massive stars and contributes, together with the $^{13}$C($\alpha$,n)$^{16}$O, to the production of neutrons for the $s$-process in Asymptotic Giant Branch (AGB) stars. However, the reaction is endothermic and competes directly with the $^{22}$Ne($\alpha,\gamma)^{26}$Mg radiative capture. The uncertainties for both reactions are large owing to the uncertainty in the level structure of $^{26}$Mg near the alpha and neutron separation energies. These uncertainties are affecting the s-process nucleosynthesis calculations in theoretical stellar models. Indirect studies in the past have been successful in determining the energies, $\gamma$-ray and neutron widths of the $^{26}$Mg states in the energy region of interest. But, the high Coulomb barrier hinders a direct measurement of the resonance strengths, which are determined by the $\alpha$-widths for these states. The goal of the present experiments is to identify the critical resonance states and to precisely measure the $\alpha$-widths by $\alpha$ transfer techniques . Hence, the $\alpha$-inelastic scattering and $\alpha$-transfer measurements were performed on a solid $^{26}$Mg target and a $^{22}$Ne gas target, respectively, using the Grand Raiden Spectrometer at RCNP, Osaka, Japan. Six levels (E$_x$ = 10717 keV , 10822 keV, 10951 keV, 11085 keV, 11167 keV and 11317 keV) have been observed above the $\alpha$-threshold in the region of interest (10.61 - 11.32 MeV). The rates are dominated in both reaction channels by the resonance contributions of the states at E$_x$ = 10951, 11167 and 11317 keV. The E$_x$ =11167 keV has the most appreciable impact on the ($\alpha,\gamma$) rate and therefore plays an important role for the prediction of the neutron production in s-process environments.

Development of a Ne gas target for (22)Na production by proton irradiation.

The article presents the design and development of a neon gas target for the production of (22)Na using a proton beam from the room temperature cyclotron in Variable Energy Cyclotron Centre, Kolkata. The target design is made to handle a beam power of 85 W (17 MeV, 5 μA). The design is based on simulation using the computer code FLUKA for the beam dump and CFD-CFX for target cooling. The target has been successfully used for the production of (22)Na in a 6 day long 17 MeV, 5 μA proton irradiation run.

Three New Low-Energy Resonances in the ^{22}Ne(p,γ)^{23}Na Reaction.

The ^{22}Ne(p,γ)^{23}Na reaction takes part in the neon-sodium cycle of hydrogen burning. This cycle affects the synthesis of the elements between ^{20}Ne and ^{27}Al in asymptotic giant branch stars and novae. The ^{22}Ne(p,γ)^{23}Na reaction rate is very uncertain because of a large number of unobserved resonances lying in the Gamow window. At proton energies below 400keV, only upper limits exist in the literature for the resonance strengths. Previous reaction rate evaluations differ by large factors. In the present work, the first direct observations of the ^{22}Ne(p,γ)^{23}Na resonances at 156.2, 189.5, and 259.7keV are reported. Their resonance strengths are derived with 2%-7% uncertainty. In addition, upper limits for three other resonances are greatly reduced. Data are taken using a windowless ^{22}Ne gas target and high-purity germanium detectors at the Laboratory for Underground Nuclear Astrophysics in the Gran Sasso laboratory of the National Institute for Nuclear Physics, Italy, taking advantage of the ultralow background observed deep underground. The new reaction rate is a factor of 20 higher than the recent evaluation at a temperature of 0.1GK, relevant to nucleosynthesis in asymptotic giant branch stars.

Electron loss of fast heavy projectiles in collision with neutral targets

The multiple electron loss of heavy projectiles in fast ion-atom collisions has been studied in the framework of the sudden perturbation approximation. Especially, a model is developed to calculate the cross sections for the loss of any number of electrons from the projectile ion, including the ionization of a single electron and up to the complete stripping of the projectile. For a given collision system, that is specified by the (type and charge state of the) projectile and target as well as the collision energy, in fact, the experimental cross sections for just three final states of the projectile are required by this model in order to predict the loss of any number, $N$, of electrons for the same collision system, or for any similar system that differs only in the energy or the initial charge state of the projectile ion. The model is simple and can be utilized for both, the projectile and target ionization, and without that large computer resources are requested. Detailed computation have been carried out for the multiple electron loss of Xe$^{18+}$ and U$^{6+,\, 10+,\, 28+}$ projectiles in collision with neutral Ar and Ne gas targets.

L-shell ionization accompanied by one-electron capture inCq+,Oq+(q=2,3)−Necollisions

The L-shell ionization processes of a Ne gas target associated with single-electron capture by bombardment of Cq+ and Oq+ (q=2,3) are investigated using the projectile-recoil-ion coincidence method in the energy range from 80 to 400 keV/u (v(p)=1.8-4 a.u.). The cross-section ratios (R-k1) of k-fold ionization to single capture are compared with the results for He2+-Ne collisions by Dubois [Phys. Rev. A 36, 2585 (1987)]. All the velocity dependences are quite similar. The ratios increase as the projectile energy increases in the lower-energy region, reach the maxima for projectile energies around E-max=160q(1/2) keV/u, and then decrease at higher energies. These results qualitatively agree with our calculations in terms of the Bohr-Lindhard model within the independent-electron approximation.

State-selective and total single-electron capture for impact of slow He2+ (E ≤ 1 keV) on H2, O2 and CO

Projectile state-selective single-electron capture for impact of slow He2+ (impact energy≤1keV) on H2, O2 and CO has been studied experimentally by means of translational energy spectroscopy. Comparative measurements were made with a Ne gas target for absolute calibration of the electron capture cross-sections for the molecular targets. We compare the importance of “direct” single-electron capture (transition of one electron only without excitation or dissociation of the ionised target molecule) versus single-electron capture accompanied by target excitation (transfer excitation, possibly leading to molecular dissociation). Our measurements are extended to lower impact energies than earlier ones and are qualitatively interpreted on the basis of Landau–Zener transitions in the collisional quasimolecules. Similar as for direct single-electron capture, transfer excitation proceeds preferentially within a “reaction window” for the involved reaction energy defect.

Statistical analysis of correlated resonance phenomena in two systems: 28Si+20Ne and 24Mg+20Ne.

A differentially pumped windowless {sup 20}Ne gas target was used to measure angle averaged excitation functions for binary decay of the {sup 28}Si+{sup 20}Ne reaction into low-lying states of {sup 28}Si+{sup 20}Ne, {sup 24}Mg+{sup 24}Mg, and {sup 32}S+{sup 16}O, and for binary decay of the {sup 24}Mg+{sup 20}Ne reaction into low-lying states of {sup 24}Mg+{sup 20}Ne and {sup 28}Si+{sup 16}O. The {sup 28}Si+{sup 20}Ne measurements span the region of excitation energy in {sup 48}Cr from 50.6 to 67.7 MeV ({sup 28}Si beam energies from 87.2 to 128.2 MeV). The reaction {sup 24}Mg+{sup 20}Ne was investigated over a region in {sup 44}Ti from 48.75 to 71.0 MeV ({sup 24}Mg beam energies from 70.8 to 119.75 MeV). Angular distribution measurements were made for {sup 20}Ne({sup 28}Si,{sup 24}Mg){sup 24}Mg at excitation energy=56.22 MeV and for {sup 20}Ne({sup 24}Mg,{sup 16}O){sup 28}Si at excitation energy=53.02 and 57.75 MeV. Both systems display evidence of statistically significant correlated resonance phenomena, but the resonances are dominated by spins comparable to the grazing angular momenta of the respective entrance channels, and are not analogs of the molecular configurations that dominate the {sup 24}Mg({sup 24}Mg,{sup 24}Mg{sup *}){sup 24}Mg{sup *} reactions.

Comparison of methods for determining the second-order detection efficiency of a VUV spectrometer

Four independent procedures were developed and tested to measure the apparatus response function of a VUV spectrometer-detector system for unpolarized 46-nm radiation dispersed in second order. These measurements were made to allow the use of continuum synchrotron radiation for the calibration of the response of the spectrometer-detector system for dispersion of 92-nm radiation in first order with full correction for the effects of synchrotron radiation dispersed in second order. In the first method, synchrotron radiation was used in combination with a thin Al foil to block out synchrotron radiation at 92 nm while allowing 46-nm radiation to enter the spectrometer. In the second as well as the third method NeII 46-nm line radiation was used to measure the response function in first and second order. The line radiation was produced by (1) an electron beam exciting a Ne gas target for which the resulting VUV light illuminated the entire grating and (2) a duoplasmatron VUV light source operating with Ne gas producing a small spot of radiation that was scanned across the surface of the spectrometer grating. In the fourth method the difference in the spectral distributions of synchrotron radiation produced by electrons with different kinetic energies was employed to deduce the second-order detection efficiency. The ratio of the second- to firstorder response function for 46-nm radiation could be determined to a precision of 6% using the bandpass filter and electron-beam methods, 10% using the duoplasmatron method, and 250% using the multiple electron energy method.

Production of [ 18F]F 2, H 18F and 18F aq− using the 20Ne(d, α) 18F process

Some experience relevant to the production of 18F in three chemical forms ([ 18F]F 2, H 18F, 18F aq −) via the 20Ne(d, α) 18F process using a Ne gas target is described. Production of [ 18F]F 2 and H 18F was done by introducing some technical modifications in standard methods. For 18F aq − production a post irradiation target wash system was used. Methods used for the characterization of various chemical forms of radiofluorine are outlined.

Studies of emission-line profiles from ion-excited neon gas targets

The widths of some atomic emission lines from ion-beam-excited Ne-gas targets have been studied. The line width is a linear function of the pressure within the pressure region studied (20–500 kPa). The line broadening depends on the spectral line studied. A reduction in the line width was observed with increasing beam current. This was interpreted as being due to a density reduction caused by the beam. The results were applied to calculate the temperature of the gas at the point of light emission.

Charge-state correlated cross sections for the production of low-velocity highly charged Ne ions by heavy-ion bombardment

We report measured cross sections for the collisional production of highly charged low-velocity Ne recoil ions resulting from the bombardment of a thin Ne gas target by highly charged 1-MeV/amu C, N, O, and F projectiles. The measurements were made using time-of-flight techniques which allowed the simultaneous identification of the final charge state of both the low-velocity recoil ion and the high-velocity projectile for each collision event. For a given incident-projectile charge state, the recoil charge-state distribution is very dependent upon the final charge state of the projectile. Single- and double-electron capture events by incident bare nuclei and projectile K-shell ionization during the collision cause large shifts in the recoil charge-state distributions toward higher charge states. A previously proposed energy-deposition model is modified to include the effects of projectile charge-changing collisions during the collision for bare and hydrogenlike projectiles and is used to discuss the present experimental results.

K-shell Auger-electron production cross sections from ion bombardment

We have measured the Ne $K$-shell electron spectra from a Ne gas target bombarded by 0.5-10 MeV ${\mathrm{H}}^{+}$ and 14-21 MeV N, 24-35 MeV O, 3-35 MeV F, and 40 and 50 MeV Cl in various incident ionic charge states. The F $K$-shell Auger-electron spectra have been measured for F-ion collisions with Ne gas from 3 to 15 MeV. Auger-electron production cross sections are deduced. ${\mathrm{H}}^{+}$ induced Ne $K$-shell Auger-electron production cross sections are compared to various theoretical Coulomb ionization predictions. The heavy-ion-induced Ne $K$-shell Auger-electron production cross sections increase with increasing projectile charge state at constant projectile energy. The F and Ne $K$-shell Auger-electron production cross sections from 3 to 15 MeV are favorably compared to calculations based on the molecular-orbital model.

K-shell Auger-electron hypersatellites of Ne

The cross section for production of excited Ne ions with double $K$-shell vacancies is measured by observing the Auger electrons from the decay of these states. A Ne-gas target is bombarded by 1-2-MeV/amu N, O, and F projectiles in various incident charge states. The $K$-shell Auger hypersatellites are observed only for bare nuclear projectiles. The projectile charge-state dependence, projectile $Z$ dependence, and energy dependence of the cross section suggest $K$-shell electron exchange as a production mechanism.

Ion-X-Ray Coincidence Experiments in SlowNe+-Ne Collisions

${\mathrm{Ne}}^{+}$ ions of 235 and 363 keV primary energy are scattered from a dilute Ne gas target. Ne $K$ x rays excited in these collisions are detected in coincidence with ions scattered through laboratory angles ranging from 3 to 36\ifmmode^\circ\else\textdegree\fi{}. The impact-parameter dependence of inner-shell excitation thus derived matches well with the Briggs-Macek theory.

Gamma Radiations ofNa23andNe20

${\mathrm{Na}}^{23}$ and ${\mathrm{Ne}}^{20}$ gamma rays were observed from the proton bombardment at various energies of thin evaporated Na and NaI targets and of a natural Ne gas target. Gamma rays involving ${\mathrm{Na}}^{23}$ states up to 4 Mev and ${\mathrm{Ne}}^{20}$ states up to 5 Mev were observed and decay schemes and branching ratios obtained. The 2.08- and 2.70-Mev states of ${\mathrm{Na}}^{23}$ are probably $\frac{7}{{2}^{+}}$ and $\frac{9}{{2}^{+}}$, respectively. Limitations on spin and parity values are given for other states. The results obtained for ${\mathrm{Na}}^{23}$ are consistent with the results of a strong-coupling collective calculation. The 4.97-Mev state of ${\mathrm{Ne}}^{20}$ has an upper limit for the ground-state branch of 4%. An upper limit of 9% is placed on the ground-state branch of the 4.2-Mev state of ${\mathrm{Ne}}^{20}$. Gamma rays from the ${\mathrm{F}}^{19}(d, n\ensuremath{\gamma}){\mathrm{Ne}}^{20}$ reaction were observed with energies of 11.4, 10.67, 10.16, 9.37, 8.37, and 7.36 Mev.