80 research outputs found
Impact of a revised Mg(p,)Al reaction rate on the operation of the Mg-Al cycle
Proton captures on Mg isotopes play an important role in the Mg-Al cycle
active in stellar H-burning regions. In particular, low-energy nuclear
resonances in the Mg(p,)Al reaction affect the production
of radioactive Al as well as the resulting Mg/Al abundance ratio.
Reliable estimations of these quantities require precise measurements of the
strengths of low-energy resonances. Based on a new experimental study performed
at LUNA, we provide revised rates of the Mg(p,)Al
and the Mg(p,)Al reactions with corresponding
uncertainties. In the temperature range 50 to 150 MK, the new recommended rate
of the Al production is up to 5 times higher than previously
assumed. In addition, at T MK, the revised total reaction rate is a
factor of 2 higher. Note that this is the range of temperature at which the
Mg-Al cycle operates in an H-burning zone. The effects of this revision are
discussed. Due to the significantly larger Mg(p,)Al
rate, the estimated production of Al in H-burning regions is less
efficient than previously obtained. As a result, the new rates should imply a
smaller contribution from Wolf-Rayet stars to the galactic Al budget.
Similarly, we show that the AGB extra-mixing scenario does not appear able to
explain the most extreme values of Al/Al, i.e. , found
in some O-rich presolar grains. Finally, the substantial increase of the total
reaction rate makes the hypothesis of a self-pollution by massive AGBs a more
robust explanation for the Mg-Al anticorrelation observed in Globular-Cluster
stars
Activation measurement of the 3He(alpha,gamma)7Be cross section at low energy
The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a
major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted
by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis
calculations. The present work reports on a new precision experiment using the
activation technique at energies directly relevant to big-bang nucleosynthesis.
Previously such low energies had been reached experimentally only by the
prompt-gamma technique and with inferior precision. Using a windowless gas
target, high beam intensity and low background gamma-counting facilities, the
3He(alpha,gamma)7Be cross section has been determined at 127, 148 and 169 keV
center-of-mass energy with a total uncertainty of 4%. The sources of systematic
uncertainty are discussed in detail. The present data can be used in big-bang
nucleosynthesis calculations and to constrain the extrapolation of the
3He(alpha,gamma)7Be astrophysical S-factor to solar energies
Ultra-sensitive in-beam gamma-ray spectroscopy for nuclear astrophysics at LUNA
Ultra-sensitive in-beam gamma-ray spectroscopy studies for nuclear
astrophysics are performed at the LUNA (Laboratory for Underground Nuclear
Astrophysics) 400 kV accelerator, deep underground in Italy's Gran Sasso
laboratory. By virtue of a specially constructed passive shield, the laboratory
gamma-ray background for E_\gamma < 3 MeV at LUNA has been reduced to levels
comparable to those experienced in dedicated offline underground gamma-counting
setups. The gamma-ray background induced by an incident alpha-beam has been
studied. The data are used to evaluate the feasibility of sensitive in-beam
experiments at LUNA and, by extension, at similar proposed facilities.Comment: accepted, Eur. Phys. J.
The 3He(alpha,gamma)7Be S-factor at solar energies: the prompt gamma experiment at LUNA
The 3He(alpha,gamma)7Be process is a key reaction in both Big-Bang
nucleosynthesis and p-p chain of Hydrogen Burning in Stars. A new measurement
of the 3He(alpha,gamma)7Be cross section has been performed at the INFN Gran
Sasso underground laboratory by both the activation and the prompt gamma
detection methods. The present work reports full details of the prompt gamma
detection experiment, focusing on the determination of the systematic
uncertainty. The final data, including activation measurements at LUNA, are
compared with the results of the last generation experiments and two different
theoretical models are used to obtain the S-factor at solar energies.Comment: Accepted for publication in Nucl. Phys.
Modification of amorphous and microcrystalline silicon film properties after irradiation with MeV and GeV protons
It is well known that the degree of crystallinity has a prominent influence on the stability of Silicon under proton irradiation. Amorphous silicon films are much more stable than mono- or polycrystalline silicon substrates or microcrystalline silicon thin films. In particular it has been shown, that in a micromorph tandem solar cell irradiated with protons in the lower MeV energy range only the microcrystalline diode showed a pronounced decrease in photocurrent after
irradiation1. The proton irradiation induced damage in thick crystalline silicon samples has a maximum at beam energies between 1MeV and 4MeV and decreases for further increasing proton energies. However, irradiating an amorphous silicon/crystalline silicon heterojunction solar cell with a relatively dose of 24GeV, we observed a very strong drop in conversion efficiency with only minor recovery after sample annealing. In literature it has been reported 2,
that the degradation of amorphous silicon is negligible for proton energies above 100MeV. In order to clarify to which extent also the thin film top layer of the hetero solar cell is affected by the proton irradiation, we exposed a variety of thin film silicon samples either to a 1.7MeV beam with a dose of 5.1012 protons/cm2 or to a 24GeV beam with a dose of 5 .1013 protons/cm2. The investigated intrinsic, p-type and n-type amorphous and microcrystalline silicon films have been deposited by conventional plasma deposition under variation of the silane / hydrogen gas phase ratio. Raman measurements have been done in order to determine the order of crystallinity obtained under various deposition conditions. We observed even at 24GeV a clear modification in the electrical characteristics of the films. Temperature dependent measurements of the dark current revealed in particular for all doped samples a significant increase of the activation energy, that might be explained by a decrease of the dopant efficiency, while for intrinsic a-Si:H layers the increasing activation energy is due to deep defect creation
Measurement of 25Mg(p; gamma)26Al resonance strengths via gamma spectrometry
The COMPTEL instrument performed the first mapping of the 1.809 MeV photons
in the Galaxy, triggering considerable interest in determing the sources of
interstellar 26Al. The predicted 26Al is too low compared to the observation,
for a better understanding more accurate rates for the 25Mg(p; gamma)26Al
reaction are required. The 25Mg(p;gamma)26Al reaction has been investigated at
the resonances at Er= 745; 418; 374; 304 keV at Ruhr-Universitat-Bochum using a
Tandem accelerator and a 4piNaI detector. In addition the resonance at Er = 189
keV has been measured deep underground laboratory at Laboratori Nazionali del
Gran Sasso, exploiting the strong suppression of cosmic background. This low
resonance has been studied with the 400 kV LUNA accelerator and a HPGe
detector. The preliminary results of the resonance strengths will be reported.Comment: Accepted for publication in Journal of Physics
First measurement of the 14N(p,gamma)15O cross section down to 70 keV
In stars with temperatures above 20*10^6 K, hydrogen burning is dominated by
the CNO cycle. Its rate is determined by the slowest process, the
14N(p,gamma)15O reaction. Deep underground in Italy's Gran Sasso laboratory, at
the LUNA 400 kV accelerator, the cross section of this reaction has been
measured at energies much lower than ever achieved before. Using a windowless
gas target and a 4pi BGO summing detector, direct cross section data has been
obtained down to 70 keV, reaching a value of 0.24 picobarn. The Gamow peak has
been covered by experimental data for several scenarios of stable and explosive
hydrogen burning. In addition, the strength of the 259 keV resonance has been
remeasured. The thermonuclear reaction rate has been calculated for
temperatures 90 - 300 *10^6 K, for the first time with negligible impact from
extrapolations
Constraining the S factor of 15N(p,g)16O at Astrophysical Energies
The 15N(p,g)16O reaction represents a break out reaction linking the first
and second cycle of the CNO cycles redistributing the carbon and nitrogen
abundances into the oxygen range. The reaction is dominated by two broad
resonances at Ep = 338 keV and 1028 keV and a Direct Capture contribution to
the ground state of 16O. Interference effects between these contributions in
both the low energy region (Ep < 338 keV) and in between the two resonances
(338 <Ep < 1028 keV) can dramatically effect the extrapolation to energies of
astrophysical interest. To facilitate a reliable extrapolation the 15N(p,g)16O
reaction has been remeasured covering the energy range from Ep=1800 keV down to
130 keV. The results have been analyzed in the framework of a multi-level
R-matrix theory and a S(0) value of 39.6 keV b has been found.Comment: 15 pages, 9 figure
Direct measurement of the 15N(p,gamma)16O total cross section at novae energies
The 15N(p,gamma)16O reaction controls the passage of nucleosynthetic material
from the first to the second carbon-nitrogen-oxygen (CNO) cycle. A direct
measurement of the total 15N(p,gamma)16O cross section at energies
corresponding to hydrogen burning in novae is presented here. Data have been
taken at 90-230 keV center-of-mass energy using a windowless gas target filled
with nitrogen of natural isotopic composition and a bismuth germanate summing
detector. The cross section is found to be a factor two lower than previously
believed.Comment: LUNA collaboration; accepted by J. Phys.
The 25Mg(p,g)Al reaction at low astrophysical energies
In the present work we report on a new measurement of resonance strengths in
the reaction 25Mg(p,gamma)26Al at E_cm= 92 and 189 keV. This study was
performed at the LUNA facility in the Gran Sasso underground laboratory using a
4pi BGO summing crystal. For the first time the 92 keV resonance was directly
observed and a resonance strength omega-gamma=(2.9+/-0.6)x10E-10 eV was
determined. Additionally, the gamma-ray branchings and strength of the 189 keV
resonance were studied with a high resolution HPGe detector yielding an
omega-gamma value in agreement with the BGO measurement, but 20% larger
compared to previous works.Comment: 6 pages, 4 figures, accepted for publication in Physics Letters
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