147 research outputs found
Alpha decay rate enhancement in metals: An unlikely scenario
It has been recently suggested that one might drastically shorten the alpha
lifetime of nuclear waste products, if these are embedded in metals at low
temperatures. Using quantum mechanical tunneling arguments, we show that such
an effect is likely to be very small, if present at all.Comment: RevTeX4. 5 pages, 1 figure. Accepted by Nucl. Phys.
Low-Energy Nuclear Astrophysics - the Fascinating Region of A=7
We discuss results and future plans for low-energy reactions that play an
important role in current nuclear astrophysics research and that happen to
concentrate around the region of A=7. The 7Be(p,gamma)8B and the
3He(4He,gamma)7Be reactions are crucial for understanding the solar-neutrino
oscillations phenomenon and the latter one plays a central role in the issue of
cosmic 7Li abundance and Big-Bang Nucleosynthesis. We also present results
regarding the host dependence of the half life of the electron-capture 7Be
radio-nuclide.Comment: 8 pages, 6 figures. Proceedings of the Erice School on Nuclear
Physics, 2006. To appear in: "Prog. Part. Nucl. Phys.
On the absence of appreciable half-life changes in alpha emitters cooled in metals to 1 Kelvin and below
The recent suggestion that dramatic changes may occur in the lifetime of
alpha and beta decay when the activity, in a pure metal host, is cooled to a
few Kelvin, is examined in the light of published low temperature nuclear
orientation (LTNO) experiments, with emphasis here on alpha decay. In LTNO
observations are made of the anisotropy of radioactive emissions with respect
to an axis of orientation. Correction of data for decay of metallic samples
held at temperatures at and below 1 Kelvin for periods of days and longer has
been a routine element of LTNO experiments for many years. No evidence for any
change of half life on cooling, with an upper level of less than 1%, has been
found, in striking contrast to the predicted changes, for alpha decay, of
several orders of magnitude. The proposal that such dramatic changes might
alleviate problems of disposal of long-lived radioactive waste is shown to be
unrealistic.Comment: 27 pages, 12 figures, accepted for publication in Nucl.Phys.A.
Revised version, including quantitative analysis of the sensitivity of
nuclear orientation experiments, discussed in this work, to changes of
alpha-decay lifetimes in metals at low temperatures. Conclusions remain
unchange
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
Comparison of the LUNA 3He(alpha,gamma)7Be activation results with earlier measurements and model calculations
Recently, the LUNA collaboration has carried out a high precision measurement
on the 3He(alpha,gamma)7Be reaction cross section with both activation and
on-line gamma-detection methods at unprecedented low energies. In this paper
the results obtained with the activation method are summarized. The results are
compared with previous activation experiments and the zero energy extrapolated
astrophysical S factor is determined using different theoretical models.Comment: Accepted for publication in Journal of Physics
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.
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
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