arXiv subjects

D. Bemmerer

Publications and source records attributed to D. Bemmerer.

At least 37 records · Page 2Linked to original sources

arXiv2022

Search for $^{22}$Na in novae supported by a novel method for measuring femtosecond nuclear lifetimes

Classical novae are thermonuclear explosions in stellar binary systems, and important sources of $^{26}$Al and $^{22}$Na. While gamma rays from the decay of the former radioisotope have been observed throughout the Galaxy, $^{22}$Na remains untraceable. The half-life of $^{22}$Na (2.6 yr) would allow the observation of its 1.275 MeV gamma-ray line from a cosmic source. However, the prediction of such an observation requires good knowledge of the nuclear reactions involved in the production and destruction of this nucleus. The $^{22}$Na($p,γ$)$^{23}$Mg reaction remains the only source of large uncertainty about the amount of $^{22}$Na ejected. Its rate is dominated by a single resonance on the short-lived state at 7785.0(7) keV in $^{23}$Mg. In the present work, a combined analysis of particle-particle correlations and velocity-difference profiles is proposed to measure femtosecond nuclear lifetimes. The application of this novel method to the study of the $^{23}$Mg states, combining magnetic and highly-segmented tracking gamma-ray spectrometers, places strong limits on the amount of $^{22}$Na produced in novae, explains its non-observation to date in gamma rays (flux < 2.5x$10^{-4}$ ph/(cm$^2$s)), and constrains its detectability with future space-borne observatories.

nucl-ex↗

arXiv2022

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

In stars, the fusion of $^{22}$Ne and $^4$He may produce either $^{25}$Mg, with the emission of a neutron, or $^{26}$Mg and a $γ$ ray. At high temperature, the ($α,n$) channel dominates, while at low temperature, it is energetically hampered. The rate of its competitor, the $^{22}$Ne($α$,$γ$)$^{26}$Mg reaction, and, hence, the minimum temperature for the ($α,n$) 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 $E$_{r}$\,=\,$334$\,$keV (corresponding to $E$_{x}$\,=\,$10949$\,$keV in $^{26}$Mg). The data reported here have been obtained using high intensity $^4$He$^+$ beam from the INFN LUNA 400 kV underground accelerator, a windowless, recirculating, 99.9% isotopically enriched $^{22}$Ne gas target, and a 4$π$ bismuth germanate summing $γ$-ray detector. The ultra-low background rate of less than 0.5 counts/day was determined using 67 days of no-beam data and 7 days of $^4$He$^+$ 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($α$,$γ$)$^{26}$Mg resonances at higher energy.

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arXiv2022

Long-term evolution of the neutron rate at the Canfranc Underground Laboratory

We report results on the long-term variation of the neutron counting rate at the Canfranc Underground Laboratory, of importance for several low-background experiments installed there, including rare-event searches. The measurement campaign was performed employing the High Efficiency Neutron Spectrometry Array (HENSA) mounted in Hall A and lasted 412 live days. The present study is the first long-term measurement of the neutron rate with sensitivity over a wide range of neutron energies (from thermal up to 0.1 GeV and beyond) performed in any underground laboratory so far. Data on the environmental variables inside the experimental hall (radon concentration, air temperature, air pressure and humidity) were also acquired during all the measurement campaign. We have investigated for the first time the evolution of the neutron rate for different energies of the neutrons and its correlation with the ambient variables.

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arXiv2021

Measurement of the neutron flux at the Canfranc Underground Laboratory with HENSA

We have performed a long-term measurement of the neutron flux with the High Efficiency Neutron Spectrometry Array HENSA in the Hall A of the Canfranc Underground Laboratory. The Hall A measurement campaign lasted from October 2019 to March 2021, demonstrating an excellent stability of the HENSA setup. Preliminary results on the neutron flux from this campaign are presented for the first time. In Phase 1 (113 live days) a total neutron flux of 1.66(2) $\times$10$^{-5}$ cm$^{-2}$ s$^{-1}$ is obtained. Our results are in good agreement with those from our previous shorter measurement where a reduced experimental setup was employed.

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arXiv2021

Direct measurement of the 13C(α,n)16O cross section into the s-process Gamow peak

One of the main neutron sources for the astrophysical s-process is the reaction 13C(α,n)16O, taking place in thermally pulsing Asymptotic Giant Branch stars at temperatures around 90 MK. To model the nucleosynthesis during this process the reaction cross section needs to be known in the 150-230keV energy window (Gamow peak). At these sub-Coulomb energies cross section direct measurements are severely affected by the low event rate, making us rely on input from indirect methods and extrapolations from higher-energy direct data. This leads to an uncertainty in the cross section at the relevant energies too high to reliably constrain the nuclear physics input to s-process calculations. We present the results from a new deep-underground measurement of 13C(α,n)16O, covering the energy range 230-300keV, with drastically reduced uncertainties over previous measurements and for the first time providing data directly inside the s-process Gamow peak. Selected stellar models have been computed to estimate the impact of our revised reaction rate. For stars of nearly solar composition, we find sizeable variations of some isotopes, whose production is influenced by the activation of close-by branching points that are sensitive to the neutron density, in particular the two radioactive nuclei 60Fe and 205Pb, as well as 152Gd

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arXiv2021

Photoexcitation of $^{76}$Ge

The dipole strength of the nuclide $^{76}$Ge was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.8 and 12.3 MeV at the $γ$ELBE facility. We identified 210 levels up to an excitation energy of 9.4 MeV and assigned spin $J$ = 1 to most of them. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical $γ$-ray cascades. The photoabsorption cross section up to the neutron-separation energy was determined and is compared with predictions of the statistical reaction model. The derived photon strength function is compared with results of experiments using other reactions.

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arXiv2021

The Future of Solar Neutrinos

In this article we review the current state of the field of solar neutrinos, including flavour oscillations, non-standard effects, solar models, cross section measurements, and the broad experimental program thus motivated and enabled. We discuss the historical discoveries that contributed to current knowledge, and define critical open questions to be addressed in the next decade. We discuss the state of the art of standard solar models, including uncertainties and problems related to the solar composition, and review experimental and model solar neutrino fluxes, including future prospects. We review the state of the art of the nuclear reaction data relevant for solar fusion in the proton-proton chain and carbon-nitrogen-oxygen cycle. Finally, we review the current and future experimental program that can address outstanding questions in this field.

hep-ph↗

arXiv2021

Measurement of the $^{2}$H($p,γ$)$^{3}$He S-factor at 265-1094keV

Recent astronomical data have provided the primordial deuterium abundance with percent precision. As a result, Big Bang nucleosynthesis may provide a constraint on the universal baryon to photon ratio that is as precise as, but independent from, analyses of the cosmic microwave background. However, such a constraint requires that the nuclear reaction rates governing the production and destruction of primordial deuterium are sufficiently well known. Here, a new measurement of the $^2$H($p,γ$)$^3$He cross section is reported. This nuclear reaction dominates the error on the predicted Big Bang deuterium abundance. A proton beam of 400-1650keV beam energy was incident on solid titanium deuteride targets, and the emitted $γ$-rays were detected in two high-purity germanium detectors at angles of 55$^\circ$ and 90$^\circ$, respectively. The deuterium content of the targets has been obtained in situ by the $^2$H($^3$He,$p$)$^4$He reaction and offline using the Elastic Recoil Detection method. The astrophysical S-factor has been determined at center of mass energies between 265 and 1094 keV, addressing the uppermost part of the relevant energy range for Big Bang nucleosynthesis and complementary to ongoing work at lower energies. The new data support a higher S-factor at Big Bang temperatures than previously assumed, reducing the predicted deuterium abundance.

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arXiv2020

Neutron flux and spectrum in the Dresden Felsenkeller underground facility studied by moderated $^3$He counters

Ambient neutrons may cause significant background for underground experiments. Therefore, it is necessary to investigate their flux and energy spectrum in order to devise a proper shielding. Here, two sets of altogether ten moderated $^3$He neutron counters are used for a detailed study of the ambient neutron background in tunnel IV of the Felsenkeller facility, underground below 45 meters of rock in Dresden/Germany. One of the moderators is lined with lead and thus sensitive to neutrons of energies higher than 10 MeV. For each $^3$He counter-moderator assembly, the energy dependent neutron sensitivity was calculated with the FLUKA code. The count rates of the ten detectors were then fitted with the MAXED and GRAVEL packages. As a result, both the neutron energy spectrum from 10$^{-9}$ MeV to 300 MeV and the flux integrated over the same energy range were determined experimentally. The data show that at a given depth, both the flux and the spectrum vary significantly depending on local conditions. Energy integrated fluxes of $(0.61 \pm 0.05)$, $(1.96 \pm 0.15)$, and $(4.6 \pm 0.4) \times 10^{-4}$ cm$^{-2}$ s$^{-1}$, respectively, are measured for three sites within Felsenkeller tunnel IV which have similar muon flux but different shielding wall configurations. The integrated neutron flux data and the obtained spectra for the three sites are matched reasonably well by FLUKA Monte Carlo calculations that are based on the known muon flux and composition of the measurement room walls.

physics.ins-det↗

arXiv2020

Setup commissioning for an improved measurement of the D(p,gamma)3He cross section at Big Bang Nucleosynthesis energies

Among the reactions involved in the production and destruction of deuterium during Big Bang Nucleosynthesis, the deuterium-burning D(p,gamma)3He reaction has the largest uncertainty and limits the precision of theoretical estimates of primordial deuterium abundance. Here we report the results of a careful commissioning of the experimental setup used to measure the cross-section of the D(p,gamma)3He reaction at the Laboratory for Underground Nuclear Astrophysics of the Gran Sasso Laboratory (Italy). The commissioning was aimed at minimising all sources of systematic uncertainty in the measured cross sections. The overall systematic error achieved (< 3 %) will enable improved predictions of BBN deuterium abundance.

physics.ins-det↗

arXiv2020

A new approach to monitor 13C-targets degradation in situ for 13C(alpha,n)16O cross-section measurements at LUNA

Direct measurements of reaction cross-sections at astrophysical energies often require the use of solid targets able to withstand high ion beam currents for extended periods of time. Thus, monitoring target thickness, isotopic composition, and target stoichiometry during data taking is critical to account for possible target modifications and to reduce uncertainties in the final cross-section results. A common technique used for these purposes is the Nuclear Resonant Reaction Analysis (NRRA), which however requires that a narrow resonance be available inside the dynamic range of the accelerator used. In cases when this is not possible, as for example the 13C(alpha,n)16O reaction recently studied at low energies at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy, alternative approaches must be found. Here, we present a new application of the shape analysis of primary gamma rays emitted by the 13C(p,g)14N radiative capture reaction. This approach was used to monitor 13C target degradation {\em in situ} during the 13C(alpha,n)16O data taking campaign. The results obtained are in agreement with evaluations subsequently performed at Atomki (Hungary) using the NRRA method. A preliminary application for the extraction of the 13C(alpha,n)16O reaction cross-section at one beam energy is also reported.

physics.ins-det↗

arXiv2019

Background in $γ$-ray detectors and carbon beam tests in the Felsenkeller shallow-underground accelerator laboratory

The relevant interaction energies for astrophysical radiative capture reactions are very low, much below the repulsive Coulomb barrier. This leads to low cross sections, low counting rates in $γ$-ray detectors, and therefore the need to perform such experiments at ion accelerators placed in underground settings, shielded from cosmic rays. Here, the feasibility of such experiments in the new shallow-underground accelerator laboratory in tunnels VIII and IX of the Felsenkeller site in Dresden, Germany, is evaluated. To this end, the no-beam background in three different types of germanium detectors, i.e. a Euroball/Miniball triple cluster and two large monolithic detectors, is measured over periods of 26-66 days. The cosmic-ray induced background is found to be reduced by a factor of 500-2400, by the combined effects of, first, the 140 meters water equivalent overburden attenuating the cosmic muon flux by a factor of 40, and second, scintillation veto detectors gating out most of the remaining muon-induced effects. The new background data are compared to spectra taken with the same detectors at the Earth's surface and at other underground sites. Subsequently, the beam intensity from the cesium sputter ion source installed in Felsenkeller has been studied over periods of several hours. Based on the background and beam intensity data reported here, for the example of the $^{12}$C($α$,$γ$)$^{16}$O reaction it is shown that highly sensitive experiments will be possible.

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arXiv2019

The muon intensity in the Felsenkeller shallow underground laboratory

The muon intensity and angular distribution in the shallow-underground laboratory Felsenkeller in Dresden, Germany have been studied using a portable muon detector based on the close cathode chamber design. Data has been taken at four positions in Felsenkeller tunnels VIII and IX, where a new 5 MV underground ion accelerator is being installed, and in addition at four positions in Felsenkeller tunnel IV, which hosts a low-radioactivity counting facility. At each of the eight positions studied, seven different orientations of the detector were used to compile a map of the upper hemisphere with 0.85° angular resolution. The muon intensity is found to be suppressed by a factor of 40 due to the 45 m thick rock overburden, corresponding to 140 meters water equivalent. The angular data are matched by two different simulations taking into account the known geodetic features of the terrain: First, simply by determining the cutoff energy using the projected slant depth in rock and the known muon energy spectrum, and second, in a Geant4 simulation propagating the muons through a column of rock equal to the known slant depth. The present data are instrumental for studying muon-induced effects at these depths and also in the planning of an active veto for accelerator-based underground nuclear astrophysics experiments.

physics.ins-det↗

arXiv2018

Direct capture cross section and the $E_p$ = 71 and 105 keV resonances in the $^{22}$Ne($p,γ$)$^{23}$Na reaction

The $^{22}$Ne($p,γ$)$^{23}$Na reaction, part of the neon-sodium cycle of hydrogen burning, may explain the observed anticorrelation between sodium and oxygen abundances in globular cluster stars. Its rate is controlled by a number of low-energy resonances and a slowly varying non-resonant component. Three new resonances at $E_p$ = 156.2, 189.5, and 259.7 keV have recently been observed and confirmed. However, significant uncertainty on the reaction rate remains due to the non-resonant process and to two suggested resonances at $E_p$ = 71 and 105 keV. Here, new $^{22}$Ne($p,γ$)$^{23}$Na data with high statistics and low background are reported. Stringent upper limits of 6$\times$10$^{-11}$ and 7$\times$10$^{-11}$\,eV (90\% confidence level), respectively, are placed on the two suggested resonances. In addition, the off-resonant S-factor has been measured at unprecedented low energy, constraining the contributions from a subthreshold resonance and the direct capture process. As a result, at a temperature of 0.1 GK the error bar of the $^{22}$Ne($p,γ$)$^{23}$Na rate is now reduced by three orders of magnitude.

nucl-ex↗

arXiv2018

Effect of beam energy straggling on resonant yield in thin gas targets: The cases $^{22}$Ne(p,γ)$^{23}$Na and $^{14}$N(p,γ)$^{15}$O

When deriving resonance strengths using the thick-target yield approximation, for very narrow resonances it may be necessary to take beam energy straggling into account. This applies to gas targets of a few keV width, especially if there is some additional structure in target stoichiometry or detection efficiency. The correction for this effect is shown and tested on recent studies of narrow resonances in the $^{22}$Ne(p,γ)$^{23}$Na and $^{14}$N(p,γ)$^{15}$O reactions.

nucl-ex↗

arXiv2018

Strong neutron pairing in core+4n nuclei

The emission of neutron pairs from the neutron-rich $N\!=\!12$ isotones $^{18}$C and $^{20}$O has been studied by high-energy nucleon knockout from $^{19}$N and $^{21}$O secondary beams, populating unbound states of the two isotones up to 15~MeV above their two-neutron emission thresholds. The analysis of triple fragment-$n$-$n$ correlations shows that the decay $^{19}$N$(-1p)^{18}$C$^*\!\rightarrow^{16}$C+$n$+$n$ is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a $^{14}$C core surrounded by four valence neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay $^{21}$O$(-1n)^{20}$O$^*\!\rightarrow^{18}$O+$n$+$n$, attributed to its formation through the knockout of a deeply-bound neutron that breaks the $^{16}$O core and reduces the number of pairs.

nucl-ex↗

arXiv2018

A high-efficiency gas target setup for underground experiments, and redetermination of the branching ratio of the 189.5 keV $\mathbf{^{22}Ne(p,γ)^{23}Na}$ resonance

The experimental study of nuclear reactions of astrophysical interest is greatly facilitated by a low-background, high-luminosity setup. The Laboratory for Underground Nuclear Astrophysics (LUNA) 400 kV accelerator offers ultra-low cosmic-ray induced background due to its location deep underground in the Gran Sasso National Laboratory (INFN-LNGS), Italy, and high intensity, 250-500 $μ$A, proton and $α$ ion beams. In order to fully exploit these features, a high-purity, recirculating gas target system for isotopically enriched gases is coupled to a high-efficiency, six-fold optically segmented bismuth germanate (BGO) $γ$-ray detector. The beam intensity is measured with a beam calorimeter with constant temperature gradient. Pressure and temperature measurements have been carried out at several positions along the beam path, and the resultant gas density profile has been determined. Calibrated $γ$-intensity standards and the well-known $E_p$ = 278 keV $\mathrm{^{14}N(p,γ)^{15}O}$ resonance were used to determine the $γ$-ray detection efficiency and to validate the simulation of the target and detector setup. As an example, the recently measured resonance at $E_p$ = 189.5 keV in the $^{22}$Ne(p,$γ$)$^{23}$Na reaction has been investigated with high statistics, and the $γ$-decay branching ratios of the resonance have been determined.

nucl-ex↗

arXiv2017

LUNA: Status and Prospects

The essential ingredients of nuclear astrophysics are the thermonuclear reactions which shape the life and death of stars and which are responsible for the synthesis of the chemical elements in the Universe. Deep underground in the Gran Sasso Laboratory the cross sections of the key reactions responsible for the hydrogen burning in stars have been measured with two accelerators of 50 and 400 kV voltage right down to the energies of astrophysical interest. As a matter of fact, the main advantage of the underground laboratory is the reduction of the background. Such a reduction has allowed, for the first time, to measure relevant cross sections at the Gamow energy. The qualifying features of underground nuclear astrophysics are exhaustively reviewed before discussing the current LUNA program which is mainly devoted to the study of the Big-Bang nucleosynthesis and of the synthesis of the light elements in AGB stars and classical novae. The main results obtained during the study of reactions relevant to the Sun are also reviewed and their influence on our understanding of the properties of the neutrino, of the Sun and of the Universe itself is discussed. Finally, the future of LUNA during the next decade is outlined. It will be mainly focused on the study of the nuclear burning stages after hydrogen burning: helium and carbon burning. All this will be accomplished thanks to a new 3.5 MV accelerator able to deliver high current beams of proton, helium and carbon which will start running under Gran Sasso in 2019. In particular, we will discuss the first phase of the scientific case of the 3.5 MV accelerator focused on the study of $^{12}$C+$^{12}$C and of the two reactions which generate free neutrons inside stars: $^{13}$C($α$,n)$^{16}$O and $^{22}$Ne($α$,n)$^{25}$Mg.

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