Development of a variable-energy, high-intensity, pulsed-mode ion source for low-energy nuclear astrophysics studies

The primary challenge in directly measuring nuclear reaction rates near stellar energies is their small cross sections. The signal-to-background ratio in these complex experiments can be significantly improved by employing high-current (mA-range) beams and novel detection techniques. Therefore, the electron cyclotron resonance ion source at the Laboratory for Experimental Nuclear Astrophysics underwent a complete upgrade of its acceleration column and microwave system to obtain high-intensity, pulsed proton beams. The new column uses a compression design with O-ring seals for vacuum integrity. Its voltage gradient between electrode sections is produced by the parallel resistance of channels of chilled, deionized water. It also incorporates alternating, transverse magnetic fields for electron suppression and an axially adjustable beam extraction system. Following this upgrade, the operational bremsstrahlung radiation levels and high-voltage stability of the source were vastly improved, over 3.5 mA of target beam current was achieved, and an order-of-magnitude increase in normalized brightness was measured. Beam optics calculations, structural design, and further performance results for this source are presented

Low-energy 23^{23}Al β\beta-delayed proton decay and 22^{22}Na destruction in novae

International audienceThe radionuclide Na22 is a target of γ-ray astronomy searches, predicted to be produced during thermonuclear runaways driving classical novae. The Na22(p,γ)Mg23 reaction is the main destruction channel of Na22 during a nova, hence, its rate is needed to accurately predict the Na22 yield. However, experimental determinations of the resonance strengths have led to inconsistent results. In this Rapid Communication, we report a measurement of the branching ratios of the Al23β-delayed protons as a probe of the key 204-keV (center-of-mass) Na22(p,γ)Mg23 resonance strength. We report a factor of 5 lower branching ratio compared to the most recent literature value. The variation in Na22 yield due to nuclear data inconsistencies was assessed using a series of hydrodynamic nova outburst simulations and has increased to a factor of 3.8, corresponding to a factor of ≈2 uncertainty in the maximum detectability distance. This is the first reported scientific measurement using the Gaseous Detector with Germanium Tagging system

GADGET: a Gaseous Detector with Germanium Tagging

International audienceThe Gaseous Detector with Germanium Tagging (GADGET) is a new detection system devoted to the measurement of weak, low-energy β -delayed proton decays relevant for nuclear astrophysics studies. It is comprised of a new gaseous Proton Detector equipped with a Micromegas readout for charged particle detection, surrounded by the existing Segmented Germanium Array (SeGA) for the high-resolution detection of the prompt γ -rays. In this work we describe in detail for the first time the design, construction, and operation of the GADGET system, including performance of the Proton Detector. We present the results of a recent commissioning experiment performed with 25 Si beam at the National Superconducting Cyclotron Laboratory (NSCL). GADGET provided low-background, low-energy β -delayed proton detection with efficiency above 95%, and relatively good efficiency for proton-gamma coincidences (2.7% at 1.37 MeV)

Measurements and computational analysis on the natural decay of 176^{176}Lu

International audienceBackground: Mainly because of its long half-life and despite its scientific relevance, spectroscopic measurements of 176Lu beta decays are very limited and lack formulation of shape factors. Direct measurement of its Q-value is also presently unreported. In addition, the description of forbidden decays provides interesting challenges for nuclear theory. The comparison of precise experimental results with theoretical calculations for these decays can help to test underlying models and can aid the interpretation of data from other experiments.Purpose: Perform the first precision measurement of 176Lu beta-decay spectrum and attempt the observation of its electron capture decays, as well as perform the first precision direct measurement of the 176Lu beta-decay Q-value. Compare the shape of the precisely determined experimental beta-spectrum to theoretical calculations, and compare the end-point energy to that obtained from an independent Q-value measurement.Method: The 176Lu beta-decay spectra and the search for electron capture decays were measured with an experimental set-up that employed lutetium-based scintillator crystals and an NaI(Tl) spectrometer for coincidence counting. The beta-decay Q-value was determined via high-precision Penning trap mass spectrometry (PTMS) with the LEBIT facility at the National Superconducting Cyclotron Laboratory. The beta-spectrum calculations were performed within the Fermi theory formalism with nuclear structure effects calculated using a shell model approach.Results: Both beta transitions of 176Lu were experimentally observed and corresponding shape factors formulated in their entire energy ranges. Search for electron captures decay branches led to an experimental upper limit of 6.3x10-6 compared to its beta decays. The 176Lu beta-decay and electron capture Q-values were measured using PTMS to be 1193.0(6) keV and 108.9(8) keV, respectively. This enabled precise beta end-point energies of 596.2(6) keV and 195.3(6) keV for the primary and secondary beta-decays, respectively, to be determined. The conserved vector current hypothesis was applied to calculate the relativistic vector matrix elements. The beta-spectrum shape was shown to significantly depend on the Coulomb displacement energy and on the value of the axial vector coupling constant gA, which was extracted according to different assumptions

25^{25}Si β+\beta^+ -decay spectroscopy

International audienceBackground: β-decay spectroscopy provides valuable information on exotic nuclei and a stringent test for nuclear theories beyond the stability line. Purpose: To search for new β-delayed protons and γ rays of Si25 to investigate the properties of Al25 excited states. Method: Si25β decays were measured by using the Gaseous Detector with Germanium Tagging system at the National Superconducting Cyclotron Laboratory. The protons and γ rays emitted in the decay were detected simultaneously. A Monte Carlo method was used to model the Doppler broadening of Mg24γ-ray lines caused by nuclear recoil from proton emission. Shell-model calculations using two newly developed universal sd-shell Hamiltonians were performed. Results: The most precise Si25 half-life to date has been determined. A new proton branch at 724(4) keV and new proton-γ-ray coincidences have been identified. Three Mg24γ-ray lines and eight Al25γ-ray lines are observed for the first time in Si25 decay. The first measurement of the Si25β-delayed γ-ray intensities through the Al25 unbound states is reported. All the bound states of Al25 are observed to be populated in the β decay of Si25. Several inconsistencies between the previous measurements have been resolved, and new information on the Al25 level scheme is provided. An enhanced decay scheme has been constructed and compared to the mirror decay of Na25 and the shell-model calculations. Conclusions: The measured excitation energies, γ-ray and proton branchings, log ft values, and Gamow-Teller transition strengths for the states of Al25 populated in the β decay of Si25 are in good agreement with the shell-model calculations, offering gratifyingly consistent insights into the fine nuclear structure of Al25

Constraining the 30^{30}P(p,γ)31p,\gamma)^{31}S reaction rate in ONe novae via the weak, low-energy, β\beta-delayed proton decay of 31^{31}Cl

The $^{30}$P$(p,\gamma)^{31}$S reaction plays an important role in understanding nucleosynthesis of $A\geq 30$ nuclides in oxygen-neon novae. The Gaseous Detector with Germanium Tagging was used to measure $^{31}$Cl $\beta$-delayed proton decay through the key $J^{\pi}=3/2^{+}$, 260-keV resonance. The intensity $I^{260}_{\beta p} = 8.3^{+1.2}_{-0.9} \times 10^{-6}$ represents the weakest $\beta$-delayed, charged-particle emission ever measured below 400 keV, resulting in a proton branching ratio of $\Gamma_p / \Gamma = 2.5^{+0.4}_{-0.3} \times 10^{-4}$. By combining this measurement with shell-model calculations for $\Gamma_{\gamma}$ and past work on other resonances, the total $^{30}$P$(p,\gamma)^{31}$S rate has been determined with reduced uncertainty. The new rate has been used in hydrodynamic simulations to model the composition of nova ejecta, leading to a concrete prediction of $^{30}$Si/$^{28}$Si excesses in presolar nova grains and the calibration of nuclear thermometers.Comment: 7 pages, 2 figures, accepted to Physical Review Letters on April 4, 202

High-precision mass measurements of the isomeric and ground states of 44^{44}V : Improving constraints on the isobaric multiplet mass equation parameters of the A=44 , 0+^+ quintet

Background: The quadratic isobaric multiplet mass equation (IMME) has been very successful at predicting the masses of isobaric analog states in the same multiplet, while its coefficients are known to follow specific trends as functions of mass number. The Atomic Mass Evaluation 2016 [Chin. Phys. C 41, 030003 (2017)]1674-113710.1088/1674-1137/41/3/030003 V44 mass value results in an anomalous negative c coefficient for the IMME quadratic term; a consequence of large uncertainty and an unresolved isomeric state. The b and c coefficients can provide useful constraints for construction of the isospin-nonconserving Hamiltonians for the pf shell. In addition, the excitation energy of the 0+,T=2 level in V44 is currently unknown. This state can be used to constrain the mass of the more exotic Cr44. Purpose: The aim of the experimental campaign was to perform high-precision mass measurements to resolve the difference between V44 isomeric and ground states, to test the IMME using the new ground state mass value and to provide necessary ingredients for the future identification of the 0+, T=2 state in V44. Method: High-precision Penning trap mass spectrometry was performed at LEBIT, located at the National Superconducting Cyclotron Laboratory, to measure the cyclotron frequency ratios of [VO44g,m]+ versus [SCO32]+, a well-known reference mass, to extract both the isomeric and ground state masses of V44. Results: The mass excess of the ground and isomeric states in V44 were measured to be −23804.9(80) keV/c2 and −23537.0(55) keV/c2, respectively. This yielded a new proton separation energy of Sp=1773(10) keV. Conclusion: The new values of the ground state and isomeric state masses of V44 have been used to deduce the IMME b and c coefficients of the lowest 2+ and 6+ triplets in A=44. The 2+c coefficient is now verified with the IMME trend for lowest multiplets and is in good agreement with the shell-model predictions using charge-dependent Hamiltonians. The mirror energy differences were determined between V44 and Sc44, in line with isospin-symmetry for this multiplet. The new value of the proton separation energy determined, to an uncertainty of 10 keV, will be important for the determination of the 0+, T=2 state in V44 and, consequently, for prediction of the mass excess of Cr44

Time Projection Chamber for GADGET II

International audienceBackground: The established GADGET detection system, designed for measuring weak, low-energy $\beta$-delayed proton decays, features a gaseous Proton Detector with MICROMEGAS readout for calorimetric particle detection, surrounded by a Segmented Germanium Array for high-resolution prompt $\gamma$-ray detection. Purpose: To upgrade GADGET's Proton Detector to operate as a compact Time Projection Chamber (TPC) for the detection, 3D imaging and identification of low-energy $\beta$-delayed single- and multi-particle emissions mainly of interest to astrophysical studies. Method: A new high granularity MM board with 1024 pads has been designed, fabricated, installed and tested. A high-density data acquisition system based on Generic Electronics for TPCs has been installed and optimized to record and process the gas avalanche signals collected on the readout pads. The TPC's performance has been tested using a $^{220}$Rn $\alpha$-particle source and cosmic-ray muons. In addition, decay events in the TPC have been simulated by adapting the ATTPCROOT data analysis framework. Further, a novel application of 2D convolutional neural networks for GADGET II event classification is introduced. Results: The GADGET II TPC is capable of detecting and identifying $\alpha$-particles, as well as measuring their track direction, range, and energy. It has also been demonstrated that the GADGET II TPC is capable of tracking cosmic-ray muons. In addition to being one of the first generation of micro pattern gaseous detectors to utilize a resistive anode applied to low-energy nuclear physics, the GADGET II TPC will also be the first TPC surrounded by a high-efficiency array of high-purity germanium $\gamma$-ray detectors. \textbf{Conclusions:} The TPC of GADGET II has been designed, fabricated, tested, and is ready for operation at the FRIB for radioactive beam-line experiments