Search

Nuclei far from the line of stability are a focal point of contemporary nuclear physics. Nuclear structure studies along the proton dripline remain an important component of the scientific program at the ATLAS facility. The Gammasphere gamma-ray array and the Fragment Mass Analyzer offer the unprecedented sensitivity and selectivity required to study nuclei at and beyond the proton dripline. Recent results in proton decay studies, in-beam γ-ray spectroscopy around 100Sn and studies of proton resonances in light nuclei involved in the astrophysical hydrogen burning are presented. Future prospects in the context of planned and on-going upgrades of ATLAS and experimental apparatus are also briefly discussed. [ABSTRACT FROM AUTHOR]

We have produced an intense and pure beam of 31Cl with the MARS Separator at the Texas A&M University and studied β-decay of 31Cl by implanting the beam into a novel detector setup, capable of measuring β-delayed protons and γ-rays simultaneously. From our data, we have established decay scheme of 31Cl, found resonance energies with 1 keV precision, have measured its half-life with under 1% accuracy, found its Isobar Analog State decay and by using the IMME obtained an improved mass excess for its ground state. In this contribution, a description of the used method along with selected preliminary experimental results are given and their relevance for novae nucleosynthesis discussed. [ABSTRACT FROM AUTHOR]

Decay spectroscopy is one of the oldest indirect methods in nuclear astrophysics. We have developed at TAMU techniques to measure beta- and beta-delayed proton decay of sd-shell, proton-rich nuclei. The short-lived radioactive species are produced in-flight, separated, then slowed down (from about 40 MeV/u) and implanted in the middle of very thin Si detectors. These allowed us to measure protons with energies as low as 200 keV from nuclei with lifetimes of 100 ms or less. At the same time we measure gamma-rays up to 8 MeV with high resolution HPGe detectors. We have studied the decay of 23Al, 27P, 31Cl, all important for understanding explosive H-burning in novae. The technique has shown a remarkable selectivity to beta-delayed charged-particle emission and works even at radioactive beam rates of a few pps. The states populated are resonances for the radiative proton capture reactions 22Na(p,γ)23Mg (crucial for the depletion of 22Na in novae), 26mAl(p,γ)27Si and 30P(p,γ)31S (bottleneck in novae and XRB burning), respectively. More recently we have radically improved the technique using a gas based detector we call AstroBox. [ABSTRACT FROM AUTHOR]

The MARS group at TAMU has developed a new experimental technique to measure very low energy protons from β-delayed proton-decay of proton-rich nuclei produced and separated with the MARS recoil spectrometer at TAMU. Recently we have investigated the β-delayed p-decays of 23Al [1], and 31Cl [2], and obtained information on the resonances in the 22Na(p,γ)23Mg and 30P(p,γ)31S reactions, respectively. These reactions are important in explosive H-burning in Novae [3]. Recently an experiment looking at the β-delayed p-decay of 20Mg was also done in order to obtain information on resonances in the 19Ne(p,γ)20Na reaction. A simple setup consisting of a telescope made of a thin double sided Si strip detector (p-detector) backed or sandwiched between two thick Si detectors (β-detectors) was designed. We studied different W1 and BB2 p-detectors made by MSL, and found that the thinner detectors with a small cell size are best to measure proton energies as low as 2-300 keV. [ABSTRACT FROM AUTHOR]

We have studied the β-decay of 23Al with a novel detector setup at the focal plane of the MARS separator at the Texas A&M University to resolve existing controversies about the proton intensities of the IAS in 23Mg and to determine the absolute proton branching ratios by combining our results to the latest γ-decay data. Experimental technique, results and the relevance for nova nucleosynthesis are discussed. [ABSTRACT FROM AUTHOR]

Classical novae are a site of explosive nucleosynthesis where hydrogen rich material from a companion giant star accretes onto the surface of a white warf. Critical to our understanding of nova explosions are proton-capture reaction rates involved in the nucleosynthesis. While, ideally, all of the relevant (p,γ) reactions would be measured directly, in practice, such measurements are very challenging and are only possible in a few cases. This provides considerable scope for indirect measurements including transfer reactions, mass measurements, beta-decay and gamma-ray spectroscopy. The latter technique, until recently largely neglected as an input in nuclear astrophysics analyses, has clear advantages in locating resonances with high energy precision and assisting in determining the spin and parity of resonances. Such information is very valuable in a complementary approach to indirect determinations of key reaction rates. [ABSTRACT FROM AUTHOR]

A search for γ-ray transitions in 101Sn, which contains only one neutron outside the 100Sn core, was carried out at the Argonne Tandem-Linac Accelerator System. 101Sn nuclei were produced using the 46Ti(58Ni,3n)101Sn reaction. In-beam γ rays were detected in the Gammasphere Ge-detector array and were correlated with 101Sn β-delayed protons using the Recoil-Decay Tagging method. A γ-ray line at 172 keV was assigned to 101Sn and was interpreted as the transition between the single-neutron g7/2 and d5/2 states. The measured νg7/2-ν d5/2 energy splitting is compared with predictions of various mean-field potentials. It is also used to calculate multi-neutron configurations in light Sn isotopes. Prospects of using β-delayed protons for tagging other exotic nuclei in the 100Sn region are discussed. [ABSTRACT FROM AUTHOR]

Proton radioactivity of 117La, was measured with much improved statistics, but no evidence is observed for the isomeric proton decay. Prompt γ-rays were identified in proton emitters 117La, 151Lu, and beta-delayed proton precursor 117Ba using recoil decay tagging method by coupling the Gammasphere Ge array at the target position with the Argonne Fragment Mass Analyzer (FMA). [ABSTRACT FROM AUTHOR]

We have investigated the low-energy states of 11N by the elastic scattering method in inverse kinematics using a 10C beam and a (CD2)n target. Recoil protons were detected at 16 laboratory angles θlab = 4.6° – 18.9° in a ΔE - E detector system. Using the R-matrix method to analyse the 10C + p elastic cross sections, we found 11N to be unbound with respect to proton emission by 1.54±0.02 MeV with a decay width of 0.83±0.03. These results are used to calculate the two-proton decay width of the 12O ground state. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

α-decay is known in a few cases just above 100Sn. The lowest isospin nuclei in which it has been observed are the Tz=1 nuclei 114Ba, 110Xe, and 106Te. For the next lower isospin with Tz=1/2, N=Z+1, we used the reactions 58Ni + 58Ni and 58Ni + 54Fe to search for the 113Ba→109Xe→105Te→101Sn α-decay chain. We found no candidates in our study. We set upper limits on the production of 113Ba via the 58Ni + 58Ni reaction and upper limits on the production of 109Xe via the 58Ni + 54Fe reaction. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Gamma rays from the transitional proton emitting nuclei 145,146,147Tm have been observed using the recoil-decay tagging technique. The ground state band of 147Tm was confirmed and extended and the unfavoured signature sequence was observed. A ground state rotational band with properties of a decoupled h11/2 band was observed in 145Tm. In addition coincidences between the proton fine structure line and the 2+→0+ γ-ray transition in 144Er were detected at the focal plane of the FMA. This is the first time that coincidences between proton radioactive decays and γ rays have been seen. The particle decay of 146Tm has been measured with improved statistics and a rotational band similar to 147Tm has been observed. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The paper reports on experimental developments for studies of decays of proton radioactive nuclei on the Argonne Fragment Mass Analyzer. The results illustrate that there is a clear proton decay rate sensitivity to the unpaired neutron orbital in highly deformed odd-odd nuclei consistent with the Influential Spectator Model of Ferreira and Maglione. No such sensitivity is observed for the proton decay rates of spherical odd-odd nuclei studied here. The first ever study of proton decay from a nucleus, 135Tb, produced via 1p6n fusion evaporation is also reported. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Several searches for new proton emitters were performed recently at ATLAS using the Argonne Fragment Mass Analyzer equipped with a Double-Sided Si Strip Detector. Proton emitting states were observed in odd-odd spherical nuclei [sup 164]Ir and [sup 170]Au. [sup 164]Ir is the fourth Ir proton emitter. A proton line was associated with the highly deformed odd-odd nucleus [sup 130]Eu. The proton emitter [sup 135]Tb was also observed. It is the first proton emitting Tb isotope. The estimated cross section for producing [sup 135]Tb is about 2 nb. The properties of the proton emitters deduced from this work helped to understand the role of core vibrations and neutrons in proton decay. In view of the present results the full delineation of the proton drip-line between Z=50 and Z=83 is feasible. In order to cope with small cross sections and short half lives expected for even more exotic proton emitters several modifications of the FMA implantation station were implemented. Measurements were also performed at the target position using a catcher to search for proton emitters with half lives down to 10 ns. [ABSTRACT FROM AUTHOR]

Production of the radioisotope [sup 18]F in novae is severely constrained by the rate of the [sup 18]SF(p, α)[sup 15]O reaction. A resonance at E[sub c.m.] = 330 keV may strongly enhance the [sup 18]F(p,α)[sup 15]O reaction rate, but its strength has been very uncertain. We have determined the strength of this important resonance by measuring the [sup 18]F(p,α)[sup 15]O cross section on- and off-resonance using a radioactive [sup 18]F beam at the ORNL Holifield Radioactive Ion Beam Facility. We find that its resonance strength is 1.48±0.46 eV, and that it dominates the [sup 18]F(p, α)[sup 15]O reaction rate over a wide range of temperatures characteristic of novae. [ABSTRACT FROM AUTHOR]

Proton radioactivity studies performed at the Holifield Radioactive Ion Beam Facility (HRIBF) within the last few years are reviewed. The discovery of five new proton radioactivities [sup140]Ho,[sup141m]Ho, [sup145]Tm, [sup150m]Lu and [sup151m]Lu is presented together with a recent observation of fine structure in proton emission from [sup146gs,m]Tm. These proton emitters were produced by means of fusion-evaporation reactions and studied with the HRIBF Recoil Mass Separator and detection system based on a Double-sided Silicon Strip Detector. For [sup113]Cs and [sup151]Lu, the studies of level structure were extended beyond the proton-emitting states via the measurements with a clover array CLARION using Recoil Decay Tagging [ABSTRACT FROM AUTHOR]

In a recent experiment to study the proton radioactivity of the highly deformed [sup131]Eu nucleus, two proton lines were detected. The higher energy one was assigned to the ground-state to ground-state decay, while the lower energy, to the ground-state to the 2[sup+] state decay. This constitutes the first observation of fine structure in proton radioactivity. With these four measured quantities, proton energies, half-life and branching ratio, it is possible to determine the Nilsson configuration of the ground state of the proton emitting nucleus as well as the 2[sup+] energy and nuclear deformation of the daughter nucleus. These results will be presented and discussed [ABSTRACT FROM AUTHOR]

The last five years have witnessed a rapid increase in the volume of data on proton decaying nuclei. The path was led by decay studies with recoil mass separators equipped with double-sided Si strip detectors. The properties of many proton decaying states were deduced, which triggered renewed theoretical interest in the process of proton decay. The decay experiments were closely followed by in-beam 7-ray studies which extended our knowledge of high-spin states of proton emitters. The unparalleled selectivity of the Recoil-Decay Tagging method combined with the high efficiency of large arrays of Ge detectors allowed, despite small cross sections and overwhelming background from strong reaction channels, the observation of excited states in several proton emitters. Recently, in-beam studies of the deformed proton emitters [sup141]Ho and [sup131]Eu have been performed with the GAMMASPHERE array of Ge detectors and the Fragment Mass Analyzer at ATLAS. Evidence was found for rotational bands in [sup141]Ho and [sup131]Eu. The deformations and the single-particle configurations proposed for the proton emitting states from the earlier proton-decay studies were confronted with the assignments deduced based on the in-beam data. It should be noted that the cross section for populating [sup131]Eu is only about 50 nb, and it represents the weakest channel ever studied in an in-beam experiment [ABSTRACT FROM AUTHOR]

Different aspects of the formation and decay of [sup 24]Mg in the collision [sup 13]N+[sup 11]B have been studied using a large solid angle and highly segmented Silicon strip detector. Results concerning the fusion cross section, the 6 α decay of [sup 24]Mg and the GDR gamma ray emission are discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In the astrophysically important reaction 19Ne(p,γ)20Na, the rate is dominated by a single key resonance at 450 keV above the proton-emission threshold in 20Na. Throughout the last few decades many experiments have been performed aimed at finding the identity of this state. Despite this, the spin-parity of the key resonant state is still up for debate. The present paper describes a new experiment studying the β-delayed proton decay of 20Mg aimed at solving this issue. [ABSTRACT FROM AUTHOR]

We have developed a new experimental technique to measure very low energy protons from β-delayed p-decay of proton-rich nuclei produced and separated with the MARS recoil spectrometer at TAMU. Recently we have investigated the β-delayed p-decays of 23Al[1] and 31Cl[2] and obtained information on the resonances in the reactions 22Na(p,γ)23Mg and 30P(p,γ)31S, respectively. These reactions are important in explosive H-burning in Novae [3]. A simple setup consisting of a telescope made of a thin double sided Si strip detector (p-detector) backed or sandwiched between two thick Si detectors (β-detectors) was designed. We studied two different p-detectors and found that the thinner detectors with a small cell size are best to measure proton energies as low as 2–300 keV. [ABSTRACT FROM AUTHOR]