Recent Research Highlights

The astrophysical rapid proton capture process (rp-process) occurs in novae and X-ray bursts and accounts for the production of elements ranging from Neon to Iron and, possibly, even of heavier nuclei. Gamma-ray satellites are currently scanning the sky in search for stellar objects where the process has occurred. To estimate their reach it is essential that the nuclear physics of the reactions involved be known with high precision. The production and destruction in stellar environments of radioactive 22Na (which emits readily detectable 1.2 MeV gamma rays) was determined accurately from a combination of measurements and techniques. For 22Na production, the mass of 22Mg and the excitation energy of the states through which proton capture from 21Na occurs were measured with great accuracy with the Canadian Penning Trap and Gammasphere, respectively.

Impact of new results on the "horizon" of the INTEGRAL satellite. Click on image to see full size.

Gammasphere also provided the information required for the determination of the probability of 22Na destruction through proton capture into 23Mg. When combined with data from the ISAC facility, an upper limit of 2 kpc was derived for the "horizon" of the INTEGRAL satellite. This puts out of reach objects such as, for example, the Crab nebula that INTEGRAL was hoping to investigate.

To read more:

1. G. Savard et al., Phys. Rev. C 70, 042501 (2004).
2. D. Seweryniak et al., Phys. Rev. Lett. 94, 032501 (2005).
3. D. Jenkins et al., . Rev. Lett. 92, 031101 (2004).

The time scale with which the astrophysical rapid proton capture process (rp process) can proceed from iron to tin depends critically on the properties of some specific nuclei, called "waiting point" nuclei. These are nuclei with the same number of protons and neutrons which lie on the proton drip line, at the very limits of nuclear stability. Their mass plays an important role in determining whether they will slow the path to synthesizing heavier species. This then in turn affects the light curve and energy output of X-ray bursters and the distribution of the elements that are finally synthesized. High precision mass measurements carried out at ATLAS with the CPT have now established that 64Ge is not a waiting point nucleus, while 68Se definitely is. The measurements are an order of magnitude more precise and free of many systematic errors that affect previous determinations.

To read more:

1. J. A. Clark et al., Phys. Rev. Lett. 92, 192501 (2004).
2. J. A. Clark et al., Phys. Rev. C 75, 032801(R) (2007).

The reactions 8Li(d,p) and 6He(d,p) have been studied with exotic 8Li and 6He beams produced at ATLAS, in order to compare the properties of states in 9Li and 7He with the results of ab-initio calculations. These calculations start from the bare nucleon-nucleon interaction and our best description of three-body forces between nucleons inside the nucleus. The new data provide an opportunity to gain new knowledge about the three-body forces in these light systems where the number of neutrons far exceeds that of protons.

To read more:

1. A. Wuosmaa et al., Phys. Rev. Lett. 94, 082502 (2005).
2. A. Wuosmaa et al., Phys. Rev. Lett. submitted.

Shell structure is the cornerstone of our understanding of nuclear structure. Yet, with every new data set on neutron-rich nuclei, we come to realize that our knowledge, based mostly on studies of stable nuclei, is only partial: shell structure can be severely affected by neutron excess. Neutron-rich Ca, Ti and Cr isotopes studied in a combination of experiments using deep inelastic reactions with Gammasphere at ATLAS and spectroscopy of fragmentation products at the NSCL indicate that new shell gaps occur because of an interaction between protons and neutrons occupying specific intrinsic states. Furthermore, in another set of experiments performed at the Yale Tandem, a study in the Sn isotopes suggests that the spin-orbit force, a crucial ingredient for shell structure, exhibits an unexpected decrease with increasing neutron number.

To read more:

1. R. V. F. Janssens et al., Phys. Lett. B546, 55 (2002).
2. S. N. Liddick et al., Phys. Rev. Lett. 92, 072501 (2004).
3. J. P. Schiffer et al., Phys. Rev. Lett. 92, 162501 (2004).

With Gammasphere and the FMA the structure of the Tm nuclei with mass A=145-147 has been mapped out and strong changes in structure as a function of mass have been uncovered. These changes in turn affect the rate of proton emission. In particular, the degree in which these deformed nuclei depart from axial symmetry (triaxiality) was quantified and the data serve as a new, severe test of calculations which attempt to take triaxiality into account in the description of the observed proton decay rate and fine structure branching ratio.

To read more:

1. D. Seweryniak et al., Phys. Rev. Lett. 86, 1458 (2001).
2. C. Davids and H. Esbensen, Phys. Rev. C 69, 034314 (2004).
3. D. Seweryniak et al., to be published.

For physicists the nucleus 6He, with 2 protons and 4 neutrons, has been intriguing for quite some time. Measurements in the eighties and nineties have indicated that, when used as a beam, the probability for it to induce a nuclear reaction on any target is much larger than that for 4He. This observation was interpreted as a strong indication that 6He is a three-body "halo" nucleus, i.e., it can be best viewed as a well bound 4He core and 2 neutrons orbiting this core at large distances. Moreover, while these three constituents of 6He form a bound system, the nuclear potential is not strong enough to bind any two of them separately. For this reason, 6He is often referred to as "Borromean" (The name derives from the heraldic emblem of the medieval princes of Borromeo, three rings interlocked in such a way that the removal of any of the rings will cause the remaining two to fall apart).

Schematic diagram of the 6He nucleus, and the heraldic emblem of the medieval pronces of Borromeo.

Because of its intriguing properties, 6He has the potential to teach us about the fundamental forces among the constituent nucleons. Indeed, the halo character can be revealed by an accurate determination of the nuclear charge radius, since the motion of the core with regard to the center of mass reflects both the radial extent of the neutrons and the correlations between these particles. The result can in turn be compared with the most modern theories ( http://dnp.nscl.msu.edu/current/exact.html) as recent advances in computational methods have made it possible to calculate the structure of few-nucleon systems from the basic interactions between the constituents.

The charge radius of 6He has been determined for the first time by measuring the atomic isotope shift between 6He and 4He using laser spectroscopy. For this work, 6He atoms were produced at ATLAS and quickly captured and cooled by an on-line laser trap. By applying laser spectroscopy on the trapped 6He atoms as well as on their 4He isotopic partner atoms, the charge radius of the 6He nucleus was determined to be 2.054 ± 0.014 fermi, approximately two millionth of a nanometer. The measurement is of such accuracy that it distinguishes between the available theoretical predictions. The data offer new insight into the dependence of three-body interactions on neutron number, which in turn is essential to the understanding of the structure of all neutron-rich systems, including neutron stars.

Schematic diagram of the laser trapping system.

In this work, 6He nuclei were produced via the 12C(7Li, 6He)13N reaction with a 100 pnA, 60 MeV beam of 7Li from ATLAS. Neutral 6He atoms diffused out of the hot graphite target and were transferred in vacuum to the nearby atomic beam assembly at a rate of approximately one million per second. Trapping helium atoms in the 23S1 metastable level was accomplished by exciting the 23S1 - 23P2 transition using laser light with a wavelength of 1083 nm. 6He atoms were mixed with a krypton carrier gas and sent through a discharge to be excited to the 23S1 level. The metastable 6He atoms were transversely cooled, decelerated with the Zeeman slowing technique, and then captured in a magneto-optical trap at a rate of approximately one atom per minute.

To read more:

1. Project homepage: http://www.phy.anl.gov/mep/atta/
2. Laser Spectroscopic Determination of the 6He Nuclear Charge Radius. L.-B. Wang et al., Phys. Rev. Lett. 93, 142501 (2004) Preprint (nucl-ex/0408008)