Greg Christian

I do nuclear physics experiments which are motivated by two main topics:

1. Structure of exotic nuclei

   Moving away from the valley of stability, a number of interesting nuclear structure changes begin to occur. These include the dissapearance of traditional "magic numbers" of enhanced stability, emergence of new magic numbers, appearance of nuclear "halos" and other pairing or clustering structures.

   A large variety of experimental tools are available to probe nuclear structure effects in exotic nuclei. One example are transfer reactions, in which the beam and target exchange a nucleon or nucleons, populating the ground state or an excited state of some new nucleus. An example would be the \( ^{18}\mathrm{O}(d,p)^{19}\mathrm{O} \) reaction, which populates states in \( ^{19}\mathrm{O} \). In a typical experiment, we measure the angles and energies of the outgoing protons. Since we are dealing with a direct reaction, these quantities are directly tied to the nuclear structure properties of the states in \( ^{19}\mathrm{O} \) accessed in the experiment. More specifically, we can deduce the energy of the states using the missing mass method, which makes use of the fact that the total four momentum is conserved in the reaction, \[ \vec{P}_{^{18}\mathrm{O}} + \vec{P}_{d} = \vec{P}_{^{19}\mathrm{O}} + \vec{P}_{p}. \] We can also learn about nuclear structure from the angular distribution of the outgoing proton. By comparing the measured cross section vs. proton angle curves with theoretical calculations, we can deduce the \( \ell \)-value of the populated state. Using this information in conjunction with predictions of the nuclear shell model, we can deduce the spin and parity of the state in question.

   Traditionally, transfer reaction experiments were carried out in "forward kinematics", where a beam of the lighter particle impinges on a target containing the heavier one. In the example above, then, we would have a beam of deuterons (\( ^{2}\mathrm{H} \) nuclei) impinging on a target containing \( ^{18}\mathrm{O} \). The outgoing protons would then be relatively forward focused, that is, they will be emitted at angles close to the beam angle in the laboratory frame. This allows them to be collected in a magnetic spectrometer such as the MDM Spectrometer at Texas A&M. However, much of the interesting physics occurs far from stability, motivating the study of reactions on radioactive nuclei, say \( ^{22}\mathrm{O}(d,p)^{23}\mathrm{O} \). This is impossible using forward kinematis since the target would decay away almost immedietly. So instead, we can do the experiment in "inverse kinemtatics", impinging a beam of the heavy nucleus (here \( ^{22}\mathrm{O} \)) on the lighter one. This creates challenges with regards to experimental detection since now the protons are emitted at a wide range of angles in the laboratory frame. One solution is to make use of large-area arrays of segmented silicon detectors to detect the outgoing protons. One example of such an array is TIARA, which will be brought to Texas A&M early in 2016.

2. Nuclear astrophysics

   The majority of the elements around us are formed by chains of nuclear reactions (and decays) occuring in astrophysical sites such as novae, supernovae, X-ray bursts, and quiescent stellar burning. To understand the nucleosynthesis occuring in these astrophysical environments and, consequently, to explain observed elemental abundances, we need to know the rates of a wide variety of nuclear reactions.

   Experiments done in the laboratory using particle accelerators can help us learn about these astrophysically-interesting reaction rates. In some cases, we can simply re-create the reaction of interest in the same energy regime as in the astrophysical environment. We then count how many reactions occur for a given number of beam particles and target atoms. These types of experiments are often referred to as direct measurements. In other cases direct measurements are not possible, perhaps because the reaction rate is too slow to measure in the lab (so you might have to run the experiment for, say, 1000 years to see a single event). In these situations we can still learn about the interesting reaction by doing other experiments that tell us about the specific nuclear structure properties associated with it. With these measurements in hand, we (or collaborators) can perform theoretical calculations that will contrain or estimate the rate of interest. These types of experiments are often called indirect measurements.

   Many of the interesting reactions for astrophysics involve radioactive nuclei. As with the nuclear structure studies, the study of these reactions requires inverse-kinematics experiments utilizing radioactive beams.

As mentioned, both of these subjects benefit immensely from the use of radioactive ion beams (or "rare isotope beams", both terms are commonly abbreviated as RIB). Facilities such as the TAMU Cyclotron Institute, TRIUMF in Vancouver, Canada, the NSCL at Michigan State University, and others around the world are capable of providing these beams. The available species, intensities, energy ranges, and emittance properties vary depending on the different techniques used to produce and accelerate the radioactive nuclei. If you want more information, this paper provides a nice overview of RIB production techniques.