YRAST Ball

YRAST Ball, with up to 9 Compton-suppressed clover detectors, is the largest university-based gamma-ray spectrometer in the U.S.   The array, which was commissioned in 1997, was designed to accomodate a wide range of detector configurations and several different auxiliary detectors.

YRAST Ball was recently reconfigured to facilitate angular correlations measurements (for determining spins, multipole mixing ratios, etc). Detectors can be placed at 90° and 42.5°  (forward and backward angles).  This modification was designed for the study of low-lying mixed symmetry states, in which the protons and neutrons are moving collectively, but out of phase with each other. The new configuration is useful for any gamma ray spectroscopy experiments in which spin identification is required.  Both in beam and beta decay experiments have been performed with this setup, and angular correlation results have been tested successfully.  

New Yale Plunger Device (NYPD)

The New Yale Plunger Device is designed to measure lifetimes of excited states in the range of about one to several hundred picoseconds using the recoil distance method. The device is surrounded by the YRASTBall array in the SPEEDY setup. The basic principle of the plunger apparatus is based on a target and a stopper foil mounted parallel to each other at a variable distance. The nucleus of interest is produced in the target foil and recoils out of it with typically 1-3% of the speed of light. A gamma-ray deexciting an nuclear level can either be emitted from the stopped nucleus in the second foil, or in flight while traversing the gap. Both possibilities are distinct by the energy difference due to the doppler shift. In order to ensure a constant distance during the experiment, the capacitance between the foils is controlled and used as a feedback to a piezo crystal adjusting the distance on the sub-micrometer scale. From measurements at various distances between the two foils the lifetime of the state can be extracted.

Moving Tape Collector (MTC)

The moving tape collector is used for beta-decay experiments. A radioactive nucleus is produced in a fusion-evaporation reaction and subsequently implanted in a tape, making use of the fact that the angular distribution of the reaction products is larger than that of the non-reacting beam particles. The beam particles are stopped in a plug, which prevents them from being implanted in the tape. The tape in which the reaction products are implanted moves periodically, and transports those nuclei to a point which is surrounded by gamma-ray detectors. After a while, this nucleus will undergo beta decay, which leads to the population of excited states in the daughter nucleus which themselves will decay by the emission gamma radiation. Those are measured. The tape is important for suppressing unwanted background. This method is ideal for the population of states with comparably large energies and low angular momenta.

SASSYER - Small Angle Separator System at Yale for Evaporation Residues

The production of heavy nuclei in fusion-evaporation reactions is difficult, since the increasing number of protons increases the probability that fusion is immediately followed by fission, and not by a heavy nucleus. Therefore, the number of successful productions of a heavy nucleus is rather small compared to the number of events which result in fission. A gas-filled separator like SASSYER is necessary to select the reaction products of interest from a large background of unwanted reactions. SASSYER allows the detection of alpha and gamma radiation from the nucleus of interest, and the gas inside the separator optimizes the transmission probability for the reaction products of interest to the focal plane, where the reaction products are detected and, using alpha decay, identified in neutron and proton number.

Rutgers setup for g factor measurements

This setup for the measurement of magnetic moments (or the corresponding g factors) of short lived excited states was installed by Rutgers University. The nuclei of interest are accelerated and hit a low mass target (e.g. carbon). The energy is not sufficient to fuse the two nuclei, but due to the exchange of virtual photons (Coulomb excitation) the impinging nucleus can be brought to an excited state. The next target layer is ferromagnetic (e.g. gadolinium) and is magnetized by the outer coils. The fast heavy ion beam hitting the solid generates a strong magnetic field (transient field) in the order of hundreds of Tesla - this is at least 10 times stronger than the fields that superconducting magnets can generate. As an excited state with some angular momentum has a magnetic moment, the nucleus will precess in that magnetic field (just like a spinning top starts "tumbling" in the field of gravity). The angular distribution of the deexciting gamma-rays will change as well, allowing the measurement of the precession angle, which gives the magnetic moment of the excited state.

SAMMY

SAMMY is the Superconducting Assembly for Magnetic Moments at Yale. g factors of excited states are measured using perturbed gamma-gamma angular correlations in a static external field. Parent nuclei are produced through a standard heavy ion fusion evaporation reaction and embedded onto a moving tape collector system. The activity is then transported to the center of a superconducting coil capable of providing fields up to 6 Telsa. The beta decay to excited states in the nuclei of interest is then observed within an external magnetic field using 8 coaxial Ge detectors. The current program at SAMMY has focused on measurements of the g factors of 2₁⁺ states in the neutron-deficient Er, Yb and Hf region.