Neutron Transfer Measurements
Single-neutron transfer reactions, such as (d,p), provide information
which is useful for nuclear structure, astrophysics and stewardship
science studies. These reactions, with stable, light ion beams, have been
used for decades to extract information about the spins and parities as
well as single-particle properties of nuclei resulting from reactions of
stable beams on targets of stable or long-lived isotopes. Now
with sufficient Radioactive Ion Beam (RIB) intensities it is possible
to perform (d,p) reactions in inverse kinematics, using a heavy
radioactive species as the beam and a deuterated polyethylene target,
allowing us to study nuclei away from the valley of stability.
At the HRIBF, we use RIBs produced from proton-induced
fission on a uranium carbide target. This production mechanism
supplies many species of neutron-rich nuclei, including those close to
the shell closures at N=50 and N=82. The 25-MV
tandem Van de Graaff accelerator is capable of accelerating beams to around the
Coulomb barrier, a favorable energy regime for transfer reactions.
The protons resulting from the (d,p) reaction are measured in
various silicon detectors including the
SIDAR array and silicon strip detector
telescopes. The beam-like recoil particles are detected in an ion
The program to date has had a number of successes, including the first spectroscopic
measurement of the r-process nucleus 83Ge and a measurement of another N=51
neutron-rich isotone, 85Se. We have recently performed three (d,p) measurements on nuclei near the double shell closure at 132Sn - 130Sn(d,p) 131Sn, 132Sn(d,p)133Sn and 134Te(d,p)135Te.
Measurements using radioactive beams require efficient detector arrays, due to the relatively low beam intensities currently obtainable (compared to stable beams) . The Oak Ridge Rutgers University Barrel Array (ORRUBA) is a large solid angle silicon detector array, capable of providing energy, angle and particle identification information. ORRUBA is comprised of two rings of 12 position-sensitive silicon detector telescopes, symmetrically covering angles forward and backward of 90 degrees (relative to the target location). Each telescope consists of a thin (65um or 140um) transmission (dE) detector, and a thick(1000um) stopping (E) detector, enabling particle identification and measurement of the energy of particles of interest. Each detector is ~8cm long, and its width is divided into four 1cm wide resistive strips, oriented parallel to the beam axis. Readouts from both ends of each strip allow measurement of the position of the interaction, allowing determination of the emission angle of the detected particles. ORRUBA is currently in its design phase. The prototype detectors are being manufactured by Micron Semiconductors Ltd, and a new chamber is being constructed to accommodate the array.
The Q-value resolution of (d,p) reactions in inverse kinematics is limited by the kinematic
conditions. However, it is possible to measure the excitation levels of excited states
populated in the reaction, as well as others in the subsequent cascade, using the emitted
also give information on the spin-parities of the states, thus yielding important nuclear structure
information. Tagging on the gamma-rays can help to clarify the proton spectra and reduce background.
As the energy resolution is now independent of the reaction it is possible to use
thicker targets to increase the yield. It is important, at the same time, to use arrays of
high resolution detectors in close geometry.
Radioactive Ion Beams (RIBs) at the HRIBF are produced via the ISOL method, where a thick target is bombarded with light ions, such as protons, deuterons or alpha particles. The radioactive species produced in the reaction has to diffuse out of the production target and is transported to the ion source where it is ionized and accelerated. These steps are highly dependent on the chemistry of the isotope. Neutron-rich beams are produced at the HRIBF via the fission of uranium. In this case, isotopes of many different elements are produced simultaneously. Hence, it is often necessary to perform chemical separation in addition to mass separation. This chemical separation has to performed in-situ and on0line by using chemical properties for the element of interest. Many beam are available at the HRIBF, however in some cases the purity or intensity needs to be improved before they can be used for transfer reactions. For instance, for some very short lived isotopes, the time required to extract and transport the ion can be long compared with the nuclear half life, leading to significant losses in intensity. The RIB development team of the Center of Excellence is working to understand all the processes which take place in a target-ion source and to optimize each of them in order to purify beams and increase their intensities. The Center of Excellence is also interested in experiments on proton-rich isotopes which are relevant to radiochemical detectors for stewardship science. This involves the development of new beams. Different target materials are under investigation for fast release of reaction products.