The LISA project

A quantitative measure of the nuclear collectivity is the B(E2), shown here are the known data for the first quadrupole excitation of even-even nuclei [1]. This property maps out the nuclear landscape and shows the regions of nuclear deformation, shell and shape changes, and their deep connection to the magic numbers. Magic and doubly magic nuclei are dominated by single-particle behavior, B(E2) values only amount to a few Weisskopf units (a measure for how many nucleons participate in the excitation). Very collective nuclei have B(E2) values up to several hundred W.u. and the simple interpretation indicates that many nucleons participate in the excitation.

\[ E_\text{lab} = E_0 \cdot \frac{\sqrt{1-\beta^2}}{1-\beta \cos{\alpha}}. \] Since both β and α depend on time, the measured Doppler-shifted energy of a γ ray contains information about the lifetime of the excited state it decayed from. If this lifetime is short as compared to the flight time through the target, the decrease of β due to the slowing-down in the target serves as a clock and allows to determine the excited-state lifetime. In contrast, for lifetimes which are long compared to the time it takes to traverse the target, the decay mainly occurs at constant β behind the target. In this case it is the emission angle α which changes with time and serves as clock.

Even employing the best γ-ray spectrometers, the uncertainties related to the position in the target and the velocity β at which the nuclear reaction occurred, are still given by the target thickness. A thinner target leads to a better defined reaction velocity and resolution, however reduces the luminosity of the experiment. The main idea of LISA is to replace the passive thick target by a stack of active targets. Active targets allow to determine the reaction vertex with much improved resolution. Knowing in which of the targets the reaction happened, allow to use the propper reaction position and velocity, i.e. α and β, for the Doppler correction. LISA thus opens new opportunities to measure lifetimes of excited states in very exotic nuclei with high precision. This is achieved by increasing the sensitivity by improving the resolution and the statistics that can be obtained in experiments with low-intensity radioactive beams.

Depending on the beam species, energy, and the target the in-beam resolution for γ rays emitted from relativistic heavy-ion beams is a few percent. For example, a FWHM of 0.9% was achieved for the 2⁺ → 0⁺ transition in ²⁸Si measured in the GRETINA array [2].
A major fraction, especially at small and large detection angles is determined by the energy loss in the target and the resulting uncertainty for the velocity β. The calculation on the right assumes a 200 MeV/nucleon beam of ⁷⁸Ni impining on a 7 mm thick carbon target. Using the active LISA target, the effective target thickness can be reduced while keeping the same luminosity, and the contribution to the resolution can be greatly suppressed. Especially for small and large detection angles, which are most sensitive to short lifetimes, the resolution is improved. The LISA project will dramatically enhance the sensitivity for lifetime measurements of exotic nuclei.