Future Perspectives for Surrogate-Reaction Studies at Storage Rings
211
nucleus [9]. The situation is very different for γ-emission probabilities. For all the
cases we have investigated the γ-probabilities induced by surrogate reactions are
much higher than neutron-induced ones. This discrepancy has been attributed to the
spin-parity mismatch, i.e. the difference between the angular momentum and parity
of the compound nucleus populated by the neutron induced and by the surrogate
reaction. Nevertheless, the γ-emission and fission probabilities are very useful to
fix the parameters of some of the key ingredients of the statistical model like level
densities, γ-ray strength functions and fission barriers. To establish how surrogate
reactions can be used to infer neutron cross sections in regions where no neutroninduced data exist, it is necessary to build the systematics of decay probabilities
involving nuclei with different structural properties located in various mass regions
and different surrogate reactions.
From a technical point of view, the measurement of surrogate reactions in
direct kinematics faces the following limitations: (1) Unavailability of targets from
short-lived nuclei. (2) High background from competing reactions with the target
contaminants and backing. (3) The heavy products of the decay of the compound
nucleus are stopped in the sample and cannot be detected with particle detectors.
The limitations (1) and (3) can be addressed using radioactive beams in inverse
kinematics. Nevertheless, radioactive beams have rather low energy and position
resolution which translates into a low excitation energy resolution. In addition, for
surrogate reactions the most promising isotopes are gases (H and He), but high
areal densities of such molecules are difficult to reach and the target container
may introduce some background and loss of energy resolution due to angular and
energy straggling in the target window and the thick target. Such difficulties can be
overcome if the surrogate experiments are performed at storage rings.
Storage rings present unique opportunities for surrogate-reaction experiments. A
heavy-ion storage ring is an ensemble of beam pipes and electro-magnetic devices
arranged in a closed geometry where the heavy ions turn with high frequencies,
about 1 MHz at 10 MeV/u. The storage of heavy ions requires to minimize the
number of atomic reactions between the stored beam and the residual gas inside
the ring. Therefore, heavy-ion storage rings are operated at ultra-high vacuum
(UHV) conditions (10 −11 to 10 −12 mbar), which poses severe constraints to inring detection. For this reason, nuclear reactions have started to be measured only
very recently at the Experimental Storage Ring (ESR) of the GSI/FAIR facility in
Darmstadt, Germany [10, 11].
The most important capability of storage rings is beam cooling, which allows
the reduction of the energy and position spread of the stored radioactive ions. Beam
cooling takes typically a few seconds, which sets the lower limit on the half-live of
the radioactive ions available. The combination of the electron cooler and the dipole
magnets ensures the quality of the stored beam in terms of emittance and purity.
The electron cooler can compensate the angular and energy straggling, and energy
loss of the beam in the gas target. Hence, the beam passes the target always with
a very low energy and position spread at the same energy. Moreover, the frequent
passing of the reaction zone allows ultra-thin gas targets (10 13 atoms/cm 2 ) to be used
and therefore no windows are necessary. This is of great advantage for surrogate
211
nucleus [9]. The situation is very different for γ-emission probabilities. For all the
cases we have investigated the γ-probabilities induced by surrogate reactions are
much higher than neutron-induced ones. This discrepancy has been attributed to the
spin-parity mismatch, i.e. the difference between the angular momentum and parity
of the compound nucleus populated by the neutron induced and by the surrogate
reaction. Nevertheless, the γ-emission and fission probabilities are very useful to
fix the parameters of some of the key ingredients of the statistical model like level
densities, γ-ray strength functions and fission barriers. To establish how surrogate
reactions can be used to infer neutron cross sections in regions where no neutroninduced data exist, it is necessary to build the systematics of decay probabilities
involving nuclei with different structural properties located in various mass regions
and different surrogate reactions.
From a technical point of view, the measurement of surrogate reactions in
direct kinematics faces the following limitations: (1) Unavailability of targets from
short-lived nuclei. (2) High background from competing reactions with the target
contaminants and backing. (3) The heavy products of the decay of the compound
nucleus are stopped in the sample and cannot be detected with particle detectors.
The limitations (1) and (3) can be addressed using radioactive beams in inverse
kinematics. Nevertheless, radioactive beams have rather low energy and position
resolution which translates into a low excitation energy resolution. In addition, for
surrogate reactions the most promising isotopes are gases (H and He), but high
areal densities of such molecules are difficult to reach and the target container
may introduce some background and loss of energy resolution due to angular and
energy straggling in the target window and the thick target. Such difficulties can be
overcome if the surrogate experiments are performed at storage rings.
Storage rings present unique opportunities for surrogate-reaction experiments. A
heavy-ion storage ring is an ensemble of beam pipes and electro-magnetic devices
arranged in a closed geometry where the heavy ions turn with high frequencies,
about 1 MHz at 10 MeV/u. The storage of heavy ions requires to minimize the
number of atomic reactions between the stored beam and the residual gas inside
the ring. Therefore, heavy-ion storage rings are operated at ultra-high vacuum
(UHV) conditions (10 −11 to 10 −12 mbar), which poses severe constraints to inring detection. For this reason, nuclear reactions have started to be measured only
very recently at the Experimental Storage Ring (ESR) of the GSI/FAIR facility in
Darmstadt, Germany [10, 11].
The most important capability of storage rings is beam cooling, which allows
the reduction of the energy and position spread of the stored radioactive ions. Beam
cooling takes typically a few seconds, which sets the lower limit on the half-live of
the radioactive ions available. The combination of the electron cooler and the dipole
magnets ensures the quality of the stored beam in terms of emittance and purity.
The electron cooler can compensate the angular and energy straggling, and energy
loss of the beam in the gas target. Hence, the beam passes the target always with
a very low energy and position spread at the same energy. Moreover, the frequent
passing of the reaction zone allows ultra-thin gas targets (10 13 atoms/cm 2 ) to be used
and therefore no windows are necessary. This is of great advantage for surrogate