Charge resetting foil
When a nanosecond isomer decays in a recoiling nucleus, the resulting
disruption of the electron cloud surrounding the nucleus may cause
several (on the order of 5) electrons to be ejected . This results in
a higher than normal charge state distribution for these nuclei.
If the RMS is set to accept the mean charge state of the other recoiling
nuclei, these reaction products would be lost in the spectrometer.
By placing a thin carbon foil (20 μg/cm2) approximately 10 cm
between the target and the RMS, charge state equilibrium of these recoils
can be restored. Thus, nuclei with isomers of lifetimes on the order of
the time it takes the recoil to reach the charge resetting foil , may be
studied with the RMS.
The RMS provides reset foils on two different frames. A standard frame is approximately 12.5 mm x 25 mm area with 2 mm sides. A RIB frame, see Fig. 1, is made of 0.1 mm thick, tantalum strips welded together and to form a 30 mm square area. The carbon foil is mounted on the "knife-edge" to minimize the cross-section of the frame exposed to Coulomb scattering of the beam from the target and to minimize the shadowing of any detectors (such as HyBall) downstream. An initial test of a charge reset foil mounted on a RIB frame with HyBall resulted in a slightly higher HyBall rate (2.5% increase) while producing a 60% higher recoil rate.
Fig. 1 - A photograph of the inside of the RMS target chamber. A target and charge reset foil mounted on a RIB frame may be observed.
ReferenceT.M. Cormier, P.M. Stwertka, H. Herman, N.G. Nicolis, Phys. Rev. C 30 , 1953 (1984).
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This file last modified Monday January 08, 2007