RF Fragment Separator

In some cases, the filtering of unwanted contaminants present in a radioactive beam provided by a fragment separator such as the A1900 can be insufficient. The Radio Frequency Fragment Separator (RFFS) can significantly improve the purity of these radioactive beams, in particular on the proton-rich side of the valley of stability, where the contamination is especially severe. By applying a transverse RF field on the particles in the beam, this device deflects them according to their phase relative to that of the cyclotrons, in effect creating a velocity selection. The unwanted particles are stopped on a set of slits, while the isotopes of interest continue on to the experimental station. The RFFS is presently located at the entrance of the S1/S2 vault, making filtered beams available for experimental setups in that area, such as the Beta-Nuclear Magnetic Resonance Apparatus, or the Beta Counting System (BCS) combined with the Segmented Gamma Array (SeGA). It has been used successfully in a number of experiments, where it increased beam purity by factors of 100 and more. For planning purposes, the effectiveness of this device in purifying beams can be checked using the program LISE++.

Expanded description

This figure illustrates the selection provided by the RFFS. The top panel shows a typical particle identification plot of energy loss versus time of flight, for fragments produced from a 112Sn beam. The bottom panel shows the vertical position at the slit location versus the same time of flight, each group corresponding to one of the time of flight lines shown in the top panel. The vertical position of the groups can be tuned by adjusting the phase of the RFFS, while the vertical slits can be closed to any position to perform the actual filtering.

The RFFS relies on the difference in the arrival times among the various isotopes selected by the fragment separator due to their different velocities and the micro-structure in time of the beam. A uniform RF electric field is applied transverse to the beam direction in the RFFS such that the ions are deflected to a greater or lesser extent depending on the phase of the applied RF during the time that they traverse the device. The phase of the RF is tuned such that a set of slits placed downstream from the RFFS blocks the bulk of the contaminants.
The figure shows an example of the effect of the RFFS, which is equivalent to that of a velocity filter, albeit modulo the period of the cyclotron RF. The data shown was taken with a reduced momentum acceptance of 0.5%, for which the time of flight lines are clearly separated. As the momentum acceptance of the A1900 is increased, these lines overlap and the separation provided by the RFFS degrades. The filtering quality of the RFFS depends on the following factors: i) the RF voltage applied between the plates, ii) the time of flight difference between the fragment of interest and the contaminants, and iii) the momentum acceptance used in producing the radioactive beam. It should be noted that this device cannot rid of contaminants that have time of flights which matches full 2π rotations of the RF phase. The program LISE++ can be used to simulate the RFFS for planning experiments.

Importance of the RF Fragment Separator

Isotopes located close to the proton drip-line cover a broad range of interest in nuclear physics as well as astrophysics. Many experiments aimed at studying these nuclei were so far impractical because of intense contamination in the beam. With the RFFS these experiments can now be performed at NSCL and take full advantage of the rare isotope production capability of the facility. An example are beta-decay studies of proton-rich nuclei close to the proton drip-line, often involved in astrophysical processes such as the rp-process. Magnetic moment measurements of proton-rich nuclei can also take advantage of the RFFS in particular to eliminate the beta-decay background caused by more abundant contaminants.

Technical information

The RFFS is composed of an RF cavity coupled to an RF system which provides the power, followed by a diagnostic box equipped with a set of continuously moveable vertical slits, where the actual filtering occurs. This box is also equipped with a pair of Parallel Plate Avalanche Counters (PPAC) to track particles at the slit location, a plastic scintillator for time-of-flight measurements, and a configurable stack of Silicon detectors for particle identification and implantation. A re-entry can located next to the Silicon stack can be used to insert a high-efficiency Germanium detector next to the implantation site for isomer tagging.
The horizontal plates of the RF cavity are 1.5 meter long and 5 cm apart. The maximum field so far has been achieved with a peak voltage of 100 kV at a frequency of 21.315 MHz. The maximum field is a function of the frequency because the quality factor (Q) of the cavity varies. The figure shows a technical drawing of the RF cavity, with its two symmetric coarse tuner drives at the top and bottom, the RF coupler on the right-hand side, and the fine tuner at the bottom left.

Status: Operational

Location: S2 vault

Contact person: Daniel Bazin

Funding acknowledgement: The construction of the RFFS was funded by the National Science Foundation through Major Research Instrumentation grant PHY-0520930.

References:

J. Stoker et al., "Commissioning Report on the NSCL RF Fragment Separator", Proceedings of the 234th ACS National Meeting, Boston, MA, USA, August 19-23, 2007

M. Doléans et al., "Status report on the NSCL RF Fragment Separator", Proceedings of the 22nd Particle Accelerator Conference (PAC2007), Albuquerque, NM, USA, June 25-29, 2007

D. Gorelov et al., "RF-Kicker System for Secondary Beams at NSCL/MSU", Proceedings of the 2005 Particle Accelerator Conference (PAC2005), Knoxville, TN, USA, May 16-20, 2005

K. Yamada et al., Nucl. Phys. A 746 (2004) 156c-160c
doi: 10.1016/j.nuclphysa.2004.09.064

Technical drawing of the RF cavity of the RFFS. The horizontal plates in between which the RF electric field is generated are the top and bottom of the two wedge-shaped electrodes, each attached to a side of the cylinder tank end caps.