RA3. Experimental Equipment - The Oak Ridge Isomer Spectrometer and Separator (ORISS)
[A. Piechaczek & K. Carter (UNIRIB), spokespersons]
In 2003, the UNIRIB Consortium set out to develop a universal means for providing high-purity radioactive beams for nuclear decay spectroscopy. High intensity backgrounds of adjacent nuclei are the principal impediment in many decay studies of such exotic nuclei. Many techniques exist for purifying RIBS, including laser ionization, molecular sideband, ranging out and others. Magnetic isobar separators are also frequently used.
The method chosen by UNIRIB was an electrostatic longitudinally dispersive separator based on the multi-turn time-of-flight technique [Cas01, Ish04]. The UNIRIB goal was initially a mass resolution of 15,000 as a separator. The technique was universal, applying equally well to all elements, and the goal of 15,000, which was beyond anything achieved at that time, is sufficient to significantly reduce other members of the mass chain.
Since that time the UNIRIB Consortium has advanced three separate technologies which when combined into one instrument will far exceed the original goal of a mass resolution of 15,000. Our instrument, ORISS, will consist of three components:
A multi-turn time-of flight spectrometer with an experimentally demonstrated mass resolving power of 110,000 [Pie08], An electrostatic gate of the Bradbury-Nielsen type [Bra36] with locally developed fast pulser electronics [Gri09] to physically separate individual isobars, A radiofrequency quadrupole (RFQ) cooler/buncher for cooling and injection of ions into the time-of-flight spectrometer [Koz09,Shc09].
Using funding from a recent DOE grant, the assembly and remaining construction will be completed. Off-line tests of ORISS should be performed within one year. ORISS will be initially commissioned and used for experiments at the online isotope separator, UNISOR, in 2011. A picture of the existing time-of-flight spectrometer is shown in Fig. RA3-1. A draft of the layout for ORISS at UNISOR is shown in Fig. RA3-2.
Figure RA3-1: View of the main component of ORISS, the multi-turn time-of-flight mass spectrometer in its vacuum chamber.
Figure RA3-2: Conceptual layout of ORISS at the UNISOR on-line test facility, where commissioning and first experiments will take place.
Experimentally determined performance data from the RFQ, the time-of-flight spectrometer, and the Bradbury-Nielsen gate together with calculations from an ion optics code we developed [Shc05] allowed us to predict the performance of ORISS when completed. We expect to obtain:
Mass resolving power of 400,000 full-width at half-maximum (consequence of ion cooling) as a mass spectrometer. Capability to produce pure beams of nuclei with a relative mass difference M/ΔM < 200,000 as a mass separator. Transmission of 50% from DC input beam to separated beam.
ORISS will provide pure beams of any isobar thus satisfying the original goal. However, the significantly improved performance of 400,000 provides a "quantum leap" in capabilities for decay spectroscopy by adding the ability to produce pure beams of isomers with a relative mass difference M/ΔM < 200,000 (corresponding to an excitation energy of an isomeric state > 470 keV in a mass A = 100 nucleus). For isomeric states with relative mass differences M/ΔM > 200,000, ORISS will be able to produce enriched beams. ORISS will provide unparalleled capabilities for nuclear structure experiments such as:
Decay experiments with pure sources, Separation of isomers and study of their decays, and Fast and efficient search for isomers in spectrometer mode.
In order to demonstrate the power of ORISS, we show in Fig. RA3-3 a
simulated time-of-flight spectrum with a mass resolving power
full-width at half-maximum of ~ 400,000 for the mass A = 77 isobaric
chain. The full width at half-maximum of the time-of-flight peaks is
14 ns. All isobars and the 772-keV isomer in 77Zn are clearly resolved
in the spectrum. Pure beams of these nuclides can be produced using
the Bradbury Nielsen time gate which can be switched from "open" to
"closed" within 10 - 15 ns. The gate will be opened for ~30 ns when
the desired nuclide arrives at the gate, and this nuclide will be
transmitted to a detector station located further down-beam. All other
nuclides arrive at the gate when it is closed, and they will be
laterally deflected into a beam dump area. The isomeric state in 77Ge
with an energy 160 keV would not be resolved in the time-of-flight
spectrum but could be identified since the resulting sum peak of 77mGe
and 77Ge (ground state) has a larger width (20 ns) than the ground-state peak alone (14 ns).
Figure RA3-3: Simulated time-of-flight spectrum for A = 77 isobars in ORISS after 300 laps, a time-of-flight of 16 ms. Energy differences [MeV] relative to 77Se (ground state) are indicated. The mass resolving power amounts to 430,000 full-width at half-maximum.
[Bra36] Absolute Values of the Electron Mobility in Hydrogen, N.E. Bradbury and R.A.Nielsen, Phys. Rev. 49 (1936) 388.
[Cas01] A multi-reflection time-of-flight mass spectrometer for in-situ measurement on acomet core, A. Casares, PhD thesis, Justus-Liebig-University Giessen (2001) unpublished.
[Gri09] Gate driver for a Bradbury Nielsen gate, B.O. Griffith, to be published, Nucl. Instr. Meth. B.
[Ish04] A time-of-flight mass spectrometer to resolve isobars, Y. Ishida et al., Nucl. Instr. and Meth. B 219-220 (2004) 468-472.
[Koz09] Study of Ion Cooling and Ejection from Two Stage Linear Quadrupole Ion Trap consisting of RFQ ion guides, V.I. Kozlovskiy et al., To be presented at the 18th International Mass Spectrometry Conference, Bremen, Germany, Aug. 30 - Sep. 04, 2009. See http://www.imsc-bremen-2009.de
[Pie08] Development of a high resolution isobar separator for study of exotic decays, A. Piechaczek et al.,, Nucl. Instr. and Meth. B 266 (2008) 4510-4514.
[Shc05] Non-Linear Beam Dynamics In High Resolution Multi-Pass Time Of Flight Mass separator*, V.A. Shchepunov, H. Wollnik, Proceedings of Particle Accelerator Conference, Knoxville, TN, May 2005, P. 4105.
[Shc09] Design of an RFQ Interface for the UNIRIB ORISS high resolution separator, V. A. Shchepunov and V. I. Kozlovskiy, to be published in Nuclear Instr. and Meth. B.