Recoil Mass Spectrometer (RMS)
The recoil mass spectrometer (RMS) is a zero degree device used to separate masses produced in nuclear reactions. The spectrometer [1-6] is 25 m long and is comprised of a momentum separator for beam rejection and the traditional electric-magnetic-electric dipole mass separator [7-10]. The initial performance of the HRIBF RMS has been reported in ref. . Other recoil separator devices may be found in ref. [12-14].
A schematic sketch of the RMS. Detectors at the target and focal plane positions may be used to achieve high selectivity of reaction products. Primary beam is usually deposited in the RMS at positions marked with 1 and 3 and in spectrometers without a momentum separator [7-10] at positions 2 and 3. The superior beam rejection capability of the RMS is due to the additional focussing elements between positions 1 and 2. In addition, the amount of beam reaching position 3 may be reduced through the use of the finger system inside Q3.
The RMS consists of three magnetic dipoles (D), seven quadrupoles (Q), two sextapoles (S), and two electric dipoles (ED). The physical properties of each element may be found in ref. . The first two quadrupoles gather the recoiling nuclei and determine the momentum focus which is located inside Q3. The first magnetic dipole separates by momentum the recoiling nuclei from the beam. The primary beam, originating from a tandem accelerator, has a well-defined momentum even after passing through the target foil and should be focussed spacially according to its charge-state distribution. The recoils have a large momentum distibution caused by the evaporation process and will thus, fill the available space. Small rods called fingers may be inserted through the split poles of Q3 to intercept the primary beam at its focus and yet, have minimal impact on the overall transport efficiency for recoil products. When used with inverse kinematic reactions where beam particles and recoils have similar rigidities within the acceptances of the RMS, the fingers are designed to provide enhanced primary beam rejection.
A schematic sketch of the fingers inside Q3 and data taken with a position sensitive detector at the achromatic focal plane. The horizonal spectrum in the middle is the normalized one-dimensional spatial distribution of recoils at the achromat with and without fingers blocking charge state 23+. The normalized count rate spectrum on the right displays the number of recoils reaching the achromat as the position of one finger inside Q3 is changed. Note the dips in this spectrum caused by the fingers intercepting the focussed beam.
The second half of the momentum separator, D1-Q4-Q5, provides a second focus which is the object of the mass separator portion of the spectrometer. This position, called the achromat, is energy independent when Q3 is correctly adjusted. The two sextapoles provide second-order corrections in the final focus.
The mass separator section contains a magnetic dipole between two electric dipoles. The electric elements separate the recoils as a function of kinetic energy and charge (E/Q) and the magnetic dipole separates as function of momentum and charge (P/Q). The net result are recoils separated as a function of mass and charge (A/Q). The magnitude of the mass dispersion is determined by the last pair of quadrupoles, Q6 and Q7. Together with Q4 and Q5, the position of the final focal plane is determined.
The performance parameters of the spectrometer, such as the size of the final image and the mass dispersion, often depend only upon specific groups of elements of the RMS. These elements may be adjusted together without affecting the field values of the other elements. Thus, several "knobs" have been incorporated into the control system so that users may adjust these parameters according to the requirements of the experiment.
Various acceptances govern the performance of the RMS and the efficiency to detect recoil products. The RMS has an nonsymmetric, overall solid-angle acceptance. The horizontal acceptance is +/- 30 mrad and the vertical acceptance is +/- 110 mrad and is determined by the positions of Q1 and Q2. The target-to-Q1 distance is approximately 75 cm. However, other aperatures in the system affect the effective overall solid angle which corresponds between 10- and 15 msr. The primary beam spot on target should be a vertical line 1 mm in width and 4 mm in length. The energy acceptance is determined, in part, by the length of the plates on the electric dipoles and is estimated to be greater than +/- 10%. The A/Q acceptance, reflecting the various aperatures throughout the flight path is near 5%. Overall mass resolution (M/dM) has been calculated to be 540 although through collimation, it is estimated that this value may approach 1000 (including software corrections).
A typical focal plane image of the RMS using the reaction 60Ni + 58Ni at 220 MeV. The masses and resolution are given in the figure. This spectrum is taken with no other requirement than an ion passing through the focal plane detector. The high primary beam suspression of the RMS is evident from the cleanliness of this spectrum although a light spray of beam particles may be observed on the far left of the spectrum. You may access a higher resolution JPEG version of this figure.
The long flight-path of the spectrometer will prohibit detection of nuclei which contain isomeric transitions with halflives ranging from approximately ten nanoseconds to a microsecond. These nuclei decay in flight through internal conversion, causing disruption  of the atomic electrons and drastic changes in charge state. In order to reduce this effect, a charge resetting foil is placed some 10 cm away from the target and charge state equilibrium is obtained for all recoils. Due to the gaussian charge state distribution, the maximum total efficiency of the RMS for each mass will be on the order of 45%. This assumes that two charge states are detected at the focal plane.
A list of focal plane detectors and other relevent features of the RMS is given below:
- RMS hardware and other features
- charge resetting foil
- RMS target area detectors
- CLARION Ge array
- RMS focal plane area detectors
- moving tape collector (MTC)
- position sensitive avalanche counter (PSAC)
- multi-channel plate detector (MCP)
- focal plane
- ionization chamber
- gamma chamber
- double-sided silicon strip detectors (DSSD)
- pair spectrometer
This device is funded by the Georgia Institute of Technology, Idaho Nuclear Engineering Laboratory, Louisianna State University, Oak Ridge Associated Universities, Oak Ridge National Laboratory, State of Tennessee, UNISOR, University of Maryland, University of Tennessee, U.S. Department of Energy, and Vanderbilt University.
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