AN ADVANCED ION GUIDE FOR BEAM COOLING AND BUNCHING FOR COLLINEAR LASER SPECTROSCOPY OF RARE ISOTOPES By Bradley Ryan Barquest A next generation beam cooler and buncher has been developed and commissioned at NSCL to provide bunched, low energy spread ion beams for collinear laser spectroscopy of rare isotopes. Low energy spread beams reduce the Doppler broadening observed in the fluorescence spectrum, thus improving the resolution achievable in a measurement. Bunched beams permit the reduction of background in fluorescence spectra, thus dramatically improving the signal to noise ratio of the measurement. The beam cooler and buncher features a novel electrode design intended to simplify construction and maintenance, as well as permit the use of large radiofrequency (RF) amplitudes for more efficient beam cooling, especially in the case of high beam currents. A series of ion optical simulations were carried out to determine the design parameters and anticipated bunched beam characteristics. Additional studies using a prototype electrode structure and ion guide system were undertaken to test the performance of the proposed design. The ion guide also serves as a component of the NSCL gas stopping facility. Based on the results of simulation, prototype and ion guide test results, the BECOLA beam cooler and buncher was developed and commissioned. Buncher efficiency, bunched beam energy spread, time width, and transverse emittance were measured using a stable ion source. In addition, the BECOLA collinear laser spectroscopy facility and the beam cooler and buncher were commissioned using a beam of the rare isotope K. The results of ion optical simulations, prototype tests, ion guide studies, and BECOLA cooler and buncher characterization measurements are presented and discussed. The results indicate a successful commissioning of the beam cooler and buncher, along with the rest of the BECOLA facility, with a rare isotope beam, and the systematic studies indicate efficient delivery of low emittance, low energy spread bunched beams well suited for collinear laser spectroscopy.

An advanced ion guide for beam cooling and bunching for collinear laser spectroscopy of rare isotopes

Tests of the unitarity of the Cabbibo-Kobayashi-Masakawa (CKM) matrix offer an important avenue for constraining the Standard Model of the electroweak interaction. Several methods are currently used to determine Vud, the largest element in the top-row normalization test. One such method is through the study of superallowed T = 1/2 mixed mirror transitions, which offers a complementary method to the current most-precise value that is determined from superallowed pure Fermi 0+ → 0+ transitions. The precision currently achievable by this method is currently limited by the very low number of transitions for which the Fermi-to-Gamow-Teller mixing ratio ρ has been measured. St. Benedict, the Superallowed Transition Beta-Neutrino Decay-Ion-Coincidence Trap, is currently under development at the University of Notre Dame’s Nuclear Science Laboratory, and intends to determine ρ for a range of new isotopes through measurements of the β-neutrino asymmetry parameter aβν using a linear Paul trap. In order to trap these ions, the fast, continuous secondary beam separated by the TwinSol twin solenoid separator must be thermalized and bunched. The system through which this will be done will feature a large-volume gas cell in which the ions will be thermalized, a double-RF-funnel-based ion guide system for the extraction of the ions, and a radiofrequency quadrupole (RFQ) to provide cooled ion bunches for capture in the Paul trap.

Stopped, bunched beams for the TwinSol facility

The N = 126 factory currently under construction at Argonne National Laboratory’s ATLAS facility will make use of multi-nucleon transfer reactions to produce nuclei around the N = 126 shell closure that are of interest for the study of the rapid neutron capture process and are not available in sufficient quantities using common particle-fragmentation, target-fragmentation, or fission production techniques. As part of this facility, a radio frequency quadrupole (RFQ) buncher will cool and accumulate the beam, converting a high-emittance, continuous beam into a low-emittance bunched beam suitable for trapping. Here, the construction of the RFQ cooler-buncher, based on the design used at the National Superconducting Cyclotron Laboratory’s BECOLA and EBIT cooler-bunchers, will be discussed. This design features injection optics optimized to maximize acceptance, separated cooling and bunching regions, and a simplified RFQ electrode construction.

A cooler-buncher for the N=126 factory at Argonne National Laboratory

We report on the successful coupling of the Penning-trap mass spectrometry setup TRIGA-TRAP to the research reactor TRIGA Mainz. This offers the possibility to perform direct high-precision mass measurements of short-lived nuclei produced in neutron-induced fission of a 235 U target located near the reactor core. An aerosol-based gas-jet system is used for efficient transport of short-lived neutron-rich nuclei from the target chamber to a surface ion source. In conjunction with new ion optics and extended beam monitoring capabilities, the experimental setup has been fully commissioned. The design of the surface ion source, efficiency studies and first results are presented.

First online operation of TRIGA-TRAP

In the development towards the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS), an optical detection region for the observation of fluorescent light is added to an electrostatic ion beam trap (EIBT). In addition to its use for highly sensitive collinear laser spectroscopy, this fluorescence detection is introduced as a diagnostics tool for the study of the ion dynamics inside an EIBT. First measurements of collision-induced fluorescence in an EIBT demonstrate the technique’s diagnostics power by tracking the evolution of an ion bunch’s temporal width over its storage time inside the ion trap. Thereby, the time-focus point of the ion bunch can be determined and the influence of space-charge effects in the EIBT can be investigated. Good qualitative agreement is obtained between the measured trend of temporal widths and the simulations of the ions’ trajectories in the trap. Particularly, the observation of self-bunching on the ion-bunch structure for many simultaneously stored ions is reproduced.

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Fluorescence detection as a new diagnostics tool for electrostatic ion beam traps

A linear Paul trap for cooling of ion beams, the former cooler for emittance elimination radiofrequency quadrupole (RFQ) at MISTRAL/ISOLDE, has been installed and commissioned at the TRIGA-SPEC experiment located at the research reactor TRIGA Mainz. It is connected to a hot-surface-ionization ion source and a subsequent mass separator for ionization and pre-separation of neutron-rich fission products as delivered from the reactor. The capability of accumulating and bunching ion beams has been implemented to provide low-emittance ion pulses of 250 ns width containing up to 106 ions. A technical description of the upgraded RFQ as well as its characterization with stable ions is presented. Its installation allows delivery of low-emittance ion bunches to the two branches of the TRIGA-SPEC experiment, namely TRIGA-TRAP and TRIGA-LASER.

W. Nörterhäuser

Papers

An RFQ cooler and buncher for the TRIGA-SPEC experiment