RFQ Beam Coolers

RFQ stands for Radio-Frequency Quadrupole (also known as a Quadrupole mass analyzer when used as a mass filter), an instrument that is used in mass spectrometry. The RFQ was invented by Prof. Wolfgang Paul in the late 50's / early 60's at the University of Bonn (Germany). Paul shared the 1989 Nobel prize in Physics for his work.

By aligning four rods and applying an RF voltage between opposite pairs, a quadrupole field is created that alternated focuses in each transverse direction. Sample for mass analysis are ionized, for example by laser (MALDI) or discharge (electrospray or Inductively Coupled Plasma, ICR) and the resulting beam is sent through the RFQ and "filtered" by scanning the operating parameters (chiefly the RF amplitude). The gives a mass spectrum, or fingerprint, of the sample. Residual gas analysers use this principle.

A "cooler" is a device that lowers the temperature of an ion beam by reducing its energy dispersion , beam spot size, and divergence - effectively increasing the beam brightness (or brilliance). Several ion beam cooling methods exist. In the case of an RFQ, the most prevalent one is buffer-gas cooling, whereby an ion beam loses energy from collisions with a light, neutral and inert gas (typically helium). Cooling must take place within a confining field in order to counteract the thermal diffusion that results from the ion-atom collisions.

Applications of ion cooling to Nuclear Physics (notably, mass measurements):

Despite its long history, high-sensitivity high-accuracy mass measurements of atomic nuclei continue to be very important areas of research for many branches of physics. Not only do these measurements provide us with a better understanding of nuclear structures and nuclear forces but they also offer insight into how matter behaves in some of Nature’s harshest environments. At facilities such as ISOLDE at CERN and TRIUMF in Vancouver, for instance, measurement techniques are now being extended to short-lived radionuclei that only occur naturally in the interior of exploding stars. Their short half-lives and very low production rates at even the most powerful facilities require the very highest in sensitivity of such measurements.

Penning traps, the central element in modern high-accuracy high-sensitivity mass measurement installations, enable measurements of accuracies approaching 1 part in 10^11 on single ions. However, to achieve this Penning traps must have the ion to be measured delivered to it very precisely and with certainty that it is indeed the desired ion. This imposes severe requirements on the apparatus that must take the atomic nucleus out of the target in which it has been created, sort it from the myriad of other ions that are emitted from the target and then direct it so that it can be captured in the measurement trap.

Cooling these ion beams, particularly radioactive ion beams, has been shown to drastically improve the accuracy and sensitivity of mass measurements by reducing the phase space of the ion collections in question. Using a light neutral background gas, typically helium, charged particles originating from on-line mass separators undergo a number of soft collisions with the background gas molecules resulting in fractional losses of the ions’ kinetic energy and a reduction of the ion ensemble’s overall energy. In order for this to be effective however, the ions need to be contained using transverse radiofrequency quadrupole (RFQ) electric fields during the collisional cooling process (also known as buffer gas cooling). These RFQ coolers operate on the same principles as quadrupole ion traps and have been shown to be particularly well suited for buffer gas cooling given their capacity for total confinement of ions having a large dispersion of velocities, corresponding to kinetic energies up to tens of electron volts. A number of the RFQ coolers have already been installed at research facilities around the world and a list of their characteristics can be found below.

----

ee also

Quadrupole mass analyzer

References

Bibliography

* cite journal
author = SHIPTRAP collaboration, J. Schönfelder, D. Ackermann, H. Backe, G. Bollen, J. Dilling, A. Dretzke, O. Engels, J. Estermann, D. Habs, S. Hofmann, F. P. Hessberger, H. -J. Kluge, W. Lauth, W. Ludolphs, M. Maier, G. Marx, R. B. Moore, W. Quint, D. Rodriguez, M. Sewtz, G. Sikler, C. Toader and Chr. Weber
title = SHIPTRAP—a capture and storage facility for heavy radionuclides at GSI
journal = Nuclear Physics A
volume = 701
issue = p. 579-582
year = 2002

* cite journal
author = G. Sikler, D. Ackermann, F. Attallah, D. Beck, J. Dilling, S. A. Elisseev, H. Geissel, D. Habs, S. Heinz, F. Herfurth et al.
title = First on-line test of SHIPTRAP
journal = Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
volume = 204
issue = p. 482-486
year = 2003
doi = 10.1016/S0168-583X(02)02116-X
pages = 482

* cite journal
author = J.B. Neumayr, L. Beck, D. Habs, S. Heinz, J. Szerypo, P.G. Thirolf, V. Varentsov, F. Voit, D. Ackermann, D. Beck et al.
title = The ion-catcher device for SHIPTRAP
journal = Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
volume = 244
issue = p. 489-500
year = 2006
doi = 10.1016/j.nimb.2005.10.017
pages = 489

* cite journal
author = J. Szerypo, A. Jokinen, V. S. Kolhinen, A. Nieminen, S. Rinta-Antila and J. Äystö
title = Penning trap at IGISOL
journal = Nuclear Physics A
volume = 701
issue = p. 588-591
year = 2002
doi = 10.1016/S0375-9474(01)01650-5
pages = 588

* cite journal
author = T. Faestermann, W. Assmann, L. Beck, H. Bongers, W. Carli, M. Groß, R. Großmann, D. Habs, P. Hartung, S. Heinz et al.
title = The Munich Accelerator for Fission Fragments - MAFF
journal = Nuclear Physics A
volume = 746
issue = p. 22-26
year = 2004
doi = 10.1016/j.nuclphysa.2004.09.106
pages = 22

* cite journal
author = D. Habs, M. Groß, W. Assmann, F. Ames, H. Bongers, S. Emhofer, S. Heinz, S. Henry, O. Kester, J. Neumayr et al.
title = The Munich accelerator for fission fragments MAFF
journal = Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
volume = 204
issue = p. 739-745
year = 2003
doi = 10.1016/S0168-583X(03)00496-8
pages = 739

* cite journal
author = F. Herfurth, J. Dilling, A. Kellerbauer, G. Bollen, S. Henry, H. -J. Kluge, E. Lamour, D. Lunney, R. B. Moore, C. Scheidenberger et al.
title = A linear radiofrequency ion trap for accumulation, bunching, and emittance improvement of radioactive ion beams
journal = Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
volume = 469
issue = p. 254-275
year = 2001
doi = 10.1016/S0168-9002(01)00168-1
pages = 254

* cite journal
author = S. Schwarz, G. Bollen, D. Lawton, A. Neudert, R. Ringle, P. Schury and T. Sun
title = A second-generation ion beam buncher and cooler
journal = Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
volume = 204
issue = p. 474-477
year = 2003
doi = 10.1016/S0168-583X(02)02114-6
pages = 474
* cite journal
author = J. Clark, R. C. Barber, C. Boudreau, F. Buchinger, J. E. Crawford, S. Gulick, J. C. Hardy, A. Heinz, J. K. P. Lee, R. B. Moore et al.
title = Improvements in the injection system of the Canadian Penning trap mass spectrometer
journal = Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
volume = 204
issue = p. 487-491
year = 2003
doi = 10.1016/S0168-583X(02)02117-1
pages = 487

External links

* [http://groups.nscl.msu.edu/lebit/ LEBIT Project NSCL/MSU]

* [http://isoltrap.web.cern.ch/isoltrap/expsetup.html ISOLTRAP Experimental Setup]

* [http://titan.triumf.ca/research/intro.shtml TITAN: TRIUMF's Ion Trap for Atomic and Nuclear science]

* [http://www.kvi.nl/~trimp/web/html/trimp.html TRIMP - Trapped Radioactive Isotopes: Micro-laboratories for fundamental Physics]

* [http://www.gsi.de/forschung/ap/projects/shiptrap/index_e.html The SHIPTRAP Experiment]

* [http://www.cern.ch/ab-div-op-iso-rfqcb The ISCOOL project]