The impact of the break-up of molecules in the stripper on AMS performance
Martin Suter1, Arnold Milenko Müller1, Hans-Arno Synal1, Marcus Christl1, Philip Gautschi1, Sascha Maxeiner2, Keith Fifield3
1Ion Beam Physics, ETH Zurich, Zurich, Switzerland, 2Ionplus AG, Dietikon, Switherland, 3Research School of Physics, The Australian National University, Canberra, Australia
When negative molecular ion beams are injected into an AMS system, the molecules break up in the stripping process in the terminal of the accelerator. If both fragments of the molecule are in positive charge states, the Coulomb force pushes them apart. Even though the resulting kinetic energy is only in the 10 eV range, the effect on the beam can be significant, because it is not energies but velocities that must be added. The transverse component of the break-up then leads to an angular spread and the longitudinal component causes an energy spread. This effect can cause significant beam losses during the transport to the final detector. This break-up process is called Coulomb explosion and was studied intensively in the period between 1975-1990 (Gemmel 1980). Already in 1983 Middleton mentioned that the Coulomb explosion of BeO in a stripper foil can lead to significant beam losses in 10Be AMS. He recommended to use a combination of gas stripping at low pressure followed by carbon foil stripping to get better transmission. However, measurement at the AMS facility Aster (Nottoli 2015) demonstrated that the effect of the coulomb explosion is also significant with gas stripping. In these experiments, measured beam profiles developed pronounced tails and the observed beam size was a factor of 2-3 times wider compared to that of atomic beams. An effective beam width of more than 10 mm was seen in the focal plane of the high energy magnet which is larger than the size of the detector entrance. Data are meanwhile available from low energy AMS, Vterm <1 MV, which also show a significant beam size enlargement in the sub-MeV range, leading to beam dimensions of ~10 mm (Gautschi, this Conference). Because the energy spread of the beam induced by the coulomb explosion enlarges the beam width, related transmission losses can be reduced by appropriate ion optics e.g. with an achromatic mass spectrometer at the high energy side of the accelerator. The size of Faraday cups, apertures and detector windows must also be optimized for best transmission.
Ways of modeling the effect of this Coulomb explosion have been investigated. Based on the bond length of BeO molecules, the Coulomb energy can be estimated for the various charge combinations of the two fragments. Under the assumption of an isotropic orientation of the molecules, the effect on the beam profile can be derived. Using the equation of motion, the separation of the fragments in time can be calculated. Foil and gas stripping lead to completely different processes. In gas stripping, the Coulomb energy is determined by the charges of the two fragments created in the single collision that breaks up the molecule. In foil stripping, by contrast, multiple collisions occur within the foil before the two fragments separate, and the mean charge of fragments within the foil is the important parameter. For thinner foils, the fragments continue to separate outside the foil, and this also plays a role.
I completed my PhD in Nuclear Physics at ETH Zurich in 1975. After postdoctoral positions at ETH and Oak Ridge, I returned to ETH in 1979 and was closely involved with developments at the AMS facility there for the next 30 years until my retirement in 2008. I was Head of the Laboratory for Ion Beam Physics (AMS and other ion beam applications) from 1992-2008. Subsequently I have been a visiting guest scientist at AMS laboratories in Seville, Vienna, Aix-en-Provence, Canberra, New Zealand and Beijing. In 2013, I cofounded Ionplus AG, and am presently a member of the Scientific Board of the company.