Purushothaman, Sivaji

English

Many scientific disciplines make use of radioactive ions in the form of a low energy ion beam or a cold ion or atom cloud. Since radioactive isotopes are often best produced at very high energy, techniques to transform high-energy ions efficiently and quickly into low-energy ones are essential. In this context, theusefulness of superfluid helium and cryogenic noble gases was investigated.The extreme purity of cryogenic noble gases allowed to demonstrate for thefirst time that the maximum efficiency in this kind of systems is determined bythe chance of survival of ions during slowing down. This principal limit, a fewtens of percent, is high enough to make the method of practical use. Experimentsin which high-density cryogenic helium was ionized by a proton beam show theimportance of a large electric field; a field that quickly pulls ions and electronsapart and thus prevents neutralization. We demonstrated that at high fields, themaximum efficiency is maintained at ionization densities many times larger thanachieved up to now with other systems.Two mechanisms play a role in the extraction of ions out of superfluid helium:thermal excitation, which is strongly temperature dependent, and an as yet unknown temperature independent mechanism. The transition between these two lies at a temperature of about 1.3 Kelvin. The combined efficiency for ion survival in and extraction out of superfluid helium lies between 1 and 10 percent; high enough for practical applications

Dissertations of the University of Groningen

   University of GroningenSuperfluid helium and cryogenic noble gases as stopping media for ion catchersPurushothaman, SivajiIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2008Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Purushothaman, S. (2008). Superfluid helium and cryogenic noble gases as stopping media for ioncatchers. s.n. https://pure.rug.nl/ws/portalfiles/portal/14485731/thesis.pdfCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 04-06-20221IntroductionThe study of atomic nuclei takes a prominent position in the quest to understandquantum-mechanical many-body systems. Studying merely the naturally oc-curring atomic nuclei imposes a severe limit since these constitute only 5 % of theabout 7000 combinations of protons and neutrons which are bound by the stronginteraction. Experimental facilities where radioactive nuclei are created and studiedare thus a necessity. The use and study of radioactive nuclei has had a great im-pact on other fields of science (ranging from particle physics to materials science)and has yielded an enormous amount of spin-off technologies and applications[3, 81, 117, 119]. For a long time, high-energy beams of only stable isotopes werereadily available. In the past 25 years, radioactive ion beam (RIB) facilities allowedfor the first time the study of many exotic nuclei with proton or neutron combina-tions very different than those of stable nuclei. This led to the discovery of manynew and unexpected phenomena, such as halo nuclei and the melting of nuclearshells. The construction of next-generation RIB facilities is of the highest priority forthe nuclear physics community. Such a next generation facility will produce beamswith several orders of magnitude higher intensity, allowing new research with awide range of nuclear species much further away from the region of stable nu-clei. In RIB facilities of the in-flight type such as FAIR (Facility for Antiproton andIon Research) at GSI, Germany [45], RIBF (Radioactive Ion Beam Factory), RIKEN,Japan [90], NSCL (National Superconducting Cyclotron Laboratory), MSU, UnitedStates of America [80] and GANIL (Grand Accélérateur National d’Ions Lourds),France [48], radioactive ions are produced and selected at high energies (100 - 1000MeV per nucleon), resulting in a radioactive ion beam of high energy and poorbeam quality (large emittance and large energy spread). Many precision studies ofexotic nuclei far from the valley of stability, such as high-resolution particle spec-troscopy or studies in atom or ion traps, need low-energy beams (typically less thana few tens of keV) of high quality (small emittance and energy spread smaller than1 eV). The same requirements hold for the re-acceleration of the radioactive ionsto a precisely defined energy needed e.g. for nuclear reaction studies with radioac-12 Introductiontive ions. This makes the transformation of a high-energy, low-quality beam intoa low-energy, high-quality one (a so-called “cold beam”) an essential part of nextgeneration RIB facilities. The essential requirements for such a transformation arespeed and efficiency because the most exotic nuclei have short half-lives (down tomilliseconds) and are produced in small quantities. The so-called “ion catcher”method to perform this transformation is being investigated in several laboratoriesfor use in virtually all existing and planned RIB facilities. It is based on the IGISOLmethod developed in the early 1980’s at the University of Jyväskylä, Finland byJ. Ärje, J. Äystö and collaborators (see [9, 34] for reviews on this topic): the high-energy ions are stopped in a chamber filled with helium gas and extracted throughan exit-hole. The size of the chambers required by the high energy of the RIB (up to2 m long with a pressure of up to 2 bar) makes the use of the gas flow to extract theions from the chamber too slow; guidance by DC electric fields or a combination ofDC and RF fields is therefore an essential, but non-trivial task.The main aim of this research programme is to check the feasibility of cryogenicnoble gases and superfluid liquid helium as stopping medium. This thesis will dis-cuss these two approaches separately. Physical processes behind both approachesare discussed in the next two chapters and the experimental techniques, results anddiscussion are presented in the following chapters.The fundamental limit of efficiencies of a noble gas ion catcher has been an openquestion for years. Near and at thermal energies, ions cannot neutralize in collisionswith noble gas atoms due to the high ionization potential of the latter. So in case ofnear zero impurity level, most of the neutralization of ions will happen during theslowing down process. This means that the relative importance of neutralizationand ionization cross sections of the ion in the noble gas during slowing down willdetermine the efficiency limit of a noble gas ion catcher. The relevant physics hasbeen explained since the early days of quantum theory of atomic collisions in thebook “The theory of atomic collisions” by N. F. Mott and H. S. W. Massey [76]. How-ever, accurate charge exchange cross sections could not be calculated accuratelybecause of the mathematical complexity involved. The pioneering work by Hugheset al. on electron capture for singly charged particles like protons and muons [58]had been ignored in the nuclear physics community for some three decades and ledto a series of misunderstandings. Some recent measurements of the average chargestate of low-energy xenon ions in helium are reported by Willmann et al. [130]: theaverage charge state of xenon ions decreases down to about 0.25 at the lowest mea-sured energy of 10 keV in full agreement with the expectations of the early modelsin the relevant range of energies.What happens to thermalized ions is determined by the presence of impuritiesand the ionization of the noble gas by the energetic ions and possibly by an accel-erator beam or radioactive decay radiation or both [6]. Impurities take part in theneutralization process via three-body recombination involving a free electron andform molecules or adducts with the ions, see e.g. Reference [65]. It is importantto note that the ionizing radiation also plays a role in re-ionizing those ions which3have been neutralized. So one of the main factors that could improve the efficiencyof the gas cells is a low impurity level. Over the past 25 years, a lot of technicaldevelopment has focused on removing impurities from and preventing ionizationof the noble gas. Sub-ppb impurity levels have been achieved in noble gas catch-ers that are built according to ultra-high vacuum standards, that are bakeable andfilled with ultra-pure noble gases, see e.g. References [65, 93]. Constructing largeultra-pure gas catchers, although possible, is far from trivial [93]. There is, however,an alternative approach to reach ultra-pure conditions: freezing out the impurities.An added benefit of cryogenic gas catchers comes from the fact that for a constantgas density, the mass flow out of a gas cell is proportional to the square-root ofthe gas temperature. This means that a cryogenic noble gas cell allows easier dif-ferential pumping for the same gas density, or, reversing this argument, allows ahigher density for a constant gas load on the extraction system. The latter meansthat higher energy ions can be stopped or that, for the same ion energy, the gas cellcan be made shorter.Chapter 4 is dedicated to describe the setup, the methods and the principlesused for the experimental study and the data analysis.Chapter 2 gives an overview of the physics involved in the processes and a liter-ature survey on the available experimental data within the context of cryogenic no-ble gases. Off-line experiments using recoil ions from a radioactive 223Ra source areperformed to study the feasibility of using a cryogenic noble gas stopping mediumfor high-energy ion beams. Based on the off-line result, an on-line experimen-tal study on the extraction of thermalized 219Rn recoils from an ionized stoppingmedium was conducted. Results from both on-line and off-line experiments and adiscussion of the observed properties are reported in chapter 5.The much larger density (factor 800) of liquid helium relative to room temper-ature helium gas at 1 bar allows the use of a very small stopping chamber. Thismakes the extraction of the ions very fast, thereby also increasing the efficiencyof ion extraction because of a reduction in neutralization and radioactive decaywhich is important for short-lived nuclei. Also, the guiding by electric fields canbe very simple. The fact that nuclear polarization is preserved in superfluid helium[100, 111, 112, 106, 109, 110] could allow the extraction of polarized beams. At theenvisaged temperatures, the low vapour pressure above the superfluid helium sur-face removes the need for pumping large volumes of helium gas. This new method,if proven successful, could be implemented in many existing and planned laborato-ries around the world. Experimental work to produce a cold radioactive ion beamusing superfluid helium was started at the Department of Physics of the Universityof Jyväskylä about 7 years ago. 219Rn ions created in the alpha decay of 223Ra andrecoiling out of the source were stopped in superfluid helium and extracted intothe vapour phase as positive ions by means of electric fields. This was the firstever observation of the extraction of positive ions from the surface of liquid helium[56, 57, 108].Different processes, most importantly the dependence of the survival of snow-4 Introductionballs on applied electric fields and the dependence of their extraction across thesuperfluid-vapor interface on temperature are studied in the framework of thisproject. Chapter 3 give an essence of the physics tools necessary to understand thephysical processes involved. The experiments described in chapter 6 aim towardsa better understanding of the ion extraction at the superfluid-vapor interface. Fur-ther, as a new idea, the possibility to enhance the ion extraction efficiency by secondsound assisted superfluid surface evaporation is also investigated.Finally chapter 7 gives concluding remarks and discusses some future researchdirections.

Superfluid helium and cryogenic noble gases as stopping media for ion catchers

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Superfluid helium and cryogenic noble gases as stopping media for ion catchers

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