Antimatter Is Difficult to Make, Let Alone Store. Jeffrey Hangst Describes
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ANTIMATTER Keeping antihydrogen: the ALPHA trap Antimatter is difficult to make, let alone store.Jeffrey Hangst describes how ALPHA, an experiment attempting to trap antihydrogen at CERN, overcomes some of the difficulties and he questions the reality of ever making more than the tiniest amounts of antimatter. Suppose, as the villain of a story, you absolutely needed to trans- port a macroscopic amount of antimatter, for whatever sinister purpose. How would you go about it and could you smuggle it, for example, into the Vatican catacombs? The truth is that we will probably never have a macroscopic amount of antimatter for such a scenario to ever become reality. According to the preface to the popular novel Angels and Demons (2000), author Dan Brown was apparently inspired by the imminent commissioning of CERN’s “antimatter factory”, the Antiproton Decelerator (AD). The real-life AD has now been fully operational for about five years, and the experiments there have produced some notable physics results. One of the big stories along the way was the synthesis in 2002 of antihydrogen atoms by the ATHENA and ATRAP collaborations (CERN Courier November 2002 p5 and December 2002 p5, respectively). This feat was an important step towards one of the ultimate goals of everyday antimatter science: precision comparisons of the spectra of hydrogen and antihydrogen. According to the CPT Fig. 1. ALPHA’s Penning trap for catching antiprotons from theorem, these spectra should be identical. To get an idea of CERN’s AD. The stack of gold-plated electrodes is placed in a what precision means in this context, take a look at the website cryostat inside a superconducting solenoid magnet generating of 2005 Nobel Laureate Theodor Hänsch, which has the follow- a field of up to 3 T. (Courtesy N Madsen/Swansea.) ing cryptic headline: f(1S–2S) = 2 466 061 102 474 851(34) Hz. This may look like a cryptic puzzle appearing in Brown’s fiction, bine, the neutral antihydrogen is no longer confined by the fields of but it simply means that the frequency of one of the n = 1 to n = 2 the Penning trap, and the precious anti-atom is lost. The ATHENA transitions in hydrogen has been measured with an absolute pre- experiment demonstrated antihydrogen production because it cision of about 1 part in 1014. This is impressive, but where do we could detect the annihilation of the anti-atoms when they escaped stand with antihydrogen? the Penning trap volume and annihilated on the walls. To study antihydrogen using laser spectroscopy, anti-atoms Storing antihydrogen need to be sustained for longer. In the 1s–2s transition mentioned Both ATHENA and ATRAP produced antihydrogen by mixing anti- above, the excited state (2s) has a lifetime of about a seventh of protons and positrons in electromagnetic “bottles” called Penning a second; while in ATHENA, an anti-atom would annihilate on the traps (figure 1). Penning traps feature strong solenoidal magnetic walls of the Penning trap within a few microseconds of its crea- fields and longitudinal electrostatic wells that confine charged tion. Thus, the next-generation antihydrogen experiments include particles. The antiprotons come from CERN’s AD, and the posi- the provision for trapping the neutral anti-atoms that are produced trons come from the radioactive isotope 22Na. The whole process in a mixture of charged constituents. involves cleverly slowing, trapping, and cooling both species of The Antihydrogen Laser Physics Apparatus (ALPHA) collabora- particles (Amoretti et al. 2002 and Gabrielse et al. 2002). But tion has recently commissioned a new device designed to trap here’s the rub: when the charged antiproton and positron com- the neutral anti-atoms. ALPHA takes the place of ATHENA at the s CERN Courier July/August 2007 13 CCJulAugALPHA13-16.indd 13 11/7/07 14:05:43 ANTIMATTER Fig. 2. Depiction of a Ioffe–Pritchard trap, surrounding the electrodes of a Penning trap for charged particles. The arrows show the direction of current flow in the magnetic coils. 0 2 4 cm external solenoid bore trap vacuum silicon detector liquid helium three layers vessel barrier liquid helium vessel wall multipole windings helium Dewar eight layers wall vacuum Fig. 4. The ALPHA octupole being wound at Brookhaven. The (mirror coils superconducting cable is fixed to an adhesive substrate using not shown) helium Dewar vacuum ultrasonic energy. The magnet has eight layers, similar to the Fig. 3. Schematic cross-section of the ALPHA device. one shown here. (Courtesy J Escallier/BNL.) AD and features five of the original groups from ATHENA (Aarhus, colder atoms, but their temperature has not yet been measured. Swansea, Tokyo, RIKEN and Rio de Janeiro) plus new contribu- (Note that the highly excited antihydrogen atoms produced in both tors from Canada (TRIUMF, Calgary, UBC and Simon Fraser), the experiments can have significantly larger magnetic moments, thus US (Berkeley and Auburn), the UK (Liverpool) and Israel (Nuclear experiencing higher trapping potentials. The trick, then, is to keep Research Center, Negev). them around while they decay to the ground state.) Both groups Neutral atoms – or anti-atoms – can be trapped because they are investigating new ways to produce colder anti-atoms, and the have a magnetic moment, which can interact with an external 2007 run at the AD (June–October) promises to be revealing. magnetic field. If we build a field configuration that has a mini- mum magnetic field strength, from which the field grows in all Designer magnets directions, some quantum states of the atom will be attracted to A second important issue facing both collaborations is the effect the field minimum. This is how hydrogen atoms are trapped for on the charged particles of adding the highly asymmetric Ioffe– studies in Bose–Einstein condensation (BEC). The usual geometry Pritchard field to the Penning trap. Penning traps depend on the is known as an Ioffe–Pritchard trap (figure 2). A quadrupole wind- rotational symmetry of the solenoidal field for their stability. As ing and two solenoidal “mirror coils” produce the field to provide ALPHA collaborator Joel Fajans of Berkeley initially pointed out, transverse and longitudinal confinement, respectively. Figure 2 the addition of transverse magnetic fields to a Penning trap can also shows the electrodes that provide the axial confinement be a recipe for disaster, leading either to immediate particle loss, in the Penning trap for the charged antiprotons and positrons. or to a slower, but equally fatal, loss due to diffusion. Fajans’ The idea is that the antihydrogen produced in the Penning trap solution, adopted by the ALPHA collaboration, is to use a higher- is “born” trapped within the Ioffe–Pritchard trap – if its kinetic order magnetic multi-pole field for the transverse confinement. A energy does not exceed the depth of the trapping potential. higher-order field can, in principle, provide the same well-depth as This is a big “if”. A ground-state hydrogen atom has a magnetic a quadrupole while generating significantly less field at the axis of moment that gives us a trap depth of only about 0.7 K for a mag- the trap, where the charged particles are confined. netic well depth of 1 T. The superconducting magnetic traps that To construct such a magnet, the ALPHA collaboration surveyed we can build and squeeze into our experiments will give 1–2 T the experts in fabrication of superconducting magnets for accel- of well-depth for neutral atoms. All antihydrogen experiments to erator applications. It turns out that the Superconducting Mag- date occur in devices cooled by liquid helium at 4.2 K, but there net Division at the Brookhaven National Laboratory (BNL) had are strong indications that the antihydrogen produced by direct previously developed a technique that is almost tailor-made to mixing of antiprotons and positrons is warmer than this, with tem- our needs. The key here is to use the proper materials in the con- peratures of at least hundreds of kelvin. ATRAP has devised a struction of the magnet. To detect antiproton annihilations, ALPHA laser-assisted method of producing antihydrogen that may give incorporates a three-layer silicon vertex detector similar to those 14 CERN Courier July/August 2007 CCJulAugALPHA13-16.indd 14 11/7/07 14:05:59 ANTIMATTER Fig. 5. The working ALPHA device. The corrugated pipe is the superconducting solenoid for the Penning traps. (Courtesy N Madsen.) used in high-energy experiments (figure 3). However, the annihila- Back to Dan Brown tion products (pions) must travel through the magnets of the atom So let’s look at what is possible in experiments with antimatter trap before reaching the silicon. Therefore, it is highly desirable to today, leaving the speculation to aficionados of sci-fi and NASA. minimize the amount of material used in the magnet construction If you wanted to take antimatter to the offices of your national to minimize multiple scattering between the vertex and the detec- funding agency, you might consider taking some antiprotons, since tor. So bulky stainless-steel collars for containing the magnetic most of the mass-energy of an antihydrogen atom is in the nucleus. forces, as used in the Tevatron or the LHC, cannot be used. This might be tempting, since our charged-particle traps are cer- The Brookhaven process uses composite materials to constrain tainly deeper than those for neutral matter or antimatter. ATRAP the superconducting cable that forms the basis of the magnet. and ALPHA initially capture antiprotons in traps with depths of a Using a specially developed 3D winding machine, the team at few kilo-electron-volts, corresponding to tens of millions of kelvin. BNL was able to wind an eight-layer octupole and the mirror coils But, density is an issue.