Taming GeV Photons & Antimatter Formation Length-Related Bremsstrahlung Effects & Controlling Slow Positrons Ta Thickness [µm] 0 5 10 15 20 γ 10 1.0 /l f0 l 9 0.9 8 0.8 7 TSF 0.7 6 0.6 ω 5 0.5 h BH LPM 4 0.4 3 0.3 2 0.2 1 BH 0.1 0 0 1 2 3 4 5 6 7 8 9 10 ∆t/lγ HEINE DØLRATH THOMSEN DEPARTMENT OF PHYSICS AND ASTRONOMY FACULTY OF SCIENCE AARHUS UNIVERSITY, DENMARK Dissertation for the degree of Doctor of Philosophy Juli 2010 Cover image: The bremsstrahlung power spectrum is plotted in the (∆t, ℓf0, ℓγ) parameter space. The investigated regimes are bordered by black lines. © 2010 Heine Dølrath Thomsen Department of Physics and Astronomy Aarhus University Ny Munkegade 120 DK-8000 Aarhus C Email: [email protected] This document was compiled and typeset in LATEX 2ε This thesis has been submitted to the Faculty of Science at the Aarhus University in partial fulfilment of the requirements for ob- taining a PhD degree in physics. The work has been carried out under the supervision of associate professor Ulrik Ingerslev Ugger- høj at the Department of Physics and Astronomy. CONTENTS Contents i Abstract & Outline of the Thesis v Acknowledgements ix List of Publications xi List of Acronyms xiii I FormationLengthRelatedBremsstrahlungPhenomena 1 1 Bremsstrahlung Basics 3 1.1 AcceleratingaCharge ............................ 3 1.2 RelevantLengths&Bethe-HeitlerYields . ..... 5 1.3 SuppressingEffects ..... ..... ...... ..... ...... ... 11 2 The NA63 Setup & Methods 15 2.1 Organic Scintillators and PhotoMultiplier Tubes . ....... 16 2.2 TotalAbsorptionCalorimeters. 17 2.3 ElectronicsandDataAcquisition . 21 2.4 BeamConditions ............................... 22 2.5 CalorimeterCalibration . 23 2.6 BackgroundCompensation . 25 3 The TSF Effect 33 3.1 Introduction .................................. 33 3.2 TheTargetAssemblies ............................ 35 3.3 Analysis .................................... 37 3.4 Results ..................................... 40 4 Theoretical Approaches to Bremsstrahlung from Thin Targets 49 4.1 Baier&Katkov ................................ 49 4.2 Shul’ga&Fomin ............................... 52 4.3 Blankenbecler&Drell ............................ 57 i ii CONTENTS 4.4 TheLogarithmicThicknessDependence . 57 5 The Sandwich Effect 63 5.1 TheoreticalFoundations . 63 5.2 TheSandwichTargets ............................ 67 5.3 Analysis&Results .............................. 69 5.4 ImprovingtheSetup ............................. 71 6 TheLow Z-LPM Effect 79 6.1 Motivation................................... 79 6.2 ThePilotStudy ................................ 81 6.3 TheFullStudy................................. 82 II Positronium – Production and Decay 87 7 The ASACUSA Positron Beam Line 89 7.1 TheRGM-1................................... 89 7.2 BehaviourinaPenningTrap. 94 7.3 TheMulti-RingTrap ............................. 97 8 Commissioning of LEPS 101 8.1 TheTools....................................101 8.2 KeyParameters ................................102 9 The Lifetime of Ortho-Positronium 111 9.1 PositroniumCharacteristics . 111 9.2 ExperimentalSetup..............................116 9.3 Analysis ....................................120 9.4 Results&Discussion .............................122 9.5 Improvements.................................125 10 Overall Conclusion 127 Appendices 131 A Green’s Function for the Wave Equations 133 A.1 TheGreen’sFunction.. ..... ...... ..... ...... .....133 A.2 TheWaveEquations .............................134 B Theory of Blankenbecler and Drell 137 B.1 Eikonalapproximation . 137 B.2 Bremsstrahlung ................................138 C Positronium in Intense Laser Fields 141 C.1 MPIandATIinIntenseLaser-Field . 141 C.2 ExperimentalSetup..............................142 CONTENTS iii D Outlook 145 D.1 MagneticBremsstrahlungSuppression. 145 D.2 BremsstrahlungfromLeadIons . 146 E Synopsis 147 Bibliography 149 ABSTRACT & OUTLINE OF THE THESIS During my PhD, I have been engaged in a small number of different fields which is also indicated by my list of publications from my time as a PhD student. I have thus actively participated in the topics of • Measuring ionization cross sections in noble gasses with slow antiprotons as the projectile particle. • Measuring the restricted energy loss of ultrarelativistic electrons in a thin Solid State Detector (SSD) and also of electron/positron pairs with creation vertex close to the SSD. • Investigating bremsstrahlung phenomena from electrons/positrons of ener- gies E0 & 150 GeV traversing amorphous foils or—to some extent—aligned crystals. • Commissioning a state-of-the-art commercial trap for low-energy positrons (. 1 eV) and later performing test experiments with the high-quality positron beam. Common to all topics is the family of interactions studied, all within the frameworks of Quantum electrodynamics (QED). Initially, the emphasis was mainly on the posi- tron trap. However, this equipment suffered firstly from a six month delay relative to the original ETA. Secondly, it arrived somewhat damaged in Aarhus in November 2007, causing a further setback. Because of this, the repairs and commissioning were not finished until late 2008. The emphasis was thus increasingly turned towards the bremsstrahlung phenomena. As the title of my thesis suggests, the work presented here will be from the bremsstrahlung experiments and also my work with the posi- tron trap, corresponding to Part I and II, respectively. Part I – Formation Length Related Bremsstrahlung Phenomena This part of the thesis deals with experimentally unexplored (or poorly explored) regions of the parameter space of the incoherent bremsstrahlung process in amor- phous targets. As R. Blankenbecler states about incoherent bremsstrahlung [Bla97b] . it is surprising that there is so much more to learn about such a well-understood process. Many of these regimes should also be relevant to systems where the interaction is much more complex, e.g. within the frameworks of Quantum chromodynam- ics (QCD). It is a great advantage to first study the phenomena in a QED system, where the interactions are known to extreme accuracy. After all, science is about v vi ABSTRACT & OUTLINE OF THE THESIS building on top of solid foundations. A theoretician, devising a QCD model for the phenomenon, could thus “see farther” by setting off from a model treating the QED system well. To find the latter, benchmark results are necessary. All experiments have been performed in bounds of the North Area 63 (NA63) Collaboration at Or- ganisation Européenne pour la Recherche Nucléaire (CERN). • Chapter 1 contains some of the basic ideas and characteristic lengths necessary to understand the suppression mechanisms of bremsstrahlung in amorphous targets. The characteristic length of the bremsstrahlung process, the formation length, is crucial in this context. • Chapter 2 gives the reader an understanding of the experimental tools and methods applied to measure the attributes of the bremsstrahlung photon spec- trum from different targets. The effects of the applied methods have been ex- amined using toy Monte Carlo simulations unveiling—in some cases—surpris- ingly large systematic effects. • Chapter 3 deals with an experiment where the targets are thin compared to the formation length. In this regime, named after Ternovski˘i-Shul’ga-Fomin (TSF), the bremsstrahlung photon yield is not linear in thickness but logarith- mic! Also, a semi-empirical logarithmic expression is presented. • Chapter 4 gives an introduction to some of the most prominent theoretical models of bremsstrahlung from thin targets. Many of the models have been implemented and compared with our experimental results of Chapter 3. The semi-empirical logarithmic expression is justified by closely resembling Blan- kenbecler’s theory of thin targets. • Chapter 5 describes the experimental search for an effect which has been pro- claimed by many independent theoretical models. The so-called “sandwich effect” should occur, when the formation length stretches between two amor- phous foils, longitudinally separated by a small gap. • Chapter 6 is based on two experiments—a preparation study and a recent full- scale experiment—aiming at a characterization of the Landau-Pomeranchuk- Migdal suppression, relevant in semi-infinite targets (in practice, thick relative to the formation length). This study focuses on target materials of low nuclear charge (Z). When considering electromagnetic air showers generated by cos- mic rays in the atmosphere, the target material consists by nature largely of low-Z materials, and the low-Z Landau-Pomeranchuk-Migdal effect is impor- tant here. Some of the effects treated in Chapter 3–6 have previously been studied by the SLAC E-146 experiment [A+97]. However, as will be discussed, they suffered from a num- ber of limitations, whence our study is—at least in the case of the TSF and sandwich effects—far superior. Unless else is specified, I have procured the results and figures shown in this part through either data analysis, drawing or simulation. vii Part II – Positronium – Production and Decay This part describes my work with an apparatus on the route towards creating anti- hydrogen (H), consisting of a positron (e+) and an antiproton (p). This is a strong candidate for a system allowing a high-precision test of the CPT-theorem [BKR99]. To produce a bound, atom-like system, the constituents’ energies must be lowered to be similar to the binding energy of the system, i.e. usually few eV. As is demon- strated, this is exactly what our apparatus does to energetic