The Advanced GAmma Tracking Array (AGATA) is a European project to develop and operate the next generation γ-ray spectrometer. AGATA is based on the technique of γ-ray energy tracking in electrically segmented high-purity germanium crystals. This technique requires the accurate determination of the energy, time and position of every interaction as a γ ray deposits its energy within the detector volume. Reconstruction of the full interaction path results in a detector with very high efficiency and excellent spectral response. The realisation of γ-ray tracking and AGATA is a result of many technical advances. These include the development of encapsulated highly segmented germanium detectors assembled in a triple cluster detector cryostat, an electronics system with fast digital sampling and a data acquisition system to process the data at a high rate. The full characterisation of the crystals was measured and compared with detector-response simulations. This enabled pulse-shape analysis algorithms, to extract energy, time and position, to be employed. In addition, tracking algorithms for event reconstruction were developed. The first phase of AGATA is now complete and operational in its first physics campaign. In the future AGATA will be moved between laboratories in Europe and operated in a series of campaigns to take advantage of the different beams and facilities available to maximise its science output. The paper reviews all the achievements made in the AGATA project including all the necessary infrastructure to operate and support the spectrometer

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Experimental results for the bremsstrahlung energy loss of 149, 207, and 287 GeV electrons in thin Ir, Ta, and Cu targets are presented. For each target and energy, a comparison between simulated values based on the Landau-Pomeranchuk-Migdal ͑LPM͒ suppression of incoherent bremsstrahlung is shown. For the electron energies investigated, the LPM effect enters the quantum regime where the recoil imposed on the electron by the emitted photon becomes important. Good agreement between simulations based on Migdal's theory and data from the experiment is found, indicating that the LPM suppression is well understood also in the quantum regime. Results from a comparison between simulations with the ''threshold'' energy E LPM as a free parameter and the data are shown. This analysis reproduces the expected trend as a function of nominal radiation length, but yields values that tend to be low compared to Migdal's theory.

Although some authors have claimed that the effect is not detectable, we show experimentally for the first time that as the quantum parameter x grows beyond 1, an increasingly large part of the hard radiation emitted arises from the spin of the electron. Results for the energy loss of electrons in the energy range 35-243 GeV incident on a W single crystal are presented. Close to the axial direction the strong electromagnetic fields induce a radiative energy loss which is significantly enhanced compared to incidence on an amorphous target. In such continuously strong fields, the radiation process is highly nonperturbative for ultrarelativistic particles and a full quantum description is needed. The remarkable effect of spin flips and the energy loss is connected to the presence of a field comparable in magnitude to the Schwinger critical field, E 0 m 2 c 3 ͞eh, in the rest frame of the emitting electron. DOI: 10.1103/PhysRevLett.87.054801 PACS numbers: 41.60. -m, 12.20. -m, 41.75.Ht, 78.70. -g Under small angles of incidence to a crystal, the strong electric fields of the nuclear constituents add coherently such as to obtain a macroscopic, continuous field of the order E Ӎ 10 11 V͞cm. This is evidenced by, e.g., the channeling phenomenon [1] or the so-called "doughnut scattering" [2]. Therefore, in the rest frame of an ultrarelativistic electron with a Lorentz factor of g Ӎ 10 5 , the field encountered becomes comparable to the critical (or Schwinger-) field, E 0 m 2 c 3 ͞eh 1.32 3 10 16 V͞cm, corresponding to a magnetic field B 0 4.41 3 10 9 T. Here, m is the rest mass of the electron, c is the speed of light, e is the elementary charge, andh is Planck's constant divided by 2p. The incident particle moves in these immensely strong fields over distances up to that of the crystal thickness, i.e., up to several mm. Thereby the behavior of charged particles in strong fields as E 0 can be investigated.Strong field effects can be investigated by other means. One example is in heavy ion collisions where the field becomes comparable to the Schwinger field, but the collision is of extremely short duration. Another -technically demanding -example is in multi-GeV electron collisions with terawatt laser pulses where nonlinear Compton scattering and so-called "Breit-Wheeler" pair production are observed [3]. In nature, near-critical fields are believed to be present in the vicinity of pulsars.However, as we point out below, in order to investigate the effect of the spin on the radiation spectrum, the electron must interact with the strong field over large distances. So crystals present unique tools for the investigation of the influence of spin on the radiation spectrum.Already in the late 1960s, Baier and Katkov [4] calculated the photon spectrum emitted by "particles of arbitrary spin moving in an arbitrary electromagnetic field." However, the realization that the spin influences the spectrum significantly in this context lay dormant for many years and was not discussed as an observable phenomenon although radiation in...

Experimental results for the radiative energy loss of 149, 207, and 287 GeV electrons in a thin Ir target are presented. From the data we conclude that at high energies the radiation length increases in accordance with the Landau-Pomeranchuk-Migdal (LPM) theory and thus electrons become more penetrating the higher the energy. The increase of the radiation length as a result of the LPM effect has a significant impact on the behavior of high-energy electromagnetic showers. DOI: 10.1103/PhysRevLett.91.014801 PACS numbers: 41.60.-m, 07.85.Fv, 29.40.Vj, 95.30.Gv In the early 1950s, Ter-Mikaelian postulated the existence of the so-called formation length [1] for the emission of radiation. This is, loosely speaking, the length it takes to separate the photon from the electron by one wavelength such that the photon can be considered ''formed.'' If for some reason the electron is influenced during this formation, the yield of photons may increase as when electrons traverse crystals [2] or it may be reduced due to destructive interference. The formation length is the basic parameter of the LandauPomeranchuk-Migdal (LPM) effect [3,4], which predicts a reduction of photon yield due to multiple scattering in the formation zone.A beautiful series of detailed experiments was performed at Stanford Linear Accelerator Center (SLAC) to examine the LPM effect by use of electrons of 8 and 25 GeV [5]. However, only the lower 500 MeV of the photon spectrum was recorded, and, thus, no conclusion could be drawn with respect to a possible variation of the radiation length X 0 based on the experimental data.The present investigation gives experimental evidence for the energy dependence of X 0 . The yield of bremsstrahlung photons is compared to theoretical calculations over essentially the complete energy range.The LPM effect -besides being interesting in itself as a basic QED process -is relevant in several connections. In the first place, it may have a significant impact on the behavior of air showers in the neighborhood of the Greisen-Zatsepin-Kuz'min cutoff of high-energy photons [6,7]. Second, the LPM effect in QED processes may have a parallel in suppression of gluons in QCD processes [8,9]. Finally, an electromagnetic shower initiated by an electron may develop over a characteristic length that is increased substantially compared to the nominal X 0 as well as having a different composition. As an example, in lead-tungstate crystals, the shower develops corresponding to a radiation length that is longer than the nominal X 0 by as much as 2.5%, 10%, or 26% for energies 0.2, 1, or 4 TeV, respectively.Since the SLAC experiments, the theory of the LPM effect has evolved substantially: several groups have calculated LPM suppression using different approaches, among these Baier and Katkov [10], Blankenbecler and Drell [11], Zakharov [12] and Shul'ga and Fomin [13].