Nuclear reactions induced by photons play a vital role for very different aspects of basic research and applications in physics. They are a key ingredient for the synthesis of nuclei in the Universe ...and provide, due to the selectivity and the model-independence of the reaction mechanism, an extremely valuable probe for researchers. The penetrability of photons in the MeV energy range makes them, in addition, an ideal tool for meeting various societal challenges. The last two decades saw a rapid development of advanced photon sources and detection methods for photonuclear reaction products. Bremsstrahlung and quasi-monoenergetic photon beams with unprecedented intensity and quality combined with state-of-the-art detector technology paved the way for new scientific discoveries and technological applications.
This review focuses on a comprehensive overview of the most important developments since the turn of the millennium restricted to the energy range between atomic and hadronic degrees of freedom. This includes a description of the formalism of photonuclear reactions below and above the particle-separation threshold. The most important techniques used to generate photon beams in the MeV energy range are presented along with selected facilities and instrumentation for diagnostics and for the analysis of photonuclear reactions. The power of photons to probe the atomic nucleus is exemplified in a number of selected examples from fundamental and applied science. New developments, facilities, and ideas promise a vivid future for photonuclear physics.
The gamma strength function and level density of 1^{-} states in ^{96}Mo have been extracted from a high-resolution study of the (pover →, pover →^{'}) reaction at 295 MeV and extreme forward angles. ...By comparison with compound nucleus γ decay experiments, this allows a test of the generalized Brink-Axel hypothesis in the energy region of the pygmy dipole resonance. The Brink-Axel hypothesis is commonly assumed in astrophysical reaction network calculations and states that the gamma strength function in nuclei is independent of the structure of the initial and final state. The present results validate the Brink-Axel hypothesis for ^{96}Mo and provide independent confirmation of the methods used to separate gamma strength function and level density in γ decay experiments.
The double-gamma (γγ)-decay of a quantum system in an excited state is a fundamental second-order process of quantum electrodynamics. In contrast to the well-known single-gamma (γ)-decay, the ...γγ-decay is characterized by the simultaneous emission of two γ quanta, each with a continuous energy spectrum. In nuclear physics, this exotic decay mode has only been observed for transitions between states with spin-parity quantum numbers J(π) = 0(+) (refs 1-3). Single-gamma decays-the main experimental obstacle to observing the γγ-decay-are strictly forbidden for these 0(+) → 0(+) transitions. Here we report the observation of the γγ-decay of an excited nuclear state (J(π) = 11/2(-)) that is directly competing with an allowed γ-decay (to ground state J(π) = 3/2(+)). The branching ratio of the competitive γγ-decay of the 11/2(-) isomer of (137)Ba to the ground state relative to its single γ-decay was determined to be (2.05 ± 0.37) × 10(-6). From the measured angular correlation and the shape of the energy spectra of the individual γ-rays, the contributing combinations of multipolarities of the γ radiation were determined. Transition matrix elements calculated using the quasiparticle-phonon model reproduce our measurements well. The γγ-decay rate gives access to so far unexplored important nuclear structure information, such as the generalized (off-diagonal) nuclear electric polarizabilities and magnetic susceptibilities.
A conceptual design is presented of a novel energy-recovering linac (ERL) facility for the development and application of the energy recovery technique to linear electron accelerators in the ...multi-turn, large current and large energy regime. The main characteristics of the powerful energy recovery linac experiment facility (PERLE) are derived from the design of the Large Hadron electron Collider, an electron beam upgrade under study for the LHC, for which it would be the key demonstrator. PERLE is thus projected as a facility to investigate efficient, high current (HC) (>10 mA) ERL operation with three re-circulation passages through newly designed SCRF cavities, at 801.58 MHz frequency, and following deceleration over another three re-circulations. In its fully equipped configuration, PERLE provides an electron beam of approximately 1 GeV energy. A physics programme possibly associated with PERLE is sketched, consisting of high precision elastic electron-proton scattering experiments, as well as photo-nuclear reactions of unprecedented intensities with up to 30 MeV photon beam energy as may be obtained using Fabry-Perot cavities. The facility has further applications as a general technology test bed that can investigate and validate novel superconducting magnets (beam induced quench tests) and superconducting RF structures (structure tests with HC beams, beam loading and transients). Besides a chapter on operation aspects, the report contains detailed considerations on the choices for the SCRF structure, optics and lattice design, solutions for arc magnets, source and injector and on further essential components. A suitable configuration derived from the here presented design concept may next be moved forward to a technical design and possibly be built by an international collaboration which is being established.
Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay ...properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink–Axel hypothesis and the related concept of so-called photon strength functions to describe γ-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasi-monochromatic photon beams and a newly implemented γ–γ coincidence setup. This technique does not assume a priori the validity of the Brink–Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus 128Te were obtained for γ-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink–Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink–Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold (Sn=8.78MeV) for the studied case of 128Te.
Considerable progress has been achieved in the experimental investigation and theoretical understanding of isovector valence shell structures in heavy nuclei that are frequently called mixed-symmetry ...states. This article attempts a review of the experimental aspects and data on mixed-symmetry states of vibrational and weakly deformed transitional nuclei. After a brief introduction the main experimental tools are described that have lately been used for the investigation of mixed-symmetry states in vibrational and transitional nuclei. The main body reviews the available data where the mixed-symmetry assignments have been made on the basis of large magnetic dipole transition rates.
Gamow-Teller (GT) transitions in atomic nuclei are sensitive to both nuclear shell structure and effective residual interactions. The nuclear GT excitations were studied for the mass number A = 42, ...46, 50, and 54 "f-shell" nuclei in ((3)He, t) charge-exchange reactions. In the (42)Ca → (42)Sc reaction, most of the GT strength is concentrated in the lowest excited state at 0.6 MeV, suggesting the existence of a low-energy GT phonon excitation. As A increases, a high-energy GT phonon excitation develops in the 6-11 MeV region. In the (54)Fe → (54)Co reaction, the high-energy GT phonon excitation mainly carries the GT strength. The existence of these two GT phonon excitations are attributed to the 2 fermionic degrees of freedom in nuclei.
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On the brink of the phase transition from spherical to deformed Zr isotopes,
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Zr shows a co-existence of low-lying level structures with different degrees of quadrupole collectivity. Experimental ...data are available, but a coherent description of the measured properties is yet missing. This paper analyses available data and critically compiles the information relevant for the interpretation of the quadrupole-collective structure of
96
Zr .
The recent observation of the competitive double-γ decay process is presented. It is a second-order electromagnetic decay mode. The 662-keV decay transition from the 11/2− isomer of 137Ba to its ...ground state has been studied. The emission of a single 662-keV γ quantum is a factor of 5 × 105 more likely than the simultaneous emission of two γ quanta instead of one. The observed angular correlation and energy distribution of these rare coincident double-γ quanta are well described by a dominant M2 - E2 and a sub-dominant E3 - M1 contribution to the double-γ decay branch.
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