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The internationally renowned Stuttgart 4 MV DYNAMITRON facility was finally shut down in April 2005 due to a decision of the Physics Faculty of the Stuttgart University. This definitely means the end of an era, of more than two decades of top research in the fields of low-energy photo-induced reactions and nuclear astrophysics. Numerous outstanding experiments, mostly performed in international collaborations with colleagues from all over the world, established the reputation of this small university accelerator facility in the nuclear physics community as the address for low-energy photon scattering and astrophysics investigations. In the future, new optical materials and technologies will be the main research topics of the former Institut fur Strahlenphysik.

After a short introduction to the nuclear resonance fluorescence technique (NRF) and a brief description of typical photon scattering facilities, the present topical review focuses on three phenomena observed in recent NRF experiments on nuclei near the shell closures. These topics of current interest in low-energy nuclear structure physics are: the systematics of 1− quadrupole–octupole-coupled two-phonon states (2+ ⊗ 3−), the study of two-phonon mixed-symmetry states and the exploration of the E1 pygmy resonance near the particle emission threshold.

Experimental indications for the nuclear phase transitions described by the new symmetries E(5) and X(5) are discussed for the stable Ba, Nd, and Gd isotopes. Their low-lying dipole excitations, systematically investigated at the bremsstrahlung facility of the 4.3-MV Stuttgart Dynamitron accelerator, provide indications for the phase transitional behavior, including the proposed critical point candidates.

Following the successful identification of mixed-symmetric one- and two-phonon states in the N=52 nuclei 94Mo and 96Ru, we have performed a photon scattering experiment on the N=52 isotone 92Zr. Experimental data and shell model calculations show that both, single particle and collective degrees of freedom are present in the low-lying levels of 92Zr. The second excited quadrupole state shows the signatures of the one-phonon mixed-symmetric 2+ state, while calculations and data indicate an almost pure neutron configuration for the 2+1 state, in contradiction with the F-spin symmetric limit. Furthermore, two strong dipole excitations, which are candidates for the two-phonon quadrupole–octupole coupled E1 excitation and for the mixed-symmetric 1+ two-phonon state, were observed.

Quite a number of proposals for a gamma-ray laser have been made over the years. One first step on the way to a gamma-ray laser is an inversion between nuclear states. For the natural isotope 103Rh we have the favorable condition that there are two low-lying nuclear levels at energies of 357 keV and 295 keV with lifetimes of 107 ps and 9.7 ps, respectively. With two Nuclear Resonance Fluorescence (NRF) measurements the population of these low-lying levels via feeding from higher-lying levels was investigated. Altogether 26 higher-lying nuclear levels that show a branching to one or even both interesting low-lying levels have been found. Summing over all contributions from these feeding levels this results in a population inversion between the levels at 357 keV and 295 keV of 103Rh.

Low-lying dipole excitations in the medium-weight vibrational nuclei of the Cd isotopic chain were investigated by means of nuclear resonance fluorescence experiments performed at the bremsstrahlung beam of the Stuttgart Dynamitron accelerator (endpoint energy 4.1 MeV). Detailed information has been obtained on excitation energies, spins, decay widths, and transition probabilities of numerous excited states in 110–114,116Cd. Additionally, the use of two Compton polarimeters enabled model-independent parity assignments for excitations in the even-even isotopes. Strongly excited J π = 1− states are found in all even-even Cd nuclei at excitation energies near the sumof the energies of the first 2+ and 3− states. These excitations are interpreted as the 1− member of the quadrupole-octupole coupled quintuplet (2+⊗3−). The fragmented strength observed in the odd isotopes 111,113Cd is compared with the strength distributions in the neighboring even-even Cd isotopes.

The confrontation of the systematized transition rates B(E1,1−1→0+1) and B(E1,1−1→2+1) in even–even nuclei around closed shells, measured in systematic photon scattering experiments, reveals some salient features. These attributes emerge as the first evidence based on the systematic behaviour of experimental data for the dipole core polarization and are supported by earlier QPM calculations.

At the Stuttgart 4: 3M V DYNAMITRON accelerator a newbremsstrahlung irradiation facility w as installed for photoactivation experiments to complement the well-established arrangement for systematic photon-scattering studies taking advantage of the high photon fluxes achievable at this high-current accelerator. The population or depopulation of isomeric states in low-energy photon-induced reactions ðEg ¼ 0:5–4:0 MeV) can be studied with sensitivities far beyond previous limits. The setup of this facility, its performance, and the absolute calibrations are described in detail. First exemplary results for a test case, the reaction 115 In ðg;g 0 Þ 115m In, are presented and analyzed in combination with data from low-energy photon-scattering experiments ( 115 In ðg;g 0 Þ 115 In). The quality of the results demonstrates the

The availability of high-flux, continuous-energy DC bremsstrahlung photon beams and the progress in modern X-ray and γ-spectroscopy have led to a real revival of both the photon scattering (Nuclear Resonance Fluorescence (NRF)) and the photoactivation techniques and enabled experiments with sensitivities far beyond previous limits. A new bremsstrahlung irradiation facility for photoactivation experiments was installed at the Stuttgart 4.3 MV Dynamitron accelerator which complements the well-established photon scattering facility. Exemplary results for a test reaction, the photoactivation of the most investigated isomer 115mIn, in combination with data from low-energy photon-scattering experiments (115In(γ,γ′)115In), demonstrate the capability of this new installation. As a first application the depopulation of the famous 180mTa-isomer by photoactivation was studied in detail using the world's stock of enriched 180Ta material. A joint Darmstadt-Karlsruhe-Munchen-Stuttgart-Youngstown - collaboration succeeded in observing the depopulation of the isomer down to excitation energies of ≈ 1 MeV. The astrophysical implications of these results, the consequences for the puzzling nucleosynthesis of nature's rarest naturally occurring isotope, 180Ta, are discussed. Future perspectives of the photoactivation technique and their application in nuclear astrophysics are outlined.

The nucleus ${}^{94}\mathrm{Mo}$ was investigated using a powerful combination of $\ensuremath{\gamma}$-singles photon scattering experiments and $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence studies following the $\ensuremath{\beta}$ decay of ${}^{94}{\mathrm{Tc}}^{m}$. The data survey short-lived ${J}^{\ensuremath{\pi}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1}^{+}{,2}^{+}$ states and include branching ratios, $E2/M1$ mixing ratios, lifetimes, and transition strengths. The proton-neutron mixed-symmetry (MS) ${1}^{+}$ scissors mode and the ${2}^{+}$ MS state are identified from $M1$ strengths. A $\ensuremath{\gamma}$ transition between MS states was observed and its rate was measured. Nine $M1$ and $E2$ strengths involving MS states agree with the O(6) limit of the interacting boson model-2 using the proton boson $E2$ charge as the only free parameter.

Photon scattering experiments have been performed on the nucleus ${}^{136}$Ba with photon energies of ${E}_{\ensuremath{\gamma}}l~4.1$ MeV and ${E}_{\ensuremath{\gamma}}l~2.8$ MeV. At 2.1 MeV clear evidence for the ${2}_{\mathrm{ms}}^{+}$ state has been found. From the measured lifetime we extract signatures for the isovector quadrupole excitation in the valence shell: a weakly collective $E2$ decay to the ground state and a strong $M1$ decay to the ${2}_{1}^{+}$ state. As the resonant photon scattering with bremsstrahlung is a complete reaction, we can conclude that the ${2}^{+}$ state at 2129 keV is a rather pure ${2}_{\mathrm{ms}}^{+}$ state with little fragmentation. This is in contrast to the ${1}^{+}$ scissors state. A comparison with an IBM-2 calculation is given.

The strength distribution of low-lying dipole excitations in the heavy odd-mass nucleus ${}^{181}\mathrm{Ta}$ was studied in nuclear resonance fluorescence experiments performed at the bremsstrahlung beam of the Stuttgart 4.3 MV Dynamitron accelerator. To increase the detection sensitivity in the whole range of excitation energies between 1.8 and 4 MeV two measurements were carried out at different bremsstrahlung end-point energies of 2.7 and 4.1 MeV using two large-volume HPGe detectors of a relative efficiency of 100%. Detailed information on excitation energies, decay widths, transition probabilities, and branching ratios of 37 new low-lying states in the energy range 1.8--3.5 MeV have been obtained. The observed dipole strength is rather fragmented, apart from a strong excitation at 2.297 MeV. The total strength in the investigated range of excitation energies (1.8--4 MeV) is reduced by a factor of $\ensuremath{\approx}3.5$ as compared to the neighboring even-even nucleus ${}^{180}\mathrm{Hf}$.

From the data on dipole excitations obtained in photon scattering experiments we extract and tabulate the mean values of the excitation energy of the ${1}^{+}$ scissors mode in nuclei of the mass region $130lAl200.$ The centers of gravity of the observed $M1$ strength distributions are always close to 3 MeV including several moderately deformed transitional nuclei. In particular, the data exhibit a weak dependence of the scissors mode energy on the deformation parameter. Including roughly the deformation dependence of pairing effects we could modify an earlier estimate based on a schematic random phase approximation and get a simple formula which yields qualitative agreement with the data.

Nuclear resonance fluorescence excited with continuous electron bremsstrahlung from the 4.3 MV Stuttgart Dynamitron accelerator is used as a nondestructive method to determine the 13C content x of bulk isotopic diamonds (12C1−x13Cx). The smallest detectable amount of 13C in carbon or low Z matrices is on the order of 0.5 mg. The relative accuracy of absolute mass determinations is about ±7%. Errors are mainly due to uncertainties in the natural widths Γ of the 13C nuclear levels at 3089 and 3684 keV used in the measurements. The results confirm a previous calibration which is based on Raman scattering and the destructive determination of x by mass spectroscopy.

A recently proposed fluctuation analysis of resonant photon scattering spectra [J. Enders et al., Phys. Rev. Lett. 79, 2010 (1997)] is applied to the ${}^{157}\mathrm{Gd}(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction. Evidence for unresolved strength in the experimental background is found which amounts to a total dipole excitation strength of $\ensuremath{\sum}g{\ensuremath{\Gamma}}_{0}^{\mathrm{red}}{=(30}_{\ensuremath{-}10}^{+6}){\mathrm{m}\mathrm{e}\mathrm{V}/\mathrm{M}\mathrm{e}\mathrm{V}}^{3}$ in the excitation energy range 2.5--4 MeV. Assuming a smooth variation of the $M1/E1$ strength ratio in the neighboring even-mass nuclei, the $M1$ part can be estimated to be ${B(M1)=(2.0}_{\ensuremath{-}0.7}^{+0.4})$ ${\ensuremath{\mu}}_{N}^{2}.$ This value is close to the expectation of sum rules. We discuss possible sources of errors which suggests that the obtained value represents a lower limit only.

A nuclear resonance fluorescence (NRF) study was performed on the heavy deformed nucleus ${}^{175}$Lu. We observe the beginning of a rotational band built on the coupling of the unpaired proton to the ${K}^{\ensuremath{\pi}}{=0}^{\ensuremath{-}}$ octupole vibration in the neighboring nucleus ${}^{174}$Yb. The dipole strength distribution is also discussed in terms of the scissors mode.

The fragmentation of low-lying dipole strength in the odd-mass nucleus {sup 133}Cs has been investigated in nuclear resonance fluorescence (NRF) experiments performed at the bremsstrahlung beam of the Stuttgart Dynamitron accelerator at an end-point energy of 4.1 MeV. In the excitation energy range 2.3 {endash} 3.7 MeV in total 22 new dipole excitations were observed. From the high-resolution {gamma}-ray spectra measured by three high-efficiency Ge detectors the reduced excitation probabilities B(E1){up_arrow} or B(M1){up_arrow} were deduced. The fragmentation and absolute total strengths of the detected dipole excitations are compared with results for the neighboring even-even, {gamma}-soft nucleus {sup 134}Ba, where both, rather strong {ital scissors mode-like} M1 and two-phonon E1 excitations are known from recent NRF experiments. {copyright} {ital 1997} {ital The American Physical Society}

Low-lying electric and magnetic dipole excitations in the transitional nucleus {sup 144}Nd have been studied in nuclear resonance fluorescence experiments performed with a bremsstrahlung beam (end point energy 4.1 MeV). The use of high-resolution {gamma}-ray spectrometers and a sectored single-crystal Compton polarimeter provided detailed information on excitation energies, spins, parities, decay widths, transition probabilities, and branching ratios of numerous new spin-1 states in {sup 144}Nd. The strong E1 excitation at 2185 keV in this N = 84 nucleus is interpreted as the quadrupole-octupole coupled two-phonon excitation which has been observed systematically in the neighboring N = 82 isotones. The decay properties of this 1{sup {minus} } state are compared with the systematics of the low-lying 1{sup {minus}} levels in the other even-even, stable Nd isotopes. The M1 excitations in {sup 144}Nd are discussed with respect to the deformation dependence of the orbital M1 scissors mode, the so-called {delta}{sup 2} law, which has been studied previously in the other stable even-even Nd nuclei and in the Sm isotopes. {copyright} {ital 1997} {ital The American Physical Society}

A photon scattering experiment was performed on the $\ensuremath{\gamma}$-soft nucleus $^{134}\mathrm{Ba}$ with photon energies of ${E}_{\ensuremath{\gamma}}l4.1$. Around 3 MeV excitation energy a fragmented ${1}^{+}$ scissors mode as well as the ${({2}^{+}\ensuremath{\bigotimes}{3}^{\ensuremath{-}})}_{{1}^{\ensuremath{-}}}$ two-phonon dipole state were identified. The dipole excitation strengths were measured. The excitation strength of the scissors mode in this $\ensuremath{\gamma}$-soft nucleus is well reproduced by an empirical $M1$ sum rule that generalizes the "${\ensuremath{\delta}}^{2}$ law" to nuclei without axial symmetry. The IBM-2 describes the mixed-symmetry states in $^{134}\mathrm{Ba}$ and their decay pattern. A large $\frac{E2}{M1}$ mixing ratio for the ${1}^{+}$\ensuremath{\rightarrow}${2}_{1}^{+}$ decay of the scissors mode to the ${2}_{1}^{+}$ state is predicted by the IBM-2. The new result from $^{134}\mathrm{Ba}$ together with a similar previous observation in $^{196}\mathrm{Pt}$ corroborates the scissors mode as a typical excitation mode of $\ensuremath{\gamma}$-soft nuclei.

Photon scattering experiments on the deformed even-even nuclei $^{164,166,168,170}\mathrm{Er}$ provided detailed information on the excitation energies, spins, transition probabilities, and K quantum numbers of low-lying dipole excitations. In the case of $^{168}\mathrm{Er}$, in addition, parities of the excited states were determined model independently by measuring the linear polarization of the scattered photons using a sectored Ge-Compton polarimeter with a BGO--anti-Compton shield. The results for M1 excitations are compared with the systematics of low-lying orbital M1 strength (``scissors mode'') in the neighboring rare-earth nuclei. The observed enhanced E1 transitions are discussed in terms of collective excitations (K=0 octupole bandheads or candidates for ${2}^{+}$\ensuremath{\bigotimes}${3}^{\mathrm{\ensuremath{-}}}$ two-phonon excitations) and in the framework of recent microscopic quasiparticle phonon model calculations. \textcopyright{} 1996 The American Physical Society.

We report on the investigation of low lying dipole excitations in heavy nuclei using real photons as an experimental probe. The latest results on the M1 Scissors Mode and on two phonon E1 excitations are summarized.

After and introduction to the subject, the formalism describing nuclear resonance scattering (NRF) is briefly summarized. Experimental techniques and developments are discussed in some detail. Recent results are presented from systematic NRF experiments studying low-lying dipole excitations in heavy nuclei. The following topics are covered: the systematics and fragmentation of the M1 Scissors Mode in even-even rare earth and actinide nuclei, its deformation dependence and saturation behaviour, the existence of the Scissors Mode in odd deformed nuclei, 2 + ⊗ 3 − two-phonon E1 excitations in N =82 isotones and Z =50 isotopes, 2 + ⊗ 3 − ⊗ particle multiplets in odd nuclei around N =82, low-lying ΔK =0 electric dipole excitations in deformed nuclei, and E1 excitations around 2.6 MeV in deformed nuclei, which are interpreted as candidates for novel two-phonon excitations (coupling of the K =1 octupole and K =2 γ-vibrations).

Photon scattering experiments on the semi magic nuclei 138Ba and 140Ce have been performed at the bremsstrahlung facility of the Stuttgart Dynamitron. Two-phonon 1− states were excited at 4.026 and 3.643 MeV excitation energy respectively as well as a number of 2+ states between 2 and 4 MeV. Lifetimes and upper limits for the decay to other low-lying states were extracted from the data and are compared to lifetimes given in the literature. The systematics of B(E1) values and energies of the 1− two-phonon states is analysed using additional data from previously published (γ, γ′) experiments on the neighbouring isotones 142Nd and 144Sm.

A photon scattering experiment on [sup 143]Nd yielded detailed information about the energies, cross sections, and absolute strengths of dipole transitions between 1.4 and 4.1 MeV. Besides some well-known levels at energies around 2 MeV we observed a number of very strong dipole excitations between 2.8 and 3.8 MeV. This is the energy region where one expects the two-phonon-(2[sup +][direct product]3[sup [minus]])[direct product] particle multiplet which arises due to the coupling of the unpaired neutron with (2[sup +][direct product]3[sup [minus]]) coupled states in the even-even core nucleus [sup 142]Nd. The summed dipole strength between 2.8 and 3.6 MeV in [sup 143]Nd, according to the sum rule, is comparable to the [ital E]1 strength of the 1[sup [minus]] member of the multiplet in the core nucleus [sup 142]Nd. Model calculations in a harmonic approximation with a quadrupole-quadrupole interaction show very good agreement with the experiment and indicate the dominant two-phonon-particle structure (2[sup +][direct product]3[sup [minus]][direct product][ital f][sub 7/2]) of the excitations in [sup 143]Nd.

The nuclei 70,72,74,76 Ge were studied by nuclear resonance fluorescence (NRF) experiments. Partially linearly polarized and unpolarized bremsstrahlung of 9 to 14 MeV endpoint energy was used at the Giessen 65 MeV electron linear accelerator; unpolarized bremsstrahlung of 4 MeV endpoint energy was used at the Stuttgart Dynamitron. The scattered photons were detected by Ge γ-ray spectrometers with high energy resolution. Multipolarities were determined by measuring the angular correlations between the beam and the scattered γ rays at different scattering angles. Precise excitation energies and ground-state decay widths of numerous (> 120) previously unknown spin-1 states were extracted. For 65 ground-state transitions (20 M1 transitions, 45 E1 transitions) parities were assigned, in a model-independent way, by polarized bremsstrahlung. A detailed distribution of electric and magnetic dipole strength in the even Ge isotopes was established.

A nuclear self-absorption measurement was carried out on the nucleus [sup 27]Al with a bremsstrahlung photon-beam endpoint energy of 4.2 MeV. For a number of states, commonly used as calibration standards in photon-scattering (NRF) experiments, the self absorption was measured. Thereby the level widths and lifetimes were determined absolutely. In addition it was found that the level energy of the 2212.01-keV level deviates from the literature value.

Systematic nuclear resonance fluorescence (NRF) experiments have been performed at the bremsstrahlung facility of the 4 MV Stuttgart Dynamitron to investigate the distributions of magnetic and electric dipole excitations in heavy nuclei. Precise excitation energies, transition strengths, spins and decay branching ratios were deduced for numerous low lying dipole excitations in heavy spherical and deformed nuclei. Measurements of the linear polarization of resonantly scattered photons using a Compton polarimeter enabled model independent parity assignments. Recent results are presented concerning: the systematics and fragmentation of the M1 “ Scissors Mode ” in even-even Rare Earth nuclei, the existence of the “ Scissors Mode ” in odd deformed nuclei, 2 + ⊗ 3 − two-phonon E1 excitations in N=82 isotones and Z=50 isotopes, 2 + ⊗ 3 − ⊗ particle multiplets in odd nuclei around N=82, low lying ΔK =0 electric dipole excitations in deformed nuclei, and E1 excitations around 2.6 MeV in deformed nuclei, which are interpreted as candidates for novel two-phonon excitations (coupling of the K =1 octupole and K =2, γ-vibrations).

Resonance fluorescence represents a standard tool to investigate composite systems like atoms or nuclei. Such systems exhibit excited states. Their excitation energies, lifetimes (or natural line widths) and angular momenta provide fundamental information on the underlying forces between the constituents of the object. Resonance fluorescence refers to the resonant excitation of an excited state by absorption of electromagnetic radiation and the subsequent decay of this level by emission of radiation. The principle advantage of this method is that both the excitation and deexcitation processes take place via the best understood interaction in physics, the electromagnetic interaction. Resonance fluoresence was very successfully used first in optics and atomic physics (see e.g. the classical textbooks by Born [33Bo0A], or Mitchell and Zemansky [34Mi0A]). Applied to the nuclear system Nuclear Resonance Fluorescence (NRF) represents the resonant absorption of a real photon exciting a nuclear level and its subsequent decay by reemission of a photon. This photon scattering off bound states has proven to be an outstanding tool to investigate low-lying dipole excitations in nuclei and to provide detailed spectroscopic information. The progress in modern γ-spectroscopy, the availability of Ge-γ-spectrometers of excellent energy resolution and high efficiency, together with the advent of high flux photon beams have led to a real renaissance of the classical NRF technique during the last three decades. The low transfer of momentum of real photons gives rise to a high selectivity in exciting low spin states (dipole transitions). The sensitivity and the spin selectivity are essential for detailed studies of the fragmentation of the strengths of specific collective dipole modes even at excitation energies where the total level density is already high. Furthermore, the use of continuous bremsstrahlung radiation enables all states with ΔJ=0,±1,±2, and with sufficient ground state γ decay widths to be excited simultaneously. The following quantities can be extracted in a completely model independent way

High-resolution nuclear resonance flourescence (NRF) experiments have been performed on the nuclei 113,114 Cd using bremsstrahlung and Ge-γ-spectrometers. Precise excitation energies, decay widths and decay branching ratios for numerous new levels (dipole excitations) in the energy range 2–4 MeV have been extracted from the measured energy spectra and angular distributions of the scattered photons. The NRF results are compared with IBA calculations and spectroscopic data from particle-induced reactions.

A summary of new results for the systematics of dipole excitations in the rare earth region is given. It is shown that the absolute strengths of the the M1 Scissors Mode is well reproduced by sum rule approach. In addition strong electric dipole transition to low lying 1 − states have been observed in spherical as well as in deformed nuclei. They can be described by an octupole excitation mechanism.

The total photofission cross section σγ,F for235U and238U has been measured in the energy range 50≤E γ≤800 MeV at the 855 MeV Mainz Microtron MAMI using energy and time tagged photons (Glasgow Tagger) and a 4π arrangement of position sensitive fragment detectors. Besides the absolute photofission cross section σγF , which almost completely exhausts the total photon absorption cross section for these nuclei, fragment mass distributions in this energy domain were determined via time of flight techniques (TOF). The results for the total photofission cross sections σγ,F normalized to the atomic numberA for both isotopes coincide, and agree in theΔ-resonance region, within the systematic errors, with the socalled“Universal Curve” σγ,T /A of the total photon absorption cross section σγ,T . At higher energies the cross sections exhibit a smooth behaviour. In particular, it is shown for the first time that there isno resonance-like shape near the D13 resonance (at ≈710 MeV) as observed for the free proton. This complete suppression of the D13 resonance in complex nuclei is not yet understood on a microscopic level. The fragment mass distributions show a predominantly mass symmetric fission. However, contributions from mass asymmetric fission at some photon energies may give a hint of an increased mass asymmetric fission after the onset of the pion and two pion channels.

In nuclear resonance fluorescence experiments with unpolarized and linearly polarized bremsstrahlung we have observed for the first time very strong electric dipole transitions at about 3.5 MeV in the spherical, semi magic 116 Sn and 124 Sn nuclei. These transitions can be attributed to possible two phonon 2 + ⊗ 3 − excitations. The measured transition strengths are compared to the results of a QRPA calculation.

Measurements of the linear polarization of resonantly scattered photons have been used for model-independent parity determinations in nuclear resonance fluorescence experiments. The experiments have been performed at the bremsstrahlung facility of the Stuttgart Dynamitron accelerator. The use of two different Compton polarimeters, a five Ge-detector arrangement and a sectored single-crystal Ge-polarimeter enabled the parity assignments. Positive parities were established for a cluster of states near 3.3 MeV (M1 transitions) to be ascribed to the so-called scissors mode. In addition an enhanced electric dipole excitation near 2.5 MeV (2471 keV, B (E1) ↑ = 3.0 ± 0.4 × 10 −3 e 2 fm 2 ) has been observed as in the neighbouring isotopes 162 Dy and 150 Nd. These 1 − states systematically exhibit an uncommon decay branching hinting at K -mixing.

A real renaissance of the classical nuclear resonance fluorescence technique (NRF) took place during the last decade. The discovery of enhanced orbital M1 excitations in deformed nuclei by A. Richter (1) in 1984 stimulated a large number of both theoretical and experimental studies (see reviews (2,3)). The NRF technique, applied at high intensity CW bremsstrahlung beams, provides unique information on low lying magnetic and electric dipole excitations in heavy nuclei. Precise excitation energies, transition widths, spins, K-numbers and parities of excited states in heavy nuclei can be determined (43). Low energy photon scattering experiments supplement in an ideal way electron scattering experiments due to the achievable high energy resolution, high sensitivity and a model independent analysis (6). The photon scattering facility at the 4 MV Stuttgart Dynamitron and the applied experimental techniques are described, and novel results are presented in this report.

Parities of nuclear levels excited in photon scattering experiments can be determined by measurements of the linear polarization of the scattered photons. The needed theoretical formalism is summarized. A fourfold sectored single crystal Ge-Compton polarimeter, its operation and performances are described. The compact polarimeter has an outstanding coincidence efficiency ecoinc of about 25% and a good energy resolution of 2.2 keV at 1.333 MeV. The polarization sensitivity of the device as determined in (γ, γ ) polarization correlation measurements and several ( p , p ′ γ ) reaction studies amounts to about 20% at photon energies of 0.5 MeV and is about 10% at 4 MeV photon energies. The polarimeter has been successfully operated in systematic photon scattering experiments at the bremsstrahlung beam at the Stuttgart Dynamitron facility to investigate enhanced magnetic and electric dipole excitations in heavy deformed nuclei. Typical results are shown and discussed.

The widths of the 3089 keV and the 3684 keV levels in [sup 13]C are precisely measured using nuclear self-absorption and employing amorphous [sup 13]C absorbers and scatterers. The role of the effective temperature of carbon is discussed in some detail. The result for the 3089 keV level was found to be larger by about 25% than the compiled value.

Near-barrier fission of 232 Th and 236 U induced by linearly polarized photons has been investigated. The experiments have been carried out at the “off-axis” bremsstrahlung facility of the Giessen 65 MeV electron linac. Fragment angular, mass and energy distributions have been measured simultaneously allowing the investigation of correlations between these fragment characteristics. A consistent assignment of the quantum numbers J π and K for the fussion channels involved in the fission process is proposed. For the first time, the polar anisotropies and azimuthal asymmetries of the fission fragment angular distributions W ( θ , φ ) have been investigated as a function of the fragment masses. The results are discussed in the framework of the double-humped fission barrier concept and the so-called “multi-exit fission channel” model. Additionally, angular distributions of heavy and light fission fragments from photofission of 236 U have been analyzed for a possible asymmetry with respect to θ = 90°.

High-resolution nuclear resonance fluorescence (NRF) experiments have been performed on the nuclei 182,184,186 W. Several new states with spin J = 1 were discovered in the energy range between 2 and 4 MeV. Ground-state transition widths and K quantum numbers are presented. The results are discussed in the light of similar studies in the rare-earth region.

A photon scattering experiment has been performed on the odd-[ital A] nucleus [sup 143]Nd. Strong dipole excitations have been detected in the energy region around 3.2 MeV. This is near the expected position of the two-phonon (2[sup +][direct product]3[sup [minus]]) multiplet in the even-even neighboring nucleus [sup 142]Nd. The summed dipole strength between 2.8 and 3.6 MeV in [sup 143]Nd is in agreement with the sum rule for weak particle coupling to the core nucleus [sup 142]Nd. Model calculations in a harmonic approximation are in a very good agreement with the experiment and they suggest a two-phonon (2[sup +][direct product]3[sup [minus]]) particle structure of the excitations in [sup 143]Nd.

Nuclear Resonance Fluorescence experiments have been performed on spherical nuclei around neutron number N=82. Detailed information on energies, cross sections and absolute transition strengths (i.e. lifetimes) of dipole excitations between 1 and 4 MeV have been obtained for the nuclei 141 Pr, 142 Nd and 143 Nd. A very strongly excited 1 − -state in 142 Nd at 3.425 MeV is assigned as a member of the two-phonon 2 + ⊗ 3 − -multiplet. Calculations in the framework of the sdf-Interacting Boson Model for a similar state in the N=82 nucleus 144 Sm confirm this assumption. In the neighbouring odd A nuclei 141 Pr and 143 Nd we found first evidence for the existence of two-phonon-particle-states which appear due to the coupling of the unpaired nucleon with the members of the 2 + ⊗ 3 − -multiplet in the core nucleus.

The cross sections for Bhabha and Moller scattering have been investigated in the energy range 2.1–2.4 MeV using monoenergetic positrons and electrons to search for hypothetical resonances superimposed to the continuum predicted by quantum electro dynamics. Bhabba-to-Mott, Moller-to-Mott and Moller-to-Bhabha cross section ratios were measured. The Bhabha-to-Mott ratios could be determined with statistical errors of typically 1% and remaining systematic errors not exceeding the statistical ones. No resonances in Bhabha scattering were observed. Limits for the intrinsic widths of hypothetical resonances are given. For the first time upper limits are deduced from our data for hypothetical resonances in the Moller scattering cross section in the MeV range. Additionally, Moller-to-Bhabha cross section ratios could be determined with good precision. A good agreement between the experimental and theoretical Moller-to-Bhabha ratios can be stated calling in question the recently predicted existence of series of narrow, unresolvable resonances in the Bhabha scattering cross section.

Systematic nuclear resonance fluorescence (NRF) experiments have been performed at the bremsstrahlung facility of the Stuttgart Dynamitron to investigate the distribution of magnetic and electric dipole strengths in deformed nuclei. Precise excitation energies, transition strengths, spins and decay branching ratios were deduced for numerous low lying dipole excitations in deformed rare earth nuclei. Measurements of the linear polarization of resonantly scattered photons using simultaneously two Compton polarimeters enabled model independent parity assignments. For the first time positive parities could be established for groups of states in the neighbouring deformed nuclei 150 Nd, 160 Gd, 162 Dy. Most of these states are concentrated near 3 MeV and should be attributed to orbital M1 excitations (“ Scissors Mode ”). The deformation dependence of the orbital M1 strength has been studied in the Nd isotopic chain. Completing previous polarization measurements on 142,150 Nd the transitional nucleus 146 Nd has been investigated. The surprising novel result of the present systematic studies, however, was the first observation of enhanced electric dipole excitations in the same deformed nuclei at excitation energies of 2.414, 2.471, and 2.520 MeV, respectively. The transition energies and the enhanced B ( E 1)↑ strengths of 3–5·10 −3 e 2 fm 2 support the interpretation in terms of the predicted new type of collective electric dipole excitations in deformed nuclei due to reflection asymmetric shapes like octupole deformations and/or cluster configurations. Furthermore, all three states systematically exhibit decay branching ratios R exp = B (1 − → 2 1 + )/ B (1 − → 0 1 + ), which hint at K -mixing. First results for the neutron-odd, deformed nucleus 163 Dy are presented.

The total photofission cross section σγ, F for 238U has been measured as a substitute for the total photon absorption cross section σtot in the energy range of the first baryon resonances (50⩽Eγ⩽800 MeV). The experiments were performed at the tagging facility of the new 855 MeV Mainz Microton MAMI B using monochromatic photons and a sophisticated 4π-arrangement of position sensitive fragment detectors. The measured cross section agrees in the Δ-resonance region with previous results and the so-called universal curve αtot/A. In the energy region of the D13-resonance near 710 MeV resonancelike structure could be observed as is seen for the free proton.

Measurements of the linear polarization of resonantly scattered photons have been used for model independent parity assignments in nuclear resonance fluorescence experiments. The novel result of the present study was the first observation of enhanced electric dipole excitations in the deformed nuclei $^{150}\mathrm{Nd}$ and $^{162}\mathrm{Dy}$ at excitation energies of 2.414 and 2.520 MeV, respectively. The transition energies and the enhanced B(E1)\ensuremath{\uparrow} strengths of 3 and 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathit{e}}^{2}$ ${\mathrm{fm}}^{2}$ support the interpretation in terms of the predicted new type of collective electric dipole excitations in deformed nuclei due to reflection asymmetric shapes like octupole deformations and/or cluster configurations.

The saturation of the {ital B}({ital E}2,0{sub 1}{sup +}{r arrow}2{sub 1}{sup +}) strengths in the actinide nuclei is described very well by an analytical formula using shell closures at {ital Z}=82,126 and {ital N}=126,184. The transition towards saturation is much more gradual than in the rare earth nuclei and reaches a saturation value of 325{plus minus}10 W.u. By assuming a similar correlation between {ital B}({ital E}2) and summed {ital B}({ital M}1) strengths as observed in the rare-earth region we predict for actinide nuclei a saturation of the summed {ital B}({ital M}1,0{sub 1}{sup +}{r arrow}1{sup +}) strengths below about 3 MeV of 3.7{plus minus}0.4 {mu}{sub {ital N}}{sup 2}. We compare our estimates for ({ital g}{sub {pi}}{minus}{ital g}{sub {nu}}){sup 2} with values obtained from extrapolations of averaged regional boson {ital g} factors.

The high selectivity of the Nuclear Resonance Fluorescence method in exciting dipole transitions in combination with the progress achieved in measuring polarization observables has led to a substantial amount of new experimental data concerning the distribution of magnetic and electric dipole strengths in deformed nuclei. Systematic experiments have been performed at the bremsstrahlung facility of the Stuttgart Dynamitron. Precise excitation energies, transition strengths, spins and decay branching ratios were deduced for numerous low lying dipole excitations in deformed rare earth nuclei. Measurements of the linear polarization of resonantly scattered photons using simultaneously two Compton polarimeters enabled model independent parity assignments. With this technique for the first time positive parities could be established for groups of states in the neighbouring deformed nuclei 150Nd, 160Gd, 162Dy. Most of these states are concentrated near 3 MeV and should be attributed to orbital isovector M1 excitations (often referred to as “Scissors Mode”). The surprising novel result of the present systematic studies, however, was the first observation of enhanced electric dipole excitations in the same deformed nuclei at excitation energies of 2.414, 2.471, and 2.520 MeV, respectively. Furthermore, all three states systematically exhibit decay branching ratios which hint at K-mixing. The transition energies and the enhanced B(E1)↑ strengths of 3 to 5.10−3e2fm2 support the interpretation in terms of the predicted new type of collective electric dipole excitations in deformed nuclei due to reflection asymmetric shapes like octupole deformations and/or cluster configurations.