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Nuclear Resonance Fluorescence

  1. What is Nuclear Resonance Fluorescence?
  2. Physical Motivation of NRF Experiments
  3. Experimental Techniques
  4. Publication

Introduction

    Nuclear Resonance Flourescence Scattering (NRF) is an excellent method with which to probe low-lying (photon energies from 0 to 10 MeV) dipole excitations in nuclei because of the extremely high selectivity of real photons, in exciting such states. This selectivity stems from the small momentum transfer of real photons, in contrast to virtual photons from electron scattering experiments. The development in recent years of high-efficiency germanium detectors with excellent energy resolution, in conjunction with high-intensity photon beams, has provided the necessary tools to study the fine structure of magnetic and electric dipole strength distributions in detail. The fundamental advantage of the photon scattering technique is that the electromagnetic interaction mechanism is the best-understood interaction in all of science. This understanding allows one to extract detailed information about the structure of nuclei and the transitions between different nuclear states in a completely model independent way. Thus results from these kinds of experiments are as robust as any results of nuclear science.

    An NRF facility was established to address some of the most fundamental questions of nuclear structure at low excitation energies. This facility, for example, will allow one to probe the transitions between three major shapes of nuclei, spherical nuclei, quadropole-deformed (football shaped) nuclei, and tri-axial nuclei (all three axes have different lengths). The proposed studies will investigate the distribution of magnetic and electric transition strength, and thus, provide rigorous tests and guidance to theoretical models. Basic data following from applicton of this technique can also be used for practical purposes such as hazardous waste assay and imaging.

    Nuclear resonance fluorescence (NRF) or resonant photon scattering denotes the process of resonant excitation of nuclear states by absorption of electromagnetic radiation (real photons) and subsequent decay of these levels by re-emission of the equivalent radiation. Those photons having the right energy will excite a target nucleus with a certain probability. The probability can be expressed by the ground state transition with, which is related to the transition strength and to the transition matrix elements. After a very short time (some fs - ps) the excited nuclei will decay either back to the ground state (elastic transition) or to some other sate with lower energy (inelastic transition). A simplified scheme of an NRF experiment is shown in fig. 1.


    Fig. 1. Sketch of Nuclear Resonance Fluorescence experiments.

    The NRF method is used for study of low-multipolarity transition (i.e. E1, M1, E2) with large partial widths to the ground state. This means that the photon has a small probability to transfer angular momentum to the atomic nucleus. Due to its low detection limit it represents an outstanding tool for examination of dipole transition. The photon scattering cross section depends on the branchng ratio of the excited level to the ground state, i.e. modes decaying to other states that the ground state or by particle emission are most likely not to be detectable in the nuclear resonance fluorescence reaction. The energies Ex of the excited states, their lifetimes t (or - what is equivalent - their energy widths dEx or G), their angular momenta J and their parities p provide important information about the nuclear structure, and - thus - about the fundamental forces between the nuclear constituents. The advantage of this method : Both the excitation and the de-excitation processes proceed via the electromagnetic interaction - the best understood interaction in physics.

    The measured values in NRF experiment are:

    • Excitation energies (Ex)
    • Spin (J)
    • Parities (p)
    • Decay widths (G0, Gf, G)
    • Transition strength (B(M1), B(E1))
    • Branching ratios (G1/G0)
    Results are completely model independent.

Physical Motivation of NRF Experiments

    Two-Phonon Excitations of Even-Even Nuclei Around Closed Shells In the domain of the electromagnetic dipole excitations, spectroscopic experiments have revealed new, unexpected phenomena in recent years: Large magnetic dipole (M1) strengths have been discovered in heavy deformed nuclei. The corresponding excitations have been associated with scissors-like oscillations of the deformed proton density distribution against the neutron distribution, and the excitation mode was, accordingly, called "scissors mode" (fig. 2).

    Fig. 2. Different modes of excitations below the neutron threshold.

    Large electric dipole (E1) transitions to the ground states have been observed in spherical nuclei near Z = 50 and N = 82. They are assumed to arise from the coupling of quadrupole and octupole vibrational modes of the nucleus. In medium-mass nuclei (A = 60 - 130), however, dipole excitations have been scarcely investigated. In this region, the existence of the scissors mode has been proven only in 94Mo and 88Sr . Here, this mode generally competes with other transitions (e.g. of spin-flip type) between shell model states. The determination of the relative weights of these two excitation mechanisms turns out to be a theoretical and experimental challenge. Variations of the nuclear shape between spherical and deformed mass distributions are expected even with minor changes in the nucleon numbers - a typical feature of medium-mass nuclei, which are candidates for future investigations in NRF experiments at HIgS. The evolution of the scissors mode with the transition from spherical to deformed nuclei will be studied. The aim of the project is to study the interplay between the collective and single-particle degrees of freedom in spherical or near spherical nuclei. The experiment is directed to measure the transition rates B(E1, 1-1 -->0+) and B(E1, 1-1 -->0+ ) in even-even nuclei around closed shells. In the semi-magic and vibrational nuclei, the coupling between the quadrupole and octopole vibrations gives rise to a quintuplet of levels with Jp = 1-,..., 5-. The level energy E1 was observed to be approximately equal to the sum E2 + E3 leading to the suggestion that the structure of the state is of two-phonon character formed by the coupling [1]. Recent experiments show the excitation of the 1- member of the quintuplet around energies 3 - 4 MeV [2]. The present measurements are concentrated on systematic properties of the E1 decay of these levels in even-even nuclei around shell closures.

    Fig. 3. Nuclear map around semi-magic number 50.

    The advent of formerly unavailable quasi monoenergetic, easily energy tunable, and 100% polarized gamma beam allows to measure excitation energies Ex, spin J, parities, decay branching ratios, ground-state transition widths G0, and hence the reduced excitation probabilities B(E1)­ in a completely model-independent way. On the other hand, this experimental information is not complete and easy to measure in traditional photo scattering experiments. Hence, using the advantages of the g-ray source at HIgS will allow us to measure systematically the decay branching ratios of the photo-excited Jp =1- states to the first Jp =2+ and the fourth Jp =3- state which were not observed before in NRF. In the future experiments will be performed on isotopes within the Cd, Te, and Sn chains. Using natural samples and a quasi monoenergetic ( ~3%) and 100% polarized g-rays with intensity of 107 g/sec simultaneously various isotopes can be studied. Four HPGe detectors (see fig. 2) in a plane perpendicular to the polarization plane will measure the parity of excited states in the energy region from 3 to 4 MeV as demonstrated in ref. [1-3].

Experimental Techniques

    There are several methods to produce photons for NRF experiments. An ideal photon source for such experiments should have the following characteristics:
    • High spectral intensity I = Ng / eV.s (number of photons per energy bin and second)
    • Good monochromaticiy dEg/Eg
    • Tunable in broad energy range
    • High degree of linear or even circular polarization (Pg=100%)
    At the present moment HIgS at the Duke Free Electron Laser Facilities (DFELL) closely meets this description. HIgS will deliver a gamma ray beams with intensities up to 109 g/sec (presently 5 107), tunable from 1 to 225 MeV (present limit is 60 MeV), untagged energy resolution better that 1%, and 100% linear polarized (with OK-5 will produce both linear and circular).

    In order to efficiently perform such experiments, low background conditions are needed in combination with highly efficient, high-resolution Ge detectors. These detectors, placed under different angles (90 and 127 degrees for instance) can observe the deexcitation of excited states and determine the spin of the intermediate levels (in double even nuclei) via the angular correlation. The set-up designed for NRF experiments with linearly polarized bremsstrahlung at HIgS is schematically displayed in fig. 4.

    NRF

    Fig. 4. HIgS polarimeter HPGe-detector array.

    5.02 MeV gamma-ray spectrum generated at HIgS facilities and measured with 123% high-purity germanium detector are shown in Fig. 3. This spectrum is presented in blue color. The area under the ful energy peak (FEP) is the generated gamma spectrum itself. The second escape peak (SE) and the backround peaks down to the low energies are the responce function of the germanium detector. In the same figure a typical bremsstrahlung spectrum generated by the MCNP4C code is shown (radiator: 3 mm W, Ee=5.64 MeV). The monoenergetic gamma beam from FEL is well targeted: only an energy region of interest is exited and contribution from the "useless", bremsstrahlung gamma quanta is minimize. Hence, more spectroscopy information can be reveal as the low gamma transition to the higher energy levels, which is usually hidden under the continuum bremsstrahlung spectrum.

    Fig. 5. A typical monoenergetic gamma spectrum generated by HIg S is shown in fig. 3 (blue spectrum). The full width at the half maximum was 100 keV at 5.02 MeV using of 1" lead colimator. For comparison the bremsstrahlung spectrum with 5.6 MeV maximum output energy is shown on the same picture.

    A typical spectrum is shown below for the case of 32S using four HPGe detectors placed 90 degree with respect to the incident beam (see fig.6).

    Fig. 7. Photon scattering spectra obtained using natural Sulfur target with detectors out-of-plane and in-plane.

Publication List

    [1] N. Pietralla, H.R. Weller, V.N. Litvinenko, M.W. Ahmed, and A.P. Tonchev. Parity Measurements of Nuclear Dipole Excitations using FEL generated Gamma Rays at HIgS. Phys. Rev. Lett. 88 (2002) 012502.

    [2] N. Pietralla, V. N. Litvinenko, S. Hartman, F. F. Mikhailov, I. V. Pinayev, G. Swift, M. W. Ahmed, J. H. Kelley, S. O. Nelson, R. Prior, K. Saburov, A. P. Tonchev, H. R. Weller. Identification of the Jp = 1- two-phonon state of 88Sr. Phys. Rev. C 65 (2002) 047305.

    [3] N. Pietralla, H. R. Weller, V. N. Litvinenko, Ahmed, A. P. Tonchev. Parity Measurements of Nuclear Dipole Excitations using FEL generated Gamma Rays at HIgS. Nucl. Instr. Meth. A 483 (2002) 556

    [4] C. Fransen, N. Pietralla, A.P. Tonchev, M.W. Ahmed, J. Chen, G. Feldman, U. Kneissl, J. Li, V.N. Litvinenko, B. Perdue, I.V. Pinayev, H.H. Pitz, R. Prior, K. Sabourov, M. Spraker, W. Tornow, H. R. Weller, V. Werner, Y. K. Wu, and S. W. Yates. Parity assignment to strong dipole excitations of 92Zr and 96Mo. Phys. Rev. C 70, (2004) 044317.

    [5] D. Savran, S. Muller, A. Zilges, M. Babilon, M.W. Ahmed, J.H. Kelley, A. Tonchev, W. Tornow, H.R. Weller, N. Pietralla, J. Li, I. V. Pinayev, Y. K. Wu. Parity assignments in 172,174Yb using polarized photons and the K quantum number in rare earth nuclei. Phys. Rev. C 71 (2005) 034304.

    [6] A.P. Tonchev, M. Boswell, C.R. Howell, H.J. Karwowski, J.H. Kelley, W. Tornow, Y.K. Wu. The High Intensity Gamma-Ray Source (HIgS) and Recent Results. Accepted in Nucl. Instr. and Methods in Phys. Res. B , 51474 (2005) to be publish.