Investigation of low-energy dipole modes in the heavy deformed nucleus 154 Sm via inelastic polarized proton scattering at zero degree

A high resolution proton scattering experiment has been performed on the heavy deformed nucleus 154Sm at extreme forward angles with 295 MeV polarized protons at RCNP, Osaka. Our scientific goal is to investigate the impact of ground state deformation on the properties of the pygmy dipole resonance and on the spin-M1 resonance in heavy deformed nuclei. The (p,p’) cross sections can be decomposed into E1 and M1 parts in two independent ways based either on a multipole decomposition of the cross sections or on spin-transfer observables as has been demonstrated for the case of 208Pb. We present the method and preliminary results from the analysis of polarization transfer observables.


Introduction
In heavy nuclei at about 1 ω excitation energies the so called spin-M1 resonance appears.Its centroid energy scales approximately with 40 • A −1/3 [1,2].In deformed nuclei the M1 strength is distributed in a double hump structure with an isoscalar part at lower excitation energies and an isovector part at higher excitation energies [3].The experimental data for the systematics for heavy deformed nuclei have been taken in the early 1990s in a proton scattering experiment with polarized protons at a bombarding energy of 223 MeV at forward angles up to 2.8 • performed at the TRIUMF facility in Canada.Figure 1 demonstrates the difficulties of the analysis [4].One assumption made was that the entire E1 strength entering the analysis is described by the giant dipole resonance i.e., the possible existence of a pygmy dipole resonance was neglected.The other problem was that the resonance structures are built on top of a significant background that originates from quasi-free scattering due to the fact that the experiment has been performed at finite scattering angles.A decomposition of the spectrum into contributions of the isovector giant dipole resonance (IVGDR, horizontally hatched), spin-M1 resonance (black, low E x ) and the isovector giant quadrupole resonance (IVGQR, black, high E x ) and a background from quasifree scattering (lower smooth line) is shown.The interpretation of the double-hump structure of the spin-M1 resonance is contradictory.While some calculations suggest an explanation based on a separation of the proton and neutron 1p-1h states, others explain it as a separation into isoscalar and isovector strength [5][6][7][8].• at TRIUMF [3,4].A decomposition of the spectrum into contributions of the IVGDR (horizontally hatched) spin-M1 resonance (black, low E x ) and the IVGQR (black, high E x ) and a background from quasifree scattering (lower smooth line) is shown.From Ref. [3].
Recently the method of polarized proton scattering at exactly 0 • has become available and this provides a new tool to reinvestigate this intriguing physics with a clearer approach.At the Research Center for Nuclear Research (RCNP) at Osaka one can study polarized proton scattering at 0 • at an energy of 295 MeV.This is optimal for spin-isospin excitations because the central term of the nucleon-nucleon cross section shows a minimum at that energy.Combined with a very good energy resolution of 25 keV at a proton energy of 295 MeV it is possible to do two independent experiments.At first a separation of E1 and M1 contributions to the cross section can be done by comparing the experimentally extracted angular distributions.In addition a spinflip/non-spinflip separation of the cross section can be performed using polarization transfer observables.As a reference case for a heavy nucleus these types of analyses have been performed for the nucleus 208 Pb and they show good consistency [9][10][11].

Experimental method and the 0 • setup at RCNP at Osaka
In this section the experimental method is briefly described.For detailed information we refer to [12].In case of inelastic scattering at 0 • the primary proton beam is transported into the scattering chamber of the Grand Raiden spectrometer.For a clean measurement, a halo-free beam as well as lateral and angular dispersion matching is necessary and the beam tuning takes up to several days.For achieving both, good scattering angle resolution and low background scattering rates, the medium under-focus mode of the Grand Raiden spectrometer is employed.At exactly polar scattering angles of 0 • , only two polarization transfer coefficients, D NN or D S S which correspond at 0 • , and D LL are non-zero and independent.The total spin transfer Σ at θ = 0 • is defined as For non-spinflip excitations (including Coulomb excitation) Σ = 0 and for spinflip excitations Σ = 1 [13].This model-independent relation is used to decompose the spinflip and non-spinflip parts of the cross section.The polarized proton beam has been preaccelerated up to 54 MeV in the AVF cyclotron and after that accelerated to an energy of 295 MeV in the RING Cyclotron.The polarization axis of the beam has been controlled by employing two solenoid magnets located in the injection line of the RING cyclotron.It has been adjusted to the normal (longitudinal) direction for the D NN (D LL ) measurement.
The two coefficients D NN and D LL were measured using a focal plane polarimeter (FPP) system.The double scattering efficiency of the FPP system is 0.04 and the effective analyzing power is 0.37.During the experiment a beam intensity of 4 nA and an average beam polarization of 0.7 has been achieved.Differential cross sections at 0 • have been measured at the same time during the polarization transfer measurements.After each two-hour run of 154 Sm-data, a short calibration run with a 26 Mg target has been performed.Prominent 1 + states of 26 Mg in the energy region of interest have been used for an excitation energy calibration since in the 154 Sm spectra no resolved transitions are observed because of the high level density.The 26 Mg data set has also been used to check the analysis of the polarization transfer coefficients because the prominent peaks should show a pure M1 character which, indeed, was observed.The analysis of the polarization transfer coefficients with the FPP data has been carried out using the method of unbiased effective estimators [14].clearly dominated by E1 strength and the structure at 6 MeV originates from electric excitations.This indicates that the strength observed in [3,4] results, at least partially, from E1 excitations.This is in conflict with the old TRIUMF data that did not extend to exactly 0 • and, hence, the low-energy E1 strength in the deformed nucleus 154 Sm escaped detection.To our best knowledge we observed for the first time the so called pygmy dipole resonance (PDR) in a heavy deformed nucleus.

Discussion and preliminary results
Figure 3 shows the photo-absorption cross section σ Absorption obtained in this experiment.The (γ,xn) cross section from [17] has been scaled to the height of the IVGDR.The two datasets are in a good 02060-p.3

INPC 2013 INPC 2013
agreement and for the first time we can show results for the photo-absorption cross section below the particle threshold in this nucleus.The previously mentioned (γ,xn) measurements are only capable to measure the cross section down to the neutron separation energy S n .The deviation of the two datasets in the vicinity of the neutron separation energy is not clear yet and needs further investigation.Also the multipole decomposition analysis as it has been carried out in [9,11] for the case of 208 Pb is currently in progress.

Figure 1 .
Figure1.Double-differential cross section of the 154 Sm( p, p ) reaction measured at θ = 2.8 • at TRIUMF[3,4].A decomposition of the spectrum into contributions of the IVGDR (horizontally hatched) spin-M1 resonance (black, low E x ) and the IVGQR (black, high E x ) and a background from quasifree scattering (lower smooth line) is shown.From Ref.[3].

3Figure 2 .
Figure 2. (color online) Upper panel: The solid line shows the total cross section in bins of 400 keV.The dashed line in blue corresponds to the spinflip cross section and the dashed line in red shows the non-spinflip cross section obtained by using polarization transfer coefficients.The arrow indicates a local enhancement of nonspinflip strength observed here for the first time.Lower panel: The experimentally obtained total spin transfer Σ.

Figure 3 .
Figure 3. (color online) γ-absorption cross section extracted from this experiment compared to (γ,xn) measurements from [17].The dashed line indicates the neutron separation energy for 154 Sm which lies at 7.97 MeV.The arrow indicates a local enhancement of E1 photo-absorption strength in the energy range of the pygmy dipole resonance.