Neutron Emission Threshold Research Articles

Revisiting the 4He(4He,4He)4He* inelastic scattering at the MAGNEX facility

The 4He(4He,4He)4He* inelastic scattering was investigated at the MAGNEX facility of INFN - LNS, aiming at shedding light on the characteristics of the isoscalar monopole resonance of 4He which lies at an energy slightly higher than the proton emission threshold and slightly lower than the neutron emission threshold. A complementary elastic scattering measurement was also performed to study the initial state interaction and set accurate coupled channel calculations. Here, the experimental setup, the data reduction and the theoretical interpretation strategy are briefly described.

New prospects for iodine detector and Solar neutrinos registration

The neutrino capture cross section σ(E) by 127I nucleus is studied with proper consideration of the nuclear strength function S(E) and its resonance structure. The calculations of giant Gamow-Teller, analog and pigmy resonances are realized based on the framework of the self-consistent finite Fermi systems theory. The results are compared with known experimental (p,n) reaction data that confirmed the significant influence of high-lying resonances on the cross section values. In the case when the energy of incident neutrino is higher than the sum of neutron emission threshold Sn and reaction threshold the 126Xe isotope is produced. Comparison of experimental yields for 127Xe and 126Xe isotopes can give the sensitive indicator of the metallicity for different Solar models. The results allow to confirm the advantages of the iodine detector for detecting hard astrophysical and accelerator neutrinos.

Supernova neutrino detection through neutron emission by nuclei

Neutrinos from core collapse supernovae can excite nuclei of some detector materials beyond their neutron emission thresholds. Detection of these neutrons can give valuable information about the supernova explosion mechanism and possibly also throw light on neutrino properties. In this article, we give a brief review of the basic physics of neutrino induced neutron emission and describe the results of some recent calculations of supernova neutrino induced neutrons for some specific target detector materials due to charged current (CC) interactions of the electron flavored neutrinos and antineutrinos as well as due to neutral current (NC) interactions of neutrinos and antineutrinos of all flavors with the detector nuclei. We highlight the fact that a detector material such as lead with a relatively large neutron excess produces neutrons dominantly through the CC interaction of the $\nu_e$s, whereas a material such as iron with small neutron excess produces neutrons dominantly through the combined NC interaction of all the six neutrino and antineutrino species. This raises the interesting possibility of probing the fraction of mu- and tau flavored neutrinos (which interact only through NC interaction) in the supernova neutrino flux by means of simultaneous detection of a supernova in a lead and an iron detector, for example.

Isospin triplet A=12: search for states with enhanced radii

Two independent methods: Asymptotic Normalization Coefficients (ANC) and Modified Diffraction Model (MDM) were applied to new and existing experimental data to search for states with enhanced radii in isobaric analogue states (IAS) of 12B and 12N. The ANC analysis of the 11B(d,p)12B reaction at Ed = 21.5 MeV has confirmed neutron halo existence for two states of 12B: 2−, 1.67 MeV and 1−, 2.62 MeV [1]. Some new results were obtained for higher excited states: halo-like states were observed for 0+, 2.72 MeV and 3−, 3.39 MeV states. It should be mentioned that the last one is unbound state, which is 19 keV above the neutron emission threshold and in both states last neutron has a non-zero orbital momentum (ln=1 and ln=2 correspondingly). We propose to use MDM to study isobaric analogue states of 12B in the mirror 12N nucleus and apply it to analyze the (3He,t) reaction data. The existing in the literature data are not completed enough to make definite conclusion about halo in the 2− and 1− states of 12N. This fact simulated us to carry out a new experiment on the 12C(3He,t)12N reaction at the end of 2018. The measurements were conducted at the University of Jyväskylä (Finland) using the K130 cyclotron to produce a 3He beam at E(3He)=40 MeV. The differential cross sections of the 12C(3He,t)12N reaction were measured in the c.m. angular range 8°–70°. Preliminary results for angular distributions are obtained. MDM analysis of preliminary (3He,t) experimental data is done.

Approaching the Gamow Window with Stored Ions: Direct Measurement of ^{124}Xe(p,γ) in the ESR Storage Ring.

We report the first measurement of low-energy proton-capture cross sections of ^{124}Xe in a heavy-ion storage ring. ^{124}Xe^{54+} ions of five different beam energies between 5.5 and 8AMeV were stored to collide with a windowless hydrogen target. The ^{125}Cs reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and γ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.

Photoneutron reaction cross section measurements on Mo94 and Zr90 relevant to the p -process nucleosynthesis

The photodisintegration cross sections for the 94Mo({\gamma},n) and 90Zr({\gamma},n) reactions have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity {\gamma}-ray Source (HI{\gamma}S) facility of the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the photoneutron reaction cross sections was measured with high precision from the respective neutron emission thresholds up to 13.5 MeV. These measurements contribute to a broader investigation of nuclear reactions relevant to the understanding of the p-process nucleosynthesis. The results are compared with the predictions of Hauser-Feshbach statistical model calculations using two different models for the dipole {\gamma}-ray strength function. The resulting 94Mo({\gamma},n) and 90Zr({\gamma},n) photoneutron stellar reaction rates as a function of temperature in the typical range of interest for the p-process nucleosynthesis show how sensitive the photoneutron stellar reaction rate can be to the experimental data in the vicinity of the neutron threshold.

First measurement of the isoscalar excitation above the neutron emission threshold of the Pygmy Dipole Resonance in 68Ni

The excitation of the Pygmy Dipole Resonance (PDR) in the 68Ni nucleus, above the neutron emission threshold, via an isoscalar probe has been observed for the first time. The excitation has been produced in reactions where a 68Ni beam, obtained by the fragmentation of a 70Zn primary beam at INFN-LNS, impinged on a 12C target. The γ-ray decay was detected using the CsI(Tl) detectors of the CHIMERA multidetector sphere. The 68Ni isotope as well as other heavy ion fragments were detected using the FARCOS array. The population of the PDR was evidenced by comparing the detected γ-ray energy spectra with statistical code calculations. The isotopic resolution of the detection system allows also to directly compare neutron decay channels with the 68Ni channel, better evidencing the PDR decay response function. This comparison allows also the extraction of the PDR cross section and the relative γ-ray angular distribution. The measured γ-ray angular distribution confirms the E1 character of the transition. The γ decay cross section for the excitation of the PDR was measured to be 0.32 mb with a 18% of statistical error.

Detecting supernova neutrinos with iron and lead detectors

Supernova (SN) neutrinos can excite the nuclei of various detector materials beyond their neutron emission thresholds through charged current (CC) and neutral current (NC) interactions. The emitted neutrons, if detected, can be a signal for the supernova event. Here we present the results of our study of SN neutrino detection through the neutron channel in lead ($^{208} {\rm Pb}$) and iron ($^{56} {\rm Fe}$) detectors for realistic neutrino fluxes and energies given by the recent Basel/Darmstadt simulations for a 18 solar mass progenitor SN at a distance of 10 kpc. We find that, in general, the number of neutrons emitted per kTon of detector material for the neutrino luminosities and average energies of the different neutrino species as given by the Basel/Darmstadt simulations are significantly lower than those estimated in previous studies based on the results of earlier SN simulations. At the same time, we highlight the fact that, although the total number of neutrons produced per kTon in a iron detector is more than an order of magnitude lower than that for lead, the dominance of the flavor blind NC events in the case of iron, as opposed to dominance of $\nu_e$ induced CC events in the case of lead, offers a complementarity between the two detector materials so that simultaneous detection of SN neutrinos in a lead and a sufficiently large iron detector suitably instrumented for neutron detection may allow estimating the fraction of the total $\mu$ and $\tau$ flavored neutrinos in the SN neutrino flux and thereby probing the emission mechanism as well as flavor oscillation scenarios of the SN neutrinos.

Neutron halo in the exotic first excited state of 9Be

Differential cross sections for elastic and inelastic 9Be + α scattering (the inelastic scattering leading to the 1.68- and 2.43-MeV states) were measured at the energies of 40 and 90 MeV. The radii of the excited states in question were determined on the basis of a modified diffraction model. The root-mean-square radius of the level at 1.68 MeV proved to be approximately 1 fm larger than the radii of both neighboring states. The reason is that, for this level, there is a neutron halo whose properties are similar to the properties of the known halo in the 11Be nucleus. This result is the first observation of a halo in an unbound state lying above the neutronemission threshold.

Polarized photon scattering offCr52: Determining the parity ofJ=1states

The photoresponse of $^{52}\mathrm{Cr}$ has been investigated in the energy range of 5.0--9.5 MeV using the photon scattering technique at the $\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$ facility of TUNL to complement previous work with unpolarized bremsstrahlung photon beams at the Darmstadt linear electron accelerator. The unambiguous parity determinations of the observed $J=1$ states provides the basis needed to better understand the structure of $E1$ and $M1$ excitations. Theoretical calculations using the quasiparticle phonon model incorporating self-consistent energy-density functional theory were performed to investigate the fragmentation pattern of the dipole strength below and around the neutron-emission threshold. These results compare very well with the experimental values.

Spatial Asymmetry of Heavy Nuclei as Essential Feature for Predictions of Compound Nuclear cross Sections and Decay Rates

Cross sections for neutron capture in the range of unresolved resonances and average level distances near the neutron emission threshold are simultaneously predicted for more than 140 spin-0 target nuclei with A >50. In nearly all these nuclei a combined description of photon strength, level density and Maxwellian averaged radiative neutron capture is presented, which only needs very few fit parameters, which are global for the full range in A. To reach that it is essential that the requirements of spherical or axial symmetry are given up.

Triaxiality of heavy nuclei as essential feature to predict radiative capture

Cross sections for neutron capture in the range of unresolved resonances and average level distances near the neutron emission threshold are simultaneously predicted for more than 140 spin-0 target nuclei with A > 50. Assuming triaxiality in nearly all these nuclei a combined description of both, photon strength and level density, is presented – needing very few fit parameters only.

Structure of unbound neutron-rich9He studied using single-neutron transfer

The 8He(d,p) reaction was studied in inverse kinematics at 15.4A MeV using the MUST2 Si-CsI array in order to shed light on the level structure of 9He. The well known 16O(d,p)17O reaction, performed here in reverse kinematics, was used as a test to validate the experimental methods. The 9He missing mass spectrum was deduced from the kinetic energies and emission angles of the recoiling protons. Several structures were observed above the neutron-emission threshold and the angular distributions were used to deduce the multipolarity of the transitions. This work confirms that the ground state of 9He is located very close to the neutron threshold of 8He and supports the occurrence of parity inversion in 9He.

Effect of neutron halos on excited states of nuclei

The differential cross sections of the inelastic scattering leading to the excitation of short-lived states in the stable ${}^{13}$C and ${}^{9}$Be nuclei as well as the radioactive ${}^{11}$Be nucleus have been analyzed. Signatures of neutron halos in the excited states located close to the neutron emission thresholds have been investigated by applying a recently developed modified diffraction model. The abnormally large rms radius was identified for the 3.089-MeV $1/{2}^{+}$ state of ${}^{13}$C. Significantly enlarged diffraction radii were found for the 1.68-MeV $1/{2}^{+}$ and the 3.05-MeV $5/{2}^{+}$ states of ${}^{9}$Be. The analysis of the diffraction radii of the weakly bound radioactive ${}^{11}$Be nuclei have shown that the 1.78-MeV $5/{2}^{+}$ and 3.41-MeV $3/{2}^{+}$ states in ${}^{11}$Be have the same diffraction radii as the $1/{2}^{+}$ ground state, which is known as the best example of a state with one-neutron halo.

Observation of abnormally large radii of nuclei in excited states in the vicinity of neutron thresholds

Differential cross sections for inelastic scattering leading to the excitation of some nuclear states situated near neutron-emission thresholds were analyzed. With the aid of a modified diffraction model, abnormally large radii were found for the 1/21+ state of the 13C nucleus at 3.09 MeV, for the first levels of positive-parity rotational bands in the 9Be (1/2+ level at 1.68 MeV and 5/2+ level at 3.05 MeV) and 11Be (5/2+ level at 1.78 MeV and 3/2+ level at 3.41 MeV) nuclei, and for the 21+ state of the 14Be nucleus at 1.54 MeV and 11− state of the 12Be nucleus at 2.7 MeV. All of these states possess signatures typical of neutron halos.

Quenching of Spectroscopic Factors for Proton Removal in Oxygen Isotopes

We present microscopic coupled-cluster calculations of the spectroscopic factors for proton removal from the closed-shell oxygen isotopes (14,16,22,24,28)O with a chiral nucleon-nucleon interaction at next-to-next-to-next-to-leading order. We include coupling-to-continuum degrees of freedom by using a Hartree-Fock basis built from a Woods-Saxon single-particle basis. This basis treats bound and continuum states on an equal footing. We find a significant quenching of spectroscopic factors in the neutron-rich oxygen isotopes, pointing to enhanced many-body correlations induced by strong coupling to the scattering continuum above the neutron emission thresholds.

Characterization of an INVS model IV neutron counter for high precision ( [formula omitted]) cross-section measurements

A neutron counter designed for assay of radioactive materials has been adapted for beam experiments at TUNL. The cylindrical geometry and 60% maximum efficiency make it well suited for ( γ , n ) cross-section measurements near the neutron emission threshold. A high precision characterization of the counter has been made using neutrons from several sources. Using a combination of measurements and simulations, the absolute detection efficiency of the neutron counter was determined to an accuracy of ±3% in the neutron energy range between 0.1 and 1 MeV. It is shown that this efficiency characterization is generally valid for a wide range of targets.

DECAY PATTERN OF PYGMY STATES OBSERVED IN NEUTRON-RICH26Ne

Coulomb excitation of the exotic neutron-rich nucleus26Ne on a208Pb target was measured at 58 A.MeV in order to search for low-lying E1 strength above the neutron emission threshold. The radioactive beam was produced by fragmentation of a 95 A.MeV Ar beam delivered by the RIKEN Accelerator Research Facility. The set-up included a NaI ( Tl ) gamma-ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the25Ne +n channel, we observe by multipole decomposition a sizable amount of E1 strength between 6 and 10 MeV excitation energy. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured.

Decay Pattern of Pygmy States Observed in Neutron-RichNe26

Coulomb excitation of the exotic neutron-rich nucleus (26)Ne on a (208)Pb target was measured at 58 MeV/u in order to search for low-lying E1 strength above the neutron emission threshold. This radioactive beam experiment was carried out at the RIKEN Accelerator Research Facility. Using the invariant mass method in the 25Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross section, a reduced dipole transition probability of B(E1)=0.49+/-0.16e(2) fm(2) is deduced, corresponding to 4.9+/-1.6% of the Thomas-Reiche-Kuhn sum rule. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured. The extracted decay pattern is not consistent with several mean-field theory descriptions of the pygmy states.

Photon data shed new light upon the GDR spreading width in heavy nuclei

A global study of the electric dipole strength in and below the isovector giant dipole resonance (GDR) is presented for mass numbers A>80. It relies on the recently established remarkably good match between data for the nuclear photo effect to novel photon scattering data covering the region below the neutron emission threshold as well as by average resonance neutron capture (ARC). From the wide energy coverage of these data the correlation of the GDR spreading width with energy can be studied with remarkable accuracy. A clear sensitivity to details of the nuclear shape, i.e. the β- and γ-deformations, is demonstrated. Based hereon a new parameterization of the energy dependence of the nuclear electric-dipole strength is proposed which — with only two new parameters — allows to describe the dipole strength in all heavy nuclei with A>80. Although it differs significantly from previous parameterizations it holds for spherical, transitional, triaxial and well deformed nuclei. The GDR spreading width depends in a regular way on the respective resonance energy, but it is independent of the photon energy.