ELI-RO-01 - Approaching day-one experiments with the GBS at ELI-NP/ADAGIO

1. Project title: Approaching day-one experiments with the GBS at ELI-NP/ADAGIO
2. Project duration: 36 months (between 2016-2019)
3. Objectives:
The goal of this project is to help preparing the program of experiments at ELI-NP by constructing a team that will have the expertise to perform experiments at the future facility.
The specific objectives of the present project are:

  1. Testing of the NE213 type and 6Li-glass neutron detectors that will be used in experiments aiming at measuring the nucleus response above neutron threshold. These types of detectors will be used at ELI-NP to monitor the gamma beam and to measure the neutron spectra for different incident energies. Before any experiments could be performed at ELI-NP, the response of these detectors has to be known. Therefore, we plan to perform a complete characterization of these detectors by performing detailed simulations using dedicated tools, and to test the detectors in-beam to confirm the results of the simulations.

  2. In-beam test of the LaBr3:Ce detectors for the first time to measure the isospin mixing in 60Zn from the gamma decay of the Giant Dipole Resonance. Theoretical predictions of the isospin mixing are mainly based on microscopic calculations and strongly depend on the theoretical approach to parameterize the nuclear interaction. Therefore, experimental data to constrain these predictions are necessarily, and a suited candidate to look for the isospin mixing is 60Zn. The decay of the GDR implies the detection of high energy γ-rays and using large LaBr3:Ce detectors is the best method available.

  3. Determine the isospin mixing between discrete states in mirror nuclei using a model-independent approach. This objective implies performing an experiment using the NRF technique at the High Intensity ?-ray Source (HIGS) to measure the decay pattern of several 5/2+ states in 35Cl to extract the isospin symmetry breaking from the relative strength of E1 transitions. This type of experiments will be also used at ELI-NP, where a whole range of applications of the NRF method are envisioned.

  4. Study of the dipole response of 142Nd using inelastic scattering of alpha particles in order to characterize the nature of the Pygmy Dipole Resonance (PDR). Such complementary investigations have the potential to reveal new features of the PDR, as it was shown already in several cases. We plan to perform an experimental investigation using the inelastic scattering of α particles at iThemba Labs in South Africa. We will use the detection in coincidence of gamma rays and scattered particles using gamma-ray detectors and a magnetic spectrometer placed at 0 degrees.

4. Title of stages and activities:
  1. Study regarding isospin symmetry breaking in 60Zn using GDR
    2. I.1. Detailed characterization of 3''x 3'' LaBr3:Ce detectors
    I.2. Experimental study of isospin mixing inferred from the γ-decay of the GDR
  2. Characterization and in-beam neutron detector testing for future experiments at ELI-NP
    3. II.1. Simulations of the production of mono-energetic neutrons from the 7Li(p,n)7Be reaction and of netron detectors, both 6Li-glass and NE213 type
    II.2 Experimental testing of neutron detectors using the 7Li(p,n)7Be reaction at the TANDEM accelerator of IFIN-HH
  3. Model independent determination of isospin mixing between discrete states in mirror nuclei excited in photonuclear reactions
    4. III.1 Study regarding B(E1) strength in 35Ar-35Cl mirror nuclei. Theoretical study of the correlation between isospin mixing in the initial and final states.
    III.2 Performing the inelastic scattering of gamma rays experiment at HIGS
    III.3 Data processing for extracting the isospin mixing in the initial and final states.
  4. IV. Study of the electric dipole response in 142Nd using the inelastic scattering of α particles
    5. IV.1 Performing the experimental investigation to study the E1 component in 142Nd
    IV.2 Preliminary analysis of the experimental data
5. Obtained results:
2016-2017:
Here we report the work performed by the group involved in this project for the period September 2016 - September 2017. This time period contains two activities, according to the working plan of the contract: i) detailed characterization of 3''x3'' LaBr3:Ce detectors and measurement of isospin mixing from the decay of the Giant Dipole Resonance (GDR) and ii) characterization and in-beam testing of NE213 and 6Li-glass neutron detectors.

The first part is dedicated to the investigation of large (3''x3'') LaBr3:Ce detectors which will be an important tool for the future ELI-GANT array of the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility. In a first step we have completely characterized the properties of the new LaBr3:Ce detectors in terms of energy and time resolution, efficiency, and linearity for different energy ranges. The excellent properties found for the LaBr3:Ce scintillation detectors are ideal for the scientific cases proposed in the Technical Design Report (TDR) of ELI-GANT. In a second step, we have performed an experimental study in collaboration with the nuclear structure group from the University of Milano to investigate the isospin symmetry breaking in 60Zn form the decay of the GDR. This kind of experiment requires the measurement of the high-energy γ-decay of the GDR. A very efficient way of performing such an experiment is to use the large LaBr3:Ce detectors in combination with high-energy resolution germanium detectors (HPGe). Therefore, several detectors of ELI-NP have been successfully used in this experiment and their properties resulted in the testing period were confirmed.

The second part reports on the detailed investigation of the neutron detectors that will be used at ELI-NP facility by the ELI-GANT team. The scientific cases proposed by this group are concentrated in the vicinity of the neutron threshold emission and therefore an important component of this system will be related to neutron detection. At ELI-NP, this is foreseen to be done with the NE213-type neutron detectors for energies above 1 MeV, while for γ-ray beams below 1 MeV the choice of detectors is based on 6Li-glass. These detectors have been purchased by ELI-NP and the best way to test them is to perform a full scale experiment at the Tandem accelerator of IFIN-HH. This task has been achieved using the 7Li(p,n)7Be reaction and the nanosecond pulsing system of the accelerator, for various incident energies of the protons. In a second step we have performed Monte Carlo simulations in order to confirm the results obtained. This task is still in progress since we have discovered that GEANT 4 has difficulties in reproducing the neutron energy spectra emitted in charged-particle induced reactions. This effect has first been corrected and afterwards the time-of-flight spectra could be reproduced.

2018:
For this year, we report an experimental investigation performed at the High Intensity Gamma-ray Source (HIGS) at Duke University using the inelastic scattering of gamma rays in order to study the isospin mixing effects in the mirror pair 35Cl-35Ar. This is one of the most prominent examples of isospin mixing, where a strong asymmetry has been observed for the decay of the first 7/2- state, which is believed to arise from a mixing of discrete states having T=1/2 and T=3/2. By assuming this scenario, it was shown that given enough experimental information on appropriate E1 transition strengths, the isospin mixing in the initial and final state could be determined in a model-independent way. Currently, the best candidate is the pair of mirror nuclei 35Cl-35Ar, where only a few reduced B(E1) strengths are missing. The main goal of this study is to investigate the complete decay of the 5/2+ states with T=3/2 in 35Cl in order to determine for the first time the ratio between the isospin mixing in the initial and final states using only experimental data.

The states of interest were populated by quasi-monochromatic, linearly polarized and intense beam of real photons delivered by the HIGS facility. The energy of the gamma rays was 8.2 MeV since we were interested in resonantly exciting a level at 8209 keV which has spin 5/2+ and T=3/2. The de-exciting gamma rays were detected with the γ3 setup consisting of four HPGe and four 3''x3'' LaBr3:Ce detectors. The HPGe detectors were placed at angles Θ=90 and 135, while having Φ=45, 90, 180, and 315; the LaBr3:Ce detectors are placed at Θ=90 and 135, with Φ=0, 135, 225, 270. Data was acquired in parallel with two acquisition systems, one working as a Multi-Channel Analyzer (MCA) and the other recording the data in an event-by even list mode. The target consisted of 6.3 g of natural LiCl, this combination of chlorine being chosen in this chemical form since the amount of level excited by the 8.2 MeV gamma-ray beam in lithium is kept to a minimum.

The main result concerns the measurement of the 5/2+, T=3/2 -> 7/2-,T=1/2 branching ratio of the level at 8209 keV. The direct decay from this state to the ground state can be seen very well in our spectra, but the branching ratio to the 7/2- state is not visible, being located below of a very pronounced background. However, one can extract an upper limit by analyzing the background, and found a value of 2% relative to the ground state decay. This value corresponds to an upper limit of the corresponding transition probability, namely B(E1; 5/2+, T=3/2 -> 7/2-,T=1/2)<1.2x10^-4 W.u. In a previous attempt to extract the magnitude of the isospin mixing in the initial and final states in 35Cl-35Ar, it was estimated that several states with spin 5/2+ with energies between 8.2 and 9.1 MeV should be considered for calculating the T=3/2 to T=1/2 transition probability and their summed intensity is above 10^-3 W.u. However, they did not take into account the level at 8209 keV level since this decay was not known at that moment. We have shown that the level at 8209 keV is in fact the only one that have to be considered for assessing the magnitude of the T=3/2 to T=1/2 decay, and found that this value is at least one order of magnitude lower than previously assumed. This has some important consequences when calculating the ratio between the mixing in the initial and final states.

2019:
This year we report results obtained concerning the determination of the isoscalar E1 response in 142Nd using the inelastic scattering of alpha particles at iThemba Labs in South Africa. Study of the E1 response in atomic nuclei is a major topic of current nuclear structure studies which showed the importance of performing experiments using complementary probes. This nucleus was studied by means of inelastic scattering experiments of gamma rays. However, it was shown in several cases that much information can be gained by studies which use complementary probes, like protons or α particles. Thus, the nucleus 142Nd was chosen for a case study in the region of the PDR.

Therefore, we have performed an experimental investigation at iThemba Labs in South Africa using the inelastic scattering reaction of α particles and detecting the γ rays in coincidence with particles by employing the BaGeL gamma array and the K600 spectrometer placed at 0◦. Gamma rays were detected with 12 HPGe clover detectors and 5 large-volume LaBr3:Ce scintillators. The K600 consists of a quadrupole, two dipoles and two trim coils. Behind the second dipole there is the focal plane position-sensitive detector. This consists of a vertical drift chamber (VDC) and a plastic scintillation detector. The latter has the main purpose of providing event trigger signals and also helping in particle identification through ∆E - ∆E method. The data was recorded using two different DAQs: Mesytec and XIA digitizers and are stored in an event-by-event format.

Most of the preliminary analysis we report is concentrated on the different corrections that have to be produced in order to obtain accurate results. These corrections include adjusting the cable length offset for the vertical drift chamber, time-of-flight and scintillator alignment, and lineshape correction for spectra involving focal plane parameters. After the corrections have been implemented, we calibrated the spectrometer using the case of 24Mg, for which the excitation energies in the energy range we consider are very well known.

The extracted calibration coefficients were further adapted for the experiment target using kinematics calculations. The final spectrum we have obtained is a preliminary gamma-ray spectrum obtained with the LaBr3:Ce detectors in coincidence with α particles, in the case of 142Nd. In order to obtain this spectrum, we have employed the corrections described earlier in the text and the calibration of the spectrometer with 24Mg. The preliminary analysis is done with about 1/10 of the total statistics collected during the experiment. Nevertheless, a clear concentration of E1 strength can be seen in the region between 4 and 7 MeV. The next steps of the analysis will confirm the nature of the observed transitions and will provide the final numbers for the cross sections. These observables will then be compared with the results obtained in the previous (γ,γ’) experiments.

6. Contact person: Dr. Sorin Gabriel Pascu (spascu @ tandem.nipne.ro)