Clover Detectors Research Articles

Reorientation-effect measurement of the 〈21+∥Ê2∥21+〉 matrix element in Ar36

The spectroscopic quadrupole moment of the first excited 21+ state, QS(21+), at 1.970 MeV in Ar36 was determined at energies well below the Coulomb barrier—where nuclear interference effects are negligible—using the Pt194(Ar36,Ar*36)Pt*194 Coulomb-excitation reaction at 134.2 MeV. Particle-gamma coincidence data were collected using the AFRODITE array—composed of eight high-purity germanium clover detectors—and an upstream double-sided silicon detector at iThemba LABS. A large diagonal matrix element of 〈21+∥E2̂∥21+〉=0.163(42) eb was determined, which yields a more accurate value of QS(21+)=+0.12(3) eb as compared with previous work, QS(21+)=+0.11(6) eb, in agreement with modern beyond mean-field and large-scale shell-model calculations. This value is consistent with the ratio of electric quadrupole moments found for other A=4n self-conjugate nuclei extracted from the reorientation effect and the rotor model, which are surprisingly equivalent to those observed in good rotors in the mass A≈160–180 region.Received 22 September 2021Accepted 28 November 2021DOI:https://doi.org/10.1103/PhysRevC.104.L061305©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsCollective modelsElectromagnetic transitionsHydrodynamic modelsNuclear structure & decaysPhotonuclear reactionsShell modelSpectroscopic factors & electromagnetic momentsProperties20 ≤ A ≤ 38TechniquesRadiation detectorsNuclear Physics

First in-beam experiment with the ELIADE detectors: a spectroscopic study of 130La

The new facility, Extreme Light Infrastructure – Nuclear Physics (ELI-NP), is a combined laser-gamma nuclear physics research facility currently undergoing its final implementation stages in Măgurele near Bucharest, Romania. It already hosts two fully-operational 10 PW laser arms and, by 2023, it will also house a γ-beam system based on laser Compton backscattering, capable of delivering a high-brilliance, low-energy beam at E γ ≲ 19.5 MeV. Owing to this unique laser-gamma instrumentation combination, several types of experiments will be possible at ELI-NP, including high precision nuclear resonance fluorescence (NRF) experiments. In this case, the main γ-beam detection system for performing NRF studies at ELI-NP is represented by the ELI Array of DEtectors (ELIADE), featuring eight high-purity germanium (HPGe) segmented clover detectors. The current work presents the characteristics of two of the ELIADE detectors, including their photopeak detection efficiency, energy resolution, and peak-to-total ratio measured using γ-ray sources, as well as the timing performance obtained via in-beam measurements. For these latter detector tests, 130La was populated via the fusion evaporation reaction 121Sb(12C,3n)130La using a beam energy of 53 MeV at the Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), also located in Măgurele. Herein, we report on the results of the ^130La linear polarization measurements taken using the ELIADE detectors as Compton polarimeters. The results obtained from the in-beam experiment were compared to several already published works and we present new information on the transition multipolarity in 130La.

Structure of collective states built on the 11/2+ isomer in Os187 : Quasiparticle-plus-triaxial-rotor model and interpretation as tilted-precession bands

Background: The shape of most nuclei is described by its quadrupole deformation (showing the deviation from spherical shape) and its triaxiality (showing the deviation from axial symmetry). Nuclei affected by triaxiality show additional collective rotational bands, called γ bands, that appear at low excitation energy. The γ bands can be caused by the precession of a nucleus with triaxial shape, but can also arise from small γ vibrations around an axially symmetric shape. Purpose: The aim of this work is to search for new collective excitations in Os187 in particular related with the γ degree of freedom. Methods: The rotational structures of Os187 were populated by the W186(He4,3n)Os187 reaction at a beam energy of 37 MeV. The measurements of γ - γ coincidences, angular distribution ratios and γ-ray intensities were performed using eleven Compton-suppressed Ge clover detectors. Results: The previously known positive-parity band built on the 11/2+ isomer has been extended up and a new excited positive-parity band built above a 15/2+ state has been observed. The 11/2+ band was assigned a νi13/2 configuration while the new 15/2+ band was associated with a coupling of the valence i13/2 neutron with the γ band of the even-even core. The quasiparticle-plus-triaxial-rotor model calculations provide a good agreement with the experimental data for both bands. They describe the 15/2+ band as a collective excitation with respect to the 11/2+ band that corresponds to a precession of the intermediate nuclear axis similarly to the precession of a rotating top. Conclusions: As shown by the calculations, the new rotational band can be understood as resulting from the three-dimensional rotation of a triaxially-deformed nucleus. However, a description based on the vibrations of a γ-soft nuclear shape should also be investigated in order to firmly establish the nature of the excited positive-parity band. Further studies able to distinguish between these alternative descriptions will be beneficial.8 MoreReceived 11 May 2021Revised 10 June 2021Accepted 5 October 2021DOI:https://doi.org/10.1103/PhysRevC.104.044326©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsElectromagnetic transitionsIsomer decaysNuclear structure & decaysNucleon distributionProperties150 ≤ A ≤ 189Nuclear Physics

Angular correlation measurements with a segmented clover detector in a close geometry

Angular correlation measurements were carried out with a segmented clover detector in a close geometry where the segments were used as individual detectors. This allowed up to 35 unique angles when only front segments are used and up to 140 unique angles when both front and back segments are used. The angular correlation was tested with 60Co and 133Ba sources. The large number of angles and the close geometry, allowing increased efficiency, produced precise angular correlation measurements and allowed the determination of several multipole mixing ratios in 133Cs with a precision comparable to that of the presently adopted values.

Detailed study of the decay of $$^{32}$$Ar

In an experiment performed at the SPIRAL1 facility of GANIL, the $$\beta $$ decay of $$^{32}$$ Ar has been studied by means of the “Silicon Cube” device associated with germanium clover detectors from the EXOGAM array. Beta-delayed protons and $$\gamma $$ rays have been observed and allowed the determination of all relevant decay branches. The Gamow–Teller strength distribution is compared to shell-model calculations and excellent agreement is found. The Fermi strength is inline with expectations. A quasi-complete decay scheme of $$^{32}$$ Ar is established.

Optimum detector arrangement of a Compton polarimeter using a clover detector for [formula omitted]-delayed [formula omitted] rays

Application of a clover Ge detector to the measurement of the linear polarization of β-delayed γ rays was investigated. The spectrometer consists of the clover and coaxial-type Ge detectors, and will be installed at an on-line isotope separator to measure short-lived nuclei. To evaluate the performance of the spectrometer, the polarization sensitivities were determined using standard radioactive sources in various detector arrangements. During optimization of the detector setup, the figure of merit was found to depend on the solid angles subtended by the detectors.

Evolution of collectivity and shape transition in Zn66

Excited states in $^{66}\mathrm{Zn}$ were investigated through the in-beam $\ensuremath{\gamma}$-ray spectroscopic techniques using the $^{52}\mathrm{Cr}(^{18}\mathrm{O}, 2p2n$) fusion-evaporation reaction at a beam energy of 72.5 MeV. The $\ensuremath{\gamma}$-rays emitted by the de-exciting nuclei were recorded in coincidence mode using the 14 Compton suppressed Ge clover detectors of the Indian National Gamma Array. With 14 new transitions being identified, the level scheme of $^{66}\mathrm{Zn}$ has been extended up to the excitation energy $\ensuremath{\approx}12.3\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ and spin $\ensuremath{\approx}17\phantom{\rule{0.28em}{0ex}}\ensuremath{\hbar}$. A rotational band, associated with the two quasineutrons from the $1{g}_{9/2}$ orbital, has been found to exhibit a band crossing with the ground-state band at a spin of $6\ensuremath{\hbar}$. The evolution of the collectivity and shape transition in this nucleus have been discussed in the framework of the total Routhian surface calculations and in comparison with the neighboring $^{68,70}\mathrm{Ge}$ nuclei.

High-spin states above the isomers in neutron-rich iodine nuclei near N=82

Excited states of neutron-rich iodine isotopes I130–134 above the high-spin isomers have been identified using prompt-delayed γ-ray spectroscopy. The iodine isotopes were produced as fission fragments of fusion-fission and transfer induced fission of Be9(U238,f) at a beam energy of 6.2 MeV/u. The complete (A,Z) identification was obtained using the large acceptance magnetic spectrometer VAMOS++. The AGATA γ-ray tracking array was used to detect the prompt γ rays while the delayed γ rays (in the time range of 100 ns to 200 μs) from the isomeric states were identified by the EXOGAM segmented clover detectors, placed behind the focal plane of the VAMOS++ spectrometer. The high-spin states above the (8−) isomers in I130,132 were populated for the first time, and a new isomer in I132 was identified. A new γ-ray transition was also assigned to the level structure of I134. Prompt transitions above the 19/2− isomer were identified in I131,133, for the first time. The level structures are interpreted in terms of the systematics of odd-Z nuclei above the Z=50 shell closure and large-scale shell model calculations.11 MoreReceived 13 May 2020Accepted 9 October 2020DOI:https://doi.org/10.1103/PhysRevC.102.044329©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsIsomer decaysNuclear structure & decaysShell modelProperties90 ≤ A ≤ 149Nuclear Physics

Intrinsic background radiation of LaBr3(Ce) detector via coincidence measurements and simulations

The LaBr$_3$(Ce) detector has attracted much attention in recent years for its superior characteristics to other scintillating materials in terms of resolution and efficiency. However, it has relatively high intrinsic radiation background due to the naturally occurring radioisotope in lanthanum, actinium and their daughter nuclei. This limits its applications in low counting rate experiments. In this paper, we identified the radioactive isotopes in the $\phi3''\times 3''$ Saint-Gobain B380 detector by a coincidence measurement using a Clover detector in a low-background shielding system. Moreover, we carried out a Geant4 simulation to the experimental spectra to evaluate the activities of the main internal radiation components. % by means of the $\alpha$, $\gamma$ and $\beta$ decays from the $^{138}$La and $^{227}$Ac. The activity of radiation background of B380 is determined to be 1.480 (69) Bq/cm$^3$, the main sources of which include $^{138}$La of 1.425 (59) Bq/cm$^3$, $^{211}$Bi of 0.0136 (15) Bq/cm$^3$, $^{219}$Rn of 0.0125 (17) Bq/cm$^3$, $^{223}$Ra of 0.0127 (14) Bq/cm$^3$, and $^{227}$Th of 0.0158 (22) Bq/cm$^3$.

Spectroscopic investigation of complex nuclear excitations in Ga66

In-beam spectroscopic technique using the fusion evaporation reaction $^{52}\mathrm{Cr}(^{18}\mathrm{O},1p3n)$, at a beam energy of 72.5 MeV, was employed to explore the structural phenomena in $^{66}\mathrm{Ga}$, mainly at intermediate and high spins. The experimental setup involved an array of 14 Compton suppressed Ge clover detectors, placed around the target position to detect emitted $\ensuremath{\gamma}$ rays from excited states. A new level scheme has been proposed, which is enriched with more than 20 new transitions and is extended up to an excitation energy $\ensuremath{\approx}12$ MeV. A few observed intermediate spin states of $^{66}\mathrm{Ga}$ are discussed in the framework of coupling of single-particle configurations with the vibrational core of $^{64}\mathrm{Zn}$. Shell model calculations have also been performed with two different interactions, viz., jj44bpn and jun45pn, for the interpretation of the observed level structure in $^{66}\mathrm{Ga}$.

Measurement and analysis of nuclear γ -ray production cross sections in proton interactions with Mg, Si, and Fe nuclei abundant in astrophysical sites over the incident energy range E=30–66 MeV

Gamma-ray production cross section excitation functions have been measured for $30$, $42$, $54$ and $66$ MeV proton beams accelerated onto targets of astrophysical interest, $^{nat}$C, C + O (Mylar), $^{nat}$Mg, $^{nat}$Si and $^{56}$Fe, at the Sector Separated Cyclotron (SSC) of iThemba LABS (near Cape Town, South Africa). The AFRODITE array equipped with 8 Compton suppressed HPGe clover detectors was used to record $\gamma$-ray data. For known, intense $\gamma$-ray lines the previously reported experimental data measured up to $E_{p}\simeq$ $25$ MeV at the Washington and Orsay tandem accelerators were extended to higher proton energies. Our experimental data for the last 3 targets are reported here and discussed with respect to previous data and the Murphy \textit{et al.} compilation [ApJS 183, 142 (2009)], as well as to predictions of the nuclear reaction code TALYS. The overall agreement between theory and experiment obtained in first-approach calculations using default input parameters of TALYS has been appreciably improved by using modified optical model potential (OMP), deformation, and level density parameters. The OMP parameters have been extracted from theoretical fits to available experimental elastic/inelastic nucleon scattering angular distribution data by means of the coupled-channels reaction code OPTMAN. Experimental data for several new $\gamma$-ray lines are also reported and discussed. The astrophysical implications of our results are emphasised.

Understanding the operation of general composite detector for gamma-ray spectroscopy

We have presented a probabilistic formalism for predicting the response of the series and parallel combination of gamma-ray detectors arranged in the form of a matrix. Assuming isotropic scattering of incident gamma-rays and considering up to four detector hit events, we have obtained expressions for the addback factor and the peak-to-total ratio (in the addback mode) in terms of only one probability. Using the experimental data of the HHS spectrometer, we have predicted the addback factor for four composite detectors. Predicted values of addback factor and fold distribution show a reasonable agreement with the experimental ones for the clover detector.

Revealing multiple band structures in Xe131 from α -induced reactions

The excited states in the transitional nucleus $^{131}\mathrm{Xe}$ have been populated by using $\ensuremath{\alpha}$-induced fusion-evaporation reaction and the de-exciting $\ensuremath{\gamma}$ rays were detected with the Compton suppressed clover detector setup of the Indian National Gamma Array coupled to digital data acquisition system. The existing level structure of $^{131}\mathrm{Xe}$ has been significantly extended with the observation and placement of 72 new $\ensuremath{\gamma}$-ray transitions. The use of light-ion ($\ensuremath{\alpha}$) beam helped to identify several new band structures in $^{131}\mathrm{Xe}$ with different quasiparticle (qp) configurations. The multipolarities of the observed $\ensuremath{\gamma}$ rays have been determined on the basis of the directional correlation from oriented states ratio and polarization asymmetry measurements. The yrast negative-parity band has been confirmed up to ${35/2}^{\ensuremath{-}}$ spin and the highly nonyrast signature partner of this band has been identified for the first time. The positive-parity band, based on the ${3/2}^{+}$ ground state, has been extended up to ${23/2}^{+}$ with the observation of a signature inversion, which signifies a pair of particle alignment around the spin of 15/2 $\ensuremath{\hbar}$. A dipole band, consisting of $M1$ transitions has been identified and assigned a 5-qp configuration. A new band structure built on a 3-qp ${23/2}^{+}$ state has been observed with a large signature splitting. A comparison in the isotopic and isotonic chains reveals the transitional nature of the $N=77$ nuclei. Total Routhian surface calculations have been performed to understand the structure of $^{131}\mathrm{Xe}$ associated with different configurations.

The [formula omitted]-ball [formula omitted]-spectrometer

The ν-ballspectrometer is an hybrid array combining high purity co-axial germanium detectors from the french-UK loan pool, clover detectors from the GAMMAPOOL, lanthanum bromide (LaBr3:Ce) scintillator detectors belonging to the FATIMA collaboration and phoswitches from the PARIS collaboration. The aim was to couple the excellent energy resolution of germanium detectors to the excellent time resolution of the LaBr3 detectors. We achieved a total photopeak efficiency of 6.7% at 1.3 MeV, and peak-to-total ratio of 50% for the germanium part of the array. Using the digital acquisition system FASTER, we achieved time resolution of about 250 ps for LaBr3. This acquisition system made also possible the use of the calorimetry for reaction selection. It makes ν-ball the first fully digital large fast timing spectrometer with time resolution similar to analogue electronics. The construction began in June 2017 and commissioning was performed in early November 2017. From November 2017 to June 2018, more than 3200 h of beam time were provided by the ALTO facility to perform eight experiments during the campaign. Among them, five weeks of beam time were dedicated to γ spectroscopy of fast neutron induced reactions. In this paper all the technical details about the spectrometer are presented. First steps of the data analysis process are also discussed.

Single crystal HPGe (80%) versus BGO shielded CLOVER detector for high precision decay rate measurements: a comparative study

In this study, various detector configurations have been investigated in order to explore the optimal condition for decay rate measurements of radioactive samples using gamma spectroscopy technique. A limitation of detecting low energy gamma rays from decaying radioactive nuclei, is the Compton background which can be significantly reduced by rejecting Compton scattered events through active Bismuth germanate (BGO) shielding. On the other hand, for a CLOVER detector without BGO shielding, one can place the radioactive samples very close to the detector for enhancing geometrical efficiency. A single crystal High Purity Germanium (HPGe) detector can also be used for decay rate measurements. In order to measure the decay rate of nuclei decaying via gamma emission with reasonable intensity, optimal close geometry options have been investigated for various HPGe detector configurations.

In-beam spectroscopic study of Zn63

Investigation on the excited states of $^{63}\mathrm{Zn}$ was done through in-beam $\ensuremath{\gamma}$-ray spectroscopic techniques using the $^{52}\mathrm{Cr}(^{18}\mathrm{O},\phantom{\rule{4pt}{0ex}}\ensuremath{\alpha}3n)$ fusion-evaporation reaction at a beam energy of 72.5 MeV. Detection of the emitted $\ensuremath{\gamma}$ rays from the excited nuclei was performed in the coincidence mode using 14 Compton suppressed Ge clover detectors of the Indian National Gamma Array. Based on the $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence data, 13 new transitions have been placed in the level scheme following their coincidence relationship and intensity balance. Spin and parity assignments of the excited levels have been carried out by extracting the directional correlation from oriented states ratio and polarization asymmetry values of the emitted $\ensuremath{\gamma}$ rays. Shell model calculations have been performed in the ${f}_{5/2}p{g}_{9/2}$ model space with a $^{56}\mathrm{Ni}$ core using the jj44bpn interaction, to interpret the observed excited states of the nucleus. A reasonable agreement is found between the experimental findings and the shell-model calculations. In order to understand the evolution of a collective shape built on the $9/{2}^{+}$ (1704 keV) state, we have performed total Routhian surface calculations with a $1{g}_{9/2}$ quasineutron and found reasonable agreement.

Investigating the aberration found in 4.438 MeV 12C photo peak of the Geant4 simulated prompt gamma spectrum

The purpose of this study is to investigate the broad 4.438 MeV 12C photo-peak found in the Geant4 simulated prompt gamma spectrum from proton beam collision on a thin carbon target. The broad 4.438 MeV 12C peak has been reported in previous work using the AFRODITE clover detector performing an absolute comparison of measured and simulated prompt gamma production for a carbon target in the proton therapeutic range. The simulated prompt gamma spectrum was obtained for a 95 MeV proton beam collision on the carbon target by counting the emitted prompt gammas over a 4ˇ solid angle using a Geant4(v10.01.p3) cross-section code with the suggested physics list for inelastic scattering in the proton therapeutic range (binary cascade model). This result was compared to prompt gamma spectra from other Geant4 inelastic scattering physics models. First, the default inelastic cross-section data set (Geisha) was compared to the Tripathi data set, producing basically identical spectra, indicating that the cross-section data set has no bearing on the binary cascade inelastic scatter model. Second, comparisons of the precompound model using an exciton number of 1 and the Geisha cross-section data set provides a 4.438 MeV photo peak that is closer to the experimental result. The precompound model with an exciton number of 1 and the default (Geisha) cross-section data set significantly narrowed the 4.438 MeV photo peak, giving the peak that most closely resembled measured data.

Single particle configurations in Ni61

Excited states of the $^{61}\mathrm{Ni}$ ($Z=28,N=33$) nucleus have been probed using heavy-ion-induced fusion evaporation reaction and an array of Compton suppressed germanium (clover) detectors as detection system for the emitted $\ensuremath{\gamma}$ rays. Seventeen new transitions have been identified and placement of six transitions have been modified with respect to the previous measurements, following which the level scheme of the nucleus has been extended up to an excitation energy ${E}_{x}\ensuremath{\approx}7$ MeV and spin $\ensuremath{\approx}10\ensuremath{\hbar}$. Higher excitations involving the ${g}_{9/2}$ orbital in the $fpg$ model space have been established. The experimental results on the level structure of the nucleus have been interpreted in the light of large basis shell model calculations that lead to an understanding of the single particle configurations underlying the level structure of the nucleus. The comparison can be suggestive of further refinements in the shell model interactions for better overlap of the theoretical results with the experimental data.

Angular Correlation Measurement with the iThemba LABS Segmented Clover Detector

CITATION: Shirinda, O., et al. 2019. Angular correlation measurement with the iThemba LABS segmented clover detector. Acta Physica Polonica B, 50(3):597-604, doi:10.5506/APhysPolB.50.597.

Gamma–gamma angular correlation analysis techniques with the GRIFFIN spectrometer

Gamma–gamma angular correlation measurements are a powerful tool for identifying the angular momentum (spin) of excited nuclear states involved in a γ-ray cascade, and for measuring the multipole orders and mixing ratios of transitions. Though the physical angular correlations are fully calculable from first principles, experimental effects can make the extraction of coefficients and thus conclusions about spins and mixing ratios difficult. In this article we present data analysis techniques developed for the clover detectors of the GRIFFIN spectrometer at TRIUMF-ISAC combined with GEANT4 simulations in order to extract accurate experimental results.