arXiv subjects

J. Ndayishimye

Publications and source records attributed to J. Ndayishimye.

8 recordsLinked to original sources

arXiv2026

Gamma-ray production cross sections in proton interactions with natMg, natSi and 56Fe targets: measurement over the energy range of $E_p = 66$-125 MeV, data analysis, results and discussion. Astrophysical implications

We have measured nuclear gamma-ray line production cross sections in interactions of highly accelerated proton beams with various target nuclei abundant in astrophysical sites. The experiments were carried out at the 200-MV Separated Sector Cyclotron (SSC) of iThemba LABS (near Cape Town, in South Africa) using a high-energy resolution and high efficiency detection system for registering the emitted gamma-ray photons. We report and discuss in this paper the collected experimental data sets for various gamma-ray lines produced in bombarding natMg, natSi and 56Fe targets with proton beams of incident energies of Ep = 66, 80, 95, 110 and 125 MeV. After describing the experimental set up and the data analysis method used, we report and discuss our total experimental cross section results in comparisons to previous counterparts from the literature, to a semi-empirical compilation and to the predictions of nuclear reaction theory via performed TALYS code calculations. Significantly improved agreements between theory and experiment are point out when using our modified optical model potential and B\^eta (lambda) level deformation parameters instead of the default input parameters built in TALYS. Finally, we put into perspective the applications of our results in nuclear physics and astrophysics with drawing relevant conclusions. gammaKeywords: Proton-induced nuclear reactions; gamma-ray production cross sections; gamma-ray spectrometry; gamma-ray spectroscopy; Astrophysical implications

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arXiv2023

Gamma-ray emission in proton-induced nuclear reactions on natC and Mylar targets over the incident energy range of Ep = 30-200 MeV. Astrophysical implications

We have measured the gamma-ray line production cross sections in proton-induced nuclear reactions on various target nuclei abundant in astrophysical sites over the incident energy range of Ep = 30 - 200 MeV. We carried out experimental campaigns in joint collaboration at the K = 200 cyclotron of iThemba LABS using a high-energy resolution, high-efficiency detection array composed of 8 Compton-suppressed clover detectors comprising 32 HP-Ge crystals for recording the gamma-ray spectra. In the current paper, we focus on de-excitation lines produced in proton irradiations of natC and Mylar targets. In particular, on the prominent 4.439 and 6.129 MeV lines of $^{12}$C and $^{16}$O which are among the strongest lines emitted in solar flares and in interactions of low-energy cosmic rays with the gas and dust of the inner galaxy. We report new gamma-ray production experimental cross section data for ten nuclear lines that we compare to previous low-energy data sets from the literature, to the predictions of the TALYS code of modern nuclear reactions and to a semi-empirical compilation. In first approach, performing calculations with default input parameters of TALYS we observed substantial deviations between the predicted cross sections and experimental data. Then, using modified optical model potential and nuclear level deformation parameters as input data we generated theoretical excitation functions for the above two main lines fully consistent with experimental data. In contrast, the experimental data sets for the other eight analyzed lines from the two proton-irradiated targets exhibit significant deviations with the predicted cross section values. We also report line-shape experimental data for the line complex observed at $E_g$ = 4.44 MeV in irradiations of the two targets. Finally, we emphasize the astrophysical implications of our results.

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arXiv2020

Spectroscopy of low spin states in 157Dy: Search for evidence of enhanced octupole correlations

Low-spin states of 157Dy have been studied using the JUROGAM II array, following the 155Gd (α, 2n) reaction at a beam energy of 25 MeV. The level scheme of 157Dy has been expanded with four new bands. Rotational structures built on the [523]5/2- and [402]3/2+ neutron orbitals constitute new additions to the level scheme as do many of the inter- and intra-band transitions. This manuscript also reports the observation of cross I- to (I-1)- and I- to (I-1)+ E1 dipole transitions inter-linking structures built on the [523]5/2- (band 5) and [402]3/2+ (band 7) neutron orbitals. These interlacing band structures are interpreted as the bands of parity doublets with simplex quantum number s = -i related to possible octupole correlations.

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arXiv2020

First candidates for γ vibrational bands built on the [505]11/2- neutron orbital in odd-A Dy isotopes

Rotational structures have been measured using the Jurogam II and GAMMASPHERE arrays at low spin following the 155Gd(α,2n)157Dy and 148Nd(12C, 5n)155Dy reactions at 25 and 65 MeV, respectively. We report high-K bands, which are conjectured to be the first candidates of a Kπ= 2+ γ vibrational band, built on the [505]11/2- neutron orbital, in both odd-A 155, 157Dy isotopes. The coupling of the first excited K=0+ states or the so-called \b{eta} vibrational bands at 661 and 676 keV in 154Dy and 156Dy to the [505]11/2- orbital, to produce a Kπ=11/2- band, was not observed in both 155Dy and 157Dy, respectively. The implication of these findings on the interpretation of the first excited 0+ states in the core nuclei 154Dy and 156Dy are also discussed.

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arXiv2020

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 $γ$-ray data. For known, intense $γ$-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 $γ$-ray lines are also reported and discussed. The astrophysical implications of our results are emphasised.

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arXiv2016

Nature of low-lying electric dipole resonance excitations in 74Ge

Isospin properties of dipole excitations in 74 Ge are investigated using the (α,α'γ) reaction and compared to (γ,γ) data. The results indicate that the dipole excitations in the energy region of 6 to 9 MeV adhere to the scenario of the recently found splitting of the region of dipole excitations into two separated parts: one at low energy, being populated by both isoscalar and isovector probes, and the other at high energy, excited only by the electromagnetic probe. Relativistic quasiparticle time blocking approximation (RQTBA) calculations show a reduction in the isoscalar E1 strength with an increase in excitation energy, which is consistent with the measurement.

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arXiv2015

Gamma ray production cross sections in proton induced reactions on natural Mg, Si and Fe targets over the proton energy range 30 up to 66 MeV

Gamma-ray excitation functions have been measured for 30, 42, 54 and 66 MeV proton beams accelerated onto C + O (Mylar), Mg, Si, and Fe targets of astrophysical interest at the separate-sector cyclotron of iThemba LABS in Somerset West (Cape Town, South Africa). A large solid angle, high energy resolution detection system of the Eurogam type was used to record Gamma-ray energy spectra. Derived preliminary results of Gamma-ray line production cross sections for the Mg, Si and Fe target nuclei are reported and discussed. The current cross section data for known, intense Gamma-ray lines from these nuclei consistently extend to higher proton energies previous experimental data measured up to Ep ~ 25 MeV at the Orsay and Washington tandem accelerators. Data for new Gamma-ray lines observed for the first time in this work are also reported.

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arXiv2011

Observation of triple-photon decay in positron-electron pair annihilation: a triple coincidence setup for the undergraduate laboratory

The positron-electron pair annihilation in two photons is known for its numerous applications using PET scanners. The decay of Positronium (Ps) from a standard sealed source in more than two photons is less likely but can be observed with a relatively simple setup. The main goal of this experiment was to verify momentum and total angular momentum conservation principles at subatomic level through the Ps annihilation. The two spin configurations of Ps are produced with a beta+ source. The decay modes are identified using gamma-ray spectroscopy techniques in a double and triple coincidence setup. Three germanium detectors (or two HPGe and one NaI(Tl) detectors) and a digital electronic system are required.

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