Characterization of a novel proton-CT scanner based on Silicon and LaBr3_3(Ce) detectors

Treatment planning systems at proton-therapy centres generally use X-ray computed tomography (CT) as primary imaging technique to infer the proton treatment doses to tumour and healthy tissues. However, proton stopping powers in the body, as derived from X-ray images, suffer from important proton-range uncertainties. In order to reduce this uncertainty in range, one could use proton-CT images instead. The main goal of this work is to test the capabilities of a newly-developed proton-CT scanner, based on the use of a set of tracking detectors and a high energy resolution scintillator for the residual energy of the protons. Different custom-made phantoms were positioned at the field of view of the scanner and were irradiated with protons at the CCB proton-therapy center in Krakow. We measured with the phantoms at different angles and produced sinograms that were used to obtain reconstructed images by Filtered Back-Projection (FBP). The obtained images were used to determine the capabilities of our scanner in terms of spatial resolution and proton Relative Stopping Power mapping and validate its use as proton-CT scanner. The results show that the scanner can produce medium-high quality images, with spatial resolution better than 2 mm in radiography, below 3 mm in tomography and resolving power in the RSP comparable to other state of the art pCT cameras

Reduction in the uncertainty of the neutron-capture cross section of 210Bi: Impact of a precise multipolarity measurement of the 2− → 1− main ground-state transition

International audience; The mixing ratio of the main 320-keV, M1 + E2 ground-state γ transition in 210Bi has been more precisely quantified, allowing a significant reduction in the uncertainty of measurements of the neutron-capture cross section to the ground state of 210Bi from 25% to 0.9%. Accurate values for neutron-capture cross sections to both the ground and long-lived 9− isomeric state at 271 keV in 210Bi are of particular importance as Pb-Bi finds increased usage in Accelerator Driven Systems

(n,γ) reactions on rare Ca isotopes: Valence-hole - Coreexcitation couplings in47Ca

Recent results on the structure of 47Ca will be presented. The nucleus of interest was populated via the cold-neutron capture 46Ca(n,\u3b3) reaction, on a rare 46Ca target, during the EXILL experimental campaign at the nuclear reactor of Institut Laue- Langevin in Grenoble. High-resolution \u3b3-ray spectroscopy, performed with a composite array of HPGe detectors, enabled the identification of new transitions deexciting states between the neutron-capture level and the ground state. Experimental data will be compared with a novel microscopic theoretical model, currently under development, specifically designed to describe the low-lying structure of odd-mass nuclei with one valence particle/hole outside a spherical doubly-magic core, using the Skyrme effective interaction self-consistently

Testing ab initio nuclear structure in neutron-rich nuclei: Lifetime measurements of second 2+ state in 16C and 20O

To test the predictive power of ab initio nuclear structure theory, the lifetime of the second 2+ state in neutron-rich 20O,τ(2+2)=150+80−30fs, and an estimate for the lifetime of the second 2+ state in 16C have been obtained for the first time. The results were achieved via a novel Monte Carlo technique that allowed us to measure nuclear state lifetimes in the tens-to-hundreds of femtoseconds range by analyzing the Doppler-shifted γ-transition line shapes of products of low-energy transfer and deep-inelastic processes in the reaction 18O(7.0MeV/u)+181Ta. The requested sensitivity could only be reached owing to the excellent performances of the Advanced γ-Tracking Array AGATA, coupled to the PARIS scintillator array and to the VAMOS++ magnetic spectrometer. The experimental lifetimes agree with predictions of ab initio calculations using two- and three-nucleon interactions, obtained with the valence-space in-medium similarity renormalization group for 20O and with the no-core shell model for 16C. The present measurement shows the power of electromagnetic observables, determined with high-precision γ spectroscopy, to assess the quality of first-principles nuclear structure calculations, complementing common benchmarks based on nuclear energies. The proposed experimental approach will be essential for short lifetime measurements in unexplored regions of the nuclear chart, including r-process nuclei, when intense beams, produced by Isotope Separation On-Line (ISOL) techniques, become available

Shape coexistence in neutron-deficient Hg-188 investigated via lifetime measurements

Shape coexistence in the $Z \approx 82$ region has been established in mercury, lead and polonium isotopes. Even-even mercury isotopes with $100 \leq N \leq 106$ present multiple fingerprints of this phenomenon, which seems to be no longer present for $N \geq 110$. According to a number of theoretical calculations, shape coexistence is predicted in the $^{188}$Hg isotope. The $^{188}$Hg nucleus was populated using two different fusion-evaporation reactions with two targets, $^{158}$Gd and $^{160}$Gd, and a beam of $^{34}$S, provided by the Tandem-ALPI accelerators complex at the Laboratori Nazionali di Legnaro. The channels of interest were selected using the information from the Neutron Wall array, while the $\gamma$ rays were detected using the GALILEO $\gamma$-ray array. The lifetimes of the excited states were determined using the Recoil Distance Doppler-Shift method, employing the dedicated GALILEO plunger device. Using the two-bands mixing and rotational models, the deformation of the pure configurations was obtained from the experimental results. The extracted transition strengths were compared with those calculated with the state-of-the-art symmetry-conserving configuration-mixing (SCCM) and five-dimentional collective Hamiltonian (5DCH) approaches in order to shed light on the nature of the observed structures in the $^{188}$Hg nucleus. An oblate, a normal- and a super-deformed prolate bands were predicted and their underlying shell structure was also discussed.Comment: v1: 13 pages, 10 figures, comparison between IBM-CM and SCCM calculations; v2: 16 pages, 13 figures, discussion on the mixing amplitudes from the experimental B(E2) values, comparison between SCCM and 5DCH calculation

The mutable nature of particle-core excitations with spin in the one-valence-proton nucleus ¹³³Sb

The γ-ray decay of excited states of the one-valence-proton nucleus ¹³³Sb has been studied using cold-neutron induced fission of ²³⁵U and ²⁴¹Pu targets, during the EXILL campaign at the ILL reactor in Grenoble. By using a highly efficient HPGe array, coincidences between γ-rays prompt with the fission event and those delayed up to several tens of microseconds were investigated, allowing to observe, for the first time, high-spin excited states above the 16.6 μs isomer. Lifetimes analysis, performed by fast-timing techniques with LaBr₃(Ce) scintillators, revealed a difference of almost two orders of magnitude in B(M1) strength for transitions between positive-parity medium-spin yrast states. The data are interpreted by a newly developed microscopic model which takes into account couplings between core excitations (both collective and non-collective) of the doubly magic nucleus ¹³²Sn and the valence proton, using the Skyrme effective interaction in a consistent way. The results point to a fast change in the nature of particle-core excitations with increasing spin

The low-spin structure of Tl-206 studied by gamma-ray spectroscopy from thermal neutron capture reaction

The low-spin structure of the 206 Tl nucleus has been investigated with the FIPPS prompt \u3b3ray spectrometer of ILL making use of the 205 Tl(n;\u3b3) 206 Tl reaction and the \u3b3coincidence technique. A large number of excitations up to the neutron binding energy (at 6.5 MeV) in 206 Tl were observed. Preliminary results of the data analysis provided the information on the decay scheme of the capture state in 206 Tl and on the multipolarities of a number of rays. The comparison of the experimental data with shell-model calculations will help describing the proton-hole and neutron-hole couplings near the doubly magic core 208Pb, benchmarking single-particle levels and two-body matrix elements of the residual interaction in this important region of the nuclear chart

Two-neutron and core-excited states in Pb-210: Tracing E3 collectivity and evidence for a new beta-decaying isomer in Tl-210

Yrast and near-yrast levels up to an I = 17h spin value and a 6-MeV excitation energy have been delineated in the "two-neutron" 210Pb nucleus following deep-inelastic reactions involving 208Pb targets and a number of heavy-ion beams at energies similar to 25% above the Coulomb barrier. The level scheme was established on the basis of multifold prompt and delayed coincidence relationships measured with the Gammasphere array. In addition to the previously known states, many new levels were identified. For most of the strongly populated states, spin-parity assignments are proposed on the basis of angular distributions. The reinvestigation of the nu(g9/2)2, 8+ isomeric decay results in the firm identification of the low-energy E2 transitions involved in the 8+-> 6+-> 4+ cascade, and in a revised 6+ level half-life of 92(10) ns, nearly a factor of 2 longer than previously measured. Among the newly identified states figure spin I = 4-10 (h) over bar levels associated with the nu g9/2i11/2 multiplet, as well as yrast states involving nu g9/2j15/2, nu i11/2j15/2, and nu(j15/2)2 neutron couplings. The highest-spin excitations are understood as 1p-lh core excitations and the yrast population is found to be fragmented to the extent that levels of spin higher than I = 17 (h) over bar could not be reached. Four E3 transitions are present in the 210Pb yrast decay; three of these involve the g9/2-> j15/2 octupole component, as reflected in the 21(2) and > 10 Weisskopf unit enhancements of the B(E3) rates of the first two. The fourth, 16+-> 13- E3 transition corresponds to the 3- core octupole excitation built on the nu i11/j15/2 state, in analogy to a similar E3 coupling to the vj15/2 level in 209Pb. Shell-model calculations performed for two-neutron states and 1p-1h 208Pb core excitations are in good agreement with the data. Evidence was found for the existence of a hitherto unknown high-spin beta-decaying isomer in 210Tl. Shell-model calculations of the 210Tl levels suggest the possibility of a 11+ long-lived, j-decaying state, and the beta-delayed yields observed in various reactions fit rather well with a 210Tl assignment.This work was supported by the Polish National Science Center, Projects No. 2012/07/N/ST2/02861 and No. 2016/21/B/ST2/02195, the US Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357 (ANL), and Grants No. DE-FG02- 97ER41041 (UNC), No. DE-FG02-97ER41033 (TUNL), and No. DE-FG02-94ER40834 (UM), NSF Grant No. PHY1404442, the Australian Research Council, and the Science and Technology Facilities Council (STFC), UK. This research used resources of ANL’s ATLAS facility, a DOE Office of Science User Facility

Excitations of one-valence-proton, one-valence-neutron nucleus 210^{210}Bi from cold-neutron capture

International audienceThe low-spin structure of one-proton, one-neutron $^{210}$Bi nucleus was investigated in cold-neutron capture reaction on$^{209}$Bi. The $\gamma$-coincidence measurements were performed with use of EXILL array consisted of 16 HPGe detectors. The experimentalresults were compared to shell-model calculations involving valence particles excitations. The $^{210}$Bi nucleus offers the potentialto test the effective proton-neutron interactions because most of the states should arise from the proton-neutron excitations. Additionally,it was discovered that a few states should come from the couplings of valence particles to the $3^-$ octupole vibration in$^{208}$Pb which provides also the possibility of testing the calculations involving the core excitations

Testing the predictive power of realistic shell model calculations via lifetime measurement of the 11/2+ state in Sb 131

The lifetime of the 11/21+ state in the Sb131 nucleus was measured at the LOHENGRIN spectrometer of the Institut Laue-Langevin via neutron-induced fission of U235 using γ-ray fast-timing techniques. The obtained value of T1/2=3(2) ps, at the edge of the sensitivity of the experimental method, is the first result for the 11/21+ state half-life in neutron-rich Sb isotopes. The corresponding quadrupole reduced transition probability to the ground state is B(E2)=1.4-0.6+1.5W.u., indicating a noncollective nature of this state. Realistic shell-model calculations performed in a large valence space reproduce well the experimental value and point to a dominant 2+(Sn130) - πg7/2 configuration for the 11/21+ state, as expected in a weak-coupling scenario. At the same time, the sum of the quadrupole strength of the multiplet states is predicted to exceed the one of the Sn130 core as a consequence of the equal contribution of the proton and the proton-neutron quadrupole matrix elements, pointing to possible development of collectivity already in the close neighborhood of Sn132