123 research outputs found
Identification and rejection of scattered neutrons in AGATA
Gamma rays and neutrons, emitted following spontaneous fission of 252Cf, were
measured in an AGATA experiment performed at INFN Laboratori Nazionali di
Legnaro in Italy. The setup consisted of four AGATA triple cluster detectors
(12 36-fold segmented high-purity germanium crystals), placed at a distance of
50 cm from the source, and 16 HELENA BaF2 detectors. The aim of the experiment
was to study the interaction of neutrons in the segmented high-purity germanium
detectors of AGATA and to investigate the possibility to discriminate neutrons
and gamma rays with the gamma-ray tracking technique. The BaF2 detectors were
used for a time-of-flight measurement, which gave an independent discrimination
of neutrons and gamma rays and which was used to optimise the gamma-ray
tracking-based neutron rejection methods. It was found that standard gamma-ray
tracking, without any additional neutron rejection features, eliminates
effectively most of the interaction points due to recoiling Ge nuclei after
elastic scattering of neutrons. Standard tracking rejects also a significant
amount of the events due to inelastic scattering of neutrons in the germanium
crystals. Further enhancements of the neutron rejection was obtained by setting
conditions on the following quantities, which were evaluated for each event by
the tracking algorithm: energy of the first and second interaction point,
difference in the calculated incoming direction of the gamma ray,
figure-of-merit value. The experimental results of tracking with neutron
rejection agree rather well with Geant4 simulations
Nucleon-induced reactions at intermediate energies: New data at 96 MeV and theoretical status
Double-differential cross sections for light charged particle production (up
to A=4) were measured in 96 MeV neutron-induced reactions, at TSL laboratory
cyclotron in Uppsala (Sweden). Measurements for three targets, Fe, Pb, and U,
were performed using two independent devices, SCANDAL and MEDLEY. The data were
recorded with low energy thresholds and for a wide angular range (20-160
degrees). The normalization procedure used to extract the cross sections is
based on the np elastic scattering reaction that we measured and for which we
present experimental results. A good control of the systematic uncertainties
affecting the results is achieved. Calculations using the exciton model are
reported. Two different theoretical approches proposed to improve its
predictive power regarding the complex particle emission are tested. The
capabilities of each approach is illustrated by comparison with the 96 MeV data
that we measured, and with other experimental results available in the
literature.Comment: 21 pages, 28 figure
Response of AGATA Segmented HPGe Detectors to Gamma Rays up to 15.1 MeV
The response of AGATA segmented HPGe detectors to gamma rays in the energy
range 2-15 MeV was measured. The 15.1 MeV gamma rays were produced using the
reaction d(11B,ng)12C at Ebeam = 19.1 MeV, while gamma-rays between 2 to 9 MeV
were produced using an Am-Be-Fe radioactive source. The energy resolution and
linearity were studied and the energy-to-pulse-height conversion resulted to be
linear within 0.05%. Experimental interaction multiplicity distributions are
discussed and compared with the results of Geant4 simulations. It is shown that
the application of gamma-ray tracking allows a suppression of background
radiation following neutron capture by Ge nuclei. Finally the Doppler
correction for the 15.1 MeV gamma line, performed using the position
information extracted with Pulse-shape Analysis, is discussed.Comment: 10 pages, 11 figure
Ultrafast all-optical switching by single photons
An outstanding goal in quantum optics is the realization of fast optical
non-linearities at the single-photon level. Such non-linearities would allow
for the realization of optical devices with new functionalities such as a
single-photon switch/transistor or a controlled-phase gate, which could form
the basis of future quantum optical technologies. While non-linear optics
effects at the single-emitter level have been demonstrated in different
systems, including atoms coupled to Fabry-Perot or toroidal micro-cavities,
super-conducting qubits in strip-line resonators or quantum dots (QDs) in
nano-cavities, none of these experiments so far has demonstrated single-photon
switching on ultrafast timescales. Here, we demonstrate that in a strongly
coupled QD-cavity system the presence of a single photon on one of the
fundamental polariton transitions can turn on light scattering on a transition
from the first to the second Jaynes-Cummings manifold with a switching time of
20 ps. As an additional device application, we use this non-linearity to
implement a single-photon pulse-correlator. Our QD-cavity system could form the
building-block of future high-bandwidth photonic networks operating in the
quantum regime
Conceptual design of the early implementation of the NEutron Detector Array (NEDA) with AGATA
The NEutron Detector Array (NEDA) project aims at the construction of a new high-efficiency compact neutron detector array to be coupled with large (Formula presented.) -ray arrays such as AGATA. The application of NEDA ranges from its use as selective neutron multiplicity filter for fusion-evaporation reaction to a large solid angle neutron tagging device. In the present work, possible configurations for the NEDA coupled with the Neutron Wall for the early implementation with AGATA has been simulated, using Monte Carlo techniques, in order to evaluate their performance figures. The goal of this early NEDA implementation is to improve, with respect to previous instruments, efficiency and capability to select multiplicity for fusion-evaporation reaction channels in which 1, 2 or 3 neutrons are emitted. Each NEDA detector unit has the shape of a regular hexagonal prism with a volume of about 3.23l and it is filled with the EJ301 liquid scintillator, that presents good neutron- (Formula presented.) discrimination properties. The simulations have been performed using a fusion-evaporation event generator that has been validated with a set of experimental data obtained in the 58Ni + 56Fe reaction measured with the Neutron Wall detector array
Lifetime measurement of neutron-rich even-even molybdenum isotopes
Background: In the neutron-rich A approximate to 100 mass region, rapid shape changes as a function of nucleon number as well as coexistence of prolate, oblate, and triaxial shapes are predicted by various theoretical models. Lifetime measurements of excited levels in the molybdenum isotopes allow the determination of transitional quadrupole moments, which in turn provides structural information regarding the predicted shape change. Purpose: The present paper reports on the experimental setup, the method that allowed one to measure the lifetimes of excited states in even-even molybdenum isotopes from mass A = 100 up to mass A = 108, and the results that were obtained. Method: The isotopes of interest were populated by secondary knock-out reaction of neutron-rich nuclei separated and identified by the GSI fragment separator at relativistic beam energies and detected by the sensitive PreSPEC-AGATA experimental setup. The latter included the Lund-York-Cologne calorimeter for identification, tracking, and velocity measurement of ejectiles, and AGATA, an array of position sensitive segmented HPGe detectors, used to determine the interaction positions of the gamma ray enabling a precise Doppler correction. The lifetimes were determined with a relativistic version of the Doppler-shift-attenuation method using the systematic shift of the energy after Doppler correction of a gamma-ray transition with a known energy. This relativistic Doppler-shift-attenuation method allowed the determination of mean lifetimes from 2 to 250 ps. Results: Even-even molybdenum isotopes from mass A = 100 to A = 108 were studied. The decays of the low-lying states in the ground-state band were observed. In particular, two mean lifetimes were measured for the first time: tau = 29.7(-9.1)(+11.3) ps for the 4(+) state of Mo-108 and tau = 3.2(-0.7)(+ 0.7) ps for the 6(+) state of Mo-102. Conclusions: The reduced transition strengths B(E2), calculated from lifetimes measured in this experiment, compared to beyond-mean-field calculations, indicate a gradual shape transition in the chain of molybdenum isotopes when going from A = 100 to A = 108 with a maximum reached at N = 64. The transition probabilities decrease for Mo-108 which may be related to its well-pronounced triaxial shape indicated by the calculations
Evidence for a spin-aligned neutron-proton paired phase from the level structure of Pd
The general phenomenon of shell structure in atomic nuclei has been
understood since the pioneering work of Goeppert-Mayer, Haxel, Jensen and
Suess.They realized that the experimental evidence for nuclear magic numbers
could be explained by introducing a strong spin-orbit interaction in the
nuclear shell model potential. However, our detailed knowledge of nuclear
forces and the mechanisms governing the structure of nuclei, in particular far
from stability, is still incomplete. In nuclei with equal neutron and proton
numbers (), the unique nature of the atomic nucleus as an object
composed of two distinct types of fermions can be expressed as enhanced
correlations arising between neutrons and protons occupying orbitals with the
same quantum numbers. Such correlations have been predicted to favor a new type
of nuclear superfluidity; isoscalar neutron-proton pairing, in addition to
normal isovector pairing (see Fig. 1). Despite many experimental efforts these
predictions have not been confirmed. Here, we report on the first observation
of excited states in nucleus Pd. Gamma rays emitted
following the Ni(Ar,2)Pd fusion-evaporation reaction
were identified using a combination of state-of-the-art high-resolution
{\gamma}-ray, charged-particle and neutron detector systems. Our results reveal
evidence for a spin-aligned, isoscalar neutron-proton coupling scheme,
different from the previous prediction. We suggest that this coupling scheme
replaces normal superfluidity (characterized by seniority coupling) in the
ground and low-lying excited states of the heaviest N = Z nuclei. The strong
isoscalar neutron- proton correlations in these nuclei are predicted to
have a considerable impact on their level structures, and to influence the
dynamics of the stellar rapid proton capture nucleosynthesis process.Comment: 13 pages, 3 figure
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