Un nouveau detecteur Micromegas vient d-etre developpe pour la premiere fois, la microet8209;grille et l-anode sont segmentees en pistes, dans des directions perpendiculaires. Ce detecteur offre ...ainsi une vraie structure 2D pour la reconstruction des trajectoires des particules chargees. De plus, ayant une masse tres faible, il est parfaitement adapte a des mesures en faisceau de neutrons moyennant l-utilisation d-un convertisseur. Le detecteur a ete teste avec succes et est dorenavant utilise comme profileur transparent du faisceau de neutrons de l-experience n_TOF au CERN.
Neutron induced reaction cross-section measurements display special interest both for fundamental research in the Nuclear Physics field and many practical applications. The Institute of Nuclear and ...Particle Physics (INP) of the National Centre for Scientific Research Demokritos (N.C.S.R. “D”) hosts the 5.5 MV T11/25 Van de Graaff accelerator, which is the only accelerator used in Greece for research purposes. This accelerator recently underwent a major upgrade, including the installation of a new pelletron charging system, two new ion sources, a new gas stripper and beam optics. This neutron facility can produce quasi-monoenrgetic neutron beams in the energy range ∼16-19 MeV via the 3 H(d,n) 4 He (D-T) reaction, employing a tritiated Titanium target (TiT). The neutron induced cross sections of a total of nine reaction channels have been experimentally measured in the present work, via the activation technique, using enriched Ge targets. These targets produce more accurate cross-section results, in comparison with the - most commonly used in bibliography - nat Ge samples, since they do not suffer from contaminating reactions that produce the same residual nucleus. Monte Carlo simulations were also performed via the combined use of MCNP5 and NeuSDesc codes for the simulation of the neutron beam.
.
The neutron time-of-flight facility n_TOF features a white neutron source produced by spallation through 20GeV/
c
protons impinging on a lead target. The facility, aiming primarily at the ...measurement of neutron-induced reaction cross sections, was operating at CERN between 2001 and 2004, and then underwent a major upgrade in 2008. This paper presents in detail all the characteristics of the new neutron beam in the currently available configurations, which correspond to two different collimation systems and two choices of neutron moderator. The characteristics discussed include the intensity and energy dependence of the neutron flux, the spatial profile of the beam, the in-beam background components and the energy resolution/broadening. The discussion of these features is based on dedicated measurements and Monte Carlo simulations, and includes estimations of the systematic uncertainties of the mentioned quantities.
At the neutron time-of-flight facility n_TOF at CERN a new vertical beam line was constructed in 2014, in order to extend the experimental possibilities at this facility to an even wider range of ...challenging cross-section measurements of interest in astrophysics, nuclear technology and medical physics. The design of the beam line and the experimental hall was based on FLUKA Monte Carlo simulations, aiming at maximizing the neutron flux, reducing the beam halo and minimizing the background from neutrons interacting with the collimator or back-scattered in the beam dump.
The present paper gives an overview on the design of the beam line and the relevant elements and provides an outlook on the expected performance regarding the neutron beam intensity, shape and energy resolution, as well as the neutron and photon backgrounds.
Several cross-section measurements of neutron-induced reactions on Ge found in literature, are performed utilizing natGe targets. The production of the same residual nucleus as the measured one might ...occur as a result of the unavoidable presence of neighboring isotopes in the same target, acting as a contamination. Corrections must be made based on theoretical calculations and models in order to resolve this problem. The accuracy and limits of a methodology for these "theoretical corrections" are investigated in this work using isotopically enriched targets, which can produce very accurate results without the need for such corrections. Experimental cross-section measurements have been made for the 76Ge(n,2n)75Ge, 72Ge(n,α)69mZn and 72Ge(n,p)72Ga reactions, via the activation technique, with the 27Al(n,α)24Na reaction used as reference, employing both a natGe and isotopically enriched Ge targets. The 3H(d,n)4He (D-T) reaction was used for producing the quasi-monoenergetic neutron beam in the 5.5 MV Tandem Accelerator Laboratory of the National Centre for Scientific Research "Demokritos" in Athens, Greece, at an incident deuteron beam energy of 2.9 MeV. Using HPGe detectors, γ-ray spectroscopy was applied to determine the induced γ-ray activity of the residual nuclei.
One of the critical aspects for the accurate determination of neutron capture cross sections when combining time-of-flight and total energy detector techniques is the characterization and control of ...systematic uncertainties associated to the measuring devices. In this work we explore the most conspicuous effects associated to harsh count rate conditions: dead-time and pile-up effects. Both effects, when not properly treated, can lead to large systematic uncertainties and bias in the determination of neutron cross sections. In the majority of neutron capture measurements carried out at the CERN n_TOF facility, the detectors of choice are the C6D6 liquid-based either in form of large-volume cells or recently commissioned sTED detector array, consisting of much smaller-volume modules. To account for the aforementioned effects, we introduce a Monte Carlo model for these detectors mimicking harsh count rate conditions similar to those happening at the CERN n_TOF 20 m flight path vertical measuring station. The model parameters are extracted by comparison with the experimental data taken at the same facility during 2022 experimental campaign. We propose a novel methodology to consider both, dead-time and pile-up effects simultaneously for these fast detectors and check the applicability to experimental data from 197Au(n, γ), including the saturated 4.9 eV resonance which is an important component of normalization for neutron cross section measurements.
Nine neutron induced reactions on Ge isotopes (
70
Ge(n,2n)
69
Ge,
76
Ge(n,2n)
75
Ge,
73
Ge(n,p)
73
Ga,
72
Ge(n,p)
72
Ga,
73
Ge(n,d/np)
72
Ga,
74
Ge(n,d/np)
73
Ga,
74
Ge(n,α)
71m
Zn,
72
Ge(n,α)
69m
...Zn,
73
Ge(n,nα)
69m
Zn) have been measured in this work at energies 16.4-18.9 MeV. For these reactions, most of the experimental datasets in literature were obtained with a
nat
Ge target. However, the residual nucleus produced by some reaction channels can also be produced from neighboring isotopes, acting as a contamination for the measured reactions. This contribution must be subtracted, based on theoretical calculations, bearing their own uncertainties. The use of enriched targets however, does not suffer from such contaminations, leading to accurate experimental results. In this scope, five highly isotopically enriched Ge samples have been used in this work. The quasi-monoenergetic neutron beams were produced via the
3
H(d,n)
4
He reaction at the 5.5 MV Tandem Van de Graaff accelerator of N.C.S.R. ‘Demokritos’. The cross section of these nine reactions were measured using the activation method, with respect to the
27
Al(n,α)
24
Na reference reaction.
This article presents a few selected developments and future ideas related to the measurement of
(
n
,
γ
)
data of astrophysical interest at CERN n_TOF. The MC-aided analysis methodology for the use ...of low-efficiency radiation detectors in time-of-flight neutron-capture measurements is discussed, with particular emphasis on the systematic accuracy. Several recent instrumental advances are also presented, such as the development of total-energy detectors with
γ
-ray imaging capability for background suppression, and the development of an array of small-volume organic scintillators aimed at exploiting the high instantaneous neutron-flux of EAR2. Finally, astrophysics prospects related to the intermediate
i
neutron-capture process of nucleosynthesis are discussed in the context of the new NEAR activation area.
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