Previous studies of proton and neutron spectra from Non-Mesonic Weak Decay of eight Λ-Hypernuclei (A=5–16) have been revisited. New values of the ratio of the two-nucleon and the one-proton induced ...decay widths, Γ2N/Γp, are obtained from single proton spectra, Γ2N/Γp=0.50±0.24, and from neutron and proton coincidence spectra, Γ2N/Γp=0.36±0.14stat−0.04sys+0.05sys, in full agreement with previously published ones. With these values, a method is developed to extract the one-proton induced decay width in units of the free Λ decay width, Γp/ΓΛ, without resorting to Intra Nuclear Cascade models but by exploiting only experimental data, under the assumption of a linear dependence on A of the Final State Interaction contribution. This is the first systematic determination ever done and it agrees within the errors with recent theoretical calculations.
The decay of Λ-hypernuclei without π emission, known as Non-Mesonic Weak Decay (NMWD), gives an effective tool to investigate ΔS=1 four-baryon interactions. It was theoretically suggested that the ...two-nucleon induced mechanism could play a substantial role in reproducing the observed NMWD decay rates and nucleon spectra, but at present no direct evidence of such a mechanism has been obtained. The FINUDA experiment, exploiting the possibility to detect both charged and neutral particles coming from the hypernucleus decay, has allowed us to deduce the relative weight of the two nucleon induced decay rate to the total NMWD rate. The value of Γ2N/ΓNMWD=0.21±0.07stat−0.02sys+0.03sys has been deduced, well consistent with the previous determinations.
This Letter is concerned with the study of the Kstop−A→π±Σ∓A′ reaction in p-shell nuclei, i.e., 6,7Li, 9Be, 13C and 16O. The π±Σ∓/Kstop− emission rates are reported as a function of A. These rates ...are discussed in comparison with previous findings. The ratio π−Σ+/π+Σ− in p-shell nuclei is found to depart largely from that on hydrogen, which provides support for large in-medium effects possibly generated by the sub-threshold Λ(1405). The continuum momentum spectra of prompt pions and free sigmas are also discussed as well as the π±Σ∓ missing mass behavior and the link with the reaction mechanism. The apparatus used for the investigation is the FINUDA spectrometer operating at the DAΦNEϕ-factory (LNF-INFN, Italy).
Two different experimental methods, based on cosmic rays measurements, have been investigated to calibrate in absolute units the scintillation light detected, near the threshold, at both ends of the ...long scintillators constituting the External Scintillator Barrel of the FINUDA experiment.
The first one is based on the selection of the cosmic rays intercepting also the vertex detectors. The second one takes into account all the cosmic rays detected by the barrel itself.
Both methods are shown to be able to evaluate with enough precision an absolute value of the threshold for neutron detection efficiency calculations.
The CSSE code, a GEANT3-based Monte Carlo simulation program, has been developed in the framework of the EXPLODET project (Nucl. Instr. and Meth. A 422 (1999) 918) with the aim to simulate ...experimental set-ups employed in Thermal Neutron Analysis (TNA) for the landmines detection. Such a simulation code appears to be useful for studying the background in the
γ-ray spectra obtained with this technique, especially in the region where one expects to find the explosive signature (the
γ-ray peak at 10.83 MeV coming from neutron capture by nitrogen). The main features of the CSSE code are introduced and original innovations emphasized. Among the latter, an algorithm simulating the time correlation between primary particles, according with their time distributions is presented. Such a correlation is not usually achievable within standard GEANT-based codes and allows to reproduce some important phenomena, as the pulse pile-up inside the NaI(Tl)
γ-ray detector employed, producing a more realistic detector response simulation. CSSE has been successfully tested by reproducing a real nuclear sensor prototype assembled at the Physics Department of Bari University.
A systematic analysis of γ-ray spectra produced by NaI(Tl) detectors of different sizes employed in nuclear sensors for Humanitarian Demining has been performed by Monte Carlo calculations. The study ...has been undertaken in the field of the
Thermal Neutron Analysis (TNA) technique, based on (n,γ) reactions. The sensor geometry and configuration as well as the detector response function have been simulated by the CSSE code, specifically developed and tested for these purposes (Nucl. Instr. and Meth., this issue; The EXPLODET Project Progress Report 1999, DEPD 1/NP/00, Report, Physics Department and INFN of Padova, 2000). The sensor is basically made of a moderated neutron source (a
252
Cf
source enclosed in a Pb sphere and a high-density polyethylene structure) and a γ-ray detectors array. Events coming from pile-up of simultaneous pulses in the detector have been investigated with particular care, since they could cause a significant background in the spectrum region where the explosive signature (
10.83
MeV
γ
-ray emerging from neutron capture by
14
N
) is found. The study has been performed for different moderator sizes and shapes and for different detectors configurations.
A large area scanning system using 14 MeV tagged neutrons has been built. The associated
4He particles emitted in the D
+
T reaction are detected in an array of Parallel Plate Avalanche Counters that ...defines a 10
×
100 cm
2 area in the scanning plane. Coincident gamma-rays are detected by 10 BaF
2 scintillators. The commissioning of the sensor has been completed. The capability of the system to determine the presence and the position of samples has been verified. Furthermore the possibility of on-line background subtraction is demonstrated.
DA Φ NE monitored by FINUDA Agnello, M.; Benussi, L.; Bertani, M. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
2007, Letnik:
570, Številka:
1
Journal Article
Recenzirano
The FINUDA spectrometer, devoted to hypernuclear physics and installed on the DA
Φ
NE two rings collider at the Laboratori Nationali di Frascati, is able to monitor the relevant machine parameters, ...as luminosity, collision vertexes, c.m. energy and transversal momentum boost, during the process of data taking to study hypernuclear physics without affecting it. The collider parameters relevant to optimize the machine performances to the needs of the experiment are measured both on-line and offline in a run-to-run basis, in an efficient, redundant way, allowing the continuous extraction of reliable and cross-checked information on the machine working conditions.
The latest results from the FINUDA experiment on the Non-Mesonic Weak Decay (NMWD) channels of p-shell Λ-hypernuclei are presented and discussed. Spectra of protons from NMWD were obtained for 5ΛHe, ...7ΛLi, 9ΛBe, 11ΛB, 12ΛC, 13ΛC, 15ΛN and 16ΛO. An estimation of the contributions of both Final State Interactions (FSI) and two-nucleon induced (2N) decay processes was performed, following a model independent approach. The previously published results are confirmed by a new analysis of the triple coincidence (π−, p, n) events.
This paper provides a summary of the first results obtained by the FINUDA Collaboration in the field of hypernuclear physics. They range from the spectroscopic study of several A-hypernuclei to the ...observation of some non-mesonic decay modes. Hints on rare hypernuclear two-body decays, on neutron-rich hypernuclei and on deeply-bound kaonic nuclei are also reported.