arXiv subjects

P. Achenbach

Publications and source records attributed to P. Achenbach.

At least 127 records · Page 7Linked to original sources

arXiv2011

Detector developments for the hypernuclear programme at PANDA

The technical design of the PANDA experiment at the future FAIR facility next to GSI is progressing. At the proposed anti-proton storage ring the spectroscopy of double Lambda hypernuclei is one of the four main topics which will be addressed by the Collaboration. The hypernuclear experiments require (i) a dedicated internal target, (ii) an active secondary target of alternating silicon and absorber material layers, (iii) high purity germanium (HPGe) detectors, and (iv) a good particle identification system for low momentum kaons. All systems need to operate in the presence of a high magnetic field and a large hadronic background. The status of the detector developments for this programme is summarized.

physics.ins-det↗

arXiv2011

A Tracking Fiber Detector based on Silicon Photomultipliers for the Kaos Spectrometer

A tracking detector based on two meters long scintillating fibers read out by silicon photomultipliers (SiPM) is being developed for the Kaos spectrometer at the Mainz Microtron MAMI. Results from a prototype setup using 2 mm square fibers and large area SiPM readout are presented. The detection efficiency of such a combination was measured to be between 83 and 100% depending on the threshold on the SiPM amplitude. A Monte Carlo simulation based on a physical model was employed in order to extract the photon detection efficiency of the SiPM devices.

physics.ins-det↗

arXiv2011

A Large-Scale FPGA-Based Trigger and Dead-Time Free DAQ System for the Kaos Spectrometer at MAMI

The Kaos spectrometer is maintained by the A1 collaboration at the Mainz Microtron MAMI with a focus on the study of (e,e'K^+) coincidence reactions. For its electron-arm two vertical planes of fiber arrays, each comprising approximately 10 000 fibers, are operated close to zero degree scattering angle and in close proximity to the electron beam. A nearly dead-time free DAQ system to acquire timing and tracking information has been installed for this spectrometer arm. The signals of 144 multi-anode photomultipliers are collected by 96-channel front-end boards, digitized by double-threshold discriminators and the signal time is picked up by state-of-the-art F1 time-to-digital converter chips. In order to minimize background rates a sophisticated trigger logic was implemented in newly developed Vuprom modules. The trigger performs noise suppression, signal cluster finding, particle tracking, and coincidence timing, and can be expanded for kinematical matching (e'K^+) coincidences. The full system was designed to process more than 4 000 read-out channels and to cope with the high electron flux in the spectrometer and the high count rate requirement of the detectors. It was successfully in-beam tested at MAMI in 2009.

physics.ins-det↗

arXiv2011

Particle tracking in kaon electroproduction with cathode-charge sampling in multi-wire proportional chambers

Wire chambers are routinely operated as tracking detectors in magnetic spectrometers at high-intensity continuous electron beams. Especially in experiments studying reactions with small cross-sections the reaction yield is limited by the background rate in the chambers. One way to determine the track of a charged particle through a multi-wire proportional chamber (MWPC) is the measurement of the charge distribution induced on its cathodes. In practical applications of this read-out method, the algorithm to relate the measured charge distribution to the avalanche position is an important factor for the achievable position resolution and for the track reconstruction efficiency. An algorithm was developed for operating two large-sized MWPCs in a strong background environment with multiple-particle tracks. Resulting efficiencies were determined as a function of the electron beam current and on the signal amplitudes. Because of the different energy-losses of pions, kaons, and protons in the momentum range of the spectrometer the efficiencies depend also on the particle species.

nucl-ex↗

arXiv2010

High-precision determination of the electric and magnetic form factors of the proton

New precise results of a measurement of the elastic electron-proton scattering cross section performed at the Mainz Microtron MAMI are presented. About 1400 cross sections were measured with negative four-momentum transfers squared up to Q^2=1 (GeV/c)^2 with statistical errors below 0.2%. The electric and magnetic form factors of the proton were extracted by fits of a large variety of form factor models directly to the cross sections. The form factors show some features at the scale of the pion cloud. The charge and magnetic radii are determined to be r_E=0.879(5)(stat.)(4)(syst.)(2)(model)(4)(group) fm and r_M=0.777(13)(stat.)(9)(syst.)(5)(model)(2)(group) fm.

nucl-ex↗

arXiv2008

The HypHI project: Hypernuclear spectroscopy with stable heavy ion beams and rare isotope beams at GSI and FAIR

The HypHI collaboration aims to perform a precise hypernuclear spectroscopy with stable heavy ion beams and rare isotope beams at GSI and fAIR in order to study hypernuclei at extreme isospin, especially neutron rich hypernuclei to look insight hyperon-nucleon interactions in the neutron rich medium, and hypernuclear magnetic moments to investigate baryon properties in the nuclei. We are currently preparing for the first experiment with $^6$Li and $^{12}$C beams at 2 AGeV to demonstrate the feasibility of a precise hypernuclear spectroscopy by identifying $^{3}_Λ$H, $^{4}_Λ$H and $^{5}_Λ$He. The first physics experiment on these hypernuclei is planned for 2009. In the present document, an overview of the HypHI project and the details of this first experiment will be discussed.

nucl-ex↗

arXiv2008

Characterisation of radiation damage in silicon photomultipliers with a Monte Carlo model

Measured response functions and low photon yield spectra of silicon photomultipliers (SiPM) were compared to multi-photoelectron pulse-height distributions generated by a Monte Carlo model. Characteristic parameters for SiPM were derived. The devices were irradiated with 14 MeV electrons at the Mainz microtron MAMI. It is shown that the first noticeable damage consists of an increase in the rate of dark pulses and the loss of uniformity in the pixel gains. Higher radiation doses reduced also the photon detection efficiency. The results are especially relevant for applications of SiPM in fibre detectors at high luminosity experiments.

nucl-ex↗

arXiv2008

A new measurement of the structure functions $P_{LL}-P_{TT}/epsilon$ and $P_{LT}$ in virtual Compton scattering at $Q^2=$ 0.33 (GeV/c)$^2$

The cross section of the $ep \to e' p' γ$ reaction has been measured at $Q^2 = 0.33$ (GeV/c)$^2$. The experiment was performed using the electron beam of the MAMI accelerator and the standard detector setup of the A1 Collaboration. The cross section is analyzed using the low-energy theorem for virtual Compton scattering, yielding a new determination of the two structure functions $P_LL}-P_{TT}/epsilon$ and $P_{LT}$ which are linear combinations of the generalized polarizabilities of the proton. We find somewhat larger values than in the previous investigation at the same $Q^2$. This difference, however, is purely due to our more refined analysis of the data. The results tend to confirm the non-trivial $Q^2$-evolution of the generalized polarizabilities and call for more measurements in the low-$Q^2$ region ($\le$ 1 (GeV/c)$^2$).

nucl-ex↗

arXiv2008

Virtual Compton Scattering measurements in the $γ^* N\to Δ$ transition

We report on new H$(e,e^\prime p)γ$ measurements in the $Δ(1232)$ resonance at $Q^2=0.06$ (GeV/c) carried out simultaneously with H$(e,e^\prime p)π^0$. It is the lowest $Q^2$ for which the virtual Compton scattering (VCS) reaction has been studied in the first resonance region. The VCS measured cross sections are well described by dispersion-relation calculations in which the multipole amplitudes derived from H$(e,e^\prime p)π^0$ data are used as input, thus confirming the compatibility of the results. The derived resonant magnetic dipole amplitude $M^{3/2}_{1+} = (40.60 \pm 0.70_{stat+sys})(10^{-3}/m_{π^+})$ at $W=$ 1232 MeV is in excellent agreement with the value extracted from H$(e,e^\prime p)π^0$ measurements.

nucl-ex↗

arXiv2008

Design criteria for multi-layered scintillating fibre arrays with inclined columns

Multi-layered scintillating fibre arrays read-out are commonly used as high resolution charged particle hodoscopes. Fibres of a column along the geometrical trajectory of incident particles are typically grouped to one pixel of a multi-channel read-out device. In some applications the incident particles will cross the detection plane with large angles w.r.t. the normal to the layers. Then, the packing of the fibres needs to be adapted to the incident particles and the columns need to be inclined. In this paper possible fibre array geometries are shown, relevant design criteria for detectors are discussed, and the effect of diverging particles incident on fibre arrays was studied using a Monte Carlo simulation.

nucl-ex↗

arXiv2008

In-beam tests of scintillating fibre detectors at MAMI and at GSI

The performance of scintillating fibre detectors was studied with electrons at the spectrometer facility of the Mainz microtron MAMI, as well as in a C-12 beam of 2 AGeV energy and in a beam of different particle species at GSI. Multi-anode photomultipliers were used to read out one or more bundles of 128 fibres each in different geometries. For electrons a time resolution of FWHM = 1 ns was measured in a single detector plane with a detection efficiency epsilon > 99%. A time resolution of 310 ps (FWHM) between two planes of fibres was achieved for carbon ions, leading to a FWHM = 220 ps for a single detector. The hit position residual was measured with a width of FWHM = 0.27 mm. The variation in the measured energy deposition was Delta E/E= 15-20% (FWHM) for carbon ions. In addition, the energy response to p/pi/d particles was studied. Based on the good detector performance fibre hodoscopes will be constructed for the KAOS/A1 spectrometer at MAMI and for the HypHI experiment at GSI.

nucl-ex↗

arXiv2007

Recoil polarization and beam-recoil double polarization measurement of ηelectroproduction on the proton in the region of the S_{11}(1535) resonance

The beam-recoil double polarization P_{x'}^h and P_{z'}^h and the recoil polarization P_{y'} were measured for the first time for the p(\vec{e},e'\vec{p})ηreaction at a four-momentum transfer of Q^2=0.1 GeV^2/c^2 and a center of mass production angle of θ= 120^\circ at MAMI C. With a center of mass energy range of 1500 MeV < W < 1550 MeV the region of the S_{11}(1535) and D_{13}(1520) resonance was covered. The results are discussed in the framework of a phenomenological isobar model (Eta-MAID). While P_{x'}^h and P_{z'}^h are in good agreement with the model, P_{y'} shows a significant deviation, consistent with existing photoproduction data on the polarized-target asymmetry.

nucl-ex↗

arXiv2007

Beam-helicity asymmetry in photon and pion electroproduction in the Delta(1232) resonance region at Q^2= 0.35 (GeV/c)^2

The beam-helicity asymmetry has been measured simultaneously for the reactions (e p \to e p γ) and (e p \to e p π^0) in the $Δ(1232)$ resonance region at $Q^2=$ 0.35 (GeV/c)$^2$. The experiment was performed at MAMI with a longitudinally polarized beam and an out-of-plane detection of the proton. The results are compared with calculations based on Dispersion Relations for virtual Compton scattering and with the MAID model for pion electroproduction. There is an overall good agreement between experiment and theoretical calculations. The remaining discrepancies may be ascribed to an imperfect parametrization of some $γ^{(*)} N \to πN$ multipoles, mainly contributing to the non-resonant background. The beam-helicity asymmetry in both channels ($γ$ and $π^0$) shows a good sensitivity to these multipoles and should allow future improvement in their parametrization.

hep-ph↗

arXiv2007

Measurement of Propagation Time Dispersion in a Scintillator

One contribution to the time resolution of a scintillation detector is the signal time spread due to path length variations of the detected photons from a point source. In an experimental study a rectangular scintillator was excited by means of a fast pulsed ultraviolet laser at different positions along its longitudinal axis. Timing measurements with a photomultiplier tube in a detection plane displaced from the scintillator end face showed a correlation between signal time and tube position indicating only a small distortion of photon angles during transmission. The data is in good agreement with a Monte Carlo simulation used to compute the average photon angle with respect to the detection plane and the average propagation time. Limitations on timing performance that arise from propagation time dispersion are expected for long and thin scintillators used in future particle identification systems.

nucl-ex↗

arXiv2006

Lowest Q^2 Measurement of the gamma*p -> Delta Reaction: Probing the Pionic Contribution

To determine nonspherical angular momentum amplitudes in hadrons at long ranges (low Q^2), data were taken for the p(\vec{e},e'p)π^0 reaction in the Delta region at Q^2=0.060 (GeV/c)^2 utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios at W=1232 MeV are: M_{1+}^{3/2} = (40.33 +/- 0.63_{stat+syst} +/- 0.61_{model}) (10^{-3}/m_{π^+}),Re(E_{1+}^{3/2}/M_{1+}^{3/2}) = (-2.28 +/- 0.29_{stat+syst} +/- 0.20_{model})%, and Re(S_{1+}^{3/2}/M_{1+}^{3/2}) = (-4.81 +/- 0.27_{stat+syst} +/- 0.26_{model})%. These disagree with predictions of constituent quark models but are in reasonable agreement with lattice calculations with non-linear (chiral) pion mass extrapolations, with chiral effective field theory, and with dynamical models with pion cloud effects. These results confirm the dominance, and general Q^2 variation, of the pionic contribution at large distances.

nucl-ex↗

arXiv2006

Performance of HPGe Detectors in High Magnetic Fields

A new generation of high-resolution hypernuclear gamma$-spectroscopy experiments with high-purity germanium detectors (HPGe) are presently designed at the FINUDA spectrometer at DAPhiNE, the Frascati phi-factory, and at PANDA, the antiproton proton hadron spectrometer at the future FAIR facility. Both, the FINUDA and PANDA spectrometers are built around the target region covering a large solid angle. To maximise the detection efficiency the HPGe detectors have to be located near the target, and therefore they have to be operated in strong magnetic fields B ~ 1 T. The performance of HPGe detectors in such an environment has not been well investigated so far. In the present work VEGA and EUROBALL Cluster HPGe detectors were tested in the field provided by the ALADiN magnet at GSI. No significant degradation of the energy resolution was found, and a change in the rise time distribution of the pulses from preamplifiers was observed. A correlation between rise time and pulse height was observed and is used to correct the measured energy, recovering the energy resolution almost completely. Moreover, no problems in the electronics due to the magnetic field were observed.

nucl-ex↗

arXiv2006

Determination of quadrupole strengths in the gamma*p-->Delta(1232) transition at Q2= 0.20 (GeV/c)2

We report new precise p$(\vec{e},e^\prime p)π^0$ measurements at the peak of the $Δ^{+}(1232)$ resonance at $Q^2=\unit[0.20](GeV/c)^2$ performed at the Mainz Microtron (MAMI). The new data are sensitive to both the electric quadrupole ($E2$) and the coulomb quadrupole ($C2$) amplitudes of the $γ^* N\toΔ$ transition. They yield precise quadrupole to dipole amplitude ratios CMR $= (-5.09 \pm 0.28_{stat+sys}\pm 0.30_{model})%$ and EMR $= (-1.96 \pm 0.68_{stat+sys} \pm 0.41_{model})%$ for $M^{3/2}_{1+} = (39.57 \pm 0.75_{stat+sys}\pm 0.40_{model})(10^{-3}/m_{π^+})$. The new results are in disagreement with Constituent Quark Model predictions and in qualitative agreement with models that account for mesonic contributions, including recent Lattice calculations. They thus give further credence to the conjecture of deformation in hadronic systems favoring the attribution of the origin of deformation to the dominance of mesonic effects.

nucl-ex↗

arXiv2006

New detectors for the kaon and hypernuclear experiments with KaoS at MAMI and with PANDA at GSI

The KaoS spectrometer at the Mainz Microtron MAMI, Germany, is perceived as the ideal candidate for a dedicated spectrometer in kaon and hypernuclei electroproduction. KaoS will be equipped with new read-out electronics, a completely new focal plane detector package consisting of scintillating fibres, and a new trigger system. First prototypes of the fibre detectors and the associated new front-end electronics are shown in this contribution. The Mainz hypernuclei research program will complement the hypernuclear experiments at the planned FAIR facility at GSI, Germany. At the proposed antiproton storage ring the spectroscopy of double Lambda hypernuclei is one of the four main topics which will be addressed by the PANDA Collaboration. The experiments require the operation of high purity germanium (HPGe) detectors in high magnetic fields (B= 1T) in the presence of a large hadronic background. The performance of high resolution Ge detectors in such an environment has been investigated.

nucl-ex↗