0000000000469686
AUTHOR
M. Biroth
Measurement of Compton scattering at MAMI for the extraction of the electric and magnetic polarizabilities of the proton
A precise measurement of the differential cross-sections $d\sigma/d\Omega$ and the linearly polarized photon beam asymmetry $\Sigma_3$ for Compton scattering on the proton below pion threshold has been performed with a tagged photon beam and almost $4\pi$ detector at the Mainz Microtron. The incident photons were produced by the recently upgraded Glasgow-Mainz photon tagging facility and impinged on a cryogenic liquid hydrogen target, with the scattered photons detected in the Crystal Ball/TAPS set-up. Using the highest statistics Compton scattering data ever measured on the proton along with two effective field theories (both covariant baryon and heavy-baryon) and one fixed-$t$ dispersion …
On the operation of silicon photomultipliers at temperatures of 1–4 kelvin
Abstract SiPM operation at cryogenic temperatures fails for many common devices. A particular type from Zecotek with deep channels in the silicon substrate instead of quenching resistors was tested at liquid helium temperature. Two similar types were thoroughly characterized from room temperature down to liquid nitrogen temperature by illuminating them with low light levels. At cryogenic temperatures the SiPMs show an unchanged rise-time and a fast recovery time, practically no after-pulses, and exhibit no increased cross-talk probability. Charge collection spectra were measured to extract the pixel gain and its variation, both comparable to room temperature at the same over-voltage. The qu…
Characterization of SiPM properties at liquid nitrogen temperature
SiPM operation at cryogenic temperatures fails for many common devices. A particular type with deep channels in the silicon substrate instead of quenching resistors was thoroughly characterized from room temperature down to liquid nitrogen temperature by illuminating it with low light levels. The devices were mounted in vacuum with the temperature stabilized to allow long-term operation. SiPM signals from a LED pulser were acquired with single-pixel resolution. Generalized fits to the charge collection spectra were used to extract properties like single-pixel gain, inter-pixel variation, breakdown voltage, and photon detection efficiency. With these measurements a deeper investigation of th…
Helicity-dependent cross sections for the photoproduction of π0 pairs from nucleons
The double-polarization observable $E$ and helicity-dependent cross sections $\sigma_{1/2}$, $\sigma_{3/2}$ have been measured for the photoproduction of $\pi^0$ pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost $4\pi$ covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the $\gamma N\rightarrow N\pi^0\pi^0$ reaction. They are compared to predictions from re…
Insight into the narrow structure in η-photoproduction on the neutron from helicity\ud dependent cross sections
The double polarization observable E and the helicity dependent cross sections σ1/2 and σ3/2 were measured for η photoproduction from quasifree protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η→2γ and η→3π0. The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ1/2 and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that …
Modeling and Characterization of SiPM Parameters at Temperatures between 95 K and 300 K
The modeling and characterization of silicon photomultipliers (SiPMs) in a wide temperature range from 95 K to 300 K is presented. The devices under study had the distinctive feature of forward-biased p-n junctions situated under each pixel as active quenching resistors making them particularly appropriate to be operated at cryogenic temperatures. The voltage drop across the diode in a forward direction was measured for a series of injected currents in this temperature range. It was observed that the characteristics of different SiPM types influence the temperature dependence of the reverse saturation current. The devices were further characterized by low-level light-pulse measurements. The…
Deuteron photodisintegration by polarized photons in the region of the $d^*(2380)$
We report the first large-acceptance measurement of the beam–spin asymmetry for deuteron photodisintegration (γ→d→pn) in the photon energy range 420<Eγ<620MeV. The measurement provides important new constraints on the mechanisms of photodisintegration above the Δ resonance and on the photocoupling of the recently discovered d⁎(2380) hexaquark.
Insight into the Narrow Structure in η Photoproduction on the Neutron from Helicity-Dependent Cross Sections
The double polarization observable E and the helicity dependent cross sections σ1=2 and σ3=2 were measured for η photoproduction from quasifree protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η → 2γ and η → 3π0. The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ1=2 and hence, most likely related to a spin-1=2 amplitude. Nucleon resonances t…
Single π0 production off neutrons bound in deuteron with linearly polarized photons
The quasifree γ→d→π0n(p) photon beam asymmetry, Σ, has been measured at photon energies, Eγ, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49∘ to 148∘. In this kinematic region, polarization observables are sensitive to contributions from the Δ(1232) and N(1440) resonances. The extracted values of Σ have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID and Bonn–Gatchina anal…
Detector response of Cherenkov radiators for calorimetry in the energy range below 14 MeV
Abstract A study of the detector response of PbF 2 crystals and three different types of lead glass blocks to electrons from a 14-MeV beam of the Mainz Microtron MAMI is presented. For the first time, signal height, signal width, and homogeneity of the response of these Cherenkov radiators were determined for energies between 10 and 14 MeV. To complement the beam tests, optical properties of the materials, in particular measured transmittances in the near UV and visible spectrum, were studied. The measured detector responses were also compared to Monte Carlo simulations of energy-loss, light production, transport, and detection. These Cherenkov radiators are considered as active material o…
First measurement of the polarization observable E and helicity-dependent cross sections in single π 0 photoproduction from quasi-free nucleons
The double-polarization observable $E$ and the helicity-dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$ have been measured for the first time for single $\pi^{0}$ photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the $\pi^0$ meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the $\pi^{0}N$ final state. A comparison to data mea…
Performance of sensl C-Series SiPM with high photoelectron resolution at cryogenic temperatures
The C-Series of silicon photomultipliers (SiPM) from SensL provides devices with a fast response and high performance at low cost. The device's ability to detect light at temperatures of liquid nitrogen (77K) and liquid helium (4 K) with high photoelectron resolution was demonstrated. Results include relative photon detection efficiency (PDE), gain, microcell capacitance, and cross-talk probability at different over-voltages, both at room and at cryogenic temperatures. At 77K the SiPM demonstrated significantly improved operating characteristics while at 4K the observed increase in break-down voltage, the reduction of PDE by a factor of 2-3, and the extensively dropped microcell capacitance…
Insight into the narrow structure in {\boldmath{$\eta$}}-photoproduction on the neutron from helicity dependent cross sections
The double polarization observable $E$ and the helicity dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$ were measured for $\eta$ photoproduction from quasi-free protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost $4\pi$ detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from $\eta\rightarrow 2\gamma$ and $\eta\rightarrow 3\pi^0$. The results show that the narrow structure previously observed in $\eta$ photoproduction from the neutron is only apparent in $\sigma_…
Novel optical interferometry of synchrotron radiation for absolute electron beam energy measurements
Abstract A novel interferometric method is presented for the measurement of the absolute energy of electron beams. In the year 2016, a pioneering experiment was performed using a 195 MeV beam of the Mainz Microtron (MAMI). The experimental setup consisted of two collinear magnetic undulators as sources of coherent optical synchrotron light and a high-resolving grating monochromator. Beam energy measurements required the variation of the relative undulator distance in the decimeter range and the analysis of the intensity oscillation length in the interference spectrum. A statistical precision of 1 keV was achieved in 1 h of data taking, while systematic uncertainties of 700 keV were present …
Photoproduction of π 0 mesons off protons and neutrons in the second and third nucleon resonance regions
Photoproduction of mesons off quasi-free nucleons bound in the deuteron allows to study the electromagnetic excitation spectrum of the neutron and the isospin structure of the excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets. Single $\pi^0$ photoproduction off quasi-free nucleons from the deuteron was experimentally studied. Nuclear effects were investigated by a comparison of the results for free protons and quasi-free protons and used as a correction for the quasi-free neutron data. The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator for photon energies between 0.45~GeV and 1.4~GeV, using an…
First measurement of helicity-dependent cross sections in Photoproduction From Quasi-free Nucleons
The helicity-dependent cross sections for the photoproduction of π0η pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball – TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the σ1/2 (photon and target spin antiparallel) and σ3/2 (parallel spins) components to be measured for quasi-free production of π0η pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute cou…
Electron beam studies of light collection in a scintillating counter with embedded fibers
The light collection of several fiber configurations embedded in a box-shaped plastic scintillating counter was studied by scanning with minimum ionizing electrons. The light was read out by silicon photomultipliers at both ends. The light yield produced by the 855-MeV beam of the Mainz Microtron showed a strong dependence on the transverse distance from the beam position to the fibers. The observations were modeled by attributing the collection of indirect light inside of the counter and of direct light reaching a fiber to the total light yield. The light collection with fibers was compared to that of a scintillating counter without fibers. These studies were carried out within the develop…
Measurement of the beam-helicity asymmetry in photoproduction of π0η pairs on carbon, aluminum, and lead
The beam-helicity asymmetry was measured, for the first time, in photoproduction of $\pi^{0}\eta$ pairs on carbon, aluminum, and lead, with the A2 experimental setup at MAMI. The results are compared to an earlier measurement on a free proton and to the corresponding theoretical calculations. The Mainz model is used to predict the beam-helicity asymmetry for the nuclear targets. The present results indicate that the photoproduction mechanism for $\pi^{0}\eta$ pairs on nuclei is similar to photoproduction on a free nucleon. This process is dominated by the $D_{33}$ partial wave with the $\eta\Delta(1232)$ intermediate state.
A low-noise and fast pre-amplifier and readout system for SiPMs
Abstract To operate silicon photomultipliers (SiPMs) in a demanding environment with large temperature gradients, different amplifier concepts were characterized by analyzing SiPM pulse-shapes and charge distributions. A fully differential 4-wire SiPM pre-amplifier with separated tracks for the bias voltage and with good common-mode noise suppression was developed and successfully tested. To achieve highest single-pixel resolutions an online after-pulse and pile-up suppression was realized with fast readout electronics based on digital filters.
Characterisation and calibration of a scintillating fibre detector with > 4000 multi-anode photomultiplier channels
In the Kaos spectrometer at the Mainz Microtron a high-resolution coordinate detector for high-energy particles is operated. It consists of scintillating fibres with diameters of 4000 multi-anode photomultiplier channels. It is one of the most modern focal-plane detectors for magnetic spectrometers world-wide. To correct variations in the detection efficiency, caused by the different gains and the different optical transmittances, a fully automated off-line calibration procedure has been developed. The process includes the positioning of a radioisotope source alongside the detector plane and the automated acquisition and analysis of the detector signals. It was possible to characterise and …
Operation and characterization of a windowless gas jet target in high-intensity electron beams
Abstract A cryogenic supersonic gas jet target was developed for the MAGIX experiment at the high-intensity electron accelerator MESA. It will be operated as an internal, windowless target in the energy-recovering recirculation arc of the accelerator with different target gases, e.g., hydrogen, deuterium , helium, oxygen, argon, or xenon. Detailed studies have been carried out at the existing A1 multi-spectrometer facility at the electron accelerator MAMI. This paper focuses on the developed handling procedures and diagnostic tools, and on the performance of the gas jet target under beam conditions. Considering the special features of this type of target, it proves to be well suited for a n…
Helicity-dependent cross sections and double-polarization observable E in eta photoproduction from quasi-free protons and neutrons
Precise helicity-dependent cross sections and the double-polarization observable $E$ were measured for $\eta$ photoproduction from quasi-free protons and neutrons bound in the deuteron. The $\eta\rightarrow 2\gamma$ and $\eta\rightarrow 3\pi^0\rightarrow 6\gamma$ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were de…
Measurement of the π0→e+e−γ Dalitz decay at the Mainz Microtron
The Dalitz decay π0→e+e−γ has been measured in the γp→π0p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the π0 electromagnetic transition form factor, aπ=0.030±0.010tot, is in agreement with existing measurements of this decay and with recent theoretical calculations. The uncertainty obtained in the value of aπ is lower than in previous results based on the π0→e+e−γ decay.
Gain stabilization and noise minimization for SiPMs at cryogenic temperatures
Abstract The performance of solid-state photon detectors such as avalanche photodiodes or silicon photomultipliers (SiPMs) is strongly affected by temperature. Important device characteristics for the detection of low light levels or single photons are photon detection efficiency, dark noise, and gain. In the present work the C-series SiPMs from SensL was characterized in cryogenic environments. At 77 K the SiPMs proved to be an excellent choice for single photon detection and an operation point with minimum noise contributions was found. At 4 K the performance was degraded, exhibiting a smaller gain and a larger noise.
Insight into the Narrow Structure in eta Photoproduction on the Neutron from Helicity-Dependent Cross Sections
The double polarization observable E and the helicity dependent cross sections σ1/2 and σ3/2 were measured for η photoproduction from quasifree protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η→2γ and η→3π0. The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ1/2 and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that …
Compton Scattering Program Studying Nucleon Polarizabilities
The A2 collaboration at the Mainz Microtron has undertaken a program of Compton scattering experiments with the goal of extracting the proton and, in the future, neutron polarizabilities. These are fundamental parameters that describe the response of the nucleon to an electromagnetic field. An extraction of these parameters will provide a crucial test of QCD theories. This talk presented the concept and status of this program, including some preliminary results.
First measurement of helicity-dependent cross sections in π0η photoproduction from quasi-free nucleons
The helicity-dependent cross sections for the photoproduction of $\pi^0\eta$ pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the $\sigma_{1/2}$ (photon and target spin antiparallel) and $\sigma_{3/2}$ (parallel spins) components to be measured for quasi-free production of $\pi^0\eta$ pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant pr…
Signatures of the d*(2380) Hexaquark in d(γ,pn→)
We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300-700 MeV provides the first measurement of recoil neutron polarization at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarization in a narrow structure centered around E_{γ}∼570 MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition sugge…
Silicon photomultiplier properties at cryogenic temperatures
Abstract The properties of different types of silicon photomultipliers (SiPMs) were studied at cryogenic temperatures. In liquid nitrogen at 77 K, problems with quenching in Hamamatsu SiPMs and with the protective epoxy layer, covering Zecotek SiPMs, were observed. Tests with one Zecotek SiPM were successful after removal of the epoxy layer. In liquid helium at 4 K, fast signals with pulse lengths shorter than 50 ns were observed, the dark count rate was below 10 Hz and no after-pulses were detected. The gain, as a function of over-voltage, was comparable to room temperature. The SiPM׳s response to photons was found to be linear with intensity for low light levels and single-photon detectio…