Pair Spectrometer Research Articles

Checking the 8Be Anomaly with a Two-Arm Electron Positron Pair Spectrometer

We have repeated the experiment performed recently by ATOMKI Laboratory (Debrecen, Hungary), which may indicate a new particle called X17 in the literature. In order to obtain a reliable and independent result, we used a different structure of the electron–positron pair spectrometer at the VNU University of Science. The spectrometer has two arms and simpler acceptance and efficiency as a function of the correlation angle, but the other conditions of the experiment were very similar to the published ones. We could confirm the presence of the anomaly measured at Ep = 1225 keV, which is above the Ep = 1040 keV resonance.

The GlueX beamline and detector

The GlueX experiment at Jefferson Lab has been designed to study photoproduction reactions with a 9-GeV linearly polarized photon beam. The energy and arrival time of beam photons are tagged using a scintillator hodoscope and a scintillating fiber array. The photon flux is determined using a pair spectrometer, while the linear polarization of the photon beam is determined using a polarimeter based on triplet photoproduction. Charged-particle tracks from interactions in the central target are analyzed in a solenoidal field using a central straw-tube drift chamber and six packages of planar chambers with cathode strips and drift wires. Electromagnetic showers are reconstructed in a cylindrical scintillating fiber calorimeter inside the magnet and a lead-glass array downstream. Charged particle identification is achieved by measuring energy loss in the wire chambers and using the flight time of particles between the target and detectors outside the magnet. The signals from all detectors are recorded with flash ADCs and/or pipeline TDCs into memories allowing trigger decisions with a latency of 3.3 μs. The detector operates routinely at trigger rates of 40 kHz and data rates of 600 megabytes per second. We describe the photon beam, the GlueX detector components, electronics, data-acquisition and monitoring systems, and the performance of the experiment during the first three years of operation.

Evidence for shape coexistence and superdeformation in 24Mg

The E0 transition depopulating the first-excited 0+ state in 24Mg has been observed for the first time, and the E0 transition strength determined by electron-positron pair and γ-ray spectroscopy measurements performed using the Super-e pair spectrometer. The E0 transition strength is ρ2×103=380(70). A two-state mixing model implies a deformation of the first-excited 0+ state of β2≈1 and a change in the mean-square charge radius of Δ〈r2〉≈1.9fm2, which suggests a significant shape change between the ground state and first-excited 0+ state in 24Mg. The observed E0 strength gives direct evidence of shape coexistence and superdeformation in 24Mg, bringing this nucleus into line with similar behaviour in nearby N=Z nuclei. This result agrees with recent theoretical work on the cluster nature of 24Mg and has potential ramifications for nuclear reactions of astrophysical importance.

Improved precision on the experimental E0 decay branching ratio of the Hoyle state

Background: Stellar carbon synthesis occurs exclusively via the 3α process, in which three α particles fuse to form C12 in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the α threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the E0 decay branching ratio of the Hoyle state, and the current 10% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The rate of the 3α process is an important input parameter in astrophysical calculations on stellar evolution, and a high precision is imperative to constrain the possible outcomes of astrophysical models. Purpose: To deduce a new, more precise value for the E0 decay branching ratio of the Hoyle state. Method: The E0 branching ratio was deduced from a series of pair conversion measurements of the E0 and E2 transitions depopulating the 02+ Hoyle state and 21+ state in C12, respectively. The excited states were populated by the C12(p,p′) reaction at 10.5 MeV beam energy, and the pairs were detected with the electron-positron pair spectrometer, Super-e, at the Australian National University. The deduced branching ratio required knowledge of the proton population of the two states, as well as the alignment of the 21+ state in the reaction. For this purpose, proton scattering and γ-ray angular distribution experiments were also performed. Results: An E0 branching ratio of ΓπE0/Γ=8.2(5)×10−6 was deduced in the current work, and an adopted value of ΓπE0/Γ=7.6(4)×10−6 is recommended based on a weighted average of previous literature values and the new result. Conclusions: The new recommended value for the E0 branching ratio is about 14% larger than the previous adopted value of ΓπE0/Γ=6.7(6)×10−6, while the uncertainty has been reduced from 9% to 5%. The new result reduces the radiative width, and hence 3α reaction rate, by 11% relative to the adopted value, and the uncertainty to 6.1%. This reduction in width and increased precision is likely to constrain possible outcomes of astrophysical calculations.6 MoreReceived 28 May 2020Accepted 29 July 2020DOI:https://doi.org/10.1103/PhysRevC.102.024320©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsNuclear astrophysicsNuclear structure & decaysSpectroscopic factors & electromagnetic momentsProperties6 ≤ A ≤ 19Nuclear Physics

The magnetic toroidal sector as a broad-band electron–positron pair spectrometer I. lepton trajectories

We report an analysis of electron-optical properties of a toroidal magnetic sector spectrometer and examine parameters for its implementation in a relativistic heavy-ion storage ring like HESR. For studies of free-free pair production in heavy-ion atom collisions this spectrometer exhibits very high efficiencies for coincident $e^+e^-$ pair spectroscopy over a wide range of momenta of emitted lepton pairs. The high coincidence efficiency of the spectrometer is the key for stringent tests of theoretical predictions for the coincident positron- and electron- emission characteristics and for the phase space correlation of lepton vector momenta in free-free pair production.

Design and construction of a high-energy photon polarimeter

We report on the design and construction of a high-energy photon polarimeter for measuring the degree of polarization of a linearly-polarized photon beam. The photon polarimeter uses the process of pair production on an atomic electron (triplet production). The azimuthal distribution of scattered atomic electrons following triplet production yields information regarding the degree of linear polarization of the incident photon beam. The polarimeter, operated in conjunction with a pair spectrometer, uses a silicon strip detector to measure the recoil electron distribution resulting from triplet photoproduction in a beryllium target foil. The analyzing power ΣA for the device using a 75 μm beryllium converter foil is about 0.2, with a relative systematic uncertainty in ΣA of 1.5%.

The silicon photomultipliers in the detector subsystems of the GlueX experiment

The subsystem detectors of GlueX experiment use silicon photomultiplier (SiPM). Around five thousand SiPM’s in total uses in the detectors of experiment. The detectors operate in condition of load level 2MHz and up to 104 pixels «fired» with ∼0.3ns time resolution. The list of such detectors: the tagger microscope, the pair spectrometer, the start counter which surrounded the liquid hydrogen target; the electromagnetic barrel calorimeter to measure energy and direction of secondary photons comes from the target. We present the results of the time resolution measurements and the relaxation time measurements for two SiPM types in experimental conditions.

Commissioning of the Pair Spectrometer of the GlueX experiment

The main goal of the pair spectrometer of the GlueX experiment at Jefferson Lab is to determine the photon beam flux and to measure beam polarization. We present the design of the pair spectrometer and the performance results during fist commissioning runs of the GlueX experiment.

Possible explanation of the electron positron anomaly at 17 MeV in 8 Be transitions through a light pseudoscalar

We estimate the values of Yukawa couplings of a light pseudoscalar A with a mass of about 17 MeV, which would explain the 8 Be anomaly observed in the Atomki pair spectrometer experiment. The resulting couplings of A to up and down type quarks are about 0.3 times the coupling of the standard Higgs boson. Then constraints from K and B decays require that loop contributions to flavour changing vertices cancel at least at the 10% level. Constraints from beam dump experiments require the coupling of A to electrons to be larger than about 4 times the coupling of the standard Higgs boson, leading to a short enough A life time consistent with an explanation of the anomaly.

Performance of the pair spectrometer of the GlueX experiment

The description of the pair spectrometer of the GlueX detector at Jefferson Lab and its performance during the first beam commissioning runs are presented. We measured the amount of light collected from each channel of the pair spectrometer hodoscopes and the time resolution of the pair spectrometer counters.

A pair spectrometer for measuring multipolarities of energetic nuclear transitions

An electron–positron pair spectrometer has been designed and constructed for the simultaneous measurement of energy- and angular correlations of e+e- pairs. Experimental results are obtained over a wide angular range for high-energy transitions in 16O, 12C and 8Be. The results showed that the angular correlations between 50 and 180°of the e+e- pairs in the energy range between 6 and 18MeV can be determined with sufficient resolution and efficiency in good agreement with the GEANT simulations.

Pair spectrometer hodoscope for Hall D at Jefferson Lab

We present the design of the pair spectrometer hodoscope fabricated at Jefferson Lab and installed in the experimental Hall D. The hodoscope consists of thin scintillator tiles; the light from each tile is collected using wave-length shifting fibers and detected using a Hamamatsu silicon photomultiplier. Light collection was measured using relativistic electrons produced in the tagger area of the experimental Hall B.

Application of the Silicon Photomultipliers for Detectors in the GlueX Experiment

The GlueX detector in Hall D at Jefferson Lab [1]is instrumented with about 5000 Silicon Photomultipliers (SiPM) manufactured by Hamamatsu Corporation [2]. These photo sensors have properties similar to conventional photomultipliers but can be operated at high magnetic fields. Silicon photomultipliers with a sensitive area of 3x3 mm2 are used to detect light from the following GlueX scintillator detectors: the tagger microscope, pair spectrometer, and start counter. Arrays of 4x4 SiPMs sensors were chosen for the instrumentation of the barrel electromagnetic calorimeter. The tagger microscope must operate at high rates (up to 2.5MHz) and provide time measurements with a resolution better than 0.3ns. The paper will describe some results of the characterization of SiPMs for various GlueX sub-detectors.

High precision photon flux determination for photon tagging experiments

The Jefferson Laboratory PrimEx Collaboration has developed and implemented a method to control the tagged photon flux in photoproduction experiments at the 1% level over the photon energy range from 4.9 to 5.5GeV. This method has been successfully implemented in a high precision measurement of the neutral pion lifetime. Here, we outline the experimental equipment and the analysis techniques used to accomplish this. These include the use of a total absorption counter for absolute flux calibration, a pair spectrometer for online relative flux monitoring, and a new method for post-bremsstrahlung electron counting.

Towards the pair spectroscopy of the Hoyle state in12C

The triple–alpha process leading to the formation of stable carbon in the Universe is one of the most important nuclear astrophysical processes. The radiative width of the so–called Hoyle state, involving the 7.654 MeV E0 and the 3.2148 MeV E2 transitions, is known with 10–12% accuracy. A novel, more direct approach to determining the width is proposed here, based on the measurement of the E0 and the E2 internal pair conversion intensities. We report on the development of a new magnetic pair spectrometer with high sensitivity for electron–positron pairs and with excellent energy resolution.

Measurements of the pair production cross section close to the threshold energy

We have measured the total cross section for production of ${e}^{+}{e}^{\ensuremath{-}}$ pairs by photons in the Coulomb field of Ge nuclei. Up to 10${}^{16}$ $\ensuremath{\gamma}$ quanta per second were produced by thermal-neutron capture reactions of a natural gadolinium target, placed inside the high-flux reactor of the ILL Grenoble. Out of this stream of $\ensuremath{\gamma}$ rays various strong discrete $\ensuremath{\gamma}$ lines were extracted using the GAMS5 Bragg spectrometer of the ILL. A well-collimated beam of reflected $\ensuremath{\gamma}$ rays irradiated a germanium detector placed inside an anti-Compton shield, consisting of eight bismuth germanate crystals, assembled as a pair spectrometer. Using this new technique we have measured, for the first time, the pair production cross section at only 18.2 keV above the 1022-keV threshold. Our measurements indicate that the pair production cross section, expressed in units of Bethe-Heitler, grows monotonously toward low energies and does not support the claim of other works of a maximum at $\ensuremath{\sim}$65 keV.

High-energy x-ray backlighter spectrum measurements using calibrated image plates

The x-ray spectrum between 18 and 88 keV generated by a petawatt laser driven x-ray backlighter target was measured using a 12-channel differential filter pair spectrometer. The spectrometer consists of a series of filter pairs on a Ta mask coupled with an x-ray sensitive image plate. A calibration of Fuji™ MS and SR image plates was conducted using a tungsten anode x-ray source and the resulting calibration applied to the design of the Ross pair spectrometer. Additionally, the fade rate and resolution of the image plate system were measured for quantitative radiographic applications. The conversion efficiency of laser energy into silver Kα x rays from a petawatt laser target was measured using the differential filter pair spectrometer and compared to measurements using a single photon counting charge coupled device.

Energy calibration of the JLab bremsstrahlung tagging system

In this report, we present the energy calibration of the Hall B bremsstrahlung tagging system at the Thomas Jefferson National Accelerator Facility. The calibration was performed using a magnetic pair spectrometer. The tagged photon energy spectrum was measured in coincidence with e+e- pairs as a function of the pair spectrometer magnetic field. Taking advantage of the internal linearity of the pair spectrometer, the energy of the tagging system was calibrated at the level of ±0.1%Eγ. The absolute energy scale was determined using the e+e- rate measurements close to the end-point of the photon spectrum. The energy variations across the full tagging range were found to be <3MeV.

Spectral microthermography for component discrimination and hot spot identification in integrated circuits

We demonstrate an efficient and versatile spectral microthermography technique for identifying hot and cold spots in the active layer of a biased integrated circuit. Hot (cold) spots are regions where heat accumulates more rapidly (slowly) than the average rate of the entire active layer. Knowledge of the hot and cold spot locations is crucial in assessing the thermal integrity of a layer structure because hot spots are locations were defects are more likely to develop. The active layer is uniformly illuminated with light from a tungsten lamp and its reflectance image r(x, y) is scanned across (x-direction) the entrance slit of a grating-prism pair (GRISM) spectrometer to produce a spectral map R(lambda; x, y) where lambda is the wavelength [450 </= lambda (nm) </= 650]. For a particular slit position x = x(1), the GRISM spectrometer outputs a one-dimensional spectral map R (lambda; x(1), y). A pair of maps R(ub) (lambda; x, y) and R(b)(lambda; x, y) are obtained from the active layer in the absence and presence of voltage bias, respectively. A reflectance gradient map R(lambda; x, y) = R(b)(lambda; x, y) - R(ub)(lambda; x, y), is derived and used to locate possible hot and cold spots because R(lambda; x, y) is proportional to the temperature gradient T(lambda; x, y). We use the technique to generate gradient maps of a photodiode array and the emitting surface of a biased light emitting diode. Two different semiconductor materials could be distinguished easily from their dissimilar reflectance spectra.

Levels of [formula omitted] populated in average resonance neutron capture

The primary γ rays and energy levels in Yb 174 were investigated in average resonance neutron capture using tailored neutron beams at 2 and 24 keV and a pair spectrometer at the High Flux Reactor at Brookhaven. A thermal neutron beam was also used. Nearly 300 levels with spin up to J = 4 are reported below 4 MeV. The distribution of radiative strength in this nucleus was derived and is analyzed with a giant dipole resonance and single-particle strength models. The neutron binding energy in Yb 174 was determined at 7464.4 ( 3 ) keV .