Photon Energy Resolution Research Articles

Design, commissioning and performance of the PIBETA detector at PSI

We describe the design, construction and performance of the PIBETA detector built for the precise measurement of the branching ratio of pion beta decay, π +→π 0e +ν e, at the Paul Scherrer Institute. The central part of the detector is a 240-module spherical pure CsI calorimeter covering ∼3 π sr solid angle. The calorimeter is supplemented with an active collimator/beam degrader system, an active segmented plastic target, a pair of low-mass cylindrical wire chambers and a 20-element cylindrical plastic scintillator hodoscope. The whole detector system is housed inside a temperature-controlled lead brick enclosure, which in turn is lined with cosmic muon plastic veto counters. Commissioning and calibration data were taken during two 3-month beam periods in 1999/2000 with π + stopping rates between 1.3·10 3 π +/ s and 1.3·10 6 π +/ s . We examine the timing, energy and angular detector resolution for photons, positrons and protons in the energy range of 5– 150 MeV , as well as the response of the detector to cosmic muons. We illustrate the detector signatures for the assorted rare pion and muon decays and their associated backgrounds.

Experimental and theoretical study of the cascade Auger transitions in Kr and Xe

The Auger cascade processes following the Kr ${3d}_{5/2}\ensuremath{\rightarrow}5p$ and Xe ${4d}_{5/2}\ensuremath{\rightarrow}6p$ excitations have been studied both theoretically and experimentally. Experimental spectra have been measured with high photon and electron energy resolutions and the results have been compared with theoretical predictions obtained by multiconfiguration Dirac-Fock (MCDF) method. The MCDF approach is able to reproduce well the cascade process of Kr, but strong discrepancies are observed in the case of Xe. Some possible reasons for that are discussed.

Photoionization ofNe+using synchrotron radiation

Absolute measurements of cross sections for photoionization of a statistical admixture of ${\mathrm{Ne}}^{+}$ in the ${}^{2}{P}_{3/2}^{o}$ ground state and the ${}^{2}{P}_{1/2}^{o}$ metastable state are reported in the energy range 40--71 eV at photon energy resolutions ranging from 22 meV to 2 meV. The experiments were performed using synchrotron radiation from an undulator beamline of the Advanced Light Source with a newly developed ion-photon-beam endstation. The data are characterized by multiple Rydberg series of autoionizing resonances superimposed upon a direct photoionization background cross section where some of the observed resonance line shapes show evidence of interference between the direct and indirect photoionization channels. The resonance features are assigned spectroscopically, and their energies and quantum defects are tabulated. The experimental photoionization cross sections are in satisfactory agreement with the predictions from theoretical calculations performed in intermediate coupling using the semirelativistic Breit-Pauli approximation with ten states. The resonances nearest to the ionization thresholds exhibit anomalous behavior with respect to their positions and strengths due to the presence of interloping resonances associated with higher-lying ionic states causing disruption of the regular Rydberg spectral pattern.

3smd1Dto3png1Fautoionization resonances of Mg

Intense ultranarrow resonances have been observed in the photoionization of Mg from the singly excited $3smd{}^{1}D$ $(m=3,4,5,6)$ states to the doubly excited $3png{}^{1}F$ $(n=5,6,7)$ autoionization states using stepwise laser excitation and time-of-flight mass spectrometry detection. The reported experimental energy resolution at an order of ${10}^{\ensuremath{-}7}$ of the photon energy represents a considerable improvement over the best photon energy resolution of about one part in ${10}^{5}$ for the synchrotron radiation light sources. The ultra-narrow $3png{}^{1}F$ resonances acquire a significant strength for the normally weak $3smd{}^{1}\stackrel{\ensuremath{\rightarrow}}{D}3png{}^{1}F$ two-electron transition due to a strong configuration interaction with an overlapping broad $3pnd{}^{1}F$ autoionization series and the $3s\ensuremath{\epsilon}f{}^{1}F$ continuum. The measured line positions and transition widths are in close agreement with the theoretical results derived from a B-spline-based configuration-interaction calculation.

Beam line I411 at MAX II—performance and first results

We report on the characteristics and first results from the soft X-ray beam line I411, based on an undulator at the third generation synchrotron facility MAX II, Sweden. The beam line is designed for high-resolution, angle-resolved electron spectroscopy on gases, liquids and solids. Main components are the modified SX700 monochromator and the end station, both of which were previously used at beam line 51 at MAX I. The end station is equipped with a rotatable SES-200 hemispherical electron-analyser. Before the end station, a one-metre section is reserved for exchangeable experimental set-ups. The usable photon energy range is 50–1500 eV and the photon flux is two orders of magnitudes higher compared to beam line 51. At 400 eV a resolving power of about 5700 in the first order of the monochromator grating could be obtained. In gas phase, a total electron energy resolution of 16 meV has been achieved. Detailed results on the undulator performance, flux, photon and electron energy resolution as well as some technical details are presented here. As an example of the capabilities of the beam line I411, we present the fully vibrationally resolved Auger resonant Raman electron spectrum of gas-phase N 2.

First results of the soft X-ray microfocus beamline U41-PGM

We present the first experimental data obtained from the new microfocus beamline U41-PGM recently put into operation. The optical layout of the beamline has been designed to conserve the high spectral brillance of the undulator source. The beamline consists of a collimated plane grating monochromator (PGM) and a subsequent refocusing stage. A toroidal mirror is used for refocusing onto the sample surface. With the 600 1/mm grating an absolute photon flux between 10 12 and 10 13 photons per second has been recorded over the whole energy range. Although the monochromator is not designed to deliver highest energy resolution, a resolving power of up to 6000 at the nitrogen K-edge has been obtained recently. The fixfocus constant c ff is variable in a wide range. According to the experimental requirements it can be set to select either high photon flux or energy resolution.

Coherent and incoherent π0 photoproduction from the deuteron

Differential cross-sections for the reactions d(gamma,pi (o))d and d(gamma,pi (o))pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV > E- > 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The (o) missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d(gamma, pi (o))X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d(gamma, pi (o))X cross-sections, on, the other hand, are significantly smaller than predicted by the quasi-free process for E-gamma greater than or similar to 250 MeV. Associating this failure with the pi (o) photoproduction on the neutron would suggest that its cross-section is up to 25% below the presently believed value.

High-Resolution Inner-Shell Photoabsorption and Dissociation of Ozone

The total-ion-yield (TIY) spectrum of ozone has been recorded in the K-edge region for the first time with high photon energy resolution and without significant O2 contamination. Spectral features have been clearly observed in the high energy region below the two ionization thresholds. The assignment for the core excitation processes from the “terminal” and “central” oxygen atoms, given for the first time in the whole spectral range, is based on high level ab initio calculations. The QDPTCI (quasidegenerate perturbation theory configuration interaction) theoretical approach provided an excitation energy pattern and photoabsorption oscillator strengths that were found in good agreement with the experimental TIY spectrum. The decay dynamics of the core excited resonant states is shown to be strongly dependent on the σ/π antibonding character of the virtual MO involved in the excitation process.

Light response of pure CsI calorimeter crystals painted with wavelength-shifting lacquer

We have measured scintillation properties of pure CsI crystals used in the shower calorimeter built for a precise determination of the π +→ π 0 e + ν e decay rate at the Paul Scherrer Institute (PSI). All 240 individual crystals painted with a special wavelength-shifting solution were examined in a custom-built detection apparatus (RASTA – radioactive source tomography apparatus) that uses a 137Cs radioactive gamma source, cosmic muons and a light-emitting diode as complementary probes of the scintillator light response. We have extracted the total light output, axial light collection nonuniformities and timing responses of the individual CsI crystals. These results predict improved performance of the 3 π sr PIBETA calorimeter due to the painted lateral surfaces of 240 CsI crystals. The wavelength-shifting paint treatment did not affect appreciably the total light output and timing resolution of our crystal sample. The predicted energy resolution for positrons and photons in the energy range of 10–100 MeV was nevertheless improved due to the more favorable axial light collection probability variation. We have compared simulated calorimeter ADC spectra due to 70 MeV positrons and photons with a Monte Carlo calculation of an ideal detector light response.

Search for narrow dibaryon resonances in neutral pion photoproduction from the deuteron

The reaction γd↦π0 X has been measured with TAPS at MAMI in the energy range E γ = 140-300 MeV. Using the Glasgow tagging spectrometer a photon energy resolution of 0.8 MeV was achieved. The energy excitation functions of integral and differential total cross-sections show no structures of statistical significance > 2σ. Upper limits for the production of narrow isoscalar or isovector dibaryons with masses m? 2100 MeV/c2 were deduced. They are in the range 2-5 μb averaged over the 0.8 MeV energy resolution.

Measurements and calculations of high-angular-momentum satellite transitions in Li1sphotoionization

Lithium $1s$ photoelectron spectra are reported in high electron and photon energy resolution, with resolved LS term structure of the ${\mathrm{Li}}^{+}$ $1snl$ satellite transitions up to $n=6.$ Branching ratios and anisotropy parameters of individual lines, determined over the 85--130 eV photon energy range, are compared with R-matrix calculations and with previous works. The high-angular-momentum satellite lines $(L>~2)$ are found to contribute significantly to the $1snl$ satellite cross sections for $n=3$ and 4, and to become the dominant terms for $n>~5.$ The high-angular-momentum lines exhibit the same photon-energy-dependence as the P-lines, providing experimental evidence that the continuum-continuum state coupling (equivalent to virtual electron collision processes) is responsible for the $L>~1$ terms in the satellite spectrum, in contrast to the electron relaxation (shake-up) mechanism responsible for the S-terms. The angular distribution of the lines in the ${\mathrm{Li}}^{+}$ $1snl,$ $n=2--6$ groups, determined at 110 eV photon energy, is in good agreement with calculations, showing more isotropic distributions for high-angular-momentum lines.

Measurement of gas bremsstrahlung at the SPring-8 insertion device beamline using PWO scintillator

Gas bremsstrahlung, generated by the interaction of stored electrons with residual gas molecules in a storage ring vacuum chamber, was measured at the insertion device beamlines of SPring-8 by using a new-type scintillator PWO (PbWO 4). The fine energy resolution for the bremsstrahlung photons was obtained, which is about three times as good as that of lead glass. The measurements were made at the two beamlines, BL11XU (high-betatron function beamline) and BL46XU (low-betatron function beamline), in relation to the characteristics of the stored electron beam. The gas bremsstrahlung was measured as a function of the aperture size of the slits, showing the apparent dependence on the conditions of the stored electron beam, as well as the beam current and the vacuum pressure. The measurements of the aperture size dependency were compared with the calculations by using the Monte Carlo code EGS4, to show a fairly good agreement. The total power of the gas bremsstrahlung was estimated as 25.8±0.8 nW/10 −8 Pa/mA and the corresponding equivalent dose rates for a scoring radius of 0.023 cm were calculated to be 15.8 nSv/s/10 −8 Pa/mA for BL11XU, 9.22 nSv/s/10 −8 Pa/mA for BL46XU. Calculations were also made to obtain 25.9 nSV/s/10 −8 Pa/mA neglecting the stored beam conditions at 40 m from the center of the straight section of SPring-8.

High-resolution angle-resolved ion-yield measurements of H 2O and D 2O in the region of O 1s to Rydberg transitions

Angle-resolved energetic-ion yield spectra have been observed in the O 1s excitation region of H 2O and D 2O with the incident photon-energy resolution better than 14 000 . Vibrational structures appear in most of the Rydberg members and are ascribed to the bending vibrations. The assignments of the electronic states are established on the basis of the angular distribution data for the energetic ions.

The bremsstrahlung tagged photon beam in Hall B at JLab

We describe the design and commissioning of the photon tagging beamline installed in experimental Hall B at the Thomas Jefferson National Accelerator Facility (JLab). This system can tag photon energies over a range from 20% to 95% of the incident electron energy, and is capable of operation with beam energies up to 6.1 GeV. A single dipole magnet is combined with a hodoscope containing two planar arrays of plastic scintillators to detect energy-degraded electrons from a thin bremsstrahlung radiator. The first layer of 384 partially overlapping small scintillators provides photon energy resolution, while the second layer of 61 larger scintillators provides the timing resolution necessary to form a coincidence with the corresponding nuclear interaction triggered by the tagged photon. The definitions of overlap channels in the first counter plane and of geometric correlation between the two planes are determined using digitized time information from the individual counters. Auxiliary beamline devices are briefly described, and performance results to date under real operating conditions are presented. The entire photon-tagging system has met or exceeded its design goals.

SB7: the new bending-magnet double-headed dragon beamline at SuperACO

A new beamline is now operational at LURE on bending-magnet 7 of the SuperACO storage ring in Orsay. The monochromator is of the ;dragon' type, designed to work in the energy range 150-1500 eV, and is equipped with a double vertical focusing mirror in order to allow for fast switching of light polarization. In this paper the results of photoion-yield experiments performed on N(2), Ar and Ne gases are presented. The observed resonance structures enable us to determine the available energy resolution: a resolving power higher than 8500 is obtained at the Ar 2p, N 1s and Ne K-edges when using all the optical elements at full aperture. The total flux as a function of the measured photon energy resolution and the characterization of the double-head behaviour in delivering circularly polarized light are reported.

Auger decay of the C 1s−12π* resonance in carbon monoxide: Vibrationally and angularly resolved spectra

Auger electron spectra from the decay of the ν=0, 1, and 2 levels of the C 1s−12π* state were measured with sufficiently high electron and photon energy resolution to completely resolve the vibrational structure of the final electronic states. The results are compared with ab initio calculations with emphasis on the analysis of the spectator Auger transitions. The anisotropy parameters extracted from angle-resolved spectra show variations within the vibrational envelopes of the participator Auger decay and exhibit a complex oscillating behavior for the spectator Auger transitions. In addition, two-dimensional (2D) imaging of the electron emission across the C 1s−12π* resonance was performed. The ratio of resonant and nonresonant ionization processes is estimated from the 2D map. Strong vibrational lifetime interference effects apparent in the 2D imaging are discussed.

High-resolution calorimetry: limitations of doped semiconductor thermometers

Small thermal calorimeters operating at cryogenic temperatures have achieved an energy resolution for single X-ray photons that is a factor of 20 better than the theoretical limit for a silicon ionization detector. To determine the potential for further improvements and decide on likely routes for achieving them, we discuss detector design optimization, first for an ideal calorimeter, and then for the case where components exhibit non-ideal behavior. Two serious non-ideal properties of doped semiconductor thermometers are electron–phonon decoupling and excess noise. These have been characterized over a range of sensitivity and operating temperature, and their effects on design optimization and ultimate performance can be evaluated.

Results from the photoemission spectroscopy beamline 2B1 at Pohang Light Source

Abstract The results of photoemission spectroscopy using molybdenum and tantalum samples have been obtained from the new beamline 2B1 at Pohang Light Source. Beamline 2B1 is based on a spherical grating monochromator (SGM) which is equipped with five gratings. The photon energy range from 184 to 1100 eV was covered in this work using two gratings (Gratings 4 and 5). The photon energy resolution has been deduced from Ta Fermi-level spectra and 3d spectra of Mo.

3d photoionization of Xe, Cs and Ba and the collapse of the 4f wavefunction

Total photoion yield spectra in the vicinity of the 3d thresholds of Xe, Cs and Ba are presented. These spectra were recorded with higher photon energy resolution than previously reported. For xenon the new experimental data reveal additional structures. Employing Hartree-Fock calculations, the origin of the prominent resonances is explained: for Xe and Cs the main absorption features are caused by shape resonances above threshold, whereas for Ba it is revealed that the 4f orbital has finally collapsed and the sharp resonances are caused by discrete excitations into bound states.

The character of the Xe resonances studied with the aid of their Auger decay

Theoretical predictions for the Xe photoexcitation are shown to depend strongly on the gauge and wavefunctions used in calculations. In order to test the theory, the resonant Auger decay of the Xe states has been measured with high photon and electron energy resolution using the Finnish beamline at MAX-lab in Lund, Sweden. The character of the excited states is investigated by comparing the observed intensity distributions of the transitions to the states with the results of multiconfiguration Dirac-Fock calculations. As the fine structure of resonant Auger transitions is sensitive to the nature of the spectator electron, the spectra are useful for the assignment of the resonant photoabsorption peaks which do not show any characteristics of the excited electron.