Instrumentation for high-resolution laser spectroscopy at the ALTO radioactive-beam facility
D. T. Yordanov, D. Atanasov, M. L. Bissell, S. Franchoo, G. Georgiev, A. Kanellakopoulos, S. Lechner, E. Minaya Ramirez, D. Nichita, L. V. Rodríguez, A. Said
IInstrumentation for high-resolution laser spectroscopy at the ALTO radioactive-beam facility
D. T. Yordanov, ∗ D. Atanasov, M. L. Bissell, S. Franchoo, G. Georgiev, A. Kanellakopoulos, S. Lechner,
2, 6
E. Minaya Ramirez, D. Nichita,
7, 8
L. V. Rodríguez, and A. Said Institut de Physique Nucléaire, CNRS-IN2P3, Université Paris-Sud, Université Paris-Saclay, Orsay, France Experimental Physics Department, CERN, Geneva, Switzerland School of Physics and Astronomy, The University of Manchester, Manchester, United Kingdom CSNSM, CNRS-IN2P3, Université Paris-Sud, Université Paris-Saclay, Orsay, France Instituut voor Kern- en Stralingsfysica, KU Leuven, Leuven, Belgium Technische Universität Wien, Wien, Austria ELI-NP, Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, Magurele, Romania Doctoral School in Engineering and Applications of Lasers and Accelerators,University Polytechnica of Bucharest, Bucharest, Romania Max-Planck-Institut für Kernphysik, Heidelberg, Germany (Dated: May 8, 2020)
Collinear laser spectroscopy is one of the essential tools for nuclear-structure studies. It allows nu-clear electromagnetic properties of ground and isomeric states to be extracted with high experimentalprecision. Radioactive-beam facilities worldwide strive to introduce such capabilities or to improveexisting ones. Here we present the implementation of collinear laser spectroscopy at the ALTO re-search laboratory, along with data from successful off-line commissioning using sodium beam. Theinstrumental constituents are discussed with emphasis on simple technical solutions and maximizeduse of standard equipment. Potential future applications are outlined.
Keywords: collinear laser spectroscopy, atomic hyperfine structure;
INTRODUCTION
Laser spectroscopy is an experimental technique prob-ing the energy levels of the atomic hyperfine structureinduced by the nuclear electromagnetic properties [ ] .Root mean square charge-radii changes, magnetic-dipoleand electric-quadrupole moments of a nucleus are thequantities typically assessed. Atomic beams with well-defined energy of the order of tens of keV in combinationwith narrow-band lasers facilitate high-resolution mea-surements limited essentially by the lifetime broadening. The relativistic Doppler effect: ν = ν ( − β cos φ ) / (cid:198) − β is used, where φ is the angle between the propagationdirection of the laser radiation and the velocity (cid:126)υ of theatomic beam ( β = | (cid:126)υ | / c ), and takes values of φ = φ = π radians for collinear and anti-collinear ge-ometry, respectively. The Doppler-shifted frequency ν isscanned to match the transitions of the hyperfine struc-ture by varying the velocity | (cid:126)υ | while the laser frequency ν is kept constant, or vice versa. In most cases, mea-surements are implemented on atomic states, which re-quire neutralisation [ ] of the singly-ionized beams usedfor acceleration. Resonant laser excitations are detectableby various means [ ] . A comprehensive overview ofthe technique and its existing implementations can befound in dedicated reviews [
4, 5 ] . The following outlinesthe newly-constructed collinear-laser-spectroscopy setupat the ALTO facility [ ] . The natural linewidth in angular frequency is inversely proportionalto the lifetime of the excited state: Γ = τ − . INSTRUMENTATION
Figure 1 shows a technical drawing of the associated vac-uum beamline. Due to space limitations in the experimen-tal hall, the realised assembly is relatively compact, with atotal length of 3.8 m. The instrument is commissioned bymeasurements in the D line of Na, as shown in Fig. 2.A brief technical description of the instrumentation is pre-sented below.
Merging bender
A cylindrical bender with a radius of800 mm, a gap of 60 mm, and a height of 180 mm fa-cilitates a 20 ◦ rotation of the ion beam. The laser beampenetrates the outer electrode collinearly to the ion beamthrough an opening of 20 mm in diameter. The bendingvoltage requirement per unit beam energy is 0.15 V / eV.Windows of fused silica at Brewster’s angle of 55.5 ◦ , eachincluding a set of apertures, are installed at both ends ofthe instrument. Electrostatic ion optics
The bender is followed by a sec-tion for ion-beam handling comprising a Faraday cup, twox-y deflectors, and a non-steering quadrupole triplet inbetween, namely the model
EQT 64-15 by NEC . The latteris tuned in conjunction with a quadrupole lens [ ] in thepreceding section of beamline to achieve maximum trans-mission to the second Faraday cup. Post-acceleration chamber
Chamber 6 in Fig. 1 is spe-cific to collinear laser spectroscopy and conceptually sim-ilar to existing instrumentation [
2, 3 ] . A five-cylinderelectrostatic lens with elements of 40 ×
80 mm in lengthand diameter, separated by 12-mm thick insulators, isused for modification of the ion-beam energy. The post-acceleration potential is linearly distributed with a voltagedivider. Optionally, the first three elements may also beoperated as an Einzel lens to obtain an additional handle a r X i v : . [ phy s i c s . i n s - d e t ] M a y xyz1 1233 45 467 FIG. 1.
Vacuum beamline.
Technical drawing with a 90 ◦ longitudinal cross section: 1.) Fused-silica windows at Brewster’s angle;2.) 20 ◦ bender for laser and ion-beam overlap; 3.) Faraday cups; 4.) Electrostatic x-y deflectors; 5.) Electrostatic quadrupole triplet;6.) Post-acceleration and charge-exchange chamber; 7.) Optical detection; Total length: 3.8 m. on the ion-beam focusing. A hot cell for vaporizing al-kali metals is held at the post-acceleration potential. Foruse with sodium vapour on a sodium ion beam, its cen-tral region of 80 mm in length is heated to about 270 ◦ Cto achieve 50% neutralization. A floating power sup-ply
FPD-40-008-10 by ISEG , matched to a
THERMOCOAX 1 NcAc heating wire, is used for the purpose. A heating andcooling oil circulator
LAUDA-ECO-RE-630-GN maintains bothends at a constant temperature of just a couple of degreesabove the melting point, hence at a 100 ◦ C for sodium.Apertures of 8 mm contain the molten metal within thecell while obstructing the propagation of stray light.
Optical detection
Two telescopes of aspheric lenses po-sitioned in the horizontal plane image the ion-beam flu-orescence onto the faceplates of photomultiplier tubes.Longitudinal aperture arrays down to 8 mm at the en-trance and 12 mm at the exit reduce the background from laser scattering. All internal surfaces are painted in col-loidal graphite. The lenses
AL100100-328-SP by THORLABS with a diameter of 100 mm are custom made from ultra-violet grade fused silica with a focal length from the flatsurface of 76.2 mm at a wavelength of 328 nm. The as-sembly is designed with spacer rings which allows adjust-ing the focal distance in steps of 5 mm to accommodatewavelengths in the range from 200 nm to 1 µ m. Head-on photomultiplier tubes for single-photon counting areused, such as the 51-mm ( (cid:48)(cid:48) ) model from ET withdomed quartz windows, magnetic shielding, and a speci-fied dark count rate of less than 300 Hz. Control and data acquisition
Analogue and digitalvoltage input-output is implemented using
PXI expresselectronic modules from
NATIONAL INSTRUMENTS . A four-quadrant source and measure unit type provides ascanning potential, software limited to ±
10 V, with high
GHz-1 0 123 ²³Na, 3/2 ⁺ ×10 ⁴ c o un t s (11) (12)(21) (22) FIG. 2.
Fluorescence spectrum of the D line in Na ( I π = / + ) . Each transition is denoted in parentheses by the total angular-momentum quantum numbers of the lower and the higher state, respectively. The frequency scale is relative to the fine-structuresplitting. stability and low ripple. The actual post-acceleration volt-age is produced by a ± / from TREK capable of ramping speedsgreater than 700 V / µ s. A digital multimeter type is used to monitor and record the applied voltage via an OHM-LABS voltage divider model
KV-10R . During a scan, themultimeter is operated in a 5 / -digit mode at 3000 S / s.An ultra-precise 7 / -digit mode at 100 S / s is used for in-dependent voltage calibrations. An identical module isavailable for monitoring the acceleration voltage. Theanalogue signals of the photomultiplier tubes are con-verted to low-voltage TTL pulses and read by a timer-counter module type , operated as a 32-bit edgecounter. The unit is also capable of digital-signal out-put for controlling auxiliary equipment. All modules arehoused in a type chassis with an -type controllerand currently operated with a user-oriented LabVIEW soft-ware.
RESULTS OF COMMISSIONING TEST
Surface-ionized sodium was accelerated to an energy of30 keV and subjected to collinear laser spectroscopy inthe transition 3 s S / → p P o1 / . The required wave-length of 589.8 nm [ ] was produced by a cw ring dyelaser using Rhodamine 590 in ethylene glycol as the ac-tive medium. An example spectrum is shown in Fig. 2fitted with the empirical lineshape from Ref. [ ] , whichaccounts for the asymmetric line broadening from colli-sional excitations in the hot cell after charge exchange.The full width at half maximum of resonances observedthroughout the experiment is about 50 MHz, determinedby a substantial Gaussian-like component. Consideringthe differential Doppler shift of 14.2 MHz / V, the latter could be attributed to a 10 − ripple on the accelerationvoltage. For faster transitions, this contribution will besurpassed by the natural linewidth, which in the abovetransition is only about 10 MHz [ ] . A magnetic hy-perfine parameter of + ( )( ) MHz is found fromthe individual fit in Fig. 2. The systematic uncertaintyin the second set of parentheses represents the standarddeviation of the sample distribution from 26 independentmeasurements. This result is in line with the value fromatomic-beam magnetic resonance [ ] . The spectrum inFig. 2 was obtained in 30 scans of 400 steps, 20 ms each.Laser power of about 1 mW was used on an incident beamof about 300 pA, measured with the first Faraday cup inFig. 1. The corresponding detection efficiency is one pho-ton in 64000 ions. For faster transitions at the peak of thedetector’s quantum efficiency of 30%, compared to 2% atthe above wavelength, the overall detection efficiency isprojected to about 1 : 1000. PERSPECTIVES
Collinear laser spectroscopy interlinks several cutting-edge technologies: from the production of exotic beams,to lasers, to high-precision measurement instrumentation.In this context, “off-the-shelf” laser spectroscopy is outof reach, certainly on the isotope production side wherededicated facilities are required. Commercially availableuser-oriented laser systems, on the other hand, are al-ready being exploited in most existing installations toenhance operations. Likewise, the instrumentation de-scribed here has been developed with an emphasis onsimple solutions, thus, maximizing the use of standardequipment to reduce cost and development time, and toimprove user accessibility. To exploit its full potential,the apparatus needs to be used in conjunction with ion-beam bunching capabilities [ ] for background suppres-sion. Techniques such as laser-induced nuclear orienta-tion [ ] , collisional ionization [ ] , and state-selectivecharge exchange [ ] may also be considered in specificcases. Possible future applications include studies of nu-clear structure or facilitating experiments with polarizedbeams for decay spectroscopy [ ] or research on funda-mental symmetries [ ] . The instrument has been con-structed along the time-line of the low-energy radioactivebeam facility DESIR [ ] , where dedicated techniques forstudies of proton-rich nuclei will be developed.This work has been supported by the Orsay NuclearPhysics Institute, the P2IO Laboratory of Excellence, theFrench National Institute for Nuclear and Particle Physics,the Paris-Sud University, and the European Union’s HORI-ZON 2020 Program under grant agreement no. 654002.We thank the ALTO technical group for their professionalassistance. Author contributions
D.T.Y. initiated the project andcontributed in securing financial resources along with S.F.,G.G., E.M.R., L.V.R., and other collaborators. Implemen-tation took place under the technical coordination of A.S.M.L.B, A.K., S.L., D.N., L.V.R., and D.T.Y. prepared the in-strumentation. D.A. developed the software for data ac-quisition. All authors contributed to the measurementsand the preparation of the manuscript.
Data availability
Technical and experimental detailsare available from the main author upon reasonable re-quest. ∗ [email protected] [ ] H. Kopfermann,
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